Source of cholesterol for the ocular lens, studied with U18666A: a cataract-producing inhibitor of lipid metabolism.

Cholesterol is the major lipid component of the ocular lens. The source of lens cholesterol during the first month of post-natal life of the rat was investigated by use of U18666A, a potent inhibitor of cholesterol biosynthesis which can also produce cataracts. Lenses from rats treated with U18666A at a level known to produce cataracts were of smaller size and accumulated total sterol at about one-half the rate of untreated controls. The lens content of phospholipid also lagged behind that of controls. Desmosterol accounted for 50-75% of the total sterol in lens of all treated rats, this paralleled the percent content of desmosterol in liver and serum of these animals. Lenses taken from 20-day-old treated rats and incubated in vitro synthesized little digitonide-precipitable sterol (DPS) from 3H2O as compared to lenses from age-matched controls. The steady state concentration of U18666A in lens was found to be 1-2 X 10(-6) M; this concentration almost completely blocked sterol synthesis in vitro when added to normal lenses. Although U18666A inhibited lens synthesis in vitro of phospholipids from 1,3-[3H]-glycerol and 32Pi, it did so only at levels much higher than those encountered in vivo. Thus, the changes seen in lens phospholipids appear secondary to the decreases in sterols. Since lenses of treated rats synthesized little if any sterol but accumulated sterol at one-half the rate of control lens, we conclude that during early post-natal development of the rat the ocular lens possesses the potential to satisfy about one-half of its sterol requirements from sources outside of the lens, perhaps from lipoproteins in aqueous humor. This conclusion is consistent with our earlier work which indicated that the rat's lens can furnish 50-100% of its total cholesterol by synthesis de novo during the first two weeks of life and less thereafter. The relationship of the inhibition of sterol synthesis to production of the U18666A-induced cataract is discussed.