The R22X mutation of the SDHD gene in hereditary paraganglioma abolishes the enzymatic activity of complex II in the mitochondrial respiratory chain and activates the hypoxia pathway

The Tricarboxylic Acid Cycle Metabolites for Cancer: Friend or Enemy

The tricarboxylic acid (TCA) cycle is capable of providing sufficient energy for the physiological activities under aerobic conditions. Although tumor metabolic reprogramming places aerobic glycolysis in a dominant position, the TCA cycle remains indispensable for tumor cells as a hub for the metabolic linkage and interconversion of glucose, lipids, and certain amino acids. TCA intermediates such as citrate, α-ketoglutarate, succinate, and fumarate are altered in tumors, and they regulate the tumor metabolism, signal transduction, and immune environment to affect tumorigenesis and tumor progression. This article provides a comprehensive review of the modifications occurring in tumor cells in relation to the intermediates of the TCA cycle, which affects tumor pathogenesis and current therapeutic strategy for therapy through targeting TCA cycle in cancer cells.

Hypoxic Signaling Pathways in Carotid Body Tumors

Carotid body tumors (CBTs) are rare tumors with a 1-2 incidence per 100,000 individuals. CBTs may initially present without apparent symptoms, and symptoms begin to arise since tumors grow bigger to compress surrounding tissue, such as recurrent laryngeal nerve and esophagus. Also, the etiology of CBTs remains unclear since it is more likely to occur in those who live in high-altitude areas or suffer from chronic hypoxic diseases such as COPD. SDH mutations and familial inheritance have been reported to be related to CBTs. SDH complexes play crucial roles in aerobic respiration, and SDH mutations in CBTs have been reported to be associated with hypoxia. Hypoxic signaling pathways, specifically hypoxic markers, have attracted more research attention in tumor exploration. However, the existing literature on these signaling and markers lacks a systematic review. Also, therapeutic approaches in CBTs based on hypoxic signaling are rarely used in clinics. In this review, we concluded the role of hypoxic signaling and markers and their potential implications in the initiation and progression of CBTs. Our findings underscore the involvement of the SDH family, the HIF family, VEGFs, and inflammatory cytokines (ICs) in tumorigenesis and treatment. Of particular interest is the role played by SDHx, which has recently been linked to oxygen sensing through mutations leading to hereditary CBTs. Among the SDH family, SDHB and SDHD exhibit remarkable characteristics associated with metastasis and multiple tumors. Besides SDH mutations in CBTs, the HIF family also plays crucial roles in CBTs via hypoxic signaling pathways. The HIF family regulates angiogenesis during mammalian development and tumor growth by gene expression in CBTs. HIF1α could induce the transcription of pyruvate dehydrogenase kinase 1 (PDK1) to inhibit pyruvate dehydrogenase kinase (PDH) by inhibiting the TCA cycle. Then, carotid body cells begin to hyperplasia and hypertrophy. At the same time, EPAS1 mutation, an activating mutation, could decrease the degradation of HIF2α and result in Pacak-Zhuang syndrome, which could result in paraganglioma. HIFs can also activate VEGF expression, and VEGFs act on Flk-1 to control the hyperplasia of type I cells and promote neovascularization. ICs also play a pivotal signaling role within the CB, as their expression is induced under hypoxic conditions to stimulate CB hyperplasia, ultimately leading to CBTs detecting hypoxic areas in tumors, and improving the hypoxic condition could enhance photon radiotherapy efficacy. Moreover, this review offers valuable insights for future research directions on understanding the relationship between hypoxic signaling pathways and CBTs.

Targeting the Metabolic Paradigms in Cancer and Diabetes

Dysregulated metabolic dynamics are evident in both cancer and diabetes, with metabolic alterations representing a facet of the myriad changes observed in these conditions. This review delves into the commonalities in metabolism between cancer and type 2 diabetes (T2D), focusing specifically on the contrasting roles of oxidative phosphorylation (OXPHOS) and glycolysis as primary energy-generating pathways within cells. Building on earlier research, we explore how a shift towards one pathway over the other serves as a foundational aspect in the development of cancer and T2D. Unlike previous reviews, we posit that this shift may occur in seemingly opposing yet complementary directions, akin to the Yin and Yang concept. These metabolic fluctuations reveal an intricate network of underlying defective signaling pathways, orchestrating the pathogenesis and progression of each disease. The Warburg phenomenon, characterized by the prevalence of aerobic glycolysis over minimal to no OXPHOS, emerges as the predominant metabolic phenotype in cancer. Conversely, in T2D, the prevailing metabolic paradigm has traditionally been perceived in terms of discrete irregularities rather than an OXPHOS-to-glycolysis shift. Throughout T2D pathogenesis, OXPHOS remains consistently heightened due to chronic hyperglycemia or hyperinsulinemia. In advanced insulin resistance and T2D, the metabolic landscape becomes more complex, featuring differential tissue-specific alterations that affect OXPHOS. Recent findings suggest that addressing the metabolic imbalance in both cancer and diabetes could offer an effective treatment strategy. Numerous pharmaceutical and nutritional modalities exhibiting therapeutic effects in both conditions ultimately modulate the OXPHOS-glycolysis axis. Noteworthy nutritional adjuncts, such as alpha-lipoic acid, flavonoids, and glutamine, demonstrate the ability to reprogram metabolism, exerting anti-tumor and anti-diabetic effects. Similarly, pharmacological agents like metformin exhibit therapeutic efficacy in both T2D and cancer. This review discusses the molecular mechanisms underlying these metabolic shifts and explores promising therapeutic strategies aimed at reversing the metabolic imbalance in both disease scenarios.

SDHi fungicides: An example of mitotoxic pesticides targeting the succinate dehydrogenase complex

Succinate dehydrogenase inhibitors (SDHi) are fungicides used to control the proliferation of pathogenic fungi in crops. Their mode of action is based on blocking the activity of succinate dehydrogenase (SDH), a universal enzyme expressed by all species harboring mitochondria. The SDH is involved in two interconnected metabolic processes for energy production: the transfer of electrons in the mitochondrial respiratory chain and the oxidation of succinate to fumarate in the Krebs cycle. In humans, inherited SDH deficiencies may cause major pathologies including encephalopathies and cancers. The cellular and molecular mechanisms related to such genetic inactivation have been well described in neuroendocrine tumors, in which it induces an oxidative stress, a pseudohypoxic phenotype, a metabolic, epigenetic and transcriptomic remodeling, and alterations in the migration and invasion capacities of cancer cells, in connection with the accumulation of succinate, an oncometabolite, substrate of the SDH. We will discuss recent studies reporting toxic effects of SDHi in non-target organisms and their implications for risk assessment of pesticides. Recent data show that the SDH structure is highly conserved during evolution and that SDHi can inhibit SDH activity in mitochondria of non-target species, including humans. These observations suggest that SDHi are not specific inhibitors of fungal SDH. We hypothesize that SDHi could have toxic effects in other species, including humans. Moreover, the analysis of regulatory assessment reports shows that most SDHi induce tumors in animals without evidence of genotoxicity. Thus, these substances could have a non-genotoxic mechanism of carcinogenicity that still needs to be fully characterized and that could be related to SDH inhibition. The use of pesticides targeting mitochondrial enzymes encoded by tumor suppressor genes raises questions on the risk assessment framework of mitotoxic pesticides. The issue of SDHi fungicides is therefore a textbook case that highlights the urgent need for changes in regulatory assessment.

Mitochondrial heterogeneity in diseases

As key organelles involved in cellular metabolism, mitochondria frequently undergo adaptive changes in morphology, components and functions in response to various environmental stresses and cellular demands. Previous studies of mitochondria research have gradually evolved, from focusing on morphological change analysis to systematic multiomics, thereby revealing the mitochondrial variation between cells or within the mitochondrial population within a single cell. The phenomenon of mitochondrial variation features is defined as mitochondrial heterogeneity. Moreover, mitochondrial heterogeneity has been reported to influence a variety of physiological processes, including tissue homeostasis, tissue repair, immunoregulation, and tumor progression. Here, we comprehensively review the mitochondrial heterogeneity in different tissues under pathological states, involving variant features of mitochondrial DNA, RNA, protein and lipid components. Then, the mechanisms that contribute to mitochondrial heterogeneity are also summarized, such as the mutation of the mitochondrial genome and the import of mitochondrial proteins that result in the heterogeneity of mitochondrial DNA and protein components. Additionally, multiple perspectives are investigated to better comprehend the mysteries of mitochondrial heterogeneity between cells. Finally, we summarize the prospective mitochondrial heterogeneity-targeting therapies in terms of alleviating mitochondrial oxidative damage, reducing mitochondrial carbon stress and enhancing mitochondrial biogenesis to relieve various pathological conditions. The possibility of recent technological advances in targeted mitochondrial gene editing is also discussed.

Succinate-Directed Approaches for Warburg Effect-Targeted Cancer Management, an Alternative to Current Treatments?

Approximately a century ago, Otto Warburg discovered that cancer cells use a fermentative rather than oxidative metabolism even though the former is more inefficient in terms of energy production per molecule of glucose. Cancer cells increase the use of this fermentative metabolism even in the presence of oxygen, and this process is called aerobic glycolysis or the Warburg effect. This alternative metabolism is mainly characterized by higher glycolytic rates, which allow cancer cells to obtain higher amounts of total ATP, and the production of lactate, but there are also an activation of protumoral signaling pathways and the generation of molecules that favor cancer progression. One of these molecules is succinate, a Krebs cycle intermediate whose concentration is increased in cancer and which is considered an oncometabolite. Several protumoral actions have been associated to succinate and its role in several cancer types has been already described. Despite playing a major role in metabolism and cancer, so far, the potential of succinate as a target in cancer prevention and treatment has remained mostly unexplored, as most previous Warburg-directed anticancer strategies have focused on other intermediates. In this review, we aim to summarize succinate's protumoral functions and discuss the use of succinate expression regulators as a potential cancer therapy strategy.

Hypoxia-Inducible Factor 2 Alpha (HIF2α) Inhibitors: Targeting Genetically Driven Tumor Hypoxia

Tumors driven by deficiency of the VHL gene product, which is involved in degradation of the hypoxia-inducible factor subunit 2 alpha (HIF2α), are natural candidates for targeted inhibition of this pathway. Belzutifan, a highly specific and well-tolerated HIF2α inhibitor, recently received FDA approval for the treatment of nonmetastatic renal cell carcinomas, pancreatic neuroendocrine tumors, and central nervous system hemangioblastomas from patients with von Hippel-Lindau disease, who carry VHL germline mutations. Such approval is a milestone in oncology; however, the full potential, and limitations, of HIF2α inhibition in the clinic are just starting to be explored. Here we briefly recapitulate the molecular rationale for HIF2α blockade in tumors and review available preclinical and clinical data, elaborating on mutations that might be particularly sensitive to this approach. We also outline some emerging mechanisms of intrinsic and acquired resistance to HIF2α inhibitors, including acquired mutations of the gatekeeper pocket of HIF2α and its interacting partner ARNT. Lastly, we propose that the high efficacy of belzutifan observed in tumors with genetically driven hypoxia caused by VHL mutations suggests that a focus on other mutations that similarly lead to HIF2α stabilization, such as those occurring in neuroendocrine tumors with disruptions in the tricarboxylic acid cycle (SDHA/B/C/D, FH, MDH2, IDH2), HIF hydroxylases (EGLN/PHDs), and the HIF2α-encoding gene, EPAS1, are warranted.

Metabolic determinants of tumour initiation

Tumours exhibit notable metabolic alterations compared with their corresponding normal tissue counterparts. These metabolic alterations can support anabolic growth, enable survival in hostile environments and regulate gene expression programmes that promote malignant progression. Whether these metabolic changes are selected for during malignant transformation or can themselves be drivers of tumour initiation is unclear. However, intriguingly, many of the major bottlenecks for tumour initiation - control of cell fate, survival and proliferation - are all amenable to metabolic regulation. In this article, we review evidence demonstrating a critical role for metabolic pathways in processes that support the earliest stages of tumour development. We discuss how cell-intrinsic factors, such as the cell of origin or transforming oncogene, and cell-extrinsic factors, such as local nutrient availability, promote or restrain tumour initiation. Deeper insight into how metabolic pathways control tumour initiation will improve our ability to design metabolic interventions to limit tumour incidence.

Inhibition of 2-arachidonoylglycerol degradation enhances glial immunity by single-cell transcriptomic analysis

BACKGROUND

2-Arachidonoylglycerol (2-AG) is the most abundant endogenous cannabinoid. Inhibition of 2-AG metabolism by inactivation of monoacylglycerol lipase (MAGL), the primary enzyme that degrades 2-AG in the brain, produces anti-inflammatory and neuroprotective effects in neurodegenerative diseases. However, the molecular mechanisms underlying these beneficial effects are largely unclear.

METHODS

Hippocampal and cortical cells were isolated from cell type-specific MAGL knockout (KO) mice. Single-cell RNA sequencing was performed by 10 × Genomics platform. Cell Ranger, Seurat (v3.2) and CellChat (1.1.3) packages were used to carry out data analysis.

RESULTS

Using single-cell RNA sequencing analysis, we show here that cell type-specific MAGL KO mice display distinct gene expression profiles in the brain. Inactivation of MAGL results in robust changes in expression of immune- and inflammation-related genes in microglia and astrocytes. Remarkably, upregulated expression of chemokines in microglia is more pronounced in mice lacking MAGL in astrocytes. In addition, expression of genes that regulate other cellular functions and Wnt signaling in astrocytes is altered in MAGL KO mice.

CONCLUSIONS

Our results provide transcriptomic evidence that cell type-specific inactivation of MAGL induces differential expression of immune-related genes and other fundamental cellular pathways in microglia and astrocytes. Upregulation of the immune/inflammatory genes suggests that tonic levels of immune/inflammatory vigilance are enhanced in microglia and astrocytes, particularly in microglia, by inhibition of 2-AG metabolism, which likely contribute to anti-inflammatory and neuroprotective effects produced by inactivation of MAGL in neurodegenerative diseases.

Succinate at the Crossroad of Metabolism and Angiogenesis: Roles of SDH, HIF1α and SUCNR1

Angiogenesis is an essential process by which new blood vessels develop from existing ones. While adequate angiogenesis is a physiological process during, for example, tissue repair, insufficient and excessive angiogenesis stands on the pathological side. Fine balance between pro- and anti-angiogenic factors in the tissue environment regulates angiogenesis. Identification of these factors and how they function is a pressing topic to develop angiogenesis-targeted therapeutics. During the last decade, exciting data highlighted non-metabolic functions of intermediates of the mitochondrial Krebs cycle including succinate. Among these functions is the contribution of succinate to angiogenesis in various contexts and through different mechanisms. As the concept of targeting metabolism to treat a wide range of diseases is rising, in this review we summarize the mechanisms by which succinate regulates angiogenesis in normal and pathological settings. Gaining a comprehensive insight into how this metabolite functions as an angiogenic signal will provide a useful approach to understand diseases with aberrant or excessive angiogenic background, and may provide strategies to tackle them.

Cancer-derived extracellular succinate: a driver of cancer metastasis

Succinate is a tricarboxylic acid (TCA) cycle intermediate normally confined to the mitochondrial matrix. It is a substrate of succinate dehydrogenase (SDH). Mutation of SDH subunits (SDHD and SDHB) in hereditary tumors such as paraganglioma or reduction of SDHB expression in cancer results in matrix succinate accumulation which is transported to cytoplasma and secreted into the extracellular milieu. Excessive cytosolic succinate is known to stabilize hypoxia inducible factor-1α (HIF-1α) by inhibiting prolyl hydroxylase. Recent reports indicate that cancer-secreted succinate enhances cancer cell migration and promotes cancer metastasis by activating succinate receptor-1 (SUCNR-1)-mediated signaling and transcription pathways. Cancer-derived extracellular succinate enhances cancer cell and macrophage migration through SUCNR-1 → PI-3 K → HIF-1α pathway. Extracellular succinate induces tumor angiogenesis through SUCNR-1-mediated ERK1/2 and STAT3 activation resulting in upregulation of vascular endothelial growth factor (VEGF) expression. Succinate increases SUCNR-1 expression in cancer cells which is considered as a target for developing new anti-metastasis drugs. Furthermore, serum succinate which is elevated in cancer patients may be a theranostic biomarker for selecting patients for SUCNR-1 antagonist therapy.

Succinate dehydrogenase and MYC-associated factor X mutations in pituitary neuroendocrine tumours

Pituitary neuroendocrine tumours (PitNETs) associated with paragangliomas or phaeochromocytomas are rare. SDHx variants are estimated to be associated with 0.3-1.8% of PitNETs. Only a few case reports have documented the association with MAX variants. Prolactinomas are the most common PitNETs occurring in patients with SDHx variants, followed by somatotrophinomas, clinically non-functioning tumours and corticotrophinomas. One pituitary carcinoma has been described. SDHC, SDHB and SDHA mutations are inherited in an autosomal dominant fashion and tumorigenesis seems to adhere to Knudson's two-hit hypothesis. SDHD and SDHAF2 mutations most commonly have paternal inheritance. Immunohistochemistry for SDHB or MAX and loss of heterozygosity analysis can support the assessment of pathogenicity of the variants. Metabolomics is promising in the diagnosis of SDHx-related disease. Future research should aim to further clarify the role of SDHx and MAX variants or other genes in the molecular pathogenesis of PitNETs, including pseudohypoxic and kinase signalling pathways along with elucidating epigenetic mechanisms to predict tumour behaviour.

Colorectal paragangliomas with immunohistochemical deficiency of succinate dehydrogenase subunit B

Recent progress in paraganglioma (PGL) revealed genotype-phenotype relationship, especially succinate dehydrogenase complex subunit B (SDHB) gene mutation-related to the extra-adrenal origin and metastasis. SDHB-immunohistochemistry can detect all types of SDH-subunit mutations, and is a useful tool to detect SDH-mutation tumors. PGLs usually occur along with sympathetic, and parasympathetic chains, however, colorectal paraganglioma is extremely rare. We have experienced one sigmoid colon PGL and one rectal PGL. These colorectal PGLs: a sigmoid colon PGL measuring 25 mm associated with a gastrointestinal stromal tumor (GIST) of the stomach, and a rectal PGL measuring 75 × 45 mm with elevated norepinephrine level were analyzed by immunohistochemistry for INSM1, chromogranin A, synaptophysin, tyrosine hydroxylase, dopamine-beta-hydroxylase, and SDHB and SDHA. The tumors were strongly positive for above markers, however, negative for SDHB. Both PGLs negative for SDHB immunohistochemistry were defined SDHB-deficient PGLs. Histologic grading of the PGLs by GAPP was well differentiated in sigmoid PGL versus poorly differentiated in rectal PGL. Although these PGLs were the same Stage II of TNM classification, the patient with sigmoid colon PGL had neither recurrence nor metastasis for 5 years after the operation, however, the patient with rectal PGL suffered the recurrent multiple metastases and expired 5 years after the operation. Herein, we compared these colorectal PGLs in regard to the patients' prognostic factors. Patient prognosis with these colorectal PGLs was mostly related to the tumor size and histologic grade under the same situation of SDH-deficiency.

[Succinate dehydrogenase in cancer]

Succinate dehydrogenase (SDH) is a mitochondrial enzyme that participates in both the tricarboxylic acid cycle and the electron transport chain. Mutations in genes encoding SDH are responsible for a predisposition to pheochromocytomas and paragangliomas, and more rarely, to gastrointestinal stromal tumors or renal cell carcinomas. A decrease in SDH activity, not explained by genetics, has also been observed in more common cancers. One of the consequences of the inactivation of SDH is the excessive production of its substrate, succinate, which acts as an oncometabolite by promoting a pseudohypoxic status and an extensive epigenetic rearrangement. Understanding SDH-related oncogenesis now makes it possible to develop innovative diagnostic methods and to consider targeted therapies for the management of affected patients.

Loss of SDHB Induces a Metabolic Switch in the hPheo1 Cell Line toward Enhanced OXPHOS

BACKGROUND

Enzymes of tricarboxylic acid (TCA) have recently been recognized as tumor suppressors. Mutations in the SDHB subunit of succinate dehydrogenase (SDH) cause pheochromocytomas and paragangliomas (PCCs/PGLs) and predispose patients to malignant disease with poor prognosis.

METHODS

Using the human pheochromocytoma cell line (hPheo1), we knocked down SDHB gene expression using CRISPR-cas9 technology.

RESULTS

Microarray gene expression analysis showed that >500 differentially expressed gene targets, about 54%, were upregulated in response to SDHB knock down. Notably, genes involved in glycolysis, hypoxia, cell proliferation, and cell differentiation were up regulated, whereas genes involved in oxidative phosphorylation (OXPHOS) were downregulated. In vitro studies show that hPheo1 proliferation is not affected negatively and the cells that survive by shifting their metabolism to the use of glutamine as an alternative energy source and promote OXPHOS activity. Knock down of SDHB expression results in a significant increase in GLUD1 expression in hPheo1 cells cultured as monolayer or as 3D culture. Analysis of TCGA data confirms the enhancement of GLUD1 in SDHB mutated/low expressed PCCs/PGLs.

CONCLUSIONS

Our data suggest that the downregulation of SDHB in PCCs/PGLs results in increased GLUD1 expression and may represent a potential biomarker and therapeutic target in SDHB mutated tumors and SDHB loss of activity-dependent diseases.

Integrative Analysis of Metabolomics and Transcriptomics Data Identifies Prognostic Biomarkers Associated With Oral Squamous Cell Carcinoma

Background

Oral squamous cell carcinoma (OSCC) is the most malignant neoplasm in oral cancer. There is growing evidence that its progression involves altered metabolism. The current method of evaluating prognosis is very limited, and metabolomics may provide a new approach for quantitative evaluation. The aim of the study is to evaluate the use of metabolomics as prognostic markers for patients with OSCC.

Methods

An analytical platform, Ultra-Performance Liquid Chromatography-Quadrupole/Orbitrap High Resolution Mass Spectrometry (UHPLC-Q-Orbitrap HRMS), was used to acquire the serum fingerprinting profiles from a total of 103 patients of OSCC before and after the operation. In total, 103 OSCC patients were assigned to either a training set (n = 73) or a test set (n = 30). The potential biomarkers and the changes of serum metabolites were profiled and correlated with the clinicopathological parameters and survival of the patients by statistical analysis. To further verify our results, we linked them to gene expression using data from the Kyoto Encyclopedia of Genes and Genomes (KEGG).

Results

In total, 14 differential metabolites and five disturbed pathways were identified between the preoperative group and postoperative group. Succinic acid change-low, hypoxanthine change-high tumor grade, and tumor stage indicated a trend towards improved recurrence-free survival (RFS), whether in a training set or a test set. In addition, succinic acid change-low, hypoxanthine change-high, and tumor grade provided the highest predictive accuracy of the patients with OSCC. KEGG enrichment analysis showed that the imbalance in the amino acid and purine metabolic pathway may affect the prognosis of OSCC.

Conclusions

The changes of metabolites before and after operation may be related to the prognosis of OSCC patients. UHPLC-Q-Orbitrap HRMS serum metabolomics analysis could be used to further stratify the prognosis of patients with OSCC. These results can better understand the mechanisms related to early recurrence and help develop more effective therapeutic targets.

Role of succinylation modification in thyroid cancer and breast cancer

The incidence of thyroid cancer and breast cancer is increasing year by year, and the specific pathogenesis is unclear. Posttranslational modifications constitute an important regulatory mechanism that affects the function of almost all proteins, are essential for a diverse and well-functioning proteome and can integrate metabolism with physiological and pathological processes. In recent years, posttranslational modifications, which mainly include metabolic enzyme-mediated protein posttranslational modifications, such as methylation, phosphorylation, acetylation and succinylation, have become a research hotspot. Among these modifications, lysine succinylation is a newly discovered broad-spectrum, dynamic, non-enzymatic protein post-translational modification, and it plays an important regulatory role in a variety of tumors. Studies have shown that succinylation can affect the synthesis of thyroid hormones, and the regulation of this post-translational modification can inhibit the apoptosis and migration of thyroid cancer cell lines, and promote breast cancer cell proliferation, DNA damage repair and autophagy-related regulation. However, the specific regulatory mechanism of succinylation in thyroid cancer and breast cancer is currently unclear. Therefore, this article mainly reviews the research progress of succinylation modification in thyroid cancer and breast cancer. It is expected to provide new directions and targets for the prevention and treatment of thyroid cancer and breast cancer.

High throughput proteomic and metabolic profiling identified target correction of metabolic abnormalities as a novel therapeutic approach in head and neck paraganglioma

Head and neck paragangliomas (HNPGLs) are rare neoplasms that represent difficult treatment paradigms in neurotology. Germline mutations in genes encoding succinate dehydrogenase (SDH) are the cause of nearly all familial HNPGLs. However, the molecular mechanisms underlying tumorigenesis remain unclear. Mutational analysis identified 6 out of 14 HNPGLs harboring clinicopathologic SDH gene mutations. The SDHB gene was most frequently mutated in these patients, and western blot showed loss of SDHB protein in tumors with SDHB mutations. The paraganglioma cell line (PGL-626) was established from a sample that harbored a missense SDHB mutation (c.649C > T). Spectrometric analysis using tandem mass tags identified 151 proteins significantly differentially expressed in HNPGLs compared with normal nerves. Bioinformatics analyses confirmed the high level of enrichment of oxidative phosphorylation and metabolism pathways in HNPGLs. The mitochondrial complex subunits NDUFA2, NDUFA10, and NDUFA4, showed the most significantly increased expression and were localized predominantly in the cytoplasm of PGL-626 cells. The mitochondrial complex I inhibitor metformin exerted dose-dependent inhibitory effects on PGL-626 cells via cooperative down-regulation of NDUFA2, 4, and 10, with a significant decrease in the levels of reactive oxygen species and mitochondrial membrane potential. Further metabolomic analysis of PGL-626 cells showed that metabolites involved in central carbon metabolism in cancer and sphingolipid signaling pathways, pantothenate and CoA biosynthesis, and tryptophan and carbon metabolism were significantly altered after metformin treatment. Thus, this study provides insights into the molecular mechanisms underlying HNPGL tumorigenesis and identifies target correction of metabolic abnormalities as a novel therapeutic approach for this disease.

Loss of SDHB Promotes Dysregulated Iron Homeostasis, Oxidative Stress, and Sensitivity to Ascorbate

Succinate dehydrogenase is a key enzyme in the tricarboxylic acid cycle and the electron transport chain. All four subunits of succinate dehydrogenase are tumor suppressor genes predisposing to paraganglioma, but only mutations in the SDHB subunit are associated with increased risk of metastasis. Here we generated an Sdhd knockout chromaffin cell line and compared it with Sdhb-deficient cells. Both cell types exhibited similar SDH loss of function, metabolic adaptation, and succinate accumulation. In contrast, Sdhb-/- cells showed hallmarks of mesenchymal transition associated with increased DNA hypermethylation and a stronger pseudo-hypoxic phenotype compared with Sdhd-/- cells. Loss of SDHB specifically led to increased oxidative stress associated with dysregulated iron and copper homeostasis in the absence of NRF2 activation. High-dose ascorbate exacerbated the increase in mitochondrial reactive oxygen species, leading to cell death in Sdhb-/- cells. These data establish a mechanism linking oxidative stress to iron homeostasis that specifically occurs in Sdhb-deficient cells and may promote metastasis. They also highlight high-dose ascorbate as a promising therapeutic strategy for SDHB-related cancers. SIGNIFICANCE: Loss of different succinate dehydrogenase subunits can lead to different cell and tumor phenotypes, linking stronger 2-OG-dependent dioxygenases inhibition, iron overload, and ROS accumulation following SDHB mutation.

Sunitinib Treatment for Advanced Paraganglioma: Case Report of a Novel SDHD Gene Mutation Variant and Systematic Review of the Literature

Background

Paragangliomas (PGLs) are neuroendocrine neoplasms arising from chromaffin cells of sympathetic or parasympathetic paraganglia. Systemic therapies have been used only in metastatic PGLs. Antiangiogenic agents, such as sunitinib, could be a viable therapeutic choice in the subgroup of patients with SDH-positive PGLs. We describe the case of a man with Familial Paraganglioma Syndrome type 1 (FPGL) related to a novel mutation in SDHD gene treated with sunitinib. Furthermore, we performed a systematic review of the literature aimed to address the following question: is sunitinib treatment effective in patients with advanced/progressive/metastatic PGL?

Methods

We performed a data search using MEDLINE, Cochrane Library, and Scopus between April 2019 and September 2020. We included studies reporting data on clinical or biological characteristics, or clinical outcomes of patients with PGLs treated with sunitinib.

Results

The search leaded to the selection of 25 publications. Data from case reports and case series showed that disease control rate (DCR = stable disease + partial response + complete response) was achieved in 34.7% of cases under sunitinib treatment. In 39% of patients DCR was followed by progressive disease (PD) or tumor relapse, 26.1% patients showed PD. Data from clinical trials showed that DCR was 83%, and the median progression free survival was 13.4 months.

Discussion

Data from the present literature review suggested that sunitinib could be a viable therapeutic option in advanced/progressive/metastatic inoperable PGLs. However, further trials on the efficacy of sunitinib in FPGL and sporadic PGL are needed.

Mitochondrial arginase-2 is essential for IL-10 metabolic reprogramming of inflammatory macrophages

Mitochondria are important regulators of macrophage polarisation. Here, we show that arginase-2 (Arg2) is a microRNA-155 (miR-155) and interleukin-10 (IL-10) regulated protein localized at the mitochondria in inflammatory macrophages, and is critical for IL-10-induced modulation of mitochondrial dynamics and oxidative respiration. Mechanistically, the catalytic activity and presence of Arg2 at the mitochondria is crucial for oxidative phosphorylation. We further show that Arg2 mediates this process by increasing the activity of complex II (succinate dehydrogenase). Moreover, Arg2 is essential for IL-10-mediated downregulation of the inflammatory mediators succinate, hypoxia inducible factor 1α (HIF-1α) and IL-1β in vitro. Accordingly, HIF-1α and IL-1β are highly expressed in an LPS-induced in vivo model of acute inflammation using Arg2^-/- mice. These findings shed light on a new arm of IL-10-mediated metabolic regulation, working to resolve the inflammatory status of the cell.

Carney Triad, Carney-Stratakis Syndrome, 3PAS and Other Tumors Due to SDH Deficiency

Succinate dehydrogenase (SDH) is a key respiratory enzyme that links Krebs cycle and electron transport chain and is comprised of four subunits SDHA, SDHB, SDHC and SDHD. All SDH-deficient tumors are caused by or secondary to loss of SDH activity. As many as half of the familial cases of paragangliomas (PGLs) and pheochromocytomas (PHEOs) are due to mutations of the SDHx subunits. Gastrointestinal stromal tumors (GISTs) associated with SDH deficiency are negative for KIT/PDGFRA mutations and present with distinctive clinical features such as early onset (usually childhood or adolescence) and almost exclusively gastric location. SDH-deficient GISTs may be part of distinct clinical syndromes, Carney-Stratakis syndrome (CSS) or dyad and Carney triad (CT). CSS is also known as the dyad of GIST and PGL; it affects both genders equally and is inherited in an autosomal dominant manner with incomplete penetrance. CT is a very rare disease; PGL, GIST and pulmonary chondromas constitute CT which shows female predilection and may be a mosaic disorder. Even though there is some overlap between CT and CSS, as both are due to SDH deficiency, CSS is caused by inactivating germline mutations in genes encoding for the SDH subunits, while CT is mostly caused by a specific pattern of methylation of the SDHC gene and may be due to germline mosaicism of the responsible genetic defect.

2-Oxoglutarate-dependent dioxygenases in cancer

2-Oxoglutarate-dependent dioxygenases (2OGDDs) are a superfamily of enzymes that play diverse roles in many biological processes, including regulation of hypoxia-inducible factor-mediated adaptation to hypoxia, extracellular matrix formation, epigenetic regulation of gene transcription and the reprogramming of cellular metabolism. 2OGDDs all require oxygen, reduced iron and 2-oxoglutarate (also known as α-ketoglutarate) to function, although their affinities for each of these co-substrates, and hence their sensitivity to depletion of specific co-substrates, varies widely. Numerous 2OGDDs are recurrently dysregulated in cancer. Moreover, cancer-specific metabolic changes, such as those that occur subsequent to mutations in the genes encoding succinate dehydrogenase, fumarate hydratase or isocitrate dehydrogenase, can dysregulate specific 2OGDDs. This latter observation suggests that the role of 2OGDDs in cancer extends beyond cancers that harbour mutations in the genes encoding members of the 2OGDD superfamily. Herein, we review the regulation of 2OGDDs in normal cells and how that regulation is corrupted in cancer.

Succinate Dehydrogenase and Ribonucleic Acid Networks in Cancer and Other Diseases

Simple Summary

Although the dysfunction of the succinate dehydrogenase complex in mitochondria leads to cancer and other diseases due to aberrant metabolic reactions and signaling pathways, it is not well known how the succinate dehydrogenase complex is regulated. Our review highlights that non-coding ribonucleic acids (RNAs), RNA editing enzymes, and RNA modifying enzymes regulate expressions and functions of the succinate dehydrogenase complex. This research will provide new strategies for treating succinate dehydrogenase-relevant diseases in a clinic.

Abstract

Succinate dehydrogenase (SDH) complex connects both the tricarboxylic acid (TCA) cycle and the electron transport chain (ETC) in the mitochondria. However, SDH mutation or dysfunction-induced succinate accumulation results in multiple cancers and non-cancer diseases. The mechanistic studies show that succinate activates hypoxia response and other signal pathways via binding to 2-oxoglutarate-dependent oxygenases and succinate receptors. Recently, the increasing knowledge of ribonucleic acid (RNA) networks, including non-coding RNAs, RNA editors, and RNA modifiers has expanded our understanding of the interplay between SDH and RNA networks in cancer and other diseases. Here, we summarize recent discoveries in the RNA networks and their connections to SDH. Additionally, we discuss current therapeutics targeting SDH in both pre-clinical and clinical trials. Thus, we propose a new model of SDH–RNA network interaction and bring promising RNA therapeutics against SDH-relevant cancer and other diseases.

Immunohistochemistry and Mutation Analysis of SDHx Genes in Carotid Paragangliomas

Carotid paragangliomas (CPGLs) are rare neuroendocrine tumors often associated with mutations in SDHx genes. The immunohistochemistry of succinate dehydrogenase (SDH) subunits has been considered a useful instrument for the prediction of SDHx mutations in paragangliomas/pheochromocytomas. We compared the mutation status of SDHx genes with the immunohistochemical (IHC) staining of SDH subunits in CPGLs. To identify pathogenic/likely pathogenic variants in SDHx genes, exome sequencing data analysis among 42 CPGL patients was performed. IHC staining of SDH subunits was carried out for all CPGLs studied. We encountered SDHx variants in 38% (16/42) of the cases in SDHx genes. IHC showed negative (5/15) or weak diffuse (10/15) SDHB staining in most tumors with variants in any of SDHx (94%, 15/16). In SDHA-mutated CPGL, SDHA expression was completely absent and weak diffuse SDHB staining was detected. Positive immunoreactivity for all SDH subunits was found in one case with a variant in SDHD. Notably, CPGL samples without variants in SDHx also demonstrated negative (2/11) or weak diffuse (9/11) SDHB staining (42%, 11/26). Obtained results indicate that SDH immunohistochemistry does not fully reflect the presence of mutations in the genes; diagnostic effectiveness of this method was 71%. However, given the high sensitivity of SDHB immunohistochemistry, it could be used for initial identifications of patients potentially carrying SDHx mutations for recommendation of genetic testing.

Multifaceted Roles of Mitochondrial Components and Metabolites in Metabolic Diseases and Cancer

Mitochondria are essential cellular components that ensure physiological metabolic functions. They provide energy in the form of adenosine triphosphate (ATP) through the electron transport chain (ETC). They also constitute a metabolic hub in which metabolites are used and processed, notably through the tricarboxylic acid (TCA) cycle. These newly generated metabolites have the capacity to feed other cellular metabolic pathways; modify cellular functions; and, ultimately, generate specific phenotypes. Mitochondria also provide intracellular signaling cues through reactive oxygen species (ROS) production. As expected with such a central cellular role, mitochondrial dysfunctions have been linked to many different diseases. The origins of some of these diseases could be pinpointed to specific mutations in both mitochondrial- and nuclear-encoded genes. In addition to their impressive intracellular tasks, mitochondria also provide intercellular signaling as they can be exchanged between cells, with resulting effects ranging from repair of damaged cells to strengthened progression and chemo-resistance of cancer cells. Several therapeutic options can now be envisioned to rescue mitochondria-defective cells. They include gene therapy for both mitochondrial and nuclear defective genes. Transferring exogenous mitochondria to target cells is also a whole new area of investigation. Finally, supplementing targeted metabolites, possibly through microbiota transplantation, appears as another therapeutic approach full of promises.

Oxidative Phosphorylation Dysfunction Modifies the Cell Secretome

Mitochondrial oxidative phosphorylation disorders are extremely heterogeneous conditions. Their clinical and genetic variability makes the identification of reliable and specific biomarkers very challenging. Until now, only a few studies have focused on the effect of a defective oxidative phosphorylation functioning on the cell’s secretome, although it could be a promising approach for the identification and pre-selection of potential circulating biomarkers for mitochondrial diseases. Here, we review the insights obtained from secretome studies with regard to oxidative phosphorylation dysfunction, and the biomarkers that appear, so far, to be promising to identify mitochondrial diseases. We propose two new biomarkers to be taken into account in future diagnostic trials.

Cancer and pulmonary hypertension: Learning lessons and real-life interplay

This article reviews the scientific reasons that support the intriguing vision of pulmonary hypertension (PH) as a disease with a cancer-like nature and to understand whether this point of view may have fruitful consequences for the overall management of PH. This review compares cancer and PH in view of Hanahan and Weinberg’s principles (i.e., hallmarks of cancer) with an emphasis on hyperproliferative, metabolic, and immune/inflammatory aspects of the disease. In addition, this review provides a perspective on the role of transcription factors and chromatin and epigenetic aberrations, besides genetics, as “common driving mechanisms” of PH hallmarks and the foreseeable use of transcription factor/epigenome targeting as multitarget approach against the hallmarks of PH. Thus, recognition of the widespread applicability and analogy of these concepts will increasingly affect the development of new means of PH treatment.

The Tricarboxylic Acid Cycle at the Crossroad Between Cancer and Immunity

Significance: The tricarboxylic acid (TCA) cycle is a housekeeping metabolic pathway essential for generation of energy and biosynthetic intermediates. Alterations of the TCA cycle play a pivotal role in oncogenesis and inflammation. As such, some metabolic vulnerabilities, imposed by TCA cycle dysfunction in cancer, have been identified. Similarly, the TCA cycle appeared as an actionable pathway in immunopathologies. Recent Advances: Metabolic changes accompanying cell transformation have been usually considered as adaptive mechanisms to malignant transformation. The identification of oncogenic mutations in some TCA cycle enzymes changed this view, indicating altered mitochondrial metabolism as an instrumental mechanism for cancer initiation. Similarly, the observation that TCA cycle-derived metabolites have multiple signaling roles in immune cells supports the idea of this pathway as a metabolic rheostat of immune responses. Critical Issues: This review summarizes the crucial role of the TCA cycle in pathophysiology describing the post-translational and epigenetic impact of oncometabolites accumulation in cancer and immune cells. Future Directions: Additional studies will be necessary to further explore the role of oncometabolites in paracrine signaling and to identify genuine metabolic and nutritional liabilities imposed by TCA cycle dysfunction in cancer, hardly to be escaped by resistance mechanisms.

Warburg and Krebs and related effects in cancer

Warburg and coworkers' observation of altered glucose metabolism in tumours has been neglected for several decades, which, in part, was because of an initial misinterpretation of the basis of their finding. Following the realisation that genetic alterations are often linked to metabolism, and that the tumour micro-environment imposes different demands on cancer cells, has led to a reinvestigation of cancer metabolism in recent years. Increasing our understanding of the drivers and consequences of the Warburg effect in cancer and beyond will help to identify new therapeutic strategies as well as to identify new prognostic and therapeutic biomarkers. Here we discuss the initial findings of Warburg and coworkers regarding cancer cell glucose metabolism, how these studies came into focus again in recent years following the discovery of metabolic oncogenes, and the therapeutic potential that lies within targeting the altered metabolic phenotype in cancer. In addition, another essential nutrient in cancer metabolism, glutamine, will be discussed.

Clinical implications of the oncometabolite succinate in SDHx-mutation carriers

Succinate dehydrogenase (SDH) mutations lead to the accumulation of succinate, which acts as an oncometabolite. Germline SDHx mutations predispose to paraganglioma (PGL) and pheochromocytoma (PCC), as well as to renal cell carcinoma and gastro‐intestinal stromal tumors. The SDHx genes were the first tumor suppressor genes discovered which encode for a mitochondrial enzyme, thereby supporting Otto Warburg's hypothesis in 1926 that a direct link existed between mitochondrial dysfunction and cancer. Accumulation of succinate is the hallmark of tumorigenesis in PGL and PCC. Succinate accumulation inhibits several α‐ketoglutarate dioxygenases, thereby inducing the pseudohypoxia pathway and causing epigenetic changes. Moreover, SDH loss as a consequence of SDHx mutations can lead to reprogramming of cell metabolism. Metabolomics can be used as a diagnostic tool, as succinate and other metabolites can be measured in tumor tissue, plasma and urine with different techniques. Furthermore, these pathophysiological characteristics provide insight into therapeutic targets for metastatic disease. This review provides an overview of the pathophysiology and clinical implications of oncometabolite succinate in SDHx mutations.

Regulation of succinate dehydrogenase and role of succinate in cancer

Succinate dehydrogenase (SDH) has been classically considered a mitochondrial enzyme with the unique property to participate in both the citric acid cycle and the electron transport chain. However, in recent years, several studies have highlighted the role of the SDH substrate, i.e. succinate, in biological processes other than metabolism, tumorigenesis being the most remarkable. For this reason, SDH has now been defined a tumor suppressor and succinate an oncometabolite. In this review, we discuss recent findings regarding alterations in SDH activity leading to succinate accumulation, which include SDH mutations, regulation of mRNA expression, post-translational modifications and endogenous SDH inhibitors. Further, we report an extensive examination of the role of succinate in cancer development through the induction of epigenetic and metabolic alterations and the effects on epithelial to mesenchymal transition, cell migration and invasion, and angiogenesis. Finally, we have focused on succinate and SDH as diagnostic markers for cancers having altered SDH expression/activity.

Role of HIF1α Regulatory Factors in Stem Cells

Hypoxia-inducible factor 1 (HIF1) is a master transcription factor that induces the transcription of genes involved in the metabolism and behavior of stem cells. HIF1-mediated adaptation to hypoxia is required to maintain the pluripotency and survival of stem cells under hypoxic conditions. HIF1 activity is well known to be tightly controlled by the alpha subunit of HIF1 (HIF1α). Understanding the regulatory mechanisms that control HIF1 activity in stem cells will provide novel insights into stem cell biology under hypoxia. Recent research has unraveled the mechanistic details of HIF1α regulating processes, suggesting new strategies for regulating stem cells. This review summarizes recent experimental studies on the role of several regulatory factors (including calcium, 2-oxoglutarate-dependent dioxygenase, microtubule network, importin, and coactivators) in regulating HIF1α activity in stem cells.

Hypoxia/pseudohypoxia-mediated activation of hypoxia-inducible factor-1α in cancer

Since the first identification of hypoxic cells in sections of carcinomas in the 1950s, hypoxia has been known as a central hallmark of cancer cells and their microenvironment. Indeed, hypoxia benefits cancer cells in their growth, survival, and metastasis. The historical discovery of hypoxia‐inducible factor‐1α (HIF1A) in the early 1990s had a great influence on the field as many phenomena in hypoxia could be explained by HIF1A. However, not all regions or types of tumors are necessarily hypoxic. Thus, it is difficult to explain whole cancer pathobiology by hypoxia, especially in the early stage of cancer. Upregulation of glucose metabolism in cancer cells has been well known. Oxygen‐independent glycolysis is activated in cancer cells even in the normoxia condition, which is known as the Warburg effect. Accumulating evidence and recent advances in cancer metabolism research suggest that hypoxia‐independent mechanisms for HIF signaling activation is a hallmark for cancer. There are various mechanisms that generate pseudohypoxic conditions, even in normoxia. Given the importance of HIF1A for cancer pathobiology, the pseudohypoxia concept could shed light on the longstanding mystery of the Warburg effect and accelerate better understanding of the diverse phenomena seen in a variety of cancers.

The Role of Immunohistochemistry and Molecular Analysis of Succinate Dehydrogenase in the Diagnosis of Endocrine and Non-Endocrine Tumors and Related Syndromes

Succinate dehydrogenase (SDH) is an enzyme complex, composed of four protein subunits, that plays a role in both the citric acid cycle and the electron transport chain. The genes for SDHA, SDHB, SDHC, and SDHD are located in the nuclear DNA, and mutations in these genes have initially been described in paragangliomas (PGL) and pheochromocytomas (PCC), which are relatively rare tumors derived from the autonomic nervous system and the adrenal medulla, respectively. Patients with SDH mutations, that are almost exclusively in the germline, are frequently affected by multiple PGL and/or PCC. In addition, other tumors have been associated with SDH mutations as well, including gastrointestinal stromal tumors, SDH-deficient renal cell carcinoma, and pituitary adenomas. Immunohistochemistry for SDHB and SDHA has been shown to be a valuable additional tool in the histopathological analysis of these tumors, and can be considered as a surrogate marker for molecular analysis. In addition, SDHB immunohistochemistry is relevant in the decision-making whether a genetic sequence variant represents a pathogenic mutation or not. In this review, we highlight the current knowledge of the physiologic and pathologic role of the SDH enzyme complex and its involvement in endocrine and non-endocrine tumors, with an emphasis on the applicability of immunohistochemistry.

Mito-Nuclear Communication by Mitochondrial Metabolites and Its Regulation by B-Vitamins

Mitochondria are cellular organelles that control metabolic homeostasis and ATP generation, but also play an important role in other processes, like cell death decisions and immune signaling. Mitochondria produce a diverse array of metabolites that act in the mitochondria itself, but also function as signaling molecules to other parts of the cell. Communication of mitochondria with the nucleus by metabolites that are produced by the mitochondria provides the cells with a dynamic regulatory system that is able to respond to changing metabolic conditions. Dysregulation of the interplay between mitochondrial metabolites and the nucleus has been shown to play a role in disease etiology, such as cancer and type II diabetes. Multiple recent studies emphasize the crucial role of nutritional cofactors in regulating these metabolic networks. Since B-vitamins directly regulate mitochondrial metabolism, understanding the role of B-vitamins in mito-nuclear communication is relevant for therapeutic applications and optimal dietary lifestyle. In this review, we will highlight emerging concepts in mito-nuclear communication and will describe the role of B-vitamins in mitochondrial metabolite-mediated nuclear signaling.

Metastatic Phaeochromocytoma: Spinning Towards More Promising Treatment Options

Phaeochromocytomas (PCC) and paragangliomas (PGL) are rare tumours arising from the chromaffin cells of the adrenal medulla (PCC) or the paraganglia located outside the adrenal gland (PGL). However, their incidence is likely to be underestimated; around 10% of all PCC/PGL are metastatic, with higher metastatic potential of PGLs compared to PCCs. If benign, surgery is the treatment of choice, but if metastatic, therapy is challenging. Here we review the currently existing therapy options for metastatic PCCs/PGLs including conventional chemotherapy (the original Averbuch scheme, but updated), radiopharmaceutical treatments (131I-MIBG, 90Y- and 177Lu-DOTATATE) and novel targeted therapies (anti-angiogenic tyrosine kinase inhibitors and mTORC1 inhibitors), emphasising future therapeutic approaches (HIF-2α and PARP inhibitors, temozolomide alone, metronomic temozolomide, somatostatin analogues) based on the oncogenic signalling pathways related to three different clusters comprising more than 20 well-characterised PCC/PGL susceptibility genes. We suggest that targeted combination therapies including repurposed agents may offer more effective future options worthy of exploration.

Anti-inflammatory mechanisms of cannabinoids: an immunometabolic perspective

A number of studies have implicated cannabinoids as potent anti-inflammatory mediators. However, the exact mechanism by which cannabinoids exert these effects remains to be fully explained. The recent resurgence in interest regarding the metabolic adaptations undergone by activated immune cells has highlighted the intricate connection between metabolism and an inflammatory phenotype. In this regard, evidence suggests that cannabinoids may alter cell metabolism by increasing AMPK activity. In turn, emerging evidence suggests that the activation of AMPK by cannabinoids may mediate an anti-inflammatory effect through a range of processes. First, AMPK may promote oxidative metabolism, which have been shown to play a central role in immune cell polarisation towards a tolerogenic phenotype. AMPK activation may also attenuate anabolic processes which in turn may antagonise immune cell function. Furthermore, AMPK activity promotes the induction of autophagy, which in turn may promote anti-inflammatory effects through various well-described processes. Taken together, these observations implicate cannabinoids to mediate part of their anti-inflammatory effects through alterations in immune cell metabolism and the induction of autophagy.

The spectrum of SDHD mutations in Russian patients with head and neck paraganglioma

AIM OF THE STUDY

It was found that the mutations in the SDHD gene, encoding one of subunits of the succinate dehydrogenase complex, lead to the development of head and neck paraganglioma (HNPGL). We analyzed this gene in 91 patients with HNPGL from Russia.

MATERIALS AND METHODS

DNA was isolated from the whole blood. A screening for mutations was performed by Sanger sequencing.

RESULTS

We revealed three missense mutations that have been described previously: p.Pro81Leu, p.His102Arg, p.Tyr114Cys. Moreover, we identified a novel potentially pathogenic variant (p.Trp105*).

CONCLUSIONS

We found that mutations in the SDHD gene were less common in Russian patients compared with the majority of European populations. It was shown that the p.His102Arg mutation is a major mutation in Russia. We confirmed the previous suggestion that a bilateral localization of the tumor and the carotid type represent a marker of the genetically determined form of HNPGL.

Succinate dehydrogenase (SDH) deficiency, Carney triad and the epigenome

In this report, we review the relationship between succinate dehydrogenase (SDH) deficiency and the epigenome, especially with regards to two clinical conditions. Carney triad (CT) is a very rare disease with synchronous or metachronous occurrence of at least three different tumor entities; gastric gastrointestinal stromal tumor (GIST), paraganglioma (PGL), and pulmonary chondroma. This condition affects mostly females and it is never inherited. Another disease that shares two of the tumor components of CT, namely GIST and PGL is the Carney-Stratakis syndrome (CSS) or dyad. CSS affects both genders during childhood and adolescence. We review herein the main clinical features and molecular mechanisms behind those two syndromes that share quite a bit of similarities, but one is non-hereditary (CT) whereas the other shows an autosomal-dominant, with incomplete penetrance, inheritance pattern (CSS). Both CT and CSS are caused by the deficiency of the succinate dehydrogenase (SDH) enzyme. The key difference between the two syndromes is the molecular mechanism that causes the SDH deficiency. Most cases of CT show down-regulation of SDH through site-specific hyper-methylation of the SDHC gene, whereas CSS cases carry inactivating germline mutations within one of the genes coding for the SDH subunits A, B, C, or D (SDHA, SDHB, SDHC, and SDHD). There is only partial overlap between the two conditions (there are a few patients with CT that have SDH subunit mutations) but both lead to increased methylation of the entire genome in the tumors associated with them. Other tumors (outside CT and CSS) that have SDH deficiency are associated with increased methylation of the entire genome, but only in CT there is site-specific methylation of the SDHC gene. These findings have implications for diagnostics and the treatment of patients with these, often metastatic tumors.

Metabolic Regulation of Hypoxia-Inducible Transcription Factors: The Role of Small Molecule Metabolites and Iron

Hypoxia-inducible transcription factors (HIFs) facilitate cellular adaptations to low-oxygen environments. However, it is increasingly recognised that HIFs may be activated in response to metabolic stimuli, even when oxygen is present. Understanding the mechanisms for the crosstalk that exists between HIF signalling and metabolic pathways is therefore important. This review focuses on the metabolic regulation of HIFs by small molecule metabolites and iron, highlighting the latest studies that explore how tricarboxylic acid (TCA) cycle intermediates, 2-hydroxyglutarate (2-HG) and intracellular iron levels influence the HIF response through modulating the activity of prolyl hydroxylases (PHDs). We also discuss the relevance of these metabolic pathways in physiological and disease contexts. Lastly, as PHDs are members of a large family of 2-oxoglutarate (2-OG) dependent dioxygenases that can all respond to metabolic stimuli, we explore the broader role of TCA cycle metabolites and 2-HG in the regulation of 2-OG dependent dioxygenases, focusing on the enzymes involved in chromatin remodelling.

Pheochromocytoma/Paraganglioma: A Poster Child for Cancer Metabolism

CONTEXT

Pheochromocytomas (PCCs) are tumors that are derived from the chromaffin cells of the adrenal medulla. Extra-adrenal PCCs called paragangliomas (PGLs) are derived from the sympathetic and parasympathetic chain ganglia. PCCs secrete catecholamines, which cause hypertension and have adverse cardiovascular consequences as a result of catecholamine excess. PGLs may or may not produce catecholamines depending on their genetic type and anatomical location. The most worrisome aspect of these tumors is their ability to become aggressive and metastasize; there are no known cures for metastasized PGLs.

METHODS

Original articles and reviews indexed in PubMed were identified by querying with specific PCC/PGL- and Krebs cycle pathway-related terms. Additional references were selected through the in-depth analysis of the relevant publications.

RESULTS

We primarily discuss Krebs cycle mutations that can be instrumental in helping investigators identify key biological pathways and molecules that may serve as biomarkers of or treatment targets for PCC/PGL.

CONCLUSION

The mainstay of treatment of patients with PCC/PGLs is surgical. However, the tide may be turning with the discovery of new genes associated with PCC/PGLs that may shed light on oncometabolites used by these tumors.

Mitochondria and Hypoxia: Metabolic Crosstalk in Cell-Fate Decisions

Alterations in mitochondrial metabolism influence cell differentiation and growth. This process is regulated by the activity of 2-oxoglutarate (2OG)-dependent dioxygenases (2OGDDs) - a diverse superfamily of oxygen-consuming enzymes - through modulation of the epigenetic landscape and transcriptional responses. Recent reports have described the role of mitochondrial metabolites in directing 2OGDD-driven cell-fate switches in stem cells (SCs), immune cells, and cancer cells. An understanding of the metabolic mechanisms underlying 2OGDD autoregulation is required for therapeutic targeting of this system. We propose a model dependent on oxygen and metabolite availability and discuss how this integrates 2OGDD metabolic signalling, the hypoxic transcriptional response, and fate-determining epigenetic changes.

Pheochromocytoma: The First Metabolic Endocrine Cancer

Dysregulated metabolism is one of the key characteristics of cancer cells. The most prominent alterations are present during regulation of cell respiration, which leads to a switch from oxidative phosphorylation to aerobic glycolysis. This metabolic shift results in activation of numerous signaling and metabolic pathways supporting cell proliferation and survival. Recent progress in genetics and metabolomics has allowed us to take a closer look at the metabolic changes present in pheochromocytomas (PHEO) and paragangliomas (PGL). These neuroendocrine tumors often exhibit dysregulation of mitochondrial metabolism, which is driven by mutations in genes encoding Krebs cycle enzymes or by activation of hypoxia signaling. Present metabolic changes are involved in processes associated with tumorigenesis, invasiveness, metastasis, and resistance to various cancer therapies. In this review, we discuss the metabolic nature of PHEOs/PGLs and how unveiling the metabolic disturbances present in tumors could lead to identification of new biomarkers and personalized cancer therapies. Clin Cancer Res; 22(20); 5001-11. ©2016 AACR SEE ALL ARTICLES IN THIS CCR FOCUS SECTION, "ENDOCRINE CANCERS REVISING PARADIGMS".

Metabolic Alterations in Cancer Cells and the Emerging Role of Oncometabolites as Drivers of Neoplastic Change

The mitochondrion is an important organelle and provides energy for a plethora of intracellular reactions. Metabolic dysregulation has dire consequences for the cell, and alteration in metabolism has been identified in multiple disease states—cancer being one. Otto Warburg demonstrated that cancer cells, in the presence of oxygen, undergo glycolysis by reprogramming their metabolism—termed “aerobic glycolysis”. Alterations in metabolism enable cancer cells to gain a growth advantage by obtaining precursors for macromolecule biosynthesis, such as nucleic acids and lipids. To date, several molecules, termed “oncometabolites”, have been identified to be elevated in cancer cells and arise from mutations in nuclear encoded mitochondrial enzymes. Furthermore, there is evidence that oncometabolites can affect mitochondrial dynamics. It is believed that oncometabolites can assist in reprogramming enzymatic pathways and providing cancer cells with selective advantages. In this review, we will touch upon the effects of normal and aberrant mitochondrial metabolism in normal and cancer cells, the advantages of metabolic reprogramming, effects of oncometabolites on metabolism and mitochondrial dynamics and therapies aimed at targeting oncometabolites and metabolic aberrations.

DIAGNOSIS of ENDOCRINE DISEASE: SDHx mutations: beyond pheochromocytomas and paragangliomas

Mutations in one of the five genes encoding the succinate dehydrogenase (SDHx) or mitochondrial complex II cause the corresponding family syndromes characterized by the occurrence of pheochromocytomas (PHEO) and paragangliomas (PGL). Recently, other solid growths, such as gastrointestinal stromal tumors (GISTs), renal cell carcinomas (RCCs) and pituitary adenomas (PAs) have been associated with these syndromes. In the absence of prospective studies assessing their frequency, at present, their occurrence seems too infrequent to suggest systematic screening for SDHx mutation carriers. However, SDHB immunohistochemistry (IHC) on tumor tissues or SDHx genetic testing on blood or tumor samples should be performed in patients affected by GISTs, RCCs or PAs with clinicopathologic phenotypes suggesting an etiologic role of SDHx genes.

Mechanisms regulating immune surveillance of cellular stress in cancer

The purpose of this review is to explore immune-mediated mechanisms of stress surveillance in cancer, with particular emphasis on the idea that all cancers have classical hallmarks (Hanahan and Weinberg in Cell 100:57-70, 67; Cell 144:646-674, 68) that could be interrelated. We postulate that hallmarks of cancer associated with cellular stress pathways (Luo et al. in Cell 136:823-837, 101) including oxidative stress, proteotoxic stress, mitotic stress, DNA damage, and metabolic stress could define and modulate the inflammatory component of cancer. As such, the overarching goal of this review is to define the types of cellular stress that cancer cells undergo, and then to explore mechanisms by which immune cells recognize, respond to, and are affected by each stress response.

Characterization and metabolic synthetic lethal testing in a new model of SDH-loss familial pheochromocytoma and paraganglioma

Succinate dehydrogenase (SDH)-loss pheochromocytoma and paraganglioma (PPGL) are tumors driven by metabolic derangement. SDH loss leads to accumulation of intracellular succinate, which competitively inhibits dioxygenase enzymes, causing activation of pseudohypoxic signaling and hypermethylation of histones and DNA. The mechanisms by which these alterations lead to tumorigenesis are unclear, however. In an effort to fundamentally understand how SDH loss reprograms cell biology, we developed an immortalized mouse embryonic fibroblast cell line with conditional disruption of Sdhc and characterize the kinetics of Sdhc gene rearrangement, SDHC protein loss, succinate accumulation, and the resultant hypoproliferative phenotype. We further perform global transcriptomic, epigenomic, and proteomic characterization of changes resulting from SDHC loss, identifying specific perturbations at each biological level. We compare the observed patterns of epigenomic derangement to another previously-described immortalized mouse chromaffin cell model of SDHB loss, and compare both models to human SDH-loss tumors. Finally, we perform analysis of SDHC synthetic lethality with lactate dehydrogenase A (LDHA) and pyruvate carboxylase (PCX), which are important for regeneration of NAD+ and aspartate biosynthesis, respectively. Our data show that SDH-loss cells are selectively vulnerable to LDH genetic knock-down or chemical inhibition, suggesting that LDH inhibition may be an effective therapeutic strategy for SDH-loss PPGL.