9-cis-retinoic acid induces neuronal differentiation of retinoic acid-nonresponsive embryonal carcinoma cells.

P19 mouse embryonal carcinoma cells differentiate into neurons and glial cells when treated with retinoic acid. In contrast, a subline of the P19 cells, RAC65, is known to show little sign of differentiation with the treatment. We treated the two embryonal carcinoma (EC) cell lines with 9-cis-retinoic acid and investigated its neuron-inducing activity. In P19 cells, 9-cis-retinoic acid showed an activity equal to that of all-trans-retinoic acid. However in RAC65 cells, 9-cis-retinoic acid induced neurons 10-fold more effectively than all-trans-retinoic acid. The order in which various retinoids appeared in P19 cells corresponded to that of retinoic acid receptors, and the order in RAC65 cells to that of retinoid X receptors (RXRs). Furthermore we found that the down-regulation of retinoid X receptor-gamma mRNA expression was associated with neuronal differentiation in both embryonal carcinoma cell lines. In addition, a synthetic RXR-selective retinoid induced neurons from both EC cells. Our findings support an intriguing possibility that the 9-cis-retinoic acid/retinoid X receptor system may play an important role in neural differentiation.