Diacylglycerol Signaling

390 L. Xiao
of the antiapoptotic Bcl-2 exclusively at the Ser-70, and PKCa appears to be an
upstream kinase responsible for the nicotine induced phosphorylation of Bcl-2
(Mai et al. 2003). Nicotine also exerts its prosurvival action through the activation
of PKCz, which in turn directly phosphorylates the proapoptotic Bax at Ser-184
(Xin et al. 2007). The activity of PKCz is required for its interaction with Bax, and
increased expression of wild-type or activated PKCz leads to sequestration of Bax
in the cytoplasm and prevents Bax from undergoing a conformational change
thereby inhibiting its proapoptotic function (Xin et al. 2007). Unlike nicotine, NNK
promotes survival through a PKCi-dependent mechanism, resulting in phosphorylation
and inactivation of the proapoptotic Bad in NSCLC cells (Jin et al. 2005).
NNK activates PKCi through a Src-dependent mechanism, and activated PKCi
directly phosphorylates Bad at multiple serine sites and promotes the disassociation
of the Bad/Bcl-X L complex thereby leading to cell survival. The existence of multiple
nicotine/NNK-PKC survival pathways suggests that the activation of specific
pathways may be controlled in a cell context-dependent manner.
Heat shock protein 27 (HSP27) has been implicated in protecting cells from apoptosis
triggered by a variety of stimuli including radiation and chemotherapeutic drugs
(Bruey et al. 2000; Rane et al. 2003). HSP27 is overexpressed in human NSCLC
tumor tissues compared to the corresponding normal lung tissues, and down-regulation
of highly expressed HSP27 in NSCLC cell lines results in enhanced apoptosis in
response to radiation or chemotherapeutic drug treatment (Kim et al. 2007).
Inhibition of PKCd kinase activity through a direct interaction between HSP27 and
PKCd contributes to HSP27-mediated chemo- and radiation-resistance in lung cancer
cells (Lee et al. 2005; Kim et al. 2007). The amino acid residues 668–674 of the
V5 region of PKCd is necessary for HSP27 binding, and treatment of NSCLC cells
with the PKCd-V5 heptapeptide containing the corresponding amino acid sequences
required for HSP27 binding restores the PKCd activity and significantly increases
cisplatin or radiation-induced cell death in vitro and in vivo (Kim et al. 2007), suggesting
that PKCd activity plays an essential role in mediating DNA damage-induced
cell death in NSCLC cells. In contrast to this finding, Clark et al. (2003) reported an
antiapoptotic function of PKCd in NSCLC cells. It was shown that rottlerin, a selective
PKCd inhibitor, effectively induces apoptosis in NSCLC cells and enhanced
chemotherapy-induced apoptosis in a cell line- and drug-specific manner. Although
rottlerin has nonspecific effects toward other kinases (Davies et al. 2000) and displays
a widespread cytotoxicity in many cancer cell lines, its antiapoptotic effect
observed in NSCLC cells appears PKCd-dependent as expression of a kinase-dead
mutant of PKCd in NSCLC cells induces apoptosis accompanied with decreased
PKCd phosphorylation (Clark et al. 2003). PKCd also functions as an antiapoptotic
kinase in mediating the prosurvival effect of the combination of two growth factors,
amphiregulin (AR) and insulin-like growth factor-1 (IGF1), in human NSCLC cells
(Hurbin et al. 2005). AR/IGF1 protects NSCLC cells from serum deprivationinduced
apoptosis associated with increased phosphorylation of PKCd at Thr-505,
and rottlerin or siRNA mediated gene silencing of PKCd abrogates this effect.
Additionally, expression of constitutively activated PKCd alone is able to inhibit
serum deprivation-induced apoptosis (Hurbin et al. 2005), suggesting the involvement