Acute effects of the sigma-2 receptor agonist siramesine on lysosomal and extra-lysosomal proteolytic systems in lens epithelial cells

PURPOSE The aim of the present study was to examine the effects of the sigma-2 receptor agonist, siramesine, on morphology, growth, cell death, lysosomal function, and effects on extra-lysosomal proteolytic systems in human lens epithelial cells. METHODS Human lens epithelial cells in culture were exposed to siramesine and examined for morphological changes using Nomarski optics or calcein. Lysosomes were evaluated using acridine orange and Magic Red (RR-cresyl violet). Nuclear morphology was studied using Hoechst 33342 and propidium iodide. Enzymatic activities in living cells or cell lysates were studied using fluorogenic substrates. RESULTS Siramesine at low concentrations increased the cytoplasmic proteolytic activity of the proteasome and the calpain system. Effects were also observed with respect to lysosomal morphology, acidity and function. In addition, activation of caspase-3 and the appearance of nuclei with an apoptotic morphology was found. CONCLUSIONS Siramesine at low concentrations affects lens epithelial cells with perturbations of the major proteolytic systems and lysosomal morphology, resulting in caspase activation and cell death. Siramesine may be a possible substance for the treatment or prevention of posterior capsular opacification (PCO).