The role of cellular proteostasis in antitumor immunity.

Immune checkpoint blockade therapy is perhaps the most important development in cancer treatment in recent memory. It is based on decades of investigation into the biology of immune cells and the role of the immune system in controlling cancer growth. While the molecular circuitry that governs the immune system in general-and antitumor immunity in particular-is intensely studied, far less attention has been paid to the role of cellular stress in this process. Proteostasis, intimately linked to cell stress responses, refers to the dynamic regulation of the cellular proteome and is maintained through a complex network of systems that govern the synthesis, folding, and degradation of proteins in the cell. Disruption of these systems can result in the loss of protein function, altered protein function, the formation of toxic aggregates, or pathologies associated with cell stress. However, the importance of proteostasis extends beyond its role in maintaining proper protein function; proteostasis governs how tolerant cells may be to mutations in protein-coding genes and the overall half-life of proteins. Such gene expression changes may be associated with human diseases including neurodegenerative diseases, metabolic disease, and cancer and manifest at the protein level against the backdrop of the proteostasis network in any given cellular environment. In this review, we focus on the role of proteostasis in regulating immune responses against cancer as well the role of proteostasis in determining immunogenicity of cancer cells.

The chaperonin CCT8 controls proteostasis essential for T cell maturation, selection, and function.

T cells rely for their development and function on the correct folding and turnover of proteins generated in response to a broad range of molecular cues. In the absence of the eukaryotic type II chaperonin complex, CCT, T cell activation induced changes in the proteome are compromised including the formation of nuclear actin filaments and the formation of a normal cell stress response. Consequently, thymocyte maturation and selection, and T cell homeostatic maintenance and receptor-mediated activation are severely impaired. In the absence of CCT-controlled protein folding, Th2 polarization diverges from normal differentiation with paradoxical continued IFN-γ expression. As a result, CCT-deficient T cells fail to generate an efficient immune protection against helminths as they are unable to sustain a coordinated recruitment of the innate and adaptive immune systems. These findings thus demonstrate that normal T cell biology is critically dependent on CCT-controlled proteostasis and that its absence is incompatible with protective immunity.

Hallmarks of T cell aging.

The aged adaptive immune system is characterized by progressive dysfunction as well as increased autoimmunity. This decline is responsible for elevated susceptibility to infection and cancer, as well as decreased vaccination efficacy. Recent evidence indicates that CD4+ T cell-intrinsic alteratins contribute to chronic inflammation and are sufficient to accelerate an organism-wide aging phenotype, supporting the idea that T cell aging plays a major role in body-wide deterioration. In this Review, we propose ten molecular hallmarks to represent common denominators of T cell aging. These hallmarks are grouped into four primary hallmarks (thymic involution, mitochondrial dysfunction, genetic and epigenetic alterations, and loss of proteostasis) and four secondary hallmarks (reduction of the TCR repertoire, naive-memory imbalance, T cell senescence, and lack of effector plasticity), and together they explain the manifestation of the two integrative hallmarks (immunodeficiency and inflammaging). A major challenge now is weighing the relative impact of these hallmarks on T cell aging and understanding their interconnections, with the final goal of defining molecular targets for interventions in the aging process.

Oxidized Low-Density Lipoprotein Drives Dysfunction of the Liver Lymphatic System.

BACKGROUND AND AIMS: As the incidence of nonalcoholic steatohepatitis (NASH) continues to rise, understanding how normal liver functions are affected during disease is required before developing novel therapeutics which could reduce morbidity and mortality. However, very little is understood about how the transport of proteins and cells from the liver by the lymphatic vasculature is affected by inflammatory mediators or during disease. METHODS: To answer these questions, we utilized a well-validated mouse model of NASH and exposure to highly oxidized low density lipoprotein (oxLDL). In addition to single cell sequencing, multiplexed immunofluorescence and metabolomic analysis of liver lymphatic endothelial cells (LEC)s we evaluated lymphatic permeability and transport both in vitro and in vivo. RESULTS: Confirming similarities between human and mouse liver lymphatic vasculature in NASH, we found that the lymphatic vasculature expands as disease progresses and results in the downregulation of genes important to lymphatic identity and function. We also demonstrate, in mice with NASH, that fluorescein isothiocyanate (FITC) dextran does not accumulate in the liver draining lymph node upon intrahepatic injection, a defect that was rescued with therapeutic administration of the lymphatic growth factor, recombinant vascular endothelial growth factor C (rVEGFC). Similarly, exposure to oxLDL reduced the amount of FITC-dextran in the portal draining lymph node and through an LEC monolayer. We provide evidence that the mechanism by which oxLDL impacts lymphatic permeability is via a reduction in Prox1 expression which decreases lymphatic specific gene expression, impedes LEC metabolism and reorganizes the highly permeable lymphatic cell-cell junctions which are a defining feature of lymphatic capillaries. CONCLUSIONS: We identify oxLDL as a major contributor to decreased lymphatic permeability in the liver, a change which is consistent with decreased protein homeostasis and increased inflammation during chronic liver disease.

The eIF2α kinase HRI in innate immunity, proteostasis, and mitochondrial stress.

The integrated stress response (ISR) is an evolutionary conserved stress response pathway that leads to a global arrest in translation as well as to the expression of specific genes, such as the transcription factor ATF4, to promote cellular recovery. The central nexus of this pathway is the phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2α) by one of the four eIF2α kinases that sense specific cellular stressors. The heme-regulated inhibitor (HRI) is one of these kinases, and it was initially reported to be activated in response to heme deprivation. Nevertheless, further studies have established that cytosolic proteotoxicity, resulting from oxidative or osmotic stress, heat shock, and proteasome inhibition, is the predominant trigger for HRI to induce the ISR. In this review, we present newly identified functions of HRI in innate immunity, proteostasis, and mitochondrial stress. Indeed, HRI-mediated signaling defines a novel cytosolic unfolded protein response (cUPR) required for the proper formation of some innate immune signalosomes and the control of toxic protein aggregates, and this eIF2α kinase also serves as a relay for mitonuclear communication after a mitochondrial stress.

Mastering organismal aging through the endoplasmic reticulum proteostasis network.

The aging process is characterized by a progressive decline in the function of most tissues, representing the main risk factor in the development of a variety of human diseases. Studies in multiple animal models have demonstrated that interventions that improve the capacity to maintain endoplasmic reticulum (ER) proteostasis prolong life and healthspan. ER stress is monitored by the unfolded protein response (UPR), a signaling pathway that mediates adaptive processes to restore proteostasis or the elimination of damaged cells by apoptosis. Here, we discuss recent advances in understanding the significance of the UPR to aging and its implications for the maintenance of cell physiology of various cell types and organs. The possible benefits of targeting the UPR to extend healthspan and reduce the risk of developing age-related diseases are also discussed.

A cullin-RING ubiquitin ligase promotes thermotolerance as part of the intracellular pathogen response in Caenorhabditis elegans.

Intracellular pathogen infection leads to proteotoxic stress in host organisms. Previously we described a physiological program in the nematode Caenorhabditis elegans called the intracellular pathogen response (IPR), which promotes resistance to proteotoxic stress and appears to be distinct from canonical proteostasis pathways. The IPR is controlled by PALS-22 and PALS-25, proteins of unknown biochemical function, which regulate expression of genes induced by natural intracellular pathogens. We previously showed that PALS-22 and PALS-25 regulate the mRNA expression of the predicted ubiquitin ligase component cullin cul-6, which promotes thermotolerance in pals-22 mutants. However, it was unclear whether CUL-6 acted alone, or together with other cullin-ring ubiquitin ligase components, which comprise a greatly expanded gene family in C. elegans Here we use coimmunoprecipitation studies paired with genetic analysis to define the cullin-RING ligase components that act together with CUL-6 to promote thermotolerance. First, we identify a previously uncharacterized RING domain protein in the TRIM family we named RCS-1, which acts as a core component with CUL-6 to promote thermotolerance. Next, we show that the Skp-related proteins SKR-3, SKR-4, and SKR-5 act redundantly to promote thermotolerance with CUL-6. Finally, we screened F-box proteins that coimmunoprecipitate with CUL-6 and find that FBXA-158 and FBXA-75 promote thermotolerance. In summary, we have defined the three core components and two F-box adaptors of a cullin-RING ligase complex that promotes thermotolerance as part of the IPR in C. elegans, which adds to our understanding of how organisms cope with proteotoxic stress.

Expanding the Organismal Proteostasis Network: Linking Systemic Stress Signaling with the Innate Immune Response.

Stress response pathways regulate proteostasis and mitigate macromolecular damage to promote long-term cellular health. Intercellular signaling is an essential layer of systemic proteostasis in an organism and is facilitated via transcellular signaling molecules that orchestrate the activation of stress responses across tissues and organs. Accumulating evidence indicates that components of the immune response act as signaling factors that regulate the cell-non-autonomous proteostasis network. Here, we review emergent advances in our understanding of cell-non-autonomous regulators of proteostasis networks in multicellular settings, from the model organism, Caenorhabditis elegans, to humans. We further discuss how innate immune responses can be players of the organismal proteostasis network and discuss how both are linked in cancer.

Identification and therapeutic rescue of autophagosome and glutamate receptor defects in C9ORF72 and sporadic ALS neurons.

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease with diverse etiologies. Therefore, the identification of common disease mechanisms and therapeutics targeting these mechanisms could dramatically improve clinical outcomes. To this end, we developed induced motor neuron (iMN) models from C9ORF72 and sporadic ALS (sALS) patients to identify targets that are effective against these types of cases, which together comprise ~90% of patients. We find that iMNs from C9ORF72 and several sporadic ALS patients share two common defects - impaired autophagosome formation and the aberrant accumulation of glutamate receptors. Moreover, we show that an anticoagulation-deficient form of activated protein C, 3K3A-APC, rescues these defects in both C9ORF72 and sporadic ALS iMNs. As a result, 3K3A-APC treatment lowers C9ORF72 dipeptide repeat protein (DPR) levels, restores nuclear TDP-43 localization, and rescues the survival of both C9ORF72 and sporadic ALS iMNs. Importantly, 3K3A-APC also lowers glutamate receptor levels and rescues proteostasis in vivo in C9ORF72 gain- and loss-of-function mouse models. Thus, motor neurons from C9ORF72 and at least a subset of sporadic ALS patients share common, early defects in autophagosome formation and glutamate receptor homeostasis and a single therapeutic approach may be efficacious against these disease processes.