Mechanism of defective sterol synthesis in human leukocytes

When human lymphocytes, granulocytes and monocyte-enriched cell preparations were incubated with [2-14C]acetate, only 10-36% of the radioactivity incorporated into the nonsaponifiable lipid fraction was present as digitonin-precipitable sterols. This percentage is considerably less than that observed for rat hepatocytes (95%) or human liver slices (68%). Even though a marked increase in the incorporation of labeled acetate into both nonsaponifiable lipids and digitonin-precipitable sterols resulted from stimulation of lymphocytes with concananvalin A or granulocytes by phagocytosis, the proportion of nonsaponifiable lipid radioactivity that was digitonin-precipitable remained low. These findings suggest that rate-limiting steps beyond the 3-hydroxy-3-methylglutaryl coenzyme A reductase reaction exist in the sterol synthetic pathway of leukocytes. Pulse-chase experiments demonstrated a precursor product relationship in leukocytes between lanosterol and cholesterol, but some squalene appears to be in a pool that is not further metabolized. A subcellular fraction prepared from mixed leukocytes was incapable of converting appreciable amounts of [3H]squalene to lanosterol or cholesterol, suggesting an enzyme deficiency in this segment of the sterol synthetic pathway. With isolated liver microsomes, 50% of the nonsaponifiable lipid radioactivity synthesized from [3H]squalene was recovered in cholesterol when the system contained added liver cytosol. By contrast, if the liver cytosol was replaced by leukocyte cytosol, 14-fold less radioactivity was incorporated into nonsaponifiable lipids, and only 17% of the radioactivity was recovered in cholesterol. When sterol-carrier protein 1, partially purified from rat liver, was added to leukocyte cytosol, [3H]squalene incorporation into lanosterol increased more than 3-fold. With the addition of both sterol-carrier protein 1 and sterol-carrier protein 2, cholesterol synthesis increased 2-fold. These results suggest that the low cholesterol synthetic activity in human leukocytes is due to a defect in one or more of the microsomal enzymes that operate between squalene and cholesterol, as well as to a deficiency of sterol-carrier proteins in leukocyte cytosol.