Theabrownin from dark tea may have the potential to treat insulin resistance

By Tarun Sai LomteSep 8 2023Reviewed by Lily Ramsey, LLM

In a recent study published in Nutrients, researchers assessed the effects of theabrownin (TB) from dark tea treatment on insulin resistance (IR).

Study: Attenuating Oxidative Stress and Modulating IRS-1/PI3K/Akt Pathway in HepG2 Cells. Image Credit: NatalliaPloskaya/Shutterstock.com

Background

Diabetes is a severe public health issue. The International Diabetes Federation estimates over 592 million cases of diabetes by 2035. In China, one in 11 individuals has diabetes.

Type 2 diabetes (T2D) is an increasingly prevalent metabolic disease, often associated with IR. Persistent hyperglycemia elevates oxidative stress and induces IR, thereby contributing to T2D.

Maintaining metabolic homeostasis in the liver depends on the regulatory functions of glucagon and insulin. High-fat- and high-glucose-induced IR is associated with increased reactive oxygen species (ROS) levels, leading to oxidative stress.

Excess ROS can also cause mitochondrial dysfunction. Therefore, decreasing oxidative stress helps improve IR and prevent progression to T2D.

Chinese dark tea is popular for its taste and aroma. Its bioactive constituent, TB, a water-soluble phenolic compound, has been demonstrated to effectively decrease IR in the skeletal muscle and fasting blood glucose levels and promote the growth of beneficial bacteria.

Nevertheless, how TB with varying fermentation periods influences IR and the underlying mechanisms are unclear.

About the study

In the present study, researchers assessed how TB affects IR. TB was extracted following fermentation for seven days (TB1) or 14 days (TB2). The ultraviolet-visible absorption spectra of the colloids were measured. Samples were also analyzed using Fourier-transform infrared (FTIR) spectroscopy.

The team established an IR model of HepG2 cells in a high-glucose medium supplemented with free fatty acids (FFA). Cell viability was ascertained by incubating cells with increasing concentrations of metformin (MET) or  TB. The team measured glucose uptake, consumption, and glycogen content by cells.

The Oil Red O (ORO) staining method was used to measure the accumulation of neutral lipids. Cellular and mitochondrial ROS and mitochondrial membrane potential were analyzed.

The team also performed real-time polymerase chain reaction (PCR) and western blotting to investigate the effects of TB treatment on target genes and proteins.

Findings

All samples showed absorption peaks at 205 nm and 275 nm. Peaks of TB1 were relatively lower than those of TB2. The degree of polyphenol polymerization was lower in TB1 samples, which may explain its reduced absorption (relative to TB2). FTIR spectroscopy revealed that samples were rich in carboxylic and hydroxyl groups.

TB2 absorption peaks were stronger than TB1 peaks, plausibly due to a higher degree of phenol polymerization during (extended) fermentation. Cell viability was unaffected with increasing concentrations of MET or TB, indicating their non-cytotoxicity.

TB1 and TB2 increased glycogen content and the uptake and consumption of glucose; however, TB2 showed superior effects.

Cellular accumulation of lipids was inhibited upon TB treatment. TB reduced total cholesterol, triglycerides, and low-density lipoprotein but increased high-density lipoprotein. The effects of 150 μg/mL of TB2 were the same as 25 μg/mL of MET.

TB treatment increased superoxide dismutase, adenosine triphosphate (ATP), and glutathione levels and reduced mitochondrial ROS synthesis. TB2 was more potent in decreasing oxidative stress than TB1.

TB treatment improved enzymatic expression to promote glucose uptake and utilization. It promoted the phosphorylation of forkhead box O1 (FOXO1) and downregulated glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase 1 (PEPCK1), inhibiting gluconeogenesis and decreasing glucose synthesis. The transcript levels of glucose transporters 2 (GLUT2) and 4 (GLUT4) increased upon TB treatment.

TB treatment significantly reduced transcript and protein levels of SREBP (sterol regulatory element-binding protein)-1C, glycerol 3 phosphate acyltransferase 1 (GPAT1), and fatty acid synthase (FASN) and increased the expression of acetyl CoA carboxylase (ACC).

The effects of TB2 on FASN and ACC were more significant than those of TB1. TB intervention downregulated SREBP-2C and 3-hydroxy 3-methylglutaryl CoA reductase (HMGCR), indicating that TB can inhibit cholesterol synthesis.

The team noted increased levels of insulin receptor substrate (IRS)-1, phosphoinositide 3 kinase (PI3K), and protein kinase B (Akt) with TB interventions; this effect was more pronounced with TB2.

In cells treated with a PI3K inhibitor, PI3K phosphorylation was inhibited, and the expression of IRS-1 and Akt was suppressed. Moreover, the phosphorylation of two Akt substrates was blocked in the presence of the inhibitor.

Conclusions

Together, TB1 and TB2 modulated IR through IRS-1/PI3K/Akt pathway, regulating enzymes involved in glucose and lipid metabolism.

Moreover, TB treatment attenuated oxidative stress, with TB2 being more potent than TB1. TB2 also had a more favorable lipid-lowering and hypoglycemic effect than TB1, plausibly due to increased fermentation that enhanced the degree of oxidation and polyphenol polymerization.

Future clinical trials are necessary to establish TB2 as a potential candidate to mitigate the global burden of IR and T2D.