Identification of signaling molecules mediating corticotropin-releasing hormone-R1α-mitogen-activated protein kinase (MAPK) interactions: The critical role of phosphatidylinositol 3-kinase in regulating ERK1/2 but not p38 MAPK activation

In most target cells, activation of the type 1 CRH receptor (CRH-R1) by CRH or urocortin (UCN I) leads to stimulation of the Gs-protein/adenylyl cyclase/protein kinase A cascade. Signal transduction of CRH-R1 also involves alternative pathways such as phosphorylation of ERK1/2 and p38 MAPK, two members of the MAPK family that mediate important pathophysiological responses. The intracellular pathways by which CRH-R1 activates these MAPK are only partially understood; here we characterized further signaling mechanisms and molecules involved in CRH-R1-mediated ERK1/2 and p38 MAPK activation. In human embryonic kidney 293 cells overexpressing recombinant CRHR1α, UCN I induced ERK1/2 and p38 MAPK activation was dependent on signaling molecules involved in agonist-induced CRH-R1α trafficking and endocytosis. Furthermore, time course studies and use of selective inhibitors demonstrated that ERK1/2 activation occured within 5 min, was sustained for at least 60 min, and was dependent on both phosphatidylinositol 3-kinase (PI3-K)/Akt activation and epidermoid growth factor receptor transactivation involving matrix metelloproteinases. UCN I effect on p38 MAPK phosphorylation was more transient, returned to basal within 40 min and was dependent on epidermoid growth factor receptor transactivation, but not PI3-K/Akt activation. Overexpression of G _α--transducin, showed that G_βγ-subunit activation is only partially required for ERK1/2 phosphorylation and does not play a role in p38 MAPK phosphorylation, whereas overexpression of a dominant-negative Ras (Ras N17) attenuated both ERK and p38 MAPK activation. In conclusion, a complex signaling network appears to mediate CRH-R1α-MAPK interactions; PI3-K might play a critical role in the regulation of CRH-R1α signaling selectivity and cellular responses.