Abstract 1651: Hypoxia Induces the Expression of Very Low-Density Lipoprotein Receptor in Human Macrophages and Vascular Smooth Muscle Cells via a HIF-1 Dependent Mechanism.
Background: Clinical studies have demonstrated that remnant lipoproteins play an important role in atherogenesis, and we have previously reported that very low-density lipoprotein receptor (VLDL-R), one of the major receptors for remnant lipoproteins, is involved in foam cell formation. Recent studies have indicated that the oxygenation state of atherosclerotic plaques vary with plaque thickness. Therefore, the goal of the present study was to determine the effect of hypoxia on VLDL-R expression in macrophages and vascular smooth muscle cells and to characterize the role of hypoxia-inducible factor-1 (HIF-1) in hypoxia-induced changes in VLDL-R expression.
Methods and Results: The expression of VLDL-R was assayed by real-time PCR and Western blot analysis in THP-1 macrophages and in human coronary artery smooth muscle cells (HCASMC). Treatment of THP-1 macrophages and HCASMC with CoCl2 resulted in upregulation of VLDL-R mRNA (4.5-fold in macrophages and 3.6-fold in HCASMC, respectively) and protein expression beginning at 8 hours. Similarly, hypoxia also resulted in upregulation of VLDL-R mRNA in these cells (3.1-fold in macrophages and 5.1-fold in HCASMC, respectively). Transfection of HCASMC with siRNA against HIF-1α resulted in a 73±3.7% decrease in the hypoxia-induced increase of VLDL-R mRNA levels. Furthermore, knockdown of the von Hippel-Lindau tumor suppressor (VHL) protein, a key molecule for the ubiquitination and degradation of HIF-1α under normoxic condition, resulted in an increase in HIF-1α protein and VLDL-R mRNA levels by 6.4±1.7-fold under normoxic condition. Finally, hypoxia enhanced foam cell formation induced by β-VLDL in THP-1 macrophages.
Conclusions: These data demonstrate that hypoxia induces VLDL-R expression in human macrophages and vascular smooth muscle cells via a HIF-1 dependent mechanism. Further, these data provide novel insights into the mechanisms of atherosclerotic plaque progression.