**2. Protein kinase C**

The protein kinase C (PKC) family of serine-threonine kinases was first described as a calcium-activated, phospholipid-dependent serine/threonine protein kinase (Takai et al. 1977). PKC is activated diacylglycerol (DAG) hydrolyzed from phosphatidylinositol (PI) by phospholipase C (PLC) under a different cell-signaling system (Nishizuka 1984, Nishizuka 1988, Nishizuka 1992, Nishizuka 1995). It has attracted attention as an intracellular receptor for tumor-promotor phorbol esters, such as 12-O-tetradecanoyl-13-phorbol acetate (TPA) (Niedel et al. 1983). Although PKC had been recognized as a protein kinase, subsequent studies have revealed that it belongs to a family of serine/threonine-specific protein kinases and is activated by diverse stimuli and participates in various cellular processes, such as growth, differentiation, apoptosis, and cellular senescence (Casabona 1997, Clemens et al. 1992, Goodnight et al. 1994, Hofmann 1997, Hug and Sarre 1993, Nishizuka 1984, Nishizuka 1988, Nishizuka 1992, Nishizuka 1995). PKC consists of at least 11 isozymes (, I, II, , , , , , , / and ) with selective tissue distribution, activators, and substrates. PKC isozymes

Role for PKCδ on Apoptosis in the DNA Damage Response 295

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,

Translocation of PKC into the nucleus has been demonstrated in various cells (Blass et al. 2002, DeVries et al. 2002, DeVries-Seimon et al. 2007, Eitel et al. 2003, Scheel-Toellner et al. 1999, Yoshida et al. 2003, Yuan et al. 1998). Recent study showed that PKC translocates to nucleus after exposure of cells with 1--D-arabinofuranosylcytosine (ara-C) (Yoshida et al. 2003). Moreover, pretreatment with PKC inhibitor, rottlerin, attenuates nuclear targeting of PKC (Yoshida et al. 2003), suggesting that its kinase activity is required for nuclear translocation. A putative nuclear localization signal has been identified at the C-terminus of the catalytic domain of PKC (DeVries et al. 2002). Numerous PKC targets and substrates, including the p53 tumor suppressor, are nuclear proteins that function in induction of

The tumor suppressor protein p53 plays a central role in mediating stress and DNA damageinduced cell cycle arrest and apoptosis (Vogelstein et al. 2000). The p53 protein controls normal responses to DNA damage and other forms of genotoxic stress and is an indispensable element in maintaining genomic stability (Vogelstein et al. 2000). In fact, *p53* is the most frequently mutated gene in human cancers (Nigro et al. 1989). The level of p53 protein is mostly undetectable in normal cells but rapidly increases in response to a variety of stress stimuli. The mechanism by which the p53 protein is stabilized is not completely understood, but post-translational modification plays a crucial role (Shieh et al. 1997). Mutations in the *p53* gene are frequently correlated with generation of human cancers; however, the p53 pathway can be also derailed by diverse oncogenic molecules (Oren et al. 2002). The *p53* gene knockedout mice develop tumors with an increased rate (Donehower et al. 1992). It is reasonable that many agents may inhibit the p53 pathway as part of the road toward tumor promotion. However, mechanisms for action of many chemical agents that promote tumor development have not been elucidated. With the central role of p53 in mind, agents that promote tumor formation might block the p53 pathway. Importantly, p53 is regulated primarily via posttranslational modifications, especially phosphorylation, and the accumulation of p53 is the first step following cellular stress (Oren 1999). The *mdm2* gene is a transcriptional target of p53, and once synthesized, the MDM2 protein can bind to p53 at its NH2 terminus leading to its rapid degradation through the ubiquitin proteasome-mediated pathway (Kubbutat and Vousden 1998, Oren 1999, Ryan et al. 2001). Upon DNA damage, p53 is phosphorylated at multiple sites at the NH2 terminus, thereby inhibiting MDM2 binding (Burns and El-Deiry 1999, Canman et al. 1998, Kubbutat and Vousden 1998, Oren 1999, Ryan et al. 2001, Siliciano et al. 1997). As a result, p53 degradation stops and p53 accumulates. p53 can also be phosphorylated at its COOH-terminal regulatory domain, which influences its DNA binding

nuclear targeting of PKC is pre-requisite for ATM-mediated full activation of PKC.

**4. Nuclear translocation of PKC in the apoptotic responses** 

**5. Role for p53 in response to DNA damage** 

apoptosis.

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 methods such as overexpression or inhibition of enzyme.
