Retinoic acid fails to induce expression of Hox genes in differentiation-defective murine embryonal carcinoma cells carrying a mutant gene for alpha retinoic acid receptor.

Murine P19 embryonal carcinoma cells irreversibly differentiate into neuroectoderm following brief exposure to retinoic acid (RA). We compared the expression of RA-responsive genes in P19 cells and in a mutant cell line from mouse, RAC65, which fails to differentiate in RA. Some RA-responsive genes were normally regulated by RA in RAC65 cells while others were not. Amongst the latter were Oct-3 and PEA-3, whose transcripts rapidly disappeared following RA treatment of P19 cells but which were lost only slowly and incompletely from RAC65 cells. Expression of the Hox 1.6, 1.4, and 1.3 transcripts was induced by RA in P19 cells but not in RAC65 cells. Nuclear run-on and transfection assays indicated that transcription of the Hox 1.6 gene was regulated by a previously identified [26] DNA sequence located 3' of the Hox 1.6 gene, probably through interaction with the alpha RA receptor (RAR alpha). Results of nuclear run-on analysis suggested that expression of the Hox 1.6 gene may also be regulated post-transcriptionally. Constitutive expression of Hox 1.6 from a heterologous promoter did not induce differentiation indicating that expression of this gene is insufficient to initiate the cascade of events that culminates in cell differentiation.