Abstract
Mitochondria are major sites of energy metabolism that influence numerous cellular events, including immunity and cancer development. Previously, we reported that the mitochondrion-specific antioxidant enzyme, manganese-containing superoxide dismutase (MnSOD), has dual roles in early- and late-carcinogenesis stages. However, how defective MnSOD impacts the chain of events that lead to cell transformation in pathologically normal epidermal cells that have been exposed to carcinogens is unknown. Here, we show that UVB radiation causes nitration and inactivation of MnSOD leading to mitochondrial injury and mitophagy. In keratinocytes, exposure to UVB radiation decreased mitochondrial oxidative phosphorylation, increased glycolysis and the expression of autophagy-related genes, and enhanced AKT Ser/Thr kinase (AKT) phosphorylation and cell growth. Interestingly, UVB initiated a prosurvival mitophagy response by mitochondria-mediated reactive oxygen species (ROS) signaling via the mammalian target of the mTOR complex 2 (mTORC2) pathway. Knockdown of rictor but not raptor abrogated UVB-induced mitophagy responses. Furthermore, fractionation and proximity-ligation assays reveal that ROS-mediated mTOC2 activation in mitochondria is necessary for UVB-induced mitophagy. Importantly, pretreatment with the MnSOD mimic MnTnBuOE-2-PyP5+ (MnP) attenuates mTORC2 activation and suppresses UVB-induced mitophagy. UVB radiation exposure also increased cell growth as assessed by soft-agar colony survival and cell growth assays, and pretreatment with MnP or the known autophagy inhibitor 3-methyladenine abrogated UVB-induced cell growth. These results indicate that MnSOD is a major redox regulator that maintains mitochondrial health and show that UVB-mediated MnSOD inactivation promotes mitophagy and thereby prevents accumulation of damaged mitochondria.
Highlights
Mitochondria are major sites of energy metabolism that influence numerous cellular events, including immunity and cancer development
Oxidative stress plays a pivotal role in the pathophysiology of various cellular and sub-cellular conditions where manganese-containing superoxide dismutase (MnSOD), a major mitochondrial antioxidant enzyme, imparts defense mechanisms against oxidative stress during cancer initiation and progression
Very little is known about the link between UVB-induced oxidatively modified MnSOD and mitochondrial metabolism in a pre-carcinogenesis stage
Summary
To directly test the effect of peroxynitrite on MnSOD activity, we exposed purified recombinant MnSOD proteins to various concentrations of peroxynitrite and measured the MnSOD activity. Consistent with the in vitro data, MnSOD activity decreases significantly in mouse skin exposed to UVB even 24 h after treatment (Fig. 1d), but MnSOD protein levels do not decrease. They remain unchanged 1 h after UVB treatment and increase 24 h after UVB treatment (Fig. 1e). These in vitro and in vivo data demonstrate that MnSOD is nitrated, and the MnSOD enzyme is inactivated following exposure to UVB
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