Introduction

Type 2 diabetes mellitus (T2DM) is becoming a crucial threat to human health, with approximately 425 million individuals worldwide in 2017, and it is estimated that the number will exhibit a 45% increase in three decades. T2DM patients might be suffering from pancreatic β-cell dysfunction and reduced insulin sensitivity in target tissues. Chronic hyperglycemia causes macrovascular and microvascular complications and remodeling in the heart, kidneys, eyes, and nerves [4–6]. Oral antidiabetic agents, including metformin, sulfonylureas, glucosidase inhibitors, thiazolidinediones, meglitinides, and inhibitors of dipeptidyl peptidase 4, have substantially improved the prognosis of most T2DM patients. However, the efficacy of Western medications remains deficient in certain patients and vastly disproportionate burden on patients in low- and middle-income countries. Hence, complementary or alternative therapies such as traditional Chinese herbal medications are required to prolong life expectancy and improve the quality of life in T2DM patients.

Although traditional Chinese herbal medications have been widely used for more than 2000 years in East Asia and previous studies suggested that some Chinese herbal medications were effective in treating T2DM [7–9], their evidence based on randomized controlled trials (RCTs) is still feeble. Tangminling pill, mainly containing Rhizoma coptidis, Rheum officinale Baill, Scutellaria baicalensis Georgi, and Bupleurum Chinense DC, might reduce blood glucose level and improve pancreatic β-cell function [8,10–12]. However, the studies focusing on the effects of Tangminling pill on T2DM were limited due to insufficient sample sizes and inconclusive results. In our study, we performed a pooled analysis to evaluate the effects of Tangminling pill on blood glucose, body weight, and β-cell function in T2DM patients.

Preparation of Tangminling pill

The pharmaceutical composition of Tangminling pill comprises extracts of the following ten traditional Chinese herbs: Rhizoma coptidis (Huanglian), Scutellaria baicalensis Georgi (Huangqin), Radix Paeoniae Alba (Baishao), Rheum officinale Baill (Dahuang), Citrus aurantium L (Zhishi), Pinellia ternate (Baixia), Crataegus pinnatifida Bunge (Shanzha), Smoked plum Fructus mume (Wumei), Radix Trichosanthis (Tianhuafen), and Bupleurum Chinense DC (Chaihu), and optionally comprises pharmaceutically acceptable excipient. The method to prepare Tangminling pill and the doses of all the above ten crude herbs have been disclosed to the public (http://www.freepatentsonline.com/8637092.html). It was manufactured and capsulated with specific processing of Chinese herbal medicine in Tasly Pharmaceutical (Tianjin, China), as described previously. There were quantitative control limits for the raw herbs in the final product (the content of berberine in Rhizoma coptidis shall not be <6.0 mg/g and the content of paeoniflorin in Radix Paeoniae Alba shall not be <3.0 mg/g). The chemical composition of Tangminling pill was further analyzed using the high-performance liquid chromatography/mass spectrometry (HPLC/MS) method.

Ethics statement

The study project conformed to the ethical rules of the Helsinki Declaration (revised edition in October 2013) and was approved by the Ethics Committee of Shandong University Qilu Hospital (KYLL-201761; date of approval: 15 July 2017).

Data search and selection

Relevant RCTs were identified from the following data sources: Medline, China VIP, China National Knowledge Internet, China Wanfang, and Cochrane library. Literature were searched with the terms (‘Tang-min-ling’ or ‘TM81’ or ‘Tangminling’ or ‘TML’) AND (‘diabetes’ or ‘T2DM’). Publication status, language, and date were not restricted.

We included studies with the following criteria: (i) compared Tangminling treatment vs. placebo treatment in T2DM patients; (ii) RCTs with more than 1-month follow-up durations; and (iii) contained available data to estimate the pooled weighted mean differences (WMDs) and their corresponding 95% confidence intervals (CIs). Studies were excluded as follows: (i) diagnosis of T2DM was not definite; (ii) studies were non-randomized and/or non-double-blinded; (iii) different traditional Chinese medications were compared between two groups; (iv) only symptomatic changes were described without objective laboratory measurements; (v) methodological quality was poor with a Jadad score < 3 as described previously; and (vi) if two articles reported results from the same trial, the article with less information was excluded.

Data extraction

We extracted the following information from retrieved reports using a standardized protocol: author names, publication dates, study designs, countries of origin, subject numbers, subject characteristics at baseline, follow-up durations, concomitant medications, and end points. The literature search and data extraction were performed blindly by two authors (J.C. and Y.L.) according to a standardized approach. All disagreements were resolved by consensus and a third reviewer (P.H.).

Statistical analysis

RevMan software version 5.3 was used for this pooled analysis. The heterogeneity was evaluated by the Cochran’s Q statistic and the inconsistency index (I2). Heterogeneity was considered statistically significant when chi-square P was <0.10 and I2 was >50%. Pooled WMDs with 95% CIs were analyzed, and the difference was considered statistically significant when two-tailed P was <0.05. Moreover, sensitivity analyses were performed to evaluate individual effects and explain possible heterogeneity. Additionally, we calculated the fail-safe number (Nfs; Nfs0.05 = (∑Z/1.64)2 – k, where k is the number of retrieved studies) to assess publication bias. Any Nfs0.05 values smaller than the number of included trials indicated that obvious publication bias existed.

Clinical end points

First, we performed a pooled analysis to evaluate the effect of Tangminling pill on blood glucose level. Blood glucose improvement was evaluated by reductions of hemoglobin A1c (HbA1c), fasting plasma glucose (FPG) and 2-h postprandial glucose (2-hr PG) levels from baseline in all four studies (Table 1). It showed that Tangminling pill significantly reduced HbA1c (1.11 vs. 0.32%, P<0.001), FPG (0.82 vs. −0.40 mM, P<0.001) and 2-hr PG levels (2.81 vs. 1.11 mM, P<0.001) in T2DM patients, as compared with placebo group.

Also, Tangminling pill significantly ameliorated obesity, which was expressed as reductions of waist circumference (WC) and body mass index (BMI) in T2DM patients (Table 1). Tangminling significantly reduced WC (1.04 vs. 0.36 cm, P<0.001) and BMI (0.37 vs. 0.11 kg/m2, P=0.05) from baseline in comparison with placebo treatment in T2DM patients.

Furthermore, it showed that Tangminling pill significantly improved pancreatic β-cell function, expressed as homeostatic model assessment (HOMA)-β decrease from baseline (Table 1). Although there was no difference in reducing HOMA-insulin resistance (IR) between Tangminling and placebo groups (0.16 vs. −0.16, P>0.05), Tangminling significantly decreased HOMA-β level from baseline as compared with placebo (4.28 vs. 0.41, P<0.001).

Furthermore, the safety of Tangminling pill was also confirmed, without serious adverse events reported in all the four RCTs.

Sensitivity analyses

Heterogeneity was well-addressed in terms of HbA1c (I2 = 0%), 2h-PG (I2 = 0%), and HOMA-β (I2 = 36%). However, significant heterogeneity was existed in terms of FPG (I2 = 77%), BMI (I2 = 90%), WC (I2 = 63%), and HOMA-IR (I2 = 81%). Alterations in pooled WMDs were not significant after excluding the study with the largest sample size (data not shown). Thereafter, subgroup analyses according to FPG, BMI, WC, and HOMA-IR were conducted (Table 2). The heterogeneity among trials was partially explained by the variability in doses of the medication and subject numbers.

Publication bias

Nfs0.05 values for HbA1c, FPG, 2-hr PG, BMI, WC, HOMA-β, and HOMA-IR were higher than the number of studies included in the corresponding analyses, indicating negligible publication bias in our analyses.

Discussion

The pooled analyses indicated that Tangminling pill was more efficacious in decreasing glucose level, body weight, and improving β-cell function, with no more drug-related adverse events, than placebo treatment in T2DM patients.

Traditional Chinese medications, mainly edible herbs, have been widely used in clinical practice in China for more than 2000 years [16–18]. Ten traditional Chinese herbs were extracted and included in Tangminling pill: Rhizoma coptidis, Scutellaria baicalensis Georgi, Radix Paeoniae Alba, Rheum officinale Baill, Citrus aurantium L, Pinellia ternate, Crataegus pinnatifida Bunge, Smoked plum Fructus mume, Radix Trichosanthis and Bupleurum Chinense DC (Table 3). Three bioactive alkaloids have been isolated from Rhizoma coptidis, including berberine, coptisine, and jatrorrhizine. Berberine has numerous pharmacological effects with anti-diabetic, anti-inflammatory, anti-hypercholesterolemia, and cardioprotective properties [19–23]. Berberine displays beneficial actions in the treatment of insulin-resistant and hyperglycemic states by up-regulating adipose triglyceride lipase in adipocytes via activating adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK) signaling pathway. It might improve glucose metabolism via inhibiting oxidation and aldose reductase. Besides, berberine might exert its anti-inflammatory action through AMPK activation, nuclear factor-κB (NF-κB) inhibition, and AP-1 pathway inhibition. Coptisine is another bioactive component of Rhizoma coptidis, and it exhibits antioxidation, anti-osteosarcoma, anti-inflammation, neuroprotection, and anti-cancer actions through preventing interleukin-1β (IL-1β) expression, activating Akt and c-Jun N-terminal kinase (JNK)/nuclear factor erythroid 2-related factor 2 (Nrf2)/NAD(P)H:quinine oxidoreductase 1 (NQO1) signaling pathways and inhibiting lipopolysaccharide (LPS)/toll-like receptor-4 (TLR-4)-mediated pathway [28–32]. Rheum officinale Baill, the root and rhizome of Rheum palmatum L, has antioxidant, anti-platelet aggregation, and anti-inflammatory properties [33–35]. The underlying mechanism might involve reductions in the mRNA expressions of acidic mammalian chitinase and chitinase 3-like protein 4 as well as inhibition of the progression of inflammation. Bupleurum Chinense DC and its bioactive ingredients, saikosaponins, exhibited immunomodulatory, antiviral, hepatoprotective, and anti-cancer effects through inducing cell cycle arrest and cell apoptosis. Moreover, albiflorin, which was extracted from Radix Paeoniae Alba or Paeonia lactiflora, exerted anti-depressant and anti-allergic effects through biosynthesis of allopregnanolone and inhibition of mast cell activation. The main active ingredients of Scutellaria baicalensis Georgi are baicalin and wogonin, which exhibited protection of diabetes and its complications. Baicalin reduced renal fibrosis through down-regulating transforming growth factor-β/Smad pathway. It also had a protective effect on T2DM via improving islet β-cell function. Furthermore, wogonin alleviated diabetic cardiomyopathy, diabetes-related cognitive deficits, and metabolic syndrome [45–47]. Wogonin improved peroxisome proliferator-activated receptor (PPAR)-α (PPAR-α) activity in adipocytes and type 1 diabetic mice. Flavanones naringenin and hesperetin from Citrus aurantium L exhibited anti-depressant, antioxidant, and anti-atherogenic effects. Pinellia ternate had anti-cancer and pro-inflammatory actions. It also attenuated mucus secretion and airway inflammation in rats with chronic obstructive pulmonary disease. Crataegus pinnatifida Bunge exerted anti-inflammatory effects and decreased the risks of cardiovascular diseases by lowering blood cholesterol [57–59]. Fructus mume exhibited neuroprotective effect through down-regulating inflammation and inhibiting TLR4 and p38 MAPK signaling. Radix Trichosanthis played anti-cancer and immunosuppressive roles. The combination of these components in Tangminling pill might exert more beneficial effects in T2DM patients. In our study, the safety of Tangminling pill was also confirmed, without serious adverse events reported in the included RCTs. However, the production of traditional Chinese medicine is very complicated, so it is necessary to test the effectiveness and safety of each plant extract and investigate easier preparation methods of the pill formulation.

T2DM is a chronic metabolic disease in which the deterioration of glycemic control is associated with progressive islet β-cell dysfunction, chronic inflammation, oxidation, and lipid metabolic disturbance. Metabolic syndrome is characterized by the presence of abdominal obesity, IR, hyperglycemia, dyslipidemia, and hypertension. Tangminling pill significantly improved pancreatic β-cell function and reduced BMI and WC in T2DM patients in our analyses. The anti-hyperglycemic effect of traditional Chinese medications might be attributed in part to the β-cell protection through activating AMPK signaling. Tangminling was found to increase glucose transporter 4 (GLUT4) level and AMPK activity in skeletal muscle and promote insulin secretion through up-regulating PPARα. It might also promote the proliferation and differentiation of pancreatic β-cells and therefore postpone the exhaustion of β-cell function. Tangminling was showed to improve insulin sensitivity, which might be another potential mechanism to regulate glucose metabolic disorder.

Hypoglycemia has been considered as a major barrier to the proper glycemic control in diabetic patients. Our analyses revealed that Tangminling reduced blood glucose mildly, without hypoglycemic events reported. Tangminling treatment might significantly decrease the risk of hypoglycemia, which result in a better quality of life for T2DM patients. Besides, significant heterogeneity existed in the analyses of FPG, BMI, WC, and HOMA-IR. There was a significant disparity in sample sizes among the four RCTs included in our pooled analyses. In fact, among the 767 patients included in the analyses, 399 (52.0%) were from one study. However, after excluding this study, no substantial alteration was found in pooled WMDs.

Several limitations should be considered in our analyses. The sample sizes were relatively small, and the number of included studies was limited, both of which might limit the power to estimate the effects of Tangminling pill on T2DM. Intervention and follow-up durations of some studies were relatively short. Therefore, more high-quality studies with more extended intervention and larger sample sizes are needed. Despite the above limitations, this systematic analysis was more convincing than any previous single study. Indeed, our analysis has some notable strengths. It strictly adhered to the guidelines and only RCT studies were included, which helps to increase the robustness of the conclusions. In addition, the parameters used to assess the clinical outcomes are strict, which can increase the validity of our study. To the best of our knowledge, this is the first pooled analysis to assess the effect of one traditional Chinese medication, Tangminling pill, on T2DM.

Conclusion

Tangminling pill is effective and safe for glycemic control and β-cell function in T2DM patients, and it could be used to treat T2DM as a promising complementary approach. However, more rigorously designed trials are warranted to investigate the impacts of Tangminling pill on diabetes and diabetic complications.