Hypoxia induces renal tubular epithelial cell apoptosis in chronic renal disease

Renal tubular atrophy characterizes chronic progressive renal disease, but the molecular mechanisms of renal tubular cell (RTC) deletion are unclear. Because glomerular sclerosis leads to impaired peritubular blood flow, we t ested the hypothesis that chronic hypoxia contributes to RTC apoptosis. Tub ule hypoxia in mice with progressive renal disease (Os/+) was assessed by i njecting EF5, a nitroimidazole compound that preferentially binds to cells undergoing anaerobic metabolism. Hypoxic tubules, as determined by direct i mmunofluorescence with anti-EFS antibodies, were identified in kidneys from Os/+ mice, but not in age-matched controls (+/+) at 12 weeks, coincident w ith the onset of glomerular pathology. Hypoxia can cause apoptosis, but apo ptotic RTCs were rare and equivalent in number in 12 week Os/+ and +/+ kidn eys. However, by 16 weeks apoptotic RTCs were significantly more frequent i n Os/+ versus +/+ mice, demonstrating that tubule hypoxia preceded RTC apop tosis. Importantly, apoptotic RTCs co-localized to hypoxic, but not normoxi c tubules, indicating that tubular atrophy may result from hypoxic stimulat ion of RTC apoptosis. We have previously demonstrated enhanced, diffuse exp ression of the Fas apoptosis receptor in Os/+ tubules, providing a potentia l intermediary between hypoxia and apoptosis. To determine whether hypoxia stimulates Fas-dependent apoptosis, RTCs were cultured within a hypoxia cha mber or in the presence of the cyanide analog, sodium azide. Both in vitro hypoxic conditions stimulated RTC plasma membrane Fas expression, and cause d RTC apoptosis upon ligation with agonistic Fas antibodies. The data sugge st that in the context of progressive renal disease, chronic hypoxia stimul ates Fas-dependent RTC apoptosis, which represents the first definitive lin k between hypoxia and tubular atrophy. We believe that hypoxic induction of RTC apoptosis provides a unifying mechanism for the pathogenesis of tubula r atrophy, and this paradigm identifies novel targets for chronic renal fai lure therapy.