Effects Of Semicarbazide-sensitive Amine Oxidase Inactivation On Atherosclerosis

Aims: SSAO is an enzyme that has inter-related adhesive and enzymatic functions in regulating physiological trafficking and inflammation. Clinical studies have demonstrated that serum semicarbazide-sensitive amine oxidase(SSAO) activities positively correlate with the progression of atherosclerosis. The aim of the present study is to investigate the effect of SSAO inactivation on the development of atherosclerosis.Methods: a) Female LDLr knockout(KO) mice were given the Western-type diet for 6 and 9 weeks to induce the formation of early and advanced lesions, and semicarbazide(SCZ, 0.125%) was added into the drinking water to inactivate SSAO in vivo; b)LDLr KO mice received a Western-type diet(WTD) for 6 weeks or 9 weeks to induce the development of atherosclerotic lesions. Thereafter, mice were given the weekly-changed drinking water containing 0.125% SCZ to inactivate SSAO in vivo. Female mice were kept on WTD for another 3 weeks to maintain the hypercholesterolemic condition, while the male ones were switched to regular chow diet for the subsequent 6 weeks to lower/normalize the plasma cholesterol levels.Results: a)Despite no impact on plasma total cholesterol levels, abrogation of SSAO by SCZ not only resulted in the enlargement of both early(1.5-fold, P=0.0043) and advanced(1.8-fold, P=0.0013) atherosclerotic lesions, but also led to reduced/increased lesion contents of macrophages/smoothmusclecells(SMCs)(macrophage:~0.74-fold,P=0.0002(early)/0.0016(advanced);SMC:~1.55-fold,P=0.0003(early)/0.0001(advanced)), respectively. Moreover, SSAO inactivation inhibited the migration of circulating monocytes into peripheral tissues and reduced the amount of circulating Ly6 Chigh monocytes(0.7-fold, P=0.0001), which may account for the reduced macrophage content in lesions. In contrast, the increased number of SMCs in lesions of SCZ-treated mice is attributed to an augmented synthetic vascular SMC phenotype switch as evidenced by the increased proliferation of SMCs and accumulation of collagens in vivo.;b) Despite no impact on plasma total cholesterol levels, the infiltration of circulating monocytes into peripheral tissues, and the size of atherosclerotic lesions, abrogation of SSAO activity resulted in the stabilization of established lesions as evidenced by the increased collagen(WTD: 1.5-fold, P<0.05;chow:1.9-fold, P<0.05)contents under both conditions. Moreover, SSAO inactivation decreased Ly6 Chigh monocytosis(blood:0.32-fold, P<0.001;peritoneal cavity :0.70-fold, P<0.05; spleen : 0.64-fold,P<0.05) and lesion macrophage(1.4-fold, P<0.05) contents in hypercholesterolemic mice, while no effect was observed in mice after normalization of hypercholesterolemia by dietary lipid lowering. Strikingly, abrogation of SSAO activity significantly increased not only the absolute numbers of smooth muscle cells(SMCs)(WTD: 1.4-fold, P<0.05;chow:1.7-fold, P<0.05), but also the percent of SMCs with a synthetic phenotype in established lesions of mice regardless of plasma cholesterol levels(WTD: 1.5-fold, P<0.05;chow:3.3-fold, P<0.05).Conclusion: SSAO inactivation by SCZ promotes the phenotypic switch of SMCs, thereby enhancing the stability of atherosclerotic plaques, despite the potential changes in the lesion size. Targeting SSAO activity thus may represent a potential treatment for patients with atherosclerosis.