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 insulin resistance and impaired glucose tolerance. Protein-tyrosine phosphatases (PTPases), which negatively regulate signaling through the insulin receptor, are potential targets of chromium action, since this transition metal may inhibit catalysis at the thiol-dependent active sites of these enzymes. Treatment of cultured rat hepatoma cells with 0.1 mM CrCl₃ for 16 h increased the insulin-stimulated tyrosine phosphorylation of high M_r insulin receptor 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-receptor effect of chromium on signal transduction. PTPase activity in cell extracts of CrCl₃-treated hepatoma cells before or after insulin stimulation was unchanged, indicating that if chromium acted via cellular PTPases, the effect was reversible and limited to the in vivo state. Chromium (Cr⁺³) ion and two organic derivatives, an oligopeptide chromium complex from bovine liver (Cr-pep), and a synthetic multinuclear complex of chromium with carboxylate ligands (Sm-Cr) were also tested for their direct in vitro inhibition of the enzymatic activity of LAR and PTP1B, two structurally variant PTPases 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 CrCl₃ to 47% above the control value (n = 12; P< 0.001). The Cr-pep and Sm-Cr complexes had no effect on PTP1B and LAR activity at the tested concentrations using the pNPP assay. These data suggest that the metabolic effects of chromium may be mediated by inhibition of PTP1B, a PTPase that negatively modulates insulin signaling, consistent with other recent studies implicating PTP1B in the regulation of the dephosphorylation of post-insulin receptor substrate proteins. J. Trace Elem. Exp. Med. 14:393–404, 2001. © 2001 Wiley-Liss, Inc.