Targeting CD8+ T cells with natural products for tumor therapy: Revealing insights into the mechanisms

BACKGROUND

Despite significant advances in cancer immunotherapy over the past decades, such as T cell-engaging chimeric antigen receptor (CAR)-T cell therapy and immune checkpoint blockade (ICB), therapeutic failure resulting from various factors remains prevalent. Therefore, developing combinational immunotherapeutic strategies is of great significance for improving the clinical outcome of cancer immunotherapy. Natural products are substances that naturally exist in various living organisms with multiple pharmacological or biological activities, and some of them have been found to have anti-tumor potential. Notably, emerging evidences have suggested that several natural compounds may boost the anti-tumor effects through activating immune response of hosts, in which CD8+ T cells play a pivotal role.

METHODS

The data of this review come from PubMed, Web of Science, Google Scholar, and ClinicalTrials (https://clinicaltrials.gov/) with the keywords "CD8+ T cell", "anti-tumor", "immunity", "signal 1", "signal 2", "signal 3", "natural products", "T cell receptor (TCR)", "co-stimulation", "co-inhibition", "immune checkpoint", "inflammatory cytokine", "hesperidin", "ginsenoside", "quercetin", "curcumin", "apigenin", "dendrobium officinale polysaccharides (DOPS)", "luteolin", "shikonin", "licochalcone A", "erianin", "resveratrol", "procyanidin", "berberine", "usnic acid", "naringenin", "6-gingerol", "ganoderma lucidum polysaccharide (GL-PS)", "neem leaf glycoprotein (NLGP)", "paclitaxel", "source", "pharmacological activities", and "toxicity". These literatures were published between 1993 and 2023.

RESULTS

Natural products have considerable advantages as anti-tumor drugs based on the various species, wide distribution, low price, and few side effects. This review summarized the effects and mechanisms of some natural products that exhibit anti-tumor effects via targeting CD8+ T cells, mainly focused on the three signals that activate CD8+ T cells: TCR, co-stimulation, and inflammatory cytokines.

CONCLUSION

Clarifying the role and underlying mechanism of natural products in cancer immunotherapy may provide more options for combinational treatment strategies and benefit cancer therapy, to shed light on identifying potential natural compounds for improving the clinical outcome in cancer immunotherapy.

Untargeted serum metabolomic profiles and breast density in young women

PURPOSE OF THE STUDY

Breast density is an established risk factor for breast cancer. However, little is known about metabolic influences on breast density phenotypes. We conducted untargeted serum metabolomics analyses to identify metabolic signatures associated with breast density phenotypes among young women.

METHODS

In a cross-sectional study of 173 young women aged 25-29 who participated in the Dietary Intervention Study in Children 2006 Follow-up Study, 449 metabolites were measured in fasting serum samples using ultra-high-performance liquid chromatography-tandem mass spectrometry. Multivariable-adjusted mixed-effects linear regression identified metabolites associated with magnetic resonance imaging measured breast density phenotypes: percent dense breast volume (%DBV), absolute dense breast volume (ADBV), and absolute non-dense breast volume (ANDBV). Metabolite results were corrected for multiple comparisons using a false discovery rate adjusted p-value (q).

RESULTS

The amino acids valine and leucine were significantly inversely associated with %DBV. For each 1 SD increase in valine and leucine, %DBV decreased by 20.9% (q = 0.02) and 18.4% (q = 0.04), respectively. ANDBV was significantly positively associated with 16 lipid and one amino acid metabolites, whereas no metabolites were associated with ADBV. Metabolite set enrichment analysis also revealed associations of distinct metabolic signatures with %DBV, ADBV, and ANDBV; branched chain amino acids had the strongest inverse association with %DBV (p = 0.002); whereas, diacylglycerols and phospholipids were positively associated with ANDBV (p ≤ 0.002), no significant associations were observed for ADBV.

CONCLUSION

Our results suggest an inverse association of branched chain amino acids with %DBV. Larger studies in diverse populations are needed.

Comparative analysis of PDZ-binding motifs in the diacylglycerol kinase family

Diacylglycerol kinases (DGKs) control local and temporal amounts of diacylglycerol (DAG) and phosphatidic acid (PA) by converting DAG to PA through phosphorylation in cells. Certain DGK enzymes possess C-terminal sequences that encode potential PDZ-binding motifs (PBMs), which could be involved in their recruitment into supramolecular signaling complexes. In this study, we used two different interactomic approaches, quantitative native holdup (nHU) and qualitative affinity purification (AP), both coupled to mass spectrometry (MS) to investigate the PDZ partners associated with the potential PBMs of DGKs. Complementing these results with site-specific affinity interactomic data measured on isolated PDZ domain fragments and PBM motifs, as well as evolutionary conservation analysis of the PBMs of DGKs, we explored functional differences within different DGK groups. All our results indicate that putative PBM sequences of type II enzymes, namely DGKδ, DGKη, and DGKκ, are likely to be nonfunctional. In contrast, type IV enzymes, namely DGKζ and DGKι, possess highly promiscuous PBMs that interact with a set of PDZ proteins with very similar affinity interactomes. The combination of various interactomic assays and evolutionary analyses provides a useful strategy for identifying functional domains and motifs within diverse enzyme families.

Diacylglycerol kinases: A look into the future of immunotherapy

Cancer still represents the second leading cause of death right after cardiovascular diseases. According to the World Health Organization (WHO), cancer provoked around 10 million deaths in 2020, with lung and colon tumors accounting for the deadliest forms of cancer. As tumor cells become resistant to traditional therapeutic approaches, immunotherapy has emerged as a novel strategy for tumor control. T lymphocytes are key players in immune responses against tumors. Immunosurveillance allows identification, targeting and later killing of cancerous cells. Nevertheless, tumors evolve through different strategies to evade the immune response and spread in a process called metastasis. The ineffectiveness of traditional strategies to control tumor growth and expansion has led to novel approaches considering modulation of T cell activation and effector functions. Program death receptor 1 (PD-1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4) showed promising results in the early 90s and nowadays are still being exploited together with other drugs for several cancer types. Other negative regulators of T cell activation are diacylglycerol kinases (DGKs) a family of enzymes that catalyze the conversion of diacylglycerol (DAG) into phosphatidic acid (PA). In T cells, DGKα and DGKζ limit the PLCγ/Ras/ERK axis thus attenuating DAG mediated signaling and T cell effector functions. Upregulation of either of both isoforms results in impaired Ras activation and anergy induction, whereas germline knockdown mice showed enhanced antitumor properties and more effective immune responses against pathogens. Here we review the mechanisms used by DGKs to ameliorate T cell activation and how inhibition could be used to reinvigorate T cell functions in cancer context. A better knowledge of the molecular mechanisms involved upon T cell activation will help to improve current therapies with DAG promoting agents.

Human sphingomyelin synthase 1 generates diacylglycerol in the presence and absence of ceramide via multiple enzymatic activities

Sphingomyelin (SM) synthase 1 (SMS1), which is involved in lipodystrophy, deafness, and thrombasthenia, generates diacylglycerol (DG) and SM using phosphatidylcholine (PC) and ceramide as substrates. Here, we found that SMS1 possesses DG-generating activities via hydrolysis of PC and phosphatidylethanolamine (PE) in the absence of ceramide and ceramide phosphoethanolamine synthase (CPES) activity. In the presence of the same concentration (4.7 mol%) of PC and ceramide, the amounts of DG produced by SMS and PC-phospholipase C (PLC) activities of SMS1 were approximately 65% and 35% of total DG production, respectively. PC-PLC activity showed substrate selectivity for saturated and/or monounsaturated fatty acid-containing PC species. A PC-PLC/SMS inhibitor, D609, inhibited only SMS activity. Mn2+ inhibited only PC-PLC activity. Intriguingly, DG attenuated SMS/CPES activities. Our study indicates that SMS1 is a unique enzyme with PC-PLC/PE-PLC/SMS/CPES activities.

The Regulation Network of Glycerolipid Metabolism as Coregulators of Immunotherapy-Related Myocarditis

Background

To date, immunotherapy for patients with malignant tumors has shown a significant association with myocarditis. However, the mechanism of metabolic reprogramming changes for immunotherapy-related cardiotoxicity is still not well understood.

Methods

The CD45⁺ single-cell RNA sequencing (scRNA-seq) of the Pdcd1^-/-Ctla4^+/- and wild-type mouse heart in GSE213486 was downloaded to demonstrate the heterogeneity of immunocyte atlas in immunotherapy-related myocarditis. The liquid chromatography-tandem mass spectrometry (LC-MS/MS) spectrum metabolomics analysis detects the metabolic network differences. The drug prediction, organelle level interaction, mitochondrial level regulatory network, and phosphorylation site prediction for key regulators have also been screened via multibioinformatics analysis methods.

Results

The scRNA analysis shows that the T cell is the main regulatory cell subpopulation in the pathological progress of immunotherapy-related myocarditis. Mitochondrial regulation pathway significantly participated in pseudotime trajectory- (PTT-) related differential expressed genes (DEGs) in the T cell subpopulation. Additionally, both the gene set enrichment analysis (GSEA) of PTT-related DEGs and LC-MS/MS metabolomics analysis showed that mitochondrial-regulated glycerolipid metabolism plays a central role in metabolic reprogramming changes for immunotherapy-related cardiotoxicity. Finally, the hub-regulated protease of diacylglycerol kinase zeta (Dgkz) was significantly identified and widely played various roles in glycerolipid metabolism, oxidative phosphorylation, and lipid kinase activation.

Conclusion

Mitochondrial-regulated glycerolipid metabolism, especially the DGKZ protein, plays a key role in the metabolic reprogramming of immunotherapy-related myocarditis.

Predicting small molecule binding pockets on diacylglycerol kinases using chemoproteomics and AlphaFold

Diacylglycerol kinases (DGKs) are metabolic kinases involved in regulating cellular levels of diacylglycerol and phosphatidic lipid messengers. The development of selective inhibitors for individual DGKs would benefit from discovery of protein pockets available for inhibitor binding in cellular environments. Here we utilized a sulfonyl-triazole probe (TH211) bearing a DGK fragment ligand for covalent binding to tyrosine and lysine sites on DGKs in cells that map to predicted small molecule binding pockets in AlphaFold structures. We apply this chemoproteomics-AlphaFold approach to evaluate probe binding of DGK chimera proteins engineered to exchange regulatory C1 domains between DGK subtypes (DGKα and DGKζ). Specifically, we discovered loss of TH211 binding to a predicted pocket in the catalytic domain when C1 domains on DGKα were exchanged that correlated with impaired biochemical activity as measured by a DAG phosphorylation assay. Collectively, we provide a family-wide assessment of accessible sites for covalent targeting that combined with AlphaFold revealed predicted small molecule binding pockets for guiding future inhibitor development of the DGK superfamily.

Lymphocytic Choriomeningitis Virus Clone 13 Infection Results in CD8 T Cell-Mediated Host Mortality in Diacylglycerol Kinase α-Deficient Mice

Diacylglycerol is a potent element of intracellular secondary signaling cascades whose production is enhanced by cell-surface receptor agonism and function is regulated by enzymatic degradation by diacylglycerol kinases (DGKs). In T cells, stringent regulation of the activity of this second messenger maintains an appropriate balance between effector function and anergy. In this article, we demonstrate that DGKα is an indispensable regulator of TCR-mediated activation of CD8 T cells in lymphocytic choriomeningitis virus Clone 13 viral infection. In the absence of DGKα, Clone 13 infection in a murine model results in a pathologic, proinflammatory state and a multicellular immunopathologic host death that is predominantly driven by CD8 effector T cells.

Optimization of the proliferation and persistency of CAR T cells derived from human induced pluripotent stem cells

The effectiveness of chimaeric antigen receptor (CAR) T-cell immunotherapies against solid tumours relies on the accumulation, proliferation and persistency of T cells at the tumour site. Here we show that the proliferation of CD8αβ cytotoxic CAR T cells in solid tumours can be enhanced by deriving and expanding them from a single human induced-pluripotent-stem-cell clone bearing a CAR selected for efficient differentiation. We also show that the proliferation and persistency of the effector cells in the tumours can be further enhanced by genetically knocking out diacylglycerol kinase, which inhibits antigen-receptor signalling, and by transducing the cells with genes encoding for membrane-bound interleukin-15 (IL-15) and its receptor subunit IL-15Rα. In multiple tumour-bearing animal models, the engineered hiPSC-derived CAR T cells led to therapeutic outcomes similar to those of primary CD8 T cells bearing the same CAR. The optimization of effector CAR T cells derived from pluripotent stem cells may aid the development of long-lasting antigen-specific T-cell immunotherapies for the treatment of solid tumours.

The expanding role for small molecules in immuno-oncology

The advent of immune checkpoint inhibition (ICI) using antibodies against PD1 and its ligand PDL1 has prompted substantial efforts to develop complementary drugs. Although many of these are antibodies directed against additional checkpoint proteins, there is an increasing interest in small-molecule immuno-oncology drugs that address intracellular pathways, some of which have recently entered clinical trials. In parallel, small molecules that target pro-tumorigenic pathways in cancer cells and the tumour microenvironment have been found to have immunostimulatory effects that synergize with the action of ICI antibodies, leading to the approval of an increasing number of regimens that combine such drugs. Combinations with small molecules targeting cancer metabolism, cytokine/chemokine and innate immune pathways, and T cell checkpoints are now under investigation. This Review discusses the recent milestones and hurdles encountered in this area of drug development, as well as our views on the best path forward.

Ca2+ -induced structural changes and intramolecular interactions in N-terminal region of diacylglycerol kinase alpha

Diacylglycerol kinases (DGKs) are multi-domain lipid kinases that modulate the levels of lipid messengers, diacylglycerol, and phosphatidic acid. Recently, increasing attention has been paid to its α isozyme (DGKα) as a potential target for cancer immunotherapy. However, little progress has been made on the structural biology of DGKs, and a detailed understanding of the Ca²⁺ -triggered activation of DGKα, for which the N-terminal domains likely play a critical role, remains unclear. We have recently shown that Ca²⁺ binding to DGKα-EF induces conformational changes from a protease-susceptible "open" conformation in the apo state to a well-folded one in its holo state. Here, we further studied the structural properties of DGKα N-terminal (RVH and EF) domains using a series of biophysical techniques. We first revealed that the N-terminal RVH domain is a novel Ca²⁺ -binding domain, but the Ca²⁺ -induced conformational changes mainly occur in the EF domain. This was corroborated by NMR experiments showing that the EF domain adopts a molten-globule like structure in the apo state. Further analyses using SEC-SAXS and NMR indicate that the partially unfolded EF domain interacts with RVH domain, likely via hydrophobic interactions in the absence of Ca²⁺ , and this interaction is modified in the presence of Ca²⁺ . Taken together, these results present novel insights into the structural rearrangement of DGKα N-terminal domains upon binding to Ca²⁺ , which is essential for the activation of the enzyme.

Overarching roles of diacylglycerol signaling in cancer development and antitumor immunity

Diacylglycerol (DAG) is a lipid second messenger that is generated in response to extracellular stimuli and channels intracellular signals that affect mammalian cell proliferation, survival, and motility. DAG exerts a myriad of biological functions through protein kinase C (PKC) and other effectors, such as protein kinase D (PKD) isozymes and small GTPase-regulating proteins (such as RasGRPs). Imbalances in the fine-tuned homeostasis between DAG generation by phospholipase C (PLC) enzymes and termination by DAG kinases (DGKs), as well as dysregulation in the activity or abundance of DAG effectors, have been widely associated with tumor initiation, progression, and metastasis. DAG is also a key orchestrator of T cell function and thus plays a major role in tumor immunosurveillance. In addition, DAG pathways shape the tumor ecosystem by arbitrating the complex, dynamic interaction between cancer cells and the immune landscape, hence representing powerful modifiers of immune checkpoint and adoptive T cell-directed immunotherapy. Exploiting the wide spectrum of DAG signals from an integrated perspective could underscore meaningful advances in targeted cancer therapy.

The Roles of Diacylglycerol Kinase α in Cancer Cell Proliferation and Apoptosis

Simple Summary

Diacylglycerol (DG) kinase (DGK) phosphorylates DG to generate phosphatidic acid (PA). DGKα is highly expressed in several refractory cancer cells, including melanoma, hepatocellular carcinoma, and glioblastoma cells, attenuates apoptosis, and promotes proliferation. In cancer cells, PA produced by DGKα plays an important role in proliferation/antiapoptosis. In addition to cancer cells, DGKα is highly abundant in T cells and induces a nonresponsive state (anergy), representing the main mechanism by which advanced cancers avoid immune action. In T cells, DGKα induces anergy through DG consumption. Therefore, a DGKα-specific inhibitor is expected to be a dual effective anticancer treatment that inhibits cancer cell proliferation and simultaneously activates T cell function. Moreover, the inhibition of DGKα synergistically enhances the anticancer effects of programmed cell death-1/programmed cell death ligand 1 blockade. Taken together, DGKα inhibition provides a promising new treatment strategy for refractory cancers.

Abstract

Diacylglycerol (DG) kinase (DGK) phosphorylates DG to generate phosphatidic acid (PA). The α isozyme is activated by Ca²⁺ through its EF-hand motifs and tyrosine phosphorylation. DGKα is highly expressed in several refractory cancer cells including melanoma, hepatocellular carcinoma, and glioblastoma cells. In melanoma cells, DGKα is an antiapoptotic factor that activates nuclear factor-κB (NF-κB) through the atypical protein kinase C (PKC) ζ-mediated phosphorylation of NF-κB. DGKα acts as an enhancer of proliferative activity through the Raf–MEK–ERK pathway and consequently exacerbates hepatocellular carcinoma progression. In glioblastoma and melanoma cells, DGKα attenuates apoptosis by enhancing the phosphodiesterase (PDE)-4A1–mammalian target of the rapamycin pathway. As PA activates PKCζ, Raf, and PDE, it is likely that PA generated by DGKα plays an important role in the proliferation/antiapoptosis of cancer cells. In addition to cancer cells, DGKα is highly abundant in T cells and induces a nonresponsive state (anergy), which represents the main mechanism by which advanced cancers escape immune action. In T cells, DGKα attenuates the activity of Ras-guanyl nucleotide-releasing protein, which is activated by DG and avoids anergy through DG consumption. Therefore, a DGKα-specific inhibitor is expected to be a dual effective anticancer treatment that inhibits cancer cell proliferation and simultaneously enhances T cell functions. Moreover, the inhibition of DGKα synergistically enhances the anticancer effects of programmed cell death-1/programmed cell death ligand 1 blockade. Taken together, DGKα inhibition provides a promising new treatment strategy for refractory cancers.

Diacylglycerol kinase ζ limits IL-2-dependent control of PD-1 expression in tumor-infiltrating T lymphocytes

Background

The inhibitory functions triggered by the programmed cell death-1 (PD-1) receptor following binding to its ligand (PD-L1) protect healthy organs from cytotoxic T cells, and neutralize antitumor T cell attack. Antibody-based therapies to block PD-1/PD-L1 interaction have yielded notable results, but most patients eventually develop resistance. This failure is attributed to CD8⁺ T cells achieving hyporesponsive states from which recovery is hardly feasible. Dysfunctional T cell phenotypes are favored by a sustained imbalance in the diacylglycerol (DAG)- and Ca²⁺-regulated transcriptional programs. In mice, DAG kinase ζ (DGKζ) facilitates DAG consumption, limiting T cell activation and cytotoxic T cell responses. DGKζ deficiency facilitates tumor rejection in mice without apparent adverse autoimmune effects. Despite its therapeutic potential, little is known about DGKζ function in human T cells, and no known inhibitors target this isoform.

Methods

We used a human triple parameter reporter cell line to examine the consequences of DGKζ depletion on the transcriptional restriction imposed by PD-1 ligation. We studied the effect of DGKζ deficiency on PD-1 expression dynamics, as well as the impact of DGKζ absence on the in vivo growth of MC38 adenocarcinoma cells.

Results

We demonstrate that DGKζ depletion enhances DAG-regulated transcriptional programs, promoting interleukin-2 production and partially counteracting PD-1 inhibitory functions. DGKζ loss results in limited PD-1 expression and enhanced expansion of cytotoxic CD8⁺ T cell populations. This is observed even in immunosuppressive milieus, and correlates with the reduced ability of MC38 adenocarcinoma cells to form tumors in DGKζ-deficient mice.

Conclusions

Our results, which define a role for DGKζ in the control of PD-1 expression, confirm DGKζ potential as a therapeutic target as well as a biomarker of CD8⁺ T cell dysfunctional states.

The expression of diacylglycerol kinase isoforms α and ζ correlates with the progression of experimental autoimmune encephalomyelitis in rats

Multiple sclerosis (MS) is characterized by neuroinflammation and neurodegeneration, whose precise processes are not fully understood. Diacylglycerol kinase (DGK) isozymes of α, β, γ and ζ expressed abundantly in the brain and/or the immune system, may be regulatory targets for MS. In this study, we analyzed the four DGK isozymes along the induction, peak and recovery phases in an experimental autoimmune encephalomyelitis (EAE) rat model of MS. The expression of these DGK isozymes and the diacylglycerol (DAG) pathway in the EAE rat brainstems were analyzed by qRT-PCR, immunohistochemistry, immunofluorescence double staining, western blotting and ELISA. Our results showed that the mRNA content of the four DGK isozymes decreased significantly, and their immunoreactivity in myelin sheathes (DGKα, β) and neurons (DGKγ, ζ) became weaker at the beginning of the induction phase. With the progressive increase in clinical signs, DGKα, DGKγ and DGKζ mRNA increased and DGKβ mRNA decreased, and microglia were involved in the formation of perivascular cuffing. In the peak phase, both DGKα and DGKζ were expressed in neurons and inflammatory cells, and DGKζ was also positive in microglia. During the recovery phase, the mRNA content and immunoreactivity of these DGK isozymes generally reached normal levels. Moreover, our results revealed that changes in DAG accumulation and PKCδ phosphorylation were almost the same as those of DGKα and DGKζ mRNA. In summary, the four DGK isozymes are involved in the EAE process. The predominant and broad presence of DGKα and DGKζ suggests that they may regulate the pathological process by attenuating DAG/PKCδ pathway signaling during EAE evolution.

Membrane morphology determines diacylglycerol kinase α substrate acyl chain specificity

Diacylglycerol kinases catalyze the ATP-dependent phosphorylation of diacylglycerol (DAG) to produce phosphatidic acid (PA). In humans, the alpha isoform (DGKα) has emerged as a potential target in the treatment of cancer due to its anti-tumor and pro-immune responses. However, its mechanism of action at a molecular level is not fully understood. In this work, a systematic investigation of the role played by the membrane in the regulation of the enzymatic properties of human DGKα is presented. By using a cell-free system with purified DGKα and model membranes of variable physical and chemical properties, it is shown that membrane physical properties determine human DGKα substrate acyl chain specificity. In model membranes with a flat morphology; DGKα presents high enzymatic activity, but it is not able to differentiate DAG molecular species. Furthermore, DGKα enzymatic properties are insensitive to membrane intrinsic curvature. However, in the presence of model membranes with altered morphology, specifically the presence of physically curved membrane structures, DGKα bears substrate acyl chain specificity for palmitic acid-containing DAG. The present results identify changes in membrane morphology as one possible mechanism for the depletion of specific pools of DAG as well as the production of specific pools of PA by DGKα, adding an extra layer of regulation on the interconversion of these two potent lipid-signaling molecules. It is proposed that the interplay between membrane physical (shape) and chemical (lipid composition) properties guarantee a fine-tuned signal transduction system dependent on the levels and molecular species of DAG and PA.

Diacylglycerol kinase α inhibition cooperates with PD-1-targeted therapies to restore the T cell activation program

BACKGROUND

Antibody-based therapies blocking the programmed cell death-1/ligand-1 (PD-1/PD-L1) axis have provided unprecedent clinical success in cancer treatment. Acquired resistance, however, frequently occurs, commonly associated with the upregulation of additional inhibitory molecules. Diacylglycerol kinase (DGK) α limits the extent of Ras activation in response to antigen recognition, and its upregulation facilitates hypofunctional, exhausted T cell states. Pharmacological DGKα targeting restores cytotoxic function of chimeric antigen receptor and CD8⁺ T cells isolated from solid tumors, suggesting a mechanism to reverse T cell exhausted phenotypes. Nevertheless, the contribution of DGKα downstream of the PD-1/PD-L1 inhibitory axis in human T cells and the consequences of combining DGKα and anti-PD-1/PD-L1 inhibitors are still unresolved relevant issues.

MATERIALS AND METHODS

We used a human triple parameter reporter cell line to investigate DGKα contribution to the PD-1/PD-L1 inhibitory pathway. We also addressed the impact of deleting DGKα expression in the growth dynamics and systemic tumor-derived effects of a PD-1-related tumor model, the MC38 colon adenocarcinoma.

RESULTS

We identify DGKα as a contributor to the PD-1/PD-L1 axis that strongly limits the Ras/ERK/AP-1 pathway. DGKα function reinforces exhausted T cell phenotypes ultimately promoting tumor growth and generalized immunosuppression. Pharmacological DGKα inhibition selectively enhances AP-1 transcription and, importantly, cooperates with antibodies blocking the PD-1/PD-L1 interrelation.

CONCLUSIONS

Our results indicate that DGKα inhibition could provide an important mechanism to revert exhausted T lymphocyte phenotypes and thus favor proper anti-tumor T cell responses. The cooperative effect observed after PD-1/PD-L1 and DGKα blockade offers a promising strategy to improve the efficacy of immunotherapy in the treatment of cancer.

Membrane shape as determinant of protein properties

Membrane lipids play a role in the modulation of a variety of biological processes. This is often achieved through fine-tuned changes in membrane physical and chemical properties. While some membrane physical properties (e.g., curvature, lipid domains, fluidity) have received increased scientific attention over the years, only recently has membrane shape emerged as an active modulator of protein properties. Biological membranes are mostly found organized into a lipid bilayer arrangement, in which the spontaneous shape is an intrinsically flat, planar morphology (in relation to the size of proteins). However, it is known that many cells and organelles have non-planar morphologies. In addition, perturbations in membrane morphology occur in a variety of biological processes. Recent studies have shown that membrane shape can modulate a variety of biological processes by determining protein properties. While membrane shape generation modulates proteins via changes in membrane mechanical properties, membrane shape recognition regulates proteins by providing the optimal surface for interaction. Hence, membranes have evolved an elegant mechanism to couple mesoscopic perturbations to molecular properties and vice-versa. In this review, the regulation of the enzymatic properties of two isoforms of mammalian diacylglycerol kinase, which play important roles in cellular signal transductions, will be used to exemplify the recent advancements in the field of membrane shape recognition, as well as future challenges and perspectives.

DGKA Mediates Resistance to PD-1 Blockade

Immunologic checkpoint blockade has been proven effective in a variety of malignancies. However, high rates of resistance have substantially hindered its clinical use. Understanding the underlying mechanisms may lead to new strategies for improving therapeutic efficacy. Although a number of signaling pathways have been shown to be associated with tumor cell-mediated resistance to immunotherapy, T cell-intrinsic resistant mechanisms remain elusive. Here, we demonstrated that diacylglycerol kinase alpha (Dgka) mediated T-cell dysfunction during anti-PD-1 therapy by exacerbating the exhaustion of reinvigorated tumor-specific T cells. Pharmacologic ablation of Dgka postponed T-cell exhaustion and delayed development of resistance to PD-1 blockade. Dgka inhibition also enhanced the efficacy of anti-PD-1 therapy. We further found that the expression of DGKA in cancer cells promoted tumor growth via the AKT signaling pathway, suggesting that DGKA might be a target in tumor cells as well. Together, these findings unveiled a molecular pathway mediating resistance to PD-1 blockade and provide a potential therapeutic strategy with combination immunotherapy.

2020 White Paper on Recent Issues in Bioanalysis: BAV Guidance, CLSI H62, Biotherapeutics Stability, Parallelism Testing, CyTOF and Regulatory Feedback (Part 2A - Recommendations on Biotherapeutics Stability, PK LBA Regulated Bioanalysis, Biomarkers Assays, Cytometry Validation & Innovation Part 2B - Regulatory Agencies' Inputs on Bioanalysis, Biomarkers, Immunogenicity, Gene & Cell Therapy and Vaccine)

The 14^th edition of the Workshop on Recent Issues in Bioanalysis (14^th WRIB) was held virtually on June 15-29, 2020 with an attendance of over 1000 representatives from pharmaceutical/biopharmaceutical companies, biotechnology companies, contract research organizations, and regulatory agencies worldwide. The 14^th WRIB included three Main Workshops, seven Specialized Workshops that together spanned 11 days in order to allow exhaustive and thorough coverage of all major issues in bioanalysis, biomarkers, immunogenicity, gene therapy and vaccine. Moreover, a comprehensive vaccine assays track; an enhanced cytometry track and updated Industry/Regulators consensus on BMV of biotherapeutics by LCMS were special features in 2020. As in previous years, this year's WRIB continued to gather a wide diversity of international industry opinion leaders and regulatory authority experts working on both small and large molecules to facilitate sharing and discussions focused on improving quality, increasing regulatory compliance and achieving scientific excellence on bioanalytical issues. This 2020 White Paper encompasses recommendations emerging from the extensive discussions held during the workshop, and is aimed to provide the Global Bioanalytical Community with key information and practical solutions on topics and issues addressed, in an effort to enable advances in scientific excellence, improved quality and better regulatory compliance. Due to its length, the 2020 edition of this comprehensive White Paper has been divided into three parts for editorial reasons. This publication covers the recommendations on (Part 2A) BAV, PK LBA, Flow Cytometry Validation and Cytometry Innovation and (Part 2B) Regulatory Input. Part 1 (Innovation in Small Molecules, Hybrid LBA/LCMS & Regulated Bioanalysis), Part 3 (Vaccine, Gene/Cell Therapy, NAb Harmonization and Immunogenicity) are published in volume 13 of Bioanalysis, issues 4, and 6 (2021), respectively.

Combined inhibition/silencing of diacylglycerol kinase α and ζ simultaneously and synergistically enhances interleukin-2 production in T cells and induces cell death of melanoma cells

The α-isozyme of diacylglycerol kinase (DGK) enhances cancer cell proliferation and, conversely, it promotes the nonresponsive immune state known as T-cell anergy. Moreover, a DGKα-selective inhibitor, CU-3, induced cell death in cancer-derived cells and simultaneously enhanced T-cell interleukin-2 production. In addition to DGKα, DGKζ is also known to induce T-cell anergy. In the present study, we examined whether combined inhibition/silencing of DGKα and DGKζ synergistically enhanced T-cell activity. Combined treatment with CU-3 or DGKα-small interfering RNA (siRNA) and DGKζ-siRNA more potently enhanced T-cell receptor-crosslink-dependent interleukin-2 production in Jurkat T cells than treatment with either alone. Intriguingly, in addition to activating T cells, dual inhibition/silencing of DGKα and DGKζ synergistically reduced viability and increased caspase 3/7 activity in AKI melanoma cells. Taken together, these results indicate that combined inhibition/silencing of DGKα and DGKζ simultaneously and synergistically enhances interleukin-2 production in T cells and induces cell death in melanoma. Therefore, dual inhibition/silencing of these DGK isozymes represents an ideal therapy that potently attenuates cancer cell proliferation and simultaneously enhances immune responses that impact anticancer immunity.

Beyond Lipid Signaling: Pleiotropic Effects of Diacylglycerol Kinases in Cellular Signaling

The diacylglycerol kinase family, which can attenuate diacylglycerol signaling and activate phosphatidic acid signaling, regulates various signaling transductions in the mammalian cells. Studies on the regulation of diacylglycerol and phosphatidic acid levels by various enzymes, the identification and characterization of various diacylglycerol and phosphatidic acid-regulated proteins, and the overlap of different diacylglycerol and phosphatidic acid metabolic and signaling processes have revealed the complex and non-redundant roles of diacylglycerol kinases in regulating multiple biochemical and biological networks. In this review article, we summarized recent progress in the complex and non-redundant roles of diacylglycerol kinases, which is expected to aid in restoring dysregulated biochemical and biological networks in various pathological conditions at the bed side.

New Era of Diacylglycerol Kinase, Phosphatidic Acid and Phosphatidic Acid-Binding Protein

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DG) to generate phosphatidic acid (PA). Mammalian DGK consists of ten isozymes (α–κ) and governs a wide range of physiological and pathological events, including immune responses, neuronal networking, bipolar disorder, obsessive-compulsive disorder, fragile X syndrome, cancer, and type 2 diabetes. DG and PA comprise diverse molecular species that have different acyl chains at the sn-1 and sn-2 positions. Because the DGK activity is essential for phosphatidylinositol turnover, which exclusively produces 1-stearoyl-2-arachidonoyl-DG, it has been generally thought that all DGK isozymes utilize the DG species derived from the turnover. However, it was recently revealed that DGK isozymes, except for DGKε, phosphorylate diverse DG species, which are not derived from phosphatidylinositol turnover. In addition, various PA-binding proteins (PABPs), which have different selectivities for PA species, were recently found. These results suggest that DGK–PA–PABP axes can potentially construct a large and complex signaling network and play physiologically and pathologically important roles in addition to DGK-dependent attenuation of DG–DG-binding protein axes. For example, 1-stearoyl-2-docosahexaenoyl-PA produced by DGKδ interacts with and activates Praja-1, the E3 ubiquitin ligase acting on the serotonin transporter, which is a target of drugs for obsessive-compulsive and major depressive disorders, in the brain. This article reviews recent research progress on PA species produced by DGK isozymes, the selective binding of PABPs to PA species and a phosphatidylinositol turnover-independent DG supply pathway.

Polyunsaturated fatty acid-containing phosphatidic acids selectively interact with L-lactate dehydrogenase A and induce its secondary structural change and inactivation

Phosphatidic acid (PA) consists of various molecular species that have different fatty acyl chains at the sn-1 and sn-2 positions; and consequently, mammalian cells contain at least 50 structurally distinct PA molecular species. However, the different roles of each PA species are poorly understood. In the present study, we attempted to identify dipalmitoyl (16:0/16:0)-PA-binding proteins from mouse skeletal muscle using liposome precipitation and tandem mass spectrometry analysis. We identified L-lactate dehydrogenase (LDH) A, which catalyzes conversion of pyruvate to lactate and is a key checkpoint of anaerobic glycolysis critical for tumor growth, as a 16:0/16:0-PA-binding protein. LDHA did not substantially associate with other phospholipids, such as phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, phosphatidylinositol, phosphoinositides and cardiolipin at physiological pH (7.4), indicating that LDHA specifically bound to PA. Interestingly, 18:0/18:0-, 18:0/20:4- and 18:0/22:6-PA also interacted with LDHA, and their binding activities were stronger than 16:0/16:0-PA at pH 7.4. Moreover, circular dichroism spectrometry showed that 18:0/20:4- and 18:0/22:6-PA, but not 16:0/16:0- or 18:0/18:0-PA, significantly reduced the α-helical structure of LDHA. Furthermore, 18:0/20:4- and 18:0/22:6-PA attenuated LDH activity. Taken together, we demonstrated for the first time that LDHA is a PA-binding protein and is a unique PA-binding protein that is structurally and functionally controlled by associating with 18:0/20:4- and 18:0/22:6-PA.

Interplay Between SNX27 and DAG Metabolism in the Control of Trafficking and Signaling at the IS

Recognition of antigens displayed on the surface of an antigen-presenting cell (APC) by T-cell receptors (TCR) of a T lymphocyte leads to the formation of a specialized contact between both cells named the immune synapse (IS). This highly organized structure ensures cell–cell communication and sustained T-cell activation. An essential lipid regulating T-cell activation is diacylglycerol (DAG), which accumulates at the cell–cell interface and mediates recruitment and activation of proteins involved in signaling and polarization. Formation of the IS requires rearrangement of the cytoskeleton, translocation of the microtubule-organizing center (MTOC) and vesicular compartments, and reorganization of signaling and adhesion molecules within the cell–cell junction. Among the multiple players involved in this polarized intracellular trafficking, we find sorting nexin 27 (SNX27). This protein translocates to the T cell–APC interface upon TCR activation, and it is suggested to facilitate the transport of cargoes toward this structure. Furthermore, its interaction with diacylglycerol kinase ζ (DGKζ), a negative regulator of DAG, sustains the precise modulation of this lipid and, thus, facilitates IS organization and signaling. Here, we review the role of SNX27, DAG metabolism, and their interplay in the control of T-cell activation and establishment of the IS.

Diacylglycerol Kinase Alpha in Radiation-Induced Fibrosis: Potential as a Predictive Marker or Therapeutic Target

Radiotherapy is an efficient tool in cancer treatment, but it brings along the risk of side effects such as fibrosis in the irradiated healthy tissue thus limiting tumor control and impairing quality of life of cancer survivors. Knowledge on radiation-related fibrosis risk and therapeutic options is still limited and requires further research. Recent studies demonstrated that epigenetic regulation of diacylglycerol kinase alpha (DGKA) is associated with radiation-induced fibrosis. However, the specific mechanisms are still unknown. In this review, we scrutinized the role of DGKA in the radiation response and in further cellular functions to show the potential of DGKA as a predictive marker or a novel target in fibrosis treatment. DGKA was reported to participate in immune response, lipid signaling, exosome production, and migration as well as cell proliferation, all processes which are suggested to be critical steps in fibrogenesis. Most of these functions are based on the conversion of diacylglycerol (DAG) to phosphatidic acid (PA) at plasma membranes, but DGKA might have also other, yet not well-known functions in the nucleus. Current evidence summarized here underlines that DGKA activation may play a central role in fibrosis formation post-irradiation and shows a potential of direct DGKA inhibitors or epigenetic modulators to attenuate pro-fibrotic reactions, thus providing novel therapeutic choices.

Diacylglycerol kinase ζ promotes actin cytoskeleton remodeling and mechanical forces at the B cell immune synapse

Diacylglycerol kinases (DGKs) limit antigen receptor signaling in immune cells by consuming the second messenger diacylglycerol (DAG) to generate phosphatidic acid (PA). Here, we showed that DGKζ promotes lymphocyte function-associated antigen 1 (LFA-1)-mediated adhesion and F-actin generation at the immune synapse of B cells with antigen-presenting cells (APCs), mostly in a PA-dependent manner. Measurement of single-cell mechanical force generation indicated that DGKζ-deficient B cells exerted lower forces at the immune synapse than did wild-type B cells. Nonmuscle myosin activation and translocation of the microtubule-organizing center (MTOC) to the immune synapse were also impaired in DGKζ-deficient B cells. These functional defects correlated with the decreased ability of B cells to present antigen and activate T cells in vitro. The in vivo germinal center response of DGKζ-deficient B cells was also reduced compared with that of wild-type B cells, indicating that loss of DGKζ in B cells impaired T cell help. Together, our data suggest that DGKζ shapes B cell responses by regulating actin remodeling, force generation, and antigen uptake-related events at the immune synapse. Hence, an appropriate balance in the amounts of DAG and PA is required for optimal B cell function.

CDP-Diacylglycerol Synthases (CDS): Gateway to Phosphatidylinositol and Cardiolipin Synthesis

Cytidine diphosphate diacylglycerol (CDP-DAG) is a key intermediate in the synthesis of phosphatidylinositol (PI) and cardiolipin (CL). Both PI and CL have highly specialized roles in cells. PI can be phosphorylated and these phosphorylated derivatives play major roles in signal transduction, membrane traffic, and maintenance of the actin cytoskeletal network. CL is the signature lipid of mitochondria and has a plethora of functions including maintenance of cristae morphology, mitochondrial fission, and fusion and for electron transport chain super complex formation. Both lipids are synthesized in different organelles although they share the common intermediate, CDP-DAG. CDP-DAG is synthesized from phosphatidic acid (PA) and CTP by enzymes that display CDP-DAG synthase activities. Two families of enzymes, CDS and TAMM41, which bear no sequence or structural relationship, have now been identified. TAMM41 is a peripheral membrane protein localized in the inner mitochondrial membrane required for CL synthesis. CDS enzymes are ancient integral membrane proteins found in all three domains of life. In mammals, they provide CDP-DAG for PI synthesis and for phosphatidylglycerol (PG) and CL synthesis in prokaryotes. CDS enzymes are critical for maintaining phosphoinositide levels during phospholipase C (PLC) signaling. Hydrolysis of PI (4,5) bisphosphate by PLC requires the resynthesis of PI and CDS enzymes catalyze the rate-limiting step in the process. In mammals, the protein products of two CDS genes (CDS1 and CDS2) localize to the ER and it is suggested that CDS2 is the major CDS for this process. Expression of CDS enzymes are regulated by transcription factors and CDS enzymes may also contribute to CL synthesis in mitochondria. Studies of CDS enzymes in protozoa reveal spatial segregation of CDS enzymes from the rest of the machinery required for both PI and CL synthesis identifying a key gap in our understanding of how CDP-DAG can cross the different membrane compartments in protozoa and in mammals.

Reprogramming fatty acyl specificity of lipid kinases via C1 domain engineering

C1 domains are lipid-binding modules that regulate membrane activation of kinases, nucleotide exchange factors and other C1-containing proteins to trigger signal transduction. Despite annotation of typical C1 domains as diacylglycerol (DAG) and phorbol ester sensors, the function of atypical counterparts remains ill-defined. Here, we assign a key role for atypical C1 domains in mediating DAG fatty acyl specificity of diacylglycerol kinases (DGKs) in live cells. Activity-based proteomics mapped C1 probe binding as a principal differentiator of type 1 DGK active sites that combined with global metabolomics revealed a role for C1s in lipid substrate recognition. Protein engineering by C1 domain swapping demonstrated that exchange of typical and atypical C1s is functionally tolerated and can directly program DAG fatty acyl specificity of type 1 DGKs. Collectively, we describe a protein engineering strategy for studying metabolic specificity of lipid kinases to assign a role for atypical C1 domains in cell metabolism.

DGK α and ζ Activities Control TH1 and TH17 Cell Differentiation

CD4⁺ T helper (T_H) cells are critical for protective adaptive immunity against pathogens, and they also contribute to the pathogenesis of autoimmune diseases. How T_H differentiation is regulated by the TCR's downstream signaling is still poorly understood. We describe here that diacylglycerol kinases (DGKs), which are enzymes that convert diacylglycerol (DAG) to phosphatidic acid, exert differential effects on T_H cell differentiation in a DGK dosage-dependent manner. A deficiency of either DGKα or ζ selectively impaired T_H1 differentiation without obviously affecting T_H2 and T_H17 differentiation. However, simultaneous ablation of both DGKα and ζ promoted T_H1 and T_H17 differentiation in vitro and in vivo, leading to exacerbated airway inflammation. Furthermore, we demonstrate that dysregulation of T_H17 differentiation of DGKα and ζ double-deficient CD4⁺ T cells was, at least in part, caused by increased mTOR complex 1/S6K1 signaling.

Biological regulation of diacylglycerol kinases in normal and neoplastic tissues: New opportunities for cancer immunotherapy

In the recent years, the arsenal of anti-cancer therapies has evolved to target T lymphocytes and restore their capacity to destroy tumor cells. However, the clinical success is limited, with a large number of patients that never responds and others that ultimately develop resistances. Overcoming the hypofunctional state imposed by solid tumors to T cells has revealed critical but challenging due to the complex strategies that tumors employ to evade the immune system. The Diacylglycerol kinases (DGK) limit DAG-dependent functions in T lymphocytes and their upregulation in tumor-infiltrating T lymphocytes contribute to limit T cell cytotoxic potential. DGK blockade could reinstate T cell attack on tumors, limiting at the same time tumor cell growth, thanks to the DGK positive input into several oncogenic pathways. In this review we summarize the latest findings regarding the regulation of specific DGK isoforms in healthy and anergic T lymphocytes, as well as their contribution to oncogenic phenotypes. We will also revise the latest advances in the search for pharmacological inhibitors and their potential as anti-cancer agents, either alone or in combination with immunomodulatory agents.

Graded diacylglycerol kinases α and ζ activities ensure mucosal-associated invariant T-cell development in mice

Mucosal-associated invariant T (MAIT) cells participate in both protective immunity and pathogenesis of diseases. Most murine MAIT cells express an invariant TCRVα19-Jα33 (iVα19) TCR, which triggers signals crucial for their development. However, signal pathways downstream of the iVα19TCR and their regulation in MAIT cells are unknown. Diacylglycerol (DAG) is a critical second messenger that relays the TCR signal to multiple downstream signaling cascades. DAG is terminated by DAG kinase (DGK)-mediated phosphorylation and conversion to phosphatidic acid. We have demonstrated here that downregulation of DAG caused by enhanced DGK activity impairs late-stage MAIT cell maturation in both thymus and spleen. Moreover, deficiency of DGKζ but not DGKα by itself causes modest decreases in MAIT cells, and deficiency of both DGKα and ζ results in severe reductions of MAIT cells in an autonomous manner. Our studies have revealed that DAG signaling is not only critical but also must be tightly regulated by DGKs for MAIT cell development and that both DGKα and, more prominently, DGKζ contribute to the overall DGK activity for MAIT cell development.

Beyond the Cell Surface: Targeting Intracellular Negative Regulators to Enhance T cell Anti-Tumor Activity

It is well established that extracellular proteins that negatively regulate T cell function, such as Cytotoxic T-Lymphocyte-Associated protein 4 (CTLA-4) and Programmed Cell Death protein 1 (PD-1), can be effectively targeted to enhance cancer immunotherapies and Chimeric Antigen Receptor T cells (CAR-T cells). Intracellular proteins that inhibit T cell receptor (TCR) signal transduction, though less well studied, are also potentially useful therapeutic targets to enhance T cell activity against tumor. Four major classes of enzymes that attenuate TCR signaling include E3 ubiquitin kinases such as the Casitas B-lineage lymphoma proteins (Cbl-b and c-Cbl), and Itchy (Itch), inhibitory tyrosine phosphatases, such as Src homology region 2 domain-containing phosphatases (SHP-1 and SHP-2), inhibitory protein kinases, such as C-terminal Src kinase (Csk), and inhibitory lipid kinases such as Src homology 2 (SH2) domain-containing inositol polyphosphate 5-phosphatase (SHIP) and Diacylglycerol kinases (DGKs). This review describes the mechanism of action of eighteen intracellular inhibitory regulatory proteins in T cells within these four classes, and assesses their potential value as clinical targets to enhance the anti-tumor activity of endogenous T cells and CAR-T cells.

DGKα in Neutrophil Biology and Its Implications for Respiratory Diseases

Diacylglycerol kinases (DGKs) play a key role in phosphoinositide signaling by removing diacylglycerol and generating phosphatidic acid. Besides the well-documented role of DGKα and DGKζ as negative regulators of lymphocyte responses, a robust body of literature points to those enzymes, and specifically DGKα, as crucial regulators of leukocyte function. Upon neutrophil stimulation, DGKα activation is necessary for migration and a productive response. The role of DGKα in neutrophils is evidenced by its aberrant behavior in juvenile periodontitis patients, which express an inactive DGKα transcript. Together with in vitro experiments, this suggests that DGKs may represent potential therapeutic targets for disorders where inflammation, and neutrophils in particular, plays a major role. In this paper we focus on obstructive respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), but also rare genetic diseases such as alpha-1-antitrypsin deficiency. Indeed, the biological role of DGKα is understudied outside the T lymphocyte field. The recent wave of research aiming to develop novel and specific inhibitors as well as KO mice will allow a better understanding of DGK's role in neutrophilic inflammation. Better knowledge and pharmacologic tools may also allow DGK to move from the laboratory bench to clinical trials.

DGKζ Plays Crucial Roles in the Proliferation and Tumorigenicity of Human glioblastoma

Glioblastoma is one of the most malignant brain cancers in adults, and it is a fatal disease because of its untimely pathogenetic location detection, infiltrative growth, and unfavorable prognosis. Unfortunately, multimodal treatment with maximal safe resection, chemotherapy and radiation has not increased the survival rate of patients with glioblastoma. Gene- and molecular-targeted therapy is considered to be a promising anticancer strategy for glioblastoma. The identification of novel potential targets in glioblastoma is of high importance. In this study, we found that both the mRNA and protein levels of diacylglycerol kinase ζ (DGKζ) were significantly higher in glioblastoma tissues than in precancerous lesions. The silencing of DGKζ by lentivirus-delivered shRNA reduced glioblastoma cell proliferation and induced G0/G1 phase arrest. Moreover, knockdown of DGKζ expression in U251 cells markedly reduced in vitro colony formation and in vivo tumorigenic capability. Further study showed that DGKζ inhibition resulted in decreases in cyclin D1, p-AKT and p-mTOR. Moreover, the rescue or overexpression of DGKζ in glioblastoma cells demonstrated the oncogenic function of DGKζ. In conclusion, these studies suggest that the suppression of DGKζ may inhibit the tumor growth of glioblastoma cells with high DGKζ expression. Thus, DGKζ might be a potential therapeutic target in malignant glioblastoma.

Phospholipids: Pulling Back the Actin Curtain for Granule Delivery to the Immune Synapse

Phosphoinositides, together with the phospholipids phosphatidylserine and phosphatidic acid, are important components of the plasma membrane acting as second messengers that, with diacylglycerol, regulate a diverse range of signaling events converting extracellular changes into cellular responses. Local changes in their distribution and membrane charge on the inner leaflet of the plasma membrane play important roles in immune cell function. Here we discuss their distribution and regulators highlighting the importance of membrane changes across the immune synapse on the cytoskeleton and the impact on the function of cytotoxic T lymphocytes.

Transcriptional Activity of FOXO Transcription Factors Measured by Luciferase Assays

The Forkhead box O (FOXO) family of transcription factors translates environmental cues into gene expression. FOXO factors are crucial for the maintenance of cell homeostasis, with important roles in cell fate decisions and differentiation. Identification of FOXO target genes requires strict validation by several methods. Luciferase-based reporters are a valuable starting point for determining the transcription-promoting capacity of potential FOXO-binding sites in candidate genes. Luciferase, an enzyme found in bioluminescent organisms catalyzes oxidation of luciferin to produce oxyluciferin together with light, which can be easily detected and measured with a luminometer. Due to their many advantages, transcriptional assays based on luciferase activity are widely used; they are easy, highly reproducible, and very sensitive. Continued improvements in luciferase-based vectors and measurement reagents confer considerable versatility. Luciferase-based reporters are also a reliable approach in the search for unknown components in the signaling pathways that control FOXO factor activity.We previously reported that FOXO transcription factors control expression of the enzyme diacylglycerol kinase α (DGKα) in T cells. DGKα consumes diacylglycerol, a lipid that activates several mitogenic pathways. Here, we describe the use of a luciferase-based promoter bearing the FOXO-binding sites of the DGKα gene to explore the relationship between the expression of this enzyme and stress conditions in NIH3T3 mouse fibroblasts. Our data support a role for FOXO factors in promoting high DGKα levels in conditions of growth factor deprivation. DGKα regulation by FOXO factors correlates with the reported alterations in DGKα expression during cell transformation and cancer progression.

Diacylglycerol Kinase Malfunction in Human Disease and the Search for Specific Inhibitors

The diacylglycerol kinases (DGKs) are master regulator kinases that control the switch from diacylglycerol (DAG) to phosphatidic acid (PA), two lipids with important structural and signaling properties. Mammalian DGKs distribute into five subfamilies that regulate local availability of DAG and PA pools in a tissue- and subcellular-restricted manner. Pharmacological manipulation of DGK activity holds great promise, given the critical contribution of specific DGK subtypes to the control of membrane structure, signaling complexes, and cell-cell communication. The latest advances in the DGK field have unveiled the differential contribution of selected isoforms to human disease. Defects in the expression/activity of individual DGK isoforms contribute substantially to cognitive impairment, mental disorders, insulin resistance, and vascular pathologies. Abnormal DGK overexpression, on the other hand, confers the acquisition of malignant traits including invasion, chemotherapy resistance, and inhibition of immune attack on tumors. Translation of these findings into therapeutic approaches will require development of methods to pharmacologically modulate DGK functions. In particular, inhibitors that target the DGKα isoform hold particular promise in the fight against cancer, on their own or in combination with immune-targeting therapies.

Deciphering T Cell Immunometabolism with Activity-Based Protein Profiling

As a major sentinel of adaptive immunity, T cells seek and destroy diseased cells using antigen recognition to achieve molecular specificity. Strategies to block checkpoint inhibition of T cell activity and thus reawaken the patient's antitumor immune responses are rapidly becoming standard of care for treatment of diverse cancers. Adoptive transfer of patient T cells genetically engineered with tumor-targeting capabilities is redefining the field of personalized medicines. The diverse opportunities for exploiting T cell biology in the clinic have prompted new efforts to expand the scope of targets amenable to immuno-oncology. Given the complex spatiotemporal regulation of T cell function and fate, new technologies capable of global molecular profiling in vivo are needed to guide selection of appropriate T cell targets and subsets. In this chapter, we describe the use of activity-based protein profiling (ABPP) to illuminate different aspects of T cell metabolism and signaling as fertile starting points for investigation. We highlight the merits of ABPP methods to enable target, inhibitor, and biochemical pathway discovery of T cells in the burgeoning field of immuno-oncology.

Diacylglycerol kinase α inactivation is an integral component of the costimulatory pathway that amplifies TCR signals

The arsenal of cancer therapies has evolved to target T lymphocytes and restore their capacity to destroy tumor cells. T cells rely on diacylglycerol (DAG) to carry out their functions. DAG availability and signaling are regulated by the enzymes diacylglycerol kinase (DGK) α and ζ, whose excess function drives T cells into hyporesponsive states. Targeting DGKα is a promising strategy for coping with cancer; its blockade could reinstate T-cell attack on tumors while limiting tumor growth, due to positive DGKα functions in several oncogenic pathways. Here, we made a side-by-side comparison of the effects of commercial pharmacological DGK inhibitors on T-cell responses with those promoted by DGKα and DGKζ genetic deletion or silencing. We show the specificity for DGKα of DGK inhibitors I and II and the structurally similar compound ritanserin. Inhibitor treatment promoted Ras/ERK (extracellular signal-regulated kinase) signaling and AP-1 (Activator protein-1) transcription, facilitated DGKα membrane localization, reduced the requirement for costimulation, and cooperated with enhanced activation following DGKζ silencing/deletion. DGKiII and ritanserin had similar effects on TCR proximal signaling, but ritanserin counteracted long-term T-cell activation, an effect that was potentiated in DGKα-/- cells. In contrast with enhanced activation triggered by pharmacological inhibition, DGKα silencing/genetic deletion led to impaired Lck (lymphocyte-specific protein tyrosine kinase) activation and limited costimulation responses. Our results demonstrate that pharmacological inhibition of DGKα downstream of the TCR provides a gain-of-function effect that amplifies the DAG-dependent signaling cascade, an ability that could be exploited therapeutically to reinvigorate T cells to attack tumors.

Unexpected positive control of NFκB and miR-155 by DGKα and ζ ensures effector and memory CD8+ T cell differentiation

Signals from the T-cell receptor (TCR) and γ-chain cytokine receptors play crucial roles in initiating activation and effector/memory differentiation of CD8 T-cells. We report here that simultaneous deletion of both diacylglycerol kinase (DGK) α and ζ (DKO) severely impaired expansion of CD8 effector T cells and formation of memory CD8 T-cells after Listeria monocytogenes infection. Moreover, ablation of both DGKα and ζ in preformed memory CD8 T-cells triggered death and impaired homeostatic proliferation of these cells. DKO CD8 T-cells were impaired in priming due to decreased expression of chemokine receptors and migration to the draining lymph nodes. Moreover, DKO CD8 T-cells were unexpectedly defective in NFκB-mediated miR-155 transcript, leading to excessive SOCS1 expression and impaired γ-chain cytokine signaling. Our data identified a DGK-NFκB-miR-155-SOCS1 axis that bridges TCR and γ-chain cytokine signaling for robust CD8 T-cell primary and memory responses to bacterial infection.

Deconstructing Lipid Kinase Inhibitors by Chemical Proteomics

Diacylglycerol kinases (DGKs) regulate lipid metabolism and cell signaling through ATP-dependent phosphorylation of diacylglycerol to biosynthesize phosphatidic acid. Selective chemical probes for studying DGKs are currently lacking and are needed to annotate isoform-specific functions of these elusive lipid kinases. Previously, we explored fragment-based approaches to discover a core fragment of DGK-α (DGKα) inhibitors responsible for selective binding to the DGKα active site. Here, we utilize quantitative chemical proteomics to deconstruct widely used DGKα inhibitors to identify structural regions mediating off-target activity. We tested the activity of a fragment (RLM001) derived from a nucleotide-like region found in the DGKα inhibitors R59022 and ritanserin and discovered that RLM001 mimics ATP in its ability to broadly compete at ATP-binding sites of DGKα as well as >60 native ATP-binding proteins (kinases and ATPases) detected in cell proteomes. Equipotent inhibition of activity-based probe labeling by RLM001 supports a contiguous ligand-binding site composed of C1, DAGKc, and DAGKa domains in the DGKα active site. Given the lack of available crystal structures of DGKs, our studies highlight the utility of chemical proteomics in revealing active-site features of lipid kinases to enable development of inhibitors with enhanced selectivity against the human proteome.

SNX27 links DGKζ to the control of transcriptional and metabolic programs in T lymphocytes

Sorting nexin 27 (SNX27) recycles PSD-95, Dlg1, ZO-1 (PDZ) domain-interacting membrane proteins and is essential to sustain adequate brain functions. Here we define a fundamental SNX27 function in T lymphocytes controlling antigen-induced transcriptional activation and metabolic reprogramming. SNX27 limits the activation of diacylglycerol (DAG)-based signals through its high affinity PDZ-interacting cargo DAG kinase ζ (DGKζ). SNX27 silencing in human T cells enhanced T cell receptor (TCR)-stimulated activator protein 1 (AP-1)- and nuclear factor κB (NF-κB)-mediated transcription. Transcription did not increase upon DGKζ silencing, suggesting that DGKζ function is dependent on SNX27. The enhanced transcriptional activation in SNX27-silenced cells contrasted with defective activation of the mammalian target of rapamycin (mTOR) pathway. The analysis of Snx27^−/− mice supported a role for SNX27 in the control of T cell growth. This study broadens our understanding of SNX27 as an integrator of lipid-based signals with the control of transcription and metabolic pathways.

PDE8 controls CD4+ T cell motility through the PDE8A-Raf-1 kinase signaling complex

The levels of cAMP are regulated by phosphodiesterase enzymes (PDEs), which are targets for the treatment of inflammatory disorders. We have previously shown that PDE8 regulates T cell motility. Here, for the first time, we report that PDE8A exerts part of its control of T cell function through the V-raf-1 murine leukemia viral oncogene homolog 1 (Raf-1) kinase signaling pathway. To examine T cell motility under physiologic conditions, we analyzed T cell interactions with endothelial cells and ligands in flow assays. The highly PDE8-selective enzymatic inhibitor PF-04957325 suppresses adhesion of in vivo myelin oligodendrocyte glycoprotein (MOG) activated inflammatory CD4⁺ T effector (Teff) cells to brain endothelial cells under shear stress. Recently, PDE8A was shown to associate with Raf-1 creating a compartment of low cAMP levels around Raf-1 thereby protecting it from protein kinase A (PKA) mediated inhibitory phosphorylation. To test the function of this complex in Teff cells, we used a cell permeable peptide that selectively disrupts the PDE8A-Raf-1 interaction. The disruptor peptide inhibits the Teff-endothelial cell interaction more potently than the enzymatic inhibitor. Furthermore, the LFA-1/ICAM-1 interaction was identified as a target of disruptor peptide mediated reduction of adhesion, spreading and locomotion of Teff cells under flow. Mechanistically, we observed that disruption of the PDE8A-Raf-1 complex profoundly alters Raf-1 signaling in Teff cells. Collectively, our studies demonstrate that PDE8A inhibition by enzymatic inhibitors or PDE8A-Raf-1 kinase complex disruptors decreases Teff cell adhesion and migration under flow, and represents a novel approach to target T cells in inflammation.

IL-33-matured dendritic cells promote Th17 cell responses via IL-1β and IL-6

IL-33 is associated with a variety of autoimmune diseases, such as sclerosis, inflammatory bowel disease, and rheumatoid arthritis. Although IL-33 is mainly involved in the induction of Th2 cells, however, the relationship between IL-33 and Th17 cells is still largely unknown. In this study, we investigated the effects of IL-33 on DC-mediated CD4⁺ T cell activation and Th17 cell differentiation because DCs are essential cells for presenting self-antigens to CD4⁺ T cells in autoimmune disease conditions. OT-II mice were injected with IL-33-treated DCs or untreated DCs that were primed by OVA_323-339 peptide, and their Th17 cell responses were compared. Th17 cell population and IL-17 expression levels were significantly increased in draining lymph nodes of mice injected with IL-33-treated DCs, compared with those in mice injected with untreated DCs. IL-33 treatment maturated DCs to present self-antigens and to increase production of proinflammatory cytokines such as IL-1β and IL-6, which have a crucial role in Th17 cell differentiation. We found that the IL-33-matured DCs enhanced the expression of an early T cell activation marker (CD69) and the Th17 master transcription factor (RORγt), but IL-33 did not directly affect CD4⁺ T cell differentiation or increase Th17 polarization. Notably, neutralizing IL-1β and/or IL-6 significantly decreased IL-17 expression levels and Th17 cell population which were increased by the coculture of CD4⁺ T cells with IL-33-matured DCs, indicating that IL-33 may induce Th17 cell responses via IL-1β and IL-6 derived from IL-33-matured DCs.

Topological organisation of the phosphatidylinositol 4,5-bisphosphate-phospholipase C resynthesis cycle: PITPs bridge the ER-PM gap

Phospholipase C (PLC) is a receptor-regulated enzyme that hydrolyses phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) at the plasma membrane (PM) triggering three biochemical consequences, the generation of soluble inositol 1,4,5-trisphosphate (IP₃), membrane-associated diacylglycerol (DG) and the consumption of PM PI(4,5)P₂ Each of these three signals triggers multiple molecular processes impacting key cellular properties. The activation of PLC also triggers a sequence of biochemical reactions, collectively referred to as the PI(4,5)P₂ cycle that culminates in the resynthesis of this lipid. The biochemical intermediates of this cycle and the enzymes that mediate these reactions are topologically distributed across two membrane compartments, the PM and the endoplasmic reticulum (ER). At the PM, the DG formed during PLC activation is rapidly converted into phosphatidic acid (PA) that needs to be transported to the ER where the machinery for its conversion into PI is localised. Conversely, PI from the ER needs to be rapidly transferred to the PM where it can be phosphorylated by lipid kinases to regenerate PI(4,5)P₂ Thus, two lipid transport steps between membrane compartments through the cytosol are required for the replenishment of PI(4,5)P₂ at the PM. Here, we review the topological constraints in the PI(4,5)P₂ cycle and current understanding how these constraints are overcome during PLC signalling. In particular, we discuss the role of lipid transfer proteins in this process. Recent findings on the biochemical properties of a membrane-associated lipid transfer protein of the PITP family, PITPNM proteins (alternative name RdgBα/Nir proteins) that localise to membrane contact sites are discussed. Studies in both Drosophila and mammalian cells converge to provide a resolution to the conundrum of reciprocal transfer of PA and PI during PLC signalling.

Diacylglycerol Kinase ζ Limits Cytokine-dependent Expansion of CD8+ T Cells with Broad Antitumor Capacity

Interleukin-2 and -15 drive expansion/differentiation of cytotoxic CD8⁺ T cells that eliminate targets via antigen-independent killing. This property is clinically relevant for the improvement of T cell-based antitumor therapies. Diacylglycerol kinase α and ζ (DGKα/ζ) metabolize the diacylglycerol generated following antigen recognition by T lymphocytes. Enhanced expression of these two lipid kinases in tumor-infiltrating CD8⁺ T cells promotes a hyporesponsive state that contributes to tumor immune escape. Inhibition of these two enzymes might thus be of interest for potentiating conventional antigen-directed tumor elimination. In this study, we sought to characterize the contribution of DGKα and ζ to antigen-independent cytotoxic functions of CD8⁺ T cells. Analysis of DGKζ-deficient mice showed an increase in bystander memory-like CD8⁺ T cell populations not observed in DGKα-deficient mice. We demonstrate that DGKζ limits cytokine responses in an antigen-independent manner. Cytokine-specific expansion of DGKζ-deficient CD8⁺ T cells promoted enhanced differentiation of innate-like cytotoxic cells in vitro, and correlated with the more potent in vivo anti-tumor responses of DGKζ-deficient mice engrafted with the murine A20 lymphoma. Our studies reveal a isoform-specific function for DGKζ downstream of IL-2/IL-15-mediated expansion of innate-like cytotoxic T cells, Pharmacological manipulation of DGKζ activity is of therapeutic interest for cytokine-directed anti-tumor treatments.

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DGKζ, a well-characterized negative regulator of TCR signals, also limits IL-2/15 function.

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DGKζ impairs cytokine-induced differentiation of cytotoxic T cell populations with innate-like ability to kill targets.

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As a result, DGKζ-deficient mice demonstrate enhanced rejection of implanted B cell lymphoma compared to wild type mice.

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Targeting DGKζ activity might be of interest to enhance cytokine-mediated antitumor therapies.

The immune system defends the body from foreign invaders. In cancer, tumors disguise as self-body cells and evade immune attack. For this reason it is important to identify the mechanism that stop T lymphocytes from recognize and destroy tumors. In this study we investigate the role of Diacylglycerol kinase zeta (DGKζ) as an inhibitor of antitumor T cell functions. We demonstrate that lymphoma cells injected in mice genetically modified to lack DGKζ expression develop smaller tumors that resolve more rapidly than those grown in normal mice. Our studies suggest that inhibition of DGKζ could help to reinforce the antitumor capacity of immune T lymphocytes.

eNOS S-nitrosylates β-actin on Cys374 and regulates PKC-θ at the immune synapse by impairing actin binding to profilin-1

The actin cytoskeleton coordinates the organization of signaling microclusters at the immune synapse (IS); however, the mechanisms involved remain poorly understood. We show here that nitric oxide (NO) generated by endothelial nitric oxide synthase (eNOS) controls the coalescence of protein kinase C-θ (PKC-θ) at the central supramolecular activation cluster (c-SMAC) of the IS. eNOS translocated with the Golgi to the IS and partially colocalized with F-actin around the c-SMAC. This resulted in reduced actin polymerization and centripetal retrograde flow of β-actin and PKC-θ from the lamellipodium-like distal (d)-SMAC, promoting PKC-θ activation. Furthermore, eNOS-derived NO S-nitrosylated β-actin on Cys374 and impaired actin binding to profilin-1 (PFN1), as confirmed with the transnitrosylating agent S-nitroso-L-cysteine (Cys-NO). The importance of NO and the formation of PFN1-actin complexes on the regulation of PKC-θ was corroborated by overexpression of PFN1- and actin-binding defective mutants of β-actin (C374S) and PFN1 (H119E), respectively, which reduced the coalescence of PKC-θ at the c-SMAC. These findings unveil a novel NO-dependent mechanism by which the actin cytoskeleton controls the organization and activation of signaling microclusters at the IS.

T cells are an essential arm of the immunity against the invasion of pathogenic agents in organisms. These specialized cells recognize foreign antigens displayed on the surface of antigen-presenting cells (APC) by means of the T cell receptor (TCR). Early signaling takes place in these cells through the specific clustering of TCRs, which trigger the recruitment of signaling molecules to the immune synapse (IS), a plasma membrane–associated intercellular domain important for T cell activation. In this location, several signaling molecules that include the protein kinase C-θ (PKC-θ) form microclusters that are translocated centripetally towards the center of the IS, following the retrograde movement of actin. In this study, we show that nitric oxide (NO) formed by endothelial nitric oxide synthase (eNOS) regulates the translocation of PKC-θ to the IS, increasing its activation. eNOS can effectively modify β-actin by S-nitrosylation on Cys374, reducing its ability to bind profilin-1 (PFN1)—a protein required for actin polymerization—polymerize and flow from the periphery to the central region of the IS. We propose that eNOS-derived NO controls actin polymerization via S-nitrosylation of actin as one of the major driving forces for the transport of PKC-θ towards the central area of the IS, which is essential for T cell activation.

DGK-α: A Checkpoint in Cancer-Mediated Immuno-Inhibition and Target for Immunotherapy

Immunotherapy is moving to the forefront of cancer treatments owing to impressive durable responses achieved with checkpoint blockade antibodies and adoptive T-cell therapy. Still, improvements are necessary since, overall, only a small percentage of patients benefit from current therapies. Here, I summarize evidence that DGK-α may represent an immunological checkpoint suppressing the activity of cytotoxic immunocytes in the tumor microenvironment. DGK-inhibitors can restore the antitumor function of tumor-suppressed adaptive and innate cytotoxic immunocytes. The activity of DGK-inhibitors lays downstream of current checkpoint blockade antibodies. Thus, synergistic effects are expected from combination strategies. Moreover, DGK-inhibitors may permit a double-strike attack on tumor cells as DGK-inhibition may not only re-instate immunological tumor attack but also may harm tumor cells directly by interfering with oncogenic survival pathways. Together, DGK-inhibitors have very promising characteristics and may be beneficially included into the armamentarium of cancer immunotherapeutics.