SGK1 Attenuates Oxidative Stress-Induced Renal Tubular Epithelial Cell Injury by Regulating Mitochondrial Function

Mitochondrial dysfunction has been implicated in the early stages or progression of many renal diseases. Improving mitochondrial function and homeostasis has the potential to protect renal function. Serum- and glucocorticoid-induced kinase 1 (SGK1) is known to regulate various cellular processes, including cell survival. In this study, we intend to demonstrate the effect and molecular mechanisms of SGK1 in renal tubular cells upon oxidative stress injury and to determine whether regulation of mitochondrial function is implicated in this process. HK-2 cells were exposed to HO, and cell viability and apoptosis were dynamically detected by the CCK-8 assay and annexin-V/PI staining. The concentrations of cellular reactive oxygen species (ROS) and adenosine triphosphate (ATP) and the expression of the SGK1/GSK3/PGC-1 signaling pathway were analyzed by flow cytometry or western blot. In addition, shRNA targeting SGK1 and SB216763 were added into the culture medium before HO exposure to downregulate SGK1 and GSK3, respectively. Cell viability and mitochondrial functions, including mitochondrial membrane potential (m), Cytochrome C release, mtDNA copy number, and mitochondrial biogenesis, were examined. Protein levels and SGK1 activation were significantly stimulated by HO exposure. HK-2 cells with SGK1 inhibition were much more sensitive to HO-induced oxidative stress injury than control group cells, as they exhibited increased apoptotic cell death and mitochondrial dysfunction involving the deterioration of cellular ATP production, ROS accumulation, mitochondrial membrane potential reduction, and release of Cytochrome C into the cytoplasm. Studies on SGK1 knockdown also indicated that SGK1 is required for the induction of proteins associated with mitochondrial biogenesis, including PGC-1, NRF-1, and TFAM. Moreover, the deleterious effects of SGK1 suppression on cell apoptosis and mitochondrial function, including mitochondrial biogenesis, were related to the phosphorylation of GSK3 and partially reversed by SB216763 treatment. HO leads to SGK1 overexpression in HK-2 cells, which protects human renal tubule cells from oxidative stress injury by improving mitochondrial function and inactivating GSK3.