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Targeting the PD-1/PD-L1 axis has transformed the field of immune-oncology. While conventional wisdom initially postulated that PD-L1 serves as the inert ligand for PD-1, an emerging body of literature suggests that PD-L1 has cell-intrinsic functions in immune and cancer cells. In line with these studies, here we show that engagement of PD-L1 via cellular ligands or agonistic antibodies, including those used in the clinic, potently inhibits the type I interferon pathway in cancer cells. Hampered type I interferon responses in PD-L1-expressing cancer cells resulted in enhanced infection with oncolytic viruses in vitro and in vivo. Consistently, PD-L1 expression marked tumor explants from cancer patients that were best infected by oncolytic viruses. Mechanistically, PD-L1 suppressed type I interferon by promoting a metabolic shift characterized by enhanced glucose uptake and glycolysis rate. Lactate generated from glycolysis was the key metabolite responsible for inhibiting type I interferon responses and enhancing oncolytic virus infection in PD-L1-expressing cells. In addition to adding mechanistic insight into PD-L1 intrinsic function and showing that PD-L1 has a broader impact on immunity and cancer biology besides acting as a ligand for PD-1, our results will also help guide the numerous efforts currently ongoing to combine PD-L1 antibodies with oncolytic virotherapy in clinical trials.Once sentence summaryPD-L1 promotes oncolytic virus efficacy.

HER2 is overexpressed in 20–30% of all breast cancers and is associated with an invasive disease and poor clinical outcome. The Ste20-like kinase (SLK) is activated downstream of HER2/Neu and is required for efficient epithelial-to-mesenchymal transition, cell cycle progression, and migration in the mammary epithelium. Here we show that loss of SLK in a murine model of HER2/Neu-positive breast cancers significantly accelerates tumor onset and decreases overall survival. Transcriptional profiling of SLK knockout HER2/Neu-derived tumor cells revealed a strong induction in the triple-negative breast cancer marker, Sox10, accompanied by an increase in mammary stem/progenitor activity. Similarly, we demonstrate that SLK and Sox10 expression are inversely correlated in patient samples, with the loss of SLK and acquisition of Sox10 marking the triple-negative subtype. Furthermore, pharmacological inhibition of AKT reduces SLK-null tumor growth in vivo and is rescued by ectopic Sox10 expression, suggesting that Sox10 is a critical regulator of tumor growth downstream of SLK/AKT. These findings highlight a role for SLK in negatively regulating HER2-induced mammary tumorigenesis and provide mechanistic insight into the regulation of Sox10 expression in breast cancer.

Over the past two decades, the mammalian Ste20-like kinase (SLK) has been characterized for its role in regulating cellular migration, proliferation and apoptosis in fibroblasts and myoblasts. In mammary epithelial cells, SLK has been shown to be required for efficient epithelial-to-mesenchymal transition and to be activated downstream of the HER2/Neu-oncogene to control chemotactic cellular migration. Here, we assessed the role of SLK in HER2/Neu-induced mammary tumorigenesis in vivo. As SLK is activated downstream of HER2/Neu, we hypothesized that the loss of SLK would significantly delay tumor progression in a mouse model of HER2-positive breast cancer. As we have shown that global attenuation of SLK kinase activity results in embryonic lethality, a conditional SLK knockout mouse model was generated. To study the role of SLK in HER2-positive breast cancer, we crossed these conditional SLK knockout mice with mice expressing HER2/Neu linked to Cre recombinase in the mammary luminal epithelium. Unexpectedly, we have demonstrated that conditional deletion of SLK significantly accelerates Neu-induced mammary tumor onset and decreases overall survival. SLK deletion results in the induction of Sox10 which drives mammary stem/progenitor activity and cooperates with HER2/Neu to drive tumor growth. Using the Cancer Genome Atlas, we have supported previous findings and validated Sox10 as a potential biomarker of the Triple-negative Breast Cancer subtype. Furthermore, we have uncovered that SLK deletion results in enhanced activation of AKT. We provide evidence that Sox10 induction requires signaling through a novel AKT/Sox9-dependent pathway following SLK deletion. Taken together, our data suggests that SLK may be required to maintain cells in a fully differentiated state and that loss of SLK in HER2/Neu-induced breast cancer drives a more basal/stem-like phenotype through the induction of Sox10. Citation Format: Khalid N Al-Zahrani, John Abou-Hamad, Ben Pryce, David P Cook, Johnathan Hodgins, Cedrik Labreche, Pascale Robineau-Charette, Christiano de Souza, John Bell, Rebecca Auer, Michele Ardolino, Barbara Vanderhyden, Daniel Schramek, Luc Sabourin. Loss of Ste20-like kinase induces a basal/stem-like phenotype in HER2-positive breast cancers [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-04-05.

Duchenne's muscular dystrophy (DMD) is a severe muscle wasting disorder characterized by the loss of dystrophin expression, muscle necrosis, inflammation and fibrosis. Ongoing muscle regeneration is impaired by persistent cytokine stress, further decreasing muscle function. Patients with DMD rarely survive beyond their early 20s, with cardiac and respiratory dysfunction being the primary cause of death. Despite an increase in our understanding of disease progression as well as promising preclinical animal models for therapeutic intervention, treatment options for muscular dystrophy remain limited and novel therapeutic targets are required. Many reports suggest that the TGFβ signalling pathway is activated in dystrophic muscle and contributes to the pathology of DMD in part by impairing the differentiation of myoblasts into mature myofibers. Here, we show that in vitro knockdown of the Ste20-like kinase, SLK, can partially restore myoblast differentiation downstream of TGFβ in a Smad2/3 independent manner. In an mdx model, we demonstrate that SLK is expressed at high levels in regenerating myofibers. Muscle-specific deletion of SLK reduced leukocyte infiltration, increased myogenin and utrophin expression and enhanced differentiation. This was accompanied by resistance to eccentric contraction-induced injury in slow fiber type-enriched soleus muscles. Finally, we found that these effects were partially dependent on the upregulation of p38 signalling. Collectively, these results demonstrate that SLK downregulation can restore some aspects of disease progression in DMD.

Invasion can be stimulated in vitro using the soluble ligand transforming growth factor-β (TGFβ) to induce a process called epithelial-to-mesenchymal transition (EMT) characterized by cell-cell junction breakdown and an invasive phenotype. We have previously demonstrated a role for Ste20-like kinase SLK cell migration and invasion. Here we show that SLK depletion in NMuMG mammary epithelial cells significantly impairs their TGFβ-induced migration and invasion. Immunofluorescence studies show that a fraction of SLK localizes to E-cadherin-positive adherens junction and that SLK impairs the breakdown of cell-cell contacts. We find that SLK-depleted cultures express significantly lower levels of vimentin protein as well as Snai1 and E-cadherin mRNA levels following TGF-β treatment. Surprisingly, our data show that SLK depletion does not affect the activation and nuclear translocation of Smad3. Furthermore, we show that expression of a dominant negative kinase does not impair tight junction breakdown and rescues Snai1 mRNA expression levels. Together these data suggest that SLK plays a novel role in TGFβ-induced EMT, independent of Smads, in a kinase activity-independent manner.