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Activation of the Lutropin/Choriogonadotropin Receptor in MA-10 Cells Leads to the Tyrosine Phosphorylation of the Focal Adhesion Kinase by a Pathway that Involves Src Family Kinases

Department of Pharmacology, Carver College of Medicine, The University of Iowa, Iowa City, Iowa 52242-1109

Address all correspondence and requests for reprints to: Dr. Mario Ascoli, Department of Pharmacology, 2-319B BSB, 51 Newton Road, The University of Iowa, Iowa City, Iowa 52242-1109. E-mail: mario-ascoli{at}uiowa.edu.

We show that activation of the endogenous or recombinant lutropin/choriogonadotropin receptor (LHR) in mouse Leydig tumor cells (MA-10 cells) leads to the tyrosine phosphorylation of the focal adhesion kinase (FAK) and one of its substrates (paxillin). Using specific antibodies to the five tyrosine residues of FAK that become phosphorylated, we show that activation of the LHR increases the phosphorylation of Tyr⁵⁷⁶ and Tyr⁵⁷⁷, but it does not affect the phosphorylation of Tyr³⁹⁷, Tyr⁸⁶¹, or Tyr⁹²⁵. Because FAK is a prominent substrate for the Src family of tyrosine kinases (SFKs) we tested for their involvement in the LHR-mediated phosphorylation of FAK-Tyr⁵⁷⁶. Src is not detectable in MA-10 cells, but two other prominent members of this family (Fyn and Yes) are present. The LHR-mediated phosphorylation of FAK-Tyr⁵⁷⁶ is readily inhibited by PP2 (a pharmacological inhibitor of SFKs) and by dominant-negative mutants of SKFs. Moreover, activation of the LHR in MA-10 cells results in the stimulation of the activity of Fyn and Yes, and overexpression of either of these two tyrosine kinases enhances the LHR-mediated phosphorylation of FAK-Tyr⁵⁷⁶. Studies involving activation of other G protein-coupled receptors, overexpression of the different G-subunits, and the use of second messenger analogs suggest that the LHR-induced phosphorylation of FAK-Tyr⁵⁷⁶ in MA-10 cells is mediated by SFKs, and that this family of kinases is, in turn, independently or cooperatively activated by the LHR-induced stimulation of G_s and G_q/11-mediated pathways.

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