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					THEJOURNALBIOLOGICAL
        OF        CHEMISTRY                                                                               Val. 266,No. 24, Issue of August 25, pp. 16098-16104,1991
0 1991 by The American Society for Biochemistry and Molecular Biology, Inc.                                                                        Printed in U.S.A.



Intracellular CalciumRegulates the Tyrosine Kinase Receptor
Encoded bythe MET Oncogene*
                                                                                                         (Received for publication, February 13, 1991)

                Lucia Gandino, Luca Munaron, Luigi Naldini, Riccardo Ferracini, Michele MagniS,and
                Paolo M. Comoglios
                From the Department of Biomedical Sciences and Oncology, University of Torino Medical School, 10126 Torino, Italy


  Previous work (Gandino, L., Di Renzo, M. F., Gior-                             receptor. In GTL-16 cells, due to the amplification of the
dano, S . , Bussolino, F., and Comoglio, P. M. (1990)                            MET oncogene (Ponzetto et al., 1991) the p190MET       receptor
Oncogene 6, 721-726) has shown that the tyrosine                                                                       5 ~ ~ ~
                                                                                 is overexpressed and the ~ 1 4 tyrosine kinase is active in
kinase activity of the receptor encoded by the MET                               the absence of a ligand (Giordano et al., 1988; Giordano et al.,
protooncogene is negatively modulated by protein ki-                             1989a). These cells provide a useful model to study positive
nase C (PKC). We now show that an increase of intra-                                                                          5 ~ ~ ~
                                                                                 and negative regulation of the ~ 1 4 kinase activity. We
cellular Ca2+ has a similar inhibitoryeffect in viuo, via                                                                  5 ~ activated by
                                                                                 have shown previously that ~ 1 4 kinase is ~ ~
a PKC-independent mechanism. In GTL-16 cells the                                 autophosphorylation on tyrosine (Naldini et al., 1991b) and
p14SMET kinase is overexpressed and constitutively
                                                                                 inhibited by protein kinase C (PKC)’-induced phosphoryla-
phosphorylated on tyrosine. A rapid and reversible
decrease of ~ 1 4 tyrosine phosphorylation was in-
                         6 ~ ~ ~                                                 tion on serine (Gandino et al., 1990). The normal endogenous
duced by treatment thewith         calcium ionophores                            cellular activator of PKC is diacylglycerol which is produced
A23187 or ionomycin. Experiments performed with                                  transiently in the cell membrane as result of the breakdown
the ionophores in absence of extracellular calcium                               of phosphatidylinositol phosphate by phospholipase C. This
showed that a rise in cytoplasmic Ca2+concentration                              enzyme concomitantly produces inositol 1,4,5-triphosphate, a
to 460 n (due to release from intracellular stores)
         M                                                                       soluble cytoplasmic second messenger causing the release of
resulted in a similar effect. These Ca2+concentrations                           calcium ions from intracellular stores (for a review, see Ber-
had no effect on p14SMET   autophosphorylation in an in                          ridge, 1987). In this paper we investigated whether both arms
vitro kinase assay. This suggests that the effect of Ca2+                        of the signal transduction pathway triggered by phospholipase
on p14SMET tyrosine    phosphorylation is not direct but                                                      5 ~ ~ ~
                                                                                 C modulate the ~ 1 4 kinase. The results indicate that the
may be mediated by Ca2+-activated protein(s).   Involve-                         increase in intracellular Ca2+concentration is inhibitory. This
ment of Ca2+-dependent tyrosine phosphatases was                                 negative regulation takes place independently from PKC, via
ruled out by experiments carried out in presence of                              activation of another serinekinase.
Na3V04. In vivo labelingwith [32P]orthophosphate
showed thattherise       of intracellular Ca2+ induces                                            EXPERIMENTALPROCEDURES
serine phosphorylation of p14SMET a specific phos-
                                     on
phopeptide. This suggests that Ca2+negatively modu-                                  Reagents and Cells-Phorbol 12-myristate 13-acetate (TPA) and
lates p14SMET  kinase through the  phosphorylation of a                          the calcium ionophores A23187 werepurchased from Sigma and fura-
                                                                                 2/AM and ionomycin from Behring Diagnostics. Triton X-100 was
critical serine residue by a Ca2+-activated serine ki-                           from Pierce Chemical Co [T-~’P]ATP(specific activity 7000 c i /
nase distinct fromPKC.                                                           mmol), [32P]orthophosphate (10 mCi/ml), and ‘”I-protein A were
                                                                                 obtained from Amersham Corp. NitrocelluIose filters for Western
                                                                                 blots were from Bio-Rad. The molecular mass markers used in SDS-
                                                                                 PAGE were 14C-methylatedmyosin (200 kDa), phosphorylase b (92
                                                                                 kDa), bovine serum albumin (69 kDa), egg albumin (46 kDa), and
                                                                                 carbonic anhydrase (30 kDa) (Amersham Corp.). Phosphotyrosine
   The protein encoded by the MET protooncogene is an a-8                        (Tyr(P))antibodies were raised in rabbits immunized with p-amino-
chain heterodimer of 190 kDa (p190MET;      Giordano et al.,                     benzenephosphonate and affinity-purified as described previously
1989a). The a-chain (50 kDa) is exposed at the cell surface                      (Comoglio et al., 1984). Anti-MET antibodies were raised in rabbits
and is linked by disulfide bonds to the @-chain (145 kDa:                        immunized against the synthetic peptide Val-Asp-Thr-Arg-Pro-Ala-
~ 1 4 5 ~Thelatter spans the plasma membrane and is
             ~ ~ ) .                                                             Ser-Phe-Trp-Glu-Thr-Sercorresponding to theamino acid sequence
                                                                                 at the C-terminal end of the predicted MET gene product and kindly
endowed with tyrosine kinase activity (Tempest et al., 1988;                     provided by M. F. Di Renzo (University of Torino). GTL-16 cells are
Gonzatti-Haces et al., 1988; Giordano et al., 1989b). These                      a clonal cell line derived from a poorly differentiated gastric carci-
                                                        is
structural and functional features suggest that p190MET a                        noma line (Motoyama et al., 1984). The cells were cultured at 37 “C,
                                                                                 under a humidified atmosphere of 95% air-5% COZ, in RPMI-1640
  * This work wassupported by grants from the Associazione Italiana              medium supplemented with 10% fetal calf serum and antibiotics
per la Ricerca sul Cancro, Consiglio Nazionale delle Ricerche, and               (penicillin, 100 units/ml; streptomycin, 100 pglrnl). Cells were used
the Ministry of Education. The costs of publication of this article              a t approximately 90% confluence. The standard solution employed
were defrayed in part by the payment of page charges. This article
must therefore be hereby marked “advertisement” in accordance with                   The abbreviations used are: PKC, proteinkinase C; TPA, phorbol
18 U.S.C. Section 1734 solelyto indicate this fact.                              12-myristate 13-acetate; SDS, sodium dodecyl sulfate; PAGE, poly-
  4 A.I.R.C. Fellow: Dept. of Pharmacology, CNR Center of Cyto-                  acrylamide gel electrophoresis; HEPES, 4-(2-hydroxyethyl)-l-piper-
pharmacology and Scientific Institute S. Raffaele, University of Mil-            azineethanesulfonic acid PBS, phosphate-buffered saline; EGTA,
ano, 20132 Milano.                                                               [ethylenebis(oxyethylenenitrilo)]tetraaceticacid; Pipes, 1,4-pipera-
    To whom correspondence should be addressed Dipartimento di                   zinediethanesulfonic acid TPCK, ~-tosylamido-2-phenylethyl     chlo-
Scienze Biomediche e Oncologia Umana, C.so M. D’Azeglio, 52-10126                romethyl ketone; HPLC, high performance liquid chromatography;
Torino, Italy. Tel. 39-11-6527739.                                               EGF, epidermal growth factor.

                                                                              16098
                                        Ca2+Inhibition of the MET-tyrosine Kinase
                                                                                Receptor                                                   16099
in thefluorimetric measurements was a Krebs-Ringer HEPES buffer               Samples were washed in water, lyophilized, and resuspended in 5%
(KRH) containing(in millimolar/l): NaCI,140;KCI,5;CaC12,                 2;   acetic acid, 0.5% pirydin, 5 m EDTA, pH 3.5 (running buffer)
                                                                                                               M
MgSO,, 1.2; glucose, 6; HEPES-NaOH, 25, pH 7.4.                               containing 1 mg/ml of each unlabeled phosphoserine, phosphothreo-
   Measurement of Intracellular Ca'+ Concentration-Intracellular              nine, and phosphotyrosine. Samples were run on cellulose plate for 1
calcium concentration ([Ca"]J was measured with the fluorescent               h a t 1.2 kV in running buffer. The plate was dried, and phosphoamino
probe fura-2 as described by Grynkiewicz et al., (1985). Monolayers           acids were localized by spraying the plate with 0.25% ninhydrin in
of two to four dishes (100-mm diameter) were washed twice with                acetone. The dried plate was then subjected to autoradiography a t
                                                          at
phosphate-buffered saline (PBS) and then treated 37 "Cwith 2 ml               -70 "C using intensifying screens for 7 days.
of the same buffer containing trypsin (0.01%) and EDTA (0.9 mM).                 Phosphopeptide Mapping-For phosphopeptide mapping the 32P-
Detachment of the cells from the dish was completed within 5 min.             labeled bands were excised from the polyacrylamide gel, washed twice
Trypsin was neutralized by resuspending the cells in growth medium            with 10% methanol in order to remove SDS, minced, and dried in a
for 3 ha t 37 "Cunder continuous stirring.Cells were then centrifuged                                                                 M
                                                                              lyophilizer. The gel slices were rehydrated with 50 m NH4COo,pH
at 800 rpm for 5 min, resuspended in RPMI 1640 plus 1% fetal calf             7.8, containing 50 pg/band of TPCK (~-tosylamido-2-phenylethyl
serum (cell concentration, 5 X 10" cell/ml) and loaded with fura-2 by         chloromethyl ketone)-trypsin and incubated for 2 h at 37 "C. The
a 30-min incubation period a t 37 "C with 3 p~ fura-2 pentacetoxy-            tryptic digestion was repeated once, and the gel slices were further
methylester (fura-Z/AM). For the fluorimetric measurement of                                     M
                                                                              eluted with 50 m NH4C03, pH 7.8. Eluates were then lyophilized
[Ca'+Ii, lofi cells were placed in the cuvette of a thermostatically          and further incubated with 100 pg/ml trypsin for 2 h a t 37 "C. The
controlled (37 rt 1 "C) cell holder. Fluorescence records were taken a t      essicated sample was then resuspended in buffer A (100% water
an excitation wavelength of 345 nm and emission of 490 nm, with               containing 0.1% trifluoroacetic acid) and filtered on a 0.2-pm Acrodisc
slits of 5 nm. The calibration of the fluorescence signal in terms of         filter (Gelman Sciences). The phosphopeptides were analyzed on a
        was
[Ca2+Ii done according to Grynkiewicz et al. (1985).                          reverse phase CZ-CI8 Superpack Pep-S column (PharmaciaLKB
   Cell Treatments and Western Blotting-Subconfluent cultures of              Biotechnology Inc.), resolved on a gradient acetonitrile inbuffer A
                                                                                                                            of
GTL-16 cells were incubated for the indicated times with TPA (160             of 0.46%/min (from 0 to 32% acetonitrile in 82 min) with a flow rate
nM) A23187(10p M if not otherwise indicated), ionomycin (10 pM, if            of 1ml/min. The eluted radioactivity was monitored by a Radiomatic
not otherwise indicated), or Na3VO4(100 p ~ ) The treatment was
                                                      .                       A-100 radioactive flow detector (Packard). The fractions collected
terminated by placing the cells on ice. Monolayers were washed twice          from radio-HPLC runs were counted in a 8-counter (Packard), and
with ice-cold PBS and cells solubilized in boiling Laemmli buffer             the fractions corresponding to the largest radioactive peaks were
(Laemmli, 1970). Samples were adjusted to a protein concentration             pooled, lyophilized, and subjected to phosphoamino acid analysis.
of 300 pglwell, run in 8% SDS-PAGE, and transferred to nitrocel-
lulose sheets. Blots were probed with 10 pg/ml of purified Tyr(P)                                          RESULTS
antibodies or with anti-MET serum (1:500) followed by '2sII-labeled
protein A as described in details elsewhere (Di Renzo et al., 1986).             Calcium Ionophores Treatment Inhibits Tyrosine Phos-
Filters were subjected to autoradiography for 24 h a t -70 "C using                                    ~ Intact
                                                                              phorylation 0 f p 1 4 5 in ~ ~ Cells-GTL-16 cells express
intensifying screens.                                                         a highamountof the p190MET        receptorwhose145-kDa p-
   Protein Kinase C and Ca2+ Depletion-For PKC depletion, subcon-             subunit is autophosphorylatedon tyrosine in vivo, as assessed
fluent cultures of GTL-16 cells were incubated for 20 h with 160 nM           by Westernblot analysis of total cellular proteins probed with
of TPA as described previously (Gandino et al., 1990). To deplete
extracellular calcium, cells were washed and incubated in fresh RPMl          Tyr(P) antibodies (Giordano et al., 1988). It has been shown
medium, containing 4 m EGTA, 2 min before the addition of
                            M                                                 previously (Gandino et al., 1990) that treatment of GTL-16
ionophores. To deplete intracellular calcium, cells were left in the          cells with TPA for 1 h at 37 "C inhibits the tyrosine phos-
presence of 4 m EGTA and 10 p~ A23187 for different times as
                  M                                                                                      ~ ~ ~ . that a 5-min treatment
                                                                              phorylation of~ 1 4 5We now show
indicated in the legend of the figures.                                       with 10 p~ calcium ionophore A23187    also reduced the extent
   Immunoprecipitation and in Vitro Autophosphorylntion Assay-                                                             ~ " ~
                                                                              of tyrosinephosphorylationof the ~ 1 4 5 subunit. The same
GTL-16 cells were washed twice with ice-cold PBS andlysed in DIM
buffer (10 m Pipes, pH 6.8, 100 m NaC1, 5 m MgCI2, 300 m
              M                         M             M                 M
                                                                              effect was observed using a different calcium ionophore,ion-
sucrose, 5 m EGTA) plus 1%
             M                   Triton X-100 and a mixture of protease                       A.
                                                                              omycin (Fig. l ) The amountofreceptorinsampleswas
inhibitors. The cell lysates were centrifuged a t 10,000 rpm a t 4 "C for     identical as assessed in Western blots probed with antibodies
30 min and incubated with anti-MET serum coupled to Sepharose-                against a synthetic peptide derived from the predicted MET
protein A. Bound proteins were washed several times in DIM buffer             sequence (anti-MET: Fig. 1B). Both in the case ofA23187
without EGTA and MgC12 and incubated in the same buffer with                  and ionomycin, the extentof tyrosine phosphorylation ofthe
different concentration ofMgC12, CaCI2, and 10 p~ of [T-""P]ATP.
The kinase reaction was performed on ice for 2 min and stopped with
                                         M
1 ml of Tris-buffered saline with 10 m EDTA and 100 p~ Na3V04.
After a brief centrifugation proteins were eluted in boiling Laemmli
buffer and subjected to 8%SDS-PAGE followed by autoradiography
for 12 h a t -70 "C using intensifying screens. The relative amount of
                                          5 ~ ~
phosphate incorporated in ~ 1 4 was estimated by measuring the
optical density of the corresponding autoradiography band with a
                                                                                     A                                 B
laser densitometer (Pharmacia LKB 2202 Ultroscan).
   P2P10rthophosphateLabeling and Immunoprecipitation-Subcon-                      200                               200 -
fluent cultures of GTL-16 cells were incubated for 4 h at 37 "C with
1 mCi/ml of [R2P]orthophosphate phosphate-free RPMI medium
                                      in                                          p145 -                            P145-     0
                                                                                                                             0 "
in the absence of serum. The cultures were placed on ice, washed
twice with cold PBS, and lysed in RlPA buffer (20 m Tris-HC1, pH
                                                        M                         92.5 -                            92.5 -
7.4, 150 m NaC1, 2 m EDTA, 0.1% SDS, 1% Triton X-100, 1%
            M            M
                                      M
deoxycholate, 100 p M Na3V04,50 m NaF) and a mixtureof protease                     69 -                             69 .
inhibitors. Cell lysates were clarified a t 30,000 rpm for 30 min a t 4 "C
and immunoprecipitated with anti-METserum coupled to Sepharose-
protein A. Bound proteins were washed several times in RlPA buffer
and eluted in boiling Laemmli buffer. Eluted proteins were subjected                46 -                             46 -
to 8% SDS-PAGE followed by autoradiography for 1 h a t -70 "C
using intensifying screens.                                                                   anti P-Tyr                        anti-met
   Phosphoamino Acid Analysis-Phosphoamino acid analysis was
performed as described by Cooper et al. (1983). Briefly, "P-labeled              FIG. 1. Inhibition of tyrosine phosphorylation of ~ 1 4 5 ~ ~ ~
proteins were excised from the dried gel and eluted in 0.05 M ammo-           Western blot of proteins solubilized from GTL-16 cells exposed to
nium bicarbonate, pH 8, in the presence of 0.1% SDS and0.716 M @-             100 ng/ml of TPA for 1 h or to calcium ionophores A23187 (10 p M )
mercaptoethanol. Trichloroacetic acid-precipitated proteins were hy-          or ionomycin (10 p ~ for 5 min. A, Western blot probed with Tyr(P)
                                                                                                     )
drolyzed in 50 pl of 6 N HCl and placed in a 110 "C bath for 1 h.             antibodies; B, Western blot probed with anti-MET serum.
16100                                                                       Receptor
                                    Ca2+Inhibition of the MET-tyrosine Kinase
                    A
                                r
                          100

                     al                                                      al
                     -
                     3
                     m
                          80                                                 m
                     >                                                       >
                     -
                     m                                                       m
                                                                            ._
                    :G    60                                                .=    60
                    -
                    .-
                     C                                                      .-
                                                                             C




                                               L
                     0                                                       0
                     8 40                                                   8     40


                          20                                                      20

                           0
                                               2      3   4                                            2    3
                                0    1   IO 10 10 10                                      0   1   10 10 10


                                A23187 concentration (nM)                              IONOMYCIN concentration (nM)


                                                                      A23187
                                                                 3 0 1' 5' 10' 30'

                                             P145 -       Y



               FIG.2. Dose dependence and time course of the inhibitionof ~ 1 4 5tyrosine phosphorylation. A,
                                                                                                ~ " ~
            relative amount of phosphotyrosinepresentin        ~145~"",  estimated by measuringtheopticaldensity       of the
                                               of
            corresponding band in radiograms Western blots probed with Tyr(P) antibodies. Values were measured after 5-
            min treatment with increasing concentrationof A23187 or ionomycin. 100% refers the amount of phosphotyrosine
            present in untreated samples.Bars indicate the meanof triplicate values ina representative experiment (standard
            deviations never exceeded k 6%).B, time course of tyrosine phosphorylation of ~ 1 4 5 ~ after cell treatment with
                                                                                                     "'
            A23187 (10 p ~ ) .Proteins were solubilized after the indicated times and analysed in Western blot    probed with
            Tyr(P) antibodies.



           ~ " ~
~ 1 4 5 decreased as a function of the concentration of the
ionophore added; the half-maximal effect, measured after 5
min, was observed at a concentration close to 100 nM (Fig.                         -
                                                                                  tZ     A23187
                                                                                        EGTAEGTA+A23187
2 A ) . Time course experiments showed that the inhibitory                        0
effect of A23187 (10 p M ) was already detectable after 30 s of                   0 5' 30' 1hr 5' 30' l h r 5' 30' 1hr
treatment andwas maximal after 5 min (Fig. 2B).
   Calcium ionophores alter the permeability to ions of both
the cell plasma membrane and the membranesintracellular
                                               of                   200 -
stores. Experiments were performed to assess whether the
release of Ca2+ from these internal stores was sufficient to
                                   ~ " ~
determine a decrease of ~ 1 4 5 tyrosine phosphorylation. In
these experiments, the extracellular calcium in the culture
medium was removed by the addition of 4 mM EGTA before
addition of A23187 to the cells. Under these conditions the          69 -
ionophore A23187 could still release the Ca2+ stored in the
                                              Ca'+
intracellular compartment. The intracellular concentra-
tions reached were effective in inhibiting the kinase activity
              ~ " 3).
of ~ 1 4 5 (Fig. ~ The inhibition was reversible. If cells           46 -
were left in thepresence of EGTA and A23187 for 30 min or
more, the ensuing calcium efflux resulted in lowering the
intracellular concentration to the pretreatment values. Under
these conditions the tyrosine                               ~
                              phosphorylation of ~ 1 4 5 was" ~
restored (Fig. 3).                                                 FIG.3. Inhibition of ~ 1 4 tyrosine phosphorylation by
                                                                                                     5 ~ ~ ~
                                          ~ " ~
   Tyrosine Phosphorylation of ~ 1 4 5 Is Inhibited at Phys- Ca2+mobilized from intracellular stores.Western blot analysis
iological Concentration of Intracellular Ca2+-The actual in- with Tyr(P) antibodies of proteins solubilized after the indicated
tracellular calcium concentrations ([Ca"];) reached under the times from cells untreated (control) or incubated with 4 mM EGTA
                                                                                                         and right panels show the
experimental conditions described above were measured by to remove the extracellularcalcium.p Centerthe presence (center)or in
                                                                effect of the ionophoreA23187 (10 M ) in
the fluorescent probe fura-2. As expected, in GTL-16 cells the absence (right)of extracellular calcium.
treated with ionomycin, [Ca2+Iibegan to increaseimmedi-
                                    Ca2+Inhibition of the MET-tyrosine Kinase Receptor                                            16101
ately, reaching a maximum after a few seconds. On the aver-              view see Nishizuka, 1986). This is not the case in GTL-16
age, the [CaZ+liincrease was from 226 (+46) to 733 (f41) nM              cells where TPA treatment did not result in any detectable
(means  +   S.D.; n = 8 and 6, respectively). Thereafter,                variation of the steady-state level of [Caz+],(Fig. 4, trace C )
[Ca2+Iiremained elevated at a plateau for at least 25 min,               nor did it affect the response to ionomycin (not shown). The
when measurements were interrupted (Fig. 4, trace A ) . On                                                             5      two
                                                                         fact that PKC and Caz+ inhibit~ 1 4 kinase~by ~ ~inde-
the other hand, when this experimentwas carried out on cells             pendent mechanisms was proved by two mirror experiments.
incubatedin the same medium containing excess (4 mM)                     GTL-16 cells were depleted of PKC by 20-h treatment with
EGTA to buffer Ca2+to very low values ( 4 nM), the initial               100 ng/ml of TPA. Inthese conditions, TPA or otherphorbol
peak was reduced of about 50% ([Ca"];. The intracellular                 esters were unable to exert anyregulatory effect on the extent
concentration increased from 210 (k35)to 450 (f20) nM                                                               On
                                                                         of tyrosine phosphorylation of p14EiMET. the contrary, in
        +
(means S.D., n = 6) and returned to resting value in about               PKC-depleted cells, the calcium ionophores A23187 or iono-
7 (+2) min (Fig. 4, trace B ) . Finally, dose-response experi-           mycin were still able to exerttheirinhibitory        effect on
ments showed that detectable increases of [Ca2+]iwere ob-                ~ 1 4 5 tyrosine phosphorylation (Fig. 5A). On the other
                                                                                    ~ " ~
served beginning with ionomycin concentration as low as 100              hand, when cells were depleted of intracellular calcium by
p M (15% of maximal response, not shown). Half-maximal                   incubation for 1.5 h with EGTA and A23187, subsequent TPA
effect was observed at 1 NM and maximal at 10 pM; however                treatment, for 30 min, was still able to inhibit the autophos-
at ionophore concentration 4 0 0 nM, these responses became              phorylation on tyrosine of p14tjMET  (Fig. 5 B ) .
more transient, even if the plateau phasepersisted for at least                                                     Is '
                                                                            Tyrosine Kinase Activity of ~ 1 4 5 ~ ~Not Inhibited in
15 (+2) min. These measurements show that the observed                    Vitro by Physiological Ca2' Concentration-Tyrosine kinase
inhibition of the p190MET  tyrosine kinase activity takes place          activity is known to be sensitive to the concentrations of
at intracellular calcium concentrations falling within the               divalent ions (Pritchard et al., 1989). In order to study the
range of physiological responses (Swope andShonbrunn,                    possible direct effect of Caz+on the activity of p145MET,    we
1988; Corps et al.,1989).                                                performed an in vitrokinase assay in the presence of increas-
   Protein Kinase C and Ca2+Inhibit ~ 1 4 5 by Independent
                                                ~ " ~                    ing concentrations of Ca'+. The reaction was carried out, at
Mechanisms-As mentioned above (Fig. l ) , TPA-induced                    physiological concentration of M F (0.5 mM), using the
PKC activation inhibits the kinase activity of ~ 1 4 5 ~It is .
                                                            "~                       5 ~ ~ ~
                                                                         ~ 1 4 kinase immunoprecipitated by specific anti-MET
known that in some cells PKC interferes with calcium chan-               antibodies. Fig. 6 shows that no inhibitory effect was meas-
nels affecting the intracellular Ca'+ concentration (for a re-           ured within the physiological range ofCa'+ concentrations.




 763-
 212-   JI
         I-    4

             lON0
                                                                  450-

                                                                  204-


                                                                     EOTA
                                                                          4         4
                                                                                  K)FIo




                                                         C                                100




260-

               4
             TPA



                                               '   2 min.     '
               FIG. 4. [Caz+Ii increase induced by ionomycin in GTL-16 cells. GTL-16 cells were loaded with the
            fluorescent Ca'+ indicator fura-2 and changes in fluorescence continuously monitored. Where indicated by the
            arrows, ionomycin ( I O N 0 1 FM, truces A and B ) , EGTA (4 mM, truce B ) , or TPA (100 ng/ml, trace C ) were
                                          0
            added to the cell suspensions. The numbers to the left of each trace refer to the [Ca'+], expressed as nanornolar
                                                                         as
            ( n M ) .Values shown in the dose-response curve are expressed percentage of the maximum value observed after
            treatment with 10 P M ionomycin (standard deviations never exceeded * 5 % ) .
16102                                                 MET-tyrosine
                                      Ca2' Inhibition of the Kinase                                    Receptor
             PKC-depleted                                     Ca++ depleted       evaluate if tyrosine-specific phosphatase(s) were involved in
                                                                                                                               5 ~ ~
                                                                                  the loss of phosphotyrosine from ~ 1 4 after treatment in
                                                                                  vivo with calcium ionophores,experiments were performed in
                                                                                  the presence of sodium orthovanadate (Na3V04), a     powerful
                                                                                  tyrosine phosphatases inhibitor with broad specificity (Jones
                                                                                  et al., 1989). GTL-16 cells were treated for 1 h with 100 pM
                                                                                  Na3V04and then incubated for different length of time with
         A                                                B                       the ionophore A23187 in the presence of the inhibitor. As
                                                                                  expected, after Na3V04 treatment the   amount of phosphoty-
 200 -                                        200 -                                                   ~ ~
                                                                                  rosine in ~ 1 4 5 was"highly increased (Fig. 7). Under these
                                             pi45 -
                                                                                  conditions the tyrosine phosphorylation of a number of other
115 -
 14
                                                                                  proteins also increased.
92.5 -                                        92.5   -                               These may be   considered candidate substrates of the
 69 -                                             69 .


                                                                                                   N%V04
                                                                                                               - - +       +   +    +
 46 -                                             46 -
                                                                                                   A23187      -    + - + + +
                                                                                                                   (5') (5') (10') (30')
   FIG.5. Ca2+and PKC inhibit ~ 1 4 tyrosine phosphory-
                                               5 ~ ~ ~
lation by independent mechanisms. Proteins solubilized and ana-
lyzed in Western blot probed with Tyr(P) antibodies. A, cells were
depleted of PKC, as described under "Experimental Procedure,"                                          200 -
washed, and incubated in the absence (control) or in the presence of
100 ng/ml TPA for 1 h or 10 p~ A23187 or ionomycin for 5 min. B,                                      p145 -
cells were depleted of calcium by prolonged treatment with 4 m    M
EGTA and 10 p~ A23187 to discharge intracellular stores and further                                   92.5-
incubated with TPA (100 ng/ml) for 30 min or A23187 or ionomycin
for 5 min.                                                                                              69 -

                                                                              1
                                                                                                        46 -

                                                                                     FIG.7. Inhibition of ~ 1 4 tyrosine phosphorylation by
                                                                                                                        5 ~ ~ ~
                                                                                  A23187 is independent from tyrosine phosphatases. Cells were
                                                                                  treated with 100 p~ NanV04for 1 h and stimulated for 5, 10, or 30
                                                                                  min (brackets) with 10 p~ A23187, as indicated. Proteins were soh-
                                                                                  bilized and analyzed in Western blot probed with Tyr(P) antibodies.




                                                                                              A

                                                                                                                            P-Ser
        M#++(mM) 0.5    0.5     .
                               15      0.5          0.5          .
                                                                05     2.5
                                                                                              200 -                         P-Thr
        Ca2' (mM)   -   1
                        6'     I o4    1o    -~                 0.1    0.1

   FIG.6. In vitro tyrosine kinase activity of       ~ 1 4 5 The ~ ~ .
                                                                  ~                         pl45 -       Qom
~ 1 4was immunoprecipitated with anti-METantibodies, and the
           5 ~ ~                                                                             92.5-                          P-Tyr
kinase reaction was performed on ice for 2 min in the presence of [ T -
'"PIATP a t the indicated concentrations of Mg2' and Ca2+.After
SDS-PAGE andautoradiography, the kinase activity was assessed by
measuring the optical density of the "P-labeled band with a laser                              69 -
densitometer. A. U.,absorbance units.

lnhibition was observed only at concentrations higher than
100 p ~ the kinase activity was restored by increasing the
           ;
concentration of M P ,suggesting that the inhibitory effect                                  46 -
of Ca2+at nonphysiological concentrations was due to com-                           FIG.8. Analysis of the amino acids phosphorylated in
petition with Mg2' for generation of ATP-divalent cation                                           ~ ~ ~
                                                                                  ~ 1 4 in5response to A23187 treatment. A, SDS-PAGE of
complexes.                                                                        proteins soIubilized from [S'P]orthophospate-labeled cells and im-
                                                                                  munoprecipitated with anti-MET antibodies. Control, unstimulated
   The Ca2+-dependent  Inhibition of ~ 1 4 Tyrosine Phos-
                                               5 ~ ~ ~                            cells; A23187, cells treated with 10 PM A23187 for 5 min. B, high
phorylation Is Not Mediated by Tyrosine Phosphatases-The                          voltage electrophoresis of phosphoamino acids hydrolyzed from the
amount of phosphotyrosine in proteins is due to thebalanced                       p145 band eluted from the SDS-PAGE shown in A. P-Ser, phospho-
activity of tyrosine kinases and tyrosine phosphatases. To                        serine; P-Thr, phosphothreonine; P-Tyr, phosphotyrosine.
                                     Ca2' Inhibition of the
                                                     MET-tyrosine                Kinase Receptor                                   16103
                                                                                              if
                                                                       8B). In order to assessthe increase in phosphoserine content
                                     CONTROL                           was duetophosphorylation of specific site(s) induced by
1000.                                                                  calcium,phosphopeptideanalysis        was performed.Reverse
                                                            . P-scr
                                                            - P-thr    phase radio-HPLC separation of '"P-labeled ~ 1 4 5 tryptic ~ " ~
5                       P-tyr                                          peptides from controlorionophore-treated           cellswas per-
                                                                       formed. P145M"Tfrom control cells yielded seven peaks: the
                          I                                            major one, eluting t 35 min, contained only phosphotyrosine
                                                                                           a
                                                       4    - P-tyr
500.
                                                                       (Fig. 9). This represent the phosphopeptide containing the
                                                                       major tyrosine phosphorylation   site.2 P145""'   immunoprecip-
                                                                       itated from ionophore-treated cells yielded the same peptides,
                                                       0               but the amount of sLzP   incorporated in the peak eluting after
                                                                       35 min was drastically reduced. Ontheotherhand,the
                                                                                                                       peak
                                                                       amount of radioactivity incorporated in the eluting after
                                                                       19 min was increased more than 20-fold. Phosphoamino acid
                                                                       analysis of this peakshowed that phosphoserinewas the only
                                                                       labeled amino acid (Fig. 9). The other peaks were virtually
                                                                       unchanged.
1000
                                                        1    - P-ser                              DISCUSSION
5                                                            . P-lhr
                                                                                                                   of
                                                                        Structural and functional properties the proteinencoded
                                                                     by the MET oncogene ( ~ 1 9 0 ~ " "indicate that this trans-
                                                                                                                )
                                                                     membrane molecule is a tyrosine kinase receptor (Gonzatti-
                                                             . P-tyr
  500                                                                Haces, 1988; Giordanoet al., 1988). Itsunique two-chain
                                                                     composition (Giordano et al., 1989a; Giordano et al., 1989b)
                                                                     suggests that p190METis the prototype of a new class of
                                                                     receptors distinct from the alreadyknown three classes that
                                                                     include epidermal growth factor (EGF), insulin, and platelet-
                                                                     derivedgrowth factorreceptors, respectively (reviewed by
         o I O 20 30 40 50 60 70 80                                  Yarden and Ullrich, 1988). In man, the ~ 1 9 0 ~ "receptor is
                                                                                                                                 '
                        elution time                                 expressed in hepatocytes, incells of epithelial origin, and in
                                                                     a variety of epithelial tumors (Prat et al., 1991). Recently,
   FIG. 9. Analysis of tryptic pepdides phosphorylated          in
~ 1 4 5 in response to A23187 treatment.
             ~ ' ~                              [:"P]Orthophospate- hepatocyte growth factor (Miyazawa et al., 1989; Nakamura
labeled SDS-PAGE bands of p145''r"     were    subjected toTPCK-     et al., 1989; Zarnegar et al., 1990) has been proposed as the
trypsin digestion in 50 m ammonium carbonate, pH 7.8, for 4 h at putative ligand (Bottaro etal., 1991; Naldini et al., 1991a).
                          M
37 "C. The cleavage products were analyzed on a RP radio-HPLC          The intrinsic tyrosine kinase activity of the /3 subunit of
Peps column developed with a 0-32% acetonitrile gradient in 82 min, the MET receptor ( ~ 1 4 5 ~ " ' ) positively regulated by auto-
                                                                                                        is
as described under "Experimental Procedures." The upper panel phosphorylation on tyrosine residues (Naldini et al., 1991b).
(CONTROL) shows the elution profile of phosphopeptides originated
               immunoprecipitated from untreated GTL-16 cells.The Previous work fromthislaboratoryhas
from ~ 1 4 5 ' ' ~                                                                                                       also shownthat
                                                                                ~ " ~
lower panel (A23187) shows the profile of ~145"""immunoprecipi- ~ 1 4 5 kinase is negatively regulated by PKC activation in
                                                                                                           by
tated from cells treated with 10 PM A23187 for 5 min. The phospho- vivo. This inhibition is mediated serine phosphorylationof
amino acid analysis of the major HPLC peaks are shown on the right. ~ 1 4 5 (Gandino et al., 1990). It is known that PKC nega-
                                                                                ~ " ~
P-ser, phosphoserine; P-thr, phosphothreonine; P-tyr, phosphotyro- tively modulates several tyrosine kinase receptors, including
sine.                                                                EGF receptor, insulin receptor, and insulin-like        growth factor
                                                                     1 receptor. In all the above cases, inhibition is mediated by
             ~ " ~
~ 1 4 5 kinase. This observation indicates that the phospha-         threonine and/or serine phosphorylation (Cochet al., 1984;     et
tases involved in the turnover of phosphate groups of the Hunter et 1984; Davis andCzech, 1985; Jacobs etal., 1983;
                                                                                al.,
             ~ " ~
~ 1 4 5 tyrosine phosphorylation sites are indeed sensitive          Takayama et al., 1988; Koshio et al., 1989). Colony-stimulat-
                                                       of
t o vanadate. Fig. 7 also shows that in the presencevanadate ing factor 1 receptor functions are also inhibited                     by PKC
                   of                      ~          ~ ~
ions the amount phosphotyrosine in 1 4 5 still"decreased activation, although through different mechanisminvolving
                                                                                                       a
with time as a consequence of increased intracellular Ca'+ activation of a specific protease (Downing etal., 1989).
concentration. This experiments suggest that calcium ions               The data reported in this paper                                ""'
                                                                                                              show t h a t ~ 1 4 5 ~ kinase
regulate the steady-statelevel of tyrosine phosphorylation by is negatively regulated also by the increase of intracellular
affecting tyrosine kinase rather       that tyrosine phosphatase     Ca'+ concentration. The inhibitory effectwasobserved                  at
activity.                                                            [Ca2+Iilevels in thephysiological range (between 200 and 760
   Increased Intracellular Ca2+ Concentration Induces Serine nM), as determined by fluorometric analysis. Inhibition was
Phosphorylation of~145~"'Kinase-After calcium ionophore observed both after ionophore-induced calcium influx from
treatment,thetotalamount             of phosphate           in
                                                   detectable        the extracellular compartment and after release from intra-
            ~
~ 1 4 5 " " was unaffected, although the level of tyrosine phos- cellular stores. The Ca"-induced inhibition wasreversible
phorylation decreased (Fig. 8A). T o identify thenewly phos- and totally independent             from PKC activation, as took placeit
phorylated aminoacids, ['"PJorthophosphate-labeled         GTL-16 also in PKC-depleted cells. On the other hand, PKC inhibits
cells were treated with A23187, solubilized, and immunopre- the ~ 1 4 5 kinase by a mechanism which is totally inde-
                                                                                     ~ " ~
           with
cipitated anti-MET       antibodies. Phosphoamino               acid pendent from intracellular Caz+.
analysis of the protein immunoprecipitated from control        cells    A Ca2+-dependent negative regulation has been observed
revealed that phosphotyrosine was the most abundant phos- recently also in the casethe EGF       of             receptor. Treatment with
phorylated amino acid. A limited amount of phosphoserine
was also detectable. After A23187 treatment the protein was              R. Ferracini, P. Longati, L. Naldini, E. Vigna, and P. M.Comoglio,
almost exclusively phosphorylatedonserine residues (Fig. submitted for publication.
16104                                 Ca2+Inhibition of the MET-tyrosineKinase Receptor
the ionophore A23187 resulted in inhibition of EGF-induced               Davis, R. J., and Czech, M. P. (1985) Proc. Natl. Acad. Sci. U. S. A .
receptor tyrosine phosphorylation and loss of high affinity                82, 1974-1978
                                                                         Di Renzo, M. F., Ferracini, R., Naldini, L., Giordano, S., and Como-
binding sites (Friedman et al., 1989). Also in this case the               glio, P. M. (1986) Eor. J. Bwchem. 158,383-391
effect of Ca2+appeared to be independent from PKC activa-                Downing, J. R., Roussel, M. F., and Sherr, C. J. (1989) Mol. Cell. Biol.
tion (Friedman et al., 1989; Verheijden et al., 1990).                     9,2890-2896
   The importance of [Ca2+Ii      oscillation in cells is due to the     Edelman, A.M., Blumenthal, D. K., and Krebs, E. G. (1987) Annu.
fact that Ca2+ acts as second messenger to affect a large                  Reu. Biochern. 56,567-613
                                                                         Friedman, B., Van Amsterdam, J., Fujiki, H., and Rosner, M. R.
number of biochemical activities in the various subcellular                (1989) Proc. Natl. Acad. Sci. U. S . A . 86,812-816
compartments (for a review see Carafoli, 1987). The precise              Gandino, L., Di Renzo, M.     F., Giordano, S., Bussolino, F., and
mechanism by which Ca2+inhibits the kinase of ~ 1 4 is 5                  ~ ~ ~ P. M. (1990) Oncogene 5 , 721-725
                                                                           Comoglio,
unknown. Ca2+does not seem to directly affect the enzymatic              Giordano, S., Di Renzo, M. F., Ferracini, R., Chiado’ Piat, L., and
                                                                           Comoglio P. M. (1988) Mol. Cell. Biol. 8 , 3510-3517
                           ~ ~ ~ .
activity of ~ 1 4 5Although it is not possible to assess the             Giordano, S., Ponzetto, C., Di Renzo, M. F.,Cooper, C., and Comoglio,
local concentration of calcium at the membrane in response                 P. M. (1989a) Nature 3 3 9 , 155-156
to ionophores, no inhibitory effect was measured within the              Giordano, S., Di Renzo, M. F., Narsimhan, R. P., Cooper, C., Rosa,
physiological range of Ca2+ concentrations in an i vitro     n             C., and Comoglio, P. M. (1989b) Oncogene 4 , 1383-1388
kinase assay; inhibition was observed only at concentrations             Gonzatti-Haces, M., Seth, A., Park, M., Copeland, T., Oroszlan, S.,
                                                                           and Vande Woude, G. F. (1988) Proc. Natl. Acad. Sci. U. S. A . 85,
higher than 100 p ~ A calcium-dependent activation of tyro-
                          .                                                21-25
sine specific phosphatase(s) could explain the observed de-              Grynkiewicz, G., Poenie, M., and Tsien, R. Y. (1985) J. Biol. Chern.
crease in tyrosine phosphorylation of ~ 1 4 5Recently, the
                                                        ~ ~ ~ .            260,3440-3450
role of phosphotyrosine phosphatases in regulating the tyro-             Hunter, T., Ling, N., and Cooper, J. A. (1984) Nature 3 1 1 , 480-483
                                                                         Jacobs, S., Shayoun, N. E., Saltiel, A. R., and Cuatrecasas, P. (1983)
sine kinase activity of receptors has been emphasized (Roome               Proc. Natl. Acad. Sci. U. S. A . 8 0 , 6211-6213
et al., 1988; Lin et al., 1988). However, all the tyrosine phos-         Jones, S. W., Erikson, R. L., lngebritsen, V. M., and lngebritsen, T.
phatases known so far are inhibited by vanadate ions (Jones                S. (1989) J. Biol. Chern. 2 6 4 , 7747-7753
et al., 1989))and such an inhibitor did not impair the calcium-          Koshio, O., Akanuma, Y., and Kasuga, M. (1989) FEBS Lett. 2 5 4 ,
                                                                           22-24
dependent inhibitory effect. A calcium-dependent activation              Laemmli, U. K. (1970) Nature 227,680-685
                           cific
of a ~ 1 4 5 ~ ~ ~ - s p e protease was also ruled out, since the        Lin, M. F., Lee, C. L., and Clinton, G.M. (1988) Mol. Cell. Biol. 8 ,
                          5 ~ ~ ~
amount of ~ 1 4 was not altered incells treated with Ca2+                  5477-5485
ionophores. Inhibition of the activity of ~ 1 4 by5 Ca2+is      ~ ~      Miyazawa, K., Tsubouchi, H., Naka, D., Takahashi, K., Okigaki., M.,
                                                                         ~
associated to serine phosphorylation of a specific phospho-                Arakadi, N., Nakayama, H., Hirono, S., Sakiyama, O., Gohoda, E.,
                                                                           Daikuhara, Y., and Kitamura, N. (1989) Biochem. Biophys. Res.
peptide. It is well known that calcium and calmodulin may                  Cornrnun. 163,967-973
activatea number of protein kinases that phosphorylate                   Motoyama, T., Hojo, H., Suzuki, T., and Oboshi, S. (1984) Acta Med.
serine and/or threonine residues on a wide spectrum of dif-                Biol. 27,49-52
ferentsubstrates (for a review, see Edelman et al., 1987).               Nakamura, T., Nishizawa, T., Hagiya, M., Seki, T., Shimomishi, M.,
                                                                           Sugimura, A., Talshiro, K., and Shimizu, S. (1989) Nature 3 4 2 ,
                    inhibited ~
Since ~ 1 4 is 5 ~ ~ by Ca2+ concentrations               within the       440-443
physiological range, the signal transduced by the MET recep-             Naldini, L., Vigna, E., Narsimhan, R., Gaudino, G., Zarnegar, R.,
tor is likely to be modulated by other receptor systems coupled            Michalopoulos, G. K., and Comoglio, P. M. (1991a) Oncogene 6,
to transient Ca2+fluxes.                                                   501-504
                                                                         Naldini, L., Vigna, E., Ferracini, R., Longati, P., Gandino, L., Prat,
  Acknowledgments-Critical discussion with Carola Ponzetto,                M., and Comoglio P. M. (1991b) Mol. Cell. Biol. 11, 1793-1803
Maria Flavia Di Renzo, Gianni Gaudino, and Anna Mondino during           Nishizuka, Y. (1986) Science 233,305-312
                                                                         Ponzetto, C., Giordano, S., Peverali, F., Della Valle, G., Abate, M.,
the course of these studies and the secretarial help of Antonella          Vaula, G., and Comoglio, P. M. (1991) Oncogene 6,553-559
Cignetto are gratefully acknowledged.                                    Prat, M., Narsimhan, R. P., Crepaldi, T., Nicotra, M. R., Natali, P.
                                                                           G., and Comoglio. P. M. (1991) Znt. J. Cancer, in press
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