The SESAME Complex Regulates Telomere Silencing And Cell Aging By Regulating H4K16 Acetylation

Acetyl-Co A is a central node in carbon metabolism and plays critical roles in regulatory and biosynthetic processes.Acs2(Acetyl-Co A synthetase 2)is an integral subunit of the SESAME(Serine-responsive SAM-containing metabolic enzyme)complex,which contains pyruvate kinase Pyk1,serine metabolic enzymes(Ser33 and Shm2),SAM synthetases(Sam1 and Sam2)and Acs2.Acs2 catalyzes acetyl-Co A production from acetate,but the precise functions of Acs2 within the SESAME complex remain unclear.Here,we show that Acs2 synthesizes acetyl-Co A to promote H4K16ac(H4 lysine 16acetylation)enrichment and the occupancy of bromodomain-containing protein,Bdf1(Bromodomain-containing transcription factor 1).Bdf1 binds at subtelomere regions,which maintains telomere silencing by antagonizing the spreading of Sir2(Silencing information regulator 2)along the telomeres.Acs2 can sense both endogenous and exogenous acetate and promote the synthesis of acetyl-Co A,thereby increases H4K16 ac and Bdf1 occupancy at subtelomere regions.Consequently,Sir2 is dissociated from telomeres by acetate,leading to compromised telomere silencing and accelerated chronological aging.Furthermore,the mammalian Acs2 homolog,ACSS2(Acetyl-Co A synthetase 2)is required for acetate to increase H4K16 ac and induce senescence in HUVEC(Human Umbilical Vein Endothelial Cell)cells.Altogether,our results reveal a novel mechanism how SESAME complex regulates telomere silencing and cell senescence and clarify a new mechanism how acetate accelerates cell senescence,and this mechanism presents in yeast and HUVEC cells.This study connects cell metabolism with telomere silencing and aging,and provides new research ideas and theoretical basis for cell metabolism and epigenetic regulation.The main conclusions of this study are as follows:(1)The SESAME complex specifically regulates H4K16 ac.Western blot and ChIP(Chromatin immunoprecipitation)experiments showed that in SESAME complex mutants,both the global levels of H4K16 ac as well as its enrichment at subtelomere regions were significantly reduced.By performing the ChIP-seq experiments,we found that SESAME binds at subtelomere regions to promote subtelomeric H4K16 ac.These indicate that the SESAME complex is required to maintain both the global levels of H4K16 ac and its enrichment at subtelomere regions.(2)The SESAME complex regulate gene transcription and telomere silencing.We performed RNA-seq(RNA-sequencing)for WT(Wild Type),acs2-ts,sam1?,sas2?and H4K16 R mutants.By comparing the transcriptomes of acs2-ts,sam1?,sas2? and H4K16 R mutants,we found that more genes were repressed instead of activated in these mutants,and down-regulated genes were significantly clustered at subtelomere regions.Further more,we confirmed the RNA-seq results by q RT-PCR(quantitative Real-time PCR)with probes specific for telomere-proximal genes,including COS8,IRC7,YCR106 W,SOR1and PHO11.The transcription of these genes was significantly reduced in acs2-ts mutant and sam1?,sas2?,H4K16 R mutants.These results show that the SESAME complex regulates telomere-proximal genes transcription and telomere silencing.(3)Acetate induces the global levels of H4K16 ac and the abundance of H4K16 ac at subtelomere regions.We found that H4K16 ac was significantly increased by acetate as blocking the endogenous acetate metabolism significantly reduced H4K16 ac.Moreover,adding exogenous acetate also significantly increased both the global levels of H4K16 ac as well as its occupancy at subtelomere regions.(4)The SESAME complex is required for acetate to promote the binding of Bdf1 at subtelomere regions.Cell growth experiments indicated a genetic interaction between ACS2 and BDF1.We performed RNA-seq for bdf1? mutant.By analyzing the number of genes repressed in bdf1?mutant relative to their distance to telomeres,we found significant clustering of repressed genes near the telomeres.The occupancy of Bdf1 at subtelomere regions was significantly reduced in acs2-ts mutant,indicating that Acs2 promotes the binding of Bdf1 at subtelomere regions.Acetate significantly increased the occupancy of Bdf1 at subtelomere regions in WT but not in acs2-ts mutant,indicating that the SESAME complex is required for acetate to promote the binding of Bdf1 at subtelomere regions.(5)The SESAME complex is required for acetate to regulate Sir2 binding and transcription silencing at subtelomere regions.Cell growth experiments indicated a negative genetic interaction between ACS2 and SIR2.Acetate significantly reduced Sir2 binding at telomere regions;however,this effect was not observed in acs2-ts mutant,indicating that acetate reduced Sir2 binding at subtelomere regions in an Acs2-dependent manner.(6)The SESAME complex is required for acetate to regulate life span.We found KAc treatment significantly reduced the chronological lifespan of WT cells but not acs2-ts mutant,suggesting that Acs2 is required for acetate to regulate cell aging in budding yeast.Acetate treatment also caused premature senescence in HUVEC cells.However,acetate did not accelerate the senescence of ACSS2 knockdown cells.These data suggest that ACSS2 is required for acetate to induce H4K16 ac and accelerate cell senescence.