Regulation of hepatic cholesterogenesis by polar steroids accumulated after cholesterol feeding

Differential effects of dietary fat on chick plasma and liver composition and HMG-CoA reductase activity

The comparative effects of diet supplementation with 10% saturated fat rich in 12:0 and 14:0 fatty acids (coconut oil), without and with 1% added cholesterol, and with 10% unsaturated fat rich in n-3 polyunsaturated fatty acids (menhaden oil) on cholesterol metabolism in neonatal chicks were examined to clarify the different mechanisms of their hyper- and hypolipidemic action. Supplementation of coconut oil produced a significant hypercholesterolemia after 7 days of treatment, with a similar increase in the amount of both free and esterified cholesterol. Supplementation of coconut oil plus cholesterol produced a higher increase of plasma cholesterol levels (approximately two to three times higher than those found with standard diet). However, supplementation of menhaden oil induced a significant decrease in total cholesterol after only 2 weeks of treatment. Levels of plasma triglycerides did not change by coconut oil addition to the diet, but a significant increase was observed after coconut oil plus cholesterol feeding. Menhaden oil produced a transient decrease in plasma triglycerides. Hepatic 3-hydroxy-3-methylglutaryl-CoA reductase activity did not change with coconut oil treatment. However, both coconut oil plus cholesterol and menhaden oil supplemented diets drastically decreased reductase activity after 1 week of dietary manipulation. These results show that different nutrients with the same inhibitory effect on reductase activity produced opposite effects on plasma cholesterol content, suggesting the existence of important differences in the regulatory mechanisms implied in cholesterol biosynthesis and its accumulation in plasma.

Regulation of mevalonate 5-pyrophosphate decarboxylase in HeLa cells. Inhibition of enzymatic protein synthesis by serum lipoproteins

Mevalonate 5-pyrophosphate decarboxylase (EC 4.1.1.33) has been considered as a secondary site of regulation of cholesterogenesis. Because of this, we have studied the regulation of decarboxylase in HeLa cells by serum lipoproteins in the cell culture medium. A first group of experiments was performed with cells grown in Eagle's medium with 10% foetal calf serum. The specific activity of decarboxylase was increased when whole foetal calf serum was replaced with lipoprotein-poor serum. This increase was clearly reduced in the presence of cycloheximide. Addition of serum lipoproteins to a medium containing lipoprotein-poor serum led to a clear decrease in the decarboxylase activity. An identical decrease was observed after the addition of lipoproteins alone or in combination with cycloheximide. These results suggest for the first time that the effect of serum lipoproteins on decarboxylase activity should be a decrease in the rate of enzymatic protein synthesis, and corroborate the important role of reactions other than those catalysed by 3-hydroxy-3-methylglutaryl-CoA reductase in the regulation of cholesterogenesis.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Progesterone stimulation of HMG-CoA reductase activity in cultured cells

In a previous study we showed that progesterone (PG) stimulated HMG-CoA reductase (HMGR) activity in rat intestinal epithelial cells (IEC-6) incubated in the presence or absence of low-density lipoprotein (LDL) [1,2]. In the present study we examined further the mechanism of this stimulation. We observed that the stimulation of HMGR activity by PG was completely prevented by cycloheximide. Turnover studies utilizing immunoprecipitation of HMGR-labeled with [35S]methionine revealed that PG increased reductase activity by inhibiting HMGR degradation without affecting the synthesis of HMGR. The stimulation of HMGR activity by progesterone could be accounted for by a continuous synthesis of HMGR while its degradation was retarded. In the presence of LDL, the activity of HMGR in IEC-6 cells was effectively inhibited, however PG was able to stimulate HMGR in the presence of LDL. This effect was not due to an interference of normal cellular metabolism of LDL, since PG had no effect on the cellular uptake and lysosomal degradation of 125I-LDL. PG did not affect of the rate of lysosomal hydrolysis of [3H]cholesteryl linoleate-LDL. The free [3H]cholesterol derived from [3H]cholesteryl linoleate-LDL moved to the cell membrane and effluxed to HDL3 in the medium at the same rate in the presence or absence of PG. Although PG did not affect LDL metabolism, pre-treatment of cells with LDL delayed the onset of HMGR stimulation by PG. In IEC-6 cells deprived of LDL for 24 h, the HMGR activity was stimulated immediately following PG addition. In cells pre-treated with LDL for 24 h, the stimulation was delayed by 4 h. Treatment of cells with 25-hydroxycholesterol completely prevented PG stimulation of HMGR activity. We propose that the stimulation of HMGR activity in the presence or absence of LDL is related to the ability of PG to attenuate the formation and/or action of intracellular HMGR repressor molecules which accelerate the degradation of HMGR.

Effect of feeding cholesterol to laying hens and chicks on cholesterol metabolism in pre- and posthatch chicks

Single Comb White Leghorn laying hens that were 60 wk of age were fed wheat and soybean meal diets containing either 0 or 1% cholesterol. Birds were artificially inseminated, and fertilized eggs were collected for incubation after a plateau of egg cholesterol content was reached. Posthatch chicks were raised with starter diets containing either 0 or .5% cholesterol. Samples of developing embryos and posthatch chicks at various stages were prepared for cholesterol analysis. As compared with controls, cholesterol content of eggs from hens fed 1.0% cholesterol diet was increased by approximately 70%. Embryos from the cholesterol-loaded eggs had significantly higher (P less than .05) cholesterol content. The plasma total cholesterol (TC) level in chicks from cholesterol-loaded eggs, when compared with TC in control eggs, was significantly higher at hatching but decreased to the same level by 2 wk after hatching. Cholesterol feeding to newly hatched chicks elevated plasma TC and high-density lipoprotein cholesterol levels. The TC contents of liver and heart, but not skeletal muscle, were significantly higher in chicks fed the .5% cholesterol starter diet than those fed the cholesterol-free diet. These results show that cholesterol metabolism in developing embryos and posthatch chicks is influenced by cholesterol in both maternal and chick diets.