The Mechanism Of Synergy Of BH3Mimetics And Paclitaxel In Chronic Myeloid Leukemia

Paclitaxel is an alternative chemotherapeutic agent for chronic myelogenous leukemia (CML) when primary or secondary resistance of tyrosine kinase inhibitors (TKI) is emerging, because paclitaxel as a microtubule inhibitor could bypass the apoptotic deficiencies linked to p53and fas ligand pathways in CML and induce apoptosis via intrinsic apoptotic pathway. The Bcl-2family anti-apoptotic and pro-apoptotic proteins regulate endogenous apoptotic through the balance of protein-protein interaction. The activations of MAPK and STAT pathways led to up-regulating anti-apoptotic Bcl-2, Bcl-xL and Mcl-1, which prevent CML cells from paclitaxel-induced apoptosis. As such, there is a need for paclitaxel to combine with other agents and antagonize anti-apoptosis Bcl-2family members, which synergize to induce apoptosis.Small-molecular BH3mimetics that mimic the key residues on the BH3-only proteins could bind in the BH3grooves of the anti-apoptotic members of Bcl-2family proteins, which lead to anti-cancer activities. ABT-737is one of BH3mimetics for clinical drugs and is a well established the most high efficient Bcl-2/Bcl-xL inhibitor, while does not target Mcl-1. We previously identified that S1is a specific BH3mimetic and exhibits a high affinity to Bcl-2/Mcl-1(310nM/58nM).In this paper, we aimed to study that the mechanism of synergy of BH3mimetics and paclitaxel in chronic myeloid leukemia cells.This study takes CML cell line K562as research object. Firstly, By means of MTT experiment and combining with combination index (CI) analysis, we found BH3mimetics (ABT-737and S1) could synergize paclitaxel to induce K562apoptosis (CI<1). Through measure activation of caspase-8/9and cytochrome c from the mitochondria, we proved both of the two BH3mimetics could efficiently synergize with paclitaxel through intrinsic apoptosis pathway. By using Bcl-2siRNA, Bcl-xL siRNA or Mcl-1siRNA, we found Mcl-1is the determinant for the synergistic effect between paclitaxel and ABT-737or S1. Western Blot and co-immunoprecipitation confirmed that paclitaxel/ABT-737and paclitaxel/S1exert synergistic function by different mechanisms:paclitaxel/ABT-737synergize to drastically upregualted Bim to replaced Bak from Mcl-1and induce apoptosis; S1directly hit Mcl-1to release both Bim and Bak and apoptosis. Furthermore, we revealed ABT-737synergize with paclitaxel enhanced the activation of JNK/Bim pathway, and upregulated the level of Bim. In addition, in primary cells from3patients with CML, we found all results supported the cooperated mechanism and molecular mechanism between paclitaxel and BH3mimetics.The study demonstrated BH3mimetics could synergize with paclitaxel to induce apoptosis and revealed the signal transmission pathway and molecular mechanism of synergy of BH3mimetics and paclitaxel. The conclusion of this study offered a new candidate for CML comination therapy and molecular.