TNF-α upregulates HIF-1α expression in pterygium fibroblasts and enhances their susceptibility to VEGF independent of hypoxia

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The clinical manifestations of pterygium are characterized by rapid growth and postoperative recurrences. We had previously proposed that hypoxia-inducible factor (HIF)-1α recruits progenitor cells during the development and progression of pterygia. Recently, it was reported that various stimuli, including inflammation, could activate HIF-1α even under normoxic conditions. The ocular surface directly faces external environments, and is thus frequently exposed to inflammatory insults. First, we examined the gene expression of HIF-1α, its downstream molecule, vascular endothelial growth factor (VEGF)-A, and VEGF receptor (VEGFR)-2 in corneal and conjunctival cells compared with cultured human umbilical vein endothelial cells. Corneal fibroblasts had high expression of VEGFR-2 in the presence of TNF-α, and HIF-1α was activated by TNF-α in diverse ocular surface cells. The HIF-1α/VEGF/VEGFR signaling pathway in response to TNF-α was evaluated in cultured human pterygium fibroblasts (HPFs) at the gene and protein levels and was compared to treatment with cobalt chloride (CoCl2), a hypoxic mimetic, to exclude the effect of hypoxia. Although VEGF-A expression was not changed by TNF-α, expression of HIF-1α and VEGFR-2 was enhanced in HPFs treated with TNF-α, independent of hypoxia conditioning. In addition, VEGF-C gene expression was activated solely by TNF-α in HPF, but VEGF-B levels were not significantly affected. These results may provide mechanistic explanations for the uniquely vigorous proliferation of pterygium fibrovascular tissue during TNF-α-induced ocular surface inflammation.

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