Lysosomal cystine storage augments apoptosis in cultured human fibroblasts and renal tubular epithelial cells

Abstract

Nephropathic cystinosis is a lethal disorder of lysosomal cystine storage due to defective lysosomal cystine transport. How lysosomal cystine causes this multisystemic disorder culminating in end-stage renal disease is not known, because the cystine is isolated from cellular metabolism by the lysosomal membrane. It is here reported that in both normal and nephropathic cystinotic fibroblasts and cultured renal proximal tubule epithelial cells, increased lysosomal cystine causes an increased rate of apoptosis. In nephropathic cystinotic fibroblasts, the rate of apoptosis is 14.8% after exposure to TNF-alpha versus 7.8% in control normal fibroblasts. Anti-Fas antibodies and UV exposure induced apoptosis in 18.1% and 17.4% of nephropathic cystinotic fibroblasts, respectively, versus 5.2% and 7.1% in normal fibroblasts when analyzed by CaspACE (P < 0.05). Similar results were found when the cells were analyzed by TdT-mediated dUTP nick end labeling (TUNEL). When the cystine content of normal fibroblasts is increased by exposure to cystine dimethylester (CDME), the apoptotic rate is increased to the rate seen in nephropathic cystinotic cells. Decreasing the cystinotic cells' cystine content by use of cysteamine results in normalization of the apoptotic rate. Renal proximal tubule epithelial (RPTE) cells are much more sensitive to CDME than fibroblasts, reaching 43.8% apoptosis 6 h after exposure to CDME alone, compared with 38.2% when exposed to TNF-alpha alone. Serum withdrawal causes an apoptotic rate of 8.7% in nephropathic cystinotic fibroblasts, compared with 6.1% in normal fibroblasts. That rate increases to 37.3% in normal fibroblasts after CDME exposure. Fibroblasts from two cystinotic variants, benign ocular and intermediate cystinosis, do not display increased apoptosis with increased lysosomal cystine. It is concluded that enhanced apoptosis resulting from lysosomal cystine storage may lead to inappropriate cell death and decreased cell numbers in many tissues and hence contribute to the nephropathic cystinotic phenotype. The variant forms may represent co-segregation or linkage of rare alleles that confer resistance to apoptosis, moderating the cell loss and causing the milder disease expression.