Interferon-γ Enhances PML Protein Expression in Acute Promyelocytic Leukemia Cells and Cooperates with All-Trans Retinoic Acid to Induce Maturation of NB4 and MR2 Cells

More than 90% patients with untreated acute promyelocytic leukemia(APL) obtain complete remission clinically by using All-trans retinoic acid(ATRA), a strong differentiation inducer. However, the rapid development of ATRA-resistance brings a new problem to the treatment of APL. Interferon(IFN), as an important cytokine, has broad biological activities. It can not only inhibit the growth of tumor cells, but also reverse the drug-resistance of chemotherapy. As proved by some research, the mechanisms of ATRA-resistance are probably related to lacking some important proteins which synthesized by Interferon-γ (IFN-γ). In order to explore a new way to solve the problem of ATRA-resistance in APL, we investigate the effect and mechanisms of IFN-γ in combination with ATRA on the proliferation/differentiation of NB4 cells(APL cell line with ATRA-sensitiveness) and MR2 cells(APL cell line with ATRA-resistance) respectively. ATRA, IFN-γ and IFN-γ in combination with ATRA were incubated with NB4 and MR2 cells respectively. The cell proliferation was tested by MTT assay, the cell differentiation was tested through light microscope, by NBT reduction test and flow cytometry(FCM). The results showed that ATRA could inhibit the growth of NB4 cells significantly (P<0.05), but it had no effect on the growth of MR2 cells (P>0.05). IFN-γ could inhibit the growth of both NB4 cells and MR2 cells slightly (P<0.05). Moreover, the growth inhibition effect of IFN-γ in combination with ATRA on NB4 and MR2 cells was obviously stronger than that of any single drug group (P<0.05). The results of cell morphology observation, NBT reduction test and CD11b antigen detection showed that ATRA could induce differentiation of NB4 cells significantly (P<0.05), but it had no effect on MR2 cells (P>0.05). Although IFN-γ alone had no effect on the differentiation of either NB4 or MR2 cells (P>0.05), it could augment the differentiation of NB4 cells induced by ATRA (P<0.05) and induce the differentiation of MR2 cells slightly (P<0.05) when it combined with ATRA. Furthermore, we have observed the expression of promyelocytic leukemia(PML) protein by indirect immune fluorescent test. The results showed that the number of fluorescent particles in both NB4 and MR2 cells’ nucleus was increased significantly (P<0.05) when they were incubated with IFN-γ respectively, which indicated IFN-γ could induce the expression of PML protein, a tumor growth inhibitor. It can be seen that IFN-γ could augment the proliferation inhibition effect of ATRA on NB4 and MR2 cells through enhancing the expression of PML protein. Moreover, IFN-γ in combination with ATRA not only can strengthen the induction differentiation effect of ATRA on NB4 cells, but also can partially induce the maturation of MR2 cells with ATRA-resistance.