Hypoxia Up-regulates CD36 Expression and Function via Hypoxia-inducible Factor-1- and Phosphatidylinositol 3-Kinase-dependent Mechanisms

Abstract

Neovascular and degenerative diseases of the eye are leading causes of impaired vision and blindness in the world. Hypoxia or reduced oxygen tension is considered central to the pathogenesis of these disorders. Although the CD36 scavenger receptor features prominently in ocular homeostasis and pathology, little is known regarding its modulation by hypoxia. Herein we investigated the role and regulation of CD36 by hypoxia and by the major hypoxia effector, hypoxia-inducible factor (HIF)-1. In vivo, hypoxia markedly induced CD36 mRNA in corneal and retinal tissue. Subsequent experiments on human retinal pigment epithelial cells revealed that hypoxia time-dependently increased CD36 mRNA, protein, and surface expression; these responses were reliant upon reactive oxygen species production. As an important novel finding, we demonstrate that hypoxic stimulation of CD36 is mediated by HIF-1; HIF-1␣ down-regulation abolished CD36 induction by both hypoxia and cobalt chloride. Sequence analysis of the human CD36 promoter region revealed a functional HIF-1 binding site. A luciferase reporter construct containing this promoter fragment was activated by hypoxia, whereas mutation at the HIF-1 consensus site decreased promoter activation. Specific binding of HIF-1 to this putative site in hypoxic cells was detected by a chromatin immunoprecipitation assay. Interestingly, inhibition of the phosphatidylinositol 3-kinase pathway blocked the hypoxia-dependent induction of CD36 expression and promoter activity. Functional ramifications of CD36 hypoxic accumulation were evinced by CD36-dependent increases in scavenging and anti-angiogenic activities. Together, our findings indicate a novel mechanism by which hypoxia induces CD36 expression via activation of HIF-1 and the phosphatidylinositol 3-kinase pathway.Hypoxia, a reduction in cellular oxygen tension, is a key determinant of tissue pathology and survival during tumor development and ischemic diseases including retinopathies, myocardial infarction, and atherosclerosis. In response to hypoxia, mammalian cells express a variety of gene products important for erythropoiesis, angiogenesis, and glycolysis, thereby improving tissue oxygenation and facilitating metabolic demands (1, 2). These adaptive responses require the concerted activation of various transcription factors including hypoxia-inducible factor-1 (HIF-1), 4 which is generally considered the master regulator of oxygen homeostasis (2-5). HIF-1 is a heterodimer comprised of an oxygen-regulated ␣ subunit (HIF-1␣) and the ubiquitous aryl hydrocarbon receptor nuclear translocator (ARNT or HIF-1␤) (4, 6, 7). HIF-1␣ protein turnover in normoxia is very rapid due to the action of prolyl hydroxylases. These oxygen-dependent enzymes hydroxylate two conserved proline residues of HIF-1␣, promoting binding of the Von Hippel-Lindau protein, ubiquitination, and subsequent proteosomal degradation. Under hypoxic conditions the prolyl hydroxylases are inhibited, thereby allowing stabilization and accumulation of the HIF-1␣ protein (4 -6,...

LPA/PKD-1-FoxO1 Signaling Axis Mediates Endothelial Cell CD36 Transcriptional Repression and Proangiogenic and Proarteriogenic Reprogramming

“…HMVECs transfected with a luciferase reporter plasmid (pGL3-CD36-luc) encoding a 3519 bp sequence of the CD36 gene immediately proximal to the transcriptional start site 12 showed significant basal luciferase activity (Figure 1A) compared to cells transfected with the control pGL3 plasmid. Activity was decreased by over 80% after LPA exposure for 24 hrs ( p < 0.01).…”

“…Despite the biological importance of CD36 in angiogenesis, little is known regarding how endothelial cell-specific CD36 transcription is regulated or how that might relate to endothelial cell heterogeneity and function. In retinal MVECs, CD36 transcription is up-regulated in response to hypoxia via HIF-1 transcription factor 12 . The antiangiogenic pathway triggered by HIF-1 signaling counteracts hypoxia-driven proangiogenic pathways, providing a “homeostatic brake” to prevent pathological neovascularization.…”

Lysophosphatidic acid suppresses endothelial cell CD36 expression and promotes angiogenesis via a PKD-1–dependent signaling pathway

“…9,17,18 In contrast, although CD36 is broadly and constitutively expressed in microvascular beds, there is surprisingly little known regarding regulation of its expression on MVECs. Mwaikambo et al 19 recently reported that retinal MVEC CD36 expression was up-regulated by hypoxia via the hypoxia-inducible factor-1 transcription factor, suggesting that up-regulation of a natural antiangiogenic pathway may accompany up-regulation of hypoxia-driven proangiogenic pathways, perhaps to provide a "brake" to prevent excess neovascularization.…”

Hypoxia worsens the impact of intracellular triglyceride accumulation promoted by electronegative low-density lipoprotein in cardiomyocytes by impairing perilipin 5 upregulation

“…In addition, LRP1 internalizes aggregated LDL (Llorente-Cortes et al, 2000). Moreover, hypoxia upregulates the expression of CD36 (Mwaikambo et al, 2009), LOX-1 (Crucet et al, 2013) and LRP1 (Castellano et al, 2011a, Cal et al, 2012a. Thus, one or several of these receptors could be involved in the increased uptake of LDL(−) by cardiomyocytes.…”