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Metabolic modification and tumor immunotherapy
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Metabolic modification and tumor immunotherapy.
In the past decade, tumor immunotherapy has made significant progress in the treatment of various cancers.
However, recent studies have uncovered the complex heterogeneity of the tumor microenvironment, leading to non-negligible treatment resistance.
A central phenomenon in malignancy is metabolic dysfunction, which reprograms metabolic homeostasis in tumor and stromal cells, thereby affecting the metabolic modification of specific proteins.
These post-translational modifications include glycosylation and palmitoylation, which often alter protein localization, stability, and function.
Many of these proteins are involved in acute or chronic inflammation and play key roles in tumor initiation and progression.
These metabolic modifications targeting immune checkpoints and inflammation thus provide an attractive therapeutic strategy for certain cancers.
Metabolic modifications in the tumor microenvironment
The tumor microenvironment ( TME ) is a complex ecosystem consisting of many cells that co-evolve with cancer cells, influencing the development and progression of cancer and acellular components.
Various complex intercellular communications exist within the TME, so many cells reprogram their metabolic pathways to adapt to an acidic hypoxic environment, provide energy support for cancer cells, help cancer cells evade immune surveillance, and spread and invade.
Over the past 20 years, there has been increasing evidence that metabolism is closely related to the development of tumors and the tumor microenvironment.
For example, cancer cells have been shown to utilize the overactivated PI3K-AKT pathway to drive glucose uptake by upregulating the glucose transporter GLUT1; this favors the glycolytic pathway and stimulates the production of large amounts of pyruvate and lactate, leading to the formation of acetyl-CoA, available for ATP synthesis or de novo lipogenesis.
On the other hand, increased uptake of glutamine and glucose by cancer cells and immune cells contributes not only to increased glycolysis but also to increased flux into the metabolic branching HBP pathway, the final product of which is uridine diphosphate ( UDP ). )-GlcNAc, a key metabolite for O-glycosylation and N-glycosylation.
In fact, there is direct evidence that (UDP)-GlcNAc is increased 12-fold in breast cancer.
In addition, tumor cells need to form a large number of new membranes, so they increase the de novo synthesis of endothelial lipids, which is called reprogramming of fatty acid metabolism.
Studies have shown that FASN ( catalyzed palmitic acid synthesis ) is elevated in many human cancers. In addition, tumor cells also increased the expression of transmembrane proteins responsible for uptake of exogenous FA, including CD36 ( also known as FAT ), fatty acid transporter protein family ( FATP ), or soluble carrier protein family 27 ( SLC27 ).
Palmitic acid is the most abundant fatty acid in cells ( 20%–30% of total fatty acids ), and abnormal concentrations will trigger higher levels of protein lipidation, such as palmitoylation.
Glycosylation of PD-L1
PD-L1 is a type I transmembrane protein normally expressed on tumor cells and host immune cells. PD-L1 interacts with its receptor PD1 through its extracellular domain to inhibit T cell attack.
Mass spectrometry and sequence analysis showed that PD-L1 was modified by N-glycans mainly at four glycosylation sites ( N35, N192, N200, N219 ) in the extracellular region, and these modifications were mainly related to its stability and interaction with PD1 Related, represents the functional form of PD-L1.
The study found that N-glycosylation of PD-L1 enhances its stability by eliminating the interaction with GSK3β, which can phosphorylate PD-L1 at T180 and S184 sites, thereby mediating the effect of β-TrCP on PD-L1. 26S proteasomal degradation.
N-glycosylation of PD-L1 is catalyzed by β-1,3-N-acetylglucosamine transferase 3 ( B3GNT3 ), which is initiated by EGFR signaling downstream of the transcription factor TCF4, and is enhances the interaction with PD-1.
Furthermore, glycosylation stabilizes PD-L1 expression not only through EGFR signaling, but also through epithelial-mesenchymal transition ( EMT ) in TNBC.
EMT transcriptionally upregulates the N-glycosyltransferase STT3 via the Wnt/β-catenin pathway, resulting in elevated PD-L1 N-glycosylation in cancer stem-like cells.
PD-L1 has been found to be glycosylated in many cancer types, including breast, melanoma, lung, colon, etc., and is a common feature of cancer.
However, the glycosylation pathway may be cancer type dependent ( via specific glycosyltransferases ).
For example, the glycosyltransferase GLT1D1 is significantly upregulated in certain B-cell lymphoma subtypes and enhances PD-L1 stability through N-glycosylation, suggesting that GLT1D1 may be a promising lymphoma biomarker .
STM108 is a specially developed antibody-drug conjugate that recognizes the N192 glycosylation of PD-L1 and causes PD-L1 internalization and degradation, resulting in potent resistance to adjacent cancer cells lacking PD-L1 expression Tumor effects and bystander killing without any detectable toxicity.
This suggests that targeting glycosylated PD-L1 is a potential immunotherapeutic strategy, and the glycosylation pathway can also serve as a target or biomarker for early diagnosis.
Glycosylation of PD-1
PD-1 is also a type I transmembrane protein, and unlike PD-L1, it is normally expressed on the cell membrane of T lymphocytes.
Similar to PD-L1, the extracellular domain of PD-1 also has four N-glycosylation sites ( N49, N58, N74, and N116 ), which are associated with stable expression on the cell surface of PD-1. Furthermore, the glycosylation of PD-1 at N58 is critical for PD-1 cell surface expression and stability, and for mediating its interaction with PD-L1.
PD-1-based glycosylation study, induction of mutation at PD-1 N74 site by adenine base editor ( ABE ) can downregulate PD-1 expression in CAR-T cells and enhance CAR in vitro and in vivo – Cytotoxic function of T cells.
Recently, a novel monoclonal antibody against N58-glycosylated PD-1, STM418, was developed, which showed higher PD-1 affinity than previous FDA-approved antibodies and induced stronger T cells against tumors immunity. Another PD-1 antibody, camrelizumab, also selectively binds N58-glycosylated PD-1 to inhibit the PD-1/PD-L1 pathway.
These studies suggest that targeting glycosylated PD-1 may improve immunotherapy response.
Although we already know the N-glycosylases and sites on PD-1/PD-L1, so far, the exact composition of the glycosyls on PD-1/PD-L1 is unknown; revealing the further Details may reveal more potential molecular targets.
Although biochemical experiments and animal models have demonstrated that targeting PD-1/PD-L1 N-glycosylation not only improves the accuracy of disease diagnosis but also improves the effectiveness of treatment, more clinical trials are still needed to verify its effectiveness and safety.
Palmitoylation of PD-L1
Palmitoylation is a lipidation process that covalently attaches palmitic acid to protein residues in three different ways.
(1) S-palmitoylation, attached to a cysteine residue via a thioester bond,
(2) O-palmitoylation, attached to a serine/threonine residue via an oxyester bond,
(3) N- Palmitoylated, linked to a primary amino group via an amide bond.
Emerging evidence suggests that S-palmitoylation regulates many biological processes by affecting protein migration, membrane localization, stability, and interactions.
In 2019, two research groups simultaneously reported that S-palmitoylation maintains PD-L1 stability and inhibits T cell cytotoxicity.
They both identified C272 as the palmitoylation site on PD-L1, and replacing C272 with alanine abolished PD-L1 palmitoylation in tumor cells.
S-palmitoylation regulates PD-L1 stability by inhibiting PD-L1 monoubiquitination, thereby preventing lysosomal degradation of PD-L1 and preventing its transport to MVB by ESCRT, resulting in the cell surface of PD-L1 The expression is increased, thereby inhibiting the cytotoxicity of T cells. Palmitoylation inhibitors may help reduce PD-L1 expression in tumor cells.
Palmitoylation modulates innate immunity and inflammation
TLR4 is a pattern recognition receptor in a class of innate immune sentinel cells such as macrophages, dendritic cells and neutrophils , belonging to the Toll-like receptor family.
TLR4 recognizes LPS of Gram-negative bacteria and participates in inflammasome activation by activating downstream NF-kB signaling to release the inflammatory factors TNF-α, IL-1β, and IL-18.
A series of studies have proved that saturated fatty acid palmitate is not a TLR4 agonist, but is involved in the palmitoylation of C113 on MYD88 downstream of TLR4.
This palmitoylation promotes the binding of MYD88 to IRAF4 and activates the downstream NF-κB signaling pathway. Changes in cellular metabolism and inflammation in neutrophils.
NOD1 and NOD2, another class of intracellular pattern recognition receptors in macrophages, belong to the NOD-like receptor family, recognize microbe-associated peptidoglycan and release the chemokine CXCL by activating downstream NF-kB and p38 MAPK signaling -1 and the inflammatory factor IL-6 are involved in host defense.
Dysregulation of NOD1/2 is known to cause severe immune and inflammatory diseases, such as Crohn’s disease ( CD ) and Blau syndrome.
Substantial evidence indicates that NOD1/2 is S-palmitoylated by ZDHHC5, and this S-palmitoylation is required for membrane localization and induction of NF-kB signaling in response to peptidoglycan.
STING is a central adaptor in the innate immune response to DNA viruses, and studies have demonstrated that STING is palmitoylated by ZDHHC3, ZDHHC7 and ZDHHC15 in the Golgi apparatus at C89/91, and this modification is critical for STING activation and subsequent IFN-β or NF-κb activation is essential.
Another advance in palmitoylation research is the STAT3 palmitoylation-depalmitoylation cycle.
DHHC7-catalyzed palmitoylation of STAT3 on Cys108 promotes membrane recruitment and phosphorylation of JAK2, followed by depalmitoylation of phosphorylated STAT3 by acyl protein thioesterase 2 ( APT2 ), which leads to nuclear translocation of p-STAT3, Ultimately promotes STAT3-mediated IL-17 transcription and TH17 cell differentiation.
TH17 cell differentiation disorder has an important pathogenic role in IBD, including ulcerative colitis and Crohn’s disease , therefore, both DHHC7 and APT2 may become new targets for IBD therapy.
Given that TME alters the metabolic pathway for lipid synthesis and results in higher palmitate concentrations, it is reasonable to speculate that, at least in some cases, cancer-associated protein targets undergo higher levels of palmitoylation, positively regulating tumor survival and Immunosuppressive.
Therefore, targeting palmitoylation to modulate inflammation may provide new avenues for cancer therapy.
The regulatory mechanism of post-transcriptional modification in tumor cells and tumor microenvironment has been a research hotspot in recent decades, and great progress has been made in the past few decades.
Basic research in this field has revealed more secrets of tumor heterogeneity in the TME, uncovered many regulatory mechanisms of tumorigenesis and immunosuppression, and suggested many new approaches for cancer treatment.
Targeting glycosylation/deglycosylation and palmitoylation/depalmitoylation pathways is promising, this includes upstream biosynthetic pathways, direct glycotransferases, palmitoylase/depalmitoylase and many Downstream effector proteins, such as PD-1/PD-L1, NOD1/2, STAT3, etc.
With the development of more advanced glycosylation and palmitoylation research methods, it is believed that there will be more breakthrough discoveries that have the potential to improve the outcomes of current immunotherapy and provide more new therapeutic opportunities.
1. Metabolic Modifications, Inflammation, and Cancer Immunotherapy. Front Oncol. 2021; 11: 703681.
Metabolic modification and tumor immunotherapy
(source:internet, reference only)