G13-dependent activation of MAPK by thyrotropin

Abstract

Stimulation of the thyrotropin receptor (TSHR) activates G proteins of all four subfamilies (Gs, Gi/o, Gq/11, and G 12/13). Whereas Gs/cAMP-dependent cellular responses upon TSHR stimulation are well established, other signaling pathways are less characterized. We evaluated TSH-elicited cellular responses in human follicular thyroid carcinoma cells stably expressing the TSHR and in primary, nonneoplastic human thyrocytes. In these cellular models, stimulation with TSH caused activation of p44/42 MAPK and subsequent induction of c-Fos. MAPK stimulation occurred independently of Gs, Gi/o, and Gq/11 signaling. Dominant negative constructs of G12 or G13 as well as shRNA-mediated suppression of Gα12 or Gα13 revealed that MAPK activation was dependent on G 13 but not on G12 signaling. Furthermore, G 13-dependent transactivation of the epidermal growth factor receptor was necessary for MAPK activation in follicular carcinoma cells, whereas EGFR was not involved in MAPK activation in nonneoplastic primary thyrocytes. The use of bacterial inhibitors of monomeric GTPases revealed that MAPK activation proceeded independently of Rho proteins but was clostridial toxin B-sensitive, suggesting involvement of Cdc42 or Rac. Thus, our data shed new light on cAMP-independent TSHR signaling and identify the first G13-dependent TSHR signaling pathway in human thyrocytes. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.