**3. PKC and apoptotic cell death upon genotoxic insults**

Novel PKC, , and are substrates for the effector caspase-3, and proteolytic activation of these novel PKCs has been associated with cell death (Datta et al. 1997, Emoto et al. 1995, Endo et al. 2000). However, recent studies have shown that PKC acts upstream of caspases to regulate cell death. For example, PKC activators enhanced caspase activation, whereas an inhibitor of PKC prevented caspase activation in response to DNA damage (Basu et al. 2001). In particular, studies with PKC–/– mice suggest that PKC plays pivotal roles in the regulation of cell proliferation and apoptosis (Humphries et al. 2006, Leitges et al. 2001). PKC is activated by a variety of stimuli including ionizing radiation, anti-cancer agents, reactive oxygen species (ROS), ultraviolet radiation, growth factors and cytokines (Carpenter et al. 2002, Chen et al. 1999, Denning et al. 1996, Konishi et al. 2001, Reyland et al. 1999, Yoshida and Kufe 2001, Yoshida et al. 2002). Molecular mechanisms such as tyrosine phosphorylation and proteolytic cleavage by caspase-3 are of importance to understand the pro-apoptotic role for PKC activation. PKC isozymes have been implicated in the growth factor signal transduction pathway (Nishizuka 1992). By contrast, activation of PKC inhibits cell cycle progression and down-regulation of PKC is linked to tumor promotion, suggesting that PKC may have a negative effect on cell survival (Lu et al. 1997, Watanabe et al. 1992). In many cases, the growth-inhibitory effects of PKC have been linked to changes in the expression of factors that influence cell cycle progression. Furthermore, PKC plays a pivotal role in the genotoxic stress response leading to apoptosis in various cell types (Brodie and Blumberg 2003, Reyland 2007, Yoshida 2007a). In addition, cells derived from PKC–/– mice were shown to be defective in mitochondria-dependent apoptosis (Humphries et al. 2006, Leitges et al. 2001). These findings thus support our proposition of a pro-genotoxic role for PKC. PKC is activated in response to diverse cellular stimuli by various processes, including membrane translocation (Joseloff et al. 2002, Wang et al. 1999), protein-protein interaction (Benes et al. 2005), tyrosine phosphorylation (Denning et al. 1996, Kaul et al. 2005), and proteolytic cleavage (Emoto et al. 1995, Ghayur et al. 1996, Yoshida 2007a, Yoshida et al. 2003). The translocation of PKC to discrete subcellular compartments and/or proteolytic cleavage can be induced by numerous stimuli, such as ceramide, TNF, UV irradiation, ionizing radiation, oxidative stress, and etoposide (DeVries et al. 2002, Majumder et al. 2000, Matassa et al. 2001, Reyland et al. 1999, Yamaguchi et al. 2007b, Yoshida 2007a, Yoshida et al. 2006a, Yoshida et al. 2002, Yoshida et al. 2003, Yoshida et al. 2006b). Importantly, recent

have been categorized into three groups: i) the classical/conventional PKCs (cPKCs: , , II ), which are calcium dependent and activated by DAG; ii) the novel PKCs (nPKCs: , , , ), which are calcium-independent and activated by DAG; and iii) the atypical PKCs (aPKCs: , ), which are calcium-independent and not activated by DAG (Casabona 1997, Goodnight et al. 1994, Hug and Sarre 1993, Nishizuka 1988, Nishizuka 1992, Nishizuka 1995). The cell-specific expression and subcellular localization of individual PKC isozymes indicate important isozyme-specific functions. To elucidate these functions, it should be necessary to study the individual features of each isozyme, such as expression, posttranslational modification, substrate specificity, subcellular localization and signaling crosstalk with other proteins. Moreover, the involvement of a PKC isozyme in a signaling pathway resulting in a specific cellular response can be investigated by diverse distinct

Novel PKC, , and are substrates for the effector caspase-3, and proteolytic activation of these novel PKCs has been associated with cell death (Datta et al. 1997, Emoto et al. 1995, Endo et al. 2000). However, recent studies have shown that PKC acts upstream of caspases to regulate cell death. For example, PKC activators enhanced caspase activation, whereas an inhibitor of PKC prevented caspase activation in response to DNA damage (Basu et al. 2001). In particular, studies with PKC–/– mice suggest that PKC plays pivotal roles in the regulation of cell proliferation and apoptosis (Humphries et al. 2006, Leitges et al. 2001). PKC is activated by a variety of stimuli including ionizing radiation, anti-cancer agents, reactive oxygen species (ROS), ultraviolet radiation, growth factors and cytokines (Carpenter et al. 2002, Chen et al. 1999, Denning et al. 1996, Konishi et al. 2001, Reyland et al. 1999, Yoshida and Kufe 2001, Yoshida et al. 2002). Molecular mechanisms such as tyrosine phosphorylation and proteolytic cleavage by caspase-3 are of importance to understand the pro-apoptotic role for PKC activation. PKC isozymes have been implicated in the growth factor signal transduction pathway (Nishizuka 1992). By contrast, activation of PKC inhibits cell cycle progression and down-regulation of PKC is linked to tumor promotion, suggesting that PKC may have a negative effect on cell survival (Lu et al. 1997, Watanabe et al. 1992). In many cases, the growth-inhibitory effects of PKC have been linked to changes in the expression of factors that influence cell cycle progression. Furthermore, PKC plays a pivotal role in the genotoxic stress response leading to apoptosis in various cell types (Brodie and Blumberg 2003, Reyland 2007, Yoshida 2007a). In addition, cells derived from PKC–/– mice were shown to be defective in mitochondria-dependent apoptosis (Humphries et al. 2006, Leitges et al. 2001). These findings thus support our proposition of a pro-genotoxic role for PKC. PKC is activated in response to diverse cellular stimuli by various processes, including membrane translocation (Joseloff et al. 2002, Wang et al. 1999), protein-protein interaction (Benes et al. 2005), tyrosine phosphorylation (Denning et al. 1996, Kaul et al. 2005), and proteolytic cleavage (Emoto et al. 1995, Ghayur et al. 1996, Yoshida 2007a, Yoshida et al. 2003). The translocation of PKC to discrete subcellular compartments and/or proteolytic cleavage can be induced by numerous stimuli, such as ceramide, TNF, UV irradiation, ionizing radiation, oxidative stress, and etoposide (DeVries et al. 2002, Majumder et al. 2000, Matassa et al. 2001, Reyland et al. 1999, Yamaguchi et al. 2007b, Yoshida 2007a, Yoshida et al. 2006a, Yoshida et al. 2002, Yoshida et al. 2003, Yoshida et al. 2006b). Importantly, recent

methods such as overexpression or inhibition of enzyme.

**3. PKC and apoptotic cell death upon genotoxic insults** 

studies have shown that genotoxin-induced PKC activation is in part dependent upon Ataxia telangiectasia mutated (ATM) (Yoshida et al. 2003). Whereas ATM activates c-Abl, and c-Abl activates PKC, a potential explanation is that DNA damage induces an ATMc-AblPKC pathway (Yoshida 2007b, Yoshida and Miki 2005, Yoshida et al. 2005). Alternatively, ATM may directly activate PKC in the DNA damage response. In either case, nuclear targeting of PKC is pre-requisite for ATM-mediated full activation of PKC.
