Metabolism of dehydroepiandrosterone by rat hippocampal cells in culture: possible role of aromatization and 7-hydroxylation in neuroprotection.

The rate of metabolism of the multifunctional neurosteroid, dehydroepiandrosterone (DHEA), by embryonic rat hippocampal cells maintained in culture was compared to that of 4-androstenedione (AD), the immediate precursor of estrone (E1). The experiments were carried out to assess the relative contribution of DHEA, its 7-hydroxylated metabolites and estrogen on their reported effects on memory and neuroprotection. The 3H-labeled steroids of high specific radioactivity were incubated for 1, 8, 24 and 48 h and the putative metabolites extracted from the culture medium with acetone-ethyl acetate before separation by TLC for radioassay. [3H]DHEA (2.0 ng/5x10(5) cells) yielded primarily the 7alpha- and 7beta-hydroxylated steroids in an almost equal ratio under conditions that resembled those used by others to study the protection of neurons by hippocampal astrocytes against excitatory amino acid-induced toxicity. The rate of conversion of DHEA to AD, and particularly to E1, was much lower. With [3H]AD as substrate, significant aromatization to estrogen occurred only after 24 h when most of [3H]DHEA had already been converted to its 7-hydroxylated products and the hydroxylase and aromatase systems would no longer be competing for the same coenzyme (NADPH). The hippocampal cells were still viable after 48 h of incubation with the steroids and were able to oxidize estradiol (E2) to E1 and reduce E1 to E2 and AD to testosterone (T). It is suggested that 7alpha- and 7beta-OHDHEA, the main metabolites formed in the rat hippocampus, might be responsible for some of the functions previously ascribed to estrogens in the brain and the reasons for this proposal are discussed.