Succinate, or butanedioic acid, was initially isolated and identified from amber through the process of dry distillation by Georgius Agricola, a German chemist, in 1,546 (Tretter et al., 2016). One of the most significant scientific breakthroughs of the 20th century was the elucidation of the TCA cycle, also known as the citric acid cycle, by Hans Adolf Krebs. In 1937, Krebs established that the succinate-fumarate-malate-oxaloacetate pathway constituted a vital component of the TCA cycle, with the inclusion of citrate, isocitrate, and α-ketoglutarate (α-KG) (Raju, 1999). For decades, the TCA cycle has been extensively employed in the microbial synthesis of citrates, glutamates, and succinates (Vuoristo et al., 2016). In the TCA cycle, succinate is synthesized through the catalytic action of succinyl-CoA synthetase from succinyl-CoA. Subsequently, succinate is promptly converted to fumarate by the enzyme SDH. In summary, succinate and SDH are of paramount importance in ATP generation and serve as pivotal cyclic pathways in metabolism, functioning as a reservoir for catabolic processes and as a point of origin for various anabolic processes (Chinopoulos, 2013).

Succinate and SDH exhibit a robust interconnection within the TCA cycle and electron transport chain (ETC.). SDH, referred to as complex II, facilitates the integration of two significant pathways within mitochondria, namely, the oxidation of succinate to fumarate as a vital constituent of the TCA cycle and the transformation of ubiquinone to ubiquinol in the mitochondrial, ETC. Both processes are indispensable for oxidative phosphorylation, which is responsible for ATP generation to sustain biogenesis (Gill, 2012; Tretter et al., 2016).

The structure of SDH is characterized by its intricate composition of multiple subunits, namely, SDHA, SDHB, SDHC, and SDHD. In addition, SDHAF1 and SDHAF2 function as assembly factors for the associated assembly process (Moosavi et al., 2019). Notably, SDHC and SDHD possess hydrophobic properties that enable their anchoring in the inner mitochondrial membrane (Her and Maher, 2015). Conversely, SDHA and SDHB extend into the matrix. SDHA is covalently linked to a flavin adenine dinucleotide (FAD) prosthetic group and harbors the binding site for succinate. In the presence of succinate, SDHA facilitates the oxidation of succinate to fumarate by bringing succinate into close proximity with the isoalloxazine ring of FAD (Hagerhall, 1997). SDHB serves as a connecting link between SDHA and SDHC and SDHD. Within SDHB, three Fe-S centers are present, which facilitate the transfer of electrons from succinate to ubiquinone for utilization in the aerobic and energy-generating respiratory chain in eukaryotic mitochondria and various prokaryotes (Yankovskaya et al., 2003). The Fe-S centers, as subunit components of SDH, offer new perspectives for the development of candidate vaccines aimed at inducing anti-embryonation and anti-fecundity immunity (Yu et al., 2007). The SDH complex is comprised of two ubiquinone-binding sites (Yankovskaya et al., 2003; Sun et al., 2005). Residues originating from SDHB, SDHC, and SDHD, which are situated in close proximity to the matrix side of the inner mitochondrial membrane, contribute to the formation of the high-affinity binding sites, thereby enhancing their efficiency (Silkin et al., 2007; Szeto et al., 2007). Conversely, the low-affinity binding site, located nearer to the intermembrane space of the inner mitochondrial membrane, is formed by SDHC and SDHD (Her and Maher, 2015). Consequently, the oxidation of succinate to fumarate by SDH results in the generation of flavin adenine dinucleotide, reduced (FADH₂). In the, ETC., only SDH (complex II) is completely encoded in nuclear deoxyribonucleic acid (DNA), and electrons from succinate oxidation are transferred to ubiquinone in the, ETC (Vuoristo et al., 2016).

Moreover, ATP can also be generated through substrate-level phosphorylation (SLP). Succinyl-CoA and adenosine diphosphate (ADP) [or guanosine diphosphate (GDP)] undergo catalysis by succinyl-CoA synthetase, resulting in the production of succinate, CoASH, and ATP [or guanosine triphosphate (GTP)]. This metabolic process, referred to as SLP, is predominantly utilized in mitochondria to generate ATP (Johnson et al., 1998). Notably, SLP in the TCA cycle, facilitated by succinyl-CoA ligase (SUCL), operates independently of the respiratory chain and the mitochondrial proton motive force. This ATP production mechanism is in addition to oxidative phosphorylation (OXPHOS) and adenylate kinase (AK) reactions (Bruns and Regina, 1977; Johnson et al., 1998; Nobumoto et al., 1998; Panayiotou et al., 2014; Komlodi and Tretter, 2017). In hence, comprehending the significance of SDH and succinate in metabolic processes can offer novel perspectives for the formulation of therapeutic approaches aimed at addressing inflammation, IR injury, and cancer (Hagerhall, 1997).

In hypoxic inflammatory microenvironments, the activation of immune cells, including macrophages and DCs, induces a metabolic shift towards glycolysis (Mills and O'Neill, 2014; Tannahill and O'Neill, 2011). This metabolic shift towards glycolysis in activated immune cells is thought to have a significant impact in low oxygen conditions, such as hypoxic inflammatory sites. The accumulation of succinate has the potential to enhance inflammatory signaling and greatly influence the immuno-inflammatory response. Various potential sources may contribute to the accumulation of succinate. Firstly, these studies have provided confirmation that succinate accumulation resulting from SDH mutations has the ability to stabilize HIF-1α in activated macrophages, particularly when the activity of the prolyl hydroxylase domain (PHD) enzyme is inhibited. This stabilization subsequently leads to the production of the proinflammatory cytokine interleukin 1β (IL-1β) through the signaling of HIF-1α (Mills and O'Neill, 2014; O'Neill and Pearce, 2016; Tannahill et al., 2013; Zhang et al., 2021; Peace and O'Neill, 2022; Williams and O'Neill, 2018) (Figure 1). In addition, it has been observed that succinate accumulation can also occur in lipopolysaccharide (LPS)-activated macrophages through the process of glutamine metabolism, which facilitates the anaplerosis of α-KG into the TCA cycle (Rubic et al., 2008) (Figure 1). Subsequently, the “GABA shunt” pathway can lead to the accumulation of succinate, as it is stimulated by the increased levels of γ-aminobutyric acid (GABA) and its transporters induced by LPS (Figure 1). This pathway can be hindered by vigabatrin which inhibits an essential enzyme of the GABA shunt, resulting in a reduction in succinate accumulation (Figure 1). Additionally, the inhibition of GABA transaminase (GABA-T), a critical enzyme of the GABA shunt, with vigabatrin can effectively prevent succinate accumulation (Tannahill et al., 2013; Ren et al., 2019) (Figure 1). Finally, succinate accumulation can arise from the inhibition of SDH activity during hypoxia and various other pathways. In addition to hypoxia, FAD-induced decrease in nicotinamide adenine dinucleotide (NAD⁺) levels caused by LPS also hampers SDH activity (Wang et al., 2021) (Figure 1). NAD⁺ demonstrates multiple anti-inflammatory properties through the activation of sirtuins, a group of NAD⁺-dependent deacetylases. Specifically, sirtuin 1 (SIRT1) suppresses glycolysis and the activity of proinflammatory transcription factors such as nuclear factor-κB (NF-κB) and HIF-1α (Schug et al., 2010; Misawa et al., 2013), while also promoting autophagy-related gene function and oxidative metabolism. Conversely, the inactivation of SIRT3 has been observed to heighten the activation of the NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome (Schug et al., 2010; Misawa et al., 2013) (Figure 1).

Succinate additionally governs the immune functions of immune cells, specifically pertaining to immune cell migration, cytokine production, and augmentation of the ability of antigen-presenting cells (APCs) to elicit adaptive immune responses (Peace and O'Neill, 2022). For instance, succinate has been found to induce migration of monocyte-derived dendritic cells (MoDCs) in vitro and act as a chemokine, thereby promoting migration to draining lymph nodes in succinate-treated DCs (Rubic et al., 2008; Haffke et al., 2019). Succinate has been found to facilitate the production of cytokines through synergistic interactions with certain ligands, including tumor necrosis factor-α (TNF-α) (Rubic et al., 2008). Furthermore, succinate stimulation with LPS has been shown to increase the expression of IL-1β in murine bone marrow-derived dendritic cells (BMDCs), in addition to TNF-α (Williams and O'Neill, 2018) (Figure 1). In addition to promoting cytokine generation, succinate can also enhance the ability of APCs to induce immune responses that result in inflammatory outcomes. Multiple studies have provided evidence of succinate’s capacity to hinder tumor growth, which will be further expounded upon in the subsequent section (Dalla Pozza et al., 2020). Of greater significance, the manipulation of succinate metabolism holds potential as a microbiological therapy in host-microbe interactions (Wei et al., 2023). These findings suggest that the manipulation of succinate metabolism could be employed to devise innovative therapeutics for the prevention and treatment of inflammation.

There have been numerous studies demonstrating the interaction between the ligand-receptor pair succinate receptor 1 (SUCNR1) (formerly known as GPR91) and succinate, beyond its role as a metabolic signal in inflammation by stabilizing HIF-1α (de Castro Fonseca et al., 2016; Ristic et al., 2017). Wittenberger et al. initially discovered several G-protein coupled receptors, including SUCNR1 (Wittenberger et al., 2001). He et al. later identified succinate as a selective ligand of SUCNR1 (He et al., 2004). SUCNR1 was initially identified and studied in the renal system, subsequently revealing significant expression levels in the hepatic, splenic, and intestinal tissues (He et al., 2004; de Castro Fonseca et al., 2016; Gilissen et al., 2016). SUCNR1 expression has been observed in white adipocytes, hematopoietic progenitor cells (Rubic et al., 2008), as well as various blood and immune cell types (Macaulay et al., 2007; Hakak et al., 2009; de Castro Fonseca et al., 2016; Krzak et al., 2021). Succinate induces the activation of SUCNR1, which subsequently initiates signaling cascades via multiple pathways. Comparable to Gβ and Gγ signaling cascades, succinate stimulation results in the mobilization of calcium ions (Ca²⁺), accumulation of inositol trisphosphate (IP₃), and phosphorylation involving extracellular regulated kinase (ERK) (He et al., 2004; Robben et al., 2009; Gilissen et al., 2016) (Figure 2). Gaining a comprehensive understanding of the signaling pathways triggered by the interaction between SUCNR1 and succinate can offer novel perspectives for the advancement of therapeutic approaches targeting specific diseases, particularly those associated with chronic inflammation.

Recent studies have provided evidence indicating the involvement of SUCNR1 in multiple succinate-dependent inflammatory processes in vivo. Firstly, it was observed that SUCNR1-deficient mice exhibited heightened migration of DCs in comparison to their wild-type counterparts. Secondly, the absence of SUCNR1 in DCs resulted in the absence of cytokine elevation, thereby supporting the notion that succinate functions as a conventional signal to enhance the antigen-presenting function of APCs. Lastly, solid organ transplantation from SUCNR1-deficient mice exhibited prolonged graft survival when compared to that from wild-type mice (Rubic et al., 2008). Peruzzotti-Jametti et al. have presented significant evidence demonstrating that the inhibition of SUCNR1 leads to a reduction in carrier-mediated succinate uptake. This suggests that the signaling of SUCNR1 promotes the expression of plasma membrane Na⁺-dependent dicarboxylic acid transporters, which facilitate the transportation of succinate across the membrane. These findings introduce intriguing possibilities regarding the involvement of succinate in the paracrine/autocrine regulation of disease development (Peruzzotti-Jametti et al., 2018). To sum up, the aforementioned findings have indicated that SUCNR1 plays a pivotal role in the immune-inflammatory response triggered by succinate. Consequently, the modulation of SUCNR1 signaling presents a promising therapeutic strategy for the management of inflammatory disorders.

Succinylation, a PTM mechanism, entails the incorporation of a succinyl group onto a protein residue, predominantly lysine residues, thereby influencing the functionality of amino groups (Hirschey and Zhao, 2015; Alleyn et al., 2018; Sreedhar et al., 2020; Chinopoulos, 2021). Recent investigations have concentrated on the succinylation modification, wherein succinyl CoA can form an amide bond with protein lysine (Zhao et al., 2023). The identification of elevated succinate levels in numerous proteins involved in the regulation of diverse cellular and biological processes underscores the significance of succinylation as a crucial biological function (Xie et al., 2012; Park et al., 2013; Hirschey and Zhao, 2015). The catalytic processes involved in succinylation mediated by multiple enzymes, remain poorly understood. Consequently, there is ongoing research aimed at elucidating these fundamental mechanisms in order to facilitate the development of innovative therapeutic interventions and associated pharmaceuticals. Numerous investigations have demonstrated that succinylation can occur in organisms through both enzymatic and non-enzymatic means. In fact, the majority of is known to occur through non-enzymatic processes, as extensively documented in the literature (Sreedhar et al., 2020; Shen et al., 2023; Zhao et al., 2023), particularly by Matthew D Hirschey et al., who have demonstrated the higher chemical reactivity of succinyl-CoA compared to other acyl-CoA species (Wagner et al., 2017). Non-enzymatic succinylation appears to be influenced by various factors, including the concentration-dependent levels of reactants within mitochondria, while the catalysis of enzymatic succinylation may be regulated by succinyl CoA. Enzymatic succinylation may be promoted via α-ketoglutarate dehydrogenase complex (KGDHC) and E1k [α-Ketoglutarate Dehydrogenase (KGDH)] beyond succinyl CoA (Yang and Gibson, 2019) (Figure 2). Succinylation is consistently mediated by KGDHC in the cytosol and nucleus. Enzymatic TCA cycle steps like succinyl-CoA synthetase (SCS) can also regulate succinyl-CoA levels, which then non-enyzmatically drive succinylation (Li et al., 2015; Carrico et al., 2018; Gut et al., 2020). The carnitine palmitoyltransferase 1A (CPT1A) G710E mutant has been observed to enhance cell proliferation in the presence of metabolic stress, while CPT1A increases lysine succinylation without any changes in succinyl CoA levels in mammalian cells (Kurmi et al., 2018; Wang et al., 2019). This finding suggests that the lysine succinyltransferase activity of CPT1A may succinylate downstream substrate proteins, thereby facilitating proliferation (Kurmi et al., 2018; Sreedhar et al., 2020). In contrast to the established mechanism of succinyl-CoA non-enzymatic lysine succinylation, the mechanism promoted by CPT1A remains controversial and has not been shown in vivo (Sreedhar et al., 2020; Shen et al., 2023). At best, this mechanism would account for a minor fraction of succinylation (Sreedhar et al., 2020). The potential impact of concentration-dependent acyl-CoA distribution on succinylation in certain tissues has been identified (Weinert et al., 2013). This finding implies that the tissue-specific regulation of acyl-CoA concentrations offers novel therapeutic strategies for modulating succinylation in certain diseases. Furthermore, the regulation of de-succinylation plays a crucial role in controlling various catalytic processes across all cellular compartments, with succinylation potentially exhibiting a balanced reciprocal relationship. Notably, the levels of NAD⁺-dependent SIRT5 have been found to influence dynamic changes in desuccinylase activity (Figure 2). Certain studies have provided evidence indicating that the inhibition of SIRT5 results in an elevation of succinylation in particular proteins, thereby contributing to the development and progression of cancer (Park et al., 2013). SIRT7, on the other hand, acts as a histone desuccinylase that is functionally associated with chromatin compaction and the maintenance of genome stability (Li et al., 2016) (Figure 2). The process of desuccinylation catalyzed by SIRT7 predominantly occurs within the nucleus and plays a crucial role in the DNA damage response and the survival of cells (Li et al., 2016).

As previously stated, the maintenance of a delicate equilibrium between succinylation and desuccinylation processes is of utmost importance for the regulation of physiological functions (Wang and Lin, 2021). Disruption of this equilibrium can lead to the development of various diseases, including inflammatory disorders, IR injury, and cancer (Choudhary et al., 2014; Sabari et al., 2017; Yang et al., 2017). The intricate involvement of SIRT5 in the process of carcinogenesis is currently under investigation by researchers (Yang et al., 2017) (Figure 2). It has been proposed that desuccinylation events facilitated by SIRT5 may play a role in the initiation and progression of tumorigenesis. The inhibition of SIRT5 has demonstrated a reduction in cell proliferation and tumor growth, implying that hyper-succinylation may exert similar effects on tumor growth by modulating SIRT5 (Li et al., 2015; Xiangyun et al., 2017). Furthermore, mounting evidence suggests that lysine succinylation is abnormally elevated during cancer development, indicating that succinylation regulates tumor energy metabolism (Song et al., 2017). Mutations in isocitrate dehydrogenase isoform 1 that impede SDH result in an elevated level of succinyl-CoA, thereby promoting cancerous metabolism (Li et al., 2015). The augmentation of succinylation under hypoxic conditions may potentially counteract the effects of IR injury in both cardiac and cerebral tissues. The proficient regulation of succinylation holds promise for unveiling novel therapeutic strategies for managing IR injury and potentially facilitating transplantation procedures (Murphy and O'Neill, 2018). In summary, comprehending the tissue-specific mechanisms implicated is imperative for the advancement of pharmacological interventions targeting succinylation and de-succinylation in hypoxia-related ailments such as inflammation, IR injury, and cancer.

Succinate has garnered growing recognition for its multifaceted involvement in both immune and cancer-related processes, in addition to its established role in immuno-inflammatory responses. The cumulative evidence from numerous interconnected studies has consistently demonstrated that succinate possesses the capacity to regulate tumorigenesis within specific intrinsic microenvironments. Consequently, succinate is now acknowledged as a classical tumorigenic signal, alongside its well-established association with inflammation (Huang and Millar, 2013; Jiang and Yan, 2017).

SDH mutations and associated succinate pathways have been demonstrated to contribute to tumorigenesis (Bardella et al., 2011). Specifically, mutations in the SDH gene have been observed to expedite tumor development and can be detected in tumor tissues and cells (Pasini and Stratakis, 2009; Gaude and Frezza, 2014). The SDH enzyme plays a crucial role in the TCA cycle by facilitating the conversion of succinate to fumarate. Mutations in the SDH gene have been identified in various tumors, establishing SDH as a recognized tumor suppressor (Bardella et al., 2011; Wallace, 2012; Wu and Zhao, 2013; Jiang and Yan, 2017). However, mutations in this gene can result in succinate accumulation, which in turn stabilizes HIF-1α and fosters tumor growth (Selak et al., 2005) (Figure 1). Hence, it is imperative to engage in a comprehensive examination of succinate accumulation in the context of cancer. Succinate accumulation can manifest through diverse pathways that contribute to the development of tumors and immune responses. The regulation of SDH activity plays a crucial role in the buildup of succinate. Notably, the presence of a mutation in the gene responsible for encoding SDH in certain cancer types has been observed to diminish SDH activity, resulting in succinate accumulation and subsequent augmentation of mitochondrial ROS generation (Drose, 2013) (Figure 1). Hence, the functional deficiency of subunits in SDH resulting from mutation and carcinogenesis has garnered significant attention in numerous studies on metabolism (Tomitsuka et al., 2010). Furthermore, the function of SDH is reliant on the presence of oxidized FAD and NAD⁺ as cofactors (Gill, 2012). However, under conditions of hypoxia where these cofactors are predominantly in their reduced forms, SDH function is hindered, leading to the accumulation of succinate in the cancer-immunity cycle (Jiang and Yan, 2017). It is noteworthy to mention that the accumulation of succinate in M1 macrophages is closely associated with the activation of toll-like receptor 4 (TLR4) signaling. Upon activation by LPS, TLR4 signaling is triggered, resulting in the disruption of the Krebs cycle (O'Neill and Pearce, 2016) (Figure 1). This disruption subsequently leads to metabolic reprogramming and the accumulation of succinate. Previous research has demonstrated that LPS-activated macrophages induce succinate accumulation by inhibiting the activity of the PHD enzyme, thereby stabilizing HIF-1α (Figure 1). The stabilization of target genes responsible for encoding glycolytic enzymes and the pro-inflammatory cytokine IL-1β, which can impede tumor cell growth, enhance tumor survival, and intensify inflammation, is facilitated by this process (O'Neill and Pearce, 2016; Selak et al., 2005; Liu et al., 2016; Aspuria et al., 2014) (Figure 1). Succinate, in aggregate, may possess two distinct functions in the development of tumors, either promoting or hindering tumor growth. Nevertheless, thus far, SDH has exclusively demonstrated its involvement in suppressing tumor growth and advancement.

In the context of the cancer-immune cycle, succinate has the potential to exert an influence on the immune responses of various immune cells, including APCs and T cells. Notably, succinate has been observed to enhance the antigen presentation capacity of DCs by eliciting an adaptive response and impeding tumor growth (Palucka and Banchereau, 2012). Furthermore, upon encountering a sequence of stimuli originating from antigens, heightened activation of antigen-specific T cells can result in the secretion of cytokines such as TNF-α and interferon-γ (IFN-γ) during immune activation (Cortese et al., 2014) (Figure 1). These cytokines possess the ability to impede the progression of cancer cells and extend the survival of patients (Hadrup et al., 2013; Jiang and Yan, 2016).

Po-Lin Tseng et al. have provided evidence indicating an inverse correlation between the malignancy of hepatocellular carcinoma (HCC) and the expression level of SDHB. Specifically, a higher degree of malignancy in HCC is associated with a lower expression of SDHB, which is significantly associated with advanced tumor stage and unfavorable prognosis (Tseng et al., 2018). Furthermore, in mouse models, experimental interventions involving the silencing and overexpression of SDHB have demonstrated the potential feasibility of regulating HCC growth and metastasis. Thus, the downregulation of SDHB expression in human HCC leads to a metabolic shift from aerobic respiration to glycolysis and the induction of the Warburg effect, ultimately facilitating tumor malignancy (Tseng et al., 2018). Studies have demonstrated that the reduction in SDH activity, resulting from the silencing of one of its subunits, promotes HCC cell proliferation and metastasis both in vitro and in vivo by increasing ROS and subsequently activating NF-κB signaling (Li et al., 2019). These findings suggest that SDH could potentially be targeted for therapeutic intervention in the treatment of HCC due to its tumor-suppressive role.

In addition to its association with HCC, the most closely correlated association with SDH is hereditary paraganglioma/phaeochromocytoma syndrome (HPGL/PCC), which is characterized by the presence of germline loss-of-function mutations in SDH genes (Brière et al., 2005). Various mechanisms of carcinogenesis have been suggested, as patients with HPGL/PCC have been found to harbor germline mutations in SDHB, SDHC, and SDHD (Eniafe and Jiang, 2021; Takács-Vellai et al., 2021). In addition to HPGL/PCC, a multitude of neuroendocrine and non-neuroendocrine neoplasms, such as gastrointestinal stromal tumors (GISTs), renal tumors, and thyroid tumors, have been discovered to exhibit a significant correlation with SDH gene mutations, alongside HPGL/PCC. The growing body of research highlights the tumor suppressor function of SDH and the oncogenic role of succinate as a metabolite in the progression of cancer (Neppala et al., 2019; Ibrahim and Chopra, 2020; Yong et al., 2020). The current research focuses on investigating the genetic and molecular mechanisms that lead to succinate accumulation as a result of SDH mutations, and how these mutations initiate neoplasm invasion and metastasis. Additionally, variants of SDH genes have been identified in thyroid C-cell hyperplasia and papillary thyroid cancers, independent of SDH mutations. Nevertheless, the importance of succinate and SDH in the development of cancer has been firmly established, resulting in the classification of succinate as an oncometabolite and SDH as a tumor suppressor.