HP1s modulate the S-Adenosyl Methionine pathway in liver cancer cells

Hepatocellular carcinoma (HCC) is the most frequent primary liver cancer in adults. Among the altered pathways leading to HCC, an increasing role is attributed to abnormal epigenetic regulation. Members of the Heterochromatin Protein (HP1) 1 family are key players in chromatin organisation, acting as docking sites for chromatin modifiers. Here, we inactivated HP1alpha in HepG2 human liver carcinoma cells and showed that HP1alpha participated in cell proliferation. HP1alpha-depleted cells have a global decrease in DNA methylation and consequently a perturbed chromatin organisation, as exemplified by the reactivation of transcription at centromeric and pericentromeric regions, eventhough the protein levels of chromatin writers depositing methylation marks, such as EZH2, SETDB1, SUV39H1, G9A and DNMT3A remained unaltered. This decrease was attributed mainly to a low S-Adenosyl Methionine (SAM) level, a cofactor involved in methylation processes. Furthermore, we showed that this decrease was due to a modification in the Methionine adenosyl transferase 2A RNA (MAT2A) level, which modifies the ratio of MAT1A/MAT2A, two enzymes that generate SAM. Importantly, HP1alpha reintroduction into HP1alpha-depleted cells restored the MAT2A protein to its initial level. Finally, we demonstrated that this transcriptional deregulation of MAT2A in HP1alpha-depleted cells relied on a lack of recruitment of HP1beta and HP1gamma to MAT2A promoter where an improper non-CpG methylation site was promoted in the vicinity of the transcription start site where HP1beta and HP1gamma bound. Altogether, these results highlight an unanticipated link between HP1 and the SAM synthesis pathway, and emphasise emerging functions of HP1s as sensors of some aspects of liver cell metabolism.