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Endocrinology Vol. 139, No. 10 4147-4154
Copyright © 1998 by The Endocrine Society

ARTICLES

{alpha}2-Heremans Schmid Glycoprotein Inhibits Insulin-Stimulated Elk-1 Phosphorylation, But Not Glucose Transport, in Rat Adipose Cells1

Hui Chen, Pothur R. Srinivas, Li-Na Cong, Yunhua Li, George Grunberger and Michael J. Quon

Hypertension-Endocrine Branch, National Heart, Lung, and Blood Institute (H.C., L.-N.C., Y.L., M.J.Q.), National Institutes of Health, Bethesda, Maryland 20892; and the Department of Internal Medicine, Wayne State University School of Medicine, Center for Molecular Medicine and Genetics (P.R.S., G.G.), Detroit, Michigan 48201

Address all correspondence and requests for reprints to: Michael J. Quon, M.D., Ph.D., Hypertension-Endocrine Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Building 10, Room 8C-103, 10 Center Drive MSC 1754, Bethesda, Maryland 20892-1754. E-mail: quonm{at}gwgate.nhlbi.nih.gov

{alpha}2-Heremans Schmid glycoprotein ({alpha}2-HSG) is a member of the fetuin family of serum proteins whose biological functions are not completely understood. There is a consensus that {alpha}2-HSG plays a role in the regulation of tissue mineralization. However, one aspect of {alpha}2-HSG function that remains controversial is its ability to inhibit the insulin receptor tyrosine kinase and the biological actions of insulin. Interestingly, some studies suggest that {alpha}2-HSG differentially inhibits mitogenic, but not metabolic, actions of insulin. However, these previous studies were not carried out in bona fide insulin target cells. Therefore, in the present study we investigate the effects of {alpha}2-HSG in the physiologically relevant rat adipose cell. We studied insulin-stimulated translocation of the insulin-responsive glucose transporter GLUT4 in transfected rat adipose cells overexpressing human {alpha}2-HSG. In addition, we measured insulin-stimulated glucose transport in adipose cells cultured with conditioned medium from the transfected cells as well as in freshly isolated adipose cells treated with purified human {alpha}2-HSG. Compared with control cells, we were unable to demonstrate any significant effect of {alpha}2-HSG on insulin-stimulated translocation of GLUT4 or glucose transport. In contrast, we did demonstrate that overexpression of {alpha}2-HSG in adipose cells inhibits both basal and insulin-stimulated phosphorylation of Elk-1 (a transcription factor phosphorylated and activated by mitogen-activated protein kinase and other related upstream kinases). Interestingly, we did not observe any major effects of {alpha}2-HSG to inhibit insulin-stimulated phosphorylation of the insulin receptor, insulin receptor substrate-1, -2, or -3, in either transfected or freshly isolated adipose cells. We conclude that {alpha}2-HSG inhibits insulin-stimulated Elk-1 phosphorylation, but not glucose transport, in adipose cells by a mechanism that may involve effector molecules downstream of insulin receptor substrate proteins. .




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