Mitochondrial Ca2+ Uptake Relieves Palmitate-Induced Cytosolic Ca2+ Overload in MIN6 Cells

Abstract

Saturated fatty acids contribute to β-cell dysfunction in the onset of type 2 diabetes mellitus. Cellular responses to lipotoxicity include oxidative stress, endoplasmic reticulum (ER) stress, and blockage of autophagy. Palmitate induces ER Ca2+ depletion followed by notable store-operated Ca2+ entry. Subsequent elevation of cytosolic Ca2+ can activate undesirable signaling pathways culminating in cell death. Mitochondrial Ca2+ uniporter (MCU) is the major route for Ca2+ uptake into the matrix and couples metabolism with insulin secretion. However, it has been unclear whether mitochondrial Ca2+ uptake plays a protective role or contributes to lipotoxicity. Here, we observed palmitate upregulated MCU protein expression in a mouse clonal β-cell, MIN6, under normal glucose, but not high glucose medium. Palmitate elevated baseline cytosolic Ca2+ concentration ([Ca2+]i) and reduced depolarization-triggered Ca2+ influx likely due to the inactivation of voltage-gated Ca2+ channels (VGCCs). Targeted reduction of MCU expression using RNA interference abolished mitochondrial superoxide production but exacerbated palmitate-induced [Ca2+]i overload. Consequently, MCU knockdown aggravated blockage of autophagic degradation. In contrast, co-treatment with verapamil, a VGCC inhibitor, prevented palmitate-induced basal [Ca2+]i elevation and defective [Ca2+]i transients. Extracellular Ca2+ chelation as well as VGCC inhibitors effectively rescued autophagy defects and cytotoxicity. These observations suggest enhanced mitochondrial Ca2+ uptake via MCU upregulation is a mechanism by which pancreatic β-cells are able to alleviate cytosolic Ca2+ overload and its detrimental consequences.