Role of adenosine receptors in the regulation of angiogenic factors and neovascularization in hypoxia

Because hypoxia increases extracellular adenosine levels and stimulates angiogenesis, we evaluated the relative roles of reduced oxygen concentrations and adenosine receptor activation in the production of angiogenic factors. In vitro, we analyzed the effects of hypoxia and adenosine on the secretion of angiogenic factors from human microvascular endothelial cells (HMEC-1). To study the effects of hypoxia alone, we scavenged adenosine from the hypoxic medium with adenosine deaminase, and we used the stable adenosine analog 5'-N-ethylcarboxamidoadenosine (NECA) to study the effects of stimulation of adenosine receptors. In the absence of adenosine, hypoxia stimulated vascular endothelial growth factor (VEGF) but not interleukin-8 (IL-8) secretion from HMEC-1. In contrast, NECA stimulated both VEGF and IL-8 secretion. VEGF secretion was increased 1.9 +/- 0.04-fold with NECA (10 microM) and 1.7 +/- 0.1-fold with hypoxia (5% O(2)) but 3.8 +/- 0.1-fold when these two stimuli were combined. Thus, adenosine receptors act in a cooperative fashion with hypoxia to stimulate VEGF and induce IL-8 secretion not stimulated by hypoxia alone. In vivo, antagonism of adenosine receptors with caffeine abrogated VEGF up-regulation induced by local injection of NECA into the mouse hind limb and produced a 46% reduction of neovascularization in a mouse ischemic hind limb model. Our study suggests that adenosine actions are not redundant but rather are complementary to the direct effects of hypoxia. Stimulation of adenosine receptors not only contributes to the overall effect of hypoxia but also has additional actions in the regulation of angiogenic factors. Thus, adenosine receptors represent a potential therapeutic target for regulation of neovascularization.

Cyclooxygenase-2 promotes early atherosclerotic lesion formation in ApoE-deficient and C57BL/6 mice

Cyclooxygenase (COX) 2 is expressed in atherosclerotic lesions. We have previously reported that selective inhibition of COX-2 reduces early atherosclerosis in LDLR deficient mice. To examine the role of COX-2 in atherosclerosis in other mouse models, we studied the effects of selective COX-2 inhibition (by rofecoxib and NS-398) and nonselective COX inhibition (by indomethacin) on early atherosclerotic lesion formation in apolipoprotein E-deficient (apoE(-/-)) mice. Selective COX-2 and nonselective COX inhibition reduced atherosclerosis in female apoE(-/-) mice by 35-38% and 38-51% in the proximal and en face aortas, respectively. Next we investigated the role of macrophage COX-2 by transplanting COX-2(-/-) fetal liver cells into C57BL/6 mice and challenging the mice with an atherogenic diet. Genetic deletion of COX-2 from hematopoietic cells reduced atherosclerosis by 51%. In addition, LPS activated COX-2(-/-) macrophages had decreased expression of monocyte chemoattractant protein-1 (MCP-1) and tumor necrosis factor-alpha (TNFalpha). The results demonstrate that selective inhibition of COX-2 and elimination of COX-2 from macrophages significantly reduces early atherosclerotic lesion formation in apoE-deficient and C57BL/6 mice. These results are compatible with COX-2 expression by macrophages having a proatherogenic role, and support the potential of anti-inflammatory therapeutic approaches for atherosclerosis.

Quantitative and Qualitative Differences in Proatherogenic NKT Cells in Apolipoprotein E–Deficient Mice

Background— Atherosclerosis is a disease marked by lipid accumulation and inflammation. Recently, atherosclerosis has gained recognition as an autoimmune-type syndrome characterized by increased activation of the innate and acquired immune systems. Natural killer T (NKT) cells have characteristics of both conventional T cells and NK cells and recognize glycolipid antigens presented in association with CD1d molecules on antigen-presenting cells. The capacity of NKT cells to respond to lipid antigens and modulate innate and acquired immunity suggests that they may play a role in atherogenesis.

Methods and Results— We examined the role of NKT cells in atherogenesis and how the atherosclerotic environment affects the NKT cell population itself. The data show that CD1d-deficiency in male apolipoprotein E–deficient (apoE 0 ) mice results in reduction in atherosclerosis, and treatment of apoE 0 mice with α-galactosylceramide, a potent and specific NKT cell activator, results in a 2-fold increase in atherosclerosis. Interestingly, we demonstrate that α-galactosylceramide–induced interferon-γ responses and numbers of NKT cells in apoE 0 mice show age-dependent qualitative and quantitative differences as compared with age-matched wild-type mice.

Conclusions— Collectively, these findings reveal that hyperlipidemia and atherosclerosis have significant effects on NKT cell responses and that these cells are proatherogenic.