Mammalian D-Cysteine is a Physiologic Down Regulator of Insulin Promoter Methylation

Endogenous D-stereoisomers of amino acids are poorly understood in mammals. Here we report the identification and function of endogenous D-cysteine in the pancreas. D-cysteine is present in substantial amounts in the eyes and pancreas of mice. Serine Racemase (SR) is the biosynthetic enzyme for D-cysteine, as it can racemize L-cysteine to D-cysteine. To investigate endogenous D-cysteine, we used SR deficient mice lacking racemizing ability and show 3.5 fold reduction in pancreatic D-cysteine. SR-/- mice produce 6-10 fold higher levels of insulin and are hypoglycemic. The excess insulin is stored in secretory vesicles and plasma exosomes as amyloid. Lack of endogenous D-cysteine results in decreased levels of cAMP. This results in reduced phosphorylation of CREB (S133) leading to lower expression of DNA methyltransferase (DNMT) 1, 3A and 3B, resulting in reduced DNMT and DNMT1 promoter activities in the pancreas and eventual decrease in DNA methylation globally and specifically of the Ins1 promoter. D-cysteine is efficiently metabolized by D-amino acid oxidase and transported by ASCT2 and Asc1. Methyl donor dietary supplementation rescues the high insulin levels and low DNMT activity in SR-/- mice. Our data show that endogenous D-cysteine in the pancreas is a physiologic down regulator of insulin promoter methylation.