Large scale analyses of transcriptome improve comprehension of complex processes such as differentiation or cell proliferation. SAGE libraries construction of the AML model U937 allowed the identification of new markers of myelomonocytic differentiation induced by retinoids and vitamin D3 (VD3) (Piquemal et al., 2002). Those molecules act through ligand-dependent transcription factors of the nuclear receptors family: RAR, RXR and VDR. Among differentially expressed members of transcriptional complexes, the most relevant was the co-regulator NCoA4 (Nuclear receptor Coactivator 4). This protein that modulates interactions between transcription factors, RNA polymerase II and chromatin remodeling factors, was initially described as a co-activator of the androgen receptor (AR). Its activity has been extended to receptors of estrogens (ER), peroxisome-proliferating activators (PPAR), retinoid X (RXR) and recently to VD3 receptor (VDR).
Using real-time semi-quantitative PCR, we found that NCoA4 is specifically expressed during the monocytic differentiation of U937 but not during the granulocytic differentiation of NB4 cell lines. These results were confirmed by analysis on normal and in vitro-differentiated leukemia primary cells. Moreover, its early induction, within 6 hours of retinoids and VD3 treatment on U937 cells, suggests that its expression may be controlled by one or several nuclear receptors. Because of cross-talks between retinoids and VD3 pathways, we used NB4-LR2 cells in which RAR is knock-down. In this cell line, NCoA4 is expressed in a VDR-dependent fashion reinforcing the hypothesis that the coregulator is specifically involved in the VD3-monocytic differentiation of leukemic cells.
Next, to explore the role of NCoA4, U937 cells were stably transfected to constitutively over-express the protein. The doubling time of this cell line (U-NCoA4) reaches to 48 hours against 24 hours in U937 cells. Concerning ligand-induced growth arrest, these cells are particularly sensitive to the RXR and VDR agonists while no significative difference was observed after treatment by the RAR agonist or by any combination. In addition, over-expression of NCoA4 induces a slow down of differentiation, as shown by expression of CD11b and CD14 myelomonocytic markers. Thus, in U-NCoA4, except for the RAR agonist, treatment for 72 hours corresponds to treatment for 48 hours levels of U937 cells.
To conclude, in term of growth arrest, NCoA4 over-expressing cells are particularly sensitive to RXR and VDR agonists. Thus, natural ligands present in the culture medium might reduce or delay proliferation, inducing the same effect on differentiation. In order to have a large view of networks, transcriptome of U-NCoA4 was analyzed by real time PCR on Low Density Array composed of a hundred messengers extracted from U937 SAGE libraries. Analysis is currently in progress.