Coronary artery disease (CAD) is a major global health concern and a leading cause of mortality worldwide [1]. Among CAD patients who underwent percutaneous coronary intervention (PCI), approximately 20% experienced adverse events within five years, with an all-cause mortality rate ranging from 2% to 11% following stent implantation [2]. Chronic inflammation is a key driver of atherosclerosis progression and plaque destabilization in diabetes mellitus (DM). This inflammatory milieu promotes adverse remodeling of coronary arteries, thereby increasing the risk of adverse events in CAD patients. Chronic low-grade inflammation plays a pivotal role in the pathogenesis of coronary artery disease [3]. The activation of the inflammatory response significantly contributes to plaque destabilization and subsequent development of acute coronary syndromes [4]. Several signaling pathways associated with the inflammatory response, including the nucleotide-binding oligomerization domain (NOD)-, leucine-rich repeat (LRR)- and pyrin domain-containing protein 3 (NLRP3) inflammasome, toll-like receptors, and Notch and Wnt signaling pathways, have been implicated in the development and subsequent regression of atherosclerosis [5]. The polarization of M1 macrophages is considered to be involved in this process. M1 macrophages are primarily activated by lipopolysaccharide (LPS) and $\gamma$-interferon (IFN$\gamma$), which engage Toll-like receptor 4 (TLR4) and $\gamma$-interferon receptor (IFN$\gamma$R). This activation leads to the up-regulation of key signaling pathways, including nuclear factor-$\kappa$B (NF-$\kappa$B), Janus kinase (JAK)-signaling, and the signal transducer and activator of transcription 1 (STAT1), which collectively enhance the secretion of pro-inflammatory factors. These mediators contribute to the promotion of inflammation, pathogen clearance, and phagocytosis [5]. Therapeutic interventions focusing on anti-inflammation have shown promising results. For example, a phase I randomized trial of dapansutrile (a specific NLRP3 inflammasome inhibitor) demonstrated its safety in heart failure patients with reduced left ventricular ejection fraction [6]. Chronic inflammation plays an important role in the development of coronary artery disease and diabetes mellitus, and finding new therapeutic targets to down-regulate inflammation appears to be a promising approach.

In recent years, the gut microbiota has received significant attention due to its potential involvement in various disease mechanisms [7, 8]. There is growing evidence to suggest that changes in the structure and function of the gut microbiota contribute to the onset and progression of CAD [9] and DM [10]. The gut microbiota can break down dietary components and generate metabolites that affect the host’s metabolism and immune responses [11]. These metabolites can influence various physiological functions and contribute to disease progression. An imbalance in the gut microbiota can increase inflammation, triggering immune activation and chronic inflammation [12]. This prolonged inflammatory state plays a crucial role in the development of numerous diseases [13]. In addition, disruptions in the gut microbiota can compromise the integrity of the gut barrier, leading to increased permeability and translocation of microbial byproducts. This process activates immune responses and may contribute to the emergence of systemic diseases. Therefore, exploring the connection between the gut microbiota and the progression of CAD and DM offers a novel perspective on understanding these diseases.

Metformin is a widely prescribed oral antidiabetic medication. In recent years, metformin was found to be beneficial for cardiovascular health. Metformin has been found to have beneficial effects on body mass index (BMI) and blood pressure, thereby helping to promote cardiovascular health [14]. Metformin also plays a crucial role in modulating the gut microbiota [15]. Metformin also affects microbial metabolism and virulence factors, which are essential for maintaining gut homeostasis. It has been suggested that metformin could influence the gut microbiota through various mechanisms. Metformin results in alterations to the structure of the gut microbiota by metabolic changes which may enhance the presence of beneficial bacteria, such as lactobacilli, and decreasing harmful bacteria [16, 17, 18]. Alternatively, metformin can affect microbial metabolism, stimulating the generation of metabolites such as short-chain fatty acids (SCFAs) which are crucial for maintenance of gut health [19]. Furthermore, a study showed that metformin may play a role in reducing the virulence of specific pathogenic bacteria, thereby decreasing inflammation [20]. However, research has primarily focused on the short-term effects of metformin on the gut microbiome. Metformin has been widely used as a first-line drug in diabetes and is often used long-term. Long-term use of metformin has a significant effect on the health of the host and their gut microbiome. However, the long-term effects of metformin have not been thoroughly investigated. Therefore, examining the mechanisms underlying CAD complicated by DM, along with alterations in gut microbiota linked to long-term metformin use, is of immense value.

Coronary artery disease is closely related to chronic and systemic inflammation [42]. Metformin, a well-known drug used in treating diabetes, has been reported to benefit cardiovascular health [43, 44]. The underlying mechanisms may include enhancing insulin sensitivity and reducing cardiovascular risk factors [45], decreasing inflammation [46], and reducing oxidative stress [47]. Although much research has been performed on metformin and host health, the long-term benefits of metformin on cardiovascular health via gut microbiome have not been studied, especially the long-term benefits associating with the gut microbiome. There have been studies which showed that there was no significant difference in the prognosis between strategies of insulin sensitization and insulin provision. Frye et al. [48] compare insulin-sensitizing strategies (including metformin) with insulin-providing strategies in CAD patients and find no significant difference in outcomes, highlighting the need for individualized treatment strategies in this population. Jung et al. [49] specifically examined the effects of glucose-lowering agents on CAD outcomes and concluded that metformin significantly reduces the risk of repeat revascularization after PCI in type 2 diabetes mellitus (T2DM) patients. This supports the role of metformin in CAD management but does not address gut microbiota or metabolomics. A meta-analysis by Griffin et al. [50] evaluated the cardiovascular outcomes of metformin and concluded that while it is considered safe and potentially beneficial for CAD, evidence from long-term studies remains inconclusive due to limitations in study design and small sample sizes. Our study provides new evidence that long-term metformin use is associated with improved five-year prognosis in CAD patients who have undergone PCI. Despite similar baseline clinical characteristics, patients in the Met CAD-DM group experienced significantly fewer MACE within five years after PCI compared to those without metformin treatment. All the patients included in our study had follow-up for over 5 years. These findings underscore the potential of metformin as a therapeutic strategy for reducing cardiovascular risk in coronary artery disease.

Studies have shown that gut microbiome plays an important role in cardiovascular health and may affect clinical outcomes [51]. It has also been reported that metformin may play important role in alterations of the gut microbiome [52]. Thus, we looked further into the gut microbiome taxonomic features and serum metabolome profiles to reveal the possible associations between metformin intake and better 5-year prognosis. Our study showed that the long-term use of metformin plays an important role in altering gut microbiome taxonomic features. The alpha diversity and beta diversity both presented different taxonomic features between the non-Met CAD-DM group and the Met CAD-DM group.

The alterations of the gut microbiome associated with metformin tends to be beneficial for the patient’s cardiovascular health. It’s worth mentioning that the healthy control group served as a base line reference rather than a focus for comparison. Specific beneficial gut microbiomes (such as B. coprocola, B. massiliensis, Clostridium III, and P. succinatutens) had the lowest abundance in the non-Met CAD-DM group and the highest abundance in HC. The Met CAD-DM showed an intermediate abundance of the above four beneficial microbes, which we attributed to the influence of metformin. This suggests that metformin may slow the decrease of these specific beneficial gut microbiota during disease progression. We specially focused on the four gut bacteria: Bacteroides coprocola (B. coprocola), Bacteroides massiliensis (B. massiliensis), Clostridium III, and Phascolarctobacterium succinatutens (P. succinatutens). These bacteria exhibited the highest abundance in the healthy controls, followed by the Met CAD-DM group and the non-Met CAD-DM group. This showed a potential association between metformin intake and the abundance of beneficial microbiota. It can be inferred that metformin may be associated with the decrease of these probiotics during disease progression. These four gut florae were reported to be beneficial to host health in several studies. B. coprocola was reported to be very promising as a potential preventive and therapeutic agent against obesity [53], which is a risk factor in both CAD and DM [54, 55]. Furthermore, B. coprocola was reported to be negatively associated with liver fibrosis among male patients with metabolic dysfunction-associated fatty liver disease (MAFLD) [56]. B. coprocola has been demonstrated to have anti-oxidative properties and an improvement in intestinal barrier function [57]. Studies have found that oxidative stress is associated with the onset and progression of coronary heart disease [58, 59] and diabetes mellitus [60, 61]. The imbalance in the generation and clearance of reactive oxygen species (ROS) can lead to extensive and permanent damage, resulting in endothelial dysfunction, and accelerating the occurrence and development of both CAD and DM [61]. These studies indicate that the abundance of B. coprocola may correlate with an anti-oxidative effect. Study on Clostridium III has shown that it is linked to the attenuation of the NF-$\kappa$B signaling pathway and the decrease of inflammatory cytokines [62]. Thus, a rise in Clostridium III within the Met CAD-DM group may play roles in suppressing the production of inflammation, leading to a better clinical prognosis. Furthermore, B. massiliensis and P. succinatutens, are capable of producing SCFAs [63, 64], which are positively associated with the use of metformin and contribute to improved disease outcomes. Extensive research has demonstrated that SCFAs possess anti-inflammatory properties [65] and inhibit interleukin (IL)-6 and IL-8 production through G-protein-coupled receptor (GPR)41/43 activation, thereby, reducing systemic inflammation and improving atherosclerosis [66]. Consequently, a higher abundance of B. massiliensis and P. succinatutens can result in increased production of these protective SCFAs, resulting in benefits for cardiovascular health. Moreover, research has shown that a deficiency of SCFAs is associated with DM [67]. Further research showed that SCFAs are associated with lower blood glucose and lipid levels [68]. Symptoms of diabetes can be alleviated by increasing SCFA-producing bacteria, promoting the production of SCFAs, and upregulating SCFA-glucagon-like peptide (GLP)1/peptide tyrosinetyrosine (PYY)-associated sensory mediators [69]. In recent years, SCFAs such as propionic and butyric acids assembled into nanoparticles and fed into a mouse model of type 2 DM showed improved symptoms [70]. P. succinatutensis a predominant gut bacteria, enhancing intestinal epithelial barrier function [71]. It was reported that P. succinatutens significantly affected several metabolic pathways such as steroid hormone biosynthesis, primary bile acid biosynthesis, phenylalanine, tyrosine and tryptophan biosynthesis, and phenylalanine metabolism [71]. P. succinatutens may play an important role in maintaining a stable symbiosis between the host and core gut microbes. The oral gavage of P. succinatutens in mice can improve host organ indexes (including the heart, spleen, and thymus). Additionally, it was reported that P. succinatutens significantly increased the crypt depths in the duodenum and ileum, regulating gut mucosal morphology [71]. Thus, metformin may be associated with the alterations of the intestinal microbiome, effectively slowing the depletion of beneficial bacterial species, which may positively influence the prognosis of CAD in patients with DM. Notably the preservation of the four beneficial bacterial species—(B. coprocola, B. massiliensis, Clostridium III, and P. succinatutens)—is closely associated with improved metabolic function and reduced disease progression. By mitigating the decrease of these beneficial bacteria, metformin contributes to better disease outcomes.

The bacterial species Eubacterium Coprostanoligenes (E. coprostanoligenes) deserves special attention. Metformin is associated with the abundance of E. coprostanoligenes, which is linked to a better prognosis of CAD-DM, and metformin is significantly associated with elevated abundance of these beneficial microbes. E. coprostanoligenes was reported to be able to alleviate intestinal mucositis by enhancing the intestinal mucus barrier [72]. This enhancement of the intestinal mucus barrier is associated with activating the aryl hydrocarbon receptor/AU-rich element RNA-binding factor 1 (AhR/AUF1) pathway, consequently enhancing Muc2 mRNA stability [72]. The intestinal mucus barrier forms the first line of defense against bacterial invasion while providing nutrients to support microbial symbiosis [73]. On the one hand, the intestinal mucus barrier plays a key role in preventing preclinical diabetes from progressing into diabetes. It was reported that the integrity of the intestinal mucus barrier avoids dysregulated crosstalk between gut microbiota and immune cells, thus preventing the progression of preclinical diabetes [74]. Loss of gut barrier integrity triggers activation of islet-reactive T cells and autoimmune diabetes [75]. On the other hand, the dysfunction of the intestinal mucus barrier is observed in coronary artery disease, and restoration of the intestinal barrier is regarded as a potential therapeutic target in CAD [76]. The improvement of the intestinal barrier’s function also attenuates atherosclerosis by decreasing toxic lipid accumulation and reducing inflammatory cytokines [77]. The dysfunction of the intestinal mucus barrier is also associated with hyperlipidemia [78], which is a well-known risk factor in CAD. Thus, the enrichment of E. coprostanoligenes may significantly promote the restoration of the intestinal mucus barrier, thus alleviating autoimmune reactions and improving the prognosis of CAD-DM. This further demonstrates that metformin exerts a protective effect and improves the prognosis of patients suffering from coronary artery disease complicated with diabetes mellitus.

Not only does the gut microbiome affect the 5-year prognosis of CAD-DM patients, but the gut microbiota-derived metabolites also play an important role in the prognosis of the disease. HAS is positively related to P. succinatutens, which is regarded as a beneficial microbe. Furthermore, HAS is negatively correlated with rehospitalization and death. Thus, HAS is potentially beneficial to CAD-DM patients and improves the 5-year prognosis of CAD-DM. It has been reported that HAS regulates gut microbiota and metabolites by affecting lipid and amino acid metabolism pathways [37, 79]. Lipid metabolism plays an important role in CAD. Regulating lipid metabolism may influence the prognosis of CAD. Study has shown that the intervention of HAS could also improve the intestinal and metabolic functions [37]. HAS has also been reported to exert anti-diabetic effects by increasing glycogen synthesis through regulation of phosphoinositide-3-kinase/protein kinase B/glycogen synthase kinase-3$\beta$/glycogen synthase (PI3K/Akt/GSK-3$\beta$/GS) signaling [80]. Additionally, HAS exhibitsvarious pharmacological properties, including analgesia effects and regulating gastrointestinal function [81]. Therefore, HAS plays an important role in improving the 5-year prognosis of CAD-DM.

Our study showed that decenoylcarnitine, a subtype of acylcarnitine, is significantly negatively related to the incidence of 5-year myocardial infarction. Decenoylcarnitine is also negatively associated with rehospitalization, recurrence of chest tightness and pain, as well as an abnormal computed tomography angiography (CTA) within five years. This implies that the higher the abundance of decenoylcarnitine, the better the 5-year prognosis of CAD-DM. Acylcarnitine is essential in fatty-acid metabolism [82]. Although research has shown that acylcarnitine is associated with the development of heart failure [83] and accelerates the progress of atherosclerosis [84], decenoylcarnitine is differs from acylcarnitine and only a few studies have been done on decenoylcarnitine.

In our study, we also discovered several pathogenic metabolites, including 4-(nitrosoamino)-1-(3-pyridinyl)-1-butanone, Tridec-10-enoic acid, 3,9-dimethyluric acid, and Z-vad-fmk. These metabolites are positively associated with a poorer 5-year prognosis. The enrichment of these four metabolites worsens the 5-year outlook of the disease. Tridec-10-enoic acid is a type of long-chain fatty acid (LCFA). These fatty acids can be derived from food or produced by certain types of gut microbiomes such as Fusimonas intestini, a commensal species of the family Lachnospiraceae [85]. Thus, LCFAs are closely associated with the gut microbiome. LCFAs are also recognized as significant triggering factors for inflammatory disease because they regulate the palmitoylation of signal transducer and activator of transcription 3 (STAT3) through cluster of differentiation 36 (CD36)-mediated endocytosis [86]. Both CAD and DM are closely associated with chronic inflammation. Consequently, LCFAs play an important role in promoting inflammation in CAD and DM, thereby worsening the 5-year prognosis. Furthermore, LCFAs also contribute to M1 macrophage polarization by activating NLRP3 inflammasome and the NF-$\kappa$B pathway [87]. The accumulation of macrophages and inflammation in atherosclerotic plaques exacerbate atherosclerosis [88], which is the underlying pathological mechanism of CAD. Macrophage polarization is closely associated with hepatic injury in diabetes by activating the protein tyrosine phosphatase 1B/signal transducer and activator of transcription 6 (PTP1B/STAT6) axis [89]. Thus, the polarization of macrophages by LCFAs aggravates both CAD and DM. Our results align with findings from other studies. Tridec-10-enoic acid, which belongs to the family of LCFAs, may be potentially pathogenic to CAD-DM patients and worsen the 5-year prognosis by promoting inflammation and cytokine accumulation. Tridec-10-enoic acid may also be considered a potential drug target.

We focused on two specific metabolites: 3,9-dimethyluric acid and Z-vad-fmk. Both these metabolites exhibited a significant negative correlation with P. succinatutens, which is a well-known beneficial microbe. 3,9-dimethyluric acid is a type of dimethyluric acid that has been linked to lipid metabolism and is known to have pro-inflammatory properties [40]. Increased exposure to inflammatory cytokines can activate the ROS-p38-p65 signaling pathway. This activation leads to endothelial cell dysfunction, resulting in coronary atherosclerotic lesions and plaque rupture [90]. Consequently, higher levels of dimethyluric acid contribute to systemic inflammation, worsening the prognosis of patients with CAD-DM. In contrast, Z-vad-fmk, has been shown to induce autophagy and necrotic cell death in smooth muscle cells via macrophage activity. Research indicates that Z-vad-fmk-treated macrophages overexpress and secrete various chemokines and cytokines, including TNF-$\alpha$. The combination of z-vad-fmk and TNF-$\alpha$ results in smooth muscle cell necrosis. Therefore, Z-vad-fmk is detrimental to atherosclerotic plaque stability as it stimulates inflammatory responses and indirectly induces the death of smooth muscle cells, leading to a worse prognosis in CAD-DM [41]. Thus, higher levels of Z-vad-fmk lead to decreased atherosclerotic plaque stability and result in a worse prognosis of CAD-DM Our results suggest that metabolites such as HAS and decenoylcarnitine may improve the 5-year prognosis of CAD-DM while 4-(nitrosoamino)-1-(3-pyridinyl)-1-butanone, tridec-10-enoic acid, 3,9-dimethyluric acid, and Z-vad-fmk may worsen the prognosis of CAD-DM over the same period.

Our results collectively demonstrate that metformin improves the 5-year prognosis of CAD-DM by influencing the gut microbiome. The potential mechanisms outlined in our analysis (Fig. 4) are as follows: (1) Metformin may play important role in influencing the abundance of beneficial microbes such as B. coprocola, B. massiliensis, Clostridium III, and P. succinatutens. (2) Metformin promotes the enrichment of SCFA-producing microbes while depleting pathogenic microbes. (3) Changes in the gut microbiome not only directly affect the 5-year clinical prognosis of CAD-DM, but also impact these outcomes by regulating the serum metabolome. (4) After metformin use, the serum metabolome shows higher levels of anti-oxidant metabolites and lower levels of pro-inflammatory metabolites. These, alterations in the gut microbiome and serum metabolome are significantly associated with lower levels of systemic inflammation, diminished macrophage activation, reduced apoptosis of smooth muscle cells, and increased atherosclerotic plaque stability. These cascading effects linked to metformin contribute to a better 5-year prognosis in patients with CAD-DM.

Fig. 4.

The potential mechanism of metformin’s beneficial effect on cardiovascular health. Metformin reshaped the gut microbiome by increasing the abundace of beneficial microbes and preserving the diversity of the gut microbiome. This change further maintained the stability of the intestinal mucus barrier and inhibited the production of inflammatory cytokines. The microbiome-derived metabolites helped stabilize atherosclerotic plaques. In contrast, individuals not taking metformin experienced disrupted gut microbiota diversity. The intestinal mucus barrier exhibited dysfunction, and the production of inflammatory factors was not inhibited. Moreover, the microbiome-derived metabolites promoted macrophage activation and further led to atherosclerotic plaque disruption. SCFAs, short-chain fatty acids; LCFAs, long-chain fatty acids; HAS, hydroxy-alpha-sanshool.

Our study emphasizes the long-term effects of metformin on the gut microbiome. While recent research has highlighted the interaction between metformin and the gut microbiome, it has largely overlooked its long-term impact. Although short-term use of metformin does alter the gut microbiome, these changes are minor and do not significantly affect disease prognosis. Only the long-term use of metformin and its lasting effects are convincing. The prolonged interaction between metformin and the gut microbiome leads to a state of homeostasis that ultimately influences clinical outcomes.

Several limitations of this study need to be acknowledged. First, the sample size of patients was relatively small and the patients were all from a single geographic area. This may affect the interpretation of gut microbiota. Second, the study population included both CAD and DM. Hence, there are two distinct disease variables. The age and BMI of the healthy control groups seemed to be younger from the diseased group. Third, untargeted metabolomics had limited accuracy in the annotation of serum metabolites. The use of Greengenes database also has limited accuracy. We also used FDR 0.2 in the whole calculation. In addition, we did not collect the individual medications (such as ACE inhibitor, beta blocker, etc.) for coronary artery disease. This study also lacked a PICRUSt2 analysis. Experimental research in animals needs to be performed to further establish the relationship between metformin and these alterations in gut microbiota and the serum metabolome.

In conclusion, CAD patients who have been on long-term metformin therapy demonstrated a significantly improved prognosis within five years following PCI. This protective effect of metformin is further associated with gut microbiome and microbiome-associated metabolites. Multi-omics analysis revealed that metformin is strongly associated with the preservation of beneficial gut microbiota such as B. coprocola, B. massiliensis, P. succinatutens, and E. coprostanoligenes. Changes observed in serum metabolome exhibit anti-inflammatory properties, characterized by an increase in HAS and decenoylcarnitine and a decrease in Tridec-10-enoic acid, 3,9-dimethyluric acid, and Z-vad-fmk. These modifications in the gut microbiome and serum metabolome induced by metformin demonstrated beneficial effects on the five-year prognosis for CAD-DM patients on metformin therapy.

The dataset supporting the results of this article has been deposited in the Sequence Read Archive under BioProject accession code SRP167862 (https://www.ncbi.nlm.nih.gov/sra).

RLZ, QYW, SYZ, and WW conceived and designed the study. RLZ wrote the manuscript. RLZ and QYW contributed to the bioinformatics analysis and made the tables and figures. QYW conducted the literature search in chief. HQ, LW, GCL, and BZ contributed to literature search. WW critically revised the manuscript. All authors contributed to the conception and editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The studies involving human participants were reviewed and approved by the Ethics Review Board at the Peking Union Medical College Hospital (protocol number is I-24PJ0927). Written informed consent was obtained from all participants and the study adhered to the principles of the Declaration of Helsinki.

Not applicable.

This work was supported by National High Level Hospital Clinical Research Funding (2022-PUMCH-D-002), Chinese Academy of Medical Sciences (CAMS) Innovation Fund for Medical Sciences (2021-I2M-1-003), and National High Level Hospital Clinical Research Funding (2022-PUMCH-B-098).

The authors declare no conflict of interest.

Supplementary material associated with this article can be found, in the online version, at https://doi.org/10.31083/RCM26835.