Systemic sclerosis is a chronic and rare autoimmune disease of the connective tissue, whose etiology remains unknown, and the pathogenesis is still partially understood, thus representing a clinical challenge and an unmet medical need (Denton and Khanna, 2017; Tsou et al., 2021). SSc is characterized by a pathogenic triad consisting of microvascular damage, innate and adaptive immune system abnormalities with autoantibody production and cell-mediated autoimmunity, and fibroblast dysfunction with excessive collagen deposition, which leads to progressive fibrosis of the skin and internal organs (Allanore et al., 2015; Pattanaik et al., 2015).

A striking characteristic of the disease is the variability within patients, with great heterogeneity in the clinical manifestations, the serological profiles, and the disease progression rate. SSc is characterized by high morbidity and mortality, higher than any other rheumatic disease (Elhai et al., 2017). Worldwide, the prevalence of SSc is approximately 17.6 cases per 100,000 population, and the incidence rate is 1.4 per 100,000 person-years; however, there is great variability among geographic populations (Bairkdar et al., 2021). Recent studies have reported a mortality rate of 1.39–5.1 times higher than the general population (Elhai et al., 2012; Rubio-Rivas et al., 2014; Hao et al., 2017; Kang et al., 2018). SSc has a disease-related mortality rate of approximately 55%, with the leading causes of death being pulmonary complications such as interstitial lung disease (ILD), followed by pulmonary arterial hypertension (PAH) (Tyndall et al., 2010). ILD has a 19%–52% prevalence in SSc patients (Perelas et al., 2020; Kuwana et al., 2022), and approximately 40% of these patients die within 10 years of diagnosis (Akter et al., 2014). The pathogenesis of SSc-ILD begins with a permanent injury to the alveolar epithelium, secondary to an activation of the immune system promoting profibrotic stimuli that induce fibroblast recruitment and differentiation to a myofibroblast phenotype and extracellular matrix (ECM) overproduction (Nihtyanova and Denton, 2020). Moreover, in SSc patients, PAH occurs with a prevalence of 5%–15% (Morrisroe et al., 2017; Naranjo and Hassoun, 2021). PAH is characterized by arterial remodeling, and an increased pulmonary vascular resistance secondary to abnormal vascular proliferation, disequilibrium in vasodilators, proliferative mediators, and thrombosis of the pulmonary vasculature resulting in right heart failure, which can eventually lead to death (Launay et al., 2017).

In relation to skin involvement, SSc can be subclassified into diffuse cutaneous systemic sclerosis (dcSSc) and limited cutaneous systemic sclerosis (lcSSc) (Young and Khanna, 2015). LcSSc is the most frequent subtype presentation (Coral-Alvarado et al., 2009), characterized by a gradual and early onset of Raynaud’s phenomenon and skin fibrosis restricted to certain areas, such as the face and distal extremities, with minor systemic involvement (Herrick, 2018). In contrast, in the dcSSc subtype, Raynaud’s phenomenon coexists with skin fibrosis extended proximally to knees, elbows, and the trunk with a more aggressive progression characterized by severe internal organ manifestations, mainly in the gastrointestinal tract, lungs, heart, and kidneys (Kowalska-Kępczyńska, 2022).

A better understanding of the pathophysiology of SSc is crucial to tackle the processes leading to disease progression and to discover effective therapies to improve the long-term survival of SSc patients. Recent studies suggest that metabolic perturbations may play an important role in SSc pathogenesis and are exhibited in different patients as a result of the disease heterogeneity and erratic course (Wishart, 2016; Cambiaghi et al., 2017; O’Reilly, 2022). Therefore, metabolomics may play an important role in understanding the pathophysiology of the disease.

Metabolic characterization represents a promising approach that can be applied for diagnosis, disease typing, and individual treatment of SSc, as well as biomarker discovery (Zhang et al., 2015a). Thus, this review aimed to identify altered metabolic pathways possibly responsible for the mechanisms associated with the appearance of SSc, in order to improve diagnosis, prognosis, and treatment. Additionally, it identifies metabolites that allow the segregation of patients with dcSSc and lcSSc.

In this study, we systematically reviewed 26 studies on the metabolomic profiling of SSc and summarized key findings on the dysregulation of major metabolic pathways in SSc, primarily amino acid-related pathways, lipid metabolism, and the TCA cycle. To our knowledge, this is the first systematic review of metabolomic analysis in SSc.

Altered amino acid metabolism was a common finding in analyzed samples of SSc patients, possibly associated with protein synthesis and catabolic processes for energy production (Akram et al., 2011). Increased Hcy levels cause vascular injury by supporting oxidative stress through the production of reactive oxygen species, thus inhibiting antioxidant enzymes, and inducing low-density lipoprotein oxidation in arterial muscle cells (Zhang et al., 2018). Moreover, it induces endothelial dysfunction by inactivating anticoagulant substances (Škovierová et al., 2016). It is noteworthy that both Szamosi et al. (2009) and ichiro et al. (2014) found no difference in Hcy levels between SSc patients and HC; however, significantly higher Hcy concentrations were found in patients with vascular or thromboembolic events in comparison to SSc patients without these manifestations (Szamosi et al., 2009). Likewise, elevated Hcy levels were positively correlated with SSc-ILD (ichiro et al., 2014).

The upregulation of amino acid metabolites such as glutamine, ornithine, proline, and citrulline can lead to the augmentation of collagen synthesis with subsequent fibrosis of the skin and internal organs (Ung et al., 2021). Urea cycle intermediates, such as ornithine and citrulline are involved in proline synthesis (Albaugh et al., 2017). Proline a key component in the synthesis of collagen and the ECM (Karna et al., 2020), is increased in transforming growth factor beta (TGFβ) stimulated fibroblasts, increasing collagen formation and accounting for fibrosis (Schwörer et al., 2020). Glutamine promotes the novo synthesis of proline and sustains collagen synthesis in fibroblasts (Kay et al., 2021). Glutaminolysis is required for the formation of a -ketoglutarate, one of the main collagen I precursors, implying that glutamine metabolism is also important in the development of fibrosis (Ge et al., 2018). Hamanaka et al. (2019) found that the conversion of glutamine to glutamate is required for collagen protein production induced by TGFβ stimulated lung fibroblasts. Additionally, Bernard et al. (2018), tested the role of glutaminolysis in TGF-β1-dependent myofibroblast development and found that TGF-1-differentiated myofibroblasts were compared to controls, glutamate concentrations increased, but glutamine levels decreased, indicating accelerated glutaminolysis. This was linked to TGF- β1 induced mRNA and protein production of the glutaminase (GLS) isoform GLS1 that converts glutamine to glutamate. In this case, extracellular glutamine depletion inhibited TGF-β induced myofibroblast differentiation. Also, glutaminolysis is considered one of the main energy sources for effector T cells and facilitates Th17 proinflammatory phenotype (Cruzat et al., 2018). Several studies have found that the level of Th17 cells in SSc patients is increased compared to HC (Maddur et al., 2012). In SSc, Th17 cells release cytokines that can promote the proliferation and migration of dermal vascular smooth muscle cells inducing endothelial inflammation (Xing et al., 2013). Nevertheless, conflicting roles in Th17 function and fibrosis development have been discussed (Wei et al., 2022). Some studies claimed that Th17 cells induced type I collagen synthesis and secretion to promote fibrosis of murine SSc models (Wilson et al., 2010; Lei et al., 2016; Ramani and Biswas, 2019); however, other researchers propose Th17 cells decrease type I collagen production by dermal fibroblasts (Brembilla et al., 2013; Chizzolini et al., 2018). Additionally, it has been revealed that increased expression of endothelial CCR6, a surface marker of Th17 cell subsets, contributes to the development of SSc vasculopathy (Ikawa et al., 2021). These actions are associated with chronic inflammation and fibrosis, bolstering Th17 cell function in SSc patients.

Furthermore, activation of arginine methyltransferases by inflammation and oxidative stress leads to increased levels of ADMA, the principal endogenous inhibitor of nitric oxide synthase (NOS) (Zhang et al., 2015b). It also leads to an impairment of nitric oxide synthesis, contributing to the augmentation of vasoconstrictor episodes and pathological changes in the vascular system, generating endothelial dysfunction and vascular remodeling (Curtiss et al., 2019). The kynurenine pathway (KP) plays an important role in autoimmune disorders (Boros and Vécsei, 2019). In conditions characterized by inflammation, proinflammatory cytokines such as interferon γ, IL-6, and tumor necrosis factor induce Trp conversion to Kyn by the immune regulatory enzyme indoleamine-2,3-dioxygenase (IDO) (Zou, 2015). Upregulation of IDO enzyme can inhibit mTOR, a regulator of T cell differentiation, and therefore can inhibit effector T cells while promoting regulatory T cells (Treg) (Kurniawan et al., 2020; Sharabi and Tsokos, 2020); however, it has also been demonstrated in vitro that Kyn can stimulate mTORC1 activity (Qin et al., 2022). On the other hand, Kyn binds to the aryl hydrocarbon receptor in T cells and dendritic cells promoting the conversion of effector T cells into Treg and promoting IDO induction, therefore establishing a loop to maintain immunotolerance (Lionetto et al., 2021). Additionally, Trp deprivation via IDO mediates cell cycle arrest in the mid-G1 resulting in T cell death and suppression of antigen-specific T cell responses (Krupa and Kowalska, 2021). The Kyn/Trp ratio is a useful marker of IDO activation that reflects the state of immune activation in proinflammatory disorders. Therefore, an increased Kyn/Trp ratio in patients with SSc can be an indicator of increased inflammation and immune system activation.

Alterations in the TCA cycle, the central metabolic pathway for aerobic metabolic processes (Cavalcanti et al., 2014; Arnold and Finley, 2023), were also observed in analyzed samples of SSc patients compared to HC. Increased levels of succinate, a TCA cycle intermediate, were also found. In a study by Tannahill et al. (2013), bone-marrow-derived macrophages stimulated with lipopolysaccharides (LPS) showed that succinate is induced by LPS, impairing propyl hydroxylase (PHD) activity that leads to hypoxia-inducible factor 1-α (HIF-1 α) stabilization and activation, enhancing IL-1b production during inflammation. Therefore, LPS-induced succinate can serve as a signal to enhance IL-1b expression via HIF-1 α. Furthermore, the accumulation of succinate in lung tissue and myofibroblast can contribute to metabolic dysregulation in fibroblasts disrupting PHD activity and enhancing HIF-1 α, promoting the development of lung fibrosis (Wang et al., 2021a). Additionally, Henderson et al. (2020) performed in vitro studies in dermal fibroblasts derived from SSc patients and stimulated isolated normal healthy dermal fibroblasts (NHDFs) with TGF-β1 to activate fibrotic pathways and measured succinate levels. After stimulation, they found significantly higher levels of this metabolite in NHDFs, as well as elevated levels of succinate receptor GPR91 in SSc dermal fibroblasts suggesting that succinate released from macrophages can activate fibroblast to undergo fibrotic changes leading to enhanced ECM. Succinate activation of GPR91 has been shown to be important for fibroblast activation and ECM formation in murine intestinal fibrosis and non-alcoholic steatohepatitis (NASH)-associated fibrosis, as well as in fibrotic lung tissue from idiopathic pulmonary fibrosis patients and bleomycin-induced mice (Macias-Ceja et al., 2019; Liu et al., 2020).

On the contrary, citrate levels were significantly decreased. As a TCA cycle intermediate, citrate is crucial for energy production (Iacobazzi and Infantino, 2014). The high consumption of citrate to meet energy charges could explain its low levels, reflecting a reduction in energy availability and increased demand under inflammatory conditions. Various studies have reported decreased levels in samples of patients with autoimmune diseases underlining its importance in immune-mediated inflammatory pathologies (Ouyang et al., 2011; Alonso et al., 2016). Moreover, Yang et al. (2015) demonstrated that a decrease in citric acid accompanied by a decrease in glucose means an increase in energy consumption. Increased glycolysis plays a critical role in fibroblast differentiation and the progression of fibrosis (Zhu et al., 2019). It has been shown that TGF-β1 can cause a rewiring of cellular metabolism, including a shift toward glycolysis, uncoupling from mitochondrial oxidative phosphorylation, and increasing glutamine metabolism (Hewitson and Smith, 2021). In experimental models of SSc, the profibrotic M2 macrophages isolated from bleomycin-induced fibrotic mouse lungs showed increased glycolysis, suggesting its importance in assuring energy efficiency (Xie et al., 2017).

In the case of lipid metabolism, SSc patients showed alterations in carnitines, FA, glycerophospholipids, glycerolipids, sphingolipids, and steroids. Acyl-carnitines play an important role in cellular energy metabolism as a transporter of FA chains into the mitochondria, where long-chain FA are further oxidized (Fielding et al., 2018). Therefore, the downregulation of acyl-carnitines leads to perturbations in fatty acids oxidation (FAO), subsequently increasing FA metabolism (Beger et al., 2018). Furthermore, perturbations of FAO can shift T helper cell differentiation towards a proinflammatory Th17 phenotype (Slack et al., 2015). Additionally, in vitro studies have demonstrated that FA accumulation in non-adipose tissues, defined as lipotoxicity, due to FAO inhibition, promotes inflammation, oxidative stress, and fibrosis in renal tubular epithelial cells (Kang et al., 2015a). Moreover, in macrophages, increased FA metabolism can induce a switch to a profibrotic M2 phenotype, playing an important role in fibrosis (Nomura et al., 2016; Wang et al., 2017). In tissues, the healing process depends on whether the initial insult persists or not. If the insult persists, chronic activation of M2 can directly regulate the development and progression of fibrotic lung diseases through the production of chemokines, tissue inhibitor of metalloproteinases, and fibronectin, as well as the capability of M2 to differentiate into fibrocyte-like cells that express collagen, opposite to their primary anti-inflammatory activity through the release of TGF-β, IL10, and arginase, controlling wound healing and tissue regeneration (Braga et al., 2015; Kishore and Petrek, 2021). In the kidney, M2 macrophages induce Th2-type immune responses, secrete large amounts of TGF-β and anti-inflammatory cytokines, transform into myofibroblasts in the injured kidney, inhibit immune responses, and promote wound healing and tissue fibrosis (Wang et al., 2021b).

Regarding sphingolipids, increased levels of sphingosine 1-phosphate (S1P) were observed. Deregulation of S1P in the pulmonary endothelium can lead to vasoconstrictive episodes and vascular remodeling increasing pulmonary vascular resistance (Gluschke et al., 2022). Moreover, S1P influences antigen uptake and presentation by dendritic cells (Arlt et al., 2014). Additionally, the S1P receptor, sphingosine-1-phosphate, modulates early fibrogenesis (Schmidt et al., 2017).

DcSSc and lcSSc subtypes are each characterized by different clinical manifestations, and disease progression (Herrick, 2018); however, the dcSSc subtype is associated with more severe and aggressive organ involvement (Smeets et al., 2020). Therefore, early identification of the disease subtype is imperative to achieve the effectiveness of therapeutic interventions. In this review, analyzed samples of dcSSc patients in comparison with lcSSc demonstrated significantly increased levels of amino acid-related pathways, involved in fibrosis, endothelial dysfunction, and gut dysbiosis. L-NAME, a NOS inhibitor was increased in the dcSSc subtype, suggesting a more severe endothelial dysfunction that could lead to vascular complications (Dooley et al., 2006). As for gut dysbiosis, phenylacetylglutamine (PAG), a gut microbiota-derived metabolite (Teufel et al., 2010), is consistently upregulated in SSc patients. A reduced number of commensal bacteria promotes an excess of substrate favoring PAG formation, suggesting that its deregulation could be an indicator of gastrointestinal involvement (Poesen et al., 2016).

In SSc patients, cardiopulmonary complications are the leading cause of death (Bruni et al., 2021). In this context, screening for PAH and ILD in SSc has emerged as an important consideration. Thakkar et al. (2016) displayed decreased levels of L-arginine, a common substrate of NOS (Cziráki et al., 2020), and increased levels of ADMA, suggesting an association with SSc-PAH. Decreased levels of amino acids with protective effects against endothelial dysfunction and anti-inflammatory effects by inhibition of proinflammatory cytokines, and oxidative stress reduction (Zhong et al., 2003; Hasegawa et al., 2012; Zhao et al., 2018), such as glycine, histidine, and betaine were also found in these patients. Furthermore, elevated levels of low-density lipoprotein in the lungs may lead to lipotoxicity, inducing inflammation and oxidative stress, which causes pulmonary vascular remodeling (Calvier et al., 2022). In SSc-ILD patients, metabolites such as Hcy, proline, glutamine, and BCAAs were elevated, which are involved in the amino acid pathways associated with fibrosis and inflammation. The upregulation of BCAAs can enhance proinflammatory phenotype by activating nuclear factor kappa B in immune cells and over-expression of IL-6 and tumor necrosis factor (Zhenyukh et al., 2017). On the other hand, phosphatidylethanolamine (PE) levels were decreased. Vazquez-de-Lara et al. (2018) demonstrated that PE could attenuate bleomycin-induced lung fibroblast, by decreasing the soluble collagen concentration in mice lungs.

To summarize, Figure 4 depicts a graphic overview of the main deregulated metabolites and metabolic pathways identified in SSc, as well as the role that these could play in the pathophysiology of the disease, leading to the appearance of clinical manifestations and complications associated with the disease.

Metabolomics, as a fast-developing technique in biomedical research, can be used to identify novel biomarkers (Kang et al., 2015b) and as a promising predictive or personalized medicine research technique (Zhou and Zhong, 2022). However, this systematic review has certain limitations, the most significant one is the challenge of comparing the metabolomic acquired across studies due to different limitation factors. Due to the scarcity of available quantitative data, the potential application of meta-analysis is limited, reducing the capacity to make more solid and generalizable conclusions. Variations in sample sources, sample preparation techniques, and metabolite detection methods may be to blame for the heterogeneity of results among studies. Another key constraint is the requirement for further research to validate metabolomics findings in multiple cohorts or independent populations. Validation of results is critical to ensuring the robustness and therapeutic usefulness of the proposed biomarkers. Likewise, most of the studies evaluated were cross-sectional, preventing us from determining a causal association in the metabolic changes associated with SSc. Despite these limitations, metabolomics remains a valuable tool, offering a unique opportunity to understand the metabolic basis of the disease and develop new diagnostic and treatment strategies.

In this review, potential biomarkers were described for the diagnosis of SSc, the identification of the dcSSc subtype, and the identification of primary pulmonary complications such as PAH, and ILD. These potential biomarkers were mainly within amino acids, nucleotides, carboxylic acids, and carbohydrate metabolism. More data are necessary concerning the specificity of biomarkers; as well as external validation studies in other and larger populations; however, we expect that metabolomics will provide more accurate and more validated biomarkers for the detection of SSc.

The data extracted from the 26 studies showed distinct metabolic profiles between SSc patients and HC and distinct profiles between SSc subtypes, generating new insights for non-invasive prognostic and early diagnostic biomarkers to improve individualized treatment and delay disease progression. Although the metabolic profile can still be affected by a series of other factors, the results obtained suggest the presence of a metabolic fingerprint of the disease. The disrupted metabolite mechanisms identified in this study, mainly, but not exclusively, involving amino acids and lipid metabolism, as well as TCA cycle dysregulation are associated with autoimmune inflammation, vascular damage, fibrosis, and gut dysbiosis, which might be relevant for the development of SSc. Nevertheless, further studies are required to evaluate the role of these alterations in the pathophysiology of the disease, as well as to assess whether these metabolomic networks have potential as treatment targets or as biomarkers not only for diagnosis but also for prognosis and treatment response.