Small molecule insulin receptor activators potentiate insulin action in insulin-resistant cells

In type 2 diabetes, impaired insulin signaling leads to hyperglycemia and o ther metabolic abnormalities. To study a new class of antidiabetic agents, we compared two small, nonpeptide molecules that activate insulin receptor (IR) P-subunit tyrosine kinase activity: Merck L7, a direct IR agonist, and Telik's TLK16998, an IR sensitizer. In rat hepatoma cells (HTCs) that over express the IR (HTC-IR), IR autophosphorylation was directly activated by L 7 in the absence of insulin. TLK16998 did not directly activate IR autophos phorylation, but it enhanced IR autophosphorylation in the presence of insu lin. Tyrosine phosphorylation of an endogenous 185-kDa IR substrate was als o significantly enhanced by both Merck L7 alone and TLK16998 plus insulin. Adding TLK16998 to L7 produced synergistic effects, further indicating that these two compounds act on the IR through separate mechanisms. We next stu died HTC-IRDelta 485-599 cells, which overexpress a mutant IR with a deleti on in the a-subunit connecting domain that does not undergo autophosphoryla tion in response to insulin binding. L7 was able to directly activate autop hosphorylation of the deletion mutant IR in these cells, whereas TLK16998 h ad no effect. Compounds were then tested in three other cell models of impa ired IR function. Both TLK16998 and Merck L7 improved IR autophosphorylatio n in cells with diminished IR signaling due to either treatment with tumor necrosis factor-a or overexpression of membrane glycoprotein PC-1. However, in TPA (tetradecanoylphorbol acetate)-treated cells, TLK16998 but not Merc k L7 was able to significantly reverse the impaired insulin-stimulated IR a utophosphorylation. In summary, these two classes of IR activators selectiv ely increased IR function in a variety of insulin-resistant cell lines.