Gut Microbiota-derived Metabolites: A New Perspective of Traditional Chinese Medicine Against Diabetic Kidney Disease

Diabetes is a serious global public health problem. The latest data from the International Diabetes Federation estimates that by 2045, about 700 million people worldwide will be living with diabetes.[1] Diabetic kidney disease (DKD), one of the most serious complications of diabetes, is a major factor of end-stage kidney disease (ESRD). At present, clinical management and treatment strategies for DKD mainly involve controlling blood glucose, blood lipids, and blood pressure as well as blocking the renin–angiotensin system (RAS); however, these strategies still cannot lower the incidence of DKD.[2] In recent years, new therapies, such as sodium-glucose transporter 2 (SGLT2) inhibitors, endothelin antagonists, glucagon-like peptide-1 (GLP-1) agonists, and mineralocorticoid receptor antagonists (MRAs), have become novel treatment options and increased renal protection for DKD patients.[3] However, patients are still at risk of ESRD. Therefore, new treatments are needed to further reduce ESRD risk. Gut microbiota disturbance plays a key role in diabetic kidney injury. The concept of the gut–kidney axis was first proposed by Meijiers in 2011 and has gained widespread attention in recent years.[4] The human gut microbiome, with its more than 100 trillion microorganisms, is a diverse and complex microbial community that plays an important role in maintaining intestinal integrity and function: it regulates immune inflammatory response, nutrient absorption and metabolism, and detoxification. These functions help maintain a dynamic balance between gut microbiota and host health.[5] An increasing amount of research evidence has shown that gut microbiota is closely related to DKD.[6–10] Due to a decrease in glomerular filtration rate in DKD, uremic toxins accumulate in patients’ circulation, leading to an imbalance in the gut microbiota. This imbalance leads to an increase in harmful bacteria and a decrease in probiotics, which results in an increase in uremic toxins, such as indoxyl sulfate (IS), p-cresol sulfate (p-CS), trimethylamine, and trimethylamine N-oxide (TMAO), and a decrease in renal protective metabolites, such as short chain fatty acids (SCFAs).[6] These uremic toxins enter the blood circulation through the damaged intestinal epithelial barrier, potentially activate intestinal mucosal immunity, and induce systemic micro-inflammatory response, which further aggravates kidney damage and ultimately leads to the progression of DKD to ESRD.[7] In particular, TMAO, as a low molecular weight substance, is mostly cleared by the kidneys through glomerular filtration and tubular secretion into urine. As DKD progresses, TMAO continues to accumulate in patients due to a decrease in glomerular filtration rate.[8] In addition, gut microbiota dysfunction in DKD patients promotes increased accumulation of TMAO. Our previous study found that 63% of the pathways involved in gut microbiota disturbance in DKD were related to metabolism.[9]Bacteroides and Elusimicrobia were positively correlated with urinary albumin/creatinine ratio (UACR), glomerulosclerosis index, and tubular injury index, while Firmicutes and Actinomyces were negatively correlated with UACR and tubular injury index scores. Moreover, Eubacteria and Actinomyces were negatively correlated with renal function.[9] Therefore, elucidating the mechanism of gut microbiota and their metabolites is of great significance for the clinical treatment of DKD. Disturbances in the gut microbiota can lead to abnormal intestinal metabolites. The metabolites produced by gut microbiota serve as chemical messengers that mediate the interaction between the microbe and host, and they can exert biological effects and enter peripheral tissues through blood circulation. SCFAs and tryptophan metabolites are the main metabolites of gut microbiota.[10] SCFAs are saturated fatty acids, which are the main byproducts of the microbial fermentation of dietary fiber. Among these fatty acids, acetic acid, propionic acid, and butyric acid constitute the most abundant SCFAs in the human body, accounting for about 90% of SCFAs. Increased SCFA levels has been reported to play an anti-inflammatory role and reduce kidney damage in patients suffering from kidney disease. SCFAs, as a ligand for G protein-coupled receptors (GPRs), bind to GPR43 to reduce pro-inflammatory cytokines in colon tissue, thereby promoting intestinal homeostasis.[11] Compared with healthy individuals, DKD patients have significant differences in terms of their relative abundance of intestinal bacteria, which is mainly manifests as a result of the decrease in SCFA-producing bacteria, including Ruminococcus, Coprococcus, Eubacterium, and Clostridium leptum. Serum and fecal SCFA levels in DKD patients are significantly reduced, and are positively correlated with renal dysfunction.[12] It has been reported that the plasma acetic acid level of patients with stage-5 CKD is significantly lower than that of patients with stage-1 CKD and stage-2 CKD.[13] Thus, reduced SCFA is associated with kidney disease progression. G protein-coupled receptors (GPCRs) are the main receptors of SCFA. They have 7 helical transmembrane receptors and are widely distributed in the intestine, kidney, and other organs. Under normal physiological conditions, SCFA can bind to GPR41, GPR43, and GPR109A receptors, promote the generation of Th1, Th17, and Treg cells, thus activating the secretion of anti-inflammatory factor interleukin (IL)-10. SCFA also reduce neutrophil recruitment, increase IL-10 levels, and decrease the levels of pro-inflammatory factors such as IL-6, IL-1β, NO, and tumor necrosis factor (TNF)-α.[14] The imbalance of flora in DKD leads to insufficient SCFA production, which promotes inflammatory response and aggravates the progression of DKD.[15] A significant amount of research has confirmed that increasing SCFA can significantly improve renal inflammation and fibrosis, either by supplementing with probiotics or by directly supplementation with SCFA. Tryptophan is an essential dietary amino acid that is a biosynthetic precursor to a large number of microbial and host metabolites. It can be directly metabolized to indole and its derivatives by gut microbiota. Indoxyl sulfate (IS), an enteric-borne uremic toxin, is tightly bound to albumin or other plasma proteins in circulation. In addition, it has a relatively large molecular weight, which is difficult to remove by dialysis and thus poses a major challenge to ESRD treatment.[16] The accumulation of uremic toxins is a common symptom of patients with kidney disease, and it has become a focus of research on the progression of DKD in recent years. Physiologically, IS and its precursor indole are beneficial to the host, enhancing intestinal barrier function and inhibiting central nervous system inflammation.[17] In DKD patients, IS levels are abnormally elevated; even after dialysis, IS levels increase by a factor of nearly 20.[18] Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is expressed in the intestine, kidney, and other organs. It is closely related to immunity, inflammation, intestinal epithelial barrier function, and gut microbiota homeostasis.[19] Intestinal microorganisms decompose tryptophan to produce indole metabolites, which can regulate the host immune system and maintain the homeostasis of the host-gut microbiota by activating AhR. When unbound to ligands, AhR exists primarily in a complex consisting of AhR, 90 kDa heat shock protein, chaperone p23, protein kinase Src, and AhR-interacting proteins. After ligand binding, AhR dissociates from the complex and enters the nucleus to form heterodimers with AhR nuclear translocation proteins, which bind to DNA response elements and initiate the expression of target genes cytochrome P450 and inflammatory factors.[20] Recent studies have confirmed that the activation of AhR can lead to progressive glomerular and tubular damage, thereby exacerbating the progression of DKD.[21] Therefore, tryptophan metabolism is an important research direction for delaying the progress of DKD. Traditional Chinese medicine (TCM) regulates gut microbiota and its metabolites to treat DKD. TCM has been widely used in many countries, especially China, to treat kidney disease. Many clinical trials and animal experiments have proved that TCM has a protective effect on the renal function of DKD. There is increasing evidence that TCM and its natural extracts can exert a kidney-protective effect by influencing the gut microbiota and its metabolites through the gut–kidney axis. The protective mechanism mainly regulates the composition of intestinal flora—especially Akkermansia, Lactobacillus, and Bacteroidetes—and the ratio of Firmicutes to Bacteroidetes. This will promote the production of SCFAs and reduce the concentration of uremic toxins, thereby restoring the intestinal barrier and inhibiting inflammation and oxidative stress.[22] For example, the Shenyan Kangfu tablet, as a marketed TCM, can reduce HBA1C and urinary microproteins and improve renal thylakoid expansion and inflammatory response in db/db mice, which may be related to the downregulation of Bacteroidetes and Firmicutes.[23] The Tangshen Formula (TSF) is a TCM used to treat DKD. Our previous multicenter randomized controlled clinical trial demonstrated that TSF reduced clinical proteinuria, increased glomerular filtration rates, and improved dyslipidemia in patients with DKD. Our recent study found that TSF treatment improved the disruption of gut microbiota and reduced the abnormal accumulation of harmful metabolites. Further explorations of the mechanism have suggested that TSF reduced inflammation and oxidative stress by upregulating intestinal metabolites such as indole derivatives.[24] Therefore, TSF can improve DKD by correcting intestinal microbiota disorders, thereby upregulating beneficial metabolites and downregulating harmful metabolites. Our study provides key evidence for TCM to alleviate renal inflammation in DKD by regulating the metabolites of gut microbiota. There are some limitations to the use of TCM to treat DKD via gut microbiota regulation. Targeting gut microbiota to treat DKD has gradually become a focus of research. Antibiotic treatment, probiotic supplementation, and fecal transplantation can correct gut microbiota disorders and treat diseases. In particular, fecal transplantation is thought to rebuild and restore the gut microbiota to an ideal state of health.[25] Fecal microbiota transplantation (FMT) showed that the microbiota of lean donors improved insulin sensitivity in patients with metabolic syndrome, accompanied by changes in the structure of gut microbiota. Furthermore, intestinal microbes from healthy donors can alleviate tubulointerstitial damage by correcting cholesterol homeostasis imbalance after bacterial fecal transplantation, which indicates that FMT can potentially be used to treat diabetes and DKD.[26] Due to the lack of direct scholarly evidence, further research is needed to determine the safety and effectiveness of FMT use in clinical treatment. Due to its complex chemical composition, TCM can be used as a prebiotic to improve the abundance of probiotics, inhibit the growth of harmful intestinal bacteria, and maintain the homeostasis of intestinal bacteria to treat diseases.[27] However, studies on TCM’s ability to regulate DKD gut microbiota have limitations. Most studies have been conducted on animal models, and few clinical studies have been done. Because of the differences in gut microbiota among different species, clinical trials are of great significance for clarifying the correlation between TCM and gut microbiota. In addition, the exact mechanism by which TCM regulates gut microbiota and its metabolites is still unclear and should be further examined in the future. In this commentary, we sorted out the mutually beneficial relationship between gut microbiota and host, and explained that DKD can lead to gut microbiota disturbance and changes in intestinal metabolites, which further aggravated the progress of DKD. Recent evidence from our study and other researchers suggests that TCM plays a key role in regulating gut microbiota and improving the structure of intestinal metabolites. These findings contribute to an improved understanding of the therapeutic mechanisms of TCM. However, most current studies are not sufficiently deep, and methods such as flora transplantation and supplementation of intestinal metabolites should be further studied to determine the molecular mechanism and effective active components of TCM’s ability to regulate intestinal flora. Furthermore, the continuous progress of multi-omics technology, microbial detection technology, and data analysis methods, will also to further elucidate the molecular mechanism of DKD from the microbiome–metabolite signaling pathway and provide new research evidence for clinical application.