In vitro assessment of antidiabetic properties of traditional medicinal plants: abrus precatorius L. and melastoma malabathricum L.

Diabetes mellitus is a chronic metabolic disorder associated with serious complications, multiple comorbidities, and high mortality. Driven by over-nutrition, increasingly sedentary lifestyles, rising obesity, and an aging population, diabetes has become a global pandemic and is predicted to affect over 700 million people by 2045. Current diabetes therapies, although effective in the short-term, lack long-term efficacy, have undesirable side-effects, and are inaccessible to patients in many parts of the world due to their high cost. Therefore, novel therapies and alternative strategies to improve the management of diabetes are urgently needed. Diabetes is primarily the result of insulin resistance (IR), especially in skeletal muscle and adipose tissue. Obesity and excess lipid accumulation is a significant cause of IR. IR impairs cellular glucose uptake and metabolism, leading to hyperglycaemia. Chronic hyperglycaemia has many detrimental effects, including increased oxidative stress, and it contributes to the long-term complications of diabetes. Improving insulin sensitivity, controlling blood glucose levels (BGL), preventing complications, and maintaining a healthy weight are the key goals of diabetes treatment. Plants have a long history of use in traditional medicine (TM) and approximately 80% of the global population rely on plants for their healthcare needs. Due to the richness of plant species, and the chemodiversity of phytochemicals they contain, plants are widely recognised as important sources of bioactive molecules and there are a multitude of documented phytochemical-derived therapeutics. Screening plants based on their ethnopharmacological use is an efficient approach for identifying plant species and bioactive phytochemicals with therapeutic potential. Abrus precatorius L. and Melastoma malabathricum L. var. alba are medicinal plants with reported ethnopharmacological use in the management of diabetes. While investigations in diabetic animal models have demonstrated that A. precatorius leaf extract lowers BGLs levels, the mechanistic details remain unknown. The antidiabetic potential of M. malabathricum var. alba has not been previously investigated. The primary aim of this research was to investigate the antidiabetic properties and the phytochemical composition of Abrus precatorius (APLE) and Melastoma malabathricum var. alba (MMLE) leaf extracts. Phytochemical profiling of APLE and MMLE (ethanol-water, 50:50) was carried out using liquid chromatography-high resolution mass spectrometry (LC-HRMS). In vitro models were utilised to investigate the effect of the extracts on insulin-stimulated glucose uptake and hyperglycaemia-induced mitochondrial superoxide levels in skeletal muscle and adipose tissue, and lipid accumulation in adipocytes. The inhibitory potential of the extracts on pancreatic α-amylase and α-glucosidase was assessed using spectrometric assays. Potential hepatotoxicity was evaluated in HepG2 hepatocytes. This research reports, for the first time, the key modes of action and molecular mechanisms responsible for the antidiabetic activity of APLE and MMLE include 1) the upregulation of insulin receptor substrate-1 (IRS-1) gene expression leading to increased insulin-stimulated glucose uptake via the Akt/PI3K pathway in C2C12 myotubes and 3T3-L1 adipocytes, 2) a reduction in hyperglycaemia-induced mitochondrial superoxide (MSO) levels, and 3) the inhibition of lipid accumulation in 3T3-L1 adipocytes. In addition. 24 and 13 phytochemicals were putatively identified from APLE and MMLE, respectively, which have not previously been reported in these plant species. LC-HRMS analysis putatively identified 55, and 54 phytochemicals from APLE and MMLE, respectively. Some of these phytochemicals have multiple antidiabetic properties documented in the literature. These antidiabetic phytochemicals included glycyrrhizin, hispidulin, cirsimaritin, and the flavonoids diosmetin, diosmin, glycitein, hispidulin, naringenin, naringin, and narirutin from APLE; and astragalin, β-sitosterol, terpenes, tannins, and anthocyanins from MMLE. APLE and MMLE significantly increased insulin-stimulated glucose uptake in C2C12 myotubes and 3T3-L1 adipocytes. In myotubes, the insulin-sensitising effect of the extracts exceeded that of rosiglitazone. qPCR analysis revealed that the improvement in insulin-stimulated glucose uptake produced by APLE in myotubes and adipocytes was primarily due to upregulated IRS-1 gene expression which leads to increased GLUT4 translocation via the Akt/PI3K pathway. Similarly, MMLE appeared to promote adipocyte glucose uptake via the upregulation of IRS-1. In myotubes, however, MMLE-induced glucose uptake was likely due to the upregulation of both IRS-1 and GLUT4 gene expression. While the extracts had no effect on MSO in the myotubes, both extracts significantly reduced hyperglycaemia-induced MSO levels in 3T3-L1 adipocytes. MMLE normalised MSO to baseline levels. The ability of MMLE to reduce MSO levels was superior to the mitochondria-targeted antioxidant MitoTEMPO. Both extracts also significantly reduced lipid accumulation in 3T3-L1 adipocytes. In addition, APLE inhibited both α-amylase (mild) and α-glucosidase, and MLE inhibited α-amylase. The α-glucosidase-inhibitory activity of APLE was comparable to acarbose. Both extracts were non-cytotoxic to HepG2 cells up to a concentration of 500 µg/mL indicating that they are unlikely to be hepatotoxic in vivo. The findings of this research demonstrate that APLE and MMLE have several antidiabetic properties offering multiple therapeutic benefits for managing diabetes. Both plants improved myotube and adipocyte insulin sensitivity and increased insulin-stimulated glucose uptake which is beneficial for reducing BGLs. Both plants also reduced hyperglycaemia-induced MSO and inhibited lipid accumulation and can therefore improve insulin sensitivity. The reduction in MSO may prevent diabetes-related oxidative stress and hyperglycaemic complications such as diabetic retinopathy, nephropathy, and neuropathy. The inhibition of lipid accumulation may prevent hypertrophic obesity, reduce ectopic lipid accumulation and lipid-induced inflammation, and thus improve insulin sensitivity. Further, the inhibition of α-glucosidase and α-amylase may prevent postprandial hyperglycaemia. Future research is recommended to explore the effects of these plants on the 5' adenosine monophosphate-activated protein kinase (AMPK) pathway; hepatic gluconeogenesis and glycogenesis pathways; adipogenesis, lipolysis, and thermogenic pathways; and on lipid accumulation in liver and skeletal muscle. Additionally, bioassay-guided fractionation in combination with comprehensive phytochemical profiling must be conducted for the definitive identification of phytochemicals responsible for the bioactivities reported in this thesis. The findings of this research support the previously reported in vivo antidiabetic activity of A. precatorius and M. malabathricum and contribute to the understanding of how these plants work. This research also provides scientific evidence for the ethnopharmacological use of these plants which may increase their acceptance in medicinal use benefiting billions of people using TM. The outcomes of this research will direct the discovery of phytochemicals for the potential development of antidiabetic therapies and form the basis for developing cost-effective nutraceuticals for the prevention and management of diabetes as well as obesity.