DNA methyltransferase 3a expression decreases during apoptosis in primary cultures of hepatocytes

Although it has been acknowledged that DNA methylation is a key factor in the epigenetic control of liver homeostasis, its role in the occurrence of hepatocyte cell death has yet been poorly documented. We therefore have investigated the expression pattern of the effectors of DNA methylation, namely DNA methyltransferase (DNMT) isoenzymes, during Fas-mediated apoptotic cell death in primary hepatocyte cultures. Cell death was assessed by in situ stainings with Annexin V, Hoechst 33342 and Propidium iodide, and measurement of caspase 3-like activity and lactate dehydrogenase release. Similar to the hepatic in vivo situation, DNMT1, DNMT2 and DNMT3b could not be detected, whereas relatively high levels of DNMT3a protein were observed in the in vitro setting, as studied by immunoblotting. Upon induction of cell death, a progressive decrease in DNMT3a protein amount was noticed, reaching a minimum level towards the final stages of the cell death process. This was preceded by parallel changes in DNMT3a mRNA production, measured by qRT-PCR analysis, which became already evident during the early stages of apoptosis. We conclude that downregulated DNMT3a protein production during Fas-mediated hepatocyte apoptosis results from inhibition of DNMT3a gene transcription. This finding further substantiates the existence of an epigenetic signature of apoptosis.