Mechanisms of nuclear Mcl-1 mediated chemoresistance

摘要

Abstract The overexpression of anti-apoptotic Bcl-2 proteins is a primary mechanism by which tumors protect themselves from chemotherapeutic interventions. Most effort in the field has focused on a specific molecular domain that is common to the anti-apoptotic Bcl-2 family proteins, which binds and prevents pro- apoptotic family members from initiating programmed cell death by the mitochondrial pathway. Our preliminary data, however, indicate that Mcl-1 protects tumors from therapeutic interventions by a second mechanism, independent of its anti-apoptotic activities. Specifically, our data indicate that Mcl-1 contains a unique loop domain that binds proteins that regulate gene expression when the molecule is translocated from the mitochondrion to the nucleus in response to chemotherapy. Specifically, the data demonstrate that once localized to the nucleus, Mcl-1 interacts with machinery known to regulate RNA stability, and directly suppresses the expression of transcripts normally upregulated in chemosensitive cells, including genes involved in the production of reactive oxygens species such as NOX4. We use colorectal cancer as our model, solid tumor because: 1) it continues to be one of the deadliest human cancers; 2) mounting data indicates that Mcl-1 plays a major role in regulating chemoresistance in colon cancer; and 3) we have access to a very large colorectal cancer tumor bank, from which we have generated numerous patient-derived models. It is our hypothesis that Mcl-1’s role in chemoresistance depends on its ability to leave the mitochondria and translocate to the nucleus, and to post-transcriptionally coordinate a stress response to cope with chemotherapy-induced oxidative stress. We thus hypothesize that to optimize targeting of Mcl-1, a combinatorial approach is necessary, whereby inhibitors of both its anti-apoptotic mechanism and the loop domains mediating its nuclear activities will provide the comprehensive strategy needed to overcome the tumor’s chemoresistance. We propose the following three related but independent aims to test our hypothesis: 1) Determine the mechanism by which chemotherapy induces Mcl-1 nuclear translocation. 2) Investigate the mechanism by which nuclear Mcl-1 mediates chemoresistance. 3) Develop strategies that target Mcl-1’s multiple mechanisms of chemoresistance. Successful completion of these aims will yield new targets for rendering tumors chemosensitive and will provide the understanding needed for anti-Mcl-1 drug design for colorectal- and ultimately other forms of cancer. As there is significant pharmaceutical interest in designing inhibitors of Mcl-1 for solid tumor therapy, our proposal is extremely timely for attaining the molecular understanding of the molecule requisite for successfully accomplishing that goal.