Alpha2-Heremans Schmid glycoprotein inhibits insulin-stimulated Elk-1 phosphorylation, but not glucose transport, in rat adipose cells.

Alpha2-Heremans Schmid glycoprotein (alpha2-HSG) is a member of the fetuin family of serum proteins whose biological functions are not completely understood. There is a consensus that alpha2-HSG plays a role in the regulation of tissue mineralization. However, one aspect of alpha2-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 alpha2-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 alpha2-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 alpha2-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 alpha2-HSG. Compared with control cells, we were unable to demonstrate any significant effect of alpha2-HSG on insulin-stimulated translocation of GLUT4 or glucose transport. In contrast, we did demonstrate that overexpression of alpha2-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 alpha2-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 alpha2-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.