Inhibition of IL-2 secretion and IL-2 receptor appearance of activated lymphocytes pretreated with hydroxylated sterols

Ligand-binding properties and catalytic activity of the purified human 24-hydroxycholesterol 7α-hydroxylase, CYP39A1

Oxysterols are derivatives of cholesterol and biologically active molecules that are involved in a number of functions, including cholesterol homeostasis, immune response, embryogenic development and pathophysiology of neurodegenerative diseases. Enzymes catalyzing their synthesis and metabolism are of particular interest as potential or evaluated drug targets. Here we report for the first time biochemical analysis of purified human oxysterol 7α-hydroxylase selective for 24-hydroxycholesterol. Binding analyses indicated a tight binding of the oxysterols and estrone. Ligand screening revealed that CYP39A1 binds with high affinity antifungal drugs and prostate cancer drug galeterone (TOK-001). Site-directed mutagenesis of conserved Asn residue in the active site revealed its crucial role for protein folding and heme incorporation. Developed protocol for expression and purification enables further investigation of this hepatic enzyme as off-target in development of specific drugs targeting cytochrome P450 enzymes.

Steroid and sterol 7-hydroxylation: ancient pathways

B-ring hydroxylation is a major metabolic pathway for cholesterols and some steroids. In liver, 7 alpha-hydroxylation of cholesterols, mediated by CYP7A and CYP39A1, is the rate-limiting step of bile acid synthesis and metabolic elimination. In brain and other tissues, both sterols and some steroids including dehydroepiandrosterone (DHEA) are prominently 7 alpha-hydroxylated by CYP7B. The function of extra-hepatic steroid and sterol 7-hydroxylation is unknown. Nevertheless, 7-oxygenated cholesterols are potent regulators of cell proliferation and apoptosis; 7-oxygenated derivatives of DHEA, pregnenolone, and androstenediol can have major effects in the brain and in the immune system. The receptor targets involved remain obscure. It is argued that B-ring modification predated steroid evolution: non-enzymatic oxidation of membrane sterols primarily results in 7-oxygenation. Such molecules may have provided early growth and stress signals; a relic may be found in hydroxylation at the symmetrical 11-position of glucocorticoids. Early receptor targets probably included intracellular sterol sites, some modern steroids may continue to act at these targets. 7-Hydroxylation of DHEA may reflect conservation of an early signaling pathway.

Cholestan-3beta,5alpha,6beta-triol, but not 7-ketocholesterol, suppresses taurocholate-induced mucin secretion by cultured dog gallbladder epithelial cells

In order to investigate oxysterol-mediated effects on the biliary system, we studied the effects of cholestan-3beta,5alpha,6beta-triol (TriolC) and 7-ketocholesterol (7KC) on gallbladder epithelial cells. We compared their cell proliferation effects in cultured dog gallbladder epithelial cells (DGBE) to their effects in cultured human pulmonary artery endothelial cells (HPAE). Oxysterols inhibited cell proliferation in a dose-dependent fashion. Oxysterols inhibited cell growth to 50% of control at a higher dose for DGBE cells than for HPAE cells. TriolC was more cytotoxic than 7KC. We also investigated the effect of oxysterols on bile salt-induced mucin secretion by DGBE cells. TriolC suppressed mucin secretion by DGBE cells, whereas 7KC did not. These findings support the hypothesis that biliary oxysterols affect gallbladder mucosal function.

Effects of oxysterols upon macrophage and lymphocyte functions in vitro

Oxygenated derivatives of cholesterol (oxysterols), found in high concentrations in atherosclerotic lesions, are potent immunosuppressive agents inhibiting T-cell responses to different stimuli. The action of oxysterols on macrophage functions and macrophage-lymphocyte interaction has been poorly investigated. In this work, the effects of 25-hydroxycholesterol (25-OHCh) and 7-ketocholesterol (7-KCh) upon some functions of murine peritoneal macrophage (PM), such as generation of reactive oxygen intermediates (ROI), secretion of neopterin and interleukin-1 (IL-1)-like activity, Fc-receptor (FcR) activity, and murine and human lymphocyte functions, participating in lymphocyte-macrophage interactions, such as macrophage-activating factor (MAF) and Ia-inducing factor (IaIF) secretion, were studied in vitro. 7-KCh in concentration of 5 micrograms/mL culture medium only, but not 25-OHCh, significantly inhibited ROI generation by zymosan-stimulated PM. Pretreatment of PM for 22 h with 25-OHCh and 7-KCh led to the decrease of IL-1-like activity secretion. 25-OHCh and 7-KCh inhibited both FcR-dependent binding and phagocytosis of sheep red blood cells (SRBC). Oxysterols did not change both spontaneous and lipopolysaccharide-stimulated secretion of neopterin by PM. 25-OHCh dose-dependently and more efficiently than 7-KCh inhibited murine splenocyte secretion of MAF, which activity was determined by the ability of splenocyte-conditioned medium to stimulate ROI generation in PM. Both 25-OHCh and 7-KCh inhibited significantly proliferative activity of human mixed lymphocyte culture (MLC), as well as lymphocyte secretion of IaIF, which stimulates the expression of HLA antigens in cultured human monocytes. Purified Ch did not alter these parameters. These data showed, that some inflammatory functions of macrophages and lymphocytes may be modified by such environmental conditions as the presence of oxysterols.

Oxysterol-induced apoptosis in human monocytic cell lines

Oxysterols constitute a large family of natural compounds, endowed with various biological activities including cholesterol regulation, immunosuppression and antitumoral potency. In the present study, we examine and compare the cytotoxic effects of two representative members of this family: 7 beta-hydroxycholesterol (7 beta-OH) and 25-hydroxycholesterol (25-OH), in two human monocytic cell lines, U-937 and HL-60. In both cell lines 7 beta-OH at 30 mu M induces cell death by apoptosis within the first hours of treatment. Under the same conditions and in contrast with results previously obtained with lymphoma cells, 25-OH is cytostatic only. It is interesting to note that the simultaneous treatment of U-937 cells by equimolar concentrations of 7 beta-OH and 25-OH leads to a considerably decreased induction of apoptosis. Such an effect is not observed with HL-60 cells. Taken together, these results indicate for the first time that: 1) oxysterols hydroxylated on the sterol nucleus are also able to induce apoptosis, 2) apoptosis can be induced by these substances in cells belonging to the myeloid lineage and 3) as far as apoptosis is concerned, a combined treatment with 7 beta-OH and 25-OH can lead to opposite effects depending on the cell type.

Circadian variations of human lymphocytes are not responsible for contradictory or variable results in studies of IL-2 production

Efforts to correlate various immunological disorders with alterations in IL-2 production in vitro have led to highly variable and often contradictory results. One possible contributing factor to these results could be the existence of circadian variations in the capacity of lymphocytes to produce IL-2 in culture. Although there were reasons to suspect that such variations might exist, the present study revealed no significant variations in 6 of 10 normal subjects in three experiments or in the mean values for all subjects at each of three sample acquisition times. In addition, no subject exhibited circadian variations in all three experiments. Since it was possible that failure to detect variations was due to selection of an inappropriate mitogen or inappropriate culture conditions, 2 mitogens and various culture conditions were used. Circadian variation was noted for only one subject out of eight under only one set of conditions. This suggests that circadian variations are not responsible for variable or contradictory results which have been reported. Therefore, the recently reported circadian variations in T helper to T suppressor ratios do not seem to significantly affect mitogen-induced IL-2 production.

Structurally distinct plasma membrane regions give rise to extracellular membrane vesicles in normal and transformed lymphocytes

Shedding of extracellular membranes from the cell surface may be one of the means through which cells communicate with one another. In an attempt to elucidate whether cell surface exfoliation is a directed or random process, we investigated the membrane lipid and protein composition and membrane lipid order of shed extracellular membranes and of plasma membranes from which they arose in normal circulating lymphocytes and in the B-lymphoblastoid cell lines Raji, WI HF2 729 and the T-lymphoblastoid cell line Jurkat. Extracellular membranes derived from transformed cell lines were more rigid as assessed by steady state polarization of 1,6-diphenylhexatriene (DPH) and were highly enriched in cholesterol when compared with the corresponding plasma membrane. The extracellular membranes from normal lymphocytes, on the other hand, were more fluid and contained more polyunsaturated acyl chains than did the plasma membranes from these cells. Our results suggest that extracellular membranes are shed from specialized regions of the lymphocyte plasma membrane and that membrane exfoliation is likely to be a directed event.

Studies on the immunosuppressive properties of 7,25-dihydroxycholesterol--I. Reduction of interleukin production by treated lymphocytes

Several oxysterols are powerful immunosuppressors. Among them 7,25-dihydroxycholesterol (7,25-OHC) seems to be particularly interesting. The effect of this synthetic sterol on murine lymphocytes, stimulated by Con A, or by allogeneic or TNP-modified syngeneic lymphocytes, was investigated. The results showed that the induction of cytotoxic T-lymphocytes (CTL) was strongly inhibited by a treatment with non-toxic concentrations of this sterol. A marked reduction of interleukin 2 (IL-2) production by alloantigen stimulated lymphocytes occurred at the same concentrations. We also showed that receptors to IL-2 did not appear on lymphocytes stimulated by Con A, when 7,25-OHC was added at the beginning of the culture. The addition of exogenous IL-2 to the culture was found to only slightly reverse the inhibition caused by 7,25-OHC. Taken together, these results indicate that an impairment at the early steps of lymphocyte activation, involving IL-2 secretion, could be responsible, at least in part for the various immunosuppressive effects of 7,25-OHC.