Citrullination of glucokinase is linked to autoimmune diabetes

Abstract

Inflammation, including reactive oxygen species and inflammatory cytokines in tissues amplify various post-translational modifications of self-proteins. A number of post-translational modifications have been identified as autoimmune biomarkers in the initiation and progression of Type 1 diabetes. Here we show the citrullination of pancreatic glucokinase as a result of inflammation, triggering autoimmunity and affecting glucokinase biological functions. Glucokinase is expressed in hepatocytes to regulate glycogen synthesis, and in pancreatic beta cells as a glucose sensor to initiate glycolysis and insulin signaling. We identify autoantibodies and autoreactive CD4+ T cells to glucokinase epitopes in the circulation of Type 1 diabetes patients and NOD mice. Finally, citrullination alters glucokinase biologic activity and suppresses glucose-stimulated insulin secretion. Our study define glucokinase as a Type 1 diabetes biomarker, providing new insights of how inflammation drives post-translational modifications to create both neoautoantigens and affect beta cell metabolism.

The virtues and vices of protein citrullination

“…A study published last year showed that PADI1- and PADI3-mediated citrullination of the glycolytic enzyme pyruvate kinase M2 (PKM2) alters its binding to amino acids and promotes glycolysis in cancer cells [ 96 ]. A more recent study showed that citrullination of glukokinase leads to reduction of its catalytic activity and results in reduced insulin secretion in response to glucose stimulation [ 97 ]. These are, to the author's knowledge, the first reports that mechanistically link protein citrullination to the regulation of metabolic pathways.…”

Biomarkers of autoimmunity and beta cell metabolism in type 1 diabetes

“…The STZ-induced diabetic mouse model of T1D exhibits decreased glucokinase expression with hyperglycemia ( 116 ). Recently, we found that citrullination decreases the catalytic activity and substrate binding affinity of human pancreatic glucokinase and diminishes glucose stimulated insulin secretion (GSIS) in INS-1E murine cells ( 10 ). In addition, citrullinated glucokinase is present in NOD pancreas prior to insulitis and in human islet beta cells exposed to inflammatory cytokines.…”

“…A major difference of metabolic machinery is in the switch of expression from high glucose affinity hexokinase 1 in immature beta cells to low glucose affinity glucokinase (also known as hexokinase 4) in mature beta cells ( 118 ). We recently demonstrated that citrullination increases the Km of glucokinase by 2-fold ( 10 ). This seemingly small change in Km is nonetheless significant since glucokinase functions in a narrow range of glucose concentration near the Km.…”

“…For example, PK activity is increased 2 to 3-fold after in vitro citrullination by PAD ( 124 ). Our recent studies demonstrate that one pan-PAD inhibitor, YW3-56, can restore cytokine-mediated suppression of insulin secretion upon pyruvate stimulation in INS-1E beta cells ( 10 ). The expression of PKM1, PKM2 and α-enolase is down regulated in renal glomeruli from patients with T1D compared to healthy control subjects ( 23 ).…”

“…Accumulating evidence has identified significant numbers of citrullinated islet proteins, including proteins in the glucose and insulin metabolic pathways. These citrulline PTM proteins elicit vigorous B and T cell autoimmune responses in both human T1D and NOD murine disease, including glutamic acid decarboxylase 65 (GAD 65), 78-kDa glucose-regulated protein (GRP78) (also called BiP, HSP5a), islet antigen-2 (IA2), islet-specific glucose 6 phosphatase catalytic subunit-related protein (IGRP), islet amyloid polypeptide (IAPP) and glucokinase ( 10 ). Indeed, several other inflammatory PTMs also play the vital roles in the progression of T1D, including deamidation, oxidation and carbonylation ( 11 ).…”