Activation of the ERK1/2 signaling pathway promotes phosphorylation and proteasome-dependent degradation of the BH3-only protein, Bim

Both the ERK and phosphatidylinositol 3'-kinase (PI3K) signaling pathways can protect cells from apoptosis following withdrawal of survival factors. We have previously shown that the ERK1/2 pathway acts independently of PI3K to block expression of the BH3-only protein, Bim(EL), and prevent serum withdrawal-induced cell death, although the precise mechanism by which ERK reduced Bim(EL) levels was unclear. By comparing Bim mRNA and Bim protein, expression we now show that the rapid expression of Bim(EL) following serum withdrawal cannot be accounted for simply by increases in mRNA following inhibition of PI3K. In cells maintained in serum Bim(EL) is a phosphoprotein. We show that activation of the ERK1/2 pathway is both necessary and sufficient to promote Bim(EL) phosphorylation and that this leads to a substantial increase in turnover of the Bim(EL) protein. ERK1/2-dependent degradation of Bim(EL) proceeds via the proteasome pathway because it is blocked by proteasome inhibitors and is defective at the restrictive temperature in cells with a temperature-sensitive mutation in the E1 component of the ubiquitin-conjugating system. Finally, co-transfection of Bim(EL) and FLAG-ubiquitin causes the accumulation of polyubiquitinated forms of Bim, and this requires the ERK1/2 pathway. Our findings provide new insights into the regulation of Bim and the role of the ERK pathway in cell survival.