Final Activity Report Summary - TELOMERASE REPRESSOR (Functional assessment of newly identified transcription factors whose expression strongly correlates with telomerase repression) Retinoids (vitamin A derivatives) have anti-cancerous and chemopreventive properties in diverse tumour tissues. These agents regulate transcription of their specific target genes through binding with their cognate receptors that act as ligand-activated transcription factors. By using the Acute Promyelocytic Leukaemia (APL) cell line NB4, in which retinoids are able to induce two fundamental processes, telomerase repression and terminal differentiation, we have developed a research project in order to identify new target genes of retinoic acid involved in these two processes. The two main objectives of our project were to identify new genes potentially involved in: - telomerase repression. Indeed, the knowledge of factors and mechanisms controlling transcription of hTERT (human telomerase reverse transcriptase) still remain to be elucidated during physiological processes in order to understand the telomerase re-activation process, a key step during human tumourogenesis, and could bring to light new molecular targets for anti-cancer therapy. - differentiation. Indeed, despite the relatively good knowledge of expressed retinoid receptors in APL cells (PML-RARa, RXRalfa, RARalfa), only very few transcription factors have so far been shown to be required for retinoid-induced-differentiation of promyelocytic cells. The knowledge of these early-induced-genes is essential to better understand the differentiation process in myeloid tissue and could also be extended to other differentiation models. By using a microarray approach, we have identified nine gene candidates early induced by retinoids in NB4 cells and encoding known or putative transcription factors. One of these genes is a novel retinoid-responsive gene encoding a nuclear factor that we named RINF (Retinoid-Inducible Nuclear Factor) and that contains a CXXC-type zinc finger motif. RINF expression correlates with retinoid-induced differentiation of leukemic cells and with cytokine-induced myelopoiesis of normal bone marrow progenitors (CD34+). Importantly, RINF invalidation by RNA interference annihilates the differentiating action of retinoids in NB4 cells and delays cytokine-induced granulocytic differentiation of normal CD34+ myeloid progenitors, thus suggesting an important role for RINF in myeloid differentiation. Interestingly, Rinf localizes to 5q31, a small region often deleted in myeloid leukaemia and suspected to harbour one or several tumour suppressor gene. If Rinf acts as a tumour suppressor gene, its expression analysis and/or sequencing in tumoural and healthy biopsies could constitute new tools with potential applications in clinical Oncology (diagnostics, prognostic and therapeutics).
