ROLE OF LYSOSOMAL CATHEPSIN ACTIVITIES IN CELL HYPERTROPHY INDUCED BYNH4CL IN CULTURED RENAL PROXIMAL TUBULE CELLS

An increase of renal ammoniagenesis has been implicated in renal hyper trophy associated with various clinical disorders such as metabolic ac idosis, 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 effec ts of NH4Cl on the activities of cathepsins B, H, and L + B, as well a s on protein synthesis and degradation in LLC-PK1 cells, were investig ated. 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 ass ociated 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 pr otein degradation rate (-59.7 +/- 4.1% after 48 h), but without enhanc ed protein synthesis. The findings were further supported with an addi tional experiment, showing that the protein synthesis inhibitor cycloh eximide (10 mu M) did not blunt NH4Cl-induced cell hypertrophy. Moreov er, NH4Cl (20 mM) resulted in a persistent elevation of the lysosomal pH, whereas the rise in the cytosolic pH was only transient. This alka linization in lysosomes may be causatively involved in the impairment of the activities of cathepsins B and L + B. In conclusion, the suppre ssion of the activities of cathepsins B and L + B and the subsequent r eduction of protein breakdown due to intralysosomal alkalin-ization co ntribute to NH4Cl-induced hypertrophy in LLC-PK1 cells.