Role of lysosomal cathepsin activities in cell hypertrophy induced by NH4Cl in cultured renal proximal tubule cells

Abstract

An increase of renal ammoniagenesis has been implicated in renal hypertrophy associated with various clinical disorders such as metabolic acidosis, diabetic nephropathy, and renal insufficiency. In vivo and in vitro studies have shown that ammonia promotes hypertrophy in tubular epithelial cells. To elucidate its role on protein turnover, the effects of NH4Cl on the activities of cathepsins B, H, and L + B, as well as on protein synthesis and degradation in LLC-PK1 cells, were investigated, The results show that NH4Cl (20 mM) induced cell hypertrophy, as defined by an increase in both cell protein content and cell volume (+25.5 ± 1.3 and +10.4 ± 0.1% after 48 h). This hypertrophy was associated with the suppression of the activities of cathepsins B and L + B (-57.0 ± 0.9 and -54.5 ± 1.5% after 48 h) and a reduction of protein degradation rate (-59.7 ± 4.1% after 48 h), but without enhanced protein synthesis. The findings were further supported with an additional experiment, showing that the protein synthesis inhibitor cycloheximide (10 μM) did not blunt NH4Cl-induced cell hypertrophy. Moreover, NH4Cl (20 mM) resulted in a persistent elevation of the lysosomal pH, whereas the rise in the cytosolic pH was only transient. This alkalinization in lysosomes may be causatively involved in the impairment of the activities of cathepsins B and L + B. In conclusion, the suppression of the activities of cathepsins B and L + B and the subsequent reduction of protein breakdown due to intralysosomal alkalinization contribute to NH4Cl-induced hypertrophy in LLC-PK1 cells.