Signal transduction in restenosis and myocardial protection by hyperoxia

Abstract

Ischemic heart disease is a major cause of morbidity and mortality in the western world. Revascularization of ischemic myocardium is essential for cell survival, and is performed by thrombolysis, percutaneous transluminal coronary angioplasty (PTCA) and by coronary artery bypass grafting. However, revascularization may paradoxically exacerbate ischemic injury, and it is desirable to develop strategies reducing this. One possible way is by evoking myocardial protection through exposure to oxygen. Revascularization by PTCA often has a poor outcome, as 20-40% of the arteries will restenose. Signal transduction pathways involving nuclear factor kappa B (NF-kappaB) have been implicated in neointima formation. However, a limitation of many experimental studies is the use of animals with healthy vessels. To improve the treatment of ischemic heart disease and restenosis, it is necessary to understand the molecular basis of disease. The present papers explore molecular mechanisms of these phenomena, with the perspective of developing future therapies.

METHODS. Carotid artery injury was induced by separate ligation of the external and internal branch. NF-kappaB activation was assessed by NF-kappaB controlled reporter gene activation in transgenic mice. Neointima formation was evaluated in mice with genetical deletion of the NF-kappaB p105 subunit (p50 precursor) and the corresponding wild types. Apolipoprotein E/LDL receptor double knockout mice (ApoE/LDLr KO), which develop atherosclerosis, were fed an atherogenic diet three months prior to experiments and lesions compared to those of wild types. For studies of heart physiology, mice or rats were exposed to different concentrations of oxygen, thereafter their hearts exised and perfused with induced global ischemia.

RESULTS (papers 1 & 2). Carotid artery ligation lead to formation of neointima four weeks later and induced NF-kappaB activation in the injured vessel OBS wall. Neointima lesions were larger in p105 KO than in wild types. NF-kappaB activation was accompanied by increased expression of inflammatory genes and basic fibroblast growth factor (real time PCR). NF-kappaB p105 knockout mice had reduced expression of the inflammatory genes and a higher percentage of basic fibroblasts growth factor positive cells. The shape and size of the lesions were reproducible in both ApoE/LDLr KO and wild types. The inflammatory reaction was more persistent in lesions from atherosclerotic mice.

(Papers 3 & 4): In vivo exposure to oxygen before heart isolation improved heart function and reduced infarct size, but the protection was dependent on exposure time and oxygen concentration, and was different in rats and mice. In mice hyperoxic exposure caused cardiac phosphorylation of the mitogen activated protein kinases p38 and ERK1/2, but not JNK. The NOS-inhibitor L-NAME, the ERK1/2 inhibitor PD98059, and the p38 inhibitor FR167653 all reduced the protection afforded by hyperoxic exposure.

CONCLUSION. Activation of the p105 subunit of NF-kappaB as a response to arterial injury may be a key regulator of the inflammatory response to injury and essential for tissue repair. We established a reproducible model of arterial injury with neointima formation in atherosclerotic mice. Hyperoxia protects the isolated rat and mouse heart against ischemia-reperfusion injury. The protection depended on both oxygen concentration in inspired air and the duration of hyperoxic exposure. Nitric oxide triggers or mediates hyperoxic protection, and ERK1/2 and p38 MAPK are involved in signalling of protection against ischemia-reperfusion injury.