Effect of the haeme oxygenase-1/endogenous carbon monoxide system on atherosclerotic plaque formation in rabbits

Abstract

Background: To investigate the effect of the haeme oxygenase-1/carbon monoxide (HO-1/CO) system on atherosclerotic plaque formation and its possible mechanism.

Methods: For 12 weeks, rabbits were given a 1.5% cholesterol diet (Ch group, n = 8) or a 1.5% cholesterol diet plus an HO-1 inducer, haemin (Hm group, n = 8), or an HO-1 inhibitor, zinc protoporphyrin IX (Znpp-IX, Zn group, n = 8) by intraperitoneal injection.

Results: Compared with the normal control group (C group, n = 8), serum levels of lipids and oxidised low-density lipoproteins (ox-LDL) increased significantly in all experimental groups (p < 0.01). However, no significant differences were observed among the three experimental groups (p > 0.01). Compared with the control group, aortic nitric oxide (NO) production and nitric oxide synthase (cNOS) activity decreased markedly, whereas carbon monoxide (CO) production and HO-1 activity increased markedly in the Ch group (p < 0.01). This was associated with an increase in the area of aortic plaque of 54.00 ± 4.16%. Compared with the Ch group, CO production and HO-1 activity increased markedly, while aortic HO activity and CO production decreased significantly in the Hm group. The area of aortic plaque was significantly reduced in the Hm group (17.88 ± 3.01%), whereas the area of aortic plaque was significantly increased in the Zn group (61.13 ± 3.50%). Compared with the Ch group, aortic endothlin-1 expression in the Hm group reduced significantly, while in the Zn group it was significantly higher than in the Ch group (p < 0.01).

Conclusion: The HO-1/CO system plays an inhibitory role in atherosclerotic plaque formation. This role was not mediated by regulating serum lipids and ox-LDL, but was related to the reciprocal relationship between the HO-1/CO and NOS/NO systems in atherosclerosis and the down-regulated expression of endothlin-1 (ET-1), which inhibits the proliferation of vascular smooth muscle cells.

Fig. 1.

Morphology of the aorta wall in the C (A), Zn (B), Ch (C) and Hm (D) groups. Except for the C and Hm groups, all other samples showed shedding of endothelial cells and the presence of foam cells. The tunica–initima was thickened and the smooth muscle layer was disorganised and extended towards the tunica–intima. (H&E staining, magnification 400 X).

Fig. 2.

Immunohistochemical detection in the C (A), Zn (B), Ch (C) and Hm (D) groups of the buffy lamellar particles of HO -1 present in the cytoplasm and cell membrane. HO -1 is mainly expressed by the endothelial, foam and smooth muscle cells of the tunica–intima (magnification 200 X).

Fig. 4.

Levels of HO -1 (left) and ET-1 (right) mRNA expression in the aorta. M: marker, C, Zn, Ch and Hm groups.

Fig. 3.

Immunohistochemical detection in the C (A), Zn (B), Ch (C) and Hm (D) groups of ET-1. Buffy lamellar particles of ET-1 were located in the endothelial cells, foam cells and smooth muscle cells of the tunica–intima of the aorta (magnification 200 X).

Fig. 5.

Diagram showing the relationship between factors involved in atherosclerotic plaque formation.