PKC alpha promotes chemoresistance in acute lymphoblastic leukemia derived REH cells by a Bcl2-dependent, MAPK-independent mechanism.

Protein kinase C (PKC) has important roles in regulating a number of cellular processes critical to cellular homeostasis. In general, activation PKC promotes signaling cascades that support cell survival. PKC modulates chemoresistance by diverse mechanisms including the activation of mitogen activated protein kinases (MAPK) pathways and by the phosphorylation of Bcl2. Activation of PKC in acute lymphoblastic leukemia (ALL) derived REH cells resulted in a) co-localization of PKC with Bcl2 in mitochondrial membranes, b) increased levels of Bcl2 phosphorylation, and c) increased chemoresistance of cells exposed to chemotherapeutic agents. It is not clear, however, what other mechanisms in addition to the regulation of Bcl2 may be involved in PKC-mediated chemoresistance in REH cells. In this report, we address this question by examining the effect of PKC overexpression on other survival mechanisms. Overexpression of PKC alone did not affect proliferation, cell cycle, or activation of MAPK in REH cells. These findings suggest that PKC -mediated chemoresistance in REH cells may indeed rely heavily on the regulation of Bcl2. Indeed, REH cells overexpressing PKC are similarly sensitive to HA-14, a drug that targets Bcl2 function directly. This finding suggests that the protective properties of PKC require Bcl2, at least in these cells. Interestingly, MAPK was not active in either REH cells or REH PKC transfectants. The subsequent discovery of ERK as another Bcl2 kinase raised the possibility that PKC could have promoted Bcl2 phosphorylation by the activation of ERK. The finding that PKC promotes Bcl2 phosphorylation in the absence of active ERK is significant since it demonstrates that PKC can act directly on Bcl2. Furthermore, overexpression of PKC inhibited mitochondrial Protein Phosphatase 2A (PP2A) activity by a mechanism that involves suppression of PP2A/B56 subunit expression. Since PP2A is a known Bcl2 phosphatase, one mechanism by which PKC may promote protection of REH cells from stress stimuli is via the inactivation of the Bcl2 phosphatase. The ability of PKC to directly target Bcl2 and the Bcl2 protein phosphatase represents a novel survival regulatory mechanism. This mechanism is the mirror image of PP2A-mediated stress pathways where PP2A targets both Bcl2 and the Bcl2 kinase.