Pregnenolone fatty acid esters incorporated into lipoproteins: substrates in adrenal steroidogenesis

The presence of fatty acid ester derivatives of pregnenolone (PFA) has been recently reported in guinea pig plasma and adrenals. Moreover, it has equally been demonstrated that PFA is present in plasma lipoproteins, including high density lipoproteins (HDL) and low density lipoproteins (LDL). We report here the conversion of LDL- and HDL-incorporated [3H]PFA in guinea pig and bovine adrenocortical cells into nonconjugated steroids, thus indicating that PFA can be used as substrate for steroid production. When guinea pig glomerulosa-fasciculata (FG) cells were incubated for 48 h in the presence of either LDL-incorporated [3H]PFA or HDL-incorporated [3H]PFA, 66 +/- 2% and 47 +/- 1% of the added radioactivity were converted to nonconjugated steroids, respectively. The transformation of LDL-incorporated [3H]PFA into nonconjugated steroids was further increased in FG cells when 10 nmol/liter ACTH were present in the culture medium. This ACTH-induced liberation of nonconjugated steroids from LDL-incorporated [3H]PFA was strongly inhibited by a 10-fold excess of unlabeled LDL, while no competitive effect of unlabeled HDL was observed on the generation of tritiated nonconjugated steroids from HDL-incorporated [3H]PFA. Moreover, nonconjugated metabolites formed during incubation of FG cells with LDL-incorporated [3H]PFA and HDL-incorporated [3H]PFA resembled those observed when FG cells were incubated with tritiated pregnenolone. The formation of nonconjugated steroids was similarly observed when lipoprotein-incorporated [3H]PFA was incubated with bovine adrenocortical cells. Our data suggest that [3H]PFA incorporated into lipoproteins can be internalized into adrenal cells, after which the [3H]PFA is hydrolyzed into nonconjugated pregnenolone, which is readily used as substrate for adrenal steroidogenesis.