Synthesis of sterol and phospholipid induced by the interaction of phytohemagglutinin and other mitogens with human lymphocytes and their relation to blastogenesis and DNA synthesis

Requirement for cell-to-cell contact for the immunosuppressive activity of human alveolar macrophages

The ability of alveolar macrophages (AM) obtained by bronchoalveolar lavage of healthy volunteers to suppress T lymphocyte responses to the mitogen phytohemagglutinin (PHA) in vitro was investigated. AM but not monocytes (MN) inhibited responses of peripheral blood mononuclear cells (PBMC) to PHA as measured by incorporation of [3H]thymidine [( 3H]TdR) and interleukin-2 (IL-2) expression. Supernatants of AM generated for various periods and with various concentrations of cells did not, however, inhibit PBMC responses to PHA. To examine the role of cell contact in the inhibitory activity of AM, AM or MN were added to PBMC in 6-well plates either directly (in co-culture) or separated by a 0.45-micron filter. MN did not inhibit PBMC blastogenic responses under either condition. AM at a 1:2 ratio with PBMC inhibited blastogenesis by 75 +/- 11% (mean +/- SD, n = 3, P less than 0.01) when cultured directly with PBMC but had no inhibitory effect on blastogenesis when physically separated from target PBMC. AM in co-culture with PBMC also inhibited PHA-stimulated IL-2 production by 70% but did not inhibit IL-2 production when AM were separated from PBMC in dual chambers. To assess the role of the cell surface in the inhibitory activity of AM, AM and MN were fixed with 2% paraformaldehyde. Neither fixed nor unfixed MN inhibited PBMC blastogenic responses, but both fixed and unfixed AM inhibited responses similarly (77 to 95%).(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular cholesterol metabolism in mitogen-stimulated lymphocytes--requirement for de novo synthesis

The relationship between cholesterol synthesis and uptake in proliferating lymphocytes has been examined. [14C]Acetate incorporation into lymphocytes cultured under lipoprotein-deficient conditions increased initially in response to mitogen, decreased after 24 h, and increased rapidly between 72 and 96 h. Addition of LDL (10 micrograms/ml) to the culture during the 'trough' period caused [14C]acetate incorporation to return rapidly to baseline, while at peak periods LDL suppression of cholesterol synthesis was minimal. Lymphocytes cultured in the presence of the HMG-CoA reductase inhibitor, mevinolin, exhibited a time-dependent increase in their capacity to incorporate [14C]acetate into cholesterol, evident when mevinolin was removed by washing prior to assay. PHA enhanced 125I-labelled LDL receptor-mediated binding by lymphocytes cultured in lipoprotein-deficient medium over a 4 day period and mevinolin augmented the effect. [3H]Thymidine incorporation into mitogen-stimulated lipoprotein-deficient cultures was inhibited up to 75% by mevinolin (1 mumol/l). LDL (2.5-10 micrograms/ml) substantially reversed this inhibition in 72 h cultures, but only partially overcame inhibition in cells cultured for 96 h. Results suggest that endogenous cholesterol synthesis may be obligatory for lymphocyte proliferation after the initial round of cell division.

Effect of AY 9944 and chlorpromazine on Concanavalin A-induced stimulation of human lymphocytes

Amphiphilic molecules AY 9944 and chlorpromazine (CPZ) inhibited DNA synthesis in Concanavalin A-stimulated lymphocytes in a dose-dependent manner. While AY 9944 strongly decreased 7-dehydrocholesterol conversion to cholesterol, CPZ did not significantly affect this reaction. Moreover, the inhibitory effect of AY 9944 and CPZ on DNA synthesis took place in the presence of cholesterol in the culture medium. These findings suggest that the mechanism of inhibition of DNA synthesis by AY 9944 or CPZ is not related to endogenous cholesterol synthesis or exogenous cholesterol supply. Results are discussed in relation to the amphiphilic properties of AY 9944 and CPZ and to the interaction of these drugs with membranes or other intracellular targets such as calmodulin.

Characterization of oxysterol-binding protein in rat embryo fibroblasts and variations as a function of the cell cycle

A cell-free system assay involving cell freeze-thawing and protein fractionation by ammonium sulfate precipitation was developed to characterize a cytosol binding protein specific for oxysterols in rat embryo fibroblasts. This protein shared common characteristics with the oxysterol-binding protein described in L cells and in normal human lymphocytes: 8 S sedimentation coefficient, sterol-protein complex of Mr 160 600, saturability, high affinity (Kd in the range of 10(-9) M) and specificity for cholesterol derivatives oxidized on the side chain. These compounds were better inhibitors of DNA synthesis than the compounds oxidized on the nucleus, whereas no difference was found between sterols oxygenated either on the side chain or on the nucleus, as far as inhibition of hydroxymethylglutaryl-CoA reductase (HMG-CoA reductase) was concerned. Macromolecular components capable of specifically binding 25-hydroxycholesterol were also detected in the fibroblast nucleus. The cytosol oxysterol-binding protein showed a sharp increase (5-fold) in the G2M phase of the cell cycle and in exponentially growing cells (maximal binding site number/cell: 43 500, versus 8850 in confluent cells). Neither the affinity nor the sedimentation coefficient of the protein changed in either situation. The quantitative (but not qualitative) variations of oxysterol-binding protein could be related to the inhibitory effect of 25-hydroxycholesterol on DNA synthesis, which becomes critical when this sterol is added in the G2M phase of the cell cycle.

Lymphocyte membrane lipid composition and mitogen responsiveness in chickens: role of membrane "fluidity"

After establishing optimal conditions for measuring the membrane lipid packing density ("fluidity") of chicken peripheral blood lymphocytes, the fluidity was modulated in vitro by incubation in cholesterol or phospholipid ("active lipid", AL)-enriched serum-free tissue culture medium. The effect of these lipids on mitogen responsiveness was then investigated, the aim being to determine whether the observed enhancement/suppression was membrane mediated, i.e. explainable by fluidity changes. Chicken peripheral blood lymphocytes exhibited no requirement for exogenous cholesterol; low concentrations did not affect the mitogen response while the higher concentrations, which induced a measurable decrease in membrane fluidity, were usually mildly suppressive. Pre-incubation did not increase this suppressive effect and we believe it not to be membrane mediated. AL, at low concentrations which induced no changes in membrane fluidity, prolonged the phytohemagglutinin response, enhancement being evident only after the peak; we interpret this as a nutrient effect. At the higher concentrations, which induced large increases in fluidity, a transient enhancement was followed by suppression; suppression was delayed in onset when AL was added 4 h after phytohemagglutinin stimulation. It is therefore an early event which may be mediated through changes in membrane fluidity.

Inhibition, post-hydroxymethylglutaryl-CoA regulation and relation to cell growth of cholesterol biosynthesis in cultured human skin fibroblasts

Cholesterol biosynthesis in cultured human skin fibroblasts was studied by [14C]acetic acid incorporation into non-saponifiable lipids and quantification of labeled cholesterol among its precursors. Synthesis was slow, in spite of a high 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA) activity and led to an accumulation of unexpected non-polar metabolites identified as C-30 sterones, but not C-27 sterol precursors. This supports a post-HMG-CoA regulation at the lanosterol demethylation step. Cholesterol biosynthesis was stimulated by cell culture in lipid depleted medium and was inhibited by pentadecane-2-one which acts mainly at two post-HMG-CoA steps: lanosterol demethylation and lathosterol isomerisation to cholesterol. A parallel pentadecane-2-one inhibition of cell growth was also observed, even when cells were cultured in the presence of whole serum. This indicates the existence of a relationship between endogenous cholesterol synthesis and cell growth and sheds additional light on the role of post-HMG-CoA regulation in this phenomenon.

Phorbol ester-induced loss of colchicine ultrasensitivity in chronic lymphocytic leukaemia lymphocytes

On exposure to the phorbol ester 12-O-tetradecanoyl-13-acetate (TPA) the pathological (non-dividing) lymphocytes of B-cell chronic lymphocytic leukaemia (CLL) lose their characteristic ultrasensitivity to the cytocidal action of colchicine in vitro. They are no longer killed in 1 day by the drug at 10(-6)M-concentration. The effect was the same whether the cells were incubated in the continuous presence of TPA, or subjected instead to pulse-treatment with it (for as little as 5 min.). Colchicine at one thousand times greater concentration was now needed to kill the cells. CLL lymphocytes already primed to undergo interphase death by pretreatment with colchicine could be prevented from doing so by early addition of TPA. A marked proportion of those CLL lymphocytes destined to undergo early spontaneous death in vitro in the absence of colchicine could be prevented from doing so by TPA. The loss of colchicine ultrasensitivity applied to cells which had not yet undergone TPA-induced morphological transformation to blast-like cells or differentiation to cells containing abundant cytoplasmic immunoglobulins (CIg). These transformed cells materialised in greatest incidence (70-80%) after 3 days of culture, an observation in agreement with others workers.

Mitogen effects on lipid metabolism during lymphocyte activation

Mitogenic and non-mitogenic lectins have been compared for their abilities to stimulate acetate and choline incorporation into mouse spleen lymphocyte lipids. The concns required for maximal acetate incorporation correspond to those required for maximal blastogenesis. The non-mitogenic lectins tested had no effect on acetate incorporation. Concanavalin A also stimulated acetate incorporation into splenic lymphocyte lipids of athymic mice which do not undergo T-cell blastogenesis. 4,4'-Diaminodiphenylsulfone (dapsone) at 150 micrograms/ml inhibited the incorporation of choline into mouse spleen lymphocyte phosphatidylcholine by 40%, but had no significant effect on concanavalin A induced mitogenesis when the inhibitor was present during the first few hours of transformation. Enhanced turnover of phospholipids appears to be a parallel but non-essential event in the early stages of mitogenesis.

DNA and cholesterol biosynthesis in synchronized embryonic rat fibroblasts. II. Effects of sterol biosynthesis inhibitors on cell division

The relationships between cholesterogenesis and cell division were studied by using two inhibitors of hydroxymethylglutaryl-CoA reductase activity--25-hydroxycholesterol and compactin. The effects of both compounds on DNA synthesis were compared in synchronized rat fibroblasts cultured in a cholesterol-containing medium. Compactin did not inhibit DNA synthesis, except after a long time of contact and at high and almost cytotoxic concentrations. 25-Hydroxycholesterol inhibited DNA synthesis (without cytotoxic effects) after only 9-16 h of contact, depending on the phase of the cell cycle at which this compound was added to the culture medium. Sensitivity of cells to 25-hydroxycholesterol was maximal at the end of the S phase/beginning of the G2M phase. The rapid effect of 25-hydroxycholesterol on DNA synthesis appears to be separate from the inhibitory effect on sterol or non-sterol mevalonate-derived compound synthesis. Indeed, under our experimental conditions, the suppression of cholesterol biosynthesis is compensated by the presence of cholesterol in the culture medium, as demonstrated by the lack of effect of compactin on DNA synthesis; moreover, addition of mevalonolactone to the culture medium did not reverse the effect of 25-hydroxycholesterol. 25-Hydroxycholesterol could inhibit DNA synthesis by a direct action on the nucleus, after transfer by the intermediary of a specific hydroxysterol-binding protein.

Suppression of endocytosis in polyclonally activated Con A-stimulated T lymphocytes by 25-hydroxycholesterol

Endocytosis in polyclonally activated, Con A-stimulated spleen cell cultures was analyzed. It was found that as lymphocytes differentiate to acquire cytotoxic capability their endocytic activity also increases, reaching a plateau at 48 hr. Inhibition of sterol synthesis reduced endocytic rates by as much as 50% when 25-hydroxycholesterol was added during the first 24 hr of culture, the time at which sterol synthesis is at its maximum. When 25-hydroxycholesterol was added after the cycle of sterol synthesis, little or no suppression of endocytosis was seen. Compactin, which is an allosteric, competitive inhibitor of HMG-CoA reductase (the rate-limiting enzyme in the sterol biosynthetic pathway), produced a similar abrogation of endocytic rate. The effect of inhibition of sterol synthesis on endocytosis can be counteracted by the addition of cholesterol to the cultures. It is hypothesized that the dynamic process of endocytosis plays a role in the reorganization of membrane components necessary for the expression of the differentiated state of cytotoxicity.

Inhibition of sterol and DNA synthesis in peripheral blood lymphocytes by AY9944

The effect of the inhibitor of cholesterol synthesis AY9944 on DNA synthesis was tested in phytohemagglutinin stimulated lymphocytes incubated in delipidated serum. AY9944 caused a decrease in cholesterol synthesis from [14C] acetate and an accummulation of [14C] 7-dehydrocholesterol. This inhibition of cholesterol synthesis resulted in an inhibition of DNA synthesis in a dose related manner. Inhibition could be partly reversed by added lipoprotein. Inhibition of DNA synthesis occurred if AY9944 was added at the early stages of blast transformation but not when the response was well established. These findings suggest that some endogenous cholesterol synthesis may be required for the complete phytohemagglutinin stimulation of lymphocytes.

A specific hydroxysterol binding protein in human lymphocyte cytosol

A discriminating system capable of recognizing the oxygenated sterols was investigated in human lymphocytes. After labelling entire cells with 25-hydroxy [3H] cholesterol (10 nM) the cytosol was ultracentrifuged on a linear sucrose density gradient. Bound 25-hydroxy [3H] cholesterol was located in a single peak with a sedimentation coefficient of 8.3 S. Pronase treatment abolished the radioactive peak. This 8.3 S protein had a low binding capacity for 25-hydroxy [3H] cholesterol and probably a high affinity. This last parameter was not determined on account of some difficulties encountered in a cell-free system relating to the physico-chemical properties of 25-hydroxycholesterol. Only the hydroxylated sterols closely related to 25-hydroxycholesterol were capable of specifically binding to the 8.3 S protein, in contrast with cholesterol. This protein differed from the binding proteins of oxygenated derivatives of vitamin D3 and glucocorticoids. With the human lymphocyte as a model and under our experimental conditions, this hydroxylated sterol-binding protein seems to be involved rather in the cell division control than in the regulation of HMG-CoA reductase activity: indeed, the hydroxysterols able to inhibit thymidine [3H] incorporation into DNA are recognized by this protein whereas the hydroxysterols active on HMG-CoA reductase activity without affecting thymidine [3H] incorporation into DNA are not.

The activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase and the rate of sterol synthesis diminish in cultures with high cell density

The specific activity of HMG-CoA reductase, the major rate-limiting enzyme in the sterol biosynthetic pathway, declined linearly with increasing cell density in four different lines of mammalian cell cultures. As expected, this caused the rates of sterol synthesis from [14C]acetate to decline in a parallel manner. The decrease in reductase activity in the dense cultures was also correlated with decreased incorporation of [14C]acetate into fatty acids and [3H]thymidine into DNA. In contrast, the activities of two enzymes, NADH dehydrogenase and 5'-nucleotidase, which are not involved in lipid synthesis, were independent of changes in cell density. The simplest explanation for these data is tht HMG-CoA reductase and the synthesis of sterol and fatty acids are regulated in concordance with the rate of cell growth and proliferation.

[Inhibition by the high density lipoprotein HDL2 and HDL3 of DNA and sterol biosynthesis in human lymphocytes stimulated with concanavalin A]

Lipoproteins HDL2 and HDL3 inhibit DNA synthesis and sterol synthesis in human Con A-stimulated lymphocytes cultured in a medium supplemented with 20 per cent lipoprotein deficient serum. On the basis of the amount of proteins added, HDL2 is more efficient on DNA and sterol synthesis than HDL3 and less efficient than LDL. However, on the basis of the amount of cholesterol added, the inhibition of sterol synthesis induced by these three lipoproteins is not significantly different. At all concentrations of these three lipoproteins, the inhibition of sterol synthesis is higher than the inhibition of DNA synthesis.

Plasma membrane cholesterol regulates human lymphocyte cytotoxic function

Cell cholesterol is believed to be confined mainly to the plasma membrane. Treatment here of human peripheral blood lymphocytes with cholesterol-free and cholesterol-containing liposomes to effect, respectively, decreases or increases in cholesterol content measureable by chemical analysis, markedly altered effector functions of the cells. Depletion of cholesterol evoked inhibition of spontaneous and phytohemagglutinin-dependent lymphocyte cytotoxicity against allogeneic target cells. Opposite effects resulted from cholesterol enrichment, with PHA-dependent and antibody-dependent cytotoxicities increasing significantly. Treatment, instead, with the known inhibitor of cholesterol biosynthesis, 25-hydroxycholesterol, had suppressive effects like those resulting from lowering the cholesterol level physically by liposome treatment. Our data suggest that the plasma membrane cholesterol content of different categories of lymphocytes in man is both essential and regulatory for their cytotoxic function.

Biphasic rate of synthesis of glycoconjugates, phospholipids and DNA in concanavalin A-stimulated mouse thymocytes. Involvement of cortisone-sensitive and -resistant subpopulations

The time course of the rate of labeling of membrane components (phospholipids, glycolipids and glycoproteins) and DNA was followed in concanavalin A-stimulated CBA/J mouse thymocyte cultures. Two peaks of stimulated biosynthetic activity were noted, the first at the beginning of the cultivation and the second about 25 h later. Both early and late peaks of biosynthesis of membrane components were accompanied by blast transformation and were unimpeded by suppression of DNA synthesis by hydroxyurea. Cortisone-sensitive and cortisone-resistant thymocytes were prepared by selective agglutination of the cortisone-sensitive cells with peanut agglutinin (Reisner et al. Cell. Immunol. 1976. 25: 129) or cortisone treatment of the animals. Cortisone-sensitive cells responded early, while the cortisone-resistant population gave only the late response. The autoradiographic patterns from sodium dodecyl sulfate polyacrylamide gels of [3H]fucose or [3H]galactose-labeled glycoproteins from early and late labeling cells, and cortisone-resistant cells, were compared. Late-labeling and cortisone-resistant cells gave indistinguishable patterns, but differed significantly in their patterns from early-labeling cells. It is concluded that the two peaks of biosynthetic activity during the course of concanavalin A stimulation of thymocytes are caused by two different cell populations which require different times for maximal response and react independently of one another.

Mixed lymphocyte reactions in serum-free medium

Tissue culture medium supplemented with bovine serum albumin (BSA), rather than human serum, was found capable of supporting the proliferative response of cryopreserved human lymphocytes to allogeneic cells in a one-way mixed lymphocyte culture. Although total tritiated thymidine uptake was lower in BSA supplemented cultures than in serum supplemented cultures, stimulation indices were equivalent. Using 0.5% (w/v) BSA, peak tritiated thymidine uptake was observed on day 5 using 2 X 10(5) responders and 4X 10(5) stimulators per microtitre well. Calf skin gelatin was also found capable of substituting for serum, but gave lower levels of proliferation. Use of BSA offers distinct advantages for investigating the proliferative responses of human lymphocytes in serum-free media.

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.