Diacylglycerol Modulation of Insulin Receptor From Cultured Human Mononuclear Cells: Effects on Binding and Internalization

Tumor-promoting phorbol esters alter binding of growth factors and hormones to their specific receptors. Action of diacylglycerols, endogenous phorbol ester analogues, on ¹²⁵I-labeled insulin binding to its receptor from human cells was therefore investigated. A variety of 1,2-diacylglycerols and 1,3-diacylglycerols inhibited ¹²⁵I-insulin binding to intact human monocytelike (U-937) and lymphoblastoid (IM-9) cells in a dose-, time-, and temperature-dependent manner within 30 sec at 37°C in a fashion analogous to that of the tumor-promoting phorbol diester 12-O-tetradecanoylphorbol-13-acetate (TPA). Inhibition of insulin binding by diacylglycerols, analyzed by Scatchard plot, seems to be due to altered binding affinity of the insulin receptor. Diacylglycerol effects were reversible, were seen regardless of the order of addition of ¹²⁵I-insulin and diacylglycerols, and were demonstrated only with occupied insulin receptors. Corresponding fatty acids or phospholipids did not affect specific insulin binding to the intact U-937 cells. Diacylglycerols also inhibited binding of ¹²⁵I-insulin—like growth factor (IGF) I but not that of ¹²⁵I-human growth hormone (HGH) to the human cells. The non-tumor-promoting phorbols (phorbol, 4-a-phorbol, phorbol-12,13-distearate) did not affect insulin binding to intact cells. Both diacylglycerols and TPA stimulated internalization of ¹²⁵I-insulin by U-937 and IM-9 cells. The ability of diacylglycerol to mimic the effects of TPA on the insulin receptor supports the concept of diacylglycerols as endogenous phorbol diester analogues even though the sole role of protein kinase C in our system is doubtful. The DAG mediated modulation of the insulin receptor in intact cells might play a role in cell differentiation, proliferation, and tumor promotion through an additional mechanism.