Sterol metabolism and lymphocyte function: Inhibition of endogenous sterol biosynthesis does not prevent mitogen-induced human T lymphocyte activation

The relationship between mitogen-induced human T lymphocyte activation and endogenous sterol biosynthesis was examined. The rate of sterol synthesis was determined from the incorporation of [1-¹⁴C]acetate into digitonin-precipitable sterols, and lymphocyte activation was assessed by the incorporation of [³H]uridine, [³H]leucine, and [³H]thymidine. Mitogenic stimulation of human peripheral blood mononuclear cells (PBM) led to a marked increase in the rate of sterol synthesis by T lymphocytes. The oxygenated sterols, 7-ketocholesterol (7-KC) and 25-hydroxycholesterol (25-HC), and the fungal derivative ML-236B significantly inhibited sterol synthesis by human PBM. These inhibitors also markedly decreased mitogen-stimulated [³H]uridine, [³H]leucine, and [³H]thymidine incorporation when these responses were assayed after a 72- or 96-hr incubation. At the concentration employed, the effect of these agents on lymphocyte responsiveness resulted specifically from inhibition of sterol synthesis, since it was prevented by supplementation of the cultures with mevalonate, the product of the inhibited enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Higher concentrations of 7-KC or 25-HC caused an inhibition of responsiveness that was not reserved by mevalonate and thus did not result from a specific action on sterol biosynthesis. In order to determine whether inhibitors of sterol synthesis affected early events in lymphocyte activation, kinetic studies were carried out. Mitogen-induced [³H]uridine and [³H]leucine incorporation measured after a 24-hr incubation were unaffected by ML-236B and mevalonate-reversible concentrations of 7-KC. These data indicate that oxygenated sterols and ML-236B do not inhibit the early events in lymphocyte activation but rather alter events occurring afterward. The results suggest that the decrease in lymphocyte responses observed later in culture is a secondary phenomenon resulting from inhibition of the capacity of the cell to synthesize adequate membrane for enlargement and subsequent division.