Childhood Acute lymphoblastic leukemia (ALL) has experienced a dramatic improvement in survival rates over the past 40 years. At this time, the likelihood of cure is greater than 70%. Despite such success, relapse still occurs in 25–30% of children. The major cause of treatment failure is intrinsic or acquired drug resistance. It has been reported that stromal cells of the bone marrow (BMSC) provide a sanctuary in which subpopulations of leukaemia cells can evade chemotherapy-induced death and acquire a drug-resistant phenotype. This explains why, when ALL cell lines are cultured on human BMSC, the apoptotic effect of chemotherapic drugs is strongly inhibited. The mechanisms of BMSC-mediated protection involve a complex interplay among stroma-derived factors, in particular the SDF-1a and the chemokine receptor CXCR4. For these reasons, CXCR4-targeting compounds are nowadays considered potential therapeutic tools in ALL (
Burger JA and Burkle A, Br J Haematol 137: 288–296, 2007
). An additive mechanism of BMSC-induced protection involeves the interaction between adhesion receptors on leukaemia cells and adhesion molecules, like fibronectin, on the surface of BMSC (Tabe Y et al, Cancer Res 67:1238–1250, 2007
). Recent evidence indicates that integrins trigger survival signals since they form macromolecular complexes with plasma membrane proteins. A novel integrin partner in such complexes is represented by ion channel proteins. The channel protein is not merely a bystander interactor, but it often feeds back by controlling integrin activation and downstream signaling (Arcangeli A et al, Trends Cell Biol 16:631–639, 2006
). This mechanisms can give a molecular explanation to the recent evidences that ion channels, especially K+ channels, mark and regulate specific stages of cancer progression, and hence may represent novel targets for cancer therapy. (Becchetti A. and Arcangeli A., J. Gen. Physiol., 28: 313–314, 2008
). Among ion channels, hERG1 channels, are capable of forming multiprotein complexes with integrins in tumor cells. We report here the discovery of a peculiar multiprotein complex formed by CXCR4 and the b1 integrin subunit, besides hERG1 channels, occurring in B-ALL cells. The assembly of the complex is stimulated by adhesion onto BMSC, and is further modulated by SDF-1a. Furthermore, the hERG1/beta1/CXCR4 complex triggers the activation of different intracellular signalling pathways (MAPK activation, pAKT phosphorylation). All the BMSC-induced signalling pathways are strongly inhibited by hERG1 channel blockers. Finally, when cultured on BMSC, all the B-ALL cell lines experienced a strong reduction of apoptosis after treatment with chemotherapic drugs (doxorubicin, prednisone, L-asparaginase, metothrexate). The addition of specific hERG1 blockers bypasses drug resistance, promptly restoring a significant apoptosis in leukemia cells. On the whole, our results allow to include hERG1 channels in a scheme where they act as upstream regulators of integrin/chemokine receptors pro survival signals. In addition data here presented indicate a potential role for hERG1 inhibitors as a novel strategy for overcoming drug resistance in the treatment of ALL.
Disclosures: No relevant conflicts of interest to declare.
This work was supported by “Associazione Noi per Voi”, Firenze and Associazione Italiana per la Ricerca sul Cancro (AIRC).
