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Mitochondrial calcium influx‐mediated superoxide generation induces cardiac fibroblast proliferation under angiotensin II stimulation

Cardiac fibroblasts (CFs) are one of the major cell types in the heart in addition to cardiomyocytes, and play an important role in regulating normal cardiac function and as well as pathological cardiac remodeling (i.e. fibrosis). It has been demonstrated that the stimulation of Gq‐protein coupled receptor (GqPCR) including angiotensin II (AngII) receptor promotes CF activation in vitro and fibrosis in vivo. In addition, GqPCR are known to increase oxidation levels in CFs, which is an important signaling component required for CF proliferation. Though it is well reported that plasma‐membrane NADPH oxidase produces reactive oxygen species (ROS) in CFs, the role of mitochondria‐derived ROS (mROS) on CF proliferation under GqPCR stimulation remains unclear. We previously demonstrated using H9C2 cardiac myoblasts, that mitochondrial Ca2+ (mtCa2+) overload via mtCa2+ uniporter (MCU) induces mitochondrial superoxide (mtSO) generation under GqPCR stimulation. We also reported that GqPCR stimulation induces mitochondrial fragmentation and fragmented mitochondria shows sustained mtCa2+ elevation upon mtCa2+ uptake in H9c2 cells. Therefore, we hypothesize that Ang‐II mediated stimulation may increase mitochondrial fission and mitochondrial Ca2+‐uptake leading to mtSO generation, which activates mROS‐dependent proliferative signaling in CFs. Using mitochondria‐targeted Ca2+ biosensor, we found that AngII (≥100 nM) stimulation significantly increases the mtCa2+ uptake in response to the Ca2+ release from the endoplasmic reticulum (ER). Furthermore, we also found that AngII stimulation leads to mitochondrial fragmentation and increase in the mtSO levels assessed by expressing mitochondria‐targeted GFP and loading mtSO‐sensitive dye MitoSox Red. We next confirmed that proliferative pathways, including ERK1/2 and p38 were activated in a time‐dependent manner on AngII stimulation in CFs and this effect was abolished by pretreatment with mitochondria‐targeted antioxidant, Mito‐tempo without affecting the profiles of AngII‐induced ER Ca2+ release and mtCa2+ uptake. Finally, the overexpression of dominant‐negative MCU also blocked AngII‐mediated activation of proliferative pathways concomitant with the inhibition of mtCa2+ uptake in response to AngII. These data suggest Ang II stimulation induces mitochondrial fragmentation and MCU‐mediated mitochondrial Ca2+ accumulation, which promotes mtSO generation and activates CF proliferative signaling pathways. In conclusion, mROS generation via MCU‐dependent mtCa2+ uptake serves as an important regulator for the proliferative pathway in CFs under Ang II stimulation.