Lysosomal Ca2+ as a mediator of palmitate-induced lipotoxicity

While the mechanism of lipotoxicity by palmitic acid (PA), an effector of metabolic stress in vitro and in vivo, has been extensively investigated, molecular details of lipotoxicity are still not fully characterized. Since recent studies reported that PA can exert lysosomal stress in addition to well-known ER and mitochondrial stress, we studied the role of lysosomal events in lipotoxicity by PA, focusing on lysosomal Ca2+. We found that PA induced accumulation of mitochondrial ROS and that mitochondrial ROS induced release of lysosomal Ca2+ due to lysosomal Ca2+ exit channel activation. Lysosomal Ca2+ release led to increased cytosolic Ca2+ which induced mitochondrial permeability transition (mPT). Chelation of cytoplasmic Ca2+ or blockade of mPT with olesoxime or decylubiquinone (DUB) suppressed lipotoxicity. Lysosomal Ca2+ release led to reduced lysosomal Ca2+ content which was replenished by ER Ca2+, the largest intracellular Ca2+ reservoir (ER → lysosome Ca2+ refilling), which in turn activated store-operated Ca2+ entry (SOCE). Inhibition of ER → lysosome Ca2+ refilling by blockade of ER Ca2+ exit channel using dantrolene or inhibition of SOCE using BTP2 inhibited lipotoxicity in vitro. Dantrolene or DUB also inhibited lipotoxic death of hepatocytes in vivo induced by administration of ethyl palmitate together with LPS. These results suggest a novel pathway of lipotoxicity characterized by mPT due to lysosomal Ca2+ release which was supplemented by ER → lysosome Ca2+ refilling and subsequent SOCE, and also suggest the potential role of modulation of ER → lysosome Ca2+ refilling by dantrolene or other blockers of ER Ca2+ exit channels in disease conditions characterized by lipotoxicity such as metabolic syndrome, diabetes, cardiomyopathy or nonalcoholic steatohepatitis.