| Acute Myeloid Leukemia(AML)is a heterogeneous hematological malignancy characterized by rapid proliferation of immature blood precursor cells in the bone marrow and peripheral blood which interfere normal blood cell growth.The annual incidence rate of AML in China is about 1.6~2.3/100,000,and annual new cases of AML are about 24,000(excluding APL).The incidence rate of AML in adults is significantly higher than that in children.The five-year survival rate for children is 65%,while that for adults is only 25%.Treatment of AML patents with cytarabine and anthracycline based chemotherapy can result in 60% remission rate,but the majority of these patients relapse.The main cause of treatment failure with traditional chemotherapy is that most AML patients develop resistance to chemotherapy at certain stage during the course of disease progression,leading to death.Therefore,there is an urgent need of well-tolerated and more effective anti-AML therapies to prolong the survival time and ultimately improve the cure rate of patients with AML.The Bcl-2 family genes encode more than 20 proteins that play important roles in cell apoptosis.Studies have shown that overexpression of the anti-apoptotic Bcl-2 family proteins(e.g.Bcl-2,Bcl-xL,and/or Mcl-1)is associated with resistance to chemotherapy and poor prognosis of patients with AML.Therefore,inhibition of the anti-apoptotic Bcl-2 family proteins may respresent a potential effective treatment for AML.ABT-199(Venetoclax)is a selective Bcl-2 inhibitor,and has been shown to bind to the hydrophobic pocket on the Bcl-2 molecule that binds to the BH3 domain,disrupting its interaction with pro-apoptotic Bcl-2 family proteins,such as Bim.Previous studies from our lab and other groups have shown that ABT-199 disrupts the interaction between Bcl-2 and Bim and releases Bim from Bcl-2.However,the resultedfreeBim is sequestered by Mcl-1,preventing Bim from activating Bax/Bak and apoptosis,representing a machnism of intrinsic resistance to ABT-199 in AML.To overcome this mechanism of intrinsic resistance to ABT-199,we proposed to use the selective Mcl-1 inhibitor S63845 in combination with ABT-199.S63845 is a BH3 mimetic that replaces Bim from Mcl-1,preventing released Bim from Bcl-2 induced by ABT-199 binding to Mcl-1,leading to enhanced Bax/Bak activation and apoptosis.Interestingly,the combination of S63845 and ABT-199 synergistically indced apoptosis in AML cell lines and primary patient samples,regardless of their sensitivities to ABT-199.After confirming the synergistic antileukemic effect of the combination of the two drugs,we investigated the mechanism of action of the combination in AML cell lines.We first determined the effect of ABT-199 and S63845,alone or combined,on the expression of Bcl-2 family proteins,and found that S63845 treatment either alone or in combination with ABT-199 resulted in increased levels of Mcl-1 protein,but had no effect on the levels of other Bcl-2 family proteins.We then determined the role of Bax and Bak in apoptosis induced by the two agents,alone or combined,in AML cell lines.Our results show that both Bak and Bax play an important role in apoptosis induced by ABT-199 and S63845,demonstrating that S63845 and ABT-199 at least partially induce intrinsic apoptosis in AML cells.In contrast,the role of Bim in apoptosis induced by the two drugs,either alone or in combination,was somewhat cell line-dependent.The results from our study demonstrate that ABT-199 and S64845 synergistically induce apoptosis in AML cells,and indicate that the combination may represent a potential treatment for AML. |
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