Effects of 25-hydroxycholesterol and 7-ketocholesterol, inhibitors of sterol synthesis, administered orally to mice

When 25-hydroxycholesterol or 7-ketocholesterol was fed to mice with the diet, growth was suppressed and mature mice lost weight. The effect of the 7-ketone upon body weight was effectively counteracted by cholesterol whereas cholestanol and beta-sitosterol were ineffective. Growth repression due to 25-hydroxycholesterol was only partially relieved by cholesterol. The effects of 25-hydroxycholesterol and 7-ketocholesterol upon body weight were related to an apparent effect upon appetite. However the sterols were not unpalatable since diets containing them were not rejected in favor of control diet. Intestinal sterol synthesis was inhibited soon after the administration of dietary 7-ketocholesterol or 25-hydroxycholesterol but inhibition decreased with prolonged feeding. When fed by gavage, the sterols suppressed intestinal sterol synthesis as soon as 2 h after administration. In contrast, cholesterol administered by gavage did not affect intestinal sterol synthesis during a 24 h test period. When fed with the diet 25-hydroxycholesterol and 7-ketocholesterol did not depress hepatic cholesterol synthesis beyond the low levels found in pair-fed controls. Inhibition of intestinal sterol synthesis was accompanied by a decrease in the concentration of cholesterol in the intestinal mucosa and, usually, by a drop in the molar ratio of cholesterol to phospholipids.

Consequences of blocked sterol synthesis in cultured cells. DNA synthesis and membrane composition

When cell cultures were incubated with certain oxygenated derivatives of cholesterol which specifically depress 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity (i.e. 25-hydroxycholesterol, 7-ketocholesterol, or 20 alpha-hydroxycholesterol), a number of consequences followed the decline in enzyme activity. DNA synthesis, expressed in terms of (methyl-3H)thymidine incorporation/mg of cellular protein, declined progressively after the addition of an inhibitor to L-cell cultures and had essentially ceased by 80 h. A decline in the rate of cell growth, as determined by the measurement of cellular protein, became apparent about 24h after the addition of the inhibitor. These effects of the inhibitory sterols were reversed when mevalonate or cholesterol was added to the medium within 48 h after the addition of the inhibitor. Specific rates of uridine incorporation into RNA and of leucine into protein were not significantly altered during 80 h of incubation with the inhibitor. Sterol synthesis was suppressed; and the concentration of desmosterol in L-cells declined to about one-half of the control level within 36 h. Since phospholipid concentrations were not altered significantly by the treatment, the molar ratio of sterol to phospholipids declined to levels lower than any previously reported for mammalian cells. Changes in the molar ratio of sterol to phospholipids in plasma membranes isolated from L-cells that had been incubated with 25-hydroxycholesterol or cholesterol were in agreement with those found in cells under similar conditions. The inhibitory sterols caused a similar but slower decline in the concentrations of unesterified and total cholesterol in fetal mouse liver cell cultures and rat hepatoma cell cultures.

Relationship between sterol synthesis and DNA synthesis in phytohemagglutinin-stimulated mouse lymphocytes

Incubation of peripheral blood or isolated lymphocytes of C57L/J mice with phytohemagglutinin stimulated the incorporation of thymidine into DNA of lymphocytes as they transformed into large lymphoblasts. DNA synthesis began after about 24 hr of incubation and reached a peak at 48 hours. The de-novo synthesis of sterols from acetate was stimulated much earlier, at 4 hr of incubation, and the rate reached a maximum at 24 hr, approximately at the time DNA synthesis began. Rates of incorporation of radioactivity from [14-C]acetate into fatty acids and into CO2 by phytohemagglutinin-treated blood were not significantly different from control values. Phytohemagglutinin stimulation of sterol synthesis could be abolished by the addition of certain oxygenated derivatives of cholesterol (e.g., 25-hydroxycholesterol and 20alpha-hydroxycholesterol) which specifically depress the activity of the regulatory enzyme in the sterol synthesis pathway, 3-hydroxy-3-methylglutaryl CoA reductase [mevalonate:NADP-nOXIDOREDUCTASE (CoA acylating); EC 1.1.1.34]. This treatment also abolished DNA synthesis and blastogenesis which otherwise followed the peak of sterol synthesis. Furthermore, DNA synthesis was repressed only if the inhibitor was added early enough to prevent sterol synthesis from reaching its usual maximum. When the compound was added after the rate of sterol synthesis had reached its maximum, DNA synthesis was not affected. These findings suggest that the synthesis of cholesterol is an essential prerequisite for successful initiation and completion of the cell cycle in lymphocytes after phytohemagglutinin activation.

Regulation of sterol synthesis in cultured cells by oxygenated derivatives of cholesterol

Sterol synthesis in liver in vivo is regulated at the site of the reaction catalyzed by 3-hydroxy-3-methylglutaryl-CoA reductase through a feedback system thought to involve either cholesterol or one or more of the products of its metabolism. Cholesterol feeding results in repression of the synthesis of the enzyme, but inactivation of the enzyme seems to precede repressroblasts is not inhibited by purified exogenous cholesterol. However, derivatives of cholesterol produced by the introduction of a ketone or hydroxyl function in the 7, 20, 22 or 25 positions effectively inhibit sterol synthesis by specifically depressing the level of HMG CoA reductase activity. As a result of this specific effect prolonged incubation of an inhibitory sterol with growing L cells results in depletion of cellular sterol. Growth of the culture then ceases and the cells die unless an appropriate sterol or a sterol precursor is supplied in the medium. The inhibitory sterols, 25-hydroxycholesterol and 7-ketocholesterol appear to be taken up by L cells through processes that involve their specific interactions with saturable cellular receptors. The uptake of cholesterol by L cells appears to be by a different process--possibly through physical diffusion.