Prolonged Mechanical Stretch is Associated with Upregulation of Hypoxia-inducible Factors and Reduced Contraction in Rat Inferior Vena Cava

Background: Decreased venous tone and vein wall dilation may contribute to varicose vein formation. We have shown that prolonged vein wall stretch is associated with upregulation of matrix metalloproteases (MMPs) and decreased contraction. Because hypoxia-inducible factors (HIFs) expression also increases with mechanical stretch, this study tested whether upregulation of HIFs is an intermediary mechanism linking prolonged vein wall stretch to the changes in MMP expression and venous contraction.

Methods: Segments of rat inferior vena cava (IVC) were suspended in tissue bath under 0.5-g basal tension for 1 hour, and a control contraction to phenylephrine (PHE, 10(-5)M) and KCl (96 mM) was elicited. The veins were then exposed to prolonged 18 hours of tension at 0.5 g, 2 g, 2 g plus HIF inhibitor U0126 (10(-5)M), 17-[2-(dimethylamino)ethyl] amino-17-desmethoxygeldanamycin (17-DMAG, 10(-5)M), or echinomycin (10(-6)M), or 2 g plus dimethyloxallyl glycine (DMOG; 10(-4)M), a prolyl-hydroxylase inhibitor that stabilizes HIF. The fold-change in PHE and KCl contraction was compared with the control contraction at 0.5-g tension for 1 hour. Vein tissue homogenates were analyzed for HIF-1α, HIF-2α, MMP-2, and MMP-9 messenger RNA (mRNA) and protein amount using real-time reverse transcription polymerase chain reaction and Western blots.

Results: Compared with control IVC contraction at 0.5-g tension for 1 hour, the PHE and KCl contraction after prolonged 0.5-g tension was 2.0 ± 0.35 and 1.1 ± 0.06, respectively. Vein contraction to PHE and KCl after prolonged 2-g tension was significantly reduced (0.87 ± 0.13 and 0.72 ± 0.05, respectively). PHE-induced contraction was restored in IVC exposed to prolonged 2-g tension plus the HIF inhibitor U0126 (1.38 ± 0.15) or echinomycin (1.99 ± 0.40). U0126 and echinomycin also restored KCl-induced contraction in IVC exposed to prolonged 2-g tension (1.14 ± 0.05 and 1.11 ± 0.15, respectively). Treatment with DMOG further reduced PHE- and KCl-induced contraction in veins subjected to prolonged 2-g tension (0.47 ± 0.06 and 0.57 ± 0.01, respectively). HIF-1α and HIF-2α mRNA were overexpressed in IVC exposed to prolonged 2-g tension, and the overexpression was reversed by U0126. The overexpression of HIF-1α and HIF-2α in stretched IVC was associated with increased MMP-2 and MMP-9 mRNA. The protein amount of HIF-1α, HIF-2α, MMP-2, and MMP-9 was also increased in IVC exposed to prolonged 2-g wall tension.

Conclusions: Prolonged increases in vein wall tension are associated with overexpression of HIF-1α and HIF-2α, increased MMP-2 and MMP-9 expression, and reduced venous contraction in rat IVC. Together with our report that MMP-2 and MMP-9 inhibit IVC contraction, the data suggest that increased vein wall tension induces HIF overexpression and causes an increase in MMP expression and reduction of venous contraction, leading to progressive venous dilation and varicose vein formation.