Background: Occlusive peripheral vascular diseases (PVDs) are common causes of morbidity. Lesions limit blood flow creating a hypoxic environment that damages distal tissue and can require therapeutic revascularization. Angiogenic growth of new vessels and arteriogenic remodeling of existing vessels attempt to restore perfusion to depleted tissues. Hypoxia inducible factors (HIFs) are key transcriptional regulators of vascular responses. Despite vascular smooth muscle cells (VSMCs) importance in vessel integrity, little is known about their responses to hypoxia in PVDs. We hypothesize that HIF in VSMCs plays a critical role in reperfusion of peripheral tissues via arteriogenesis and angiogenesis.
Methods and Results: The role of VSMC HIF in regulating adult responses to peripheral ischemia was examined in a murine femoral artery ligation model. Mice with smooth muscle specific deletion of Aryl Hydrocarbon Receptor Nuclear Translocator (ARNT, HIF1B), required for HIF transcriptional activity, (ArntSMKO) demonstrate decreased reperfusion compared to controls (p<0.01 Day 3 through Day 21). Histologic evaluation of ligated limbs from ArntSMKO mice show significantly increased tissue damage (62.5±16.7% vs. 16.3±12.9%, p<0.05) and hypoxia (14.7±5.9% vs. 1.7±1.6% p<0.05) in gastrocnemius muscles (GM) at Day 7. Compared to controls, collateral dilation in adductors is substantial (2.9±0.20 vs. 1.8±0.01 fold increase ligated to unligated p<0.05) and damaged areas of GMs have increased endothelial density (310±24 vs. 227±26 CD31+ vessels p<0.05) in ArntSMKO mice; however, these responses are insufficient to restore perfusion. Characterization of human primary peripheral VSMCs indicate that hypoxia promotes proliferation (3.35±0.38 vs. 2.40±0.27 fold increase at 48 hrs p<0.05) and survival under oxidative stress (64.7±0.04% vs. 30.4±0.08% survival p<0.05).
Conclusion: Our data suggest that robust vascular responses required for tissue reperfusion in peripheral ischemia depend on VSMC ARNT.