Barberry in the treatment of obesity and metabolic syndrome: possible mechanisms of action

Introduction

Obesity is a consequence of an imbalance between energy intake and energy expenditure. The WHO has identified obesity as a major public health problem because of its associated risk for a number of noncommunicable diseases, which include type 2 diabetes mellitus, hypertension, dyslipidemias and cardiovascular disease, and a reduction in life expectancy.¹ Overweight, obesity and morbid obesity are usually defined using body mass index (defined as the weight in kilograms divided by the height in meters squared) of 25–29.9, 30–39.9 and >40, respectively. Other more sophisticated methods for directly assessing body fat, or adiposity, include bioelectrical impedance analysis, computed tomography and MRI.² Abdominal obesity is an independent predictor of several cancer- and cardiovascular-related outcomes and can be assessed relatively simply by measuring waist circumference or waist-to-hip ratio.³ Obesity is a common public health problem, which is causing an estimated around 2.6 million deaths in the world.² It affects all age, sex, ethnic and socioeconomic groups in both developed and developing countries.⁴ Obesity is a multifactorial disorder due to an interaction between genetic, socioeconomic, environmental and behavioral influences.⁵ Differences in energy metabolism may play a role in long-term body weight regulation and the pathogenesis of human obesity. Several prospective studies have shown that relatively low energy expenditure and high respiratory quotient due to a low fat to carbohydrate oxidation rate predict body weight gain.⁶ Obesity is often accompanied by the metabolic syndrome (MetS). MetS is determined by a series of factors including physiological, biochemical, clinical and metabolic parameters that together enhance the risk of atherosclerotic cardiovascular disease, type 2 diabetes mellitus and other causes of mortality.⁷ There are several definitions of MetS, but some of the most popular definitions are from the WHO⁸ and the National Cholesterol Education Program Adult Treatment Panel III.⁹

According to the National Cholesterol Education Program Adult Treatment Panel III definition for MetS, the presence of any three of the following five features including waist circumference ≥102 cm in men or ≥88 cm in women, ≥130/85 mmHg (includes hypertensive patients and patients who received antihypertensive medications), triglyceride (TGs) ≥150 mg/dL, high-density lipoprotein cholesterol <40 mg/dL in men or <50 mg/dL in women and fasting blood glucose ≥100 mg/dL (includes diabetes).

The prevalence of MetS varies widely (10%–84%) and depends on the region, urban or rural setting, individual characteristics, including sex, age, race and ethnicity of the population studied, and the definition of the syndrome used.^10,11 The International Diabetes Federation estimates that MetS affects approximately one-quarter of the world’s adult population.¹² Therefore, many strategies for prevention are being developed to reduce the future MetS burden. Nowadays, there is a growing preference to use plant-based agents instead of synthetic drugs to manage chronic diseases such as MetS because plant-based agents have fewer adverse effects and lesser costs and are more accessible than synthetic drugs in some countries.¹³ Studies show that formulations of one-quarter of all medical prescriptions are derived from plant synthetic analogs, and according to the WHO, 80% of the world population, especially in developing countries, trust in plant-based medicines.¹³

Berberis vulgaris (zereshk in Persian) is widely cultivated in Iran (particularly in South Khorasan province), central and southern Europe and northeastern US.^14,15 B. vulgaris has been used in herbal therapy for >2,500 years, mainly by Russian therapists.¹⁶ The antioxidants present in berberry include berberine, berberrubine and berbamine.¹⁷ Berberine is an alkaloid that is used in Chinese medicine. This herbal ingredient exists in the stem, roots, rhizomes and seven plant species such as Berberis vulgaris, Berberis aristata, Mahonia aquifolium, Hydrastis canadensis, Xanthorhiza simplicissima, and Phellodendron amurense (barberry).¹² Berberine affects the glucose metabolism by increasing glycolysis and increasing insulin sensitivity,¹⁸ and a recent study has suggested that berberine is both anti-inflammatory and anti-adipogenic. The anti-adipogenic effect may be a result of the downregulation of transcription factors and adipogenic enzymes.¹⁹ Berberine is also used in the treatment of diarrhea, stomatitis, dysentery and hepatitis.²⁰

The polyphenols in barberry juice have been reported to have a positive effect on lipid profiles and diabetes complications.^21,22

The chemical structure of berberine is shown in Figure 1.

Figure 1 Chemical structure of berberine.

Inclusion of studies

The keywords including “berberine”, “diabetes”, “insulin-resistance”, “glucose”, hemoglobin A1C”, “dislipidemia”, “obesity” and “atherosclerosis” were used for an electronic search strategy. All the identified studies and review articles were reviewed. Subsequently, the eligible trials were selected. These are papers reporting original studies on the antidiabetic, antihyperlipidemic and anti-adipogenic mechanisms of action of berberine and almost all available clinical trials. But we mentioned this study is not systematic review.

The mechanisms of action of berberine in obesity

Adipocyte differentiation requires several transcription factors, and key regulators in the process are two adipogenic factors, CCAAT/enhancer-binding protein α (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ). The PPARs function as lipid sensors and regulate lipid and lipoprotein metabolism, glucose homeostasis, adipocyte proliferation and differentiation.²³ C/EBPa-deficient cells are capable of adipocyte differentiation and are also insulin resistant.²⁴ It has been discovered that GATA factors play a role in adipogenesis.²⁵ GATA-2 and GATA-3 are expressed in adipocyte precursors and control the preadipocyte to adipocyte transition. Berberine reduces the expression of C/EBPa and PPARγ and it has been shown to inhibit the differentiation of 3T3-L1 cells in murine-derived preadipocytes and human white preadipocytes.²⁶ Furthermore, berberine increases the expression of GATA-2 and GATA-3 and these suppress adipocyte differentiation.²⁴ This mechanism is directly through binding to the PPARγ promoter and suppression of basal activity of PPARγ. Also, GATA-2 and GATA-3 form a protein complex with C/EBPa that suppresses the transcriptional activity of PPARγ.²⁷ It has been reported that berberine reduces weight, the food intake, the ratio of epididymal fat to total weight, as well as the ratio of liver weight to total weight in a murine model. In addition, blood glucose, TG and total cholesterol levels were all reduced in high fat–fed obese mouse, revealing that berberine acts as a potential natural compound for the treatment of obesity.²⁶ The accumulation of TG in adipose tissue leads to obesity, and therefore, reducing the amount of TG in adipocytes may be a way for decreasing obesity. One study showed that berberine increased the adipose TG lipase expression in 3T3-L1 adipocytes and it led to reduced TG accumulation in adipocytes. Berberine also reduced the numbers and size of lipid droplets in 3T3-L1 adipocytes.²⁸

Several studies have shown that berberine regulates TG, cholesterol and glucose metabolism via AMP-activated protein kinase (AMPK) expression,²⁹ an important regulator of lipolysis in adipose tissue. Hormone-sensitive lipase and adipose TG lipase are phosphorylated by AMPK.^30,31

The effects of berberine on insulin resistance and glycemic control

Berberine has been shown to regulate glucose and lipid metabolism in vitro and in vivo (see Table 1). It enhances insulin secretion, stimulating glycolysis, suppressing adipogenesis by inhibiting PPARγ and C/EBPα function,³² activating the AMPK pathway and increasing glycokinase activity. Berberine stimulates pancreatic G protein-coupled receptor 40 (GPR40), reduces intestinal glucose absorption by inhibiting α-glucosidase activity, and also upregulates the expression of glucose transporter-4 (GLUT-4) and stimulates glucose uptake, and upregulates glucagon like peptide-1 (GLP-1) genes.^33,34

Table 1 Glycemic control mechanisms of berberine Abbreviations: AMPK, 5′ adenosine monophosphate-activated protein kinase; GLP-1, glucagon-like peptide-1; GLUT-4, glucose transporter type 4; GPR40, G protein-coupled receptor 40.

Secretagogue effect of berberine on insulin

Berberine increases glucose-stimulated insulin secretion in a dose-dependent manner, enhancing both mRNA expression of hepatic nuclear factor 4-alpha (HNF-4α) and glycokinase activity; this provides an insulinotropic effect different from sulfonylureas.³⁵

Berberine and insulin resistance

In 3T3-L1 adipocytes, with insulin resistance, berberine could inhibits IKB kinase beta (IKKbeta) Ser(181) and IRS-1 Ser(307) phosphorylation, and could progress glucose transport induced by insulin and rebound insulin resistance due to free fatty acids.³⁶ It also improves insulin sensitivity by upregulating IRS-2 mRNA expression in the expression of PPAR-α/δ and decreases the expression of PPAR-γ in the liver of diabetic rat³⁷ and is also related to the suppression of adipocyte differentiation.³⁸ Also, berberine enhances insulin sensitivity by reducing the homeostatic model assessment-insulin resistance value up to 50%³⁴

Antihyperlipidemic activity of berberine

Berberine stabilizes low-density lipoprotein (LDL)-R mRNA by reducing the LDL-R mRNA 3′ UTR binding of heterogeneous nuclear ribonucleoprotein I and KH-type splicing regulatory protein, which are key modulators of LDL-R mRNA stability in liver cells.³⁹ Berberine also enhances the transcriptional activity of LDL-R promoter by c-Jun N-terminal kinase (JNK) pathway⁴⁰ and reduces the transcription of the PCSK9 gene, the product of which induces LDL-R degradation.⁴¹ Furthermore, berberine decreases the activities of PPAR-γ, fatty acid synthase, acetyl-CoA carboxylase, acyl-CoA synthase and lipoprotein lipase.⁴² Also, berberine decreases lipid profiles by improving the liver function and bile acid secretion, and polyphenolic compound in barberry juice can justify the positive effects on lipid profiles.⁴³

Antihypertensive and endothelium protection effects of berberine

The effects of berberine on vasodilation of vessels has been shown in different rat models.^44,45 Berberine affects both endothelium and vascular smooth muscle. The vasorelaxant effect of berberine is endothelium dependent at low concentrations (<1×10⁶ M), but endothelium independent at higher concentrations.^46,47 Barberry juice contains some vasodilator factors such as berberine, this substance effects on the central nervous system and reduce blood pressure.⁴³ Regarding the possible mechanism of barberry on blood pressure, it is suggested that barberry may induce the production of nitric oxide and, thus, vasodilatation.⁴⁸ Berberine enhances the expression of endothelial nitric oxide synthase mRNA and suppresses the expression of inducible nitric oxide synthase mRNA in gastric tissue⁴⁹ and inhibits angiotensin converting enzyme.⁵⁰

Berberine-induced endothelium-independent relaxation appears to occur by the activation of tetrapentylammonium-4-aminopyridine- and Ba(2+)-sensitive K(+) channels, inhibition of intracellular Ca(2+) release from caffeine-sensitive pools or as a direct relaxant effect.⁴⁶

In rat models, the nephroprotective effect of berberine was shown to reduce hypertension.⁴⁰ Berberine preserved endothelial functions by maintaining the reactive and elastic features of vessels. In this connection, berberine suppresses platelet-derived growth factor (PDGF)-induced vascular smooth muscle cell growth by activating AMPK/p53/p21 (Cip1) signaling and Ras/Rac1/Cyclin D/Cdks and inhibiting PDGF-stimulated migration by Rac1 and also inhibiting Cdc42, resulting in antiproliferative and antimigratory activity.⁵¹ Berberine suppresses the synthesis of fibronectin and collagen by p38 MAPK pathway⁴⁰ and prevents the migration and regrowth of smooth muscle cells via inactivating mitogen-activated protein kinase/extracellular signal regulated kinase/early growth response gene 1 (MEK1,2/ERK/Egr1 signaling pathway) and decreasing Erg1, cFos, cyclin-D and PDGF-A levels.⁵² Lysophosphatidylcholine-stimulated vascular smooth muscle cell proliferation and migration is inhibited by activation of ERK1/2 pathway (one of the three MAPK groups).⁵³

In Table 2, some human and animal studies related to the effect of berberine and barberry in obesity and MetS are summarized. Some clinical trials used berbeine and some used barberry juice. Most of the studies that explained the molecular mechanisms were conducted in animal models and the molecular mechanisms in humans are not explained.

Table 2 Clinical efficacy of barberry Abbreviations: ATF-2, activating transcription factor 2; BMI, body mass index; FBG, fasting blood glucose; HbA1c, hemoglobin A1c; HDL-c, high-density lipoprotein-cholesterol; HOMA-R: homeostatic model assessment-insulin resistance; LDL-c, low-density lipoprotein-cholesterol; MMP-2, matrix metalloproteinase 2; NAFLD, nonalcoholic fatty liver disease; TC, total cholesterol; TGs, triglycerides.

Conclusion

According to this study, berberine as a major antioxidant component of B. vulgaris, suppresses adipocyte differentiation and decreases obesity. Also, it regulates glucose metabolism via decreasing insulin resistance and increasing insulin secretion. Berberine also has antihyperlipidemic and antihypertensive effects and protects the endothelium, thereby reducing the risk factors of MetS. In conclusion, B. vulgaris may be effective in treatment of obesity and MetS, but more human clinical trials are needed to judge about its effectiveness in this issue because most of the studies explaining the molecular mechanisms were conducted in animal models. Also, some clinical trials used berbeine and some others used barberry juice, so we suggest that studies should be conducted to compare the efficacy of berbeine and barberry juice. We do not know whether these effects of berberine are dose dependent or not; therefore, to clarify the effective doses, more studies are needed.

The perspective studies work on the isolation or neo-synthesis of berberine analogs with a higher bioavailability. The scientist can then clarify the mechanism of action of berberine on glucose and lipid metabolism. The possible action of berberine is as a modulator of a gene involved in more processes or as a gene regulator (eg, the upstream stimulatory factors). Anyway, the effect of berberine on PCSK9 modulation is another potential therapeutic target, because berberine strengthens the effects of available antihyperlipidemic drugs. The antihyperlipidemic and antidiabetic effects of berberine are studied through improvement in organ damage in humans (ie, by evaluating their effect on microalbuminuria or arterial stiffness).

Disclosure

The authors report no conflicts of interest in this work.

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