Inhibitory Mechanism of Epigallocatechin Gallate on Fibrillation and Aggregation of Amidated Human Islet Amyloid Polypeptide.

The abnormal fibrillation of human islet amyloid polypeptide (hIAPP) is associated with development of type II diabetes mellitus (T2DM). (-)-Epigallocatechin gallate (EGCG) can bind amyloid proteins to inhibit the fibrillation of these proteins. However, the mechanic detail of EGCG inhibiting amyloid formation is still unclear at the molecular level. In the present work, we sought to investigate the effect of EGCG on amidated hIAPP (hIAPP-NH2 ) fibrillation and aggregation by using spectroscopic and microscopic techniques, and also sought to gain insights into the interaction of EGCG and hIAPP22-27 by using spectroscopic experiments and quantum chemical calculations. ThT fluorescence, real-time NMR, and TEM studies demonstrated that EGCG inhibits the formation of hIAPP-NH2 fibrils, while promoting the formation of hIAPP-NH2 amorphous aggregates. Phenylalanine intrinsic fluorescence and NMR studies of the EGCG/hIAPP22-27 complex revealed three important binding sites including the A ring of EGCG, residue Phe23, and residue Ile26. DFT calculations identified the dominant binding structures of EGCG/Phe23 and EGCG/Ile26 complexes, named structure I and structure II, respectively. Our study demonstrates the inhibitory mechanism of EGCG on fibrillation and aggregation of hIAPP-NH2 in which EGCG interacts with hIAPP-NH2 through hydrogen bonding and π-π interactions between the A ring and residue Phe23 as well as hydrophobic interactions between the A ring and residue Ile26, which can thus inhibit the interpeptide interaction between hIAPP-NH2 monomers and finally inhibit fibrillation of hIAPP-NH2 . This study agrees with and reinforces previous studies and offers an intuitive explanation at both the atomic and molecular levels. Our findings may provide an invaluable reference for the future development of new drugs in the management of diabetes.

Influence of Aluminium and EGCG on Fibrillation and Aggregation of Human Islet Amyloid Polypeptide.

The abnormal fibrillation of human islet amyloid polypeptide (hIAPP) has been implicated in the development of type II diabetes. Aluminum is known to trigger the structural transformation of many amyloid proteins and induce the formation of toxic aggregate species. The (-)-epigallocatechin gallate (EGCG) is considered capable of binding both metal ions and amyloid proteins with inhibitory effect on the fibrillation of amyloid proteins. However, the effect of Al(III)/EGCG complex on hIAPP fibrillation is unclear. In the present work, we sought to view insight into the structures and properties of Al(III) and EGCG complex by using spectroscopic experiments and quantum chemical calculations and also investigated the influence of Al(III) and EGCG on hIAPP fibrillation and aggregation as well as their combined interference on this process. Our studies demonstrated that Al(III) could promote fibrillation and aggregation of hIAPP, while EGCG could inhibit the fibrillation of hIAPP and lead to the formation of hIAPP amorphous aggregates instead of the ordered fibrils. Furthermore, we proved that the Al(III)/EGCG complex in molar ratio of 1 : 1 as Al(EGCG)(H2O)2 could inhibit the hIAPP fibrillation more effectively than EGCG alone. The results provide the invaluable reference for the new drug development to treat type II diabetes.

Berberine ameliorates COX-2 expression in rat small intestinal mucosa partially through PPARγ pathway during acute endotoxemia.

Berberine hydrochloride (BBR), a plant alkaloid, has been used to treat intestinal inflammation or infection for years. Cyclooxygenase-2 (COX-2) is pro-inflammatory mediator and involved in the induction of gut inflammation. The expression of COX-2 in small bowel mucosa was determined and the mechanism by which BBR modulated COX-2 expression was explored in a rat model of endotoxemia induced by lipopolysaccharide (LPS). The results showed that without LPS stimulation COX-2 was constitutively expressed at low levels in control rats. LPS challenge rapidly induced COX-2 gene transcription resulting in high levels of inducible COX-2 expression in endotoxemic rats. BBR pre- and post-treatment had no marked effect on constitutive COX-2 expression but inhibited inducible COX-2 overexpression. LPS challenge increased the expression and phosphorylation of peroxisome proliferator-activated receptor gamma (PPARγ), p38 and activating transcription factor 2 and 3 (ATF2, ATF3), but the effects of LPS were inhibited by BBR treatment. GW9662 did not influence constitutive COX-2 expression but enhanced inducible COX-2 overproduction. Besides, GW9662 abolished the inhibitory effect of BBR on inducible COX-2, p38, ATF2, 3 expression and phosphorylation. Collectively, these results indicated that BBR gavage could attenuate the overexpression of inducible COX-2, not constitutive COX-2, in ileal mucosa during acute endotoxemia in part via activation of PPARγ pathway, which negatively interfered with p38/ATFs cascade.