How do small GTPase signal transduction
pathways regulate cell cycle entry?
Chris Marshall, Current Opinion Cell Biology 1999 Dec; 11(6)
INTRODUCTION SIGNAL TRANSDUCTION AND CELL CYCLE PROGRESSION
This poster provides an outline of how growth-factor-activated small GTPase signalling pathways Cellular Ras function is required throughout most of G1 in order for growth factors to stimulate
control entry into the cell cycle in mammalian somatic cells. Signal transduction pathways from the quiescent cells to enter DNA synthesis. Ras function is also required for cell cycle progression in
small GTPases of the Ras and Rho family play an important role in cell cycle control. The entry of asynchronously growing cycling cells, and Ras function is actually required in the previous cell
quiescent G0 cells into S-phase is also dependent on the activities of the G1 cyclin-dependent cycle presumably to provide cell cycle components, such as cyclin D1, for the following cell cycle.
REGULATION OF CYCLIN D1 EXPRESSION
G1 cyclin-dependent kinases (Cdks) consist of: Cdk4 and Cdk6 complexed with the D-type cyclins A key event in stimulating cell cycle entry in quiescent cells is the activation of the kinases Cdk4
D1, D2, D3, and Cdk2 complexed with cyclin E. A key regulatory event mediated by the G1 Cdks is and Cdk6. In quiescent cells, expression of the D-type cyclins is low and is stimulated by mitogenic
the phosphorylation of the retinoblastoma protein pRb105 , which is the product of the Rb tumour growth factor treatment. Cyclin D1 expression can be affected by: 1) ERK MAP kinase, 2) PI3-
suppressor locus. In its hypophosphorylated form pRb105 associates with the E2F family of kinase, 3) Rac and Ral GTPases signalling pathways. The regulation of cyclin D1 expression
transcription factors and actively promotes transcriptional repression or sequesters E2F from genesprovides an important model (Figure 2 ) for how signal pathway integration can occur, since some
required for entry into S-phase. Following phosphorylation of pRb105 by the G1 Cdks, E2F is signals may regulate transcription, whereas others regulate protein stability or translation.
released from pRb105 and transcription results.
The regulation of the activity of these kinases requires:
u an interplay between the synthesis of the cyclins,
u formation of cyclin/Cdk complexes,
u association with the cyclin-dependent-kinase inhibitors (CKIs),
u phosphorylation of Cdks by activating kinases (CAKs),
u transport of cyclin Cdk complexes to the nucleus (Figure 1).
Some of these events may be regulated directly by signal transduction pathways, whereas others
are a consequence of the cell cycle machinery becoming activated. Thus, in trying to understand
how signal transduction pathways interface with the cell cycle machinery it is important to identify
which cell cycle events are directly regulated by intracellular signals.
Figure 2 Small GTPase signalling and the regulation of G1 Cdk activity. Inhibitory pathways are indicated with
a hammerhead, and stimulatory pathways are indicated with an arrow. Ras activates the PI3-kinase (PI3-K)
signalling pathway, resulting in cyclin D1 transcription and stability, and degradation of p27Kip1. Cyclin D1
transcription is also activated by the Ras-activated Ral and MEK/ERK signalling pathways. It is not clear
whether Cdc42 acts transcriptionally or post-transcriptionally to regulate cyclin D1 expression. Activation of the
ERK (extracellular-signal-regulated kinase) pathway leads to expression of p21Waf1, which inhibits
cyclinE/Cdk2 complex activity. Rho signalling is thought to control the cell's response to activation of the ERK
SIGNAL TRANSDUCTION AND CKIs
As well as the regulation of Cdk4 and Cdk6 activity through the synthesis of D-type cyclins, the cell
cycle machinery is regulated by the CKIs. In quiescent cells the levels of the CKI p27Kip1 are high.
The degradation of CKI p27Kip1 is an important control point for entry into the cell cycle and may be
a key regulator of cyclinE/Cdk2 activity. The downregulation of p27Kip1 that occurs late in G1 phase
requires Ras and is mediated by: 1) PI3-kinase pathway, 2) ERK MAP kinase pathway 3) the small
GTPase p21RhoA pathway. On the other hand, Rho signalling results in elevated levels of CKI
Figure 1 Relationship between cyclins and CKIs in the regulation of Cdk activity, pRb105 p21Waf1, which inhibits cyclinE/Cdk2 activity and blocks p27Kip1 degradation (Figure 2).
phosphorylation and S-phase entry. The cyclinD/Cdk4/6 and the cyclinE/Cdk2 complexes
phosphorylate pRb, which releases the transcription factor E2F, resulting in entry into S-phase.
The activity of Cdk4/6 is inhibited by the p16 family of CKIs, whereas the p21 family (p21Waf1,p27Kip1) CONCLUSIONS
inhibits Cdk2 and Cdk4/6. The cyclinE/Cdk2 complex phosphorylates p27Kip1, which is then degraded Multiple signal transduction pathways from small GTPases have an influence on the activation of
by p45Skp2 (SKP2). Degradation of p27Kip1 is an important control point for entry into the cell cycle. The the cell cycle machinery. Cell cycle progression can be viewed as the total of signalling pathways.
double-headed arrow between Cdk4/6 and the p21 family of CKIs reflects the potential dual role of these Requirement for the activation of multiple signalling pathways may act as a checkpoint to ensure
CKIs in assembly of active cyclinD/Cdk4/6 complexes and in their inhibition. that cell proliferation only occurs when it is appropriate.