P53 and histone modification regulation on cellular metabolism and DNA damage

Abstract

Post-translational modifications of proteins alter the characteristics of protein, supporting its original function or perform another. Histone, which binds DNA and consist chromatin structure undergoes diverse post-translation modifications. These modifications mainly take place on N-terminal tails of core histones, and alter the electrostatic nature of histones regulating the physical binding of transcription factors or providing a binding spot of transcription factor complex on itself, playing central role in transcription regulation. p53, especially, is called guardian of genome which plays key role in suppressing cellular oncogenesis and promotes genomic stability. p53 also undergoes various post-translational modifications to regulate cell-cycle arrest, DNA-repair, apoptosis. First, novel histone modification, which takes place under DNA-damaging conditions, was explored. We found that histone H3 threonine 45 (H3-T45) was phosphorylated by cellular signal transduction protein AKT under various DNA-damaging agents. AKT is a serine/threonine kinase which transfers hormone or growth factor signal to induce cellular proliferation and is a key molecule involved in cellular oncogenesis. AKT is also known to be phosphorylated and activated under DNA-damaging conditions and induces CDKN1A transcription. By genome-wide chromatin immunoprecipitation sequencing (ChIP-seq) analysis, H3-T45 phosphorylation was distributed throughout the DNA damage responsive gene locus, particularly immediately after the transcription termination site (TTS). H3-T45 phosphorylation overlapped extensively with RNA polymerase II C-terminal domain (CTD) serine 2 phosphorylation, which establishes the transcription termination signal. AKT1 was more effective than AKT2 in H3-T45 phosphorylation. Blocking H3-T45 phosphorylation by inhibiting AKT or through amino acid substitution decreased RNA decay downstream of mRNA cleavage sites and RNA polymerase II release from chromatin. Our findings suggest that AKT-mediated phosphorylation of H3-T45 correlates with the termination of transcription on DNA damage. Second, ATP citrate lyase (ACLY), which controls cellular senescence through p53, was studied. ACLY catalyzes cytosolic citrate into acetyl-coA, involved in cellular fatty acid synthesis and ACLY inhibition caused retarded proliferation of cancer cell. Recent study revealed that ACLY affects overall cellular gene expression profiles though regulating histone acetylation by acetyl-CoA production. In this study, we found that inhibition of ACLY in normal cells showed senescent morphology which is caused by p53. Acetyl-CoA back-up in ACLY knockdown cells was not sufficient to recover from the senescent phenotype. Moreover, ACLY directly interacted with the catalytic subunit of AMPK and suppressed its activity. As well as normal cells, knockdown of ACLY in cancer cells showed increased p53 level which resulted in induced apoptosis under DNA-damaging condition. Throughout this study, we identified a novel function of ACLY which interconnects cellular energy sensor AMPK and tumor suppressor p53 in cellular senescence and tumorigenesis.