Effects of stimulation of AMP-activated protein kinase on insulinlike growth factor 1-and epidermal growth factor-dependent extracellular signal-regulated kinase pathway

AMP-activated protein kinase (AMPK) is tightly regulated by the cellular AMP:ATP ratio and plays a central role in the regulation of energy homeostasis. Previously, AMPK was reported to phosphorylate serine 621 of Raf-l in vitro, In the present study, we investigated a possible role of AMPK in extracellular signal-regulated kinase (Erk) cascades, using 5-aminoimidazole-4-carboxamide-1-beta -D-ribofuranoside (AICAR), a cell-permeable activator of AMPK and antisense RNA experiments. Activation of AMPK by AICAR in NIH-3T3 cells resulted in drastic inhibitions of Ras, Raf-l, and Erk activation induced by insulin-like growth factor 1 (IGF-1), Expression of an antisense RNA for the AMPK catalytic subunit decreased the AMPK activity and significantly diminished the AICAR effect on IGF-l-induced Ras activation and the subsequent Erk activation, indicating that its effect is indeed mediated by AMPK. Phosphorylation of Raf-l serine 621, however, was not involved in AMPK-mediated inhibition of Erk cascades. In contrast to IGF-1, AICAR did not block epidermal growth factor (EGF)-dependent Raf-l and Erk activation, but our results demonstrated that multiple Raf-l upstream pathways induced by EGF were differentially affected by AICAR: inhibition of Ras activation and simultaneous induction of Ras-independent Raf activation. The activities of IGF-1 and EGF receptor were not affected by AICAR, Taken together, our results suggest that AMPK differentially regulate Erk cascades by inhibiting Ras activation or stimulating the Ras-independent pathway in response to the varying energy status of the cell.