Post-translational protein modifications in type 1 diabetes: A role for the repair enzyme protein-l-isoaspartate (D-aspartate) O-methyltransferase?

Abstract

Aims/hypothesis Post-translational modifications, such as isomerisation of native proteins, may create new antigenic epitopes and play a role in the development of the autoimmune response. Protein-L-isoaspartate (D-aspartate) O-methyltransferase (PIMT), encoded by the gene PCMT1, is an enzyme that recognises and repairs isomerised Asn and Asp residues in proteins. The aim of this study was to assess the role of PIMT in the development of type 1 diabetes.

Materials and methods Immunohistochemical analysis of 59 normal human tissues was performed with a monoclonal PIMT antibody. CGP3466B, which induces expression of Pcmt1, was tested on MIN6 and INS1 cells, to assess its effect on Pcmt1 mRNA and PIMT levels (RT-PCR and western blot) and apoptosis. Forty-five diabetes-prone BioBreeding (BB) Ottawa Karlsburg (OK) rats were randomised to receive 0, 14 or 500 μg/kg (denoted as the control, low-dose and high-dose group, respectively) of CGP3466B from week 5 to week 20.

Results A high level of PIMT protein was detected in beta cells. CGP3466B induced a two- to threefold increase in Pcmt1 mRNA levels and reduced apoptosis by 10% in MIN6 cells. No significant effect was seen on cytokine-induced apoptosis or PIMT protein levels in INS1 cells. The onset of diabetes in the BB/OK rats was significantly delayed (85.6 ± 9.0 vs 84.3 ± 6.8 vs 106.6 ± 13.5 days, respectively; p < 0.01 for high-dose vs low-dose and control groups), the severity of the disease was reduced (glucose 22.2 ± 3.2 vs 16.9 ± 2.6 vs 15.8 ± 2.7 mmol; p < 0.01 for high- and low-dose groups vs control group) and residual beta cells were more frequently identified (43% vs 71% vs 86%; p < 0.05 for high-dose vs control group) in the treated animals.

Conclusions/interpretation The results support a role for post-translational modifications and PIMT in the development of type 1 diabetes in the diabetes-prone BB rat, and perhaps also in humans.