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 monocyte-like (U-937) and lymphoblastoid (IM-9) cells in a dose-, time-, and temperature-dependent manner within 30 sec at 37 degrees 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-alpha-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.(ABSTRACT TRUNCATED AT 250 WORDS)