Retinoic acid and alcohol/retinol dehydrogenase in the mouse adrenal gland: a potential endocrine source of retinoic acid during development.

Retinoid signaling requires the conversion of retinol to retinoic acid by a two-step process, the first of which can be catalyzed in vitro by class I and class IV alcohol dehydrogenases (ADH). These enzymes may participate in local retinoic acid synthesis in some target tissues, although other studies suggest retinoic acid may also be supplied to tissues via the bloodstream, much like an endocrine hormone. Here we have analyzed the expression of these two ADHs as well as retinoic acid production in the adrenal gland, an organ known to be an endocrine source of other hormones. In situ hybridization revealed high levels of both class I and class IV ADH messenger RNAs in adrenal glands of 16.5-day mouse embryos and adults. Class I ADH protein was immunohistochemically detected in embryonic and adult adrenal glands, the latter primarily in the zona fasiculata of the cortex. Abundant class IV ADH protein was detected in the embryonic adrenal as well as in the zona glomerulosa and zona fasiculata of the adult adrenal cortex. Interestingly, class IV ADH protein was found in only a subset of adult cortical cells arranged in radial columns, thus providing further evidence for centripetal cell migration during adrenocortical differentiation. Using a retinoic acid bioassay, adrenal glands from 16.5 day embryos were found to have significantly higher levels of retinoic acid than embryonic liver. The adult adrenal was found to have approximately 15.5 pmol/g of retinoic acid, whereas the adult liver had 24.8 pmol/g, and brain, heart, and spleen each had less than 1.0 pmol/g. Because previous findings indicate that the adrenal gland is not a retinoid target tissue, our detection of both alcohol/retinol dehydrogenases and significant amounts of retinoic acid in this organ suggests that it functions as a potential endocrine source of this hormone during mouse development.