Role of RasGRP in Phorbol Ester Response in EL4 Lymphoma Cells

Abstract

EL4, a murine thymoma cell line, exists in phorbol ester-sensitive, resistant, and intermediate variants. Sensitive cells respond to phorbol 12-myristate 13-acetate (PMA) with robust activation of Erk MAPKs. In contrast, in resistant cells, Erk activation is weak, delayed, and requires higher concentrations of PMA. Intermediate cells exhibit partial PMA responsiveness. We hypothesized that PMA-induced Erk activation in EL4 cell lines is mediated by RasGRP (Ras guanyl releasing protein), a phorbol ester binding protein that regulates activation of Ras. Ras activation was assessed by a Ras-GTP pulldown assay, while Erk activation was tested by immunoblotting with anti-phospho-Erk. Our studies showed that RasGRP protein is expressed in PMA-sensitive cells, but not in PMA-resistant or intermediate cells. Both Ras and Erk are robustly activated by PMA in all PMA-sensitive clones. In contrast, there is no detectable PMA-induced Ras activation in PMA-resistant clones, correlating with the defect in PMA-induced Erk activation. Interestingly, the intermediate clones activate Erk to a moderate extent, but do not express RasGRP and do not activate Ras in response to PMA. Thus, the results obtained for intermediate clones indicate that alternative pathways for PMA-induced Erk activation exist. Transfection of RasGRP enhances PMA-induced Erk activation in resistant cells, while the PMA response is inhibited by siRNA targeted at RasGRP in sensitive cells. We conclude that the differential effects of PMA on Erk activation in EL4 cell lines reflect differences in Erk activation pathways, and that RasGRP is one of the proteins responsible for PMA response. (Supported by NIH CA094144-01)