Phosphorylation of Bim-EL by Erk1/2 on serine 69 promotes its degradation via the proteasome pathway and regulates its proapoptotic function

Abstract

Bim is a proapoptotic member of the Bcl-2 family that shares only the BH3 domain with this family. Three Bim proteins Bim-EL, Bim-L and Bim-S are synthesized from the same transcript. We report here that Bim-EL when phosphorylated by Erk1/2 is rapidly degraded via the proteasome pathway. Using different cellular models we evidence that serine 69 is both necessary and sufficient for Erk1/2-mediated phosphorylation and degradation of Bim-EL. In K562 cells, Phorbol 12-myristate 13-acetate activates Erk1/2 and consequently increases Bim-EL phosphorylation and degradation by the proteasome, resulting in cell survival, while the Bcr-Abl inhibitor imatinib abrogates Bim-EL phosphorylation and degradation and induces caspase activation and apoptosis. We also show that Bim-EL(S69G) promotes apoptosis more efficiently than Bim-EL-WT in K562 cells. Altogether, our findings demonstrate that phosphorylation of Bim-EL by Erk1/2 on serine 69 selectively leads to its proteasomal degradation and therefore represents a new and important mechanism of Bim regulation.

BimEL is phosphorylated at mitosis by Aurora A and targeted for degradation by βTrCP1

“…As BimEL degradation occurred concomitantly with phosphorylation, we examined two phosphorylation sites on BimEL previously shown to regulate proteolysis. Erk 1 and 2 were shown to phosphorylate BimEL on S69 10,12,18 and promote degradation and phosphorylation on S93/94/98 has been shown to induce degradation through the Skp1-Cullin1-F-Box (SCF) complex. 20 In order to assess the importance of these sites for BimEL degradation in mitosis, phosphospecific antibodies were used to monitor these modifications during mitosis.…”

“…4,7,8 Post-transcriptional mechanisms that have been shown to control Bim expression levels include both mRNA stability 9 and protein stability. [10][11][12] Signaling downstream of receptor tyrosine kinases (RTKs) including epidermal growth factor receptor induce the proteasomal degradation of Bim. [13][14][15][16] There are three splice variants of Bim: BimEL, BimL and BimS.…”

“…17 Phosphorylation by extracellular signal-regulated kinase 1/2 (Erk1/2) on Serine-69 (S69) of human BimEL has been shown to induce degradation of the protein. 12,18,19 More recently, phosphorylation at S69 was shown to promote phosphorylation at additional sites on Serines 93/94/98 (S93/94/98) by ribosomal S6 kinase (Rsk1/2) within a conserved phosphodegron motif recognized by the F-box protein beta-transducin repeat containing E3 ubiquitin protein ligase (bTrCP). 20 In addition to being phosphorylated in response to growth factor signaling, BimEL is also phosphorylated during mitosis.…”

Regulatory phosphorylation of Bim: sorting out the ERK from the JNK

“…Exon 3 includes an ERK1/2 docking domain (FSF) and ERK1/2 phosphorylation sites, including Ser69. [29][30][31]33 The presence of exon 3 in Bima1, Bimb1 and Bimb2 also correlates with their ERK1/2-dependent phosphorylation in vivo. 33 ERK1/2-dependent phosphorylation of Bim EL targets it for proteasomal degradation and may prevent binding to Bax.…”

The BCL-2 protein family, BH3-mimetics and cancer therapy

“…Phosphorylation of BIM by ERK was reported to be critical for the antiapoptotic activity of this kinase. [56][57][58][59][60] However, a recent study has shown that ERK-mediated direct phosphorylation of BIM does not have a major role in the control of the proapoptotic activity of this BH3-only protein within the whole animal. 61 Both BIM and BMF were shown to be sequestered by binding to elements of the cytoskeleton, thereby restraining their pro-apoptotic activity.…”