RETINOIC ACID-MEDIATED ACTIVATION OF HNF-3-ALPHA DURING EC STEM-CELL DIFFERENTIATION

We present evidence demonstrating that the liver-enriched transcriptio n factor HNF-3 alpha is activated upon retinoic acid-induced different iation of mouse F9 embryonal carcinoma cells. We have detected increas es in the DNA binding activity and mRNA level of HNF-3 alpha. Both are reflections of the actual activation mechanism at the level of transc riptional initiation, which we showed with the help of HNF-3 alpha pro moter constructs. Time course studies clearly show that HNF-3 alpha ac tivation is a transient event. Employing Northern blots, HNF-3 alpha m RNA can be detected between 16 and 24 hours post-differentiation, reac hes its zenith at approximately 1 day, and then declines to virtually undetectable levels. F9 cells can give rise to three distinct differen tiated cell types; visceral endoderm, parietal endoderm, and primitive endoderm. We have clearly shown that HNF-3 alpha stimulation occurs u pon primitive endoderm formation. In addition, the transcription facto r is also activated during the induction of cell lineages that give ri se to parietal and visceral endoderm. HNF-3 alpha stimulation upon vis ceral endoderm differentiation is accompanied by the activation of HNF -3 target genes such as transthyretin, suggesting that HNF-3 alpha is involved in the developmental activation of this gene. In contrast, HN F-3 alpha target genes in parietal and primitive endoderm have yet to be identified. However, the stimulation of HNF-3 alpha during primitiv e endoderm formation, which is an extremely early event during murine embryogenesis, points towards a role for the factor in crucial determi nation processes that occur early during development.