Sin1-mediated mTOR signaling in cell growth, metabolism and immune response

Paeoniflorin protects hepatocytes from APAP-induced damage through launching autophagy via the MAPK/mTOR signaling pathway

BACKGROUND

Drug-induced liver injury (DILI) is gradually becoming a common global problem that causes acute liver failure, especially in acute hepatic damage caused by acetaminophen (APAP). Paeoniflorin (PF) has a wide range of therapeutic effects to alleviate a variety of hepatic diseases. However, the relationship between them is still poorly investigated in current studies.

PURPOSE

This work aimed to explore the protective effects of PF on APAP-induced hepatic damage and researched the potential molecular mechanisms.

METHODS

C57BL/6J male mice were injected with APAP to establish DILI model and were given PF for five consecutive days for treatment. Aiming to clarify the pharmacological effects, the molecular mechanisms of PF in APAP-induced DILI was elucidated by high-throughput and other techniques.

RESULTS

The results demonstrated that serum levels of ALP, γ-GT, AST, TBIL, and ALT were decreased in APAP mice by the preventive effects of PF. Moreover, PF notably alleviated hepatic tissue inflammation and edema. Meanwhile, the results of TUNEL staining and related apoptotic factors coincided with the results of transcriptomics, suggesting that PF inhibited hepatocyte apoptosis by regulated MAPK signaling. Besides, PF also acted on reactive oxygen species (ROS) to regulate the oxidative stress for recovery the damaged mitochondria. More importantly, transmission electron microscopy showed the generation of autophagosomes after PF treatment, and PF was also downregulated mTOR and upregulated the expression of autophagy markers such as ATG5, ATG7, and BECN1 at the mRNA level and LC3, p62, ATG5, and ATG7 at the protein level, implying that the process by which PF exerted its effects was accompanied by the occurrence of autophagy. In addition, combinined with molecular dynamics simulations and western blotting of MAPK, the results suggested p38 as a direct target for PF on APAP. Specifically, PF-activated autophagy through the downregulation of MAPK/mTOR signaling, which in turn reduced APAP injury.

CONCLUSIONS

Paeoniflorin mitigated liver injury by activating autophagy to suppress oxidative stress and apoptosis via the MAPK/mTOR signaling pathway. Taken together, our findings elucidate the role and mechanism of paeoniflorin in DILI, which is expected to provide a new therapeutic strategy for the development of paeoniflorin.

DEPDC5 protects CD8+ T cells from ferroptosis by limiting mTORC1-mediated purine catabolism

Peripheral CD8+ T cell number is tightly controlled but the precise molecular mechanism regulating this process is still not fully understood. In this study, we found that epilepsy patients with loss of function mutation of DEPDC5 had reduced peripheral CD8+ T cells, and DEPDC5 expression positively correlated with tumor-infiltrating CD8+ T cells as well as overall cancer patient survival, indicating that DEPDC5 may control peripheral CD8+ T cell homeostasis. Significantly, mice with T cell-specific Depdc5 deletion also had reduced peripheral CD8+ T cells and impaired anti-tumor immunity. Mechanistically, Depdc5-deficient CD8+ T cells produced high levels of xanthine oxidase and lipid ROS due to hyper-mTORC1-induced expression of ATF4, leading to spontaneous ferroptosis. Together, our study links DEPDC5-mediated mTORC1 signaling with CD8+ T cell protection from ferroptosis, thereby revealing a novel strategy for enhancing anti-tumor immunity via suppression of ferroptosis.

Discrete Mechanistic Target of Rapamycin Signaling Pathways, Stem Cells, and Therapeutic Targets

The mechanistic target of rapamycin (mTOR) is a serine/threonine kinase that functions via its discrete binding partners to form two multiprotein complexes, mTOR complex 1 and 2 (mTORC1 and mTORC2). Rapamycin-sensitive mTORC1, which regulates protein synthesis and cell growth, is tightly controlled by PI3K/Akt and is nutrient-/growth factor-sensitive. In the brain, mTORC1 is also sensitive to neurotransmitter signaling. mTORC2, which is modulated by growth factor signaling, is associated with ribosomes and is insensitive to rapamycin. mTOR regulates stem cell and cancer stem cell characteristics. Aberrant Akt/mTOR activation is involved in multistep tumorigenesis in a variety of cancers, thereby suggesting that the inhibition of mTOR may have therapeutic potential. Rapamycin and its analogues, known as rapalogues, suppress mTOR activity through an allosteric mechanism that only suppresses mTORC1, albeit incompletely. ATP-catalytic binding site inhibitors are designed to inhibit both complexes. This review describes the regulation of mTOR and the targeting of its complexes in the treatment of cancers, such as glioblastoma, and their stem cells.

The tRNA-GCN2-FBXO22-axis-mediated mTOR ubiquitination senses amino acid insufficiency

Mammalian target of rapamycin complex 1 (mTORC1) monitors cellular amino acid changes for function, but the molecular mediators of this process remain to be fully defined. Here, we report that depletion of cellular amino acids, either alone or in combination, leads to the ubiquitination of mTOR, which inhibits mTORC1 kinase activity by preventing substrate recruitment. Mechanistically, amino acid depletion causes accumulation of uncharged tRNAs, thereby stimulating GCN2 to phosphorylate FBXO22, which in turn accrues in the cytoplasm and ubiquitinates mTOR at Lys2066 in a K27-linked manner. Accordingly, mutation of mTOR Lys2066 abolished mTOR ubiquitination in response to amino acid depletion, rendering mTOR insensitive to amino acid starvation both in vitro and in vivo. Collectively, these data reveal a novel mechanism of amino acid sensing by mTORC1 via a previously unknown GCN2-FBXO22-mTOR pathway that is uniquely controlled by uncharged tRNAs.

Role of the p38/AKT Pathway in the Promotion of Cell Proliferation by Serum Heat Inactivation

Serum is a common biomaterial in cell culture that provides nutrients and essential growth factors for cell growth. Serum heat inactivation is a common treatment method whose main purpose is to remove complement factors and viruses. As serum contains many heat-labile factors, heat inactivation may affect cell proliferation, migration, differentiation, and other functions. However, the specific mechanism of its effect on cell function has not been studied. Thus, we investigate the exact effects of heat-inactivated FBS on the viability of various cells and explore the possible molecular mechanisms. We treated HCT116, HT-29, and HepG2 cell lines with heat-inactivated (56 °C for 30 min) medium, DMEM, or fetal bovine serum (FBS) for different times (0, 10, 15, 30, 60, or 90 min); we found that heat-inactivated FBS significantly promoted the viability of these cells, whereas DMEM did not have this effect. Moreover, heat-inactivated FBS stimulated cells to produce a small amount of ROS and activated intracellular signaling pathways, mainly the p38/AKT signaling pathway. These results indicate that heat-inactivated FBS may regulate the p38/AKT signaling pathway by promoting the production of appropriate amounts of ROS, thereby regulating cell viability.

Unmasking the tumourigenic role of SIN1/MAPKAP1 in the mTOR complex 2

BACKGROUND

Although the PI3K/AKT/mTOR pathway is one of the most altered pathways in human tumours, therapies targeting this pathway have shown numerous adverse effects due to positive feedback paradoxically activating upstream signaling nodes. The somewhat limited clinical efficacy of these inhibitors calls for the development of novel and more effective approaches for targeting the PI3K pathway for therapeutic benefit in cancer.

MAIN BODY

Recent studies have shown the central role of mTOR complex 2 (mTORC2) as a pro-tumourigenic factor of the PI3K/AKT/mTOR pathway in a number of cancers. SIN1/MAPKAP1 is a major partner of mTORC2, acting as a scaffold and responsible for the substrate specificity of the mTOR catalytic subunit. Its overexpression promotes the proliferation, invasion and metastasis of certain cancers whereas its inhibition decreases tumour growth in vitro and in vivo. It is also involved in epithelial-mesenchymal transition, stress response and lipogenesis. Moreover, the numerous interactions of SIN1 inside or outside mTORC2 connect it with other signaling pathways, which are often disrupted in human tumours such as Hippo, WNT, Notch and MAPK.

CONCLUSION

Therefore, SIN1's fundamental characteristics and numerous connexions with oncogenic pathways make it a particularly interesting therapeutic target. This review is an opportunity to highlight the tumourigenic role of SIN1 across many solid cancers and demonstrates the importance of targeting SIN1 with a specific therapy.

Regulation of CD8+ T memory and exhaustion by the mTOR signals

CD8+ T cells are the key executioners of the adaptive immune arm, which mediates antitumor and antiviral immunity. Naïve CD8+ T cells develop in the thymus and are quickly activated in the periphery after encountering a cognate antigen, which induces these cells to proliferate and differentiate into effector cells that fight the initial infection. Simultaneously, a fraction of these cells become long-lived memory CD8+ T cells that combat future infections. Notably, the generation and maintenance of memory cells is profoundly affected by various in vivo conditions, such as the mode of primary activation (e.g., acute vs. chronic immunization) or fluctuations in host metabolic, inflammatory, or aging factors. Therefore, many T cells may be lost or become exhausted and no longer functional. Complicated intracellular signaling pathways, transcription factors, epigenetic modifications, and metabolic processes are involved in this process. Therefore, understanding the cellular and molecular basis for the generation and fate of memory and exhausted CD8+ cells is central for harnessing cellular immunity. In this review, we focus on mammalian target of rapamycin (mTOR), particularly signaling mediated by mTOR complex (mTORC) 2 in memory and exhausted CD8+ T cells at the molecular level.

PI3K/AKT/mTOR Dysregulation and Reprogramming Metabolic Pathways in Renal Cancer: Crosstalk with the VHL/HIF Axis

Renal cell carcinoma (RCC) represents 85-95% of kidney cancers and is the most frequent type of renal cancer in adult patients. It accounts for 3% of all cancer cases and is in 7th place among the most frequent histological types of cancer. Clear cell renal cell carcinoma (ccRCC), accounts for 75% of RCCs and has the most kidney cancer-related deaths. One-third of the patients with ccRCC develop metastases. Renal cancer presents cellular alterations in sugars, lipids, amino acids, and nucleic acid metabolism. RCC is characterized by several metabolic dysregulations including oxygen sensing (VHL/HIF pathway), glucose transporters (GLUT 1 and GLUT 4) energy sensing, and energy nutrient sensing cascade. Metabolic reprogramming represents an important characteristic of the cancer cells to survive in nutrient and oxygen-deprived environments, to proliferate and metastasize in different body sites. The phosphoinositide 3-kinase-AKT-mammalian target of the rapamycin (PI3K/AKT/mTOR) signaling pathway is usually dysregulated in various cancer types including renal cancer. This molecular pathway is frequently correlated with tumor growth and survival. The main aim of this review is to present renal cancer types, dysregulation of PI3K/AKT/mTOR signaling pathway members, crosstalk with VHL/HIF axis, and carbohydrates, lipids, and amino acid alterations.

Inferring cancer common and specific gene networks via multi-layer joint graphical model

Cancer is a complex disease caused primarily by genetic variants. Reconstructing gene networks within tumors is essential for understanding the functional regulatory mechanisms of carcinogenesis. Advances in high-throughput sequencing technologies have provided tremendous opportunities for inferring gene networks via computational approaches. However, due to the heterogeneity of the same cancer type and the similarities between different cancer types, it remains a challenge to systematically investigate the commonalities and specificities between gene networks of different cancer types, which is a crucial step towards precision cancer diagnosis and treatment. In this study, we propose a new sparse regularized multi-layer decomposition graphical model to jointly estimate the gene networks of multiple cancer types. Our model can handle various types of gene expression data and decomposes each cancer-type-specific network into three components, i.e., globally shared, partially shared and cancer-type-unique components. By identifying the globally and partially shared gene network components, our model can explore the heterogeneous similarities between different cancer types, and our identified cancer-type-unique components can help to reveal the regulatory mechanisms unique to each cancer type. Extensive experiments on synthetic data illustrate the effectiveness of our model in joint estimation of multiple gene networks. We also apply our model to two real data sets to infer the gene networks of multiple cancer subtypes or cell lines. By analyzing our estimated globally shared, partially shared, and cancer-type-unique components, we identified a number of important genes associated with common and specific regulatory mechanisms across different cancer types.

RORα is critical for mTORC1 activity in T cell-mediated colitis

Inflammatory bowel disease (IBD) is multi-factorial chronic intestinal inflammation driven by pathogenic T cells, among which a large portion of patients are resistant to current anti-inflammatory regimes. The mechanisms underlying colitis pathogenicity and drug resistance are not fully understood. Here, we demonstrate that RORα is highly expressed in active UC patients, particularly in those non-responsive to anti-TNF treatment. Rorα deficiency in CD4⁺ T cells greatly reduced colitis development. Mechanistically, RORα regulated T cell infiltration in colon and inhibited T cell apoptosis. Meanwhile, genome-wide occupancy and transcriptome analysis revealed that RORα promoted mTORC1 activation. mTORC1 signaling, also hyperactivated in active UC patients, is necessary for T cell-mediated colitis. Our results thus demonstrate a crucial role of the RORα-mTORC1 axis in CD4⁺ T cells in promoting IBD, which may be targeted in human patients.

mTORC2-mediated direct phosphorylation regulates YAP activity promoting glioblastoma growth and invasive characteristics

The Hippo and mTOR signaling cascades are major regulators of cell growth and division. Aberrant regulation of these pathways has been demonstrated to contribute to gliomagenesis and result in enhanced glioblastoma proliferation and invasive characteristics. Several crosstalk mechanisms have been described between these two pathways, although a complete picture of these signaling interactions is lacking and is required for effective therapeutic targeting. Here we report the ability of mTORC2 to directly phosphorylate YAP at serine 436 (Ser436) positively regulating YAP activity. We show that mTORC2 activity enhances YAP transcriptional activity and the induction of YAP-dependent target gene expression while its ablation via genetic or pharmacological means has the opposite affects on YAP function. mTORC2 interacts with YAP via Sin1 and mutational analysis of serine 436 demonstrates that this phosphorylation event affects several properties of YAP leading to enhanced transactivation potential. Moreover, YAP serine 436 mutants display altered glioblastoma growth, migratory capacity and invasiveness both in vitro and in xenograft experiments. We further demonstrate that mTORC2 is able to regulate a Hippo pathway resistant allele of YAP suggesting that mTORC2 can regulate YAP independent of Hippo signaling. Correlative associations between the expression of these components in GBM patient samples also supported the presence of this signaling relationship. These results advance a direct mTORC2/YAP signaling axis driving GBM growth, motility and invasiveness.

RAS interaction with Sin1 is dispensable for mTORC2 assembly and activity

RAS proteins are molecular switches that interact with effector proteins when bound to guanosine triphosphate, stimulating downstream signaling in response to multiple stimuli. Although several canonical downstream effectors have been extensively studied and tested as potential targets for RAS-driven cancers, many of these remain poorly characterized. In this study, we undertook a biochemical and structural approach to further study the role of Sin1 as a RAS effector. Sin1 interacted predominantly with KRAS isoform 4A in cells through an atypical RAS-binding domain that we have characterized by X-ray crystallography. Despite the essential role of Sin1 in the assembly and activity of mTORC2, we find that the interaction with RAS is not required for these functions. Cells and mice expressing a mutant of Sin1 that is unable to bind RAS are proficient for activation and assembly of mTORC2. Our results suggest that Sin1 is a bona fide RAS effector that regulates downstream signaling in an mTORC2-independent manner.