Molecular Mechanisms Of Histone Acetylation Modification In Regulation Of Hsp Gene Expression And Lifespan In Drosophila Melanogaster

Aging is a universal but the most complex biological process in multicellularorganisms. It governs the lifespan of an organism and is influenced by both genetic andenvironmental factors. So far the underlying causes and molecular events that lead to agingstill remain mysterious. It would be of great biological interest and practical importance ifwe could uncover the molecular mechanisms of aging, and find a way to delay the agingprocess. Heat shock proteins (HSPs) are evolutionary conserved molecular chaperones that areinvolved in various complex cellular functions. They are induced in response to proteindamage caused by heat and other stresses, and have been shown to be major determinantsin the acquisition of thermotolerance and stress resistance. Aging is associated with theaccumulation of abnormal and/or malfolded proteins and oxidatively damaged proteins inmany organisms, including nematodes, flies and mammals. Induction of hsp genes duringDrosophila aging might have a beneficial effect on the lifespan of flies. Expression of hsp genes represents a special model of gene regulation involving basaland inducible expression. The reversible acetylation/deacetylation modification of corehistone tails is involved in the modulation of chromatin structure and function that leads tothe activation/suppression of gene expression. The histone deacetylase inhibitor (HDI) canraise the acetylation level of core histones by suppressing histone deacetylase (HDAC)activity and change certain gene expression. Data arising from the experiments in this study clarified the correlation between theelevated expression of hsp genes and the longevity in Drosophila melanogaster. Theresults revealed the higher hsp basal expression level, higher thermotolerance and higherresponsivity to heat shock, but lower hsp22 induction to continuous high temperature inlong-lived than in short-lived flies. Drosophila lifespan extension could be achieved byfeeding the flies with HDIs, trichostatin A (TSA) and sodium butyrate (BuA), and/orrepeated mild heat shock. However, the optimal concentrations of HDIs, and probably themechanisms of their actions, vary with the genetic background. Meanwhile, HDACinhibitor treatment promoted the hsp basal expression, but with the time of aging, theextent of this increase declined gradually. Chromatin modulation may be involved inHDI-mediated hsp gene activation, since hyperacetylation was detected in core histone H3upon TSA and BuA treatments. Thus, a close correlation among histone acetylation, hspgene regulation and aging in D. melanogaster can be established. On the other hand, by using HDI feeding of Drosophila larvae, real-time RT-PCRanalysis, and chromatin immunoprecipitation (ChIP), etc., we investigated the impacts ofhistone acetylation and its molecular mechanisms of action on hsp gene expression. Theresults demonstrated that the basal expression of hsp22 and hsp70 increased, while thebasal expression of hsp26 declined following HDI treatment. However, HDI treatment canpromote their inducible expression. HDIs TSA and BuA elevated the acetylation level ofhistone H3, and ChIP assay located the HDI-induced H3 hyperacetylation at both thepromoter and the downstream of RNA polymeraseâ…¡of the transcribing hsp genes.Meanwhile, the elevated acetylation level increased the accessibility of heat shock factor(HSF) to target cis-acting regulatory sites, enhanced the recruitment of RNA polâ…¡to hsp22and hsp70 promoters under the normal conditions, and promoted the transcriptionalelongation of hsp22, hsp26 and hsp70 genes by RNA polâ…¡under stress conditions. Inaddition, during heat shock, the acetylation level of histone H3 at the promoter of hsp26and hsp70 exhibited a kinetic change, and the change profiles were different at the differentregulatory elements at the hsp26 promoter. Meanwhile, during heat shock, expression ofhsp26 underwent a flexuous induction pattern. These results suggest that histoneacetylation was involved in a precise and complex regulatory mechanism of bothtranscription initiation and elongation of h...