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					                                                                       Journal of Neuroscience Research 77:532–539 (2004)




Effects of Apolipoprotein E on the Human
Immunodeficiency Virus Protein Tat in
Neuronal Cultures and Synaptosomes
Chava B. Pocernich,1,2 Rukhsana Sultana,1,2 Eugene Hone,3 Jadwiga Turchan,4
Ralph N. Martins,3 Vittorio Calabrese,5 Avindra Nath,4 and D. Allan Butterfield1,2,6*
1
  Department of Chemistry, University of Kentucky, Lexington, Kentucky
2
  Center of Membrane Sciences, University of Kentucky, Lexington, Kentucky
3
  Sir James McCusker Alzheimer’s Disease Research Unit, School of Psychiatry and Clinical Neurosciences,
University of Western Australia, Hollywood Private Hospital, Perth, Western Australia, Australia
4
  Department of Neurology, Johns Hopkins University, Baltimore, Maryland
5
  Department of Chemistry, Division of Biochemistry, University of Catania, Catania, Italy
6
  Sanders-Brown Center in Aging, University of Kentucky, Lexington, Kentucky

Human immunodeficiency virus type 1 (HIV-1)-associated             lipoproteins (Mahley and Rall, 2000). ApoE is well known
dementia is observed in 20 –30% of patients with acquired         for its role in lipid and cholesterol homeostasis but has also
immunodeficiency syndrome (AIDS). The 4 allele of the              been demonstrated to have immunomodulatory properties
apolipoprotein E (APOE) gene currently is thought to play a       in vitro and may regulate smooth muscle and endothelial
role as a risk factor for the development of HIV dementia.        cell growth and differentiation (Laskowitz et al., 1998).
The HIV protein Tat is neurotoxic and binds to the same           The brain is a major site of synthesis for apoE, which is
receptor as apoE, the low-density lipoprotein receptor-           produced primarily by astrocytes. In humans, three pre-
related protein (LRP). In this study, we investigated the role    dominant isoforms of the apoE protein exists, apoE2,
apoE plays in Tat toxicity. Synaptosomes from wild-type           apoE3, and apoE4, which are thought to have varying
mice treated with Tat had increased reactive oxygen spe-
                                                                  degrees of antioxidant properties (Miyata and Smith, 1996;
cies (ROS), increased lipid and protein oxidation, and de-
creased mitochondrial membrane potential. Synaptosomes
                                                                  Lauderback et al., 2002). Inheritance of the 4 allele of the
from APOE-knockout mice also had increased ROS, in-               APOE gene has been implicated as a major genetic risk
creased protein oxidation, and decreased mitochon-                factor for late-onset Alzheimer’s disease (AD; Strittmatter
drial membrane potential, but to a significantly lesser de-        et al., 1993; Martins et al., 1995).
gree. Treatment of synaptosomes with heparinase and Tat                  Human immunodeficiency virus type 1 (HIV-1)-
increased Tat-induced oxidative stress, consistent with the       associated dementia is observed in 20 –30% of patients
notion of Tat requiring interaction with neuronal mem-            with acquired immunodeficiency syndrome (AIDS;
branes to induce oxidative damage. Human lipidated                Kolson and Gonzalez-Scarano, 2000). HIV infection in
apoE3 greatly protected neurons from Tat-induced toxicity,        the brain occurs mainly in macrophages, microglia, and
whereas human lipidated apoE4 showed no protection. We            multinucleated giant cells (Lee et al., 1993; Bagasra et
demonstrated that human apoE3 has antioxidant proper-             al., 1996). There are conflicting reports concerning the
ties against Tat-induced toxicity. Taken together, the data       inheritance of APOE4 and the development of HIV
suggest that murine apoE and human apoE4 act similarly            dementia. Corder et al. (1998) states that twice as many
and do not protect the cell from Tat-induced toxicity. This       HIV patients that carry the apoE4 protein were de-
would allow excess Tat to remain outside the cell and             mented or had peripheral neuropathy compared with
interact with synaptosomal membranes, leading to oxida-           apoE4-negative HIV patients. In contrast, Dunlop and
tive stress and neurotoxicity, which could contribute to          collegues (1997) reported no correlation between HIV
dementia associated with HIV. We show that the antioxi-           dementia or encephalitis and APOE genotypes.
dant properties of apoE3 greatly outweigh the competition
for clearance in deterring Tat-induced oxidative stress.
© 2004 Wiley-Liss, Inc.                                           *Correspondence to: Prof. D. Allan Butterfield, Department of Chemistry
                                                                  and Center of Membrane Sciences, 125 Chemistry-Physics Building, Uni-
Key words: LRP receptor; oxidative stress; ApoE3;                 versity of Kentucky, Lexington, KY 40506. E-mail: dabcns@uky.edu
ApoE4; HIV dementia; Tat
                                                                  Received 27 February 2004; Revised 14 April 2004; Accepted 16 April
                                                                  2004
      Apolipoprotein E (apoE) is a small secreted protein         Published online 8 June 2004 in Wiley InterScience (www.
of   34 kDa and a component of several different types of         interscience.wiley.com). DOI: 10.1002/jnr.20182

© 2004 Wiley-Liss, Inc.
                                                                                    Effects of ApoE on HIV Protein Tat            533

       The HIV-1 protein Tat transactivates viral and cel-          buffer. One hundred microliters of sample (200 g of synapto-
lular gene expression; is actively secreted into the extra-         somes) incubated with DCFH-DA were placed in triplicate in a
cellular environment, mainly from astrocytes, microglia,            96-well plate, and the resulting fluorescence was measured
and macrophages; and is taken up by neighboring, unin-              ( ex 485 nm, em 530 nm). Intracellular esterases convert
fected cells, such as neurons (Sabatier et al., 1991; Ensoli et     DCFH-DA to the anionic DCFH, which is unable to diffuse
al., 1993; Chang et al., 1997; Kruman et al., 1998). Tat            out of the synaptosomes. Reaction with ROS converts DCFH
protein is neurotoxic in vitro (Kruman et al., 1998; New            to dichlorofluoroscein (DCF), which is highly fluorescent.
et al., 1998; Chauhan et al., 2003) and in vivo (Jones et al.,
1998; Rappaport et al., 1999). Increased protein oxidation          Assessment of Protein and Lipid Oxidation
is observed in the cerebrospinal fluid (CSF; Turchan et al.,                The level of protein oxidation was assayed as protein
2003) and increased peroxynitrite, 4-lipid, and protein             carbonyl levels, an index of protein oxidation (Butterfield and
oxidation in the brain of HIV-demented patients (Boven              Stadtman, 1997). An Oxidized Protein Detection Kit (Oxyblot;
et al., 1999; Turchan, 2003). Tat has been reported to bind         Oncor, Gaithersburg, MD; catalog No. S7150-Kit) was used, as
to the low-density lipoprotein receptor-related protein             previously described (Butterfield et al., 1999; Lauderback et al.,
(LRP), efficiently internalized and transported into neu-            2001, 2002). Samples were incubated for 20 min with 12%
ronal nuclei in a biologically active form (Liu et al., 2000).      sodium dodecyl sulfate (SDS) and 2,4-dinitrophenylhydrazine
ApoE4 is also a competitive LRP physiological ligand.               (DNPH) in 10% trifluoroacetic acid, vortexed every 5 min, and
Because Tat is associated with oxidative stress, and the            then neutralized with Oxyblot Neutralization solution.
APOE4 allele may be a risk factor for HIV dementia, the                    HNE and protein carbonyl samples were diluted to ap-
current study investigated the effect apoE and its alleles          propriate concentrations, and 600 ng of protein were loaded
have on Tat-induced oxidative stress in synaptosomes and            directly onto nitrocellulose paper by the slot blotting technique.
neuronal cultures.                                                  Membranes were blocked with 3% bovine serum albumin [BSA;
                                                                    in phosphate-buffered saline (PBS) with 0.01% sodium azide
             MATERIALS AND METHODS                                  and 0.2% Tween-20] for 30 min at room temperature. The
Materials                                                           nitrocellulose membrane was incubated with primary rabbit
                                                                    anti-DNPH protein antibody from Oncor Oxyblot (1:150
      Tat (1–72 aa) was prepared as previously described and        working dilution) or anti-HNE antibody (1:4,000) for 90 min
was 98% pure (Conant et al., 1996). Six-week-old wild-type          and then to a secondary antibody (anti-rabbit IgG coupled to
(WT) C57BL/6J inbred mice (stock 000664) and age-matched            alkaline phosphatase) diluted in the blocking solution 1:15,000,
apoE-knockout (KO) B6.129P2-Apoe            tm1Unc      (stock      for 1 hr at room temperature. Membranes were washed after
002052) mice (no apoE is present) were purchased from Jackson       every step in washing buffer (PBS with 0.01% sodium azide and
Laboratories (Bar Harbor, ME). Antibody against the lipid           0.2% Tween-20). The nitrocellulose paper was then developed
peroxidation product 4-hydroxy-2-trans-nonenal (HNE),               by Sigmafast tablet. Blots were analyzed by using the computer-
HNE11-S, was purchased from Alpha Diagnostics. Heparinase 1         assisted imaging software Scion Imaging (Lauderback et al.,
from Flavobacterium heparinium was purchased from Sigma (St.        2001).
Louis, MO).
                                                                    Mitochondrial Membrane Potential
Synaptosomal Preparation                                                  Mitochondrial membrane potential was determined with
      Cortical synaptosomes were prepared from apoE-KO and          the JC-1 dye. Synaptosomes were incubated for the last 30 min
WT mice by ultracentrifugation on discontinuous sucrose gra-        of treatment with 10 M JC-1 dye in the dark at 37°C, then
dients as described previously (Lauderback et al., 2001). Protein   washed in Locke’s buffer and centrifuged for 3 min at 3,000g.
concentration was determined by the Pierce BCA method, and          Synaptosomes (1 mg) were resuspended in Locke’s buffer
synaptosomes were adjusted to 1 mg/ml in Locke’s buffer             (500 l) and a 100- l aliquot was placed in triplicate in a
(154 mM NaCl, 5.6 mM KCl, 2.3 mM CaCl2, 1.0 mM MgCl2,               96-well plate, and the resulting fluorescence was measured
3.6 mM NaHCO3, 5 mM glucose, 5 mM HEPES). Samples                   ( ex 525 nm, em 590 nm).
were treated with 500 nM Tat (1–72) and Tat with 1 U/ml
heparinase 1 for 3 hr at room temperature, with continuous          Lipoprotein Vesicles
shaking.                                                                  The main advantage to synthesizing lipoproteins is that
                                                                    they do not contain any of the other proteins that are found on
Reactive Oxygen Species Measurement                                 lipoproteins in vivo, yet closely mimic the behavior of their
      The measurement of reactive oxygen species (ROS) for-         animal-derived counterparts (Bowler et al., 1991; Mamo et al.,
mation in synaptosomes treated with Tat (1–72) was performed        1991; Redgrave et al., 1993). Lipoprotein vesicles were prepared
with dichlorodihydrofluoroscin-diacetate (DCFH-DA) as pre-           as previously described (Martins et al., 1998). Briefly, triolen
viously described (Kanski et al., 2001). DCFH-DA was dis-           (4.5 mg), free cholesterol (0.9 mg), cholesterol oleate (0.5 mg),
solved in ethanol to a concentration of 10 mM. Aliquots of          and egg-yolk phosphatydyl choline (2.5 mg) were combined
synaptosomes (1 mg protein level) were incubated with 10 M          and purged with nitrogen gas and then desicated overnight.
DCFH-DA in 1 ml of Locke’s buffer for 30 min at 37°C. Excess        Lipids were mixed with 8.5 ml of 2.2% glycerol in distilled water
dye was removed by washing three times with Locke’s buffer,         and sonicated for 1 hr at 55–56°C with nitrogen purging over
and then synaptosomes were resuspended in 0.5 ml of Locke’s         the top. The volume was readjusted to 8 ml by using 2.2%
534     Pocernich et al.




Fig. 1. Tat significantly increases ROS formation in both wild-type
(WT) and apoE-KO mouse synaptosomes as assessed by DCF fluor-         Fig. 2. Mouse synaptosomal protein oxidation is elevated by Tat. Tat
scence. In comparison with corresponding controls, ROS is more       significantly increased protein carbonyl levels in both WT and
elevated in WT synaptosomes treated with Tat compared with           apoE-KO mouse synaptosomes. In comparison with corresponding
apoE-KO synaptosomes treated with Tat (P .05). Data presented are    controls, protein oxidation is significantly more elevated in WT syn-
means and SEM (n 6; *P .05 vs. corresponding control, Student’s      aptosomes treated with Tat compared with apoE-KO synaptosomes
t-test). See text.                                                   treated with Tat (P .05). An increase in protein carbonyl levels was
                                                                     seen in synaptosomes from WT and apoE-KO mice treated with
                                                                     1 U/ml heparinase and 500 nM Tat compared with synaptosomes
                                                                     treated with 500 nM Tat. Data presented are means and SEM (n 6;
glycerol, and 1.14 g dibasic sodium phosphate was added. Lipids      *P .05 vs. respective control; #P .05, Student’s t-test). See text.
were layered on top of NaCl gradients (1.065, 1.04, 1.02,
1.006 g/ml) and spun at 35,000 rpm for 1 hr at 20°C with a
Beckman SW41 rotor in a Beckman L8-70M ultracentrifuge.                    ROS generation leads to increased protein and lipid
Lipids were collected, adjusted to 2 ml with distilled water, and    oxidation (Butterfield and Stadtman, 1997). Protein oxi-
stored at room temperature with nitrogen purged above. Lipid         dation was measured by the levels of protein carbonyls.
vesicle size was analyzed by electron microscope. ApoE was           HNE is a marker of lipid peroxidation and is formed by
added to the lipid solution and sonicated three times for 1 sec      free radical oxidation of unsaturated lipids (Esterbauer et
each.                                                                al., 1991). Consistent with the ROS findings, both wild-
                                                                     type and APOE-KO Tat-treated synaptosomes had signif-
                        RESULTS                                      icant increases (P .05) in protein carbonyls (Fig. 2). An
      In this study, the role of apoE in Tat-induced oxi-            increase in HNE levels was seen only in the wild-type
dative damage to synaptosomes and neurons in culture was             synaptosomes treated with Tat, not the APOE-KO syn-
investigated. Synaptosomes isolated from WT mice con-                aptosomes treated with Tat (Fig. 3). Thus, these results
tain apoE as shown by Lauderback et al. (2001) and Keller            demonstrate that Tat directly increased protein and lipid
et al. (2000), and WT and apoE-KO mice have similar                  oxidation in vitro. Tat-treated wild-type synaptosomes
levels of the LRP receptor (data not shown), which is                had a significant percentage increase over controls in pro-
consistent with previous reports (Games et al., 1995). To            tein carbonyl levels (75%) and HNE levels (50%) com-
measure ROS formation by Tat and the influence of apoE                pared with APOE-KO Tat-treated synaptosomes (34%
in Tat-induced formation of ROS, synaptosomes were                   and 2%, respectively).
incubated with nonfluorescent DCFH-DA, which, upon                          Mitochondrial membrane potential as measured by
oxidation, produces the highly fluorescent DCF. After                 JC-1 fluorescence was decreased in both wild-type and
treatment with 500 nM Tat for 3 hr at room temperature               apoE-KO synaptosomes treated with Tat. Wild-type syn-
(Fig. 1), there is a significant increase in ROS in both              aptosomes had a significantly larger decrease in mitochon-
wild-type and apoE-KO synaptosomes (P          .05), as as-          drial membrane potential (P          .05) compared with
sessed by increased fluorescence. Tat directly induced                apoE-KO synaptosomes (Fig. 4).
ROS formation in vitro in synaptosomes. However, wild-                     Both Tat and apoE require the presence of the
type synaptosomes containing apoE and treated with Tat               protein heparan sulfate proteoglycan (HSPG) in order to
exhibited a significant 44% increase (P       .05) in ROS             be taken up by the LRP receptor (Liu et al., 2000), and
compared with the control, whereas apoE-KO synapto-                  Tat binds tightly to heparan sulfate (Ziegler and Seelig,
somes treated with Tat only increased DCF fluorescence                2004). We added the enzyme heparinase to cleave HSPG,
by 29% (P .05). The 44% increase in wild-type vs. the                thus inhibiting the uptake of Tat and apoE and exposing
29% increase of ROS in apoE-KO is significantly different             Tat to the outer membrane for a longer period. Protein
(P .01).                                                             carbonyl and HNE levels significantly increased in both
                                                                                            Effects of ApoE on HIV Protein Tat             535




Fig. 3. Mouse synaptosomal lipid peroxidation is induced by Tat. A
significant increase in the lipid peroxidation product HNE was found in
WT synaptosomes treated with Tat but not in apoE-KO synaptosomes           Fig. 5. Lipidated human apoE3 partially protects hippocampal neurons
treated with Tat. An increase in HNE levels was observed in synapto-       from Tat-induced protein oxidation and lipid peroxidation compared
somes from WT and apoE-KO mice treated with 1 U/ml heparinase              with lipidated human apoE4. Primary hippocampal neurons were
and 500 nM Tat compared with synaptosomes treated with 500 nM              treated with 10 g/ml lipidated specific human apoE isoform and
Tat. Data presented are means and SEM (n              6; *P     .05 vs.    500 nM Tat for 24 hr. Data are mean and SEM (n 3; *P .05 vs.
corresponding control, #P         .05 vs. WT-Tat, Student’s t-test). See   E4/Tat and Tat, Student’s t-test). See text.
text.

                                                                           neurons. Lipidated human apoE3 displayed antioxidant
                                                                           properties by partially protecting neurons from Tat-
                                                                           induced protein and lipid oxidation compared with the
                                                                           effects of Tat and human lipidated apoE4 (Fig. 5).
                                                                                                  DISCUSSION
                                                                                  The present study assessed the effects that Tat have
                                                                           on synaptosomes from WT and APOE-KO mice and the
                                                                           effects of lipidated human apoE3 and apoE4 alleles on
                                                                           Tat-induced oxidative stress. Increased ROS formation,
                                                                           elevated protein and lipid oxidation, and decreased mito-
                                                                           chondrial membrane potential in Tat-incubated synapto-
                                                                           somes from WT mice support data suggesting that Tat
                                                                           induces oxidative stress and is neurotoxic (Chauhan et al.,
                                                                           2003; Turchan et al., 2003). WT synaptosomes treated
                                                                           with Tat displayed a greater increase in ROS, protein
Fig. 4. Synaptosomes from WT mice are more vulnerable to Tat-              carbonyls, and HNE levels and a greater decrease in mi-
induced alterations in mitochondrial membrane potential than synap-        tochondrial membrane potential compared with
tosomes from apoE-KO mice. Synaptosomes from WT or apoE-KO                 APOE-KO synaptosomes treated with Tat. Not all pro-
mice were treated with Tat (500 nM) and analyzed for alterations in        teins and peptides produce ROS in synaptosomes. For
mitochondrial membrane potential by using the fluorescent indicator         example, whereas Alzheimer’s amyloid -peptide (1– 42)
JC-1. Both WT and apoE-KO synaptosomes showed a significant                 induces ROS, substitution of the methionine residue of
decrease from corresponding controls, WT mitochondrial membrane            this peptide by norleucine, which is not neurotoxic, does
potential being more affected by Tat (P    .05). Data are mean and         not produce ROS (Varadarajan et al., 2000). We recently
SEM (n 3; *P .05 vs. corresponding control, Student’s t-test). See
text.
                                                                           have shown that Tat, incubated with primary cortical rat
                                                                           neurons, also increases protein and lipid oxidation (C.B.
                                                                           Pocernich and D.A. Butterfield, unpublished observa-
                                                                           tions). These data suggest two things: first, that competi-
WT and apoE-KO mice treated with 1 U/ml heparinase                         tion for the LRP receptor between apoE and Tat increases
and 500 nM Tat compared with synaptosomes treated just                     the length of time for which Tat is in contact with the
with Tat (Figs. 2, 3).                                                     membrane, thereby increasing Tat-induced oxidative
      The effects of the different human apoE alleles on                   stress and neurotoxicity, and, second, that murine apoE
Tat-induced toxicity were investigated by the addition of                  confers no antioxidant properties against Tat-induced ox-
lipidated human apoE and Tat to primary cortical rat                       idative stress. Alternatively, it is possible that murine apoE,
536     Pocernich et al.

which acts like human apoE4, actually potentiates Tat-          toxicity, rat cortical neurons were incubated with lipidated
induced oxidative injury to neurons.                            human apoE3 and apoE4 isoforms and Tat. Lipidated
       ApoE4 and Tat are competitive ligands for the LRP        human apoE was used, because lipidation is the natural
(Liu et al., 2000). Neuronal uptake of Tat through the          state of this lipoprotein in plasma. Human lipidated
LRP receptor-mediated pathway reportedly inhibits neu-          apoE3 greatly attenuated Tat-induced protein and lipid
ronal clearance of other LRP ligands, such as apoE4 (Liu        oxidation, whereas human lipidated apoE4 seemed to have
et al., 2000). Both apoE and Tat require initial binding to     no such properties against Tat-induced oxidative effects
the protein HSPG before uptake by the LRP receptor (Liu         (Fig. 5). These data confirm that murine apoE acts simi-
et al., 2000). Recently, it has been demonstrated that          larly to human apoE4, offering no protection against Tat-
heparan sulfate can tightly and rapidly bind Tat (Ziegler       induced toxicity.
and Seelig, 2004). The enzyme heparinase cleaves HSPG,                 Tat-induced neurotoxicity is thought to be mediated
not allowing apoE or Tat to bind to HSPG and thereby            through excitotoxic mechanisms. Tat specifically binds to
inhibiting uptake of both by the LRP receptor. Hepari-          rat brain synaptosomal membranes with moderate affinity
nase is not known to cleave Tat (Liu et al., 2000). To test     (Kd 2 M; Sabatier et al., 1991). Tat is capable of depo-
the hypothesis that Tat-induced oxidative stress increases      larizing rat CA1 hippocampal neurons and human cortical
with increased contact with the membrane, we incubated          neurons (Magnuson et al., 1995), increasing intracellular
synaptosomes with heparinase. Tat- and heparinase-              Ca2 (Nath et al., 1996), and inducing neuronal death
treated synaptosomes from WT and apoE-KO mice both              (Tardieu et al., 1992; Hayman et al., 1993; Magnuson et
displayed increased protein and lipid peroxidation com-         al., 1995). In neurons, Tat leads to the activation of
pared with that induced by Tat alone (Figs. 2, 3). These        phosphatidylinositol 3-kinase (Milani et al., 1996), to in-
data suggest that Tat-induced toxicity involves contact         creased levels of inositol triphosphate (IP3), to calcium
with the membrane and that murine apoE does not act as          release from IP3-sensitive endoplasmic reticulum (ER)
an antioxidant. It is not known whether Tat binding to the      internal stores (Haughey et al., 1999), and to increased
LRP receptor results in toxicity. Our data show that            activity of the protein kinase C isoforms , , and
uptake of Tat by the LRP receptor results in decreased          (Borgatti et al., 1998). Tat-induced neurotoxicity is pre-
oxidative stress, which could be interpreted as promoting       vented by antagonists of phopholipase C and IP3-sensitive
neuronal survival, but, upon internalization, Tat enters the    ER calcium release (Haughey et al., 1999). Tat toxicity is
nucleus of the cell, where it stimulates the transcription of   also related to glutamate receptor activation; antagonists of
dormant viral genes. This scenario is particularly impor-       N-methyl-D-aspartate (NMDA) and non-NMDA recep-
tant, insofar as, in the nervous system, the LRP receptor is    tors partially protect neurons from the toxic effects of Tat
abundantly expressed on neurons (Herz and Strickland,           (Hayman et al., 1993; Magnuson et al., 1995; Nath et al.,
2001), and internalized Tat may accelerate HIV dementia.        1996; Haughey et al., 2001). Tat introduced intracellularly
       Lipid oxidation in apoE-KO synaptosomes treated          through patch recording pipettes does not alter neuronal
with Tat was not significantly increased, although a small       membrane potentials (Cheng et al., 1998). The above-
yet significant increase is observed after the addition of Tat   described findings suggest that Tat is capable of directly
and heparinase, albeit to a significantly lesser extent com-     exciting neurons and causing excitotoxicity by interacting
pared with synaptosomes treated in the same way from            with the cell membrane and causing detrimental down-
WT mice. The increased ROS formation, which is                  stream effects.
known to lead to protein and lipid oxidation (Butterfield               The 4 allele of APOE is a risk factor for AD and also
and Stadtman, 1997), was measured in the synaptosomal           possibly for HIV dementia. As noted above, some re-
cytosol and not in the lipid bilayer. The extended time in      searchers report that twice as many HIV patients that carry
which Tat interacts with the membrane after heparinase          the apoE4 protein were demented or had peripheral neu-
treatment may allow for greater interaction with the lipid      ropathy compared with apoE4-negative HIV patients
bilayer and increased lipid oxidation, although our data        (Corder et al., 1998). Other researchers reported no cor-
may suggest that Tat has to interact with apoE to induce        relation of HIV dementia or encephalitis with APOE
oxidation in the lipid bilayer. Many proteins in the lipid      genotypes (Dunlop et al., 1997). Recently, it has been
bilayer protrude partially into the extracellular space and     demonstrated that there are increased HNE levels, a
are accessible to other proteins and ROS that are pro-          marker of lipid peroxidation, in brain and CSF of HIV
duced; consequently, such exposed proteins can be easily        patients with dementia vs. HIV patients without dementia,
oxidized. This might partially explain why Tat-treated          and this finding correlates with the 4 allele of APOE (A.
WT and KO synaptosomes had a greater increase in pro-           Nath, unpublished observations). The three alleles for
tein oxidation compared with lipid peroxidation. HNE            human APOE have differential antioxidant capabilities,
binds to proteins by Michael addition, thereby adding a         E2      E3      E4 (Lauderback et al., 2002; Miyata and
carbonyl. Thus, HNE contributes to protein carbonyl             Smith, 1996), and the reverse order displays increased
levels in oxidative stress.                                     injury from stroke, head injury, and amyloid peptide-
       ApoE is thought to have antioxidant capabilities in      induced toxicity in brain (Chen et al., 1997; Sheng et al.,
brain (Lauderback et al., 2002). To test the antioxidant        1998; Lauderback et al., 2002). Our findings suggest that
capabilities of human apoE alleles against Tat-induced          human apoE3 does have antioxidant properties against
                                                                                   Effects of ApoE on HIV Protein Tat                  537

Tat-induced toxicity, supporting the previous studies            oxidative stress and neurotoxicity (Butterfield et al., 2001,
listed above. The APOE alleles differ from each other by         2002). ApoE binds A in an allele-specific manner (LaDu
two amino acids. ApoE2 has cysteine residues at positions        et al., 1994; Yang et al., 1997) and clears this toxic peptide
112 and 158, E3 has a cysteine at 112 and an arginine at         from the extracellular space through the LRP receptor in
158, and E4 has an arginine at both positions. The simple        an allele-specific manner (Beffert et al., 1999; Yang et al.,
amino acid differences in alleles could account for the          1999; Kang et al., 2000). The removal of A by an
difference in antioxidant capabilities that apoE is thought      LRP-mediated pathway is thought to be a mechanism to
to posses (Lauderback et al., 2002). Only one apoE iso-          prevent protein and lipid oxidation induced by A inter-
form has been identified in mice and is most analogous to         action with membranes (Lauderback et al., 2001). Simi-
the human apoE4 protein (Rajavashisth et al., 1985),             larly, we have shown that Tat interaction with the cell
although functionally there are clear species differences        membrane allows for oxidative stress to occur, possibly
(Fagan et al., 2002). Murine apoE is similar to human            through interactions with other receptors, such as the
apoE4 in that it possesses an arginine at position 112           NMDA receptor, and clearance of Tat from the mem-
(Rajavashisth et al., 1985). The similar levels of increase in   brane prevents continued oxidative stress. Consistent with
protein carbonyls and HNE suggest that murine apoE is            previous studies, human apoE3 acts as an antioxidant,
analogous to human apoE4 in not having antioxidant               whereas human apoE4 and mouse apoE, which is similar
capabilities toward Tat-induced toxicity. However, this          to human apoE4, do not provide protection against Tat-
notion should be tempered, in that murine apoE does not          induced toxicity. Further investigations are currently un-
posses an arginine at 61, unlike human apoE4.                    derway in our laboratory to gain insights into the molec-
       There is isoform-specific binding of apoE to the           ular pathways by which Tat induces oxidative stress and
LRP/apha 2 macroglobulin and low-density lipoprotein             neurotoxicity and the relationship of apoE alleles to HIV
(LDL) receptors, which might help to explain the isoform-        dementia.
specific protection against Tat-induced oxidative stress.
Affinity to the apoE receptors decreases according to apoE                      ACKNOWLEDGMENTS
genotype, E4 E3 E2, with E2 having a much greater                      This work was supported in part by NIH grants
decrease in affinity than E3 (Bohnet et al., 1996; Mamotte        MH-64409, AG-10836, and AG-05119 to D.A.B. and
et al., 1999). ApoE3 and -E2 have a decreased affinity to         RO1 NS-39253 and P20 RR-15592 to A.N. We thank
the receptor, which facilitates continued competition for        Dr. Mark Kindy for useful discussions. R.N.M. and E.H.
the receptor, thereby increasing Tat interaction with the        are supported by the McCusker Foundation for Alzhei-
membrane, likely resulting in increased oxidative stress.        mer’s Disease Research.
However, the antioxidant properties of apoE2 and apoE3,
resulting from the cysteine residues or possible continued                                 REFERENCES
interaction of apoE2 and E3 with Tat, appear to outweigh         Bagasra O, Bachman SE, Jew L, Tawadros R, Cater J, Boden G, Ryan I,
competition for the receptor, thereby reducing Tat-               Pomerantz RJ. 1996. Increased human immunodeficiency virus type 1
induced oxidative stress.                                         replication in human peripheral blood mononuclear cells induced by
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astrocytes. Neuronal apoE is most likely internalized from       Beffert U, Aumont N, Dea D, Lussier-Cacan S, Davignon J, Poirier J.
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