Phosphorylation of Bcl2 and regulation of apoptosis

Members of the Bcl2 family of proteins are important regulators of programmed cell death pathways with individual members that can suppress (eg Bcl2, Bcl-XL) or promote (eg Bax, Bad) apoptosis. While the mechanism(s) of Bcl2's anti-apoptotic function is not yet clear, introduction of Bcl2 into most eukaryotic cell types will protect the recipient cell from a wide variety of stress applications that lead to cell death. There are, however, physiologic situations in which Bcl2 expression apparently fails to protect cells from apoptosis (eg negative selection of thymocytes). Further, Bcl2 expression in patient tumor samples does not consistently correlate with a worse outcome or resistance to anticancer therapies. For example, patient response and survival following chemotherapy is independent of Bcl2 expression at least for pediatric patients with ALL. These findings indicate that simple expression of Bcl2 may not be enough to functionally protect cells from apoptosis. The finding that Bcl2 is post-translationally modified by phosphorylation suggests another level of regulation of function. Recent studies have shown that agonist-activated phosphorylation of Bcl2 at serine 70 (single site phosphorylation), a site within the flexible loop domain (FLD), is required for Bcl2's full and potent anti-apoptotic function, at least in murine IL-3-dependent myeloid cell lines. Several protein kinases have now been demonstrated to be physiologic Bcl2 kinases indicating the importance of this post-translational modification. Since Bcl2 phosphorylation has been found to be a dynamic process involving both a Bcl2 kinase(s) and phosphatase(s), a mechanism exists to rapidly and reversibly regulate Bcl2's activity and affect cell viability. In addition, multisite Bcl2 phosphorylation induced by anti-mitotic drugs like paclitaxel may inhibit Bcl2 indicating the potential wide range of functional consequences that this post-translational modification may have on function. While post-translational mechanisms other than phosphorylation may also regulate Bcl2's function (eg ubiquitination), this review will focus on the regulatory role for phosphorylation and discuss its potential clinical ramifications.