Retinoic acid regulates ornithine decarboxylase gene expression at the transcriptional level

Retinoic acid (RA) is important for normal mammalian development and growth. Ornithine decarboxylase (ODC) is the first and rate-limiting enzyme in the biosynthesis of the polyamines, and we have previously shown that ODC mRNA levels are suppressed by RA in human skin cells. Using HeLa cells, we now show that treatment with 0.5 microM RA for 24 h suppresses endogenous ODC mRNA levels and the expression of a transfected ODC/chloramphenicol acetyltransferase plasmid (Kpn-ODCCAT), containing sequences from -1450 to +810 of the human ODC gene. Co-transfection with either the alpha-RA receptor (alpha-RAR) or a chimeric alpha-RA/oestrogen receptor (alpha-RAER) followed by treatment with the cognate hormone suppresses expression of Kpn-ODCCAT and Not-ODCCAT, which contains sequences from -250 to +514. Liganded alpha-RAR suppresses the activity of Kpn-ODCCAT more markedly than does liganded alpha-RAER (98% and 80% suppression, respectively), whereas both receptors have very similar effects on Not-ODCCAT expression (73% and 67% suppression, respectively). The unliganded alpha-RAR suppresses Kpn-ODCCAT by 76%, whereas unliganded alpha-RAER has no significant effect. These data show that RA regulates ODC-gene expression at the transcriptional level, and that alpha-RAR, but not alpha-RAER, can confer full hormonal responsiveness. This suggests that the activating function present in the alpha-RAR ligand-binding domain is required for full transcriptional regulation.