Retinoic Acid Signaling in P19 Stem Cell Differentiation.

Retinoic acid (RA), especially all-trans retinoic acid is the most potent natural metabolite of vitamin A. RA is involved in a variety of biological functions including embryogenesis, cell differentiation and apoptosis. RA acts through its nuclear receptors to induce transcription of specific target genes. Mouse P19 embryonic carcinoma (EC) stem cells (ES) are one of the most studied in vitro systems for RA-induced differentiation. P19 ES cells can differentiate to endodermal-like, mesodermal-like, and neuronal-like phenotypes in response to specific morphogens including RA and dimethyl sulfoxide (DMSO). At low concentrations, RA directs P19 ES cells to differentiate into cells displaying an endodermal phenotype, whereas at higher concentrations it induces differentiation to neuroectoderm. In the past, many RA---regulated genes have been discovered in EC and ES cells and efforts are ongoing to elucidate the exact mechanisms of RA-induced ES cell differentiation and apoptosis. In the RA-triggered differentiation process of the P19 ES cells, several proteins belonging to different families participate, some being obligatory while others, dispensable. Revealing the mechanisms behind RA-induced effects on ES cells has a bearing on understanding how cells proliferate, differentiate and undergo apoptosis that can provide greater insight into cancer biology and therapy. In addition to summarizing the reports on gene/protein targets of RA in stem cells, the signaling pathways driven by some of the specific class of proteins in the presence or absence of RA in P19 ES cell differentiation, especially to an endodermal phenotype, are the focus of this review.