Berberine-induced browning and energy metabolism: mechanisms and implications

Introduction

Obesity is a growing health concern worldwide, primarily driven by an imbalance between energy intake and expenditure (Mayoral et al., 2020). This condition is associated with numerous metabolic disorders, including diabetes, cardiovascular diseases, and non-alcoholic fatty liver disease. Addressing obesity requires innovative approaches to increase energy expenditure while reducing energy storage. Brown adipose tissue (BAT) and the browning of white adipose tissue have emerged as promising therapeutic targets due to their roles in thermogenesis. This process converts stored energy into heat, thereby increasing total energy expenditure (Marlatt & Ravussin, 2017).

Berberine is a natural compound extensively utilized in traditional medicine across many Asian countries, especially China (Wang et al., 2017). It is an isoquinoline alkaloid derived from several plants, such as Coptis chinensis, Berberis aquifolium, Berberis vulgaris, and Berberis aristate. Plants containing berberine have been used since ancient times. Coptis chinensis was used approximately 2,200 years ago for many health issues, particularly digestive system diseases (Song, Hao & Fan, 2020). Approximately 1,500 years ago, Hongjing Tao mentioned the antidiabetic properties of berberine plants in the book “Note of Elite Physicians” (Zhang et al., 2014a). With technological advancements, the active component in these plants was identified as berberine, and consequently, the number of studies on berberine has recently increased. Berberine is believed to have numerous effects, including anti-obesity, hypoglycemic, hypolipidemic, hypotensive, and anti-inflammatory effects (Hesari et al., 2018; Pirillo & Catapano, 2015; Yarla et al., 2016).

Berberine is regarded as a potential anti-obesity agent because of its beneficial health effects. Berberine may increase thermogenesis, positively affect carbohydrate and lipid metabolism, suppress appetite, regulate intestinal permeability and hepatic gluconeogenesis, and modulate the microbiota (Ilyas et al., 2020; Park, Jung & Shim, 2020; Rong et al., 2021; Wu et al., 2019; Zhang et al., 2020, 2014c). Taking 500 mg of berberine three times a day for 12 weeks may result in an average weight loss of approximately 2.3 kg (5 pounds) in individuals with obesity (Hu et al., 2012). Berberine’s effect on thermogenesis is one of the most researched topics in this area. Although the exact mechanism is not yet fully understood, berberine induces adipose tissue browning and thermogenesis through various pathways (Zhang et al., 2015, 2008). This browning effect is considered a key mechanism contributing to berberine’s potential role in weight loss, as it promotes increased energy expenditure and thermogenesis, addressing the energy imbalance central to obesity. Beyond its metabolic effects, berberine is relatively safe. While berberine toxicity is rarely observed in animals, human studies have reported some mild side effects, such as gastrointestinal disturbances like diarrhea or constipation (Imenshahidi & Hosseinzadeh, 2019; Zhang et al., 2010). Plants rich in berberine have been reported to be safe, showing no adverse effects on creatinine levels or liver function (Linn et al., 2012). The side effects of berberine vary depending on the route of administration, dosage, and duration of use.

This review aims to clarify the mechanisms of adipose tissue browning, which are essential for preventing obesity and its associated conditions, as well as to examine the effects of berberine on these mechanisms. There are different reviews in the literature examining the health effects of berberine. However, the number of articles presenting up-to-date data on the effects of berberine on adipose tissue browning and BAT activation is limited. Due to its low bioavailability, most studies on berberine are in vitro. Methods that could address this issue are provided in this review.

Survey methodology

In this review, articles containing the keywords “berberine” along with “brown adipose tissue,” “browning,” and “thermogenesis” in their titles or abstracts were searched in the PubMed, Science Direct, and Scopus databases. Since most of the relevant sources were recently published, no year restriction was applied. Only articles in English are considered. Research articles and reviews were included. A search using these criteria resulted in 278 research articles. After applying search filters, the titles of the resulting articles were reviewed first, followed by their abstracts. Articles with abstracts relevant to the research topic were then examined in detail. Studies that met the search criteria but were not relevant to the topic, did not provide sufficient data, or were inaccessible in full text were excluded. As a result, 10 studies relevant to the purpose of this review were included. Limiting the search to articles in English resulted in the exclusion of studies written in native languages from Asian countries, where the use of berberine is more prevalent. This can be considered a limitation of this study.

Berberine and its pharmacokinetic properties

Berberine (2,3-methylenedioxy-9,10-dimethoxyprotoberberine chloride) is yellow, odorless, and has a bitter taste (Feng et al., 2019). It is more soluble in organic solvents and has low water solubility. Its molecular weight is 336.36 g/mol. It can be extracted from its source plants, or it can be synthesized (Feng et al., 2019).

Metabolism

Berberine administered orally undergoes primary metabolism in the liver and intestines. The liver enzymes responsible for metabolizing berberine include cytochrome (CY) P2D6 and CYP1A2 subtypes of CYP450 (Fig. 1) (Li et al., 2011). In vivo studies showed that the primary metabolic pathways of berberine include demethylation, demethylenation, reduction, and hydroxylation (Liu et al., 2009). These processes lead to phase 1 metabolites of berberine. Phase 2 metabolites form through the conjugation of these metabolites with sulfuric acid or glucuronic acid.

Berberine also undergoes metabolism in the intestines, where its structure and content can be altered by intestinal flora (Han et al., 2021). This alteration occurs through demethoxylation and hydrogenation pathways, involving nitroreductases produced by intestinal flora. Dihydroberberine, a form that can be absorbed in the intestines, is produced through hydrogenation. After absorption, this form oxidizes back to berberine and enters circulation (Han et al., 2021).

Berberine is metabolized into four primary metabolites: berberrubine, thalifendine, demethyleneberberine, and jatrorrhizine (Hu et al., 2018). After oral ingestion, berberine is distributed throughout the body, including the small intestine (undergoing presystemic elimination), liver (where it accumulates), kidneys, muscles, heart, and pancreas. The primary metabolic pathways include oxidative demethylation leading to the production of berberrubine, followed by glucuronidation. After intravenous administration, berberine undergoes oxidative demethylation, resulting in the production of demethyleneberberine, followed by glucuronidation of demethyleneberberine (Hu et al., 2018).

Types of adipose tissue and browning

Adipose tissue consists of white adipose tissue (WAT) and BAT, composed mainly of white and brown adipocytes, respectively (Kurylowicz & Puzianowska-Kuznicka, 2020). The origins, morphologies, anatomical locations, and nearly all functions of these two types are different from each other (Table 1). White adipocytes consist of a single large lipid droplet with a non-centrally located nucleus, and very few mitochondria (Bargut et al., 2017). Brown adipocytes contain many small lipid droplets, have a centrally located nucleus and are dark in color due to the high number of mitochondria. Adipocyte precursor cells, also known as adipose stem cells, can differentiate into white, beige, or brown adipocytes (Xue et al., 2015b). The expression of myogenic factor-5 determines the difference between white and brown adipocytes. Myogenic factor-5 is associated with thermogenic activities and present in brown adipocyte precursors but not in white adipocytes (Xue et al., 2015b).

White adipose tissue begins to develop during the second trimester of pregnancy, and BAT starts to develop towards the end of the second trimester (Cypess, 2022). In newborns, both WAT and BAT are fully developed to perform their functions. WAT is categorized into two main types: visceral and subcutaneous and its primary function is to store energy. Energy stored in the form of triglycerides undergoes lipolysis when required, resulting in the release of fatty acids as fuel. Fifty years ago, it was thought that BAT, which was known to be present in infants, was not present in adults due to insufficient imaging techniques. With the advancement of positron emission tomographic and computed tomographic (PET/CT) imaging technique, active BAT was initially observed in the supraclavicular region of adult humans (Cypess et al., 2009). Later, the presence of BAT was also identified in the neck, axillary, abdominal, and paraspinal regions in adults (Keuper & Jastroch, 2021). There is a high concentration of mitochondria in brown adipocytes. Uncoupling protein 1 (UCP1) in the inner membrane of mitochondria is essential for browning and thermogenesis mechanisms (Kurylowicz & Puzianowska-Kuznicka, 2020). Uncoupling protein 1 releases energy as heat instead of chemical energy by uncoupling mitochondrial respiration from adenosine triphosphate (ATP) synthesis. Therefore, it facilitates thermogenesis and promotes energy expenditure. Consequently, the identification of BAT in adults represents a promising avenue for combating the obesity epidemic. Additionally, “beige/brite” adipocytes are morphologically resemble white adipocytes but exhibit brown adipocyte function under adequate stimuli (Cheng et al., 2021). Phytochemicals such as berberine, resveratrol, and curcumin, and dietary components like fish oil and retinoic acid, along with cold exposure, exercise, and β-adrenergic factors, stimulate beige adipocytes through a process known as “browning” (Cheng et al., 2021; Okla et al., 2017). As browning progresses, beige adipocytes, which have a morphology similar to white adipocytes, begin to perform functions similar to brown adipocytes. As mitochondrial biogenesis and UCP1 expression increase, energy expenditure through thermogenesis will also increase. This is very important in combating obesity, which is mainly caused by an imbalance between energy intake and expenditure. The beige adipocytes lose their brown characteristics and return to the characteristics of the white adipocytes when the stimulus is removed (Ziqubu et al., 2023). This process, called “whitening,” is considered the opposite of browning. It can be seen in obesity and during aging (Graja, Gohlke & Schulz, 2019; Ziqubu et al., 2023).

Browning mechanisms and bat activation

There are two ways to increase thermogenesis through adipose tissue. The first is to increase browning and the second is to increase the already existing BAT activation. Browning can occur in two ways (Kurylowicz & Puzianowska-Kuznicka, 2020). The first is by differentiating from precursor/stem cells and the second is by transdifferentiating from mature adipocytes. Subcutaneous adipocytes are more likely to brown than visceral adipocytes due to their ability to differentiate (Gustafson & Smith, 2015).

Beta-adrenergic receptor activation is considered a key stimulus for browning. The receptors involved in this system may vary between species. For example, in rodents, the β-3 adrenergic receptor (β3-AR) is involved in browning, whereas in humans the β2-AR is involved (Blondin et al., 2020). Cold exposure, similar to β-adrenergic agonists, activates the sympathetic nervous system and releases norepinephrine. When the β-adrenergic receptor is stimulated, it activates cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA).

Protein kinase A activates cAMP response element-binding protein (CREB), p38 mitogen-activated protein kinase (p38-MAPK), and mechanistic target of rapamycin (mTOR) phosphorylation. cAMP response element-binding protein and p38-MAPK increase the transcription of peroxisome proliferator-activated receptor γ-co-activator-1α (PGC-1α), which activates transcription factors that induce mitochondrial biogenesis (Deng et al., 2019). Mechanistic target of rapamycin is also important for mitochondrial biogenesis (Wei et al., 2015).

Sympathetic activation is central to the complex mechanisms that lead to mitochondrial biogenesis, browning, and thermogenesis. Current research highlights several stimuli that enhance these processes, including exercise, specific dietary components, and pharmacological agents. For instance, exercise stimulates the production and release of irisin, which activates the p38-MAPK and extracellular signal-regulated kinase (ERK) pathways, thereby increasing UCP1 expression (Zhang et al., 2014b). Exercise also increases the expression of fibroblast growth factor-21 (FGF21) in the liver and adipose tissue. Its increase in adipose tissue induces UCP1 expression in white adipocytes (Reilly et al., 2021).

Additionally, exercise-induced reactive oxygen species (ROS) and its effects on the nervous system also play a role in adipose tissue browning (Mu et al., 2021). Chronic administration of β-adrenergic agonists and leptin increases sympathetic innervation and stimulates thermogenesis (Jimenez et al., 2003; Wang et al., 2020).

Other factors also play a role in promoting browning. For example, the lipid-lowering agent fenofibrate increases thermogenesis by activating peroxisome proliferator-activated receptor (PPAR)-α (Rachid et al., 2015). Similarly, PPAR agonists, agents that activate the Adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway, and substances such as nicotine (but not smoking) can stimulate browning by promoting mitochondrial biogenesis (Gaidhu et al., 2009; Yoshida et al., 1999).

Examining the expression and/or protein levels of transcription factors is one of the primary methods used to assess browning. These factors interact with each other, influencing adipogenesis and browning (Table 2). Among these markers, UCP1 is considered a definitive indicator of browning and thermogenic activity. Uncoupling protein 1 activity is regulated by free fatty acids, which enhance its activity, and purine nucleotides, which inhibit it (Macher et al., 2018). Which regulatory protein binds to the gene determines the transcriptional regulation of UCP1 (Villarroya, Peyrou & Giralt, 2017). In the absence of UCP1, lipogenesis and liver steatosis increase (Winn et al., 2017).

PPARγ-co-activator-1α is one of the most effective factors in stimulating mitochondrial biogenesis in both muscle and brown adipocytes (Deng et al., 2019). Another important browning factor is cell death-inducing DNA fragmentation factor-like effector A (CIDEA), which prevents the downregulation of UCP1 by inhibiting liver X receptors (LXRs) (Jash et al., 2019). PR domain containing 16 (PRDM16) can activate thermogenic genes in WAT (Ishibashi & Seale, 2015). It activates PGC-1α and is necessary for the browning of subcutaneous WAT. Low expression of PRDM16 can reverse browning and convert beige adipocytes back to white adipocytes (Harms et al., 2014). PR domain containing 16 is thus critical for maintaining beige adipocytes and their thermogenic activity.

Peroxisome proliferator-activated receptor-γ is another key transcription factor, influencing both fat and carbohydrate metabolism. It interacts with LXR and receptor-interacting protein 140 (RIP140) to downregulate UCP1 (Wang et al., 2008). Applying PPARγ agonists can increase insulin sensitivity and browning but may also increase visceral adiposity and unwanted body weight (Machado et al., 2022). Therefore, PPARγ plays a crucial role in both browning and whitening processes.

Cold exposure increases browning partly by enhancing noradrenergic stimulation, which increases iodothyronine deiodinase-2 (DIO2), converting thyroxin (T4) to triiodothyronine (T3) (Kurylowicz & Puzianowska-Kuznicka, 2020). Elevated T3 levels stimulate the sympathetic nervous system, thereby increasing UCP1 expression. Fibroblast growth factor-21 increases UCP1 expression by upregulating PGC-1α (Fisher et al., 2012). It also enhances browning by increasing intracellular Ca⁺⁺ levels. Forkhead box C2 (FoxC2), which is expressed in adipose tissue, mediates a thermogenic effect via the PKA pathway by increasing the expression of PGC-1α and UCP1 (Kajimura, Seale & Spiegelman, 2010).

Both browning and the activation of brown adipose tissue are triggered by similar stimuli (Kurylowicz & Puzianowska-Kuznicka, 2020). The method used to determine BAT activation is 2-deoxy-2-[¹⁸F] fluoro-D-glucose ([¹⁸F]FDG)-PET/CT imaging. This imaging technique allows for the tracking of the presence and size of brown adipose tissue (van der Lans et al., 2014).

Effects of berberine on browning and bat activation

A significant portion of berberine’s effects on browning and BAT activation occurs via the AMPK pathway. Beyond this long-studied area, new pathways are being explored, with recent attention on growth differentiation factor 15 (GDF15). The effects of berberine on brown adipose tissue are shown in Fig. 2. Studies investigating the effects of berberine on browning are summarized in Table 3.

Future implications

Berberine chloride and sulfate salts are more soluble, but the clinical use of the free form is limited due to its hydrophobic nature, low gastrointestinal absorption, and rapid metabolism. Different strategies have been developed to address the issue of low bioavailability in phytochemicals like berberine (Li et al., 2021; Mirhadi, Rezaee & Malaekeh-Nikouei, 2018; Qiao et al., 2018; Yu et al., 2017). These methods can be broadly categorized into three main approaches. These include:

• Changing the administration method (nanotechnology methods)

• Altering the chemical structure (organic acid salts of berberine)

• Co-administration with P-glycoprotein inhibitors (e.g., glycine, cyclosporine A)

Among these methods, nanotechnology methods are the most used, especially in clinical studies. Nanoparticles are particles with diameters ranging from 10 to 1,000 nm. With these methods, particle size is reduced as much as possible, surface properties are optimized, and the biologically active material is released at an optimal level. This structure, also known as a nano-carrier, helps berberine reach target tissues while preserving its properties. Nano-carriers can include materials such as micelles, carbon-based compounds, liposomes, and polymers (Behl et al., 2022). Some nanoencapsulation methods used to enhance the bioavailability of berberine are shown in Fig. 3.

Conclusions

Berberine, a herbal compound used in Asia for centuries, has recently gained attention for its health benefits, particularly in weight management. While studies generally report positive effects, the underlying mechanisms remain unclear. Key mechanisms include AMPK pathway activation, increased browning markers like UCP1, and appetite-regulating markers such as GDF15. Given the importance of adipose tissue browning and BAT activation in preventing obesity, berberine’s potential to enhance energy expenditure is critical.

However, its therapeutic potential is limited by low stability and poor bioavailability. For berberine to exert its effects, it must achieve and maintain effective concentrations in circulation when taken orally. Nanotechnological approaches, which improve stability and bioavailability, represent a promising solution. Despite their benefits, these methods face challenges such as high production costs, scalability issues, and regulatory hurdles. Advances in manufacturing techniques and cost-reduction strategies are essential for integrating nanotechnology-based therapies into routine clinical practice.

Future research should focus on developing new methods suitable for oral administration that enhance encapsulation efficiency and loading capacity. Additional clinical trials are needed to address the low bioavailability and insufficient toxicity data, which currently prevent the U.S. Food and Drug Administration (FDA) from classifying berberine as a drug. Comprehensive studies in diverse populations are crucial to fully establish berberine’s efficacy, optimal dosage, and clinical application.