Unwinding the loop of Bcl-2 phosphorylation

Abstract

Recent evidence indicates that anti-apoptotic functions of Bcl-2 can be regulated by its phosphorylation. According to the 'mitotic arrest-induced' model, multi-site phosphorylation of the Bcl-2 loop domain is followed by cell death. In contrast, in cytokine-dependent cell lines, cytokines mediate phosphorylation of Bcl-2 on S70, preventing apoptosis. As discussed in this review, these models are not mutually exclusive but reflect different cellular contexts. During mitotic arrest, signal transduction is unique and is fundamentally different from classical mitogenic signaling, since the nucleus membrane is dissolved, gene expression is reduced, and numerous kinases and regulatory proteins are hyperphosphorylated. Hyperphosphorylation of Bcl-2 mediated by paclitaxel and other microtubuleactive drugs is strictly dependent on targeting microtubules that in turn cause mitotic arrest. In addition to serine-70 (S70), microtubule-active agents promote phosphorylation of S87 and threonine-69 (T69), inactivating Bcl-2. A major obstacle for identification of the mitotic Bcl-2 kinase(s) is that inhibition of putative kinase(s) by any means (dominant-negative mutants, antisense oligonucleotides, pharmacological agents) may arrest cycle, preventing mitosis and Bcl-2 phosphorylation. The role of Bcl-2 phosphorylation in cell death is discussed. Leukemia (2001) 15, 869-874.

Mitotic Arrest and Cell Fate: Why and How Mitotic Inhibition of Transcription Drives Mutually Exclusive Events

“…77 Hyper-phosphorylation of the transcription apparatus is one of the mechanisms of transcription shutdown during mitosis. Phosphorylation of proteins of the nuclear membrane precludes their assembly.…”

“…Phosphorylation of Bcl-2 abrogates its anti-apoptotic functions (Bcl-2 is long-lived protein). 77 Inhibition of Bcl-2 by phosphorylation may collaborate with depletion of Mcl-1. p53 is also phosphorylated during mitosis.…”

Sequential activation and inactivation of G2 checkpoints for selective killing of p53-deficient cells by microtubule-active drugs

“…Immunoblot for apoptotic markers (Caspase-3 and PARP cleavage) and mitotic markers (Raf-1 and Bcl-2 band shift) were performed as described in Materials and methods. (c) After 2 days, MTT assay was performed as described in Materials and methods immunoblot), as markers of mitotic arrest (Blagosklonny, 2001). In turn, Raf-1 and Bcl-2 phosphorylation was accompanied by caspase-3 and PARP cleavage, as markers of apoptosis (Figure 3b), followed by cell death (Figure 3c).…”

MCL1 is phosphorylated in the PEST region and stabilized upon ERK activation in viable cells, and at additional sites with cytotoxic okadaic acid or taxol

“…A variety of BCL2 family members undergo posttranslational phosphorylation, a modification that is of particular interest as it could provide an avenue for directly targeting these proteins (Ito et al, 1997;Scheid et al, 1999;Wang et al, 1999;Blagosklonny, 2001;Harada et al, 2001;Luciano et al, 2003). Phosphorylation has been well characterized in the case of proapoptotic BAD, but is incompletely understood in the case of BCL2 itself, the prototype antiapoptotic family member.…”