The effect of high glucose and PPAR-γ agonists on PPAR-γ expression and function in HK-2 cells

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Abstract
Peroxisome proliferator-activated receptor-γ (PPAR-γ) are ligand-activated transcription factors that regulate cell growth, inflammation, lipid metabolism, and insulin sensitivity. PPAR-γ in the human kidney has been described. However, the role of PPAR-γ in proximal tubular cells with respect to cell growth and inflammation in diabetic nephropathy is largely unknown. We evaluated the effect of high (30 mM) d-glucose, thiazolidinedione pioglitazone (10 μM), and the selective PPAR-γ agonist L-805645 (8 μM) on PPAR-γ expression, growth, and inflammatory parameters in the proximal tubular model of HK-2 cells. PPAR-γ was present in HK-2 cells and upregulated with 30 mM d-glucose to 177 ± 31.2% of control ( P < 0.05). PPAR-γ activation was induced by pioglitazone to a similar level to that observed by exposure to high glucose but maximally induced by the selective agonist L-805645. However, L-805645 reduced cell viability in both 5 and 30 mM d-glucose to 73.8 ± 3.1 and 77.6 ± 1.4% of control (both P < 0.0001). In parallel, thymidine incorporation was reduced with L-805645 in both 5 and 30 mM d-glucose to 33.3 ± 3.4 and 37.9 ± 2.2%, respectively (both P < 0.0001). Flow cytometry demonstrated increased apoptosis and G1phase arrest in association with an increase in p21cip1/waf1in cells exposed to L-805645. Exposure to 30 mM d-glucose did not significantly change AP-1 promoter activity (89.0 ± 5.5% of control); however, the addition of L-805645 significantly reduced it to 62.2 ± 2.7% of control ( P < 0.0001). Thirty nanomolar d-glucose induced transforming growth factor-β1to 137.7 ± 16.9% of control ( P < 0.05), and L-805645 was able to suppress this to 68.7 ± 5.7% of control ( P < 0.01 vs. d-glucose). Exposure to 30 mM d-glucose reduced monocyte chemoattractant protein 1 levels to 78.6 ± 7.1% ( P < 0.05) of control, with the reduction more marked in the presence of either pioglitazone ( P < 0.01) or L-805645 ( P < 0.01). In summary, high glucose upregulates PPAR-γ and when significantly induced demonstrates anti-proliferative and anti-inflammatory effects.