Release of a sodium transport inhibitor (inhibitin) from cultured human cancer cells.

In this study we investigated whether the sodium transport inhibitor, inhibitin, originally isolated from leukemic promyelocytes, was also elaborated by some other neoplastic cells in culture. Like culture medium from the leukemic promyelocytes (HL60), the media from two other leukemic cell lines (erythroblasts K562 and monoblasts U937) also showed significant inhibitory activity on ouabain-insensitive sodium efflux rate constant in normal erythrocytes. Similarly, culture media from three neoplastic cell lines (H.Ep2, MRC5, and HX99) also showed significant inhibitin-like inhibitory activity. Using high-performance liquid chromatography to isolate inhibitin, culture media from HL60 and H.Ep2 cells were identically treated, and inhibitin isolated from H.Ep2 cells had the same retention time as that shown by promyelocyte inhibitin. H.Ep2 inhibitin reduced ouabain- and bumetanide-insensitive sodium efflux rate constant from 0.1510 +/- 0.0275 (SD) to 0.0988 +/- 0.0110 (P less than 0.005). Like promyelocyte inhibitin, H.Ep2 inhibitin reduced sodium efflux and influx by equivalent amounts suggesting thereby that it is a sodium/sodium exchange inhibitor. These studies show that a factor exhibiting inhibitory activity on sodium/sodium exchange is secreted by a variety of leukemic and neoplastic cells in culture.