Differentiation of myeloid cell lines correlates with a selective expression of RIZ protein

Background: The retinoblastoma-interacting zinc-finger gene RIZ is expresse d in two forms (RIZ1 and RIZ2) that differ for the presence near the N-term inus of RIZ1 of a conserved domain, defined PR (PRDI-BF1-RIZ homology), hom ologous to a similar domain present in other proteins recognized as tumor s uppressor gene products. The RIZ1 form is usually absent or expressed at lo w levels in tumor cells, whereas RIZ2 is frequently expressed. We investiga ted a possible involvement of RIZ1 in differentiation control using a myelo id cell maturation model that is easily modulated by retinoids and other ag ents. Materials and Methods: HL60 or NB4 cell lines or patients' leukemic promyel ocytes were treated with all-trans-retinoic acid or other agents to induce differentiation. RIZ gene expression was determined with reverse transcript ase polymerase chain reaction (RT-PCR) and RNase protection assay. Immunocy tochemistry was performed to assess variation of the intracellular distribu tion of RIZ protein on all-traps-retinoic acid treatment. Forced expression of RIZ1 protein was obtained with a recombinant adenovirus containing RIZ1 cDNA. Results: Treatment with retinoic acid induced a selective expression of RIZ 1 in HL60 cell line. Retinoic acid effect was maximal at 7 days and correla ted to the granulocytic differentiation of cells. A similar effect was obta ined in retinoic acid-sensitive NB4 cell line or in patients' leukemic prom yelocytes, but not in the retinoic acid-resistant cell line NB4.007/6 or in the U937 cell line. Selective expression of RIZ1 was also induced by 12-O- tetradecanoyl-phorbol-13-acetate in the U937 and HL60 cell lines and by 1,2 5-dihydroxyvitamin D-3 only in HL60 cells. In HL60 cells, RIZ1 was also ind uced by activation of a retinoid a receptor-independent maturation pathway based on retinoid X receptor agonist and protein kinase A synergism. In add ition, retinoic acid produced a redistribution of the antigen within the nu cleus in these cells. Forced expression of RIZ1 protein induced growth arre st and death of HL60 cells. Conclusions: The correlation between the selective expression of RIZ1 induc ed by retinoic acid, 12-O-tetradecanoyl-phorbol-13-acetate, or 1,25-dihydro xyvitamin D-3 and differentiation suggested that RIZ protein was involved i n myeloid cell differentiation induced by these agents.