Lysosomal Ca2+ as a mediator of palmitate-induced lipotoxicity

Abstract

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.

Inhibition of Hepatocellular Carcinoma by Endogenous Lipids: Induction of Pyroptosis by Geranylgeranoic Acid—A Comparison with Palmitic Acid and Retinoic Acid

“…ATRA treatment results in decreased mitochondrial number and average volume, as well as disappearance of mitochondrial cristae after 48 h [49]. Additionally, loss of ΔΨm is observed after 1 h of GGA (10 µM) treatment [18], 24-96 h of ATRA (1 µM) treatment [55], and 24 h of PA (200 µM) treatment [46], indicating mitochondrial permeability transition (mPT) (Table 1). Furthermore, hyperproduction of superoxide in mitochondria is confirmed after treatment with GGA (10µM, 15 min) [14], PA (500 µM, 3-24 h) [46,56], or ATRA (10 µM, 48 h) [49].…”

“…The final concentrations of PA added to the medium range from 200 to 3000 µM, which is 10 to several hundred times higher than GGA or ATRA. Although there are many reports, cell death is confirmed within 24 h after PA treatment [11,12,[43][44][45][46]. Molecules that exert effects at concentrations of several µM like GGA or ATRA are thought to act as signaling molecules.…”

“…Additionally, loss of ΔΨm is observed after 1 h of GGA (10 µM) treatment [18], 24-96 h of ATRA (1 µM) treatment [55], and 24 h of PA (200 µM) treatment [46], indicating mitochondrial permeability transition (mPT) (Table 1). Furthermore, hyperproduction of superoxide in mitochondria is confirmed after treatment with GGA (10µM, 15 min) [14], PA (500 µM, 3-24 h) [46,56], or ATRA (10 µM, 48 h) [49]. Free fatty acids like PA are proposed to induce superoxide production by directly binding to complexes I and III of the respiratory chain, inhibiting electron transfer and leading to increased superoxide production [57].…”

Linking the unfolded protein response to bioactive lipid metabolism and signalling in the cell non‐autonomous extracellular communication of ER stress

“…There is increasing interest in the interaction between lipotoxicity‐inducing free fatty acids such as palmitate and ER calcium handling in the initiation of UPR‐independent ER stress. Lysosomal calcium release has recently been implicated in a mechanism of hepatic lipotoxicity induced by palmitic acid [17] . Evidence from β‐cells also suggests that palmitic acid depletes ER calcium stores resulting in ER stress [18] .…”

“…Lysosomal calcium release has recently been implicated in a mechanism of hepatic lipotoxicity induced by palmitic acid. [17] Evidence from β-cells also suggests that palmitic acid depletes ER calcium stores resulting in ER stress. [18] In addition, increased extracellular palmitate concentrations impair the ER's ability to maintain calcium stores in hepatocytes, resulting in ER stress-mediated cellular dysfunction.…”