FTY720 Increases Doxorubicin Sensitivity in Resistant Acute Myeloid Leukemia Cells

Chemoresistance remains a major barrier to effective treatment of acute myeloid leukemia, partly because resistant leukemic cells reduce intracellular drug exposure and evade apoptosis. FTY720, a sphingosine-1-phosphate receptor modulator, has reported antiproliferative and chemosensitizing activity in several tumor models. However, whether FTY720 increases doxorubicin sensitivity in doxorubicin-resistant acute myeloid leukemia cells, and whether this effect is associated with changes in drug-efflux transporters and protein kinase B/mechanistic target of rapamycin signaling, remains insufficiently defined. This study evaluated the effects of FTY720 alone and in combination with doxorubicin in parental acute myeloid leukemia cell lines (HL60 and K562) and their doxorubicin-resistant derivatives (HL60/A and K562/A). Cell viability was assessed using Cell Counting Kit-8 assays, apoptosis was measured by Annexin V/propidium iodide flow cytometry, intracellular doxorubicin accumulation was quantified by flow cytometry, and P-glycoprotein, multidrug resistance-associated protein 1, protein kinase B/mechanistic target of rapamycin pathway proteins, and apoptosis-related proteins were examined by flow cytometry and Western blotting. FTY720 showed dose-dependent cytotoxicity and increased doxorubicin sensitivity in resistant leukemia cells. Combined treatment reduced cell viability, increased apoptotic cell fractions, enhanced intracellular doxorubicin accumulation, and decreased P-glycoprotein and multidrug resistance-associated protein 1 expression. FTY720 treatment was also associated with reduced phosphorylation of protein kinase B and mechanistic target of rapamycin, whereas the protein kinase B activator SC79 partially reversed the observed changes in signaling, transporter expression, and apoptosis. These findings indicate that FTY720 increases doxorubicin sensitivity in HL60/A and K562/A cells and that this effect is associated with reduced protein kinase B/mechanistic target of rapamycin signaling, decreased drug-efflux transporter expression, enhanced intracellular doxorubicin accumulation, and increased apoptosis. Because this study was limited to in vitro cell-line models, further validation in primary leukemia samples and in vivo systems is required before therapeutic conclusions can be drawn.