High dietary methionine plus cholesterol exacerbates atherosclerosis formation in the left main coronary artery of rabbits
Introduction
Hypercholesterolemia is a well-established risk factor for the development of cardiovascular disease. Although hyperhomocysteinemia has also been classified as a risk factor for cardiovascular disease [1], [2], its role in the development of coronary artery atherosclerosis has been disputed [3], with several epidemiological studies reporting either a weak [4] or no correlation between plasma homocysteine and the extent of coronary atherosclerosis [3], [5], [6], even though elevated homocysteine levels have been suggested to predict mortality [4].
Homocysteine is formed by the demethylation of the amino acid methionine, it may then enter into two pathways, the remethylation pathway or the trans-sulfuration pathway. The remethylation process reverts homocysteine into methionine using folate as a co-substrate and vitamin B12 as co-factors, whereas the trans-sulfuration pathway converts homocysteine into cystathionine that is then degraded into smaller molecules and excreted in the urine [7].
Studies have shown that approximately 30% of the population in some countries have the thermolabile form of the enzyme methylenetetrahydrofolate reductase, a gene defect that leads to raised plasma levels of homocysteine [8], and epidemiological studies have shown that over 50% of the population have high plasma levels of cholesterol [9]. These atherogenic risk factors have been linked to both genetic and dietary causes, and in some circumstances, both may be elevated. For example, methionine, the precursor of homocysteine, is usually consumed in combination with cholesterol, as animal products such as eggs, meat and milk contain both cholesterol and methionine.
The exact mechanisms whereby homocysteine might elicit its atherogenicity are unclear, but evidence suggests that endothelial damage and dysfunction could be potential mechanisms [10], [11], [12], [13], [14]. For instance, homocysteine may decrease NO bio-availability [11], [15]. As well, homocysteine may stimulate smooth muscle cell proliferation in vitro [16], [17], oxidize lipoproteins [18] and stimulate the clotting cascade [19].
In support of this idea, we have previously shown that hyperhomocysteinemia due to a high methionine diet leads to endothelial dysfunction and intimal proliferation in the abdominal aorta of rabbits [20]. As it is unknown whether moderate hyperhomocystenemia induces coronary artery disease over a short period of time, we tested the hypothesis that hyperhomocystenemia due to a high methionine diet leads to atherosclerosis in the left main coronary artery, and is exacerbated when combined with high dietary cholesterol. To this end, a stereological approach was used to quantitate the volume of the intima, media and lumen of the LMCA in rabbits fed either a control diet, a high methionine diet (to raise plasma homocysteine levels), a high cholesterol diet or a the combination of both diets.
Section snippets
Animals
Male New Zealand White rabbits at three months of age (Animal Services Department, Monash University, Gippsland Campus, Victoria, Australia) were randomly allocated into four groups, and fed their respective diet for twelve weeks (n = 8/group). Group 1 (control) was fed a normal rabbit chow diet (Glen-Forrest Stockfeeders, Perth, Australia), group 2 (Meth) received a normal rabbit chow diet supplemented with 1% methionine, group 3 (Chol) received a normal rabbit chow diet supplemented with 0.5%
Plasma cholesterol
The plasma cholesterol in the Chol and MethChol group were significantly increased (P < 0.001) compared to control (45 ± 4 mmol/L, 35 ± 6 mmol/L versus 0.6 ± 0.1 mmol/L, respectively). There was no difference between the Meth and control groups (0.5 ± 0.1 mmol/L versus 0.6 ± 0.1 mmol/L, respectively).
Plasma homocysteine
The plasma homocysteine in the Meth group was significantly increased (P < 0.001) compared to control (54 ± 2 μmol/L versus 15 ± 1 μmol/L, respectively). There was no difference between the Chol and
Discussion
The main findings of the present study were that: (i) high dietary cholesterol but not methionine, administered to rabbits for 12 weeks, led to the development of atherosclerosis in the LMCA, and that (ii) the combination of high dietary cholesterol plus methionine increased the level of atherosclerosis present in the LMCA. Thus, our data suggest that moderate hyperhomocysteinemia per se does not cause atherosclerosis in the coronary arteries over a short period of time, but high dietary
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