Natural product inhibitors of Acetyl-CoA Carboxylase – A drug target in Type 2 Diabetes Mellitus

Abstract

Type-2 diabetes mellitus (T2DM) is approaching epidemic proportions and threatens to become a global health issue, representing 9.9% of mortality worldwide. Raised plasma fatty acids are associated with obesity and are a cause of insulin resistance in T2DM. Consequently, there is a growing need to search for new, more efficient therapeutic approaches that not only decrease blood glucose, but also reduce the insulin resistance of diabetic patients. Acetyl-CoA carboxylase (ACC) is a crucial enzyme of fatty acid metabolism in mammals and consists of two isoforms. It catalyses the formation of malonyl-CoA, the precursor of fatty acid synthesis (ACC1) and inhibitor of fatty acid oxidation (ACC2). Recent studies in mice deficient in one of two main ACC isoforms (ACC2) demonstrated increased fatty acid oxidation with markedly improved insulin sensitivity. Therefore, ACC inhibitors represent a viable approach for the treatment of T2DM. Natural products have been recognised as a rich source of compounds with structural diversity for therapeutical potential. The aim of this study was to screen Middle Eastern plant extracts and identify their bioactive compounds responsible for ACC1/ACC2-inhibitory effects. Medicinal plants are well recognised as a source of therapeutic agents used by traditional healers and indigenous people in the treatment of various diseases based on ethnobotanical evidence. Eight Middle Eastern plants were selected for their published use to treat diabetes including Teucrium polium L., Crataegus azarolus L, Opuntia ficus-indica, Rosa damascena Mill, Achillea arabica kotschy, Pistacia falcata Becc. Ex Martell, Acacia ehrenbergiana Hayne, and Acacia asak (forss K). The plant extracts were prepared using sequential solvent extraction, and then assayed for inhibitory activity against purified ACC1 and ACC2 in vitro. The chloroform extract of A. Arabica and the methanolic extracts of R. Damascena, C. Azarolus and A. Asak showed the most promising ACC inhibition, so were subjected to chemical characterisation using LC-ESI-MS/MS. Flavonoids, acylated spermidines, and benzoic acid derivatives were tentatively identified in the methanolic extract of Rosa damascena, while the chloroform extract of Achillea arabica contained sesquiterpene lactones and methylated flavones. Crataegus azarolus predominantly contained procyanidins and sugars, while Acacia asak was rich in methylated flavonoids, dihydroflavonols, and hydroxybenzoic acid derivatives. Bioguided fractionation was performed on the selected active extracts employing partition chromatography, solid phase extraction (SPE) and thin layer chromatography. LC-ESI-MS/MS and NMR analysis of the sub-fractions shown to inhibit ACC1 led to the identification of tri-p-coumaroyl spermidine, astragalin, afzelin and tiliroside from Rosa damascena; hyperoside, procyanidin B2, catechin and epicatechin from Crataegus azarolus; taxifolin, isorhamnetin, aromadendrin and rhamnocitrin from Acacia asak and chrysosplenol D from Achillea arabica. This study supports the anti-diabetic potential of isolates or fractions from the selected Middle Eastern plants as evidenced by their ability to inhibit ACC, a key anti-diabetic drug target. The investigation of the compounds identified as potential lead compounds for the discovery of anti-T2DM drugs would a desirable consequence of the current study.