In this study, we included 120 samples, including acne with IR (n = 51) and acne without IR (n = 69). By LC-MS/MS analysis, 18 significantly differential metabolites were identified. In the positive ion mode, the upregulated substances were creatine, sarcosine, D-proline, uracil, Phe-Phe, L-pipecolic acid, and DL-phenylalanine. The downregulated substances were tridecanoic acid (tridecylic acid), L-lysine, cyclohexylamine, sphingomyelin (d18:1/18:0), epsilon - (gamma - L-glutamyl)-L-lysine, and 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine. In the negative ion mode, the upregulated substance was cholesterol sulfate, and the downregulated substances were D(-)-beta-hydroxybutyric acid, myristic acid, D-galacturonic acid, and dihydrothymine. Cholesterol sulfate showed the most significant expression among all differential metabolites (VIP = 7.3411). Based on the KEGG database, necroptosis and ABC transporters were the most significantly enriched metabolic pathways in this experiment.
The pathogenesis of acne and IR is not fully understood, and current studies have found that insulin/IGF-1 may lead to the occurrence and development of acne by directly or indirectly regulating androgen levels (Melnik and Schmitz, 2009). When the body becomes tolerant to insulin and the blood insulin level increases, insulin/IGF-1 can activate the PI3K/Akt or AMPK pathways, thereby increasing the androgen content in the body (Fan et al., 2007). High levels of androgen binding to the androgen receptor (AR) activate Wnt/β-catenin and mammalian target of rapamycin1 (mTORC1) signaling pathways. It induces the differentiation of cortical cells and promotes adipogenesis and sebaceous duct obstruction, thereby inducing acne (Melnik, 2014). In addition, IGF-1 can promote the production of inflammatory factors by activating the NF-κB pathway, which causes skin inflammation (Cong et al., 2019). In addition to insulin/IGF-1, AR play a key role in the development of acne. AR can bind to a variety of regulatory factors through its domains or ligands to exert its biological functions and is an important hub in signaling pathways(van de Wijngaart et al., 2012). When AR function is decreased, the effect of androgens on the skin and accessory organs will also be inhibited (van de Wijngaart et al., 2012).
Cholesterol sulfate (CS) was the most significant differential metabolite in the negative ion mode found in this study, and it has endogenous steroidogenic potential (Fritsch et al., 2001). Steroidogenesis in human skin can begin with cholesterol (Slominski et al., 2004; Thiboutot et al., 2003). Cholesterol sulfur transferase forms cholesterol sulfate in the basal and spinous layers of the epidermis and is converted to cholesterol by steroid sulfatase in the stratum corneum (Elias et al., 1984; Epstein et al., 1984). The most important physiological roles of CS are keratinocyte differentiation and epidermal barrier development (Jetten et al., 1989). Increased CS levels lead to retention hyperkeratosis, which is a factor in the development of acne (Elias et al., 1984; Williams and Elias, 1981). In addition, cholesterol-sulfur transferase is stimulated by peroxisome proliferator-activated receptor (PPAR) activators in human keratinocytes (Jiang et al., 2005), which is known to increase sebum production (Trivedi et al., 2006). Moreover, PPAR ligands enhance androgen activation and lipid synthesis in human sebocytes (Makrantonaki and Zouboulis, 2007). It has also been shown that CS can alleviate diabetes in STZ-induced diabetic mice by increasing pancreatic β-cell number and function, which provides a physiological pathway to preserve islet viability and prevent diabetes development (Zhang et al., 2022). In this study, cholesterol sulfate was significantly higher in patients with acne and IR than in the acne alone group, suggesting the possibility of cholesterol sulfate as a biomarker for acne with IR.
Creatine and sarcosine were the most significantly differentially expressed metabolites in the positive ion mode, and creatine supplementation has been shown to improve glycemic control and IR in both healthy and diabetic patients (Solis et al., 2021). Normally, insulin is secreted by pancreatic beta cells in response to energy substrates (e.g., glucose, fatty acids, and amino acids), hormones, and changes in energy demand (e.g., fasting-feeding cycles, exercise, and stress) to maintain euglycemic levels (Defronzo et al., 2015). However, IR may lead to β-cell failure, resulting in a progressive decrease in insulin secretion (Abdul-Ghani et al., 2006; Defronzo and Tripathy, 2009). IR is often associated with inhibition of the PI3K pathway, with increased serine phosphorylation and inhibition of tyrosine phosphorylation of IRS proteins (Copps and White, 2012). IRS protein degradation also appears to occur under certain conditions (Bouzakri et al., 2006). Higher plasma creatine concentrations were associated with an increased incidence of T2DM in a prospective cohort study including more than 4700 participants (Post et al., 2021). The authors suggested that higher extracellular creatine concentrations and lower intracellular phosphorylated creatine/creatine content may be associated with impaired intracellular energy status, suggesting mitochondrial dysfunction, a hypothesized mechanism involved in the pathophysiology of T2DM (Post et al., 2021). In this study, creatine and sarcosine levels in acne patients with IR were significantly higher than those in the acne group, which was consistent with the positive correlation between creatine concentration and IR.
The necroptosis pathway was the most significantly enriched metabolic pathway in this study, and necrosis is the most typical form of programmed necrosis and is characterized by apoptosis and necrosis (Tang et al., 2019). The relationship between the necroptosis metabolic pathway and IR is not fully understood. Necrostatin-1 (NEC-1) is a necrosis inhibitor. NEC-1 improves cardiac function in prediabetic rats by directly inhibiting necrosis and myocardial mitochondrial dysfunction and increasing mitochondrial fusion independent of peripheral metabolic function (Apaijai et al., 2021). It has also been shown that chronic HFD intake leads to prediabetic conditions and cognitive decline by causing several brain pathologies including IR, inflammation, microglial hyperactivity, mitochondrial dysfunction, oxidative stress, and necrosis. However, NEC-1 treatment directly improves cognitive function in prediabetic patients by inhibiting necrosis and reducing brain inflammation and mitochondrial dysfunction but does not improve insulin sensitivity (Jinawong et al., 2020).
The ABC transporter metabolic pathway was another significant metabolic pathway enriched in this experiment, and is characterized by the use of ATP-hydrolyzed energy to transport specific compounds across cell membranes, including lipids, amino acids, carbohydrates, vitamins, ions, glucuronate conjugates, and xenobiotics (Dean et al., 2001). It has been shown that the metabolic pathways most closely related to patients with moderate to severe acne include ATP-binding cassette transporter (ABC) and sphingolipid signaling pathways (Yu et al., 2022). In the present study, the shear trend in ABC transporter expression was not evident, which may be related to the fact that we did not group discussions of severe conditions in patients with acne vulgaris. It has been shown that ABCA1 is important for controlling monocytosis, secretion of adipose and hepatic macrophages, and insulin sensitivity when mice become obese and require substantial lipid buffering, raising the possibility of enhancing ABCA1 in hematopoietic cells as a potential target for the treatment of IR and atherosclerosis (Tang et al., 2016).
This study explored the relationship between metabolic pathways and some potential biomarkers and acne with IR, but there are still some shortcomings: 1) both the experimental and control groups were classified on the basis of acne patients. There were highly correlated metabolites and metabolic pathways in the body itself, so few species with significant metabolic differences were obtained; 2) among the metabolites screened in the positive and negative ion modes of this experiment, the number of unclassified metabolites was large, accounting for 40.17%, which may be limited by the number of metabolites in the database or the inability of the current technology to identify relevant metabolites; 3) further experiments were not performed to verify the findings of the study, such as detecting the concentrations of differential metabolites in the two groups of patients, analyzing their correlation with HOMA, and conducting ROC analysis.