The Nrf2 Activator Increases TFAM and Suppresses STING in Human Renal Proximal Tubular Cells

Recent study has demonstrated the Nrf2 (NF-E2-related factor 2) activator bardoxolone methyl improved renal function in diabetic kidney disease (DKD). On the other hand, mitochondrial transcription factor A (TFAM) activation inhibits mitochondrial DNA (mtDNA) release into the cytoplasm and suppresses stimulator of interferon genes (STING)-mediated inflammation in renal proximal tubular cells. Sulforaphane (SFN), an isothiocyanate found in cruciferous vegetables, is recognized as activators of Nrf2. In this study, we investigated the effects of SFN on TFAM and STING expression and its mitochondrial protective and anti-inflammatory effects. Because mitochondrial damage caused by fatty acids bound to albumin is involved in pathogenesis of DKD due to increased albumin reabsorption, we treated human renal proximal tubular cells (Lonza) with palmitic acid (PA, 200µM). Total RNA, DNA and protein were extracted and analyzed by qRT-PCR and western blotting. To quantify the mitochondrial DNA released into the cytoplasm, DNA was extracted from cytoplasmic and nuclear fractions. SFN (10µM) induced Nrf2 downstream target gene, NAD(P)H:quinone oxidoreductase 1 (NQO-1) mRNA expression to 359% (p<0.05). SFN alone increased TFAM mRNA expression to 132% (p<0.05) and suppressed STING mRNA expression by 26% (p<0.05). Whereas, PA had no effect on TFAM mRNA expression but increased STING mRNA expression to 135% (p<0.05). SFN suppressed PA-induced STING mRNA expression by 34% (p<0.05) and STING protein expression by 48%. PA increased cytoplasmic mtDNA to 237% and SFN decreased PA-induced cytoplasmic mtDNA release by 41%. PA increased IL-6 mRNA expression to 139% (p<0.05) and MCP-1 mRNA expression to 138% (p<0.05). SFN suppressed PA-induced IL-6 mRNA expression by 33% (p<0.05) and MCP-1 mRNA expression by 47% (p<0.05). Together, the Nrf2 activators may have renoprotective effects through mitochondrial protection and anti-inflammatory effect by activating TFAM and inhibiting STING. Disclosure R. Bessho: None. T. Takiyama: None. S. Promsuwan: None. H. Kitsunai: None. K. Sawamoto: Other Relationship; Self; Abbott, Boehringer Ingelheim Pharmaceuticals, Inc., Merck & Co., Inc., Ono Pharmaceutical Co., Ltd., Roche Diagnostics K. K., Taisho Pharmaceutical Co., Ltd. Y. Takeda: None. Y. Takiyama: Other Relationship; Self; Gilead Sciences, Inc., Research Support; Self; Boehringer Ingelheim Pharmaceuticals, Inc., Mochida Pharmaceutical Co., Ltd., Roche Diabetes Care, Taisho Pharmaceutical Co., Ltd.