This study found that MS-275, a novel synthetic benzamide histone deacetylase inhibitor (HDACI), blocked Akt/mTOR signaling in acute myelogenous leukemia (AML) HL60 and acute promyelocytic leukemia (APL) NB4 cells, as assessed by decreased levels of the phosphorylated (p)-Akt, p-p70 ribosomal S6 kinase (p70S6K), and p-S6K by Western blot analysis. Interestingly, further inactivation of mTOR by rapamycin analogue RAD001 (everolimus) significantly enhanced MS-275-mediated growth inhibition and apoptosis of these cells in parallel with enhanced upregulation of p27 kip1 and downregulation of c-Myc. In addition, RAD001 potentiated the ability of MS-275 to induce differentiation of HL60 and NB4 cells, as measured by expression of CD11b cell surface antigens, as well as reduction of nitroblue tetrazolium. Importantly, RAD001 potentiated the ability of MS-275 to induce expression of the myeloid differentiation-related transcription factor CCAAT enhancer binding protein e in these cells in association with enhanced acetylation of histone H3 on its promoter. Furthermore, RAD001 (5 mg/kg) significantly enhanced the effects of MS-275 (10 mg/kg) to inhibit proliferation of HL60 tumor xenografts in nude mice without adverse effects. Taken together, concomitant administration of a HDACI and a mTOR inhibitor may be a promising treatment strategy for the individuals with a subset of human leukemia.

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