The SETD8/PR-Set7 Methyltransferase Functions as a Barrier to Prevent Senescence-Associated Metabolic Remodeling.

Cellular senescence is an irreversible growth arrest that contributes to development, tumor suppression, and age-related conditions. Senescent cells show active metabolism compared with proliferating cells, but the underlying mechanisms remain unclear. Here we show that the SETD8/PR-Set7 methyltransferase, which catalyzes mono-methylation of histone H4 at lysine 20 (H4K20me1), suppresses nucleolar and mitochondrial activities to prevent cellular senescence. SETD8 protein was selectively downregulated in both oncogene-induced and replicative senescence. Inhibition of SETD8 alone was sufficient to trigger senescence. Under these states, the expression of genes encoding ribosomal proteins (RPs) and ribosomal RNAs as well as the cyclin-dependent kinase (CDK) inhibitor p16INK4A was increased, with a corresponding reduction of H4K20me1 at each locus. As a result, the loss of SETD8 concurrently stimulated nucleolar function and retinoblastoma protein-mediated mitochondrial metabolism. In conclusion, our data demonstrate that SETD8 acts as a barrier to prevent cellular senescence through chromatin-mediated regulation of senescence-associated metabolic remodeling.

Retinoblastoma protein promotes oxidative phosphorylation through upregulation of glycolytic genes in oncogene-induced senescent cells.

Metabolism is closely linked with cellular state and biological processes, but the mechanisms controlling metabolic properties in different contexts remain unclear. Cellular senescence is an irreversible growth arrest induced by various stresses, which exhibits active secretory and metabolic phenotypes. Here, we show that retinoblastoma protein (RB) plays a critical role in promoting the metabolic flow by activating both glycolysis and mitochondrial oxidative phosphorylation (OXPHOS) in cells that have undergone oncogene-induced senescence (OIS). A combination of real-time metabolic monitoring, and metabolome and gene expression analyses showed that OIS-induced fibroblasts developed an accelerated metabolic flow. The loss of RB downregulated a series of glycolytic genes and simultaneously reduced metabolites produced from the glycolytic pathway, indicating that RB upregulates glycolytic genes in OIS cells. Importantly, both mitochondrial OXPHOS and glycolytic activities were abolished in RB-depleted or downstream glycolytic enzyme-depleted OIS cells, suggesting that RB-mediated glycolytic activation induces a metabolic flux into the OXPHOS pathway. Collectively, our findings reveal that RB essentially functions in metabolic remodeling and the maintenance of the active energy production in OIS cells.

Hmga1 is differentially expressed and mediates silencing of the CD4/CD8 loci in T cell lineages and leukemic cells.

High-mobility group A1 (Hmga1) protein is an architectural chromatin factor, and aberrant Hmga1 expression in mice causes hematopoietic malignancies with defects in cellular differentiation. However, the functional involvement of Hmga1 in hematopoietic development and leukemic cells remains to be elucidated. Using Hmga1-green fluorescent protein (GFP) knock-in mice that endogenously express an Hmga1-GFP fusion protein, we examined Hmga1 expression in undifferentiated and differentiated populations of hematopoietic cells. During early T cell development in the thymus, Hmga1 is highly expressed in CD4/CD8-double negative (DN) cells and is transiently downregulated in CD4/CD8-double positive (DP) cells. Consistently, Hmga1 directly binds to cis-regulatory elements in the CD4/CD8 loci and the heterochromatin foci in DN-stage cells, but not in DP cells. Interestingly, CD4/CD8 expression in DN-stage leukemic cells is induced by inhibition of Hmga1 binding to nuclear DNA or RNA interference-mediated Hmga1 knockdown. In addition, Hmga1-depleted leukemic T cells markedly diminish proliferation, with transcriptional activation of cyclin-dependent kinase inhibitor genes as a direct target of Hmga1. The data in the present study reveal a role of Hmga1 in transcriptional silencing in T cell lineages and leukemic cells.

Simian virus 40 large T antigen targets the microtubule-stabilizing protein TACC2.

The large T antigens of polyomaviruses target cellular proteins that control fundamental processes, including p53 and the RB family of tumor suppressors. Mechanisms that underlie T-antigen-induced cell transformation need to be fully addressed, because as-yet unidentified target proteins might be involved in the process. In addition, recently identified polyomaviruses are associated with particular human diseases such as aggressive skin cancers. Here, we report that simian virus 40 (SV40) large T antigen interacts with the transforming acidic coiled-coil-containing protein TACC2, which is involved in stabilizing microtubules in mitosis. T antigen directly binds TACC2 and induces microtubule dysfunction, leading to disorganized mitotic spindles, slow progression of mitosis and chromosome missegregation. These mitotic defects are caused by N-terminal-deleted T antigen, which minimally interacts with TACC2, whereas T-antigen-induced microtubule destabilization is suppressed by overexpressing TACC2. Thus, TACC2 might be a key target of T antigen to disrupt microtubule regulation and chromosomal inheritance in the initiation of cell transformation.