Study On Mechanism Of Apoptosis Induced By Histone Deacetylase Inhibitor Combined With ATRA And As₂O₃ In NB4 Cell Line

Acurate regulation of gene transcription is requisite for normal cells to react to outer signals.Transformed cells have escaped normal growth regulatory signals and may be blocked at discrete states of differentiation prior to the stage of terminal cell differentiation and/or apoptosis.DNA in eukaryocytic cell exists in the form of mucleosome , which also contains histone octomer:H2A,H2B,H3, H4.Among several modifications of histone ,acetylation and deacetylation have been highlighted as one of the crucial events that're severely related to the transcriptional response of the cells.The acetylation state of histone is reversibly regulated by histone acetyltransferase(HAT) and histone deacetylase(HDAC).The acetylation of histone by HAT neutralizes the positive charge of lysine residue,which results in a decreased interaction between histone and DNA.This conformational change in chromatin activates the transcription by promoting the access of transcription factor.Reversibly,these events are restored when the histones are deacetylated by HDAC.Recent extensive studies on these two enzymes revealed that HAT and HDAC play key roles in carcinogenesis.For example,mutation of HAT is the major cause of Rubinstein-Taybi Syndrome,which is characterized by high potential of carcinogenesis.The expression of HDAC in tumors is much higher than that in normal tissues.Several oncogenic transcription factor repress expression of specific genes due to aberrant recruitment of HDAC.This repression of gene expression appears to be an important step in the oncogenic action of these transcription factors.For example,aberrant recruitment of HDAC activity by PML-RARαfusion protein has been reported in cell lines derived from patients with acute promyelocytic leukemia,and this aberrant recruitment results in th repression of PML transcription,which is related to normal differentiation.Another example is actue myelogenous leukemia of M2 subtype associated with t(8;21) chromosomal translocation involving the AML1-ETO fusion protein.The AML1-ETO fusion protein,unlike the AML1 protein,which is a transcriptional activator ,is a potent dominant transcriptional repressor.This repression appears to be mediated by the recruitment of HDACs。Histone deacetylase inhibitor(HDACi) can inhibit the activity of HDAC,and can cause the elevation of acetylated histones at both molecular level and cell level,together with expression of a lot of genes relative to cell differentiation,cell cycle arrest and apoptosis.The changes in gene expression following exposure to HDACi are not global,rather the expression of only a small subset of genes is affected by HDAC inhibition.So far,there're four kinds of HADCi discovered :1 short-chain fatty acid,such as sodium butyrate and phenylbutyrate,etc 2 aliphatic compounds containing hydroxamic acid,such as trichostatin and SAHA 3 cyclic tetrapeptide compounds,such as FR901228 4 bezamide-bearing compounds,such as MS-27-275.HDACi are inducers of transformed cell differentiation cell cycle arrest and/or apoptosis in a wide range of tumor cells both in vitro and in vivo.Majority of research about HDACi show p21 expression in tumor cells increase after HDACi exposure independent of p53,and p21 is the cause of cell cycle arrest.p21 is a member of the family of cell cycle cyclin-dependent kinase(cdk) inhibitor,whose expression results in arrest of cell cycle progression.Whether caspase participates in the process of HDACi mediated apoptosis is still in debate.Many reports support the idea that caspase does participate in the apoptosis induced by HDACi independent of p53.But there still exist some researches that show that calpain activation, Bid cleavage and subsequent reactive oxygen play key roles in this apoptosis.As2O3 is traditional Chinese medicine.During the past a few years, As2O3 has been widely used to treat APL,for As2O3 can induce apoptosis of APL cells and it's also effective for the ATRA-resistant APL cell lines and patients.But the pathways through which As2O3 induce apoptosis is still unclear.The possi...