THE MYCOTOXIN OCHRATOXIN-A IMPAIRS PROTEIN-UPTAKE IN CELLS DERIVED FROM THE PROXIMAL TUBULE OF THE KIDNEY (OPOSSUM KIDNEY-CELLS)

Proximal tubule-derived opossum kidney (OK) cells are a suitable model to study proximal tubular protein endocytosis by using fluorescein-is othiocyanate-albumin as substrate. We used OK cells to investigate sev eral steps of the endocytotic process in control cells and in ochratox in A (OTA)-treated cells. OTA is a mycotoxin which causes proteinuria. When OTA was present only during the 15-min period in which uptake wa s studied, it had no effect on albumin endocytosis. Preincubation of O K cells with OTA (10 mu mol/l) for 24 hr led to a reduction of transpo rt capacity (J(max) to similar to 50% of control) and of apparent affi nity (K-m to similar to 200% of control). Specific binding of albumin to the apical cell surface was reduced also. Maximum binding capacity was reduced to 72% of control. By contrast, endocytotic uptake of the fluid-phase marker dextran was not affected by OTA. Preincubation of O K cells for 24 hr with 10 mu mol/l of OTA reduced degradation of fluor escein-isothiocyanate-albumin to trichloroacetic acid-soluble fluoresc ence to 59% of control. We could not detect any difference in endosoma l pH (6.13 +/- 0.05 in controls vs. 6.04 +/- 0.10 in OTA-treated cells ). Furthermore, the rate of re-exocytosis of albumin taken up was sign ificantly greater in OTA-treated cells. We conclude that: 1) OK cells are a suitable model to study several steps of the endocytotic process separately and thus to investigate the pathophysiology of reduced tub ular protein reabsorption and 2) 24-hr exposure to OTA reduces protein uptake because of a decrease of specific binding sites and of enhance d exocytosis. Thus, OTA-induced proteinuria may at least be in part of tubular origin.