Effect of tyrosine kinase inhibitors on luteinizing hormone-releasing hormone (LHRH)-induced gonadotropin release from the anterior pituitary.

A range of selective tyrosine kinase inhibitors, piceatannol, methyl-2,5-dihydroxycinnamate (MDC), genistein, psi-tectorigenin and lavendustin A, all reduced luteinizing hormone-releasing hormone (LHRH)-induced luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release from pro-oestrous rat hemipituitaries incubated in vitro. In general, both 'initial' release and the augmented release resulting from LHRH self-priming, were reduced in parallel in a concentration-dependent fashion. The effects of piceatannol were independent of the steroidal status of the pituitary tissue. Both piceatannol and MDC greatly reduced LH release by ionomycin and a protein kinase C (PKC) activator, phorbol 12,13-dibutyrate (PDBu), suggesting that the tyrosine kinase(s)-dependent step is in the later stages of the stimulus-secretion pathway activated by the LHRH receptor. These data were supported by immunoblots for phosphotyrosine showing that in the gonadotrope-derived alpha T3-1 cell line, treatment with LHRH caused piceatannol-sensitive increases in specific tyrosine phosphorylation of several proteins (major bands at 65-75 and 120-130 kDa). Treatment of cells with PDBu mimicked the tyrosine phosphorylations evoked by LHRH whereas the PKC inhibitor, GF109203X, partially reduced both LHRH- and PDBu-induced tyrosine phosphorylations. Direct effects of MDC and piceatannol on PKC were assessed in an in vitro PKC assay; piceatannol, but not MDC, inhibited PKC activity but at considerably higher concentrations than required for inhibition of LHRH-induced gonadotropin secretion. These data support a role for tyrosine kinase activation in LHRH-induced secretion.