Cancer metastasis: Molecular mechanisms and clinical perspectives.

Metastatic progression combined with non-responsiveness towards systemic therapy often shapes the course of disease for cancer patients and commonly determines its lethal outcome. The complex molecular events that promote metastasis are a combination of both, the acquired pro-metastatic properties of cancer cells and a metastasis-permissive or -supportive tumor micro-environment (TME). Yet, dissemination is a challenging process for cancer cells that requires a series of events to enable cancer cell survival and growth. Metastatic cancer cells have to initially detach themselves from primary tumors, overcome the challenges of their intravasal journey and colonize distant sites that are suited for their metastases. The implicated obstacles including anoikis and immune surveillance, can be overcome by intricate intra- and extracellular signaling pathways, which we will summarize and discuss in this review. Further, emerging modulators of metastasis, like the immune-microenvironment, microbiome, sublethal cell death engagement, or the nervous system will be integrated into the existing working model of metastasis.

Non-lethal outcomes of engaging regulated cell death pathways in cancer.

Regulated cell death (RCD) is essential for successful systemic cancer therapy. Yet, the engagement of RCD pathways does not inevitably result in cell death. Instead, RCD pathways can take part in diverse biological processes if the cells survive. Consequently, these surviving cells, for which we propose the term 'flatliners', harbor important functions. These evolutionarily conserved responses can be exploited by cancer cells to promote their own survival and growth, with challenges and opportunities for cancer therapy.

Caspase-8 and FADD prevent spontaneous ZBP1 expression and necroptosis.

The absence of Caspase-8 or its adapter, Fas-associated death domain (FADD), results in activation of receptor interacting protein kinase-3 (RIPK3)- and mixed-lineage kinase-like (MLKL)-dependent necroptosis in vivo. Here, we show that spontaneous activation of RIPK3, phosphorylation of MLKL, and necroptosis in Caspase-8- or FADD-deficient cells was dependent on the nucleic acid sensor, Z-DNA binding protein-1 (ZBP1). We genetically engineered a mouse model by a single insertion of FLAG tag onto the N terminus of endogenous MLKL (MlklFLAG/FLAG), creating an inactive form of MLKL that permits monitoring of phosphorylated MLKL without activating necroptotic cell death. Casp8-/-MlklFLAG/FLAG mice were viable and displayed phosphorylated MLKL in a variety of tissues, together with dramatically increased expression of ZBP1 compared to Casp8+/+ mice. Studies in vitro revealed an increased expression of ZBP1 in cells lacking FADD or Caspase-8, which was suppressed by reconstitution of Caspase-8 or FADD. Ablation of ZBP1 in Casp8-/-MlklFLAG/FLAG mice suppressed spontaneous MLKL phosphorylation in vivo. ZBP1 expression and downstream activation of RIPK3 and MLKL in cells lacking Caspase-8 or FADD relied on a positive feedback mechanism requiring the nucleic acid sensors cyclic GMP-AMP synthase (cGAS), stimulator of interferon genes (STING), and TBK1 signaling pathways. Our study identifies a molecular mechanism whereby Caspase-8 and FADD suppress spontaneous necroptotic cell death.

Sublethal cytochrome c release generates drug-tolerant persister cells.

Drug-tolerant persister cells (persisters) evade apoptosis upon targeted and conventional cancer therapies and represent a major non-genetic barrier to effective cancer treatment. Here, we show that cells that survive treatment with pro-apoptotic BH3 mimetics display a persister phenotype that includes colonization and metastasis in vivo and increased sensitivity toward ferroptosis by GPX4 inhibition. We found that sublethal mitochondrial outer membrane permeabilization (MOMP) and holocytochrome c release are key requirements for the generation of the persister phenotype. The generation of persisters is independent of apoptosome formation and caspase activation, but instead, cytosolic cytochrome c induces the activation of heme-regulated inhibitor (HRI) kinase and engagement of the integrated stress response (ISR) with the consequent synthesis of ATF4, all of which are required for the persister phenotype. Our results reveal that sublethal cytochrome c release couples sublethal MOMP to caspase-independent initiation of an ATF4-dependent, drug-tolerant persister phenotype.

Targeting the innate immunoreceptor RIG-I overcomes melanoma-intrinsic resistance to T cell immunotherapy.

Understanding tumor resistance to T cell immunotherapies is critical to improve patient outcomes. Our study revealed a role for transcriptional suppression of the tumor-intrinsic HLA class I (HLA-I) antigen processing and presentation machinery (APM) in therapy resistance. Low HLA-I APM mRNA levels in melanoma metastases before immune checkpoint blockade (ICB) correlated with nonresponsiveness to therapy and poor clinical outcome. Patient-derived melanoma cells with silenced HLA-I APM escaped recognition by autologous CD8+ T cells. However, targeted activation of the innate immunoreceptor RIG-I initiated de novo HLA-I APM transcription, thereby overcoming T cell resistance. Antigen presentation was restored in interferon-sensitive (IFN-sensitive) but also immunoedited IFN-resistant melanoma models through RIG-I-dependent stimulation of an IFN-independent salvage pathway involving IRF1 and IRF3. Likewise, enhanced HLA-I APM expression was detected in RIG-Ihi (DDX58hi) melanoma biopsies, correlating with improved patient survival. Induction of HLA-I APM by RIG-I synergized with antibodies blocking PD-1 and TIGIT inhibitory checkpoints in boosting the antitumor T cell activity of ICB nonresponders. Overall, the herein-identified IFN-independent effect of RIG-I on tumor antigen presentation and T cell recognition proposes innate immunoreceptor targeting as a strategy to overcome intrinsic T cell resistance of IFN-sensitive and IFN-resistant melanomas and improve clinical outcomes in immunotherapy.

BRN2 suppresses apoptosis, reprograms DNA damage repair, and is associated with a high somatic mutation burden in melanoma.

Whether cell types exposed to a high level of environmental insults possess cell type-specific prosurvival mechanisms or enhanced DNA damage repair capacity is not well understood. BRN2 is a tissue-restricted POU domain transcription factor implicated in neural development and several cancers. In melanoma, BRN2 plays a key role in promoting invasion and regulating proliferation. Here we found, surprisingly, that rather than interacting with transcription cofactors, BRN2 is instead associated with DNA damage response proteins and directly binds PARP1 and Ku70/Ku80. Rapid PARP1-dependent BRN2 association with sites of DNA damage facilitates recruitment of Ku80 and reprograms DNA damage repair by promoting Ku-dependent nonhomologous end-joining (NHEJ) at the expense of homologous recombination. BRN2 also suppresses an apoptosis-associated gene expression program to protect against UVB-, chemotherapy- and vemurafenib-induced apoptosis. Remarkably, BRN2 expression also correlates with a high single-nucleotide variation prevalence in human melanomas. By promoting error-prone DNA damage repair via NHEJ and suppressing apoptosis of damaged cells, our results suggest that BRN2 contributes to the generation of melanomas with a high mutation burden. Our findings highlight a novel role for a key transcription factor in reprogramming DNA damage repair and suggest that BRN2 may impact the response to DNA-damaging agents in BRN2-expressing cancers.

LC3-Associated Phagocytosis in Myeloid Cells Promotes Tumor Immune Tolerance.

Targeting autophagy in cancer cells and in the tumor microenvironment are current goals of cancer therapy. However, components of canonical autophagy play roles in other biological processes, adding complexity to this goal. One such alternative function of autophagy proteins is LC3-associated phagocytosis (LAP), which functions in phagosome maturation and subsequent signaling events. Here, we show that impairment of LAP in the myeloid compartment, rather than canonical autophagy, induces control of tumor growth by tumor-associated macrophages (TAM) upon phagocytosis of dying tumor cells. Single-cell RNA sequencing (RNA-seq) analysis revealed that defects in LAP induce pro-inflammatory gene expression and trigger STING-mediated type I interferon responses in TAM. We found that the anti-tumor effects of LAP impairment require tumor-infiltrating T cells, dependent upon STING and the type I interferon response. Therefore, autophagy proteins in the myeloid cells of the tumor microenvironment contribute to immune suppression of T lymphocytes by effecting LAP.

Regulation of apoptosis by an intrinsically disordered region of Bcl-xL.

Intrinsically disordered regions (IDRs) of proteins often regulate function upon post-translational modification (PTM) through interactions with folded domains. An IDR linking two α-helices (α1-α2) of the antiapoptotic protein Bcl-xL experiences several PTMs that reduce antiapoptotic activity. Here, we report that PTMs within the α1-α2 IDR promote its interaction with the folded core of Bcl-xL that inhibits the proapoptotic activity of two types of regulatory targets, BH3-only proteins and p53. This autoregulation utilizes an allosteric pathway whereby, in one direction, the IDR induces a direct displacement of p53 from Bcl-xL coupled to allosteric displacement of simultaneously bound BH3-only partners. This pathway operates in the opposite direction when the BH3-only protein PUMA binds to the BH3 binding groove of Bcl-xL, directly displacing other bound BH3-only proteins, and allosterically remodels the distal site, displacing p53. Our findings show how an IDR enhances functional versatility through PTM-dependent allosteric regulation of a folded protein domain.

MOMP, cell suicide as a BCL-2 family business.

Apoptosis shapes development and differentiation, has a key role in tissue homeostasis, and is deregulated in cancer. In most cases, successful apoptosis is triggered by mitochondrial outer membrane permeabilization (MOMP), which defines the mitochondrial or intrinsic pathway and ultimately leads to caspase activation and protein substrate cleavage. The mitochondrial apoptotic pathway centered on MOMP is controlled by an intricate network of events that determine the balance of the cell fate choice between survival and death. Here we will review how MOMP proceeds and how the main effectors cytochrome c, a heme protein that has a crucial role in respiration, and second mitochondria-derived activator of caspase (SMAC), as well as other intermembrane space proteins, orchestrate caspase activation. Moreover, we discuss recent insights on the interplay of the upstream coordinators and initiators of MOMP, the BCL-2 family. This review highlights how our increasing knowledge on the regulation of critical checkpoints of apoptosis integrates with understanding of cancer development and has begun to translate into therapeutic clinical benefit.

Spatiotemporally restricted arenavirus replication induces immune surveillance and type I interferon-dependent tumour regression.

Immune-mediated effector molecules can limit cancer growth, but lack of sustained immune activation in the tumour microenvironment restricts antitumour immunity. New therapeutic approaches that induce a strong and prolonged immune activation would represent a major immunotherapeutic advance. Here we show that the arenaviruses lymphocytic choriomeningitis virus (LCMV) and the clinically used Junin virus vaccine (Candid#1) preferentially replicate in tumour cells in a variety of murine and human cancer models. Viral replication leads to prolonged local immune activation, rapid regression of localized and metastatic cancers, and long-term disease control. Mechanistically, LCMV induces antitumour immunity, which depends on the recruitment of interferon-producing Ly6C+ monocytes and additionally enhances tumour-specific CD8+ T cells. In comparison with other clinically evaluated oncolytic viruses and to PD-1 blockade, LCMV treatment shows promising antitumoural benefits. In conclusion, therapeutically administered arenavirus replicates in cancer cells and induces tumour regression by enhancing local immune responses.

IL-10 Induces T Cell Exhaustion During Transplantation of Virus Infected Hearts.

BACKGROUND/AIMS: Unexpected transmissions of viral pathogens during solid organ transplantation (SOT) can result in severe, life-threatening diseases in transplant recipients. Immune activation contributes to disease onset. However mechanisms balancing the immune response against transmitted viral infection through organ transplantation remain unknown. Methods & RESULTS: Here we found, using lymphocytic choriomeningitis virus (LCMV), that transplantation of LCMV infected hearts led to exhaustion of virus specific CD8+ T cells, viral persistence in organs and survival of graft and recipient. Genetic depletion of Interleukin-10 (IL-10) resulted in strong immune activation, graft dysfunction and death of mice, suggesting that IL-10 was a major regulator of CD8+ T cell exhaustion during SOT. In the presence of memory CD8+ T cells, virus could be controlled. However sufficient antiviral immune response resulted in acute rejection of transplanted heart. CONCLUSION: We found that virus transmitted via SOT could not be controlled by naïve mice recipients due to IL-10 mediated CD8+ T cell exhaustion which thereby prevented immunopathology and graft failure whereas memory mice recipients were able to control the virus and induced graft failure.

High Frequencies of Anti-Host Reactive CD8+ T Cells Ignore Non-Hematopoietic Antigen after Bone Marrow Transplantation in a Murine Model.

BACKGROUND: Graft versus host disease (GvHD) occurs in 20% of cases with patients having an MHC I matched bone marrow transplantation (BMT). Mechanisms causing this disease remain to be studied. METHODS: Here we used a CD8+ T cell transgenic mouse line (P14/CD45.1+) and transgenic DEE mice bearing ubiquitously the glycoprotein 33-41 (GP33) antigen derived from the major lymphocytic choriomeningitis virus (LCMV) epitope to study mechanisms of tolerance in anti-host reactive CD8+ T cells after BMT. RESULTS: We found that anti-host reactive CD8+ T cells (P14 T cells) were not negatively selected in the thymus and that they were present in wild type (WT) recipient mice as well as in DEE recipient mice. Anti-host reactive CD8+ T cells ignored the GP33 antigen expressed ubiquitously by host cells but they could be activated ex vivo via LCMV-infection. Lipopolysaccharides (LPS) induced transient cell damage in DEE mice bearing anti-host reactive CD8+ T cells after BMT, suggesting that induction of host inflammatory response could break antigen ignorance. Introducing the GP33 antigen into BM cells led to deletion of anti-host reactive CD8+ T cells. CONCLUSION: We found that after BMT anti-host reactive CD8+ T cells ignored host antigen in recipients and that they were only deleted when host antigen was present in hematopoietic cells. Moreover, LPS-induced immune activation contributed to induction of alloreactivity of anti-host reactive CD8+ T cells after BMT.

Quantitative real-time ARMS-qPCR for mitochondrial DNA enables accurate detection of microchimerism in renal transplant recipients.

The presence of microchimerism in peripheral blood of solid organ transplant recipients has been postulated to be beneficial for allograft acceptance. Kinetics of donor cell trafficking and accumulation in pediatric allograft recipients are largely unknown. In this study, we implemented SNPs of the HVRs I and II of mitochondrial DNA to serve as molecular genetic markers to detect donor-specific cell chimerism after pediatric renal transplantation. Serial dilution of artificial chimeric DNA samples showed a linear correlation coefficient of R > 0.98 and a detection sensitivity of 0.01% with high reproducibility. Longitudinal semiquantitative analysis of donor-specific SNPs was then performed in peripheral blood mononuclear cells samples up to two yr post-transplant. Quantity of donor-specific cell chimerism in peripheral blood was highest in the early post-transplant period reaching values of ~10% after liver-kidney and 2.8% after renal transplantation. From one wk after transplantation, renal transplant patients exhibited an amount of donor-specific mtDNA ranging from 0.01% to 0.1%. We developed a highly accurate, sensitive, and rapid real-time quantitative PCR method using sequence-specific primers and fluorescent hydrolysis probes for the detection of at least 0.01% donor-specific cells in the recipient's peripheral blood after renal transplantation.