Sequential activation of HOX2 homeobox genes by retinoic acid in human embryonal carcinoma cells.

RETINOIC acid has been implicated as a natural morphogen in chicken1-4 and frog5 embryogenesis, and is presumed to act through the gene regulatory activity of a family of nuclear receptors6-10. Homeobox genes, which specify positional information in Drosophila and possibly in vertebrate embryogenesis11-21, are among the candidate responsive genes4,5,14. We previously reported that retinoic acid specifically induces human homeobox gene (HOX) expression in the embryonal carcinoma cell line NT2/D1 (ref. 22). We now show that the nine genes of the HOX2 cluster are differentially activated in NT2/D1 cells exposed to retinoic acid concentrations ranging from 10-8 to 10-5 M. Genes located in the 3' half of the cluster are induced at peak levels by 10-8 M retinoic acid, whereas a concentration of 10-6 to 10-5 M is required to fully activate 5' genes. At both high and low retinoic acid concentrations, HOX2 genes are sequentially activated in embryonal carcinoma cells in the 3' to 5' direction.