To understand the signaling and growth-inhibitory effects of
retinoids, we have examined the metabolism of [3H]
retinol in a number of
estrogen receptor-positive (ER+) and
estrogen receptor-negative (ER-) human
breast cancer cell lines. We have also assayed the metabolism of [3H]
retinol in normal human mammary epithelial cells. The ER+
breast cancer cell lines MCF-7 and T47D produce [3H]4-oxoretinol from [3H]
retinol; the production of [3H]4-oxoretinol is increased by initial culture in the presence of nonradiolabeled
retinoic acid (RA) or
N-(4-hydroxyphenyl)retinamide, indicating that these drugs enhance [3H]
retinol metabolism to [3H]4-oxoretinol. No metabolism of [3H]
retinol to [3H]RA in these ER+
tumor lines was detected. ER-
breast cancer lines MDA-MB-231, MDA-MB-468, and BT20 do not metabolize [3H]
retinol to [3H]4-oxoretinol. In the ER-
tumor lines, most of the [3H]
retinol remains unmetabolized during the 24-h culture period; MDA-MB-468 and BT20 metabolize some [3H]
retinol to [3H]RA. Unlike the majority of the
tumor lines, the normal human breast epithelial cell strains AD074 and MCF10A rapidly metabolize [3H]
retinol to [3H]
retinyl esters. No detectable [3H]RA is produced from [3H]
retinol in AD074 and MCF10A cells. Thus, the normal breast epithelial strains, the ER+
tumor lines and the ER-
tumor lines differ greatly in their pathways of [3H]
retinol metabolism. The levels of
cellular retinol binding protein-I mRNA expression are not correlated with the levels or types of various
retinol metabolites. Whereas the normal breast epithelial cells and the ER+
tumor lines are growth inhibited by RA,
N-(4-hydroxyphenyl)retinamide, and
4-oxoretinol, only the
4-oxoretinol is growth inhibitory in the ER-
tumor lines. The
cellular retinoic acid-binding protein II mRNA levels are not correlated with the growth inhibition by RA or
4-oxoretinol in the normal and
tumor lines.