The B56α PP2A B Regulatory Subunit Promotes Mitochondrial PP2A Activity and Supports Etoposide-Induced Cell Death in Human ALL Derived REH Cells.

Protein Phosphatase 2A (PP2A) has broad regulatory effects on diverse signaling pathways so it is not surprising that PP2A is emerging as a possible tumor suppressor. The PP2A inhibitor okadaic acid has been found to promote tumors. Cellular transformation caused by viruses has been shown to involve dysregulation of PP2A pathways (e.g. the SV40 small T antigen). Mutations in PP2A subunit genes have been reported in lung cancer and breast cancer. PP2A does have a role in chronic myeloid leukemia (CML) tumorigenesis. While a role for PP2A in CML is emerging, the significance of PP2A signaling pathways in acute lymphoblastic leukemia (ALL) is currently not clear. PP2A has been shown to negatively regulate BCL2 function in human ALL derived REH cells. Considering that BCL2 is a potent anti-apoptotic molecule, a possibility arises that PP2A may regulate chemoresistance in ALL cells via a mechanism involving BCL2. Supporting such a notion, low dose okadaic acid treatment of REH cells results in robust BCL2 phosphorylation and promotes resistance to chemotherapeutic drugs such as etoposide. A difficulty in studying PP2A-mediated signaling is that the enzyme has a multimeric structure. PP2A is a heterotrimer comprising a catalytic subunit (C), a scaffold subunit (A), and a regulatory subunit (B). There are 2 highly homologous isoforms of the A subunit, 2 highly homologous isoforms of the C subunit, and there are 3 major B subunit families (i.e. B55, B56, and PR72) that at present include 21 proteins. Recent studies indicate that PP2A substrate specificity and sub-cellular localization are mediated by the B subunit. Thus PP2A is not a single enzyme but rather a family of protein phosphatase isoforms defined by which B regulatory subunit controls its function. Little is known about the mechanisms regulating B subunit function. It has been found that a ceramide activated mitochondrial PP2A isoform containing the B56 family member B56α acts as the BCL2 phosphatase in REH cells. In the present study, it was found that over-expression of exogenous B56α in REH cells promoted sensitivity to the chemotherapeutic drug etoposide. B56α was found to promote mitochondrial but not nuclear PP2A activity. B56α was found to promote dephosphorylation of PKCα but not PKCε. This finding suggests that regulation of PKC signaling by PP2A is dependent on different B subunits. As PKCα is a physiologic BCL2 kinase, the ability for B56α to control the PP2A isoform that targets both BCL2 and at least one BCL2 kinase suggests PP2A regulation of BCL2 function is multi-tiered. Finally, over-expression of B56α promoted changes in the composition of proteins present in the mitochondrial membranes as determined by 2D gel electrophoresis. A broad range of proteins were shown to be affected by B56α over-expression (mitochondrial proteins varied in size from < 18kd to > 100kd), PP2A stress signaling mediated by B56α may involve a number of targets. These findings suggest that B56α regulation of PP2A stress signaling is complex but an understanding of this process may result in new strategies for the treatment of ALL.