Ezh2 promotes TR beta lysine methylation-mediated degradation in hepatocellular carcinoma

Background Post-translational modification (PTM) of proteins controls various cellular functions of transcriptional regulators and participates in diverse signal transduction pathways in cancer. The thyroid hormone (triiodothyronine, T3) plays a critical role in metabolic homeostasis via its direct interaction with the thyroid hormone receptor beta (TR beta). TR beta is involved in physiological processes, such as cell growth, differentiation, apoptosis, and maintenance of metabolic homeostasis through transcriptional regulation of target genes. Objective This study was performed to characterize the specific PTM of TR beta is an active control mechanism for the proteasomal degradation of TR beta in transcriptional signaling pathways in hepatocellular carcinoma cells. Methods Based on a previous study, we predicted that the lysine methyltransferase and methylation sites of TR beta by comparing the amino acid sequences of histone H3 and TR beta. Methyl-acceptor site of TR beta was confirmed by point mutation. TR beta protein stability was evaluated by ubiquitination assay with MG132. For glucose starvation, HepG2 cells were incubated in media without D-glucose. Proliferation-related proteins were detected by western blotting. MicroRNA level and autophagy marker were measured by real-time qPCR. Results The presence of enhancer of zeste homolog 2 (Ezh2), a methyltransferase of H3 lysine 27, as a methyltransferase of TR beta also revealed that direct lysine methylation and consequent stimulated protein degradation of TR beta underlies the negative correlation between Ezh2 and TR beta. Notably, glucose starvation significantly increased lysine methylation, and methylated TR beta showed further protein instability leading to an increase in the proliferation and growth of hepatocellular carcinoma cells. Conclusions TR beta functions as a tumor suppressor in various cancers; therefore, we evaluated the effect of TR beta degradation on oncogenesis during glucose starvation. These data clearly define a functional model and provide a link between metabolism and cancer by regulating methyl-dependent protein levels of tumor suppressors. Taken together, maintaining TR beta against methyl-dependent degradation is considered a possible therapeutic target for cancer progression.