Inhibition of
protein kinase C (PKC) activity in transformed cells and
tumor cells containing activated
p21(RAS) results in apoptosis. To investigate the pro-apoptotic pathway induced by the
p21(RAS)
oncoprotein, we first identified the specific PKC
isozyme necessary to prevent apoptosis in the presence of activated
p21(RAS). Dominant-negative mutants of PKC,
short interfering RNA vectors, and PKC
isozyme-specific chemical inhibitors directed against the PKCdelta
isozyme demonstrated that PKCdelta plays a critical role in p21(RAS)-mediated apoptosis. An activating
p21(RAS) mutation, or activation of the
phosphatidylinositol 3-kinase (PI3K) Ras effector pathway, increased the levels of PKCdelta
protein and activity in cells, whereas inhibition of
p21(RAS) activity decreased the expression of the PKCdelta
protein. Activation of the Akt survival pathway by oncogenic Ras required PKCdelta activity. Akt activity was dramatically decreased after PKCdelta suppression in cells containing activated
p21(RAS). Conversely, constitutively activated Akt rescued cells from apoptosis induced by PKCdelta inhibition. Collectively, these findings demonstrate that
p21(RAS), through its downstream effector PI3K, induces PKCdelta expression and that this increase in PKCdelta activity, acting through Akt, is required for cell survival. The
p21(RAS) effector molecule responsible for the initiation of the apoptotic signal after suppression of PKCdelta activity was also determined to be PI3K. PI3K (p110(C)(AAX), where AA is aliphatic
amino acid) was sufficient for induction of apoptosis after PKCdelta inhibition. Thus, the same
p21(RAS) effector, PI3K, is responsible for delivering both a pro-apoptotic signal and a survival signal, the latter being mediated by PKCdelta and Akt. Selective suppression of PKCdelta activity and consequent induction of apoptosis is a potential strategy for targeting of
tumor cells containing an activated
p21(RAS).