IRES-dependent translational control of Cbfa1/Runx2 expression

The P1 and P2 promoters of the Cbfa1/Runx2 gene produce Type I and II mRNAs with distinct complex 5'-untranslated regions, respectively designated UTR1 and UTR2. To evaluate whether the 5'-UTRs impart different translational efficiencies to the two isoforms, we created SV40 promoter-UTR-luciferase reporter (luc) constructs in which the translational potential of the 5'-UTR regions was assessed indirectly by measurement of luciferase activity in transfected cell lines in vitro. In MC3T3-E1 pre-osteoblasts, UTR2 was translated approximately twice as efficiently as the splice variants of UTR1, whereas translation of unspliced UTR1 was repressed. To determine if the UTRs conferred internal ribosome entry site (IRES)-dependent translation, we tested bicistronic SV40 promoter-Rluc-UTR-Fluc constructs in which Fluc is expressed only if the intercistronic UTR permits IRES-mediated translation. Transfection of bicistronic constructs into MC3T3-E1 osteoblasts demonstrated that both UTR2 and the spliced forms of UTR1 possess IRES activity. Similar to other cellular IRESs, activity increased with genotoxic stress induced by mitomycin C. In addition, we observed an osteoblastic maturation-dependent increase in IRES-mediated translation of both UTR2 and the spliced forms of UTR1. These findings suggest that Cbfa1 UTRs have IRES-dependent translational activities that may permit continued Cbfa1 expression under conditions that are not optimal for cap-dependent translation.