We have previously demonstrated proteasomal degradation of DNMT1 in mammalian cells following treatment with several DNA hypomethylating agents. Here, we demonstrate dose-dependent degradation of Dnmt1 in mouse embryonic stem (ES) cells expressing catalytic site mutant (cys-ser), confirming that the covalent bond formation between Dnmt1 and decitabine-incorporated DNA is not essential for this process. DNMT1o, the oocyte-specific isoform that lacks the N-terminal 118-amino acid domain, did not undergo decitabine-mediated degradation, which further proves the requirement of multiple domains including nuclear localization signal, KEN box, and BAH domains for this process. Analysis of glycerol density gradient fractions of micrococcal nuclease-digested nuclei showed that both nucleosomal and nucleoplasmic DNMT1 are degraded upon decitabine treatment. Among different inhibitors tested, the inhibitors of the proteasomal pathway and several protein kinases impeded decitabine-induced DNMT1 degradation. The maximal effect caused by inhibiting protein kinase C (PKC) persuaded us to investigate further its role in decitabine-mediated DNMT1 degradation. Blockage of the degradation process after treatment with rottlerin, an inhibitor of PKCδ, or after siRNA-mediated depletion of PKCδ, indicated that this protein kinase is involved in decitabine-mediated depletion of DNMT1. PKCδ interacted with and phosphorylated DNMT1 in vitro. Moreover, rottlerin inhibited both basal and decitabine-induced phosphorylation of DNMT1. These studies provide substantial evidence that decitabine-induced degradation of the maintenance methyltransferase DNMT1 does not require covalent bond formation with the substrate and also elucidate its underlying molecular mechanism.
Keywords: DNA methyltransferase inhibitor; DNMT1; DNMT1o; PKCδ; azacytidine; decitabine; epigenetics.