Molecular Mechanisms by Which Natural Products Regulate Adipose Browning

The core of white adipose tissue browning is not simply a reduction in lipid storage but the reconstruction of mitochondrial oxidative and thermogenic programs. Natural products such as Hyperforin, Capsaicin, Resveratrol, Berberine, and Fucoxanthin can affect the PGC-1α-UCP1 thermogenic axis at different levels, including metabolic enzymes, ion channels, energy sensing, and mitochondrial transcriptional regulation.

Keywords: adipose browning; white adipose tissue; UCP1; AMPK; PGC-1α; Hyperforin; non-shivering thermogenesis; natural products

1 Metabolic Basis of Adipose Browning

1.1 White Adipose Tissue, Brown Adipose Tissue, and Beige Adipose Tissue

White adipose tissue (WAT) mainly functions in triglyceride storage and fatty acid mobilization. Mature adipocytes generally contain large unilocular lipid droplets and have relatively low mitochondrial density and basal oxidative capacity. Brown adipose tissue (BAT), by contrast, contains multilocular lipid droplets and abundant mitochondria, with UCP1-mediated non-shivering thermogenesis as its major metabolic characteristic. Cold exposure, β-adrenergic stimulation, and certain natural bioactive molecules can induce beige adipocytes with high UCP1 expression, increased mitochondrial abundance, and enhanced fatty acid oxidation in specific WAT depots. Therefore, adipose browning is essentially a metabolic reprogramming process in which adipocytes shift from a high-energy-storage phenotype toward a high-oxidation, high-substrate-consumption, and high-thermogenesis phenotype.

Table 1 Functional Differences Among White, Brown, and Beige Adipose Tissue

Adipose Type	Major Metabolic Function	Lipid Droplet Structure	Mitochondrial Characteristics	UCP1	Key Research Indicators
White adipose tissue	Triglyceride storage and fatty acid release	Mainly unilocular lipid droplets	Relatively low density	Low or absent expression	Adipogenesis, lipolysis, and lipid droplet size
Brown adipose tissue	Basal non-shivering thermogenesis	Multilocular lipid droplets	Abundant with strong oxidative capacity	High expression	UCP1, OCR, fatty acid oxidation, and thermogenesis
Beige adipose tissue	Inducible thermogenesis	Becomes multilocular after stimulation	Increased number and activity	Induced expression	UCP1, PGC-1α, PRDM16, CIDEA, and OCR

1.2 UCP1 and Mitochondrial Proton Leak

The mitochondrial respiratory chain pumps protons out of the matrix through Complexes I, III, and IV, generating an electrochemical proton gradient across the inner mitochondrial membrane. Under normal conditions, protons flow back through ATP synthase to drive ATP synthesis. When UCP1 (Uncoupling Protein 1) is activated, proton leak across the inner mitochondrial membrane increases, partially uncoupling substrate oxidation from ATP production, and the energy stored in the proton gradient is ultimately released as heat. Therefore, functional browning requires not only increased UCP1 expression but also enhanced substrate oxidation and respiration. An increase in UCP1 mRNA alone cannot demonstrate that adipocytes have acquired effective thermogenic capacity.

1.3 PGC-1α-Driven Mitochondrial Program

PGC-1α is encoded by PPARGC1A and is a central transcriptional coactivator in adipose thermogenesis research. PGC-1α does not directly bind DNA but cooperates with PPARs, ERRs, and other nuclear receptors and transcription factors to promote mitochondrial biogenesis, respiratory-chain components, fatty acid oxidation, and expression of thermogenic genes such as UCP1. PRDM16 is more involved in maintaining brown/beige adipocyte transcriptional identity. Therefore, increased PGC-1α mainly reflects activation of oxidative and mitochondrial programs, while whether beige adipocytes are ultimately formed still requires evaluation of UCP1, PRDM16, CIDEA, and cellular respiratory status.

1.4 β-Adrenergic Thermogenic Signaling

Classical cold-induced thermogenesis is initiated by sympathetic release of Norepinephrine. Activation of β-adrenergic receptors in adipocytes promotes cAMP production by Adenylyl Cyclase and subsequently activates PKA. On one hand, PKA increases free fatty acid supply through lipolysis; on the other hand, it enhances thermogenic gene transcription through signaling pathways including p38 MAPK and PGC-1α. Released fatty acids can serve as substrates for mitochondrial β-oxidation and can also participate in relieving purine-nucleotide-mediated inhibition of UCP1. Therefore, “lipolysis-fatty acid oxidation-UCP1 uncoupling” together constitutes the substrate-supply and execution system required for sustained thermogenesis.

2 Hyperforin: DLAT-AMPK Thermogenic Regulation

2.1 Functional Positioning of Hyperforin

Hyperforin (HPF) is a natural active component of Hypericum perforatum. Its traditional research has involved TRPC6-associated neuronal signaling, while adipose-metabolism studies have further shown that HPF can promote WAT browning and BAT thermogenesis. Unlike many natural products for which only increased UCP1 has been observed, HPF research identified Dihydrolipoamide Acetyltransferase (DLAT) as a direct binding protein and demonstrated through target intervention that DLAT is associated with subsequent AMPK activation and thermogenic phenotypes, providing a relatively complete evidence chain of “direct target-signaling axis-functional phenotype.”

2.2 DLAT and Pyruvate Oxidation

DLAT is the E2 core component of the Pyruvate Dehydrogenase Complex (PDC) and carries out the acetyl-transfer reaction, directing pyruvate-derived carbon from glycolysis toward Acetyl-CoA and the TCA cycle. Therefore, DLAT is located at an important metabolic interface between cytoplasmic glycolysis and mitochondrial carbon oxidation. HPF binding to DLAT does not activate AMPK simply by inhibiting the catalytic function of DLAT. Subsequent studies have shown that HPF enhances the interaction between DLAT and TRPV3, increases cytosolic Ca²⁺, and activates CaMKKβ, thereby promoting AMPK phosphorylation and the UCP1-associated thermogenic program. The current mechanistic pathway can therefore be more completely described as the DLAT–TRPV3–Ca²⁺–CaMKKβ–AMPK signaling axis.

2.3 DLAT-AMPK-PGC-1α-UCP1 Signaling Axis

(1) Ca²⁺-CaMKKβ-Dependent AMPK Activation

AMPK can respond to changes in cellular energy charge but can also be regulated by upstream kinases such as CaMKKβ. HPF has minimal effects on cellular AMP, ADP, and ATP levels, and its activation of AMPK is primarily associated with a DLAT–TRPV3-mediated increase in cytosolic Ca²⁺. Ca²⁺ subsequently activates CaMKKβ and promotes AMPK phosphorylation. DLAT intervention, intracellular Ca²⁺ chelation, or CaMKKβ inhibition attenuates HPF-induced AMPK activation and UCP1 expression, supporting the positioning of the DLAT–TRPV3–Ca²⁺–CaMKKβ pathway upstream of AMPK-mediated thermogenic signaling.

(2) Mitochondrial Remodeling

After AMPK activation, PGC-1α-associated mitochondrial transcriptional programs can be promoted, enhancing oxidative phosphorylation, fatty acid utilization, and mitochondrial biogenesis. HPF can promote PGC-1α-associated thermogenic programs in C3H10T1/2-derived adipocytes, human mesenchymal stem cell-derived adipocytes, and primary adipocytes, indicating that its effect is not limited to a single adipocyte model.

(3) UCP1-Dependent Thermogenesis

HPF can increase UCP1 expression in adipocytes and inguinal WAT while enhancing thermogenesis-associated phenotypes in BAT. When UCP1 function is restricted, some of the thermogenic and metabolic effects induced by HPF are markedly weakened, indicating that UCP1 is not only a molecular marker after HPF treatment but also an important effector of increased energy dissipation.

 

Figure 1. Hyperforin promotes adipose browning and thermogenesis through the DLAT–AMPK–PGC-1α–UCP1 axis

2.4 Technical Value of Hyperforin Research

HPF is suitable for establishing a multilevel mechanism-validation system. Upstream, DLAT knockdown or other genetic interventions can be used to confirm target dependence; at the intermediate level, AMPK phosphorylation and PGC-1α changes can be measured; downstream, UCP1 and mitochondrial respiration can be assessed. Compared with evaluating only body weight or lipid droplet reduction, this design can distinguish “direct target action,” “metabolic signaling response,” and “functional mitochondrial thermogenesis,” and represents a more convincing experimental approach for research on natural-product-induced adipose browning.

3 Capsaicin: TRPV1- and Ca²⁺-Dependent Thermogenic Regulation

3.1 Molecular Basis of Capsaicin Action

Capsaicin is a classical agonist of TRPV1 (Transient Receptor Potential Vanilloid 1). TRPV1 is a nonselective cation channel whose opening causes influx of Ca²⁺ and other cations. Therefore, Capsaicin first alters membrane electrical activity and intracellular Ca²⁺ signaling rather than acting directly on UCP1. TRPV1 is widely involved in sensory-neuron and energy-metabolism regulation, allowing Capsaicin to influence both local adipocyte signaling and sympathetic regulation between nerves and adipose tissue.

3.2 TRPV1-Ca²⁺ Signaling and Browning

The Ca²⁺ signal generated by TRPV1 opening can connect Ca²⁺-dependent kinases, AMPK, and mitochondrial regulatory processes and promote PGC-1α- and UCP1-associated thermogenic programs. At the adipose-tissue level, TRPV1-mediated sensory-neuron signaling can also influence sympathetic output, allowing β-adrenergic thermogenesis to intersect with local Ca²⁺ signaling. Therefore, the browning effect of Capsaicin is more appropriately interpreted as “TRPV1 activation-Ca²⁺/neuro-metabolic signaling changes-enhanced PGC-1α/UCP1 program” rather than directly describing Capsaicin as a UCP1 agonist.

3.3 Validation of TRPV1 Dependence

The most important mechanistic control in Capsaicin experiments is TRPV1 blockade. If Capsaicin-induced UCP1, PGC-1α, OCR, and fatty acid oxidation all decrease after the use of antagonists such as Capsazepine or genetic loss of TRPV1, this supports TRPV1 dependence. If the compound still reduces lipid droplets or body weight but UCP1 and mitochondrial respiration no longer respond, its effects should be considered to arise mainly from food intake, adipogenesis, or other metabolic processes rather than continuing to attribute all effects to adipose browning.

4 Resveratrol: Energy Sensing and Mitochondrial Remodeling

4.1 Characteristics of Resveratrol Action

Resveratrol is a polyphenolic natural product whose effects on adipose metabolism have clear multitarget characteristics. AMPK, NAD⁺ metabolism, SIRT1-associated deacetylation, and PGC-1α may all participate. Resveratrol should not be simply described as “directly activating SIRT1 and thereby promoting browning,” because intracellular SIRT1 regulation is also affected by NAD⁺ levels, AMPK, and other metabolic processes. Its adipose thermogenic effect is more accurately represented as integrated remodeling of energy sensing and mitochondrial transcriptional programs.

4.2 AMPK-SIRT1-PGC-1α Coupling

AMPK activation can promote fatty acid oxidation and enhance NAD⁺-related metabolism, whereas SIRT1 can regulate the acetylation state of PGC-1α and affect its transcriptional coactivation function. This creates an interconnected metabolic network among AMPK, NAD⁺/SIRT1, and PGC-1α. After Resveratrol treatment, some adipose models show increases in PGC-1α, UCP1, PRDM16, and mitochondrial oxidation-related genes together with enhanced adipocyte oxidative capacity. The core change is increased mitochondrial substrate utilization rather than simply reduced lipid droplets.

4.3 Distinguishing Oxidative Metabolism from Browning

Resveratrol can simultaneously improve inflammation, oxidative stress, and glucose metabolism. Therefore, “reduced fat mass” is not an effective endpoint for determining its browning effect. To demonstrate that Resveratrol promotes beige adipocyte formation, the UCP1/PGC-1α program, mitochondrial density, and OCR should be observed simultaneously, and whether these changes disappear after AMPK or PGC-1α intervention should be verified. This makes it possible to distinguish general metabolic improvement from true thermogenic reprogramming.

5 Berberine: Mitochondrial Energy Stress and AMPK Activation

5.1 Metabolic Characteristics of Berberine

Berberine is an isoquinoline alkaloid, and its AMPK activation should not simply be understood as direct binding to and activation of AMPK. Berberine can affect mitochondrial respiration, particularly in relation to Complex I function and changes in cellular energy charge, thereby producing mild energy stress and promoting an AMPK response. This upstream mechanism can explain its multiple effects on fatty acid oxidation, lipid synthesis, glucose metabolism, and mitochondrial adaptation.

5.2 AMPK-PGC-1α-UCP1 Thermogenic Program

After AMPK activation, ACC-associated fatty acid synthesis can be inhibited, favoring entry of fatty acids into mitochondria for β-oxidation, while PGC-1α-mediated mitochondrial remodeling is promoted. In BAT, Berberine can enhance activity of the existing UCP1 thermogenic system; in WAT such as inguinal fat, increased PGC-1α, UCP1, and other beige-fat markers may occur. Therefore, the effects of Berberine on adipose tissue include two distinct but interconnected processes: BAT activation and WAT browning.

5.3 Dose Dependence of Mitochondrial Effects

The effects of Berberine on mitochondria are concentration-dependent. A moderate reduction in respiratory efficiency may generate metabolic stress sufficient to activate AMPK, whereas excessive mitochondrial inhibition can cause ATP depletion, impaired respiration, and even cytotoxicity. Therefore, studies of Berberine-induced browning need to measure cell viability, ATP status, and OCR simultaneously to avoid incorrectly interpreting an energy crisis caused by mitochondrial dysfunction as effective thermogenesis.

6 Fucoxanthin: Induction of UCP1 and Oxidative Programs in WAT

6.1 Metabolic Characteristics of Fucoxanthin

Fucoxanthin is a xanthophyll carotenoid found in brown algae and some diatoms. After entering the body, it can also be converted into metabolites such as Fucoxanthinol, so its in vivo metabolic effects are not necessarily mediated entirely by the parent compound. The particular value of Fucoxanthin in adipose-browning research is that, in some rodent models, it can induce UCP1 and mitochondrial oxidative programs in WAT that originally has low UCP1 levels rather than only enhancing thermogenic function in existing BAT.

6.2 PGC-1α and Fatty Acid Oxidation Programs

Fucoxanthin-associated WAT remodeling can be accompanied by changes in PGC-1α, PPARα, ERRα, CIDEA, DIO2, β3-AR, and other molecules. PGC-1α and ERRα jointly participate in mitochondrial oxidative programs, PPARα promotes expression of genes associated with fatty acid β-oxidation, DIO2 enhances thermogenic transcription through local thyroid-hormone activation, and changes in β3-AR affect adipose-tissue sensitivity to sympathetic stimulation. Simultaneous changes in multiple nodes provide stronger evidence for overall remodeling of WAT oxidative capacity than an increase in UCP1 alone.

6.3 Coupling of Substrate Oxidation and UCP1 Thermogenesis

UCP1-mediated uncoupling must be supported by sustained substrate oxidation. If fatty acid oxidation capacity is insufficient, simply increasing UCP1 protein cannot maintain high thermogenic flux. Therefore, in Fucoxanthin research, it is more meaningful to analyze PPARα and fatty acid oxidation, PGC-1α and mitochondrial biogenesis, and UCP1 and uncoupled respiration together to determine whether a complete functional chain of “fatty acid supply-mitochondrial oxidation-proton leak-heat generation” has been established.

6.4 Species and Metabolite Differences

Fucoxanthin-induced UCP1 expression in rodent WAT is relatively prominent, but human adipocytes do not consistently reproduce the same degree of UCP1 or OCR increase. In addition, Fucoxanthin undergoes metabolic conversion during absorption, so direct addition of Fucoxanthin to cells does not completely reproduce the effective exposure forms after oral administration in animals. When comparing animal and human models, adipocyte type, administration form, active metabolites, and tissue-exposure differences should all be considered rather than making simple cross-species extrapolations.

7 Metabolic Regulatory Characteristics of Other Natural Products

7.1 Curcumin

Curcumin can affect AMPK, ACC, adipogenesis, and inflammatory networks, and in some models is also accompanied by increased PGC-1α and UCP1. Its technical value lies in the ability to simultaneously observe two directions: “reduced lipid synthesis” and “enhanced oxidative metabolism,” but this also increases the difficulty of result interpretation. If Curcumin only reduces PPARγ, C/EBPα, and lipid droplet formation without enhancing OCR or uncoupled respiration, the result is more consistent with inhibition of adipogenesis rather than functional adipose browning.

7.2 EGCG

EGCG (Epigallocatechin Gallate) can reduce the tendency toward fatty acid synthesis and promote fatty acid oxidation through AMPK-ACC-associated metabolic regulation. In some adipose-tissue models, changes in UCP1 or other thermogenic indicators may also occur, but its action is more appropriately positioned as enhancement of substrate oxidation and energy expenditure rather than specific activation of UCP1. Experimental design should focus on ACC phosphorylation, fatty acid oxidation, OCR, and thermogenic proteins to determine whether the effect of EGCG remains mainly at the oxidative-metabolism level or extends to the beige-fat program.

7.3 Quercetin

Quercetin can affect AMPK, PGC-1α, and mitochondrial oxidative status, and some models show increased UCP1 and fatty acid oxidation-related indicators. Its effect is clearly model-dependent, so “improved mitochondrial function” and “induced adipocyte identity conversion” should be distinguished. Only when UCP1, PRDM16/CIDEA, mitochondrial respiration, and multilocular lipid droplet phenotypes appear simultaneously does the evidence more strongly support a complete browning effect.

Table 2 Comparison of Adipose Thermogenic Mechanisms of Major Natural Products

Natural Product	Major Entry Point	Upstream Mechanism	Thermogenic Connecting Node	Major Functional Changes
Hyperforin	DLAT	DLAT–TRPV3–Ca²⁺–CaMKKβ-mediated AMPK activation	AMPK-PGC-1α-UCP1	WAT browning and BAT thermogenesis
Capsaicin	TRPV1	Ca²⁺ and neuro-adipose signaling	PGC-1α and UCP1	Enhanced lipid oxidation and thermogenesis
Resveratrol	AMPK and NAD⁺/SIRT1-related processes	Energy sensing and deacetylation regulation	PGC-1α and UCP1	Mitochondrial remodeling and beige-fat phenotype
Berberine	Mitochondrial energy charge/AMPK	Mild energy stress and AMPK response	PGC-1α-UCP1	BAT activation and WAT browning
Fucoxanthin	Mitochondrial and thermogenic transcriptional programs	PGC-1α, PPARα, ERRα, and related pathways	UCP1	Enhanced WAT oxidative and thermogenic programs
Curcumin	AMPK-ACC-related processes	Changes in the balance between lipid synthesis and oxidation	PGC-1α/UCP1-related programs	Reduced lipid accumulation and changes in thermogenesis
EGCG	AMPK-ACC	Reduced fatty acid synthesis and enhanced oxidation	Mitochondrial metabolism	Substrate oxidation and energy expenditure
Quercetin	AMPK and mitochondrial networks	Remodeling of oxidative metabolism	PGC-1α/UCP1	Mitochondrial function and browning-associated phenotypes

8 Products Related to Adipose Browning Research

Table 3 Natural Products and Compounds Related to Adipose Thermogenesis

Product Name	CAS No.	Major Target/Characteristics	Major Research Direction
Hyperforin	11079-53-1	DLAT–TRPV3–Ca²⁺/CaMKKβ–AMPK-related processes	DLAT–TRPV3–AMPK signaling and adipose thermogenesis
Capsaicin	404-86-4	TRPV1	TRPV1-dependent adipose browning
Resveratrol	501-36-0	AMPK- and SIRT1-related processes	PGC-1α and mitochondrial remodeling
Berberine	2086-83-1	AMPK-related network	BAT activation and WAT browning
Berberine Chloride	633-65-8	AMPK-related network	Glucose and lipid metabolism and thermogenesis
Fucoxanthin	3351-86-8	PGC-1α- and UCP1-related processes	WAT oxidation and thermogenesis
Curcumin	458-37-7	AMPK-ACC-related processes	Lipid synthesis, oxidation, and thermogenesis
EGCG	989-51-5	AMPK-ACC-related processes	Fatty acid oxidation
Quercetin	117-39-5	AMPK- and mitochondria-related processes	Lipid oxidation and browning
Genistein	446-72-0	AMPK- and PPAR-related processes	Lipid and mitochondrial metabolism
Naringenin	480-41-1	AMPK- and PPAR-related processes	Fatty acid oxidation
Luteolin	491-70-3	AMPK-related processes	Mitochondrial and lipid metabolism
6-Gingerol	23513-14-6	AMPK- and lipid-metabolism-related processes	Lipid oxidation and energy metabolism
6-Shogaol	555-66-8	Metabolism- and inflammation-related processes	Lipid metabolism and thermogenesis

Table 4 Compounds Related to Mechanistic Validation of Thermogenic Signaling

Product Name	CAS No.	Major Target	Major Research Use
Compound C (Dorsomorphin)	866405-64-3	AMPK-related processes	Validation of AMPK dependence
AICAR (Acadesine)	2627-69-2	AMPK-related processes	AMPK activation control
H-89 Dihydrochloride	130964-39-5	PKA	Validation of cAMP-PKA signaling
KT5720	108068-98-0	PKA	PKA-dependence research
Capsazepine	138977-28-3	TRPV1	Validation of the Capsaicin/TRPV1 mechanism
Forskolin	66575-29-9	Adenylyl Cyclase	Activation of the cAMP-PKA thermogenic pathway
IBMX	28822-58-4	PDE	Enhancement of cAMP signaling
SB203580	152121-47-6	p38 MAPK	Validation of p38 MAPK dependence
Rapamycin	53123-88-9	mTORC1	AMPK-mTOR metabolic relationship
Nicotinamide	98-92-0	NAD⁺/Sirtuin-related processes	Sirtuin-related metabolic research

Table 5 Materials Related to Mitochondrial Respiration and Fatty Acid Oxidation Research

Product Name	CAS No.	Major Target	Major Research Use
FCCP	370-86-5	Proton-gradient uncoupling	Evaluation of maximal respiratory capacity
Oligomycin A	579-13-5	ATP Synthase	Evaluation of ATP-coupled respiration
Rotenone	83-79-4	Respiratory Chain Complex I	Research on Complex I-dependent respiration
L-Carnitine	541-15-1	Mitochondrial fatty acid transport	Fatty acid oxidation research
Palmitic Acid	57-10-3	Long-chain fatty acid substrate	β-Oxidation substrate supply
Sodium Pyruvate	113-24-6	Pyruvate oxidation	Mitochondrial carbon-oxidation research
2-Deoxy-D-Glucose	154-17-6	Glycolysis	Energy stress and AMPK-response research

The key to natural-product-induced adipose browning is establishing a causal link between upstream sites of action and mitochondrial thermogenesis. Whether different entry points such as DLAT, TRPV1, and AMPK ultimately increase substrate oxidation and UCP1-dependent thermogenesis is central to evaluating their browning mechanisms.

References

[1] Baskaran P, et al. Capsaicin induces browning of white adipose tissue and counters obesity by activating TRPV1 channel-dependent mechanisms. Br J Pharmacol. 2016;173(15):2369-2389.

[2] Wang S, et al. Resveratrol induces brown-like adipocyte formation in white fat through activation of AMP-activated protein kinase (AMPK) α1. Int J Obes (Lond). 2015;39(6):967-976.

[3] Zhang Z, et al. Berberine activates thermogenesis in white and brown adipose tissue. Nat Commun. 2014;5:5493.

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