Tyrosine nitration of glucagon impairs its function: Extending the role of heme in T2D pathogenesis

New studies raise the possibility that the higher glucagon (GCG) level present in type 2 diabetes (T2D) is a compensatory mechanism to enhance β-cell function, rather than induce dysregulated glucose homeostasis, due to an important role for GCG that acts directly within the pancreas on insulin secretion by intra-islet GCG signaling. However, in states of poorly controlled T2D, pancreatic α cell mass increases (overproduced GCG) in response to insufficient insulin secretion, indicating decreased local GCG activity. The reason for this decrease is not clear. Recent evidence has uncovered a new role of heme in cellular signal transduction, and its mechanism involves reversible binding of heme to proteins. Considering that protein tyrosine nitration in diabetic islets increases and glucose-stimulated insulin secretion (GSIS) decreases, we speculated that heme modulates GSIS by transient interaction with GCG and catalyzing its tyrosine nitration, and the tyrosine nitration may impair GCG activity, leading to loss of intra-islet GCG signaling and markedly impaired insulin secretion. Data presented here elucidate a novel role for heme in disrupting local GCG signaling in diabetes. Heme bound to GCG and induced GCG tyrosine nitration. Two tyrosine residues in GCG were both sensitive to the nitrating species. Further, GCG was also demonstrated to be a preferred target peptide for tyrosine nitration by co-incubation with BSA. Tyrosine nitration impaired GCG stimulated cAMP-dependent signaling in islet β cells and decreased insulin release. Our results provided a new role of heme for impaired GSIS in the pathological process of diabetes.

Inhibitory activities of flavonoids from Scutellaria baicalensis Georgi on amyloid aggregation related to type 2 diabetes and the possible structural requirements for polyphenol in inhibiting the nucleation phase of hIAPP aggregation

Human islet amyloid polypeptide (hIAPP)-mediated cytotoxicity is identified as a potential target for developing new anti-diabetic molecules. Herein, we investigated the effect of the major bioactive compounds of Scutellaria baicalensis Georgi (S. baicalensis), including baicalein, baicalin, wogonin and oroxylin A, on hIAPP aggregation. We found that all of these compounds inhibited hIAPP fibril formation in a dose-dependent manner. But baicalein and baicalin, especially baicalein are more effective than wogonin and oroxylin A in stabilizing hIAPP monomers and eliminating toxic hIAPP assembly, suggesting that flavonoids with ortho-hydroxyl group on the A-ring exhibited higher anti-hIAPP nucleation potential than those without this structure. This stimulated our interest in further studying the possible structure-activity relationship between polyphenol and hIAPP aggregation inhibition. Our results demonstrated that flavonoids with ortho-hydroxyl group on the B-ring are also more effective against hIAPP nucleation than those without this structure. These results suggest that the ortho-hydroxybenzene structure is a key structural feature required for polyphenols to effectively inhibit hIAPP nucleation. This was further confirmed by the effect of polyphenoland phenols in inhibiting hIAPP nucleation. The conclusion that pyrogallol-type polyphenols are potential lead inhibitors may provide a valuable structural template for the further development of polyphenol-based inhibitor of amyloid peptides.