PARylation modulates the demethylation process in type 2 diabetes mellitus

DNA methylation can influence the genetic susceptibility to type 2 diabetes mellitus (T2DM) and the progression of the disease. Our previous studies demonstrated that the regulation of the DNA methylation pattern involved the PARylation process, a post-translational modification of proteins. As in a case - control study we showed an association between PAR levels and global and site-specific DNA demethylation in T2DM, here we aim to gain more insights into the mechanisms of cell response to high glucose involving PARylation and demethylation processes by exploiting HepG2 cells. After exposure to 40 mM glucose for 96 hours, HepG2 cells presented higher PAR levels and increased accumulation of demethylation intermediates 5-hydroxymethylcytosine (5hmC) and 5-formylcytosine (5fC) in the genome. Also, at level of the specific loci of diabetes- related genes, HepG2 cells showed activation of DNA demethylation process with increased 5hmC. Treatment of cells with olaparib, inhibitor of PARP activity, counteracted the increase in PAR and did not lead to any change of 5hmC induced by high glucose both at a global and gene-specific level. Our results suggest that prolonged treatment of HepG2 cells with high glucose caused activation of the PARylation process which, in turn, promoted the DNA demethylation cascade.