Intermittent hypobaric hypoxia promotes atherosclerotic plaque instability in ApoE-deficient mice

Abstract

Aim: Sudden cardiac death is one of the most frequent causes of death at high altitude. It has been reported that the intermittent normobaric hypoxia experienced by patients with obstructive sleep apnea may enhance the development of atherosclerosis. However, the effect of hypobaric hypoxia, which mimics the ambient air at high altitude, in the development of atherosclerosis has not been investigated.

Methods: Twenty male ApoE-deficient mice, 8 weeks of age, were subjected to either control conditions or intermittent hypobaric hypoxia (IHH) for 8 weeks. Mice in the IHH group were exposed to a hypobaric chamber that replicated the hypobaric hypoxia conditions found at 4000 m altitude for 8 hours a day.

Results: IHH-exposed mice did not significantly differ from control mice in plasma lipid levels, including triglyceride, total cholesterol, low-density lipoprotein, and high-density lipoprotein. The hematoxylin and eosin-stained sections of the aortic root showed similar plaque size between the groups. However, IHH-treated mice exhibited significantly decreased plaque collagen content, a feature of atherosclerotic plaque stability. Additionally, matrix metalloproteinase (MMP)-9 protein expression was significantly increased, whereas tissue inhibitor of MMP (TIMP)-2 expression was decreased after exposure to IHH.

Conclusion: IHH may promote atherosclerotic plaque instability in ApoE-deficient mice by changing the balance of MMPs and TIMPs.

FIG. 1.

The effects of IHH on the metabolic character of ApoE^-/- mice. The daily food intake (A), body weight (B), triglyceride (C), total cholesterol (D), LDL-cholesterol (E), and HDL-cholesterol (F) of ApoE^-/- mice from the control (open bars) and IHH (solid bars) groups after 8 weeks of treatment. Data are expressed as mean±SEM. n=7 mice per group; **p<0.01 compared with the control group. HDL, high-density lipoprotein; IHH, intermittent hypobaric hypoxia; LDL, low-density lipoprotein.

FIG. 2.

The effects of IHH on atherosclerotic plaque size in ApoE^-/- mice. (A) Representative photographs of thoraco-abdominal lesions in ApoE^-/- mice exposed to control and IHH conditions. Lesions are stained with Oil Red O and shown at 10X magnification. (B) The plaque size was measured as a percentage of the total aortic surface. (C) Representative photographs of aortic root lesions stained with hematoxylin and eosin. (D) The plaque size of the aortic root is expressed as μm². Data are expressed as mean±SEM. n=5 mice per group; IHH, intermittent hypobaric hypoxia.

FIG. 3.

The effects of IHH on plaque instability in ApoE^-/- mice. (A) Representative photographs of aortic lesions stained with Masson's Trichrome. (B) Collagen deposition was expressed as the percentage of positively stained area. Data are expressed as mean±SEM. n=5 mice per group; **p<0.01 compared with the control group. IHH, intermittent hypobaric hypoxia.

FIG. 4.

The effects of IHH on MMP-2/-9/-14 and TIMP-2 expression in aortas from ApoE^-/- mice. (A) Representative immunohistochemical stains showing the expression of MMP-2, MMP-9, MMP-14, and TIMP-1 in the aorta. (B) The levels of protein expression were expressed as the percentage of positively stained area. Data are expressed as mean±SEM. n=5 mice per group; **p<0.01 compared with the control group. IHH, intermittent hypobaric hypoxia; MMP, matrix metalloproteinase; TIMP, tissue inhibitors of metalloproteinase.