Rooibos phytochemicals as beta cell protectants

Over 537 million people worldwide have type 2 diabetes (T2D) and this number is still rising. T2D is characterized by insulin resistance in peripheral tissues like the liver, adipose tissue and muscles.
These organs will require more insulin secretion in order to take up glucose from the blood. In an effort to compensate, the β cells will secrete more insulin which is reflected in a first stage of the disease: hyperinsulinemia and hyperglycemia. Chronic hyperglycemia will eventually lead to β cell toxicity, which is further aggravated by elevated levels of fatty acids and oxidative stress. The continuous synthesis and secretion of insulin will lead to ER stress which also contributes to β cell death. On the long term, this will reduce the β cell mass and the insulin secretion capacity will be decreased. This brings us to the second stage of the disease: hypoinsulinemia and hyperglycemia.
In contrast to other cell types, β cells have a low expression of antioxidant enzymes which makes them prone for high glucose and free fatty acid induced oxidative stress. Oxidative stress is one of the major contributors in the development of T2D. Despite the discovery of new insulin sensitizing agents, today’s treatment still relies on symptomatic alleviation of the disease: insulin therapy or lowering glycemic levels in the blood. Despite some improvements in drug development, there is still a great need for more effective treatments, in particular treatments that include β cell protection.
Since the pancreas has a weak defense mechanism against oxidative stress, it is suggested that external addition of antioxidant molecules may be used for the treatment and prevention of T2D.
Many new drugs are derived from natural products. Aspalathus linearis (Rooibos) is a plant endemic to South Africa. Rooibos tea is already used for many years in traditional medicine. The importance of this plant has grown upon the discovery of its antioxidative effects. The main polyphenol present in the Rooibos plant is aspalathin, a dihydrochalcone glycoside. We demonstrated that aspalathin
protects INS1E β cells and primary rat islets against oxidative stress and glucotoxicity by upregulating antioxidant genes including superoxide dismutase 1 (Sod1), heme oxygenase 1 (Hmox1) and NAD(P)H quinone dehydrogenase 1 (Nqo-1), via the NRF2 (Nuclear Factor-E2-Related Factor 2)/ KEAP1 (Kelch-Like Epichlorohydrin-Associated Protein 1) pathway. One of the proposed mechanisms of NRF2/KEAP1 activation is that aspalathin leads to KEAP1 sequestering by P62 protein and thus leading to release and nuclear translocation of NRF2.
Isoorientin, a metabolite of aspalathin, might also yield anti-diabetic properties. Furthermore, it was reported that aglycones are more easily taken up by the β cell. Thus, the protective effect of aspalathin was compared with its metabolite isoorientin, its precursor and aglycone 3-hydroxyphloretin and a standardized Rooibos extract GRT against different stress inducing agents:
streptozotocin, H2O2 and palmitate (lipotoxicity) in the INS1E β cell line, while also including the antidiabetic drug exendin-4 and ROS scavenger NAC as reference molecules. We demonstrated protection against oxidative stress with the molecules aspalathin, 3-hydroxyphloretin and GRT, whereas aspalathin’s metabolite isoorientin did not offer any protection. Furthermore, these
molecules also showed to be protective against lipotoxicity-induced β cell death. Their protection was associated with the upregulation of NRF2/KEAP1/ARE pathway related genes such as Sod1, Hmox1 and Nqo-1 and the downregulation of Txnip mRNA. The latter indicates that also the thioredoxin antioxidant system was involved, which is associated with enhanced antioxidant defense and β cell
survival. Aspalathin, 3-hydroxyploretin and GRT potently downregulated Txnip expression. Lastly, it was shown that nor aspalathin, nor 3-hydroxyploretin inhibited the GLUT2 transporter, which indicates that their protection was not due to GLUT2 transporter inhibition during streptozotocin challenge.
Lastly, quercetin, a naturally occurring flavonoid in many vegetable foods, was investigated for its cytoprotective potential in the INS1E β cell line. Quercetin completely abolished H2O2-induced β cell death, compared to aspalathin, and offered also significant protection against streptozotocin-induced β cell death. However, its main metabolite quercetin-3-O-glucoronide, did not offer any protection, attributing the protective effect solely to quercetin itself. Quercetin was shown to induce NRF2 translocation with its subsequent activation of ARE response genes: Nqo-1 and Hmox1. Furthermore, quercetin led to a significant downregulation of Txnip, which is being recognized as a master regulator
in β cell survival and function. Quercetin was not found to inhibit the GLUT2 transporter and showed to be taken up by the β cells in a dose-dependent way as was demonstrated through DPBA staining.
Taken together, administration of these molecules protect β cells against oxidative stress-induced β cell death and lipotoxicity, with quercetin and aspalathin exerting the highest protection. Both molecules induce the NRF2/KEAP1 antioxidant pathway and downregulate Txnip. Further research is
needed to elucidate the exact working mechanisms of these molecules, especially in vivo. It can be concluded that they are promising nutraceuticals to be used in the prevention and treatment of T2D.