Increased responsiveness of hypoxic endothelial cells to FGF2 is mediated by HIF-1alpha-dependent regulation of enzymes involved in synthesis of heparan sulfate FGF2-binding sites.

Binding of basic fibroblast growth factor (FGF2) to its high affinity receptors requires the presence of specific heparan sulfate (HS) moieties on the cell surface that act as coreceptors. To determine the contribution of cell-surface HS to modulation of FGF2-dependent cell growth, we studied the changes in the cell mass and FGF2 binding of endothelial cell HS under normoxic and hypoxic conditions in vitro. Both large vein and cardiac microvascular endothelial cells cultured under hypoxic conditions demonstrated an increase in the ratio of cell-surface HS to chondroitin sulfate (CS), as well as an increase in the number of low affinity (HS-associated) binding sites for FGF2 with no change in the apparent K(d). This increase in the number of HS-FGF2 binding sites, in the absence of a significant change in FGF receptor expression, resulted in enhanced responsiveness of hypoxic, compared with normoxic, endothelial cells to FGF2 stimulation. Gene expression studies demonstrated increased expression of the key regulatory enzyme responsible for HS chain synthesis, 1,4 GlcNAc transferase (GlcNAcT-I), as well as increased expression of 2-O sulfotransferase (HS2ST), the enzyme responsible for sulfation of IdoA, a crucial part of the HS-FGF2 binding site. Transduction of cells with an adenovirus encoding a HIF-1alpha expression construct resulted in a similar increase in GlcNAcT-I and HS2ST expression. We conclude that hypoxia increases endothelial cell responsiveness to FGF2 by promoting preferential synthesis of HS rather than CS chains and increasing the number of FGF2-binding sites on HS chains. Both of these events are mediated by a HIF-1alpha-dependent increase in expression of the enzymes GlnNAcT-I and HS2ST. This shift in cell-surface HS composition results in enhanced cell sensitivity to FGF2-induced growth stimulation.