Delayed transactivation of the receptor for nerve growth factor is required for sustained signaling and differentiation by α2-adrenergic receptors in transfected PC12 cells

α2-Adrenergic receptors have been reported to induce subtype-specific neuronal differentiation in vitro, but the signaling mechanisms that mediate this effect have not been characterized. In the present study we found that stimulated α2-ARs induce delayed transactivation of TrkA in PC12 cells. The transactivation of TrkA was sensitive to the PP1 inhibitor of the Src family kinases and required prior transactivation of the EGF receptor. Moreover, α2-adrenergic receptors induced sustained activation of MAPK and Akt. The sustained activation of Akt, but not of MAPK, was subtype-specific and correlated with the neuronal differentiation of PC12 cells, with the order α2A < α2B < α2C. Furthermore, stimulated α2-ARs induced an increased over time expression of the cell cycle associated proteins, p21WAF1 and Cyclin D1 and led to cell cycle arrest in a similar subtype-specific manner. Contrary to sustained activation of MAPK, the persistent activation of Akt and of p21WAF1 and Cyclin D1 as well as neurite outgrowth and expression of the neuronal marker peripherin, were all blocked by K252a an inhibitor of TrkA activity. Together these results demonstrate a novel outcome following α2-AR-mediated EGFR transactivation, being the consecutive transactivation of TrkA, and that this event may mediate the subtype-specific differentiation of α2-AR-expressing PC12 cells.