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The centrality of the transcriptional regulator Snail in epithelial-to-mesenchymal transformation (EMT), known to occur in models of diabetic nephropathy, has not been established. Transforming growth factor beta-1 (TGFbeta1) is induced in diabetic nephropathy and induces both Snail and EMT. Hypoxia inducible factors (HIFs) are known to induce Snail, independent of TGFbeta1. Notch induction is integral to Snail induction and EMT in tumour cells, but its role in the kidney is unknown. The present study was undertaken to determine the upstream regulators of Snail in the kidney in high glucose and hypoxic conditions. HK-2 cells were cultured in normoxic, hypoxic, high glucose and combined hypoxic/high glucose conditions. The expression of HIF1alpha, NotchIC, Snail, Lysyl oxidase-like 2 (Loxl2), and Hairy and Enhancer Split-1 (Hes1) were measured. We found that hypoxia increased HIF1alpha expression; however, concurrent exposure to high glucose blunted this effect. A similar pattern was observed in Lox12 expression, suggesting that Loxl2 was downstream of HIF1alpha, which was confirmed using siRNA techniques. Snail was upregulated by hypoxia and high glucose and in combination the effect was additive, suggesting independent upstream activation pathways by the two stimuli. Hes1 was upregulated by high glucose and to a lesser extent by hypoxia, but the effect of the combined stimuli was no greater than that observed with high glucose alone. NotchIC was downregulated by both hypoxia and high glucose, and in combination the effect was additive. Therefore, this study suggests that hypoxia and high glucose induce Snail expression through distinct pathways, independent of Notch signalling.

Peroxisome proliferator-activated receptor-γ (PPARγ) are ligand-activated transcription factors that regulate cell growth, inflammation, lipid metabolism, and insulin sensitivity. We recently demonstrated that PPARγ agonists limit high glucose-induced inflammation in a model of proximal tubular cells (PTC; Panchapakesan U, Pollock CA, and Chen XM. Am J Physiol Renal Physiol 287: F528–F534, 2004). However, the role of PPARγ in the excess extracellular matrix production is largely unknown. We evaluated the effect of 24- to 48-h 8 μM l-805645 or 10 μM pioglitazone on 25 mM d-glucose-induced markers of fibrosis in HK-2 cells. High d-glucose induced nuclear binding of activator protein-1 (AP-1) to 140.8 ± 10.9% ( P < 0.05), which was attenuated with L-805645 and pioglitazone to 82.3 ± 14.4 ( P < 0.01 vs. high d-glucose) and 99.3 ± 12.2% ( P < 0.05 vs. high d-glucose), respectively. High d-glucose increased total production of transforming growth factor (TGF)-β1139.6 ± 6.5% ( P < 0.05), which was reversed with L-805645 and pioglitazone to 68.73 ± 5.7 ( P < 0.01 vs. high d-glucose) and 112 ± 13.6% ( P < 0.05 vs. high d-glucose). L-805645 and pioglitazone reduced high d-glucose-induced fibronectin from 156.0 ± 24.9 ( P < 0.05) to 81.9 ± 16.0 and 57.4 ± 12.7%, respectively (both P < 0.01 vs. high d-glucose). Collagen IV was not induced by high d-glucose. L-805645 and pioglitazone suppressed collagen IV to 68.0 ± 14.5 ( P < 0.05) and 46.5 ± 11.6% ( P < 0.01) vs. high d-glucose, respectively. High d-glucose increased the nuclear binding of NF-κB to 167 ± 22.4% ( P < 0.05), which was not modified with PPARγ agonists. In conclusion, PPARγ agonists exert antifibrotic effects in human PTC in high glucose by attenuating the increase in AP-1, TGF-β1, and the downstream production of the extracellular matrix protein fibronectin.

Transforming growth factor-β1 (TGF-β1) functions as an important immunomodulatory cytokine in human kidney. Evidence suggests that connective tissue growth factor (CTGF) is an important downstream mediator of the profibrotic effects of TGF-β1. However, the role of CTGF in TGF-β1-induced chemokine production remains unknown. This study was undertaken to determine whether CTGF is involved in mediating TGF-β1-induced chemokine production in renal proximal tubular (HK-2) cells. Interleukin-8 (IL-8) and macrophage chemoattractant protein-1 (MCP-1) were measured. TGF-β1 induced an increase in IL-8 and MCP-1 (both P < 0.05) compared with control levels. CTGF was effectively silenced using small interference RNA (siRNA) in HK-2 cells. RT-PCR and real-time PCR confirmed a 94% reduction in CTGF mRNA. In the CTGF-silenced cells, TGF-β1-stimulated IL-8 and MCP-1 secretion was not altered compared with control cells. Similarly, basal secretion of IL-8 and MCP-1 was not changed in CTGF-silenced cells. The direct effect of CTGF (20, 200, and 400 ng/ml) on IL-8 and MCP-1 was assessed at 24-, 48-, and 72-h time points and no stimulation was observed. Our studies further demonstrate that in the CTGF gene-silenced cells, CTGF partially mediates TGF-β1-induced fibronectin and collagen IV secretion. These data suggest that TGF-β1 induced IL-8 and MCP-1 via CTGF-independent pathway. TGF-β mediates both fibrosis and chemokine production in the proximal tubule of the kidney. However, CTGF plays a more specific role as a downstream mediator of TGF-β1-induced fibrosis.