| BackgroundCoronary heart disease (CHD) is one of the most common diseases leading to deformity or even to death worldwide. Therefore it has been a significant worldwide research issue to explore the mechanism, diagnosis and therapy scheme of atherosclerosis. Acute coronary syndrome (ACS) is the emergent syndrome in CHD, which covers a range of clinical pathophysiological condition, including unstable angina pectoris (UAP), acute myocardial infarction (AMI) and sudden death. Available data suggest that atherosclerotic plaques destabilization with subsequent disruption and acute thrombus formation represents the principal pathophysiology underlying ACS. To some extent, factors which influence the plaque's stability may also influence the clinic presentation and prognosis of CHD. Autopsy studies suggest that these atherosclerotic lesions, which are prone to rupture, typically consist of a necrotic/lipid core, covered by a thin fibrous cap with severe infiltration of macrophages in the shoulder regions. So it is very important to identify new molecular biomarkers and regulators for this specific plaque phenotype and search for the methods for the reversal and stabilization of vulnerable plaque. AS is a multifactorial and complex pathological process of large arteries, and its accurate mechanism has remained unclear. Overproduction of reactive oxygen species from all types of vascular cells is integral in the development of AS and mediates not only vascular inflammation but also antioxidative and anti-inflammatory responses. One such protective and stress-induced protein is heme oxygenase (HO). Heme Oxygenase catalyzes degradation of heme to generate carbon monoxide, biliverdin(subsequently converted to bilirubin), and ferrous iron (which leads to the up-regulation of ferritin). HO-1 is the inducible isoform that can be induced by diverse stimuli, including a variety of proatherogenic factors, such as increased blood pressure, cigarette smoke, inflammatory cytokines and oxidative lipids. HO-1 expression was observed throughout the development of atherosclerotic lesions, both in humans and several experimental animals. The concept that HO-1 may be causally related to atherogenesis has been suggested by animal studies. In spite of the success in inhibiting early lesion formation, there has been rare data in modeling the progression and stability of already established advanced lesions, as they occur in humans.Objective(1) To set up an animal model of unstable atherosclerotic plaque and plaque rupture;(2) To determine the effect of HO-1 on AS progression and plaque stabilization;(3) To observe the effect of HO-1 on plaque pathologic condition and vascular remodeling pattern by intravascular ultrasound (IVUS);(4) To determine the effect of HO- on levels of MMP-9, TNF-a and IL-6 in plaques;(5) To examine the effect of HO-1 on atherosclerotic plaque in rabbits and to explore the possible mechanism underlying its beneficial properties, so as to provide theoretical bases for preventing ACS.MethodsAfter abdominal aortic balloon injury,45 male New Zealand White rabbits were fed an atherogenic diet for 24 weeks. From week 12 to week 24, animals were randomized into three groups, receiving intraperitoneal injections of hemin to induce HO-1, Sn-protoporphyrinⅨ(SnPP) to inhibit HO-1, or vehicle as control respectively. Vulnerable plaques were established by local transfection with p53 gene, and were challenged with injection of Russell's viper venom and histamine to induce plaque rupture. IVUS was performed to observe plaque pathologic condition and vascular remodeling pattern. Then rabbits were all killed and their abdominal aortae were taken out to undergo pathological examination, and immunohistochemistry and real-time RT-PCR to examine the expression of matrix metalloproteinase-9, interleukin-6 and tumor necrosis factor-a.Results(1) HO-1 mRNA and protein levels, and HO activity were increased significantly by hemin treatment compared with controls (all P< 0.01). On the other hand, SnPP treatment resulted in an decrease of HO-1 protein (P< 0.01) and mRNA levels (P< 0.05), and HO activity (P< 0.01).(2) In comparison with control animals, rabbits in Hemin group presented a thicker fibrous cap of the abdominal aortic plaque and thinner IMT, whereas SnPP-treated rabbits possessed a thinner cap and thicker IMT (all P< 0.01). After pharmacological triggering, the incidence of plaque disruption in SnPP group was significantly higher than that in control group (79% vs.33%, P< 0.05), while no plaque in hemin group developed disruption (0% vs.33%, P< 0.05).(3) More negative remodeling in Hemin group (72/175,41% vs.49/182,27%; P< 0.01) and more positive remodeling in SnPP group (84/177,47% vs.59/182,32%; P<0.01) than the control group were observed. In addition, the mean remodeling index was smaller in Hemin group, but was higher in SnPP group than that in the control group (0.93±0.14 vs.1.02±0.14,1.11±0.15 vs.1.02±0.14, respectively, P< 0.01).(4) Compared with the control group, macrophages and lipid deposition in the abdominal aortic lesions was also less pronounced in Hemin group, whereas rabbits in Hemin group had higher levels of intimal VSMCs collagen (all P< 0.01). Conversely, in the SnPP group compared with the control, a significant increase in the intraplaque macrophage and lipid accumulation, and a marked decrease in VSMCs and collagen contents were observed (all P< 0.01). The vulnerability index was significantly lower in Hemin group, but higher in SnPP group than that in the control group (0.77±0.09 vs. 2.08±0.50 and 4.02±0.73 vs.2.08±0.50 respectively, both P< 0.01). (5) The percentage of positive-stained area and the expression of mRNAs for TNF-a, MMP-9 and IL-6 in lesions were substantially lower in Hemin group but higher in the SnPP group than the control group (all P< 0.01).ConclusionsHO-1 induction could delay the progression of advanced AS, and enhance plaque stability and prevent plaque from disruption. The possible mechanism underlying its protective properties may include the following effects.(1) HO-1 inhibited positive remodeling of the rabbit abdominal aortae;(2) HO-1 modulated plaque morphology and composition to be a more stable phenotype;(3) HO-1 attenuated intraplaque inflammatory response. |
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