Enhancement of DPP-IV Inhibitory Activity and GLP-1 Release Through RADA16-assisted Molecular Designed Rapeseed Peptide Nanogels

Objectives To produce and evaluate the microstructure and rheological behavior of a nanogel of pentapeptide IPQVS (RAP1) and octopeptide ELHQEEPL (RAP2), derived from rapeseed napin, through RADA16-assisted molecular design. In addition, to determine in vitro the dipeptidyl-peptidase IV (DPP-IV) inhibitory properties, glucagon-like peptide-1 (GLP-1) secretion through activation of calcium-sensing receptor (CaSR) and the intracellular calcium ion mobilization and receptor protein cAMP concentration. Methods An innovative nanogel, which loaded the RAP1 and RAP2 into RADA16 scaffold, respectively, was synthesized. The linker of double Gly was used in the connection of RADA16 and the functional oligopeptide region. DPP-IV inhibitory activity was evaluated in the Caco-2 cell monolayer. The microstructure and rheological behavior of RADA16-RAP1 and RADA16-RAP2 were characterized. The GLP-1 secretion through activation of CaSR receptor and the intracellular calcium ion mobilization and cAMP concentration were determined in STC-1 cells. Results DPP-IV inhibitory activity was reduced (more potent) by over 65% in the presence of 250 μM of the two formed nanogels RADA16–GG-IPQVS and RADA16- GG-ELHQEEPL (p < 0.05). The two nanogel peptides had high stability at low temperature or body temperature and high dispersibility in water. Stable β-sheet structures increased by 5.6-fold and 5.2-fold, respectively, than the original oligopeptides, with a self-assembled fibrous morphology. The nanogels RADA16-RAP1 and RADA16-RAP2 might exhibit good rheological properties for potential injectable applications; storage modulus (G’) was 10 times higher than low modulus (G’’). Furthermore, the RAP2 and its RADA16-assisted nanogel peptide at the concentration of 250 μM significantly (p < 0.05) increased the release of GLP-1 by 35.46% through calcium-sensing receptor (CaSR) pathway in the enteroendocrine STC-1 cells. Conclusions The innovated nanogel with the sequence of RADA16-GG-Xn has the possibility of oral and injection to management type 2 diabetes. DPP-IV inhibitory peptides IPQVS and ELHQEEPL can form a nanogel with high water content through the self-assembly gelation of RADA16. They may attach to the brush border of human intestinal epithelial cells to inhibit DPP-IV in the intestinal lumen. Funding Sources USDA-HATCH [grant number 1017440].