Hypoxia-mediated regulation of mitochondrial transcription factors: Implications for hypertensive renal physiology

Abstract

Kidneys have a high resting metabolic rate and low tissue partial pressure of oxygen due to enhanced mitochondrial oxygen consumption and ATP production for active solute transport. Enhanced mitochondrial activity leads to progressive hypoxia from the renal cortex to renal medulla. Renal tubulointerstitial hypoxia (TiH) is severe in hypertensive rats due to increased sodium reabsorption within their nephrons. Additionally, these rats display increased energy demand and therefore, require healthy mitochondria for adequate salt reabsorption. Hence, we sought to study the regulation of mitochondrial biogenesis and expression of mitochondrial transcription factors (mtTFs, viz. Tfam, Tfb1m and Tfb2m) during hypoxic conditions and in rodent models of genetic hypertension. We report that the expressions of HIF-1α (hypoxia inducible factor-1α), PGC-1α (peroxisome proliferator activated receptor-γ co-activator-1α), mtTFs and OXPHOS proteins are elevated in hypertensive rats as compared to their normotensive counterparts. Additionally, studies in cultured kidney cells show that acute hypoxia augments the expression of these genes. We also observe a positive correlation between HIF-1α and mtTFs transcripts in human tissues. Furthermore, we report for the first time to our knowledge, that HIF-1α binds to promoters of Tfam, Tfb1m and Tfb2m genes and augments their promoter activities in NRK52e cells subjected to acute hypoxia. Taken together, this study suggests that acute hypoxia may enhance mitochondrial function to meet the energy demand in renal tubular epithelial cells and in young/pre-hypertensive SHR kidneys. Translational Statement Our results suggest that tubulointerstitial hypoxia (TiH) prevailing in prehypertensive rats augments the expression of mitochondrial transcription factors and proteins of electron transport chain. Moreover, previous reports indicate that ATP synthesis in these rats are elevated. Thus, our study provides insights into the molecular mechanism of such enhanced mitochondrial function. We propose that during early stages of kidney diseases (marked by mild TiH) an enhancement of mitochondrial function via stimulation of HIF-1α/PGC-1α production may delay renal tubular damage.