A comparative study of the in vitro antidiabetic properties, cytotoxicity and mechanism of action of Albuca bracteata and Albuca setosa bulb extracts

Abstract

The search for cheap, non toxic and readily available antidiabetic drugs has been a challenge for researchers and the pharmaceutical industries. Diabetes mellitus is a metabolic disease characterized by defects in the synthesis of insulin and/or insensitivity to the action of insulin at the target cells. The disease has been on the increase mostly in developing countries where large proportions of the population have little access to good medical care due to either accessibility or non availability of synthetic drugs. This has led to the use of medicinal plants to treat diabetes because it is safe, cheap and with few side effects. There is little scientific evidence on the dosages, active compounds, mechanisms of action and toxicity of these traditionally used plants. Two of the most frequently used plants; Albuca setosa and Albuca bracteata were investigated in this study. The qualitative analysis of different extractions of these plants revealed the presence of phenolics, alkaloids, tannins and saponins. The antioxidant properties of aqueous, acetone and methanollic extracts of Albuca setosa and Albuca bracteata were investigated using models such as Diphenyl-1-Picrylhydrazyl (DPPH), 2, 2’-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), Ferric ion reducing antioxidant potential (FRAP), Nitric Oxide and Hydrogen Peroxide (H2O2). Both plants revealed inhibitions against DPPH in a concentration - dependent manner with Albuca setosa (0.330 mg/ml) showing higher activity than Albuca bracteata (0.647 mg/ml) determined from the IC50. The aqueous extract of Albuca setosa showed a higher inhibition against DPPH radical compared to the Albuca bracteata aqueous extract at all concentrations investigated. The isolated saponins from Albuca bracteata had a higher DPPH scavenging activity than the crude methanolic extract of the plant in a concentration - dependent manner but are significantly different from each other at 0.4, 0.6 and 1.0 mg/ml only. The IC50 of the saponins was also observed to be higher than the crude extracts and standards. The Albuca setosa aqueous extract showed a higher percentage inhibition of ABTS radicals than Albuca bracteata at all the concentrations investigated. Overall, the Albuca setosa aqueous extract (0.0809 mg/ml) showed maximum activity against ABTS radicals. The iron reducing power was significantly higher (P < 0.05) in the methanolic extract of both plants compared to the aqueous counterpart. Overall, the Albuca bracteata aqueous extract (0.344 mg/ml) showed maximum activity as indicated by the IC50. The aqueous extracts of both plants also revealed percentage inhibitions in a concentration - dependent manner against NO2. The aqueous extract of Albuca bracteata bulb was more active against nitric oxide and hydrogen peroxide inhibition. In this study, the cytotoxicity of the extracts was evaluated at a high dose of 100 μg/ml on Chang liver cells and determined using MTT, crystal violet, glucose consumption, lactate production and lactate dehydrogenase release and FRAP. The aqueous extracts of both Albuca setosa and Albuca bracteata were non-toxic on Chang liver cells at the concentrations investigated. The MTT revealed that the aqueous extract of Albuca setosa bulb had the optimum cell viability of 108.09% while the acetonic extract of Albuca bracteata showed the least cell viability (37.72%) compared with the control. The crystal violet test also revealed the acetone extract of Albuca bracteata to have the least percentage of cell viability at 31.47%, while the aqueous extract of Albuca setosa showed the maximum cell viability at 112.5%.The aqueous extracts of both plants showed higher percentage cell density on the second day of incubation from the proliferation assay. All the tested samples were observed to consume more glucose than the blank except for the methanollic and acetone extracts of Albuca bracteata bulb. The aqueous and methanolic extracts of Albuca setosa bulbs produced the highest lactate with 120.2 μg/ml and 113.7 μg/ml respectively. The acetone extracts of both Albuca setosa and Albuca bracteata revealed toxicity with a higher lactate dehydrogenase release compared to the control. The aqueous extracts of both Albuca setosa and Albuca bracteata bulbs showed reducing power of 18.7 and 22.1 μmol/l of FeSO4 respectively. The in vitro antidiabetic potentials investigated revealed that the aqueous extracts of both plants inhibited the activity of α- glucosidase but have weak inhibition against α- amylase. The acetone extract of Albuca bracteata showed the highest inhibition against the alpha amylase followed by the acetone extract of Albuca setosa. Both plant extracts also showed weak inhibition against the dipeptidylpeptidase-IV (DPP-IV) at the concentration investigated. The glucose uptake in HepG2 for both aqueous and methanol extracts of Albuca setosa were not significantly different from the control but the MTT result of the treatments in HepG2 for both Albuca setosa and Albuca bracteata aqueous extracts revealed low cell density compared to the control. The aqueous extract of Albuca setosa demonstrated glucose utilization in L6 cells with a response of 188.93% of the control at 25 μg/ml while the aqueous extract of Albuca bracteata demonstrated glucose utilisation in L6 with a response of 89.17% of the control. The MTT of the treatments in L6 revealed no sign of cytotoxicity, as the cell density of all the treatments were not significantly different from the control. The aqueous extracts of both Albuca setosa and Albuca bracteata also showed high glucose utilization in 3T3-L1 cells with 123.21% and 131.56% respectively. In conclusion, Albuca setosa possesses better in vitro strong antidiabetic activity compared to Albuca bracteata. The mechanism of glucose utilization in L6 and 3T3-L1 of Albuca setosa and Albuca bracteata may be due to the activation of some molecules in the insulin signaling pathway, while the in vitro antidiabetic activity of Albuca bracteata may also involve its inhibitory effect on α-glucosidase. The antioxidant properties of both plants may also play an important role in ameliorating the complications of diabetes. These findings therefore support the folkloric usage of these plants for the management of diabetes mellitus.