Biological activity of some oxygenated sterols.

A group of oxygenated sterols has been identified as potent and specific inhibitors of sterol biosynthesis. The ability of these compounds to inhibit sterol synthesis in cultured cells and the ineffectiveness of cholesterol under the same conditions suggest that feedback regulation of sterol biosynthesis may be brought about by an oxygenated sterol rather than by cholesterol. The nature of the regulatory sterol may vary in different cells with their specific requirements for cholesterol as a structural component or as a precursor of other steroid products. The use of oxygenated sterols to block sterol synthesis in cultured cells provides new information regarding the role of sterol in cell membrane structure and function. For example, de novo sterol synthesis is required for DNA synthesis and cell division by some cultured cells. Studies with cultured cells, and with rats and mice in vivo, suggest that oxygenated sterols could be of value in the treatment of several important human diseases.

Effect of allelic substitutions at the hairless locus on endogenous ecotropic murine leukemia virus titers and leukemogenesis.

Leukemia-prone hairless (HRS/J; hr/hr) mice had significantly higher leukemia virus titers than did the leukemia-resistant nonmutants (hr/ + and +/+). This difference was ascribed to the allelic substitution at a single gene locus; at 6 months of age it averaged 13-fold, immediately preceding the large divergence in leukemia incidence between the mutant and nonmutant mice.

Cholesterol and its biosynthesis in normal and malignant lymphocytes.

The presently available information on the role and function of cholesterol in plasma membranes of mammalian cells has been reviewed extensively before. This paper restricts itself to briefly summarized some observations gathered in our laboratory and in those of other investigators which directly address themselves to the regulation of the biosynthesis of cholesterol and its possible significance in immunocompetent cells. It is suggested that de novo synthesis of cholesterol represents a critical metabolic step for proliferation and, possibly also, differentiation of lymphoid cells such as cytotoxic T-cells. De novo synthesis of cholesterol is regulated specifically by a feedback mechanism involving oxygenated derivatives of cholesterol. Some of these oxidation products are known to be generated spontaneously from cholesterol, which in itself is not affecting the activity of the rate-limiting enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34). At present, it is not known to what extent oxidized derivatives of cholesterol contaminate the human diet. If they do, their effects on the immune system may be significant, and further investigations on such minor yet very potent and naturally occurring compounds in the diet are needed to understand the etiology of several human diseases. These compounds may also be of therapeutic value in the treatment of some malignant disorders.

Heritability of the phytohemagglutinin responsiveness of lymphocytes and its relationship to leukemogenesis.

Upon analyses of 59 inbred strains, F-1 hybrids, and congenic-resistant mouse strains, the strain distribution pattern of the stimulation of perippheral mouse lymphocytes by phytohemagglutinin was established using a micromethod. The family of DBA mice were the lowest responders of phytohemagglutinin, whereas the C57 family responded best. Strain PL/J exhibited the best response. The response of lymphocytes to the lectin is governed by more than two but less than five major genes of unknown linkage. No direct association to the H-2 histocompatibility complex was found, although an indirect influence of this locus could not be excluded. All high-leukemia strains are good responders to phytohemagglutinin. None of the low-responder-group strains exhibit spontaneous leukemia. No correlation of the response of lymphocytes to the expression of the type C RNA genome could be established. Cell suspensions from animals exhibiting clinical signs of leukemia responded only weakly or not all to the lectin.

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.

Transcortin and vitamin D-binding protein levels in mouse serum.

The influence of age, sex and strain on the serum concentration of transcortin (corticosteroid-binding globulin) and vitamin D-binding protein (DBP) in mice was investigated. The effect of age was studied in two strains, C57BL/6JPfd and BALB/cmHeAPfd. The concentration of transcortin and DBP increased with age. In young animals the concentration of each protein showed a significant strain difference, which disappeared in older mice for DBP, but not for transcortin. In 7-day-old animals, no sex difference was observed for either protein, but in older animals a clear sex difference was found for transcortin. Adult males tended to have somewhat higher levels of DBP than adult females, but this difference was significant only on day 70. The variation in transcortin and DBP levels was further investigated in a large number of mouse strains. The DBP concentration did not markedly vary among strains (5.98-9.65 mumol/l in males and 5.08-8.85 mumol/l in females). Transcortin, however, showed marked strain variations, ranging from 0.72 to 2.06 mumol/l in males and from 1.02 to 4.55 mumol/l in females and there was a significant correlation (r = 0.66, n = 26, P less than 0.001) between the mean transcortin levels in males and females of different strains. Interstrain variation was much higher than intrastrain variation or variation among related strains, suggesting that the transcortin concentration is largely controlled by genetically determined factors. There was a significant correlation (r = 0.82, n = 9, P less than 0.01) between the mean corticosterone and transcortin concentrations (measured at 21.00 h).(ABSTRACT TRUNCATED AT 250 WORDS)

High affinity Concanavalin A binding to sterol-depleted L cells.

Binding of Concanavalin A to mouse L cells which were grown in serum free, chemically defined medium and depleted of their membrane sterol by blocking their de novo sterol synthesis was investigated. Kinetic analysis of binding data implied positive cooperativity, with two different affinities for Con A, in both experimental and control cultures. The amount of Con A bound to the cell surface at saturation was approximately 0.5 picomoles per mg cellular protein in controls and approximately 1.0 picomoles per mg cellular protein in 25-hydroxycholesterol treated cultures (which had a reduced sterol concentration of up to 50% in their plasma membranes). This phenomenon was reversed when cholesterol or mevalonate was added to the inhibited cultures to compensate for their inability to synthesize sterol. Our findings indicate that lectin binding to specific glycoprotein receptors is influenced by membrane lipid composition.

Pinocytosis in L cells: its dependence on membrane sterol and the cytoskeleton.

Pinocytosis in L-cells, grown in serum-free medium, was depressed when cultures were treated with oxygenated derivatives of cholesterol which inhibited sterol synthesis and reduced the sterol concentration of the plasma membranes. Noninhibitory sterols, such as cholesterol or desmosterol counteracted the effects of the inhibitors. Treatment with polylysine increased the rate of pinocytosis in sterol-depleted cells to a level similar to the enhanced rate found in polylysine treated control cells. Drugs which interfere with cytoskeletal systems (microfilaments, microtubules) also depressed pinocytosis but their effect could not be overcome by treatment with polylysine.

25-Hydroxycholesterol-induced elevations in 45Ca uptake: permeability changes in P815 cells.

Certain oxysterols are capable of suppressing the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase. We have previously demonstrated that treatment of P815 cells with 1 microgram 25-hydroxycholesterol/ml culture results in a rapid influx of 45Ca, and supplemental cholesterol prevents this from occurring. In this paper, we report on investigations into the means whereby this influx of calcium takes place. Through the use of respiratory inhibitors which prevent mitochondrial retention of calcium it was determined that the large increase in slow phase (intracellular) calcium uptake caused by 25-hydroxycholesterol treatment was related to mitochondrial uptake. The effects of various inhibitors of calcium uptake into cells, including verapamil, diltiazem, quinidine, ruthenium red, Co++, Mn++, were tested. Of these only Co++ and ruthenium red had any effect on 45Ca uptake. 25-Hydroxycholesterol has been shown to be capable of membrane insertion and this could result in plasma membrane permeability changes. To test this hypothesis P815 cells were treated with 1 microgram 25-hydroxycholesterol/ml or 5 micrograms mevinolin/ml culture. Mevinolin, being a water soluble competitive inhibitor of HMG-CoA reductase, should be unable to disrupt membrane architecture in a manner analogous to 25-hydroxycholesterol. While both inhibitors rapidly suppressed the synthesis of digitonin-precipitable sterols, only 25-hydroxycholesterol was able to increase 45Ca influx. The implications of these findings are discussed.

Cholesterol synthesis in polyclonally activated cytotoxic lymphocytes and its requirement for differentiation and proliferation.

The kinetics of sterol synthesis and DNA synthesis in polyclonally activated, concanavalin A-stimulated spleen cell cultures were analyzed. Inhibition of DNA synthesis by 1-beta-D-arabinofuranosylcytosine (Ara-C) did not abrogate the formation of cytotoxic effector cells. However, inhibition of sterol synthesis by 25-hydroxycholesterol inhibited formation of cytotoxic effector cells as well as cellular proliferation. The inhibition of cytotoxicity correlated well with the dose of 25-hydroxycholesterol administered and was dependent on the time of administration. The agent had to be present when sterol synthesis occurred normally during the time lapse before DNA synthesis began. Compactin had the same effect as 25-hydroxycholesterol. The effects of inhibition of sterol biosynthesis on cytotoxicity could be counteracted by addition of cholesterol-containing liposomes. Based on these experiments, the links between proliferation and differentiation in lymphocytes are discussed.

25-Hydroxycholesterol-induced elevation in 45Ca uptake: correlation with depressed DNA synthesis.

The mechanism whereby 25-hydroxycholesterol, an inhibitor of the synthesis of cholesterol, depresses DNA synthesis in cycling P815 mastocytoma cells was investigated. The uptake of 45Ca into P815 cells treated with 1 microgram/ml 25-hydroxycholesterol began to rise above control levels by 6 hours after initiation of treatment and was increased tenfold by 15 hours. Kinetic data of calcium uptake indicated the presence of at least two components of calcium uptake, fast and slow. The fast phase of calcium exchange at the cell surface was changed little by treatment with 25-hydroxycholesterol. The slow phase of calcium exchange with the intracellular compartment was markedly affected by treatment with the inhibitor, there being a large increase in the flux and half-time of uptake, and a fall in the rate constant. This resulted in a large elevation of the intracellular compartment size. Incorporation of [3H]thymidine into DNA began to decline between 9 and 12 hours posttreatment in these cultures. Uptake of calcium and depression of DNA synthesis were shown to be directly related to the dose of 25-hydroxycholesterol used. The changes in 45Ca uptake and DNA synthesis due to 25-hydroxycholesterol treatment were abolished by addition of exogenous cholesterol to the incubation medium. The results are consistent with the hypothesis that 25-hydroxycholesterol, by inhibiting cholesterol production, depresses DNA synthesis via an elevation in the uptake of calcium into the cell to a level incompatible with continued DNA replication.

Specific binding of cholesterol to chromatin prepared from mouse spleen cells.

Mouse spleen cell suspensions were incubated with tritiated cholesterol for various time intervals. The chromatin of these cells was then isolated, washed with Triton X-100, and fractionated on Sephadex columns. It was found that cholesterol binds specifically to the chromatin. The binding was saturated after 45 min of incubation and also displayed characteristics typical of dose-dependent binding. The number of cholesterol molecules bound per nucleus was estimated to be on the order of 10 000. Oxygenated sterols, such as 25-hydroxycholesterol and 7-ketocholesterol, did not compete for the binding with [3H]cholesterol if added in 20-fold molar excess to the incubation medium of the cells. Chromatographic analyses on Sephadex columns displayed a distinct peak of radioactivity. The protein-sterol complex had an apparent molecular weight of 180 000 +/- 27 000. Using extensive digestion with DNase I (EC 3.1.21.1) it could be concluded that DNA, binding to the complex, did not influence the estimate of the molecular weight, whereas digestion with pronase or treatment with sodium dodecyl sulfate destroyed the complex. Additional experiments using sucrose density gradients (5-20%) showed also, that [3H]cholesterol was bound to chromatin by one or several proteins.