Metabolism of lipoidal derivatives of estradiol-17β in human mammary cancer tissue and cell lines

Abstract

Estradiol-17β (E 2) is converted exclusively to intracellular metabolites, termed lipoidal estrogens [long chain fatty acid 17β-esters (E 2-L)], by human mammary cancer tissue and cell lines. In order to further evaluate the biological role of lipoidal estrogens, rates of saturation of the estrogen receptor (ER) along with formation of [ 3H]E 2-L have been measured in human mammary cancer cells exposed to 5 nM [ 3H]E 2. Extensive specific binding of E 2 to ER in MCF-7 cells (≈ 37%) and ZR-75-1 cells (≈ 62%) occurred before appreciable synthesis of E 2-L was evident and the maximum level of E 2-L attained was only 3–9% of the E 2 specifically bound to ER. In these ER positive cell lines, and in the ER negative cell line MDA-MB-231, an initial rise in the rate of E 2-L formation was followed by a decrease at ≈6 min and re-establishment of a new rate, indicating turnover of the E 2-L fraction by esterification-de-esterification reactions. This data does not support the concept that E 2-L acts in the transport of E 2 to nuclear receptors, but rather that liberation of E 2 from E 2-L could serve to maintain occupancy of ER necessary for initiation of DNA synthesis. The esterase, as studied in pooled human mammary cancer tissue, was found to hydrolyse E 2-17β-long chain fatty acid esters at different rates—the enzyme being less active towards E 2-17β-stearate compared to E 2-17β-oleate,-linoleate and -linolenate. Esterase activity was significantly higher in MDA-MB-231 cells compared to MCF-7 cells. Treatment of MCF-7 cells with E 2 did not alter the specific activity of the esterase towards E 2-17β-oleate as substrate. Similarly, addition of dibutyryl c-AMP to ZR-75-1 cell cultures was without effect on E 2-L, both during the time when E 2-L was accumulating, or during a subsequent phase when E 2-L was decreasing following transfer to medium lacking E 2. Calcitonin, which increases endogenous c-AMP in MCF-7 cells, had no effect on E 2-L in this latter phase using this cell line. Thus, no evidence could be provided that the esterase was under E 2 control, or control by polypeptide hormones which utilize c-AMP as a second messenger.