Rapamycin interacts synergistically with idarubicin to induce T-leukemia cell apoptosis in vitro and in a mesenchymal stem cell simulated drug-resistant microenvironment via Akt/mammalian target of rapamycin and extracellular signal-related kinase signaling pathways.

T-cell acute lymphoblastic leukemias (T-ALLs) are clonal lymphoid malignancies with a poor prognosis, and still a lack of effective treatment. Here we examined the interactions between the mammalian target of rapamycin (mTOR) inhibitor rapamycin and idarubicin (IDA) in a series of human T-ALL cell lines Molt-4, Jurkat, CCRF-CEM and CEM/C1. Co-exposure of cells to rapamycin and IDA synergistically induced T-ALL cell growth inhibition and apoptosis mediated by caspase activation via the intrinsic mitochondrial pathway and extrinsic pathway. Combined treatment with rapamycin and IDA down-regulated Bcl-2 and Mcl-1, and inhibited the activation of phosphoinositide 3-kinase (PI3K)/mTOR and extracellular signal-related kinase (ERK). They also played synergistic pro-apoptotic roles in the drug-resistant microenvironment simulated by mesenchymal stem cells (MSCs) as a feeder layer. In addition, MSCs protected T-ALL cells from IDA cytotoxicity by up-regulating ERK phosphorylation, while rapamycin efficiently reversed this protective effect. Taken together, we confirm the synergistic antitumor effects of rapamycin and IDA, and provide an insight into the potential future clinical applications of combined rapamycin-IDA regimens for treating T-cell malignancies.

[Synergistic cytotoxic effects of rapamycin and idarubicin on human acute T-cell lymphoblastic leukemia Jurkat cells].

OBJECTIVE: To investigate the cytotoxic effects of mTOR inhibitor rapamycin (Rapa) and idarubicin (IDA) on human T-cell acute lymphoblastic leukemia Jurkat cell line. METHODS: The proliferation of Jurkat cells was observed by CCK-8 assay. The combined index was analyzed by Isobologram method. Apoptosis was detected by electron microscopy and flow cytometry with Annexin V/PI staining. Protein expressions of Caspase 3, PARP, Caspase 8, Caspase 9, Akt, p-Akt, P85S6K, p-P85S6K, P70S6K, p-P70S6K, ERK1/2 and p-ERK1/2 were determined by Western blotting. RESULTS: The IC(50) of IDA for Jurkat cells was significantly reduced when combined with 10 nmol/L rapamycin. The combined index was <1. Both electron microscopy and Annexin V/PI staining flow cytometry revealed that rapamycin significantly increased apoptotic sensitivity to IDA. The combination of IDA with rapamycin enhanced the expressions of Caspase 3, PARP, Caspase 8 and Caspase 9. Rapamycin significantly inhibited mTOR signaling upstream Akt and downstream S6K activation triggered by IDA, and the combination of the two agents led to synergistic inhibition of ERK phosphorylation. CONCLUSION: Combination of IDA with rapamycin exerted a synergistic anti-proliferative effect and promoted apoptosis by both extrinsic and intrinsic apoptotic pathways in Jurkat cells. Inhibition of ERK phosphorylation and mTOR signaling by rapamycin may play a certain role in this synergistic effect.