Let’s start with what cholesterol is and what it does.
In previous investigations, we found high rates of cholesterol synthesis in human fetal liver tissue, second only to rates in fetal adrenal tissue. Previous estimates of the amount of cholesterol in the fetus derived from the maternal compartment are in the range of 20%. Thus, the liver may be the principal source of circulating lipoproteins in the human fetus, as is true in the human adult. Low density lipoprotein is the major source of cholesterol used for fetal adrenal steroidogenesis; therefore, it follows that factors regulating cholesterol synthesis in the human fetal liver may indirectly control the rate of steroid secretion by the adrenal cortex. The purpose of the present investigation was to determine if hormones, particularly those produced by the fetal-placental unit, might serve to stimulate cholesterol synthesis in the human fetal liver. The rate of cholesterol biosynthesis was determined by measuring the rate of incorporation of [3H]water into [3H]cholesterol in hepatocytes maintained in culture or by determination of the specific activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase in microsomal preparations from human fetal liver. The addition of dexamethasone (10-10-10-6 M) stimulated cholesterol synthesis up to 2- to 4-fold between days 2 and 6 of exposure. When human fetal liver cells were maintained in the presence of dexamethasone (10-7 M), the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase in microsomal fractions was stimulated 4-fold compared to that in control cells. Cortisol also stimulated cholesterol biosynthesis in a concentration-dependent manner. The addition of 17β-estradiol (E2) to the culture medium resulted in stimulation of cholesterol biosynthesis in a concentration-dependent manner from 10-10-10-7 M. The rate of cholesterol synthesis when E2 was present (10-7 M) was 4-fold greater than in untreated cells. Stimulation of cholesterol synthesis by E2 was maintained between 2-7 days of incubation with E2. Estrone, estriol, and E2 (10-6 M) caused similar increases (3- to 4-fold) in the rates of cholesterol synthesis in human fetal hepatocytes. Finally, progesterone in concentrations greater than 10-6 M significantly stimulated cholesterol synthesis in human fetal liver cells. In contrast, other hormones and factors, including insulin, glucagon, PRL, GH, dehydroepiandrosterone and its sulfate, epidermal growth factor, fibroblast growth factor, T3, (Bu)2cAMP, and cholera toxin, had no effect on the rate of cholesterol synthesis in human fetal liver cells. In summary, estrogens, glucocorticoids, and progesterone produced by the fetal-placental unit stimulate the rate of cholesterol synthesis in human fetal liver cells.
Cholesterol synthesis can also be turned off when ..
The process of cholesterol synthesis ..
Thus, feedback inhibition of cholesterol biosynthesis by dietary cholesterol seems to occur primarily at the point of synthesis of mevalonate from hydroxymethylglutarate.
Cholesterol synthesized by the liver, ..
Thus, feedback inhibition of cholesterol biosynthesis by dietary cholesterol seems to occur primarily at the point of synthesis of mevalonate from hydroxymethylglutarate.">