Hypoxia reduces the expression and anti-inflammatory effects of peroxisome proliferator-activated receptor-g in human proximal renal tubular cells

Background. Peroxisome proliferator-activated receptor (PPAR)-g may counteract tissue fibrosis via its anti-inflammatory actions, while hypoxia, a new pro-fibrotic factor, reportedly modifies PPAR-g expression. However, the effects of hypoxia on the expression and anti-inflammatory actions of PPAR-g have yet remained to be clarified in renal tubular cells. Methods. Confluent human proximal renal tubular epithelial cells (HPTECs) were exposed to hypoxia (1% O2) and/or TNF-a at 10 ng/ml for up to 48 h. The cells were incubated with PPAR-g agonists, 15d-PGJ2 or pioglitazone, for 30min before stimulation. Precise amounts of PPAR-g and MCP-1 mRNA and protein were measured by TaqMan quantitative PCR and immunoblot or ELISA, respectively. Results. A cDNA array analysis identified PPAR-g as one of the hypoxia-affected genes in HPTECs. Hypoxia reduced mRNA levels of PPAR-g at 24 and 48 h and protein levels at 6 and 48 h. Knockout of hypoxia-inducible factor-1a (HIF-1a) with its dominant negative form did not block the hypoxia-induced reduction in PPAR-g expression. PPAR-g’s activation with 15d-PGJ2 or pioglitazone reduced basal and TNF-a-stimulated MCP-1 expression at mRNA and protein levels at 24 h under normoxia. MCP-1 reduction rates at basal mRNA and protein levels were slightly but significantly lower during hypoxia than normoxia (9 vs 69% and 36 vs 42%, respectively, for 15d-PGJ2, and 0 vs 34% and 12 vs 21%, respectively, for pioglitazone). Finally, a specific inhibitor for PPAR-g, GW9662, weakened the MCP-1-decreasing effect of 15d-PGJ2 by about 30%, under basal conditions, while it abolished the effect of pioglitazone almost completely. Conclusions. Hypoxia-induced loss of function of PPAR-g reduces anti-inflammatory effects of PPAR-g activation, possibly modulating inflammatory responses in the diseased kidney.