Metabolism of high density lipoproteins reconstituted with [3H]cholesteryl ester and [14C]cholesterol in the rat, with special reference to the ovary.

In order to study the metabolism of high density lipoprotein (HDL)-carried sterol in the rat, human HDL was reconstituted with [14C]cholesterol and [3H]cholesteryl ester. After iv injection into immature PMSG-human CG primed rats pretreated with 4-aminopyrazolopyrimidine and aminoglutethimide, there was time-dependent accumulation of 3H and 14C in various organs which reached a maximum by 15-90 min. On a milligram wet weight basis, uptake of 3H and 14C was greatest in the adrenals, next in ovaries, followed by the liver, with little uptake by kidneys and spleen. On an organ basis, accumulation was greatest by the liver. At 15-45 min post injection, 60% of the 3H in the ovary was in free sterol, indicating hydrolysis of the accumulated cholesteryl esters, whereas 95% of the 3H in serum remained in sterol esters associated with HDL. Coadministration of excess unlabeled HDL, but not human low density lipoprotein, reduced accumulation of radioactivity by the ovaries and adrenals by 60%, indicating a specific and saturable uptake process. Granulosa cells cultured in lipoprotein-deficient medium with reconstituted HDL formed 3H- and 14C-labeled 20 alpha-hydroxypregn-4-en-3-one. Over a 24-h period, utilization of both [14C]cholesterol and [3H]cholesteryl ester was linear, but rates of utilization of the two sterol moieties were not parallel. There was preferential uptake and utilization of free sterol. A dose-response study demonstrated a Michaelis-Menten constant (Km) of 40-60 micrograms sterol/ml for both free and esterified cholesterol. Lysosomotropic agents (chloroquine and NH4Cl) had no effect on utilization of either free or esterified cholesterol for steroidogenesis but reduced degradation of 125I-labeled low density lipoprotein apoprotein. These findings lend further support to the concept of a distinct HDL pathway in steroidogenic cells of the rat, which involves 1) preferential uptake and utilization of free cholesterol from HDL and 2) does not require lysosomal activity.