Supplemental Oxygen Reverses Hypoxia-induced Smooth Muscle Cell Proliferation by Modulating HIF-alpha and VEGF Levels in a Rabbit Arteriovenous Fistula Model

Abstract

OBJECTIVES Numerous mechanisms for the formation of intimal hyperplasia have been proposed but none have been proven or accepted. Our research focuses on the potential role of Hypoxia Inducible Factors (HIFs), VEGF, and PDGF as well as the ERK, PI3-K/AKT pathway in hypoxia mediated intimal hyperplasia processes. We hypothesize that HIF and VEGF will be down regulated with supplemental oxygen in our arteriovenous fistula rabbit model. METHODS Rabbits were randomized into different experimental groups with varying oxygen exposure (21% O2 or 30% O2), and receipt of surgery (surgery with fistula formation, no surgery, or sham operation with skin incision only). Plasma samples were collected at designated intervals in which cytokines and smooth muscle cell proliferation were measured. In addition, cell specimens were exposed to hyperoxic, normoxic, and hypoxic environments with cytokines measured at various time points. RESULTS Placement of an arteriovenous fistula resulted in hypoxia induced HIF stabilization with a concurrent increase in VEGF levels. There was a 4.2-fold induction in HIF-1α levels in animals that were placed in normal air following surgery when compared to animals that were exposed to hyperoxic air. Also VEGF significantly increased post-surgery in the normoxic group, reaching a maximum VGEF level of 959 pg/mL. Plasma VEGF levels in the surgery plus supplemental oxygen group were significantly lower than the normoxic surgery group with almost a 45% reduction in plasma VEGF levels (524pg/mL). Activation of VEGF receptors on smooth muscle cells through ERK1 and AKT pathways resulted in significant smooth muscle cell proliferation and migration. These effects are dramatically reduced in animals that are exposed to a hyperoxic environment of 30% oxygen. CONCLUSIONS Our results suggest that short-term administration of supplemental oxygen inhibits HIFs and VEGF signaling to reduce smooth muscle proliferation in the local blood vessel. These results provide strong support for the therapeutic use of supplemental oxygen following arterial surgery to reduce intimal hyperplasia. These findings also provide a nidus for future clinical trials to determine whether this is clinical applicable in humans.

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“…Earlier work in rabbits had shown that supplemental oxygen reduces IH after stent deployment [107]. Additional work in rabbits indicated that supplemental oxygen inhibits IH and SMC proliferation after creation of an arterio-venous fistula and prolongs patency [114].…”

The Biology of Hemodialysis Vascular Access Failure

“…Ischemia and the release of ROS activate early response elements such as hypoxia-inducible factor 1-alpha (HIF-1α) and immediate-early responsive gene (IEX-1). [29][30][31] These in turn activate endothelial cells (ECs), inflammatory cells, and vascular SMCs. HIF-1α, ROS, and hypoxia have been shown to upregulate vascular endothelial growth factors (VEGFs), which increase inflammation and promote EC proliferation.…”

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