Protein degradation in kidney proximal tubule cell monolayers.

Isolated proximal tubule cells have been labelled with L-[4,5-3H]leucine prior to cell division. Histochemical staining demonstrated the purity of the cultures. The bicarbonate ion or a collagen support was required for cell growth. Different culture growth rates were established by varying these parameters. The proximal tubule marker enzyme, gamma-glutamyl transpeptidase, was expressed throughout the culture period (7-10 days) and the cells undergo a glycolytic shift, shown by an increase in the levels of lactate dehydrogenase. The specific activities of these enzymes were related to the growth conditions. Exponential rates of protein degradation were observed. The uptake of labelled exogenous hepatocyte proteins in proximal tubule cell cultures was completely suppressed in the presence of serum (10%, v/v) showing that endocytosis did not contribute to the observed measurements of intracellular protein degradation. The increased growth rates seen in cultures were accompanied by decreased rates of protein degradation. Use of the inhibitors of proteolysis, leupeptin and ammonium chloride, showed that the decrease was at the lysosomal level. The results suggest that targeting of inhibitors of lysosomal proteolysis, via low-molecular-weight proteins, may be useful in stimulating tubular regeneration in kidney disease.