Abundance of PD-L1, the ligand of the anti-cancer immunotherapy target PD-1, is negatively regulated by poly-ubiquitination via the cyclin D–CDK4/cullin 3–SPOP axis and PD-1 blockade treatment in mice improved survival when combined with CDK4/6 inhibitors. Immune checkpoint blockade therapy with PD-1 or PD-L1 inhibitors has been approved for the treatment of several types of cancer, but only a small percentage of patients respond to this from of treatment. Wenyi Wei and colleagues report that the abundance of the PD-L1 protein is negatively regulated during cell cycle progression, in part through the activity of the upstream cyclin D–CDK4–SPOP–FZR1 signalling pathway. CDK4/6 inhibitor treatment increases PD-L1 levels and promotes immune evasion in vitro and in vivo. On the other hand, combined treatment with a CDK4/6 inhibitor and a PD-1 blocker has greater anti-tumour efficacy than treatment with each drug alone in mouse models. This combined approach has the potential to improve the treatment of patients with cancer. Treatments that target immune checkpoints, such as the one mediated by programmed cell death protein 1 (PD-1) and its ligand PD-L1, have been approved for treating human cancers with durable clinical benefit1,2. However, many patients with cancer fail to respond to compounds that target the PD-1 and PD-L1 interaction, and the underlying mechanism(s) is not well understood3,4,5. Recent studies revealed that response to PD-1–PD-L1 blockade might correlate with PD-L1 expression levels in tumour cells6,7. Hence, it is important to understand the mechanistic pathways that control PD-L1 protein expression and stability, which can offer a molecular basis to improve the clinical response rate and efficacy of PD-1–PD-L1 blockade in patients with cancer. Here we show that PD-L1 protein abundance is regulated by cyclin D–CDK4 and the cullin 3–SPOP E3 ligase via proteasome-mediated degradation. Inhibition of CDK4 and CDK6 (hereafter CDK4/6) in vivo increases PD-L1 protein levels by impeding cyclin D–CDK4-mediated phosphorylation of speckle-type POZ protein (SPOP) and thereby promoting SPOP degradation by the anaphase-promoting complex activator FZR1. Loss-of-function mutations in SPOP compromise ubiquitination-mediated PD-L1 degradation, leading to increased PD-L1 levels and reduced numbers of tumour-infiltrating lymphocytes in mouse tumours and in primary human prostate cancer specimens. Notably, combining CDK4/6 inhibitor treatment with anti-PD-1 immunotherapy enhances tumour regression and markedly improves overall survival rates in mouse tumour models. Our study uncovers a novel molecular mechanism for regulating PD-L1 protein stability by a cell cycle kinase and reveals the potential for using combination treatment with CDK4/6 inhibitors and PD-1–PD-L1 immune checkpoint blockade to enhance therapeutic efficacy for human cancers.

Ubiquitination, an important type of protein posttranslational modification (PTM), plays a crucial role in controlling substrate degradation and subsequently mediates the “quantity” and “quality” of various proteins, serving to ensure cell homeostasis and guarantee life activities. The regulation of ubiquitination is multifaceted and works not only at the transcriptional and posttranslational levels (phosphorylation, acetylation, methylation, etc.) but also at the protein level (activators or repressors). When regulatory mechanisms are aberrant, the altered biological processes may subsequently induce serious human diseases, especially various types of cancer. In tumorigenesis, the altered biological processes involve tumor metabolism, the immunological tumor microenvironment (TME), cancer stem cell (CSC) stemness and so on. With regard to tumor metabolism, the ubiquitination of some key proteins such as RagA, mTOR, PTEN, AKT, c-Myc and P53 significantly regulates the activity of the mTORC1, AMPK and PTEN-AKT signaling pathways. In addition, ubiquitination in the TLR, RLR and STING-dependent signaling pathways also modulates the TME. Moreover, the ubiquitination of core stem cell regulator triplets (Nanog, Oct4 and Sox2) and members of the Wnt and Hippo-YAP signaling pathways participates in the maintenance of CSC stemness. Based on the altered components, including the proteasome, E3 ligases, E1, E2 and deubiquitinases (DUBs), many molecular targeted drugs have been developed to combat cancer. Among them, small molecule inhibitors targeting the proteasome, such as bortezomib, carfilzomib, oprozomib and ixazomib, have achieved tangible success. In addition, MLN7243 and MLN4924 (targeting the E1 enzyme), Leucettamol A and CC0651 (targeting the E2 enzyme), nutlin and MI‐219 (targeting the E3 enzyme), and compounds G5 and F6 (targeting DUB activity) have also shown potential in preclinical cancer treatment. In this review, we summarize the latest progress in understanding the substrates for ubiquitination and their special functions in tumor metabolism regulation, TME modulation and CSC stemness maintenance. Moreover, potential therapeutic targets for cancer are reviewed, as are the therapeutic effects of targeted drugs.

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.