All-trans retinoic acid potentiates the inhibitory effects of interferon α on chronic myeloid leukemia progenitors in vitro

Abstract

All-trans retinoic acid (ATRA) has recently been shown to synergise with the inhibitory effect of interferon alpha (IFNα) on the growth of malignant cells isolated from solid tumors. We investigated whether ATRA could potentiate the inhibitory effects of IFNα on the proliferation of leukemic progenitors in chronic myeloid leukemia (CML). CD34⁺ cells from chronic phase, newly diagnosed patients, were incubated in short-term liquid culture with ATRA, IFNα or a combination of both molecules and then plated on semi-solid cultures for colony-forming cell assay. IFNα was found to inhibit preferentially the generation of late progenitors. ATRA at a concentration of 10⁻⁸ M was found strongly to inhibit CFU-M colonies. Addition of ATRA to IFNα dramatically potentiated the inhibitory effects of INFα on CFU-GM growth. In the presence of both molecules the inhibition of day 14 CFU-GM from CD34⁺ cells was lowered to 27 ± 4% of control. CFU-M colonies were completely inhibited. RT-PCR analysis of the colonies resulting from the action of the combination IFNαplus ATRA showed the presence of an increased number of BCR-ABL-negative colonies relatively to what was observed with IFNα alone. FISH analysis showed a higher percentage of Ph-negative cells in the ATRA plus IFNα -treated samples, confirming PCR experiments. These results indicate that, in vitro, the combination of IFNα and ATRA effectively inhibits CFU-GM colony formation in CML and suggest that it has a potential interest for the treatment of CML.