Antidiabetic effect of green rooibos (Aspalathus linearis) extract in cultured cells and type 2 diabetic model KK-A(y) mice

Effects of Herbal Tea (Non-Camellia sinensis) on Glucose Homeostasis and Serum Lipids in Type 2 Diabetes Mellitus: A Systematic Review and Meta-Analysis

CONTEXT

Hyperglycemia and hyperlipidemia increase the risk for diabetes and its complications, atherosclerosis, heart failure, and stroke. Identification of safe and cost-effective means to reduce risk factors is needed. Herbal teas may be a vehicle to deliver antioxidants and polyphenols for prevention of complications.

OBJECTIVE

This systematic review and meta-analysis were conducted to evaluate and summarize the impact of herbal tea (non-Camellia sinensis) on glucose homeostasis and serum lipids in individuals with type 2 diabetes (T2D).

DATA SOURCES

PubMed, FSTA, Web of Science, CINAHL, MEDLINE, and Cochrane Library databases were searched from inception through February 2023 using relevant keyword proxy terms for diabetes, serum lipids, and "non-Camellia sinensis" or "tea."

DATA EXTRACTION

Data from 14 randomized controlled trials, totaling 551 participants, were included in the meta-analysis of glycemic and serum lipid profile end points.

RESULTS

Meta-analysis suggested a significant association between drinking herbal tea (prepared with 2-20 g d-1 plant ingredients) and reduction in fasting blood glucose (FBG) (P = .0034) and glycated hemoglobin (HbA1c; P = .045). In subgroup analysis based on studies using water or placebo as the control, significant reductions were found in serum total cholesterol (TC; P = .024), low-density lipoprotein cholesterol (LDL-C; P = .037), and triglyceride (TG; P = .043) levels with a medium effect size. Meta-regression analysis suggested that study characteristics, including the ratio of male participants, trial duration, and region, were significant sources of FBG and HbA1c effect size heterogeneity; type of control intervention was a significant source of TC and LDL-C effect size heterogeneity.

CONCLUSIONS

Herbal tea consumption significantly affected glycemic profiles in individuals with T2D, lowering FBG levels and HbA1c. Significance was seen in improved lipid profiles (TC, TG, and LDL-C levels) through herbal tea treatments when water or placebo was the control. This suggests water or placebo may be a more suitable control when examining antidiabetic properties of beverages. Additional research is needed to corroborate these findings, given the limited number of studies.

Translation of preclinical ethnomedicine data in LMICs: the example of rooibos

All disease, but especially non-communicable diseases, are related to dysfunction of one or more regulatory systems. In developing countries, long-term management of patients with chronic diseases has many challenges and is generally not financially viable, but Africa in particular, which is rich in diverse ethnomedicines presents a more feasible long-term therapeutic approach in this niche. However, despite comprehensive preclinical investigations on numerous plant-derived candidate medicines, only a small portion of these reach the patient as recognised medicines. In this review, we use the example of rooibos (Aspalathus linearis (Burm.f.) R. Dahlgren)-which is globally consumed as aromatic, caffeine-free tea-to illustrate the hurdles that need to be overcome in the low-to middle-income countries, before progression of ethnomedicines to official treatment regimens can be achieved. In terms of methodology, regulatory system focused rooibos papers indexed on PubMed for the past three decades (n = 112) were accessed. Papers reporting duplication of previous results were excluded, as well as review papers. Topics covered includes the high standard of ethnomedicine drug discovery and efficacy testing research performed in Africa (and South Africa in particular in the case of rooibos), the potential bias in terms of preclinical research focus, ethnomedicine ownership and the requirement for independent clinical trial coordination and/or management.

Radical oxygen species: an important breakthrough point for botanical drugs to regulate oxidative stress and treat the disorder of glycolipid metabolism

Background: The incidence of glycolipid metabolic diseases is extremely high worldwide, which greatly hinders people's life expectancy and patients' quality of life. Oxidative stress (OS) aggravates the development of diseases in glycolipid metabolism. Radical oxygen species (ROS) is a key factor in the signal transduction of OS, which can regulate cell apoptosis and contribute to inflammation. Currently, chemotherapies are the main method to treat disorders of glycolipid metabolism, but this can lead to drug resistance and damage to normal organs. Botanical drugs are an important source of new drugs. They are widely found in nature with availability, high practicality, and low cost. There is increasing evidence that herbal medicine has definite therapeutic effects on glycolipid metabolic diseases. Objective: This study aims to provide a valuable method for the treatment of glycolipid metabolic diseases with botanical drugs from the perspective of ROS regulation by botanical drugs and to further promote the development of effective drugs for the clinical treatment of glycolipid metabolic diseases. Methods: Using herb*, plant medicine, Chinese herbal medicine, phytochemicals, natural medicine, phytomedicine, plant extract, botanical drug, ROS, oxygen free radicals, oxygen radical, oxidizing agent, glucose and lipid metabolism, saccharometabolism, glycometabolism, lipid metabolism, blood glucose, lipoprotein, triglyceride, fatty liver, atherosclerosis, obesity, diabetes, dysglycemia, NAFLD, and DM as keywords or subject terms, relevant literature was retrieved from Web of Science and PubMed databases from 2013 to 2022 and was summarized. Results: Botanical drugs can regulate ROS by regulating mitochondrial function, endoplasmic reticulum, phosphatidylinositol 3 kinase (PI3K)/protein kinase B (AKT), erythroid 2-related factor 2 (Nrf-2), nuclear factor κB (NF-κB), and other signaling pathways to improve OS and treat glucolipid metabolic diseases. Conclusion: The regulation of ROS by botanical drugs is multi-mechanism and multifaceted. Both cell studies and animal experiments have demonstrated the effectiveness of botanical drugs in the treatment of glycolipid metabolic diseases by regulating ROS. However, studies on safety need to be further improved, and more studies are needed to support the clinical application of botanical drugs.

Pharmacological activity of Aspalathus linearis extracts: pre-clinical research in view of prospective neuroprotection

OBJECTIVES

Rooibos tea, a very popular everyday beverage made of Aspalathus linearis plant material and containing multiple polyphenolic compounds, reveals an expectation to positively affect various processes observed in the pathogenesis of neurodegenerative diseases as in the case of consumption of other polyphenol-abundant food products.

METHODS

This review is based on available data from pre-clinical in vitro and in vivo studies and presents a broad report on the pharmacological activity of the A. linearis extracts relevant for neurodegenerative diseases.

RESULTS

Flavonoids present in herbal infusions are absorbed from gastro-intestinal tract and may affect the central nervous system. The experimental investigations yield the results indicating to supporting role of A. linearis in the prevention of neurodegeneration, primarily owing to anti-oxidative and anti-inflammatory properties, anti-hyperglycaemic and anti-hyperlipidaemic effects as well as favourable impact on neurotransmission with following cognitive and behavioural after-math.

DISCUSSION

The multiple pharmacological activities and safety of Aspalathus linearis extracts are commented in the manuscript. The continuous rooibos tea consumption seems to be safe (despite anecdotal liver irritation); however, there is a risk of herbal-drug interactions.

Near-Physiological Concentrations of Extracellular Pyruvate Stimulated Glucose Utilization along with Triglyceride Accumulation and Mitochondrial Activity in HepG2 Cells

Pyruvate, a key intermediate in energy and nutrient metabolism, probably plays important roles in these regulations. In previous reports using cell lines, extracellular pyruvate of supraphysiological concentrations inhibited the glucose uptake by myotubes while being stimulated by adipocytes. As the effect of pyruvate on the glucose utilization is unclear in cultured hepatocytes. We have investigated the effects of extracellular pyruvate on the glucose utilization and the subsequent metabolic changes using the cell line HepG2. In a 24 h culture, pyruvate enhanced the glucose consumption more potently than 1 μM insulin, and this enhancement was detectable at a near-physiological concentrations of ≤1 mM. For metabolic changes following glucose consumption, the conversion ratio of glucose and pyruvate to extracellular lactate was approximately 1.0 without extracellular pyruvate. The addition of pyruvate decreased the conversion ratio to approximately 0.7, indicating that the glycolytic reaction switched from being an anaerobic to a partially aerobic feature. Consistent with this finding, pyruvate increased the accumulation of intracellular triglycerides which are produced through substrate supply from the mitochondria. Furthermore, pyruvate stimulated mitochondria activity as evidenced by increases in ATP content, mitochondrial DNA copy number, enhanced mitochondria-specific functional imaging and oxygen consumption. Interestingly, 1 mM pyruvate increased oxygen consumption immediately after addition. In this study, we found that near-physiological concentrations of extracellular pyruvate exerted various changes in metabolic events, including glucose influx, lactate conversion rations, TG accumulation, and mitochondrial activity in HepG2 cells.

The effects of a green Rooibos (Aspalathus linearis) extract on metabolic parameters and adipose tissue biology in rats fed different obesogenic diets

Current pharmaceutical treatments addressing obesity are plagued by high costs, low efficacy and adverse side effects. Natural extracts are popular alternatives, but evidence for their anti-obesity properties is scant. We assessed the efficacy of a green (minimally-oxidized) Rooibos (Aspalathus linearis) extract (GRT) to ameliorate the effects of obesogenic feeding in rats, by examining body weight, metabolic measures, adipose tissue cellularity and tissue-resident adipose stem cells (ASCs). Furthermore, we performed statistical correlations to explore the relationships and interactions between metabolic and adipose tissue measures. Using an in vivo/ex vivo study design, male Wistar rats were maintained for 17 weeks on one of 3 diets: CON (laboratory chow), OB1 (high-sugar, medium fat) or OB2 (high-fat, high-cholesterol) (n = 24 each). From weeks 11-17, half of the animals in each group received oral GRT supplementation (60 mg per kg body weight daily). Blood and tissue samples were collected, and ASCs from each animal were cultured. Diets OB1 and OB2 induced divergent metabolic profiles compared to CON, but metabolic measures within dietary groups were mostly unaffected by GRT supplementation. Notably, diets OB1 and OB2 uncoupled the positive association between visceral adiposity and insulin resistance, while GRT uncoupled the positive association between elevated serum cholesterol and liver damage. Obesogenic feeding and GRT supplementation induced adipocyte enlargement in vivo, but lipid accumulation in cultured ASCs did not differ between dietary groups. Larger adipocyte size in subcutaneous fat was associated with favourable glucose metabolism measures in all GRT groups. In conclusion, GRT affected the associations between systemic, adipose tissue-level and cellular measures against the background of obesogenic diet-induced metabolic dysregulation.

Aspalathin and Other Rooibos Flavonoids Trapped α-Dicarbonyls and Inhibited Formation of Advanced Glycation End Products In Vitro

The excessive dietary intake of simple sugars and abnormal metabolism in certain diseases contribute to the increased production of α-dicarbonyls (α-DCs), such as methylglyoxal (MGO) and glyoxal (GO), the main precursors of the formation of advanced glycation end products (AGEs). AGEs play a vital role, for example, in the development of cardiovascular diseases and diabetes. Aspalathus linearis (Burman f.) R. Dahlgren (known as rooibos tea) exhibits a wide range of activities beneficial for cardio-metabolic health. Thus, the present study aims to investigate unfermented and fermented rooibos extracts and their constituents for the ability to trap MGO and GO. The individual compounds identified in extracts were tested for the capability to inhibit AGEs (with MGO or GO as a glycation agent). Ultra-high-performance liquid chromatography coupled with an electrospray ionization mass spectrometer (UHPLC-ESI-MS) was used to investigate α-DCs' trapping capacities. To evaluate the antiglycation activity, fluorescence measurement was used. The extract from the unfermented rooibos showed a higher ability to capture MGO/GO and inhibit AGE formation than did the extract from fermented rooibos, and this effect was attributed to a higher content of dihydrochalcones. The compounds detected in the extracts, such as aspalathin, nothofagin, vitexin, isovitexin, and eriodictyol, as well as structurally related phloretin and phloroglucinol (formed by the biotransformation of certain flavonoids), trapped MGO, and some also trapped GO. AGE formation was inhibited the most by isovitexin. However, it was the high content of aspalathin and its higher efficiency than that of metformin that determined the antiglycation and trapping properties of green rooibos. Therefore, A. linearis, in addition to other health benefits, could potentially be used as an α-DC trapping agent and AGE inhibitor.

Network Pharmacology- and Molecular Dynamics Simulation-Based Bioprospection of Aspalathus linearis for Type-2 Diabetes Care

The medicinal herb Aspalathus linearis (rooibos) is globally recognized in type-2 diabetes mellitus (T2DM) treatment due to its known and distinctive compounds. This work utilized network pharmacology (NP) coupled with molecular dynamics simulation in gaining new insight into the anti-diabetic molecular mechanism of action of rooibos teas. It looked at the interactions between rooibos constituents with various relevant protein receptors and signaling routes associated with T2DM progression. The initial analysis revealed 197 intersecting gene targets and 13 bioactive rooibos constituents linked to T2DM. The interactions between proteins and compounds to the target matrix were generated with the Cystoscope platform and STRING database. These analyses revealed intersecting nodes active in T2DM and hypoxia-inducible factor 1 (HIF-1) as an integral receptors target. In addition, KEGG analysis identified 11 other pathways besides the hub HIF-1 signaling route which may also be targeted in T2DM progression. In final molecular docking and dynamics simulation analysis, a significant binding affinity was confirmed for key compound-protein matrices. As such, the identified rooibos moieties could serve as putative drug candidates for T2DM control and therapy. This study shows rooibos constituents' interaction with T2DM-linked signaling pathways and target receptors and proposes vitexin, esculin and isovitexin as well as apigenin and kaempferol as respective pharmacologically active rooibos compounds for the modulation of EGFR and IGF1R in the HIF-1 signaling pathway to maintain normal homeostasis and function of the pancreas and pancreatic β-cells in diabetics.

Traditional Use, Phytochemical Profiles and Pharmacological Properties of Artemisia Genus from Central Asia

The flora of Kazakhstan is characterized by its wide variety of different types of medicinal plants, many of which can be used on an industrial scale. The Traditional Kazakh Medicine (TKM) was developed during centuries based on the six elements of ancient Kazakh theory, associating different fields such as pharmacology, anatomy, pathology, immunology and food nursing as well as disease prevention. The endemic Artemisia L. species are potential sources of unique and new natural products and new chemical structures, displaying diverse bioactivities and leading to the development of safe and effective phytomedicines against prevailing diseases in Kazakhstan and the Central Asia region. This review provides an overview of Artemisia species from Central Asia, particularly traditional uses in folk medicine and the recent numerous phytochemical and pharmacological studies. The review is done by the methods of literature searches in well-known scientific websites (Scifinder and Pubmed) and data collection in university libraries. Furthermore, our aim is to search for promising and potentially active Artemisia species candidates, encouraging us to analyze Protein Tyrosine Phosphatase 1B (PTP1B), α-glucosidase and bacterial neuraminidase (BNA) inhibition as well as the antioxidant potentials of Artemisia plant extracts, in which endemic species have not been explored for their secondary metabolites and biological activities so far. The main result of the study was that, for the first time, the species Artemisia scopiformis Ledeb. Artemisia albicerata Krasch., Artemisia transiliensis Poljakov, Artemisia schrenkiana Ledeb., Artemisia nitrosa Weber and Artemisia albida Willd. ex Ledeb. due to their special metabolites, showed a high potential for α-glucosidase, PTP1B and BNA inhibition, which is associated with diabetes, obesity and bacterial infections. In addition, we revealed that the methanol extracts of Artemisia were a potent source of polyphenolic compounds. The total polyphenolic contents of Artemisia extracts were correlated with antioxidant potential and varied according to plant origin, the solvent of extraction and the analytical method used. Consequently, oxidative stress caused by reactive oxygen species (ROS) may be managed by the dietary intake of current Artemisia species. The antioxidant potentials of the species A. schrenkiana, A. scopaeformis, A. transiliensis and Artemisia scoparia Waldst. & Kitam. were also promising. In conclusion, the examination of details between different Artemisia species in our research has shown that plant materials are good as an antioxidant and eznyme inhibitory functional natural source.

Neuroprotective and anxiolytic potential of green rooibos (Aspalathus linearis) polyphenolic extract

South African rooibos (Aspalathus linearis) tea is globally consumed for its health benefits and caffeine free nature, but no information is available on the neuroprotective capacity of (unfermented) green rooibos. Our aim was to investigate the cytoprotective activity of green rooibos in neuronal cells, including probing antioxidant and enzyme inhibitory properties that could explain observed effects in these cells. We also investigated the anxiolytic potential of green rooibos using zebrafish larval models. Green rooibos extract (Green oxithin™) was assessed for its neuroprotective potential in Neuro-2a cells treated with different concentrations of the extract (12.5-25-50-100 μg mL^-1) and different concentrations of hydrogen peroxide (250 or 125 μM) as oxidizing agent. Cell viability (MTT) and redox status (intracellular ROS) were also quantified in these cells. Antioxidant properties of the extract were quantified using cell-free systems (DPPH, ORAC and xanthine/xanthine oxidase), and potential neuroprotection evaluated in terms of its potential to inhibit key enzymes of the CNS (monoamine oxidase A (MOA-A), acetylcholinesterase (AChE) and tyrosinase (TYR)). Results demonstrated that green rooibos extract exerted significant cytoprotective properties in Neuro-2a cells, particularly when exposed to lethal 250 μM hydrogen peroxide, increasing cell survival by more than 100%. This may be ascribed (at least partially) to its capacity to limit intracellular ROS accumulation in these cells. Data from cell-free systems confirmed that green rooibos was able to scavenge free radicals (synthetic and physiological) in a dose dependent manner with a similar profile activity to vitamins C and E. Green rooibos also acted as a moderate MAO-A inhibitor, but had no significant effect on AChE or TYR. Finally, zebrafish larvae treated with lower doses of green rooibos demonstrated a significant anxiolytic effect in the light-dark anxiety model. Using the PTZ excitotoxicity model, green rooibos was shown to rescue GABA receptor signalling, which together with its demonstrated inhibition of MAO-A, may account for the anxiolytic outcome. Current data confirms that green rooibos could be considered a "functional brain food" and may be a good option as starting ingredient in the development of new nutraceuticals.

Cardioprotective Function of Green Rooibos (Aspalathus linearis) Extract Supplementation in Ex Vivo Ischemic Prediabetic Rat Hearts

Diabetic patients develop ischemic heart disease and strokes more readily. Following an ischemic event, restoration of blood flow increases oxidative stress resulting in myocardial damage, termed ischemia/reperfusion injury. Aspalathus linearis (rooibos), rich in the antioxidant phenolic compound aspalathin, has been implicated as cardioprotective against ischemia/reperfusion injury with undefined mechanism in control rats. Primarily, the therapeutic potential of Afriplex green rooibos extract to prevent ischemia/reperfusion injury in cardiovascular disease-compromised rats was investigated. Additionally, Afriplex Green rooibos extract's cardioprotective signaling on metabolic markers and stress markers was determined using western blotting. Three hundred male Wistar rats received either 16-wk standard diet or high-caloric diet. During the final 6 wk, half received 60 mg/kg/day Afriplex green rooibos extract, containing 12.48% aspalathin. High-caloric diet increased body weight, body fat, fasting serum triglycerides, and homeostatic model assessment of insulin resistance - indicative of prediabetes. High-caloric diet rats had increased heart mass, infarct size, and decreased heart function. Afriplex green rooibos extract treatment for 6 wk lowered pre-ischemic heart rate, reduced infarct size, and improved heart function pre- and post-ischemia, without significantly affecting biometric parameters. Stabilized high-caloric diet hearts had decreased insulin independence via adenosine monophosphate activated kinase and increased inflammation (p38 mitogen-activated protein kinase), whereas Afriplex green rooibos extract treatment decreased insulin dependence (protein kinase B) and conferred anti-inflammatory effect. After 20 min ischemia, high-caloric diet hearts had upregulated ataxia-telangiectasia mutated kinase decreased insulin independence, and downregulated insulin dependence and glycogen synthase kinase 3 β inhibition. In contrast, Afriplex green rooibos extract supplementation downregulated insulin independence and inhibited extracellular signal-regulated kinase 1 and 2. During reperfusion, all protective signaling was decreased in high-caloric diet, while Afriplex green rooibos extract supplementation reduced oxidative stress (c-Jun N-terminal kinases 1 and 2) and inflammation. Taken together, Afriplex green rooibos extract supplementation for 6 wk preconditioned cardiovascular disease-compromised rat hearts against ischemia/reperfusion injury by lowering inflammation, oxidative stress, and heart rate.

New Insights into the Efficacy of Aspalathin and Other Related Phytochemicals in Type 2 Diabetes-A Review

In the pursuit of bioactive phytochemicals as a therapeutic strategy to manage metabolic risk factors for type 2 diabetes (T2D), aspalathin, C-glucosyl dihydrochalcone from rooibos (Aspalathus linearis), has received much attention, along with its C-glucosyl flavone derivatives and phlorizin, the apple O-glucosyl dihydrochalcone well-known for its antidiabetic properties. We provided context for dietary exposure by highlighting dietary sources, compound stability during processing, bioavailability and microbial biotransformation. The review covered the role of these compounds in attenuating insulin resistance and enhancing glucose metabolism, alleviating gut dysbiosis and associated oxidative stress and inflammation, and hyperuricemia associated with T2D, focusing largely on the literature of the past 5 years. A key focus of this review was on emerging targets in the management of T2D, as highlighted in the recent literature, including enhancing of the insulin receptor and insulin receptor substrate 1 signaling via protein tyrosine phosphatase inhibition, increasing glycolysis with suppression of gluconeogenesis by sirtuin modulation, and reducing renal glucose reabsorption via sodium-glucose co-transporter 2. We conclude that biotransformation in the gut is most likely responsible for enhancing therapeutic effects observed for the C-glycosyl parent compounds, including aspalathin, and that these compounds and their derivatives have the potential to regulate multiple factors associated with the development and progression of T2D.

Therapeutic effects of an aspalathin-rich green rooibos extract, pioglitazone and atorvastatin combination therapy in diabetic db/db mice

Oral therapeutics used to treat type 2 diabetes and cardiovascular disease often fail to prevent the progression of disease and their comorbidities. Rooibos (Aspalathus linearis), an endemic South African plant used as an herbal tea, has demonstrated positive effects on glycemia and hypercholesterolemia. However, the treatment efficacy of rooibos extract in combination with conventional hypoglycemic and hypolipidemic medications on blood glucose and lipid profiles has not been established. This study aimed to investigate the effects of combining an aspalathin-rich green rooibos extract (Afriplex GRT™) with pioglitazone and atorvastatin, on blood glucose and lipid levels in obese diabetic (db/db) mice. Six-week-old male db/db mice and their nondiabetic lean littermate controls (db+) were divided into 8 experimental groups (n = 6/group). Db/db mice were treated daily either with pioglitazone (25 mg/kg), atorvastatin (80 mg/kg) and GRT (100 mg/kg), a combination of either drug with GRT or a combination of GRT-pioglitazone and atorvastatin for 5 weeks. Untreated vehicle controls were given dimethyl sulfoxide (0.1%) and phosphate buffered saline solution. At termination, serum and liver tissue were collected for lipid and gene expression analysis. Treatment with GRT, pioglitazone and atorvastatin combination effectively lowered fasting plasma glucose (FPG) levels in db/db mice (p = 0.02), whilst increasing body weight, liver weight, and reducing retroperitoneal fat weight. Atorvastatin monotherapy was effective at reducing cholesterol (from 4.00 ± 0.12 to 2.93 ± 0.13, p = 0.0003), LDL-C (from 0.58 ± 0.04 to 0.50 ± 0.00, p = 0.04), HDL-C (from 2.86 ± 0.05 to 2.50 ± 0.04, p = 0.0003) and TG (from 2.77 ± 0.50 to 1.48 ± 0.23, p = 0.04), compared to the untreated diabetic control. The hypotriglyceridemic effect of atorvastatin was enhanced when used in combination with both GRT and pioglitazone. The addition of pioglitazone to GRT significantly lowered FPG and TG. In db/db mice, Apoa1 was significantly downregulated in the liver, whilst Pparγ was significantly upregulated compared to their db+ counterparts. GRT monotherapy downregulated Apoa1 expression (p = 0.02). Atorvastatin combined with GRT significantly downregulated mRNA expression of Apoa1 (p = 0.03), whilst upregulating the expression of Pparγ (p = 0.03), Pparα (p = 0.002), Srebp1 (p = 0.002), and Fasn (p = 0.04). The GRT-pioglitazone-atorvastatin combination therapy downregulated Apoa1 (p = 0.006), whilst upregulating Fasn (p = 0.005), Pparα (p = 0.041), and Srebp1 (p = 0.03). Natural products can improve the efficacy of current drugs to prevent diabetes-associated complications. GRT in combination with pioglitazone enhanced the reduction of FPG, whilst the addition of atorvastatin to the combination, significantly lowered triglyceride levels. However, when GRT was used in combination with atorvastatin only cholesterol levels were affected. Although these results confirm both glucose- and lipoprotein-lowering biological effects of GRT in combination with pioglitazone and atorvastatin, increased expression of genes involved in lipogenesis, cholesterol, and fatty acid transport, β-oxidation, and synthesis and storage of fatty acids, may exacerbate the hepatotoxic effects of atorvastatin.

Comprehensive in vitro antidiabetic screening of Aspalathus linearis using a target-directed screening platform and cellomics

The antidiabetic potential of Aspalathus linearis has been investigated for over a decade, however, its characterisation remains incomplete with results scattered across numerous journals making the information difficult to compare and integrate. To explore whether any potential antidiabetic mechanisms for A. linearis have been neglected and to compare the suitability of extracts of green and "fermented" A. linearis as potential antidiabetic treatment strategies, this study utilised a comprehensive in vitro antidiabetic target-directed screening platform in combination with high content screening and analysis/cellomics. The antidiabetic screening platform consisted of 20 different screening assays that incorporated 5 well-characterised antidiabetic targets i.e. the intestine, liver, skeletal muscle, adipose tissue/obesity and pancreatic β-cells. Both the green and fermented extracts of A. linearis demonstrated very broad antidiabetic mechanisms as they revealed several promising activities that could be useful in combatting insulin resistance, inflammation, oxidative stress, protein glycation and pancreatic β-cell dysfunction and death - with a strong tendency to attenuate postprandial hyperglycaemia and the subsequent metabolic dysfunction which arises as a result of poor glycaemic control. The green extract was more successful at combatting oxidative stress in INS-1 pancreatic β-cells and enhancing intracellular calcium levels in the absence of glucose. Conversely, the fermented extract demonstrated a greater ability to inhibit α-glucosidase activity as well as palmitic acid-induced free fatty acid accumulation in C3A hepatocytes and differentiated L6 myotubes, however, further studies are required to clarify the potentially toxic and pro-inflammatory nature of the fermented extract.

The use of South African botanical species for the control of blood sugar

ETHNOPHARMACOLOGICAL RELEVANCE

Diabetes mellitus (DM) is one of the most prevalent diseases globally and is of considerable concern to global health. Approximately 425 million people are estimated to have DM globally and this is predicted to increase to >642 million by 2040. Whilst the prevalence of DM in South Africa is slightly lower than the global average, it is expected to rise rapidly in future years as more South Africans adopt a high calorie "westernised" diet. Traditional medicines offer an alternative for the development of new medicines to treat DM and the usage of South African plants is relatively well documented.

AIM OF THE STUDY

To critically review the literature on the anti-diabetic properties of South African plants and to document plant species used for the treatment of DM. Thereafter, a thorough examination of the related research will highlight where research is lacking in the field.

MATERIALS AND METHODS

A review of published ethnobotanical books, reviews and primary scientific studies was undertaken to identify plants used to treat DM in traditional South African healing systems and to identify gaps in the published research. The study was non-biased, without taxonomic preference and included both native and introduced species. To be included, species must be recorded in the pharmacopeia of at least one South African ethnic group for the treatment of DM.

RESULTS

One hundred and thirty-seven species are recorded as therapies for DM, with leaves and roots most commonly used. The activity of only 43 of these species have been verified by rigorous testing, and relatively few studies have examined the mechanism of action.

CONCLUSION

Despite relatively extensive ethnobotanical records and a diverse flora, the anti-diabetic properties of South African medicinal plants is relatively poorly explored. The efficacy of most plants used traditionally to treat DM are yet to be verified and few mechanistic studies are available. Further research is required in this field.

The Effect of Rooibos (Aspalathus linearis), Honeybush (Cyclopia intermedia) and Sutherlandia (Lessertia frutescens) on Testicular Insulin Signalling in Streptozotocin-Induced Diabetes in Wistar Rats

BACKGROUND

Testicular insulin signalling is altered in diabetic (DM) males. While unravelling the mechanism through which DM exert these detrimental effects, studies have shown the importance of insulin regulation in glucose homeostasis, and how a lack in insulin secretion indirectly led to reduced male fertility. The current study aimed to investigate the role of rooibos, honeybush and Sutherlandia on insulin signalling in the testicular tissue of type I diabetic rats.

METHODS

Animals (n=60) were randomly divided into six groups. The groups include a control group, a vehicle group, and diabetes was induced in the remainder of animals via a single intraperitoneal injection of STZ at 45mg/kg. The remaining four groups included a diabetic control (DC), diabetic + rooibos (DRF), diabetic + honeybush (DHB) and diabetic + Sutherlandia group (DSL). Animals were sacrificed after seven weeks of treatment, and blood and testes were collected.

RESULTS

All diabetic groups (DC, DRF, DHB, DSL) presented with a significant increase in blood glucose levels after diabetes induction compared to the control and vehicle (p<0.001). The DC animals presented with decreased testicular protein expression of IRS-1, PkB/Akt and GLUT4 compared to controls. DRF and DHB animals displayed an acute upregulation in IRS-1, while the DSL group showed improvement in IRS-2 compared to DC. Although, DRF animals presented with a decrease in PkB/Akt, DHB and DSL animals displayed upregulation (22.3%, 48%) compared to controls, respectively.

CONCLUSION

The results taken together, it can be suggested that these infusions may enhance insulin signalling through diverse pathways.

Transcriptomics of the Rooibos (Aspalathus linearis) Species Complex

Rooibos (Aspalathus linearis), widely known as a herbal tea, is endemic to the Cape Floristic Region of South Africa (SA). It produces a wide range of phenolic compounds that have been associated with diverse health promoting properties of the plant. The species comprises several growth forms that differ in their morphology and biochemical composition, only one of which is cultivated and used commercially. Here, we established methodologies for non-invasive transcriptome research of wild-growing South African plant species, including (1) harvesting and transport of plant material suitable for RNA sequencing; (2) inexpensive, high-throughput biochemical sample screening; (3) extraction of high-quality RNA from recalcitrant, polysaccharide- and polyphenol rich plant material; and (4) biocomputational analysis of Illumina sequencing data, together with the evaluation of programs for transcriptome assembly (Trinity, IDBA-Trans, SOAPdenovo-Trans, CLC), protein prediction, as well as functional and taxonomic transcript annotation. In the process, we established a biochemically characterized sample pool from 44 distinct rooibos ecotypes (1–5 harvests) and generated four in-depth annotated transcriptomes (each comprising on average ≈86,000 transcripts) from rooibos plants that represent distinct growth forms and differ in their biochemical profiles. These resources will serve future rooibos research and plant breeding endeavours.

Isoorientin: A dietary flavone with the potential to ameliorate diverse metabolic complications

Isoorientin is a natural C-glucosyl flavone that is generating a lot of interest due to its multiple pharmacological activities. Increasing experimental data have shown that the robust antioxidant and anti-inflammatory properties of isoorientin remain important in ameliorating a number of metabolic complications. In fact, plants rich in isoorientin have demonstrated strong ameliorative properties against complications such as hyperglycemia, hyperlipidemia, and insulin resistance. However, while such evidence is accumulating, it has not been reviewed to better inform on the therapeutic potential of this flavone in improving human health. This review examines and extrapolates available literature on the potential beneficial or detrimental effects associated with the use of isoorientin in mitigating metabolic diseases, with a specific focus on diabetes, obesity, and insulin resistance, including associated complications. The discussion includes effective doses in various experimental settings and proposed molecular mechanisms by which isoorientin may exert its therapeutic effects. In addition, the protective effects of extracts of a number of isoorientin-rich plants against metabolic complications will be highlighted.

Model development for predicting in vitro bio-capacity of green rooibos extract based on composition for application as screening tool in quality control

Mounting evidence of the ability of aspalathin to target underlying metabolic dysfunction relevant to the development or progression of obesity and type 2 diabetes created a market for green rooibos extract as a functional food ingredient. Aspalathin is the obvious choice as a chemical marker for extract standardisation and quality control, however, often the concentration of a single constituent of a complex mixture such as a plant extract is not directly related to its bio-capacity, i.e. the level of in vitro bioactivity effected in a cell system at a fixed concentration. Three solvents (hot water and two EtOH-water mixtures), previously shown to produce bioactive green rooibos extracts, were selected for extraction of different batches of rooibos plant material (n = 10). Bio-capacity of the extracts, tested at 10 μg ml^-1, was evaluated in terms of glucose uptake by C2C12 and C3A cells and lipid accumulation in 3T3-L1 cells. The different solvents and inter-batch plant variation delivered extracts ranging in aspalathin content from 54.1 to 213.8 g kg^-1. The extracts were further characterised in terms of other major flavonoids (n = 10) and an enolic phenylpyruvic acid glucoside, using HPLC-DAD. The 80% EtOH-water extracts, with the highest mean aspalathin content (170.9 g kg^-1), had the highest mean bio-capacity in the respective assays. Despite this, no significant (P≥ 0.05) correlation existed between aspalathin content and bio-capacity, while the orientin, isoorientin and vitexin content correlated moderately (r≥ 0.487; P < 0.05) with increased glucose uptake by C2C12 cells. Various multivariate analysis methods were then applied with Evolution Program-Partial Least Squares (EP-PLS) resulting in models with the best predictive power. These EP-PLS models, based on all quantified compounds, predicted the bio-capacity of the extracts for the respective cell types with RMSECV values ≤ 11.5, confirming that a complement of compounds, and not aspalathin content alone, is needed to predict the in vitro bio-capacity of green rooibos extracts. Additionally, the composition of hot water infusions of different production batches of green rooibos (n = 29) at 'cup-of-tea' equivalence was determined to relate dietary supplementation with the extract to intake in the form of herbal tea.

Modulation of Glucose Metabolism by Leaf Tea Constituents: A Systematic Review of Recent Clinical and Pre-clinical Findings

Leaf teas are widely used as a purported treatment for dysregulated glucose homeostasis. The objective of this study was to systematically evaluate the clinical and cellular-metabolic evidence, published between January 2013 and May 2019, and indexed on PubMed, ScienceDirect, and Web of Science, supporting the use of leaf teas for this purpose. Fourteen randomized controlled trials (RCTs) (13 on Camellia sinensis teas) were included, with mixed results, and providing scant mechanistic information. In contrast, 74 animal and cell culture studies focusing on the pancreas, liver, muscle, and adipose tissue yielded mostly positive results and highlighted enhanced insulin signaling as a recurring target associated with the effects of teas on glucose metabolism. We conclude that more studies, including RCTs and pre-clinical studies examining teas from a wider variety of species beyond C. sinensis, are required to establish a stronger evidence base on the use of leaf teas to normalize glucose metabolism.

Pharmacokinetic Interaction of Green Rooibos Extract With Atorvastatin and Metformin in Rats

An aspalathin-rich green rooibos extract (Afriplex GRT™) has demonstrated anti-diabetic and hypolipidemic properties, while also moderately inhibiting CYP3A4 activity, suggesting a potential for herb-drug interaction. The present study, therefore, evaluated the effects of orally administered GRT on the pharmacokinetics of atorvastatin and metformin in Wistar rats. Wistar rats were orally treated with GRT (50 mg/kg BW), atorvastatin (40 mg/kg BW) or metformin (150 mg/kg BW) alone or 50 mg/kg BW GRT in combination with 40 mg/kg BW atorvastatin or 150 mg/kg BW metformin. Blood samples were collected at 0, 10, and 30 min and 1, 2, 4, 6, and 8 h and plasma samples obtained for Liquid chromatography-mass spectrometry (LC-MS/MS) analyses. Non-compartment and two-compartment pharmacokinetic parameters of atorvastatin and metformin in the presence or absence of GRT were determined by PKSolver version 2.0 software. Membrane transporter proteins, ATP-binding cassette sub-family C member 2 (Abcc2), solute carrier organic anion transporter family, member 1b2 (Slco1b2), ATP-binding cassette, sub-family B (MDR/TAP), member 1A (Abcb1a), and organic cation transporter 1 (Oct1) mRNA expression were determined using real-time PCR expression data normalized to β-actin and hypoxanthine-guanine phosphoribosyltransferase (HPRT), respectively. Co-administration of GRT with atorvastatin substantially increased the maximum plasma concentration (C_max) and area of the plasma concentration-time curve (AUC_0-8) of atorvastatin by 5.8-fold (p = 0.03) and 5.9-fold (p = 0.02), respectively. GRT had no effect on the plasma levels of metformin. GRT increased Abcc2 expression and metformin downregulated Abcb1a expression while the combination of GRT with atorvastatin or metformin did not significantly alter the expression of Slco1b1 or Oct1 did not significantly alter the expression of Sclo1b2 or Oct1. Co-administration of GRT with atorvastatin in rats may lead to higher plasma concentrations and, therefore, to an increase of the exposure to atorvastatin.

A histomorphometric study on the hepatoprotective effects of a green rooibos extract in a diet-induced obese rat model

Obesity, type two diabetes mellitus and insulin resistance are associated with increased oxidative stress and inflammation. Unfermented green rooibos is an aspalathin rich variant of traditional fermented rooibos (Aspalathus linearis) and has a high polyphenol content. The present study aimed to determine the histologically observable effects of a commercially produced, aspalathin-rich green rooibos extract, Afriplex GRT™ (GRE) in a diet-induced obese rat model. Male Wistar rats (N = 28) were randomly assigned to four study groups (n = 7): control (C), green rooibos (GRT), high-fat diet (HFD) and experimental (HFD-GRT) group. Body mass was determined prior to euthanasia and liver mass was determined after death. The left lateral lobe of the liver was processed to wax and stained using haematoxylin and eosin (H & E), Masson's trichrome stain, Gordons and Sweet's reticulin impregnation and periodic acid-Schiff stain. Frozen liver tissue sections were used for Oil red O staining. Morphometric quantification of steatosis, semiquantitative pathology grading and scoring were performed and verified by a veterinary histopathologist. A significant increase in body and liver mass was observed in the HFD groups while co-treatment with green rooibos significantly reduced both. The volume and area of steatosis were significantly increased in the HFD groups while the area of steatosis significantly reduced with green rooibos co-treatment. The percentage, location and type of steatosis as well as presence of inflammation and hepatocellular injury were reduced in the HFD group co-treated with GRE. These findings suggest that a GRE has potential as an anti-steatotic, anti-inflammatory and weight reducing agent in vivo.

Exploring African Medicinal Plants for Potential Anti-Diabetic Compounds with the DIA-DB Inverse Virtual Screening Web Server

Medicinal plants containing complex mixtures of several compounds with various potential beneficial biological effects are attractive treatment interventions for a complex multi-faceted disease like diabetes. In this study, compounds identified from African medicinal plants were evaluated for their potential anti-diabetic activity. A total of 867 compounds identified from over 300 medicinal plants were screened in silico with the DIA-DB web server (http://bio-hpc.eu/software/dia-db/) against 17 known anti-diabetic drug targets. Four hundred and thirty compounds were identified as potential inhibitors, with 184 plants being identified as the sources of these compounds. The plants Argemone ochroleuca, Clivia miniata, Crinum bulbispermum, Danais fragans, Dioscorea dregeana, Dodonaea angustifolia, Eucomis autumnalis, Gnidia kraussiana, Melianthus comosus, Mondia whitei, Pelargonium sidoides, Typha capensis, Vinca minor, Voacanga africana, and Xysmalobium undulatum were identified as new sources rich in compounds with a potential anti-diabetic activity. The major targets identified for the natural compounds were aldose reductase, hydroxysteroid 11-beta dehydrogenase 1, dipeptidyl peptidase 4, and peroxisome proliferator-activated receptor delta. More than 30% of the compounds had five or more potential targets. A hierarchical clustering analysis coupled with a maximum common substructure analysis revealed the importance of the flavonoid backbone for predicting potential activity against aldose reductase and hydroxysteroid 11-beta dehydrogenase 1. Filtering with physiochemical and the absorption, distribution, metabolism, excretion and toxicity (ADMET) descriptors identified 28 compounds with favorable ADMET properties. The six compounds—crotofoline A, erythraline, henningsiine, nauclefidine, vinburnine, and voaphylline—were identified as novel potential multi-targeted anti-diabetic compounds, with favorable ADMET properties for further drug development.

Aspalathin-Rich Green Rooibos Extract Lowers LDL-Cholesterol and Oxidative Status in High-Fat Diet-Induced Diabetic Vervet Monkeys

Type 2 diabetic patients possess a two to four-fold-increased risk for cardiovascular diseases (CVD). Hyperglycemia, oxidative stress associated with endothelial dysfunction and dyslipidemia are regarded as pro-atherogenic mechanisms of CVD. In this study, high-fat diet-induced diabetic and non-diabetic vervet monkeys were treated with 90 mg/kg of aspalathin-rich green rooibos extract (Afriplex GRT) for 28 days, followed by a 1-month wash-out period. Supplementation showed improvements in both the intravenous glucose tolerance test (IVGTT) glycemic area under curve (AUC) and total cholesterol (due to a decrease of the low-density lipoprotein [LDL]) values in diabetics, while non-diabetic monkeys benefited from an increase in high-density lipoprotein (HDL) levels. No variation of plasma coenzyme Q10 (CoQ₁₀) were found, suggesting that the LDL-lowering effect of Afriplex GRT could be related to its ability to modulate the mevalonate pathway differently from statins. Concerning the plasma oxidative status, a decrease in percentage of oxidized CoQ₁₀ and circulating oxidized LDL (ox-LDL) levels after supplementation was observed in diabetics. Finally, the direct correlation between the amount of oxidized LDL and total LDL concentration, and the inverse correlation between ox-LDL and plasma CoQ₁₀ levels, detected in the diabetic monkeys highlighted the potential cardiovascular protective role of green rooibos extract. Taken together, these findings suggest that Afriplex GRT could counteract hyperglycemia, oxidative stress and dyslipidemia, thereby lowering fundamental cardiovascular risk factors associated with diabetes.

Safety Assessment of Phytochemicals Derived from the Globalized South African Rooibos Tea ( Aspalathus linearis) through Interaction with CYP, PXR, and P-gp

Rooibos tea ( Aspalathus linearis) is a well-known South African herbal tea enjoyed worldwide. Limited reports indicate the potential of rooibos tea to alter the activity of certain cytochrome P450 (CYP450) isozymes. In this study, the phytochemical investigation of MeOH extract of A. linearis (leaves and stems) resulted in the isolation and characterization of 11 phenolic compounds. The MeOH extract exhibited significant inhibition of the major human CYP450 isozymes (CYP3A4, CYP1A2, CYP2D6, CYP2C9, and CYP2C19). The strongest inhibition was observed by the extract for CYP3A4 (IC₅₀ 1.7 ± 0.1 μg/mL) followed by CYP2C19 (IC₅₀ 4.0 ± 0.3 μg/mL). Among the tested phytochemicals, the most potent inhibitors were isovitexin on CYP3A4 (IC₅₀ 3.4 ± 0.2 μM), vitexin on CYP2C9 (IC₅₀ 8.0 ± 0.2 μM), and thermopsoside on CYP2C19 (IC₅₀ 9.5 ± 0.2 μM). The two major, structurally related compounds aspalathin and nothofagin exhibited a moderate pregnane-X receptor (PXR) activation, which was associated with increased mRNA expression of CYP3A4 and CYP1A2, respectively. These results indicate that a high intake of nutraceuticals containing rooibos extracts may pose a risk of herb-drug interactions when consumed concomitantly with clinical drugs that are substrates of CYP enzymes.

Aspalathin-Enriched Green Rooibos Extract Reduces Hepatic Insulin Resistance by Modulating PI3K/AKT and AMPK Pathways

We previously demonstrated that an aspalathin-enriched green rooibos extract (GRE) reversed palmitate-induced insulin resistance in C2C12 skeletal muscle and 3T3-L1 fat cells by modulating key effectors of insulin signalling such as phosphatidylinositol-4,5-bisphosphate 3-kinase/protein kinase B (PI3K/AKT) and AMP-activated protein kinase (AMPK). However, the effect of GRE on hepatic insulin resistance is unknown. The effects of GRE on lipid-induced hepatic insulin resistance using palmitate-exposed C3A liver cells and obese insulin resistant (OBIR) rats were explored. GRE attenuated the palmitate-induced impairment of glucose and lipid metabolism in treated C3A cells and improved insulin sensitivity in OBIR rats. Mechanistically, GRE treatment significantly increased PI3K/AKT and AMPK phosphorylation while concurrently enhancing glucose transporter 2 expression. These findings were further supported by marked stimulation of genes involved in glucose metabolism, such as insulin receptor (Insr) and insulin receptor substrate 1 and 2 (Irs1 and Irs2), as well as those involved in lipid metabolism, including Forkhead box protein O1 (FOXO1) and carnitine palmitoyl transferase 1 (CPT1) following GRE treatment. GRE showed a strong potential to ameliorate hepatic insulin resistance by improving insulin sensitivity through the regulation of PI3K/AKT, FOXO1 and AMPK-mediated pathways.

Aspalathus linearis (Rooibos) - a functional food targeting cardiovascular disease

Increasing consumer bias toward natural products and the considerable wealth of indigenous knowledge has precipitated an upturn in market-driven research into potentially beneficial medicinal plants. In this context, Aspalathus linearis (Rooibos) has been identified to be a promising candidate which may impact cardiovascular disease (CVD), which is one of the most widely studied chronic diseases of modern times. Despite these efforts, ischemic heart disease remains the number one cause of mortality globally. Apart from genetic predisposition and other aetiological mechanisms specific to particular types of CVD, co-factors from interlinked systems contribute significantly to disease development and the severity of its clinical manifestation. The bioactivity of Rooibos is directed towards multiple therapeutic targets. Experimental data to date include antioxidant, anti-inflammatory and anti-diabetic effects, as well as modulatory effects in terms of the immune system, adrenal steroidogenesis and lipid metabolism. This review integrates relevant literature on the therapeutic potential of Rooibos in the context of CVD, which is currently the most common of non-communicable diseases. The therapeutic value of whole plant extracts versus isolated active ingredients are addressed, together with the potential for overdose or herb-drug interaction. The body of research undertaken to date clearly underlines the benefits of Rooibos as both preventative and complementary therapeutic functional food in the context of CVD.

The Potential of South African Herbal Tisanes, Rooibos and Honeybush in the Management of Type 2 Diabetes Mellitus

Diabetes mellitus is a metabolic disease that can lead to high morbidity, mortality and long-term complications. Available treatment strategies, which are mainly based on treating hyperglycemia, with insulin and other pharmacological agents are not completely efficient and can even lead to development of unwanted side effects. Scientific evidence suggests that bioactive compounds from teas and other plant-based foods, which are known source of natural antioxidants, could be an attractive strategy to preferentially treat and manage type 2 diabetes mellitus (T2DM) and thus, have significant therapeutic implications. In this review, we attempt an in-depth analysis and discussion of the current progress in our understanding of the antidiabetic potential of two commercialized South Africa herbal tisanes—Rooibos and Honeybush and their polyphenols.

Ethanolic Extract of Folium Sennae Mediates the Glucose Uptake of L6 Cells by GLUT4 and Ca2

In today’s world, diabetes mellitus (DM) is on the rise, especially type 2 diabetes mellitus (T2DM), which is characterized by insulin resistance. T2DM has high morbidity, and therapies with natural products have attracted much attention in the recent past. In this paper, we aimed to study the hypoglycemic effect and the mechanism of an ethanolic extract of Folium Sennae (FSE) on L6 cells. The glucose uptake of L6 cells was investigated using a glucose assay kit. We studied glucose transporter 4 (GLUT4) expression and AMP-activated protein kinase (AMPK), protein kinase B (PKB/Akt), and protein kinase C (PKC) phosphorylation levels using western blot analysis. GLUT4 trafficking and intracellular Ca²⁺ levels were monitored by laser confocal microscopy in L6 cells stably expressing IRAP-mOrange. GLUT4 fusion with plasma membrane (PM) was observed by myc-GLUT4-mOrange. FSE stimulated glucose uptake; GLUT4 expression and translocation; PM fusion; intracellular Ca²⁺ elevation; and the phosphorylation of AMPK, Akt, and PKC in L6 cells. GLUT4 translocation was weakened by the AMPK inhibitor compound C, PI3K inhibitor Wortmannin, PKC inhibitor Gö6983, G protein inhibitor PTX/Gallein, and PLC inhibitor U73122. Similarly, in addition to PTX/Gallein and U73122, the IP3R inhibitor 2-APB and a 0 mM Ca²⁺-EGTA solution partially inhibited the elevation of intracellular Ca²⁺ levels. BAPTA-AM had a significant inhibitory effect on FSE-mediated GLUT4 activities. In summary, FSE regulates GLUT4 expression and translocation by activating the AMPK, PI3K/Akt, and G protein–PLC–PKC pathways. FSE causes increasing Ca²⁺ concentration to complete the fusion of GLUT4 vesicles with PM, allowing glucose uptake. Therefore, FSE may be a potential drug for improving T2DM.

A Beneficial Role of Rooibos in Diabetes Mellitus: A Systematic Review and Meta-Analysis

In a rapid increase in cases of diabetes mellitus worldwide, there has been interested in the use of plant-derived polyphenols as nutraceuticals to prevent the onset and progression of diabetes mellitus and its associated complications. Aspalathus linearis, commonly known as rooibos, is a rich source of uncommon glycosylated plant polyphenols with various critical health-promoting properties, including the prevention and treatment of diabetes mellitus (DM). This study aimed to examine these effects by meta-analyzing the current evidence in diabetic rodent models. Peer-reviewed studies written in English from two databases, PubMed and Embase, were searched up to 28 February 2018. Studies reporting blood glucose levels in diabetic rodents with and without receiving rooibos extracts or their major phenolic compounds are included. Twelve studies enrolling 88 diabetic rodents treated with rooibos extracts or their polyphenols and 85 diabetic control males reported blood glucose levels. The pooled effect size was -0.89 (95% CI: -1.44 to -0.35) with a substantial heterogeneity (I² = 67.0%). This effect was likely to be modified by type of rooibos extracts and their polyphenols and treatment period. Blood glucose levels were significantly lower in diabetic rodent models treated with the phenolic compound rich in rooibos extracts, PPAG.

Metabolic Characterization of the Anthocyanidin Reductase Pathway Involved in the Biosynthesis of Flavan-3-ols in Elite Shuchazao Tea (Camellia sinensis) Cultivar in the Field

Anthocyanidin reductase (ANR) is a key enzyme in the ANR biosynthetic pathway of flavan-3-ols and proanthocyanidins (PAs) in plants. Herein, we report characterization of the ANR pathway of flavan-3-ols in Shuchazao tea (Camellia sinesis), which is an elite and widely grown cultivar in China and is rich in flavan-3-ols providing with high nutritional value to human health. In our study, metabolic profiling was preformed to identify two conjugates and four aglycones of flavan-3-ols: (-)-epigallocatechin-gallate [(-)-EGCG], (-)-epicatechin-gallate [(-)-ECG], (-)-epigallocatechin [(-)-EGC], (-)-epicatechin [(-)-EC], (+)-catechin [(+)-Ca], and (+)-gallocatechin [(+)-GC], of which (-)-EGCG, (-)-ECG, (-)-EGC, and (-)-EC accounted for 70-85% of total flavan-3-ols in different tissues. Crude ANR enzyme was extracted from young leaves. Enzymatic assays showed that crude ANR extracts catalyzed cyanidin and delphinidin to (-)-EC and (-)-Ca and (-)-EGC and (-)-GC, respectively, in which (-)-EC and (-)-EGC were major products. Moreover, two ANR cDNAs were cloned from leaves, namely CssANRa and CssANRb. His-Tag fused recombinant CssANRa and CssANRb converted cyanidin and delphinidin to (-)-EC and (-)-Ca and (-)-EGC and (-)-GC, respectively. In addition, (+)-EC was observed from the catalysis of recombinant CssANRa and CssANRb. Further overexpression of the two genes in tobacco led to the formation of PAs in flowers and the reduction of anthocyanins. Taken together, these data indicate that the majority of leaf flavan-3-ols in Shuchazao's leaves were produced from the ANR pathway.

Minimising variation in aspalathin content of aqueous green rooibos extract: optimising extraction and identifying critical material attributes

BACKGROUND

High levels of aspalathin, an antidiabetic dihydrochalcone, in green rooibos underpins interest in the production of a standardised extract. Elements of a quality-by-design approach were applied to optimise extraction conditions, aiming at the delivery of a dry matter yield (DMY) ≥ 160 g kg^-1 and an extract with an aspalathin content (AC) ≥ 80 g kg^-1 .

RESULTS

Hot water extraction parameters, namely extraction time, extraction temperature and water-to-plant material ratio, were optimised for DMY and aspalathin extraction efficiency (AEE) using Design of Experiments. Good polynomial prediction models were obtained and multiresponse desirability plots indicated 37 min, 93 °C and 23:1 as optimal conditions. Even when using 30 min and 10:1 instead for practical reasons, the target DMY and AC values could be achieved with the caveat that plant material with an AC ≥ 30 g kg^-1 is used. Particle size distribution and stem content were identified as contributing to variation in the AC of raw material.

CONCLUSION

By setting raw material specifications in terms of AC, as well as applying practical optimum extraction conditions, 160 g kg^-1 extract with an AC ≥ 80 g kg^-1 could be consistently achieved from green rooibos plant material. © 2017 Society of Chemical Industry.

Hyperglycemia-induced oxidative stress and heart disease-cardioprotective effects of rooibos flavonoids and phenylpyruvic acid-2-O-β-D-glucoside

Diabetic patients are at an increased risk of developing heart failure when compared to their non-diabetic counter parts. Accumulative evidence suggests chronic hyperglycemia to be central in the development of myocardial infarction in these patients. At present, there are limited therapies aimed at specifically protecting the diabetic heart at risk from hyperglycemia-induced injury. Oxidative stress, through over production of free radical species, has been hypothesized to alter mitochondrial function and abnormally augment the activity of the NADPH oxidase enzyme system resulting in accelerated myocardial injury within a diabetic state. This has led to a dramatic increase in the exploration of plant-derived materials known to possess antioxidative properties. Several edible plants contain various natural constituents, including polyphenols that may counteract oxidative-induced tissue damage through their modulatory effects of intracellular signaling pathways. Rooibos, an indigenous South African plant, well-known for its use as herbal tea, is increasingly studied for its metabolic benefits. Prospective studies linking diet rich in polyphenols from rooibos to reduced diabetes associated cardiovascular complications have not been extensively assessed. Aspalathin, a flavonoid, and phenylpyruvic acid-2-O-β-D-glucoside, a phenolic precursor, are some of the major compounds found in rooibos that can ameliorate hyperglycemia-induced cardiomyocyte damage in vitro. While the latter has demonstrated potential to protect against cell apoptosis, the proposed mechanism of action of aspalathin is linked to its capacity to enhance the expression of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) expression, an intracellular antioxidant response element. Thus, here we review literature on the potential cardioprotective properties of flavonoids and a phenylpropenoic acid found in rooibos against diabetes-induced oxidative injury.

Potential of rooibos, its major C-glucosyl flavonoids, and Z-2-(β-D-glucopyranosyloxy)-3-phenylpropenoic acid in prevention of metabolic syndrome

Risk factors of type 2 diabetes mellitus (T2D) and cardiovascular disease (CVD) cluster together and are termed the metabolic syndrome. Key factors driving the metabolic syndrome are inflammation, oxidative stress, insulin resistance (IR), and obesity. IR is defined as the impairment of insulin to achieve its physiological effects, resulting in glucose and lipid metabolic dysfunction in tissues such as muscle, fat, kidney, liver, and pancreatic β-cells. The potential of rooibos extract and its major C-glucosyl flavonoids, in particular aspalathin, a C-glucoside dihydrochalcone, as well as the phenolic precursor, Z-2-(β-D-glucopyranosyloxy)-3-phenylpropenoic acid, to prevent the metabolic syndrome, will be highlighted. The mechanisms whereby these phenolic compounds elicit positive effects on inflammation, cellular oxidative stress and transcription factors that regulate the expression of genes involved in glucose and lipid metabolism will be discussed in terms of their potential in ameliorating features of the metabolic syndrome and the development of serious metabolic disease. An overview of the phenolic composition of rooibos and the changes during processing will provide relevant background on this herbal tea, while a discussion of the bioavailability of the major rooibos C-glucosyl flavonoids will give insight into a key aspect of the bioefficacy of rooibos.

Inhibitory Interactions of Aspalathus linearis (Rooibos) Extracts and Compounds, Aspalathin and Z-2-(β-d-Glucopyranosyloxy)-3-phenylpropenoic Acid, on Cytochromes Metabolizing Hypoglycemic and Hypolipidemic Drugs

Rooibos extract, due to its glucose and lipid lowering effects, has potential as a nutraceutical for improvement of metabolic dysfunction. Potential herb-drug interactions as a result of the use of natural products are of increasing concern. Cytochrome P450 enzymes, CYP2C8, CYP2C9, and CYP3A4, are important in the metabolism of hypoglycemic drugs, such as thiazolidinediones (TZDs) and sulfonylureas, and hypocholesterolemic drugs, such as atorvastatin. This study investigated the effects of rooibos extracts, prepared from "unfermented" and "fermented" rooibos plant material and two of the major bioactive compounds, Z-2-(β-d-glucopyranosyloxy)-3-phenylpropenoic acid (PPAG) and aspalathin (ASP), on Vivid^® recombinant CYP450 enzymes. Unfermented (GRT) and fermented (FRE) rooibos extracts inhibited the activity of CYP2C8 (7.69 ± 8.85 µg/mL and 8.93 ± 8.88 µg/mL, respectively) and CYP3A4 (31.33 ± 4.69 µg/mL and 51.44 ± 4.31 µg/mL, respectively) based on their respective IC₅₀ concentrations. Both extracts dose- and time-dependently inhibited CYP2C8 activity, but only time-dependently inhibited CYP2C9. CYP3A4 showed concentration-dependent inhibition by ASP, GRT, and FRE at 25, 50, and 100 µg/mL concentrations. ASP, GRT, and FRE time-dependently inhibited CYP3A4 activity with GRT and FRE showing a more potent time-dependent inhibition, comparable to erythromycin. These findings suggest that herb-drug interactions may occur when nutraceuticals containing rooibos extracts are co-administered with hypoglycemic drugs such as TZDs, sulfonylureas, and dyslipidemic drug, atorvastatin.

Antidiabetic effect of enterolactone in cultured muscle cells and in type 2 diabetic model db/db mice

Enterolactone (ENL) is formed by the conversion of dietary precursors like strawberry lignans via the gut microbiota. Urinary concentrations of lignan metabolites are reported to be significantly associated with a lower risk of Type 2 diabetes (T2D). In the present study, antidiabetic effect of ENL and its modes of action were studied in vitro and in vivo employing a rat skeletal muscle-derived cell line, L6 myocytes in culture, and T2D model db/db mice. ENL dose-dependently increased glucose uptake in L6 myotubes under insulin absent condition. This increase by ENL was canceled by compound C, an inhibitor of 5'-adenosine monophosphate-activated protein kinase (APMK). Activation (=phosphorylation) of AMPK and translocation of glucose transporter 4 (GLUT4) to plasma membrane in L6 myotubes were demonstrated by Western blotting analyses. Promotion by ENL of GLUT4 translocation to plasma membrane was also visually demonstrated by immunocytochemistry in L6 myoblasts that were transfected with glut4 cDNA-coding vector. T2D model db/db mice were fed the basal 20 % casein diet (20C) or 20C supplemented with ENL (0.001 or 0.01 %) for 6 weeks. Fasting blood glucose (FBG) levels were measured every week and intraperitoneal glucose tolerance test (IPGTT) was conducted. ENL at a higher dose (0.01 % in 20C) suppressed the increases in FBG levels. ENL was also demonstrated to improve the index of insulin resistance (HOMA-IR) and glucose intolerance by IPGTT in db/db mice. From these results, ENL is suggested to be an antidiabetic chemical entity converted from dietary lignans by gut microbiota.

Short-Term and Sub-Chronic Dietary Exposure to Aspalathin-Enriched Green Rooibos (Aspalathus linearis) Extract Affects Rat Liver Function and Antioxidant Status

An aspalathin-enriched green rooibos (Aspalathus linearis) extract (GRE) was fed to male Fischer rats in two independent studies for 28 and 90 days. The average dietary total polyphenol (TP) intake was 75.6 and 62.7 mg Gallic acid equivalents (GAE)/kg body weight (bw)/day over 28 and 90 days, respectively, equaling human equivalent doses (HEDs) of 12.3 and 10.2 GAE mg/kg bw/day. Aspalathin intake of 29.5 mg/kg bw/day represents a HED of 4.8 mg/kg bw/day (90 day study). Consumption of GRE increased feed intake significantly (p < 0.05) compared to the control after 90 days, but no effect on body and organ weight parameters was observed. GRE significantly (p < 0.05) reduced serum total cholesterol and iron levels, whilst significantly (p < 0.05) increasing alkaline phosphatase enzyme activity after 90 days. Endogenous antioxidant enzyme activity in the liver, i.e., catalase and superoxide dismutase activity, was not adversely affected. Glutathione reductase activity significantly (p < 0.05) increased after 28 days, while glutathione (GSH) content was decreased after 90 days, suggesting an altered glutathione redox cycle. Quantitative Real Time polymerase chain reaction (PCR) analysis showed altered expression of certain antioxidant defense and oxidative stress related genes, indicative, among others, of an underlying oxidative stress related to changes in the GSH redox pathway and possible biliary dysfunction.