We have reported hypertrophic proximal tubules in obese mice which implies an inefficient oxygen supply. We examined whether hypoxic condition in proximal tubules are involved the pathogenesis of obesity-induced renal injury. We also test the hypothesis that this injury can be ameliorated by molecular intervention of prolyl hydroxylase domains (PHDs), sensors for tissue oxygen levels that is a crucial molecule for tissue response to hypoxia.Design and method:
Tissue hypoxic conditions were assessed by pimonidasole immunostaining. Peritubular capillaries (PTCs) were evaluated by counting CD34 stained vessels. Tamoxifen (Tam)-inducible proximal tubules-specific PHD2 deficient mice were created by crossing PHD2flox/flox mice and Tam-inducible N-myc downstream-regulated gene-1-Cre mice on C57BL/6J backgrounds. These inducible conditional knock out (CKO) miceand their wild-type littermates (WT) were fed a high fat diet (HFD) or a low fat diet (LFD) for 12 weeks. The expressions of PHD2 and VEGF were measured by real-time PCR and immunohistochemistry.Results:
The WT mice on HFDmanifested renal histological changes, including cellular enlargement of proximal tubules and a rarefaction of PTCs, which were consistent with more hypoxic area in proximal tubules than in WT mice on LFD. Urinary albumin and NGAL excretion were higher in HFD-fed mice, indicating ischemic tissue damage in proximal tubular area. However, expression of either PHD2 or VEGF was unchanged in HFD-fed WT mice, suggesting the lack of hypoxic tissue response in HFD-fed WT. Injecting Tam to HFD-fed CKO mice downregulated PHD2 in proximal tubules, increased VEGF expression, increased the number of PTCs, decreased hypoxic area, and attenuated proximal tubular damages and albuminuria.Conclusions:
Hypoxic condition due to enlarged cell with vascular rarefaction is evident in the proximal tubular area of obese mice whereas tissue reaction to hypoxic damages failed to properly compensate. The early reduction of PHD2 specifically in the proximal areamay constitute a novel strategy againstthe progression process from an early stage of obesity-induced kidney injury.