Possible role of mevalonate in the hypercholesterolemia seen in experimental chronic renal failure.

Hypercholesterolemia may contribute to the pathogenesis of atherosclerosis associated with chronic renal failure (CRF). The mechanism underlying CRF-induced hypercholesterolemia, however, is still unknown. Mevalonate is the direct product of the rate-limiting step in cholesterol synthesis which is catalyzed by 3-hydroxy-3-methylglutaryl coenzyme A reductase. We studied the changes in mevalonate metabolism in a mouse model of CRF in which serum total cholesterol levels are directly correlated with the degree of severity of the disease as measured by serum urea levels. The results of these experiments indicated that in CRF mice, the urine mevalonate levels were significantly lower, while serum mevalonate and total cholesterol levels were significantly higher than in normal mice. We believe that by restricting the normal urinary excretion of mevalonate CRF results in more of this precursor being available for direct cholesterol synthesis. In addition, an increase in circulating mevalonate may upregulate the shunt pathway of mevalonate metabolism in the liver and peripheral tissues, thus providing increased levels of the substrates acetoacetate and acetyl coenzyme A for cholesterol synthesis.

Effect of lovastatin on hypercholesterolemia in chronic renal failure.

Using a mouse model of chronic renal failure (CRF), the possibility of using lovastatin to treat hypercholesterolemia associated with CRF was examined. Renal failure was induced in 5 week-old, C57BL/6J mice by electrocoagulation of the right kidney surface followed by left nephrectomy 2 weeks later. Five weeks post-nephrectomy, BUN and serum total cholesterol levels were assessed and lovastatin treatment commenced. Upon sacrifice 4 weeks later, BUN, serum total cholesterol levels and hepatic HMG-CoA reductase activity were measured. Results showed that CRF induced significant increases in serum total cholesterol levels and enzyme activity. Treatment with lovastatin led to a dose-dependent reduction in serum total cholesterol levels without affecting the enzyme activity. These results suggest that the hyper-cholesterolemia in CRF is partly due to an increase in de novo cholesterol synthesis in the liver and that the lipid-lowering effect of lovastatin is mediated by an action other than the direct reduction of hepatic HMG-CoA reductase activity.

Pathology of atherosclerosis in cholesterol-fed, susceptible mice.

In recent years the C57BL/6J mouse has gained popularity as a model for studying the genetics of diet-induced atherosclerosis. After 10-20 weeks of consuming a diet enriched with saturated fat and cholesterol, it develops fatty streak-like lesions in the valve sinus region of the ascending aorta. The current study shows that continuing the atherogenic diet for a further 15 weeks leads to the development of fibro-fatty lesions which have many of the characteristics of human atheromatous plaques. This finding lends credence to the use of C57BL/6J mice for studying the pathogenesis of atherosclerosis.

The effect of a high-fat diet on murine macrophage activity.

The phagocytic, oxygen free radical generating and cytotoxic activities of macrophages from C57BL/6J mice fed either a normal or an atherogenic high-fat diet have been investigated. Phagocytosis of aggregated low density lipoprotein (LDL) was only slightly inhibited by the high-fat diet although phorbol myristate acetate (PMA)-induced hydrogen peroxide (H2O2) and superoxide anion (O2-) production was significantly reduced. Activation of tumoricidal activity against L929 target cells by lipopolysaccharide (LPS) or lymphocyte-derived macrophage-activating factor (MAF), but not N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP), was also significantly reduced in macrophages from mice fed the high-fat diet. These results indicate that an atherogenic diet is capable of significantly affecting the responsiveness of macrophages to a number of stimulatory agents which act via specific membrane receptors.

Macrophage secretory activity and atherosclerosis during chronic renal failure.

Accelerated atherosclerosis is a serious complication of chronic renal failure (CRF) treated by peritoneal dialysis. In order to study the pathological mechanisms underlying its development we are using an animal model, namely the C57BL/6J mouse, which develops foam cell-type atherosclerotic lesions after surgical induction of CRF. During atherogenesis, monocyte/macrophages move from the circulation to the blood vessel wall, migrate through the endothelium, imbibe lipid and transform into foam cells. Migration through the endothelium involves proteolysis by plasminogen activator (PA) and uptake of lipids involves hydrolysis of lipoproteins by lipoprotein lipase (LPL). Both of these enzymes are secreted by macrophages. In this paper we report the results of studies on the effect of uremia on the secretion of PA and LPL by macrophages from C57BL/6J mice. The secretion of PA and LPL by macrophages from uremic mice (as defined by BUN levels) was higher than that by cells from control animals. Furthermore, whereas macrophage secretion of PA and LPL was significantly less in normal mice fed a high fat diet than in mice fed rodent chow, it was increased above control levels in uremic animals fed the atherogenic diet. We conclude that increased secretion of PA and LPL by macrophages may contribute to atherogenesis in uremic C57BL/6J mice.

Atherosclerosis in uremic mice.

To study the possibility that susceptibility to the development of atherosclerosis during chronic renal failure is genetically determined, aortic lesion formation and changes in serum lipid levels in mice of 2 different inbred strains, C57BL/6J and A/J, were examined. Chronic uremia was induced by electrocoagulation of the right renal cortex and left nephrectomy. The mice were then fed either normal rodent chow or a saturated fat and cholesterol-enriched diet for 6 weeks. None of the A/J mice developed atherosclerosis, whereas the aortas of chow-fed uremic C57BL/6J mice were found to contain a number of fatty lesions, the severity of which was increased by a high fat diet. Uremia had different effects on serum cholesterol, triglycerides, and lipoproteins in the two strains. The results suggest that the resistance of A/J mice to uremia-induced atherosclerosis may be attributed to their ability to maintain high serum HDL-cholesterol levels and/or low serum triglyceride levels. The results further indicate that the development of atherosclerotic lesions in uremic mice is both genetically determined and affected by diet.

ATH-3, a new gene for atherosclerosis in the mouse.

Mice derived from resistant A/J (A) and susceptible C57BL/6J (B) strains have been used to study the genetics of diet-induced atherosclerosis. A comparison of lesion scores between the parental strains, the F1 offspring of A x B and B x A matings and the offspring of (B x A)F1 hybrids backcrossed to either parent strain, indicates that a single major gene with alleles for resistance and susceptibility is responsible for the difference in response of A/J and C57BL/6J mice to a high fat, high cholesterol diet. By comparing the strain distribution pattern of susceptibility with known genetic markers in 30 A x B and B x A recombinant inbred (RI) strains, this gene, designated Ath-3, has been mapped close to a coat colour gene, c, on chromosome 7. Although a single gene may be primarily responsible for the difference in susceptibility to diet-induced atherosclerosis in these mice, the results of the breeding experiments indicate that its expression is probably modified by one or more additional genes.

Genetically determined susceptibility and resistance to diet-induced atherosclerosis in inbred strains of mice.

To determine whether recombinant inbred strains derived from C57BL/6J and A/J mice would provide a good model in which to study the genetics of diet-induced atherosclerosis, male mice of the parent strains were compared in a number of experiments designed to correlate various biochemical changes with susceptibility or resistance to the disease. In both strains fed an atherogenic diet containing 27% coconut oil and 4.5% cholesterol, there was a significant rise in serum very low-density plus low-density lipoprotein cholesterol levels, but only C57BL/6J mice developed discernible fatty lesions in the aortic wall. In A/J mice a significant rise in high-density lipoprotein cholesterol was also observed, which corresponded to the appearance of a second species of high-density lipoprotein in the serum, but in C57BL/6J mice there was a fall. In susceptible C57BL/6J mice, free cholesterol is secreted into bile, which becomes supersaturated, leading to the formation of gallstones. In the resistant strain, however, dietary cholesterol accumulates in the liver. A difference in hepatic cholesterol metabolism between the two strains may thus be a factor in determining their different susceptibilities to diet-induced atherosclerosis.

Genetic control of susceptibility to atherosclerosis (with special emphasis on the role of the macrophage).

The pathogenesis of atherosclerosis involves a complex interplay of arterial endothelium, multiple cellular constituents, and circulating lipo/apolipoproteins. Early work implicated a single etiology--either excess of circulating lipids, disruption of endothelial integrity, or clonal proliferation of smooth muscle cells. However, current research supports a hypothesis unifying endothelial disruption with lipid imbibation. The macrophage is recognized as having an integral role in atherogenesis due to its capacity to adhere to endothelium, internalize and metabolize lipids, and liberate proteases, apolipoproteins, and mitogens. Genetically determined susceptibility or resistance to atherosclerosis has long been described in animals and man, and differences in lipo/apolipoproteins have been proposed to explain these variations. Genetically defined variants of inbred mice have been used to pursue hereditary influences in atherogenesis. Strain variations in lipid metabolism have been advanced as one possibility to explain a polygenic mode of inheritance--those strains most replete in cholesterol-enriched lipoproteins demonstrating the greatest susceptibility. However, there is now a substantial body of evidence suggesting that the diverse functions of the macrophage may be the mechanism underlying the genetic polymorphism in atherosclerosis. Recombinant inbred strains of mice may serve as a vehicle through which this concept can be explored.