Prosurvival and proapoptotic functions of ERK1/2 activation in murine thymocytes in vitro

The extracellular signal-regulated kinases 1/2 (ERK1/2) are serine/threonine-selective protein kinases involved in proliferation and differentiation of cells, including thymocytes. The requirement of ERK1/2 for thymocyte differentiation and maturation has been well established; however, their role in regulating thymocyte survival and apoptosis has not been resolved. Here, we asked whether ERK1/2 affected thymocyte survival in vitro in response to apoptotic stimuli. The results show that phorbol 12-myristate 13-acetate (PMA) treatment (with or without ionomycin) and serum starvation (s/s) induced sustained ERK1/2 activation in murine thymocytes. Importantly, pharmacological treatment of thymocytes with the MEK inhibitor UO126 revealed that PMA-induced ERK1/2 activation was proapoptotic, whereas serum starvation-induced ERK1/2 activation inhibited apoptosis and promoted cell survival. While basal MEK activity was required for both s/s- and PMA-induced ERK1/2 activation, MEK activity increased only in response to PMA. The results show that the suppression of ERK1/2 phosphatases was responsible for s/s-induced sustained ERK1/2 activation. Unexpectedly, neither s/s-induced proapoptotic nor PMA-induced anti-apoptotic functions of ERK1/2 depended on the Bcl-2 family phosphoprotein Bim(EL), which was previously implicated in thymocyte apoptosis. Lastly, etoposide treatment of immature thymocytes induced both p53 and ERK1/2 activation, but ERK1/2 activity did not affect the phosphorylation and stabilization of p53. Thus, ERK1/2 has a dual role in promoting cell survival and cell death in thymocytes in the context of different stimuli.