Enhancement of post-receptor insulin signaling by trivalent chromium in hepatoma cells is associated with differential inhibition of specific protein-tyrosine phosphatases

Various molecular forms of chromium have been implicated in the regulation of glucose metabolism, and chromium deficiency can be associated with insul in resistance and impaired glucose tolerance. Protein-tyrosine phosphatases (PTPases), Which negatively regulate signaling through the insulin recepto r, are potential targets of chromium action, since this transition metal ma y inhibit catalysis at the thiol-dependent active sites of these enzymes. T reatment of cultured rat hepatoma cells with 0.1 mM CrCl3 for 16 h increase d the insulin-stimulated tyrosine phosphorylation of high M-r insulin recep tor substrate (IRS) proteins by 49% to 7.3-fold over basal (n = 7; P = 0.03 ), without altering basal insulin receptor or IRS tyrosine phosphorylation or insulin-stimulated receptor autophosphorylation, suggesting a post-recep tor effect of chromium on signal transduction. PTPase activity in cell extr acts of CrCl3-treated hepatoma cells before or after insulin stimulation wa s unchanged, indicating that if chromium acted via cellular PTPases, the ef fect was reversible and limited to the in vivo state. Chromium (Cr+3) ion a nd two organic derivatives, an oligopeptide chromium complex from bovine li ver (Cr-pep), and a synthetic multinuclear complex of chromium with carboxy late ligands (Sm-Cr) were also tested for their direct in vitro inhibition of the enzymatic activity of LAR and PTP1B, two structurally variant PTPase s that have been implicated in regulation of the insulin signaling pathway. PTP1B (rat and human) was strongly inhibited by CrCl3 to 21-33% of control (n = 4-6; P < 0.001). In contrast, LAR activity was actually enhanced by C rCl3 to 47% above the control value (n = 12 P < 0.001). The Cr-pep and Sm-C r complexes had no effect on PTP1B and LAR activity at the tested concentra tions using the pNPP assay. These data suggest that the metabolic effects o f chromium may be mediated by inhibition of PTP1B, a PTPase that negatively modulates insulin signaling, consistent with other recent studies implicat ing PTP1B in the regulation of the dephosphorylation of post-insulin recept or substrate proteins. J. Trace Elem. Exp. Med. 14:393-404, 2001. (C) 2001 Wiley-Liss, Inc.