Diacylglycerol Signaling

164 J.D. Black
the ERK/MAPK signaling pathway (Clark et al. 2004; Salabat et al. 2006; Zezula
et al. 1997; Lin et al. 2002; Akashi et al. 1999; Liu et al. 1996; Esposito et al. 1997).
PKC agonists promote increased association of p21 Waf1/Cip1 with Cdk2/cyclin E
(Frey et al. 2000; Livneh et al. 1996; Ashton et al. 1999; Coppock et al. 1995;
Asiedu et al. 1997) and Cdk2/ cyclin A (Frey et al. 2000) complexes, leading to
attenuation of Cdk2 activity and cell cycle blockade. The importance of p21 Waf1/Cip1
in mediating PKC-induced G 1 arrest has been demonstrated directly using several
approaches. For example, in contrast to its effects in wild-type mouse embryo fibroblasts
(MEFs), PMA was unable to block PDGF-induced DNA synthesis in MEFs
isolated from p21 Waf1/Cip1 -null embryos (Balciunaite and Kazlauskas 2002). In addition,
recent studies using p21 Waf1/Cip1 siRNA have confirmed a requirement for
p21 Waf1/Cip1 in PKCd-induced inhibition of G1→S progression in lung adenocarcinoma
cells (Nakagawa et al. 2005). Similar effects have been observed in coronary
smooth muscle cells (Bowles et al. 2007), where testosterone promoted cell cycle
arrest via PKCd-mediated induction of p21 Waf1/Cip1 . Although upregulation of
p21 Waf1/Cip1 generally results in cell cycle blockade, it should be noted that accumulation
of this CKI can be required to promote rather than inhibit cell cycle progression
in some systems. Antisense strategies revealed that PKCa-mediated induction
of p21 Waf1/Cip1 is necessary for cyclin/Cdk complex formation and increased proliferation
in glioma cells (Besson and Yong 2000).
PKC signaling can also lead to a reduction in p21 Waf1/Cip1 levels and accelerated
mitogenesis (Walker et al. 2006). Recent studies in MEFs identified a role for
PKCd in mediating posttranscriptional destabilization of p21 Waf1/Cip1 via a proteasome-dependent
mechanism, an effect that was associated with G 1 →S progression.
Loss of PKCd, on the other hand, increased p21 Waf1/Cip1 levels and reduced entry into
S phase, effects not observed in p21 Waf1/Cip1 -null cells. Destabilization of p21 Waf1/Cip1
can also be induced by PKCz, as demonstrated in HeLa cells, where the effect is
dependent on PDK1 (Scott et al. 2002). In addition, inhibition of PKCe signaling
in NSCLC cells by expression of kinase inactive, dominant negative enzyme led to
p53-independent induction of p21 Waf1/Cip1 and cell growth arrest; similar effects were
noted in fibroblasts following combined loss of PKCa and q (Deeds et al. 2003).
Finally, there is evidence that PKCs can mediate phosphorylation of p21 Waf1/Cip1 to
regulate its stability, activity, and/or localization (Kashiwagi et al. 2000; Scott et al.
2002; Agell et al. 2006; Rodriguez-Vilarrupla et al. 2005).
PKC activation can also lead to increased expression of the CKI p27 Kip1 11, 107 ,
although induction is generally delayed compared with that of p21 Waf1/Cip1 (Black
2000; Frey et al. 1997, 2000; Tibudan et al. 2002; Asiedu et al. 1997). Limited
evidence indicates that p27 Kip1 can be the only Cip/Kip CKI induced in response to
PKC activation in some systems (e.g., Fukumoto et al. 1997; Asiedu et al. 1997).
Although the mechanism(s) underlying PKC-induced accumulation of p27 Kip1 have
not been extensively studied, posttranscriptional or posttranslational mechanisms
appear to be involved (Asiedu et al. 1997). p27 Kip1 has been shown to accumulate in
both cyclin E- and cyclin A-Cdk2 complexes, indicating that the molecule could
potentially mediate PKC-induced suppression of Cdk2 activity in some cases
(Frey et al. 2000; Asiedu et al. 1997). However, the role of p27 Kip1 in the cell