The insulin receptor of a human monocyte-like cell line: characterization and function

Most clinical studies of the insulin receptor in man have been carried out in circulating cells, mainly monocytes. It is important to establish whether the function of the insulin receptor in blood cells reflects that of the major target tissues. Since peripheral monocytes do not divide, they cannot be studied under continuous cell culture conditions. We, therefore, studied the insulin receptor of a monocyte-like human cell line (U-937) to determine whether it could serve as a model for further investigation of the function of the insulin receptor. We found that the U-937 cells bind [125I]insulin specifically and in a time-, temperature-, concentration-, and pH-dependent fashion, similar to circulating monocytes. In addition, they internalize the hormone-receptor complex rapidly and extensively at 37 C in a fashion analogous to that of blood monocytes and hepatocytes. U-937 cells show a similar affinity for insulin as monocytes. Unlike hepatocytes, these cells do not appear to release extensive insulin-degrading activity and do not degrade cell surface-associated ligand during initial incubations. [125I] Insulin extracted or dissociated spontaneously from the cell surface shows full rebindability to fresh cells. The insulin receptor of the U-937 cells can be down-regulated as in the major target cells. As in cultured human lymphocytes, preincubation of U-937 cells with prednisolone results in significantly increased insulin binding. Incubation with a phorbol ester tumor promoter, in contrast, inhibits the extent of specific [125I]insulin binding to U-937 cells by altering the receptor affinity. The insulin receptor of the U-937 monocyte-like cell line mirrors the insulin receptor of blood monocytes and target cells; it is a useful tool for further in vitro studies of the insulin receptor.