Diacylglycerol Signaling

16 PKC Isozymes and Skin Cancer
active investigation. Chronic inflammation, hyperplasia, and long-term PKC
isozyme downregulation are all considered to be important, but the key process
remains elusive (Hansen et al. 1990; Moore et al. 1999). Direct activation of PKC
by TPA has profound and complex effects on epidermal KC cell proliferation. TPA
treatment of normal mouse skin initially causes an inhibition of DNA synthesis,
followed by several waves of increased proliferation (Raick 1973; Raick et al.
1972). After 24–48 h, the resulting epidermis is hyperplastic, with increased cell
numbers in all suprabasal layers, including the stratum corneum. This complex
response to PKC activation may be due to differential responses of KCs at different
stages of maturation, or compensatory proliferative response of the epidermis
to rapid induction of differentiation (Reiners and Slaga 1983; Yuspa et al. 1982).
In addition, TPA treatment induces the activation/proliferation of hair follicle stem
cells, a critical target cell population for skin carcinogenesis (Trempus et al. 2007).
TPA is a potent inducer of normal KC growth arrest and terminal differentiation in
culture (Tibudan et al. 2002). PKC activation by endogenous activators such as
diacylglycerol, or pharmacological activators such as phorbol esters like TPA
stimulate the granular layer differentiation program and cornification of normal
KCs while simultaneously inhibiting the spinous layer differentiation (Denning
et al. 1995a; Dlugosz and Yuspa 1993; Efimova et al. 1998). PKC also becomes
activated by inducers of differentiation such as calcium (Denning et al. 1995a;
Chakravarthy et al. 1995) or confluency (Lee et al. 1998; Yang et al. 2003), and
inhibition of PKC activity can block the induction of differentiation gene products
(Dlugosz and Yuspa 1993; Denning et al. 1995a; Lee et al. 1998; Yang et al.
2003).
Throughout this chapter, the utility of dissecting PKC isozyme function in normal
cells will be demonstrated to be a very fruitful and informative approach for understanding
PKC isozyme function in neoplastic cells. In normal KCs, five PKC
isozymes have been described at the protein and mRNA level: PKC alpha (a), PKC
delta (d), PKC epsilon (e), PKC eta (h), and PKC zeta (z) (Dlugosz et al. 1992;
Denning et al. 1993; Longthorne and Williams 1997). These PKC isozymes can
be classified as calcium-dependent (PKCa), calcium-independent (PKCd, e, h),
and phorbol ester-independent (PKCz) based upon structural and regulatory features.
The diversity of PKC regulatory mechanisms and large number of isozymes has
prompted investigation into distinct functions for individual PKC isozymes (Fig. 16.1).
The ability of TPA to trigger KC differentiation and growth arrest may seem at
odds with its activity as a potent mouse skin tumor promoter. However, normal KCs
transduced with active Ha-Ras, mouse KC cell lines derived from papillomas, and
human SCC cell lines are all resistant to the differentiating effects of TPA, and in
fact TPA can stimulate DNA synthesis in Ras-transformed mouse KCs (Yuspa et al.
1985). Given this divergent response between normal and neoplastic KC, it is clear
that TPA would provide a strong selective advantage for clonal expansion of KCs
with activating Ha-Ras mutations. Mechanisms responsible for this differential
response of normal and Ras-initiated KC to TPA are not entirely understood, but
selective inactivation or downregulation of the proapoptotic and prodifferentiating
PKCd isozyme in Ras transformed KCs may be part of the explanation (Denning
et al. 1993; Geiges et al. 1995).
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