Endotoxin-Induced Inhibition of Growth Hormone Receptor Signaling in Rat Liver in Vivo1.

Abstract

The bacterial lipopolysaccharide endotoxin induces a catabolic response characterized by resistance to multiple anabolic hormones. The objective of this study was to determine the effects of endotoxin on the GH signaling pathway in rat liver in vivo. After the iv injection of Escherichia coli endotoxin (1 mg/kg), there was a progressive decrease in liver STAT5 (signal transducer and activator of transcription-5) tyrosine phosphorylation in response to GH (40% decrease 6 h after endotoxin), which occurred in the absence of a change in abundance of the STAT5 protein. Endotoxin resulted in a rapid 40-fold increase in liver Janus family kinase-2 (JAK2) messenger RNA, followed by a 2-fold increase in JAK2 protein abundance. This was associated with a 50% decrease in phosphorylated/total JAK2 after GH stimulation. GH receptor abundance was unchanged, suggesting a postreceptor site of endotoxin-induced GH resistance. Rat complementary DNAs for three members of the suppressor of cytokine signaling gene family were cloned [cytokine-inducible sequence (CIS), suppressor of cytokine signaling-2 (SOCS-2), and SOCS-3] and, using these probes, messenger RNAs for SOCS-3 and CIS were shown to be increased 10and 4-fold above control values, respectively, 2 h after endotoxin infusion. The finding of endotoxin inhibition of in vivo STAT5 tyrosine phosphorylation in response to a supramaximal dose of GH in the absence of a change in GH receptor abundance or total GH-stimulated JAK2 tyrosine phosphorylation provides the first demonstration of acquired postreceptor GH resistance. We hypothesize that this may occur through a specificity-spillover mechanism involving the induction of SOCS genes by cytokines released in response to endotoxin and subsequent SOCS inhibition of GH signaling. (Endocrinology 140: 5505–5515, 1999) R TO GH has been demonstrated in the liver and other tissues after bacterial sepsis, trauma, burns, major surgery, hypoglycemia, or starvation (1–3). Consistent with hepatic resistance to endogenous GH, insulin-like growth factor-I (IGF-I) production and plasma IGF-I concentrations, which are determined largely by GH-induced IGF-I synthesis in the liver (2, 4, 5), are decreased in critically ill and septic patients despite elevated plasma GH levels (6, 7). This GH resistance may contribute to the catabolic response that is characteristic of severe illness and, ultimately, to an unfavorable disease outcome. The administration of GH to critically ill patients has been shown to promote tissue anabolism under some circumstances, but exogenous GH may produce untoward side-effects and is not uniformly capable of inducing an anabolic response (8, 9). Therefore, it is important to understand the mechanism of GH resistance in catabolic states. The immediate signaling events following GH binding to its receptor have only recently been defined (10). In target tissues, such as liver and skeletal muscle, GH binding results in dimerization of the GH receptor and consequent activation of the intracellular receptor-associated tyrosine kinase Janus kinase-2 (JAK2) (11, 12). This is followed by JAK2 autophosphorylation and tyrosine phosphorylation of the GH receptor and members of the signal transducer and activator of transcription (STAT) protein family (13). Phosphorylation of the STAT proteins leads to their dimerization and translocation to the nucleus, where they modulate gene transcription (10, 14). Although studies in cell culture and experimental animals have provided evidence for GH-stimulated tyrosine phosphorylation of a number of cellular signaling proteins, including various STATs (10, 15), insulin receptor substrate-1 (IRS-1), IRS-2, IRS-3 (13, 16, 17), and the SHC proteins (18), STAT5 appears to be the dominant intracellular signaling protein phosphorylated by GH-activated JAK2 in adult rat liver and muscle in vivo (12, 14). GH resistance in catabolic states has been related at least in part to decreased tissue abundance of GH receptors (19–21). In other hormonal pathways, such as the insulin signaling pathway, diseaserelated hormone resistance has been shown to result from changes in both receptor abundance and postreceptor signaling responses (22–24). It is not known whether GH resistance also may result from alterations in postreceptor steps in the GH signaling pathway. In many experimental studies, infusion of the Gram-negative bacterial cell wall lipopolysaccharide, endotoxin, has been used to generate a model of catabolic disease states. Endotoxin is an important contributor to the fever and tissue Received May 27, 1999. Address all correspondence and requests for reprints to: Robert J. Smith, M.D., Joslin Diabetes Center, Harvard Medical School, One Joslin Place, Boston, Massachusetts 02215. E-mail: robert.smith@joslin. harvard.edu. * This work was supported in part by NIH Grants DK-50411 and DK-43038, Diabetes and Endocrinology Research Center Grant DK-36836, and the Markey Charitable Trust. Portions of this work have been communicated in oral presentations at the 80th Annual Meeting of The Endocrine Society, New Orleans, Louisiana, 1998, and the 81st Annual Meeting of The Endocrine Society, San Diego, California, 1999. 0013-7227/99/$03.00/0 Vol. 140, No. 12 Endocrinology Printed in U.S.A. Copyright © 1999 by The Endocrine Society

DOI: 10.1210/endo.140.12.7212

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@article{Mao1999EndotoxinInducedIO, title={Endotoxin-Induced Inhibition of Growth Hormone Receptor Signaling in Rat Liver in Vivo1.}, author={Yilei Mao and Pei-ra Ling and Timothy P. Fitzgibbons and Karen C McCowen and Gerd Frick and Bruce Ryan Bistrian and Robert J. Smith}, journal={Endocrinology}, year={1999}, volume={140 12}, pages={5505-5515} }