Zn²⁺ protect cardiac H9c2 cells from endoplasmic reticulum stress by preventing mPTP opening through MCU

Zn²⁺ regulates endoplasmic reticulum stress (ERS) and is essential for myocardial protection through gating the mitochondrial permeability transition pore (mPTP). However, the underlining mechanism of the mPTP opening remains uncertain. Cells under sustained ERS induce unfolded protein responses (UPR) and cell apoptosis. Glucose regulatory protein 78 (GRP 78) and glucose regulatory protein 94 (GRP 94) are marker proteins of ERS and regulate the onset of apoptosis through the endoplasmic reticulum signaling pathway. We found tunicamycin (TM) treatment activates ERS and significantly increases intracellular Ca²⁺ and mitochondrial reactive oxygen species (ROS) levels in H9c2 cardiomyocyte cells. Zn²⁺ markedly decreased protein level of GRP 78/94 and suppressed intracellular Ca²⁺ and ROS elevation. Mitochondrial calcium uniporter (MCU) is an important Ca²⁺ transporter protein, through which Zn²⁺ enter mitochondria. MCU inhibitor ruthenium red (RR) or siRNA significantly reversed the Zinc effect on GRP 78, mitochondrial Ca²⁺ and ROS. Additionally, Zn²⁺ prevented TM-induced mPTP opening and decreased mitochondrial Ca²⁺ concentration, which were blocked through inhibiting or knockdown MCU with siRNA. In summary, our study suggests that Zn²⁺ protected cardiac ERS by elevating Ca²⁺ and closing mPTP through MCU.