Abstract
Hypoxia modulates gene expression and affects multiple aspects of endothelial cell biology. Fibulin-5 (FBLN5) is an extracellular matrix protein essential for elastic fiber assembly and vasculogenesis that participates in vascular remodeling and controls endothelial cell adhesion, motility, and proliferation. In this context, we aimed to analyze FBLN5 regulation by hypoxia in endothelial cells. Hypoxia (1% O(2)) increased FBLN5 mRNA levels in endothelial cells in a time-dependent manner. Maximal induction (∼2.5-fold) was achieved after 24 h of hypoxia. This effect paralleled an increase in both intracellular and extracellular FBLN5 protein levels. The increase in FBLN5 mRNA levels observed in hypoxic cells was blocked by inhibitors of the PI3K/Akt/mTOR pathway (LY294002 and rapamycin) and mimicked by dimethyl oxal glycine, which prevents proline hydroxylase-mediated degradation of HIF-1α. Silencing of HIF-1α completely prevented hypoxia-induced FBLN5 up-regulation. Accordingly, both hypoxia and HIF-1α overexpression increased FBLN5 transcriptional activity. Serial promoter deletion and mutagenesis studies revealed the involvement of a putative hypoxia response element (HRE) located at -78 bp. In fact, EMSA and ChIP assays demonstrated increased HIF-1 binding to this site in hypoxic cells. Interestingly, the rate of endothelial cells undergoing apoptosis in cultures exposed to hypoxia increased in FBLN5 knockdown cells, suggesting that hypoxia-induced FBLN5 expression contributes to preserve cell survival. These results provide evidence that HIF-1 signaling underlies the increase of FBLN5 expression elicited by hypoxia in endothelial cells and suggest that FBLN5 induction could be involved in the adaptive survival response of endothelial cells to hypoxia.
Highlights
The hypoxia-inducible factor-1 (HIF-1) is a critical mediator of cellular responses to hypoxia [2]
These results provide evidence that HIF-1 signaling underlies the increase of FBLN5 expression elicited by hypoxia in endothelial cells and suggest that FBLN5 induction could be involved in the adaptive survival response of endothelial cells to hypoxia
We found that FBLN5 expression was enhanced in a time-dependent manner in BAEC exposed to hypoxia (1% O2)
Summary
The hypoxia-inducible factor-1 (HIF-1) is a critical mediator of cellular responses to hypoxia [2]. Under hypoxic conditions, HIF-1␣ hydroxylation is inhibited leading to increased HIF-1␣ levels that, in turn, results in an enhancement of HIF-1-dependent transcriptional responses [5,6,7]. HIF-1 controls multiple aspects of endothelial behavior including cell proliferation, chemotaxis, ECM penetration, and wound healing [8]. Hypoxia Induces FBLN5 Expression motes endothelial cell adhesion via its Arg-Gly-Asp (RGD) motif [10, 11, 15]. FBLN5 has been associated with different vascular processes involving ECM remodeling; little is known about the mechanisms underlying its regulation in vascular cells. This study identifies FBLN5 as a hypoxia-responsive gene in endothelial cells, dissects the molecular mechanisms underlying this regulation, and delineates its biological significance
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