Multidrug Resistance Protein 4 (MRP4) Controls Tumor Growth In Human Acute Myeloid Leukemia In Vivo Model.

COPSEL S; BRUZZONE A; GARCÍA C; WARGON V; RUSSEL FG; SHAYO C; DAVIO C

Noordwijk aan zee

Encuentro; 19th MDO and 12th European ISSX Meeting.; 2012

International Society for the Study of Xenobiotics

Less than a third of adults with acute myeloid leukemia (AML) are cured by current treatments, emphasizing the need for new approaches to therapy. Resistance to drug therapy can arise from diverse mechanisms, among them drug export from tumor cells or alterations in intracellular signaling. Both mechanisms can be mediated by blast expression of members of the multidrug resistance-associated proteins (MRPs). In particular, MRP4/ABCC4 is implicated in cAMP efflux and also confers resistance to nucleoside analogs, cytostatic drugs used in the treatment of AML. We have recently demonstrated in AML cell lines that MRP4 has a relevant role in the regulation of intracellular cAMP levels, leading to the inhibition of cell proliferation and promoting differentiation. Thus, we have postulated that the inhibition of cAMP efflux by blocking MRPs, mainly MRP4, may lead to a decrease in proliferation and tumor growth. To test this, a human AML model was established by subcutaneous injection of U937 cells into nu/nu mice. In order to investigate the effect of  the regulation of cAMP levels, mice were treated with probenecid (MRP inhibitor, 50 mg/kg), rolipram (phosphodiesterase 4 inhibitor, 1.5 mg/kg), probenecid+rolipram or with vehicle for 2 weeks. Interestingly, all treatments showed an inhibition on tumor growth compared with vehicle-treated mice (P<0.01). A significant decrease in the mitotic index was observed with all the treatments (P<0.001). Moreover, treatment with rolipram+probenecid was even more effective in decreasing the mitotic index compared with rolipram or probenecid-treated mice. However, no differences in apoptosis were observed. To further investigate the specific role of MRP4 in tumor growth and proliferation,  nu/nu mice were injected with U937 cells expressing short hairpin RNA (shRNA) against MRP4 (U937-shMRP4) or scrambled shRNA (U937-scramble). A third group of mice carrying U937-scramble tumor was treated with rolipram+probenecid for 2 weeks. Remarkably, blockage of MRP4 strongly reduces tumor growth and tumor size was even smaller than in the rolipram+probenecid-treated group (P<0.05). In addition, a decrease in the mitotic and apoptotic index was observed. In conclusion, these results show for the first time the importance of MRP4 in tumor growth in a human AML in vivo model, providing the basis for a novel promising target for leukemia therapy