title: Insulin reverses growth hormone-induced homologous desensitization.
abstract: Growth hormone (GH) is secreted in a pulsatile pattern to promote body growth and metabolism. GH exerts its function by activating several signaling pathways, including JAK2/STAT and MEK/ERK. ERK1/2 activation by GH plays important roles in gene expression, cell proliferation, and growth. We previously reported that in rat H4IIE hepatoma cells after an initial GH exposure, a second GH exposure induces STAT5 phosphorylation but not ERK1/2 phosphorylation (Ji, S., Frank, S. J., and Messina, J. L. (2002) J. Biol. Chem. 277, 28384-28393). In this study the mechanisms underlying GH-induced homologous desensitization were investigated. A second GH exposure activated the signaling intermediates upstream of MEK/ERK, including JAK2, Ras, and Raf-1. This correlated with recovery of GH receptor levels, but was insufficient for GH-induced phosphorylation of MEK1/2 and ERK1/2. Insulin restored the ability of a second GH exposure to induce phosphorylation of MEK1/2 and ERK1/2 without altering GH receptor levels or GH-induced phosphorylation/activation of JAK2 and Raf-1. GH and insulin synergized in promoting cell proliferation. Further investigation suggested that insulin increased the amount of MEK bound to KSR (kinase suppressor of Ras) and restored GH-induced tyrosine phosphorylation of KSR. Previous GH exposure also induced desensitization of STAT1 and STAT3 phosphorylation, but this desensitization was not reversed by insulin. Thus, insulin-regulated resensitization of GH signaling may be necessary to reset the complete response to GH after a normal, physiologic pulse of GH.
authors.affiliation: Department of Pathology, Division of Molecular and Cellular Pathology, University of Alabama at Birmingham, Birmingham, Alabama 35294-0019.
authors.forename: Jie
authors.initials: J
authors.lastname: Xu
authors.affiliation: Department of Pathology, Division of Molecular and Cellular Pathology, University of Alabama at Birmingham, Birmingham, Alabama 35294-0019.
authors.forename: Zhongyu
authors.initials: Z
authors.lastname: Liu
authors.affiliation: Department of Pathology, Division of Molecular and Cellular Pathology, University of Alabama at Birmingham, Birmingham, Alabama 35294-0019.
authors.forename: Thomas L
authors.initials: TL
authors.lastname: Clemens
authors.affiliation: Department of Pathology, Division of Molecular and Cellular Pathology, University of Alabama at Birmingham, Birmingham, Alabama 35294-0019. Electronic address: messina@path.uab.edu.
authors.forename: Joseph L
authors.initials: JL
authors.lastname: Messina
mesh_terms: D000818:Animals; D002417:Cattle; D049109:Cell Proliferation; D002478:Cells, Cultured; D013006:Growth Hormone / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D007328:Insulin / Q000502:physiology*; D053614:Janus Kinase 2; D008297:Male; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051381:Rats; D011986:Receptors, Somatotropin / Q000378:metabolism; D015398:Signal Transduction
chemical_list: D007328:Insulin; D011518:Proto-Oncogene Proteins; D011986:Receptors, Somatotropin; D013006:Growth Hormone; D011494:Protein Kinases; D011505:Protein-Tyrosine Kinases; C507927:Jak2 protein, rat; D053614:Janus Kinase 2