SCFFBW7 regulates cellular apoptosis by targeting MCL1 for ubiquitylation and destruction

Abstract

Loss of the tumour suppressor FBW7 is frequently observed in various types of human cancers, but its mechanism of action as a tumour suppressor remains unclear. Two groups demonstrate that in several cancer types, including ovarian cancer and T-cell leukaemias, the apoptosis regulator MCL1 is targeted for degradation by FBW7. Inuzuka et al. find that this mechanism can determine the response to drugs targeting BCL2 family apoptosis factors, and Wertz et al. show that it is activated during mitotic arrest and determines the response to anti-tubulin chemotherapeutics. Deletion or mutation of FBW7 in patients with cancer can therefore render tumours resistant to these therapies. This is one of two papers demonstrating that in several cancer types including ovarian cancer and T-cell leukaemias, the apoptosis regulator MCL1 is targeted for degradation by the FBW7 tumour suppressor. This study finds that this mechanism can determine the response to drugs targeting BCL2 family apoptosis factors. Deletion or mutation of FBW7 found in cancer patients therefore can render tumours resistant to these therapies. The effective use of targeted therapy is highly dependent on the identification of responder patient populations. Loss of FBW7, which encodes a tumour-suppressor protein, is frequently found in various types of human cancer, including breast cancer, colon cancer1 and T-cell acute lymphoblastic leukaemia (T-ALL)2. In line with these genomic data, engineered deletion of Fbw7 in mouse T cells results in T-ALL3,4,5, validating FBW7 as a T-ALL tumour suppressor. Determining the precise molecular mechanisms by which FBW7 exerts antitumour activity is an area of intensive investigation. These mechanisms are thought to relate in part to FBW7-mediated destruction of key proteins relevant to cancer, including Jun6, Myc7, cyclin E8 and notch 1 (ref. 9), all of which have oncoprotein activity and are overexpressed in various human cancers, including leukaemia. In addition to accelerating cell growth10, overexpression of Jun, Myc or notch 1 can also induce programmed cell death11. Thus, considerable uncertainty surrounds how FBW7-deficient cells evade cell death in the setting of upregulated Jun, Myc and/or notch 1. Here we show that the E3 ubiquitin ligase SCFFBW7 (a SKP1–cullin-1–F-box complex that contains FBW7 as the F-box protein) governs cellular apoptosis by targeting MCL1, a pro-survival BCL2 family member, for ubiquitylation and destruction in a manner that depends on phosphorylation by glycogen synthase kinase 3. Human T-ALL cell lines showed a close relationship between FBW7 loss and MCL1 overexpression. Correspondingly, T-ALL cell lines with defective FBW7 are particularly sensitive to the multi-kinase inhibitor sorafenib but resistant to the BCL2 antagonist ABT-737. On the genetic level, FBW7 reconstitution or MCL1 depletion restores sensitivity to ABT-737, establishing MCL1 as a therapeutically relevant bypass survival mechanism that enables FBW7-deficient cells to evade apoptosis. Therefore, our work provides insight into the molecular mechanism of direct tumour suppression by FBW7 and has implications for the targeted treatment of patients with FBW7-deficient T-ALL.