Structure of a HG-type pectin from lotus seedpod (Receptaculum Nelumbinis) and its anti-diabetic effect via the FoxO signaling pathway, metabolites, and gut microbiota in streptozotocin-induced rats

In this study, we aimed to purify a fraction (LSP2) from a previously obtained refined lotus seedpod polysaccharide, characterize its structure, and evaluate its anti-diabetic effect in streptozotocin-induced rats. Its mechanism of action was investigated using multi-omics approaches. Results showed that LSP2 was a pectin with a molecular weight of 14.619 kDa, primarily composed of galacturonic acid (GalA, 73.49%). LSP2 was identified as a high-ester HG-type pectin, with a backbone consisting of →4)-α-D-GalpA-6-OMe-(1→ and →4)-α-D-GalpA-(1→ residues in an approximate molar ratio of 2:1. LSP2 exhibited scavenging activities on 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), 1,1-diphenyl-2-picryl-hydrazyl (DPPH), and hydroxyl (OH) radicals, along with α-glucosidase activity inhibitory effect (half-maximal inhibitory concentration: 0.193 mg/mL). LSP2 alleviated hyperglycemia and dyslipidemia, and enhanced insulin secretion and antioxidant levels in diabetic rats. Transcriptomic and western blot analyses showed that LSP2 altered gene expressions of liver tissues, notably downregulating the forkhead box O (FoxO) signaling pathway, as evidenced by marked reductions in protein expressions of hub targets FOXO1, phosphoenolpyruvate carboxykinase 1 (PCK1), and glucose-6-phosphatase catalytic subunit (G6PC). Metabolomic profiling revealed that LSP2 altered liver and fecal metabolisms, with eight and eleven metabolic biomarkers, respectively (e.g., 1-stearoyl-2-hydroxy-sn-glycero-3-phosphocholine, SOPC, phosphorylcholine; glutamic acid, L-glutamine, tryptophan), and significantly modulated four and six metabolic pathways respectively (e.g., choline metabolism in cancer, biosynthesis of unsaturated fatty acids, linoleic acid metabolism; protein digestion and absorption, glutathione metabolism, glutamatergic synapse). 16S rRNA sequencing showed that LSP2 regulated fecal gut microbiota in, particularly affecting ten differential genera bacteria (Muribaculaceae, Streptococcus, Lactobacillus, etc.). Significant correlations were observed among differential metabolites, differential gut microbiota and biochemical indexes (FBG, TG, HDL-C, liver SOD, liver MDA, FOXO1, PCK1, and G6PC) affected by LSP2. These findings suggest that LSP2 exerts an anti-diabetic effect in streptozotocin-induced rats, potentially through regulation of the gut-liver axis. This study supports the potential of LSP2 in diabetes mellitus intervention.