E3 ligase Ligand 14

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 benefit. 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 understood. Recent studies revealed that response to PD-1-PD-L1 blockade might correlate with PD-L1 expression levels in tumour cells. 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.

Targeted protein degradation (TPD) strategies have revolutionized how scientists tackle challenging protein targets deemed undruggable with traditional small molecule inhibitors. Many promising campaigns to inhibit proteins have failed due to factors surrounding inhibition selectivity and targeting of compounds to specific tissues and cell types. One of the major improvements that PROTAC (proteolysis targeting chimera) and molecular glue technology can exert is highly selective control of target inhibition. Multiple studies have shown that PROTACs can gain selectivity for their protein targets beyond that of their parent ligands via optimization of linker length and stabilization of ternary complexes. Due to the bifunctional nature of PROTACs, the tissue selective nature of E3 ligases can be exploited to uncover novel targeting mechanisms. In this review, we provide critical analysis of the recent progress towards making selective PROTAC molecules and new PROTAC technologies that will continue to push the boundaries of achieving selectivity. These efforts have wide implications in the future of treating disease as they will broaden the possible targets that can be addressed by small molecules, like undruggable proteins or broadly active targets that would benefit from degradation in specific tissue types.

Dysfunctional T cells in the tumour microenvironment have abnormally high expression of PD-1 and antibody inhibitors against PD-1 or its ligand (PD-L1) have become commonly used drugs to treat various types of cancer1-4. The clinical success of these inhibitors highlights the need to study the mechanisms by which PD-1 is regulated. Here we report a mechanism of PD-1 degradation and the importance of this mechanism in anti-tumour immunity in preclinical models. We show that surface PD-1 undergoes internalization, subsequent ubiquitination and proteasome degradation in activated T cells. FBXO38 is an E3 ligase of PD-1 that mediates Lys48-linked poly-ubiquitination and subsequent proteasome degradation. Conditional knockout of Fbxo38 in T cells did not affect T cell receptor and CD28 signalling, but led to faster tumour progression in mice owing to higher levels of PD-1 in tumour-infiltrating T cells. Anti-PD-1 therapy normalized the effect of FBXO38 deficiency on tumour growth in mice, which suggests that PD-1 is the primary target of FBXO38 in T cells. In human tumour tissues and a mouse cancer model, transcriptional levels of FBXO38 and Fbxo38, respectively, were downregulated in tumour-infiltrating T cells. However, IL-2 therapy rescued Fbxo38 transcription and therefore downregulated PD-1 levels in PD-1+ T cells in mice. These data indicate that FBXO38 regulates PD-1 expression and highlight an alternative method to block the PD-1 pathway.