Fiber-specific cholesterol changes in murine dystrophy

Cholesterol alterations in young dystrophic mice

Cholesterol and cholesteryl ester concentrations and cholesteryl ester fatty acid substituents have been measured during the first 10 weeks of life in tissues of normal and dystrophic mice. In normal Swiss and 129ReJ(+/?) mice the concentrations of both cholesterol and cholesteryl esters remain essentially constant in liver, increase in brain and fall sharply in both thigh (mixed fiber type muscles) and chest muscles (predominantly slow oxidative muscles) over this period. In all cases the concentration of free cholesterol exceeds that of esterified cholesterol. In dystrophic mice, similar patterns are found in brain and liver. In both thigh and chest muscles, however, the developmental pattern is significantly different. After an initial decrease the concentrations of cholesterol and cholesteryl esters increase rapidly with the largest increase occurring in the concentration of cholesteryl esters which by 10 weeks of age exceeds the concentration of cholesterol in chest muscle. During the same period the pattern of esterified fatty acids changes gradually in dystrophic tissues towards an increasing ratio of unsaturated/saturated fatty acids. By 10 weeks of age this ratio is significantly higher in dystrophic tissues than normal in all tissues tested.

Cholesterol metabolism in dystrophic mice. II. Altered enzyme activities

In a previous study we found that free cholesterol (FC) and cholesterol ester (CE) concentrations in fast-glycolytic (FG) muscle tissue from dystrophic mice are significantly higher than normal. This increase is not due to an increased capacity for de novo cholesterol biosynthesis. HMG-CoA reductase (HMGR) (the enzyme which catalyzes the rate limiting step) activity is significantly decreased in dystrophic muscle compared to normal. This decrease is paralleled by an increased capacity for both CE production and hydrolysis, i.e., both Acyl-CoA:cholesterol acyltransferase (ACAT) activity and the activities of both lysosomal and sarcoplasmic cholesterol ester hydrolases (CEH) are greatly increased. These enzyme changes in dystrophic FG muscle are similar those observed in normal tissues with elevated levels of cholesterol, which suggests that such changes are not the cause of the altered cholesterol concentrations but are rather the response of the tissue to elevated levels of cholesterol.

Oxidative stress and muscular dystrophy

Oxidative stress may be the fundamental basis of many of the structural, functional and biochemical changes characteristic of the inherited muscular dystrophies in animals and humans. The presence of by-products of oxidative damage, and the compensatory increases in cellular antioxidants, both indicate oxidative stress may be occurring in dystrophic muscle. Changes in the proportions and metabolism of cellular lipids, abnormal functions of cellular membranes, altered activity of membrane-bound enzymes such as the SR Ca2+-ATPase, disturbances in cellular protein turnover and energy production and a variety of other changes all indicate that these inherited muscular dystrophies appear more like the results of oxidative stress to muscle than any other type of underlying muscle disturbance. Particular details of these altered characteristics of dystrophic muscle, in combination with current knowledge on the processes of oxidative damage to cells, may provide some insight into the underlying biochemical defect responsible for the disease, as well as direct research towards the ultimate goal of an effective treatment.