Characterization of a specific receptor for somatomedin C (SM-C) on cultured human lymphocytes: evidence that SM-C modulates homologous receptor concentration.

A membrane receptor for somatomedin C (SM-C) on cultured IM-9 cells has recently been reported. The current studies were undertaken to further characterize this intact cellular receptor in terms of both kinetics and specificity and to investigate the ability of SM-C to induce homologous receptor loss. The binding of [125I]iodo-SM-C was rapid, achieving a steady state within 90 min and was greater than 95% reversible. Specific binding at 15C averaged 25-30% for 20 X 10(6) cells/ml. Competition for binding was observed at SM concentrations as low as 2 mU/ml, with half-maximal displacement of [125I]iodo-SM-C at SM concentrations of 33 mU/ml (3.33 X 10(-9) M). Insulin-like growth factor I (IGF-I) and a purified SM-C preparation were approximately equipotent in their ability to displace [125I]iodo-SM-C from the IM-9 receptor. The relative potencies of other growth-related peptides, in comparison with SM-C/IGF-I, were IGF-II (1:10), multiplication stimulating activity (1:10), insulin (1:100), and hGH (nonreactive). Preexposure of IM-9 cells to SM-C at 37 C resulted in a time- and concentration-dependent reduction in [125I]iodo-SM-C. A 10-15% decrease in binding was observed after preincubation with SM in concentrations of 7 mU/ml. Preincubation with 100 mU/ml SM resulted in 50% reduction in binding, but no further decreases were observed after preincubation with higher concentrations of SM (up to 1 U/ml). Scatchard analysis indicated that the reduction in binding was due to a loss of available specific receptors on the cell membrane. These data indicate that the IM-9 cell possesses a specific SM receptor and that SM-C and IGF-I compete equally for occupancy. Furthermore, the data provide a direct demonstration in cell culture that SM-C, like insulin and growth hormone, can modulate homologous receptor concentrations and potentially alter target-cell sensitivity.