| Murine F9 embryonal carcinoma stem cells grown in culture resemble the blastocyst inner cell mass (ICM) cells in early mouse embryogenesis. Retinoic acid (RA), a derivative of vitamin A (retinol), induces F9 cells to differentiate into non-cancerous cells with characteristics of extraembryonic endoderm, which develops in the embryo from the ICM. Because of this response, F9 cells are a model system for studying differentiation in the early embryo and the mechanisms of action of RA.;Rex-1 message has been detected in a limited range of mouse cells and tissues: undifferentiated embryonic stem (ES) cells, blastocyst stage embryos, trophectoderm, and meiotic male germ cells. This expression pattern suggests that Rex-1 is involved in trophoblast development and spermatogenesis, and is a useful marker for studies of early cell fate determination in the ICM.;The Rex-1 (standard nomenclature = Zfp-42) gene structure was characterized, and the gene mapped to mouse Chromosome 8. An upstream region required for Rex-1 promoter activity in F9 stem cells contains an octamer motif (ATTTGCAT), a binding site for octamer transcription factors of the POU domain family of DNA binding proteins. Rex-1 reporter plasmids with the octamer site exhibited reduced expression in F9 cells treated with RA. Two base changes introduced into the octamer site reduced activity of the reporter plasmid in stem cells, and the altered construct did not demonstrate a substantial RA-associated decrease in activity.;Thus, the octamer motif is a regulatory element required for activity of the Rex-1 promoter in F9 stem cells, and it contributes to the negative regulation by RA of Rex-1 gene transcription. As an initial confirmation of in vivo function, a larger Rex-1 promoter fragment, also containing the octamer site, promoted expression of the bacterial LacZ gene in mouse morula stage embryos.;F9 stem cells express mRNA for Rex-1, a gene whose deduced protein product contains a potential acidic domain and four repeats of the zinc finger DNA binding motif, features which suggest that Rex-1 encodes a transcriptional regulator. The transcription rate for the Rex-1 gene decreases when F9 cells are exposed to RA, causing reduced levels of Rex-1 mRNA in differentiated cells. |
