Differential Signaling in the Regulation of LH- and GH-Cells in Goldfish
John P Chang*
Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E9
Goldfish luteinizing hormone (LH)- and growth hormone (GH)-cell activities are controlled by multiple neuroendocrine factors. Interestingly, hormone release and gene expression in gonadotropes and somatotropes are stimulated by both endogenous gonadotropin (GTH)-releasing hormones (chicken GnRH-II, GnRH2 and salmon GnRH, GnRH3), through G-protein coupled receptors (GPCRs). We have previously shown that protein kinase C (PKC), extracellular Ca2+ entry through voltage-sensitive channels, extracellular signal-regulated protein kinase (ERK), and cyclic GMP/PKG mediate GnRH2- and GnRH3-induced hormone release; however, pharmacologically distinct intracellular Ca2+ stores, mitochondrial Ca2+ buffering, nitric oxide production, and arachidonic acid differentially participate in a GnRH- and cell-type-selective manner. In addition, ERK, but not PKC, mediates GnRH-stimulated increases in GTH subunit and GH mRNA levels, whereas increases in intracellular Ca2+ generally reduce GTH and GH gene expression. Recently, we showed that unique suites of the phosphatidylinositol-3,4,5-P3 (PtdIns(3,4,5)P3)-generating class I phosphoinositide 3-kinases (PI3Ks), including the GPCR-insensitive PI3Kd, as well as the Gbg-sensitive PI3Kb and PI3Kg isoforms, are selectively involved in GnRH2 and GnRH3 signaling in LH and GH release; whereas PI3Ka only affects basal LH secretion. The relationship between PI3Ks and GnRH-induced changes in intracellular free Ca2+ levels also differs between LH and GH release. Downstream of class I PI3Ks activation, results indicate the recruitment of non-canonical effectors possessing PtdIns(3,4,5)P3-binding pleckstrin homology domains, as well as demonstrate the selective participation of the canonical PI3K-dependent targets, protein kinase B and Bruton’s tyrosine kinase, in GnRH-stimulated hormone release. PI3K- and ERK-dependent signaling also differentially mediate GnRH-selective effects on elevations in cellular LH and GH contents in a time-dependent manner. These results, when taken as a whole, support the hypothesis that biased GnRHR signaling forms the basis for ligand-, function-, and cell-type specific neuroendocrine control of gonadotrope and somatotrope activities by native GnRH peptides.
Acknowledgement: Supported by NSERC.
Key Words: Biased receptor signal transduction, GnRH isoforms, PI3K signaling
Gonadotropes, somatotropes
John P Chang*
Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E9
Goldfish luteinizing hormone (LH)- and growth hormone (GH)-cell activities are controlled by multiple neuroendocrine factors. Interestingly, hormone release and gene expression in gonadotropes and somatotropes are stimulated by both endogenous gonadotropin (GTH)-releasing hormones (chicken GnRH-II, GnRH2 and salmon GnRH, GnRH3), through G-protein coupled receptors (GPCRs). We have previously shown that protein kinase C (PKC), extracellular Ca2+ entry through voltage-sensitive channels, extracellular signal-regulated protein kinase (ERK), and cyclic GMP/PKG mediate GnRH2- and GnRH3-induced hormone release; however, pharmacologically distinct intracellular Ca2+ stores, mitochondrial Ca2+ buffering, nitric oxide production, and arachidonic acid differentially participate in a GnRH- and cell-type-selective manner. In addition, ERK, but not PKC, mediates GnRH-stimulated increases in GTH subunit and GH mRNA levels, whereas increases in intracellular Ca2+ generally reduce GTH and GH gene expression. Recently, we showed that unique suites of the phosphatidylinositol-3,4,5-P3 (PtdIns(3,4,5)P3)-generating class I phosphoinositide 3-kinases (PI3Ks), including the GPCR-insensitive PI3Kd, as well as the Gbg-sensitive PI3Kb and PI3Kg isoforms, are selectively involved in GnRH2 and GnRH3 signaling in LH and GH release; whereas PI3Ka only affects basal LH secretion. The relationship between PI3Ks and GnRH-induced changes in intracellular free Ca2+ levels also differs between LH and GH release. Downstream of class I PI3Ks activation, results indicate the recruitment of non-canonical effectors possessing PtdIns(3,4,5)P3-binding pleckstrin homology domains, as well as demonstrate the selective participation of the canonical PI3K-dependent targets, protein kinase B and Bruton’s tyrosine kinase, in GnRH-stimulated hormone release. PI3K- and ERK-dependent signaling also differentially mediate GnRH-selective effects on elevations in cellular LH and GH contents in a time-dependent manner. These results, when taken as a whole, support the hypothesis that biased GnRHR signaling forms the basis for ligand-, function-, and cell-type specific neuroendocrine control of gonadotrope and somatotrope activities by native GnRH peptides.
Acknowledgement: Supported by NSERC.
Key Words: Biased receptor signal transduction, GnRH isoforms, PI3K signaling
Gonadotropes, somatotropes