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Prolonged treatment by phosphoramidon modulates the number of endothelin receptors in cultured Swiss 3T3 fibroblasts.

Endothelin (ET) is generated from prepro-ET initially by dibasic pair proteolysis, followed by a specific proteolytic cleavage between Trp21 and Val22. Currently, intense research is focused on the investigation of a metalloprotease that is inhibited by phosphoramidon (PHOS) only, but not by other protease inhibitors. In this report, we show that ET binding was increased significantly in cultured Swiss 3T3 fibroblasts with PHOS pretreatment. The effect of PHOS was dose- and time-dependent. Other protease inhibitors, such as thiorphan, pepstatin-A, E-64, phenylmethyl sulfonyl fluoride, and aprotinin, failed to exert a similar effect. Control experiments indicated that the effect of PHOS was not due to inhibition of 125I-ET-1 degradation. Binding studies using whole cells or membranes prepared from cell show that ET binding sites increased from 23,000 to 133,000 sites/cell in control versus PHOS-treated cells. The effect of PHOS treatment on the ET receptor may be due to the inhibition of a protease responsible for ET-receptor processing. This effect is likely to complicate interpretation of results from studies using PHOS to block the putative ET-converting enzyme.