Diacylglycerol Signaling

204 M.E. Reyland and A.P. Bradford
Caspase Cleavage of PKCd
Early studies by Emoto et al. showed that irradiation activated a 40 kD myelin basic
protein kinase that was subsequently identified as a stable, proteolytically cleaved, yet
catalytically competent fragment of PKCd (Emoto et al. 1996). Caspase cleavage of
PKCd occurs in the hinge region of the kinase and effectively separates the regulatory
domain from the catalytic domain, resulting in the release of a constitutively active
catalytic fragment (dCF; Matassa et al. 2001). Expression of dCF is sufficient to
induce cell death, and Sitailo et al. have shown that expression of dCF is associated
with activation of the pro-apoptotic protein, Bax, and cytochrome c release (Sitailo
et al. 2004). Furthermore, a caspase resistant mutant of PKCd protects keratinocytes
from UV-induced apoptosis (D’Costa and Denning 2005). Studies from the Reyland
lab indicate that PKCd is cleaved in the nucleus, and that nuclear import of PKCd and
activated caspase-3 are temporally linked (DeVries-Seimon et al. 2007). These studies
also suggest that it is the nuclear accumulation of PKCd, and not caspase cleavage
per se, which is critical for the apoptotic response as targeting a caspase-resistant
mutant of PKCd to the nucleus is also able to induce apoptosis (DeVries-Seimon
et al. 2007). Although initially largely nuclear, in the later stages of apoptosis dCF
also can be found in the cytoplasm, consistent with studies that a role for PKCd at
the mitochondria in apoptotic cells (Majumder et al. 2000; Sitailo et al. 2004).
9.4.3 Contribution of Atypical Isoforms of PKC
The atypical isoforms, PKCi (its murine counterpart PKCl) and PKCz, have been
shown to be critical components of cell survival signal transduction pathways,
downstream of PI3K (Akimoto et al. 1996; Cataldi et al. 2003). Atypical PKCs also
suppress apoptosis by activation of pro-survival NFkB and MAPK signaling (Berra
et al. 1993; Diaz-Meco et al. 1993). PKCi phosphorylation and subsequent degradation
of IKKab is thought to play a role in NFkB activation and survival of
androgen-independent (DU-145) prostate cancer cells (Win and Acevedo-Duncan
2008). In addition, the atypical PKCs may directly target mediators and regulators
of apoptotic signaling pathways. In NSCLC cells, evidence suggests that PKCz
functions as a Bax kinase; phosphorylation resulting in cytoplasmic sequestration
and inhibition of the pro-apoptotic functions of this Bcl-2 family member (Xin
et al. 2007). Expression of a dominant negative, kinase inactive PKCz construct
resulted in increased Bax expression and downregulation of the antiapoptotic Bcl-2
in leukemic cells (Filomenko et al. 2002). PKCz may also phosphorylate FADD,
inhibiting formation of DISC and thereby conferring resistance to Fas mediated
apoptosis in leukemic cells (de Thonel et al. 2001). Finally, PKCz can protect
against UV-induced apoptosis by inhibition of acid sphingomyolinase-dependent
production of ceramide (Charruyer et al. 2007).
Consistent with their role in cell survival, atypical PKCs are frequently activated
or upregulated in response to apoptotic stimuli and are thought to mediate protective