ZR-75-1 breast cancer cells generate nonconjugated steroids from low density lipoprotein-incorporated lipoidal dehydroepiandrosterone

Abstract

Fatty acid esters of dehydroepiandrosterone (DHEA-FA) are present in the circulation, although no physiological function has yet been attributed to these metabolites. They are formed directly in serum and are predominantly localized in association with lipoproteins. The objective of this study was to determine the capacity of these lipoprotein-incorporated DHEA metabolites to generate nonconjugated steroids after incubation with cells in culture. A method for studying DHEA-FA using a radiolabeling technique that marks human low density lipoproteins (LDL) with tritiated DHEA-FA was elaborated. Analysis of the fatty acid composition of tritiated DHEA-FA-labeled LDL ([3H] DHEA-FA-LDL) indicated the prevalence of DHEA-linoleate/palmitoleate and DHEA-oleate. Incubation of [3H]DHEA-FA-LDL with ZR-75-1 breast cancer cells produced a time-dependent increase in labeled nonconjugated steroids in the cell culture medium, whereas the levels of tritiated DHEA-FA decreased. Lipoidal radioactivity in cells increased with time, but nonconjugated radioactivity associated with the cells showed no such increase. HPLC analysis of the culture medium indicated the presence of DHEA and androst-5-ene-3 beta,17 beta-diol. The endogenous levels of lipoidal DHEA were also determined in human plasma and its lipoprotein components to reveal that these metabolites circulate naturally in the range of 6.5 +/- 0.4 nM. Approximately 90% of this concentration was associated with the lipoprotein components, namely among the LDL and high density lipoprotein fractions. These results suggest that lipoidal DHEA may indeed act as a substrate for potent steroid formation after their entry into steroid target cells.