The effect of exercise training on cholesterol and bile acid metabolism in ovariectomized rats
There has been accumulating evidence in recent years that the estrogen deficient state in ovariectomized (Ovx) animals and in postmenopausal women results in substantial liver triglyceride (TG) accumulation. However, evidence of disturbances in cholesterol metabolism in link with estrogen deficiency is limited to observations of higher plasma total cholesterol levels found in human as well as in animals. In fact, the impact of estrogen withdrawal on liver cholesterol metabolism has received little attention and shows some controversies. Therefore, the three studies presented in this thesis have been conducted in Ovx rats, as an animal model of post-menopausal women, to investigate the effects of estrogen withdrawal on key molecular markers of cholesterol and bile acid metabolism in liver and in transintestinal cholesterol excretion (TICE), and also to determine the potential role of exercise training as a positive alternative intervention. It has been shown that exercise training can improve plasma cholesterol levels. An enhanced transport of peripheral cholesterol toward the liver for subsequent secretion into bile and excretion from the body has been suggested; however, the underlying mechanism for this action is not fully understood.  In the first study, we showed that estrogen withdrawal was associated with higher (P < 0.05) liver total cholesterol under the standard diet and the standard diet + cholesterol diet, while liver triglyceride (TG) content was higher in Ovx than in Sham rats in all three dietary conditions which are the standard diet, the standard diet + cholesterol and the high fat diet + cholesterol. Surprisingly, the standard diet + cholesterol was associated with lower (P < 0.001) plasma total cholesterol and TG levels in Ovx than in Sham rats, suggesting a decrease in very low-density lipoprotein (VLDL) secretion. Accordingly, several transcripts of key markers of VLDL synthesis including microsomal triglyceride transfer protein (MTP) and apoB-100 were decreased (P < 0.05) in Ovx compared to Sham rats under the three dietary conditions and even more so for MTP and apoB-100 when rats were fed the standard diet + cholesterol. To go one step further, in the second study we determined the effects of exercise training on hepatic key markers of farnesoid X receptor (FXR)-small heterodimer partner (SHP)-cholesterol 7 alpha-hydroxylase (CYP7A1) (FXR-SHP-CYP7A1) pathway, involved in cholesterol conversion into bile acid and excretion from the body, in Ovx cholesterol fed rats. Our main experimental group was Ovx rats fed a high cholesterol diet (Ovx-Chol) that was compared, on one hand, to a group of Ovx rats fed a standard diet (Ovx-SD) to observe the effects of the diet and, on the other hand, compared to a group of Sham operated rats fed the cholesterol diet (Sham-Chol) to observe the effect of estrogen withdrawal. Results of this study showed that plasma and liver total cholesterol levels were not affected by exercise training in any of the experimental conditions. Cholesterol feeding in both Sham and Ovx rats resulted in significantly (P<0.001) higher hepatic cholesterol accumulation than in Ovx-SD rats. A main effect of training (P< 0.05) was, however, found for transcripts of SHP and CYP7A1. The SHP and CYP7A1 transcripts were increased by training. These results suggest that exercise training through up-regulation of genes involved in bile acid formation may modulate cholesterol metabolism in Ovx animals. Finally, a recent growing body of evidence suggests that reverse cholesterol transport (RCT) can also proceed through a non-biliary pathway known as transintestinal cholesterol excretion (TICE). Indeed, both liver and intestine are involved in excretion of the excess cholesterol from the body. Based on this concept, we expanded our research to determine whether exercise training has an effect on intestinal membrane cholesterol receptors involved in TICE pathway in intact and Ovx rats fed a normal and a high cholesterol diet. Results of the third study showed that exercise training increased (P< 0.01) transcripts of intestinal LDL-R and PCSK9, which are involved in intestinal cholesterol uptake from circulation, and their nuclear transcription factor, intestinal sterol regulatory element-binding protein 2 (SREBP2) (P< 0.05) in both Sham and Ovx rats compared to rats remaining sedentary (Sed). On the other hand, hepatic LDL-R and PCSK9 gene expression was suppressed (P< 0.01) by cholesterol feeding but not affected by exercise training. Flavin monooxygenase 3 (FMO3) gene expression, as a cholesterol balance regulator in liver, was significantly decreased (P<0.01) by cholesterol feeding in both Sham and Ovx rats compared to rats were fed the SD diet but unchanged following exercise training and estrogen withdrawal. An up-regulation of intestinal gene expression of LDL-R and PCSK9 following voluntary wheel running in intact and Ovx rats suggests that exercise training may contribute to increased cholesterol elimination through the TICE pathway. Overall, our results indicate that a high cholesterol diet and ovariectomy combine to decrease the gene expression of key markers of VLDL synthesis suggesting a reduction in cholesterol excretion from the liver. Alternatively, it seems that reduced hepatic LDL-R transcript found in Ovx animals might be due to hepatic cholesterol accumulation. Moreover, our findings introduced exercise training as an appropriate non-pharmacological intervention to stimulate the excretion of the excess cholesterol from the body through upregulation of genes involved in bile acid biosynthesis in liver and intestinal basolateral cholesterol transporters in TICE.
Rat LDL-R PCSK9 CYP7A1 Ovariectomy Rat High cholesterol diet VLDL assembly LDL-R PCSK9 Hepatic cholesterol accumulation Exercise training CYP7A1 TICE pathway