调控组蛋白H3K27me3水平对砷致肝细胞凋亡过程中BCL2表达的影响

Role of histone H3K27me3 on BCL2 expression in arsenic-induced hepatocyte apoptosis

目的探讨在砷诱导肝细胞凋亡过程中,组蛋白H3第27位赖氨酸三甲基化(H3K27me3)水平对抗凋亡蛋白B淋巴细胞瘤-2(BCL2)基因表达的影响。方法体外培养大鼠肝细胞BRL-3A,根据砷处理因素将实验分为两部分,未染砷时分为正常、转染试剂、阴性转染、JMJD3(H3K27me3特异性去甲基化酶)小干扰RNA(siRNA)转染、EZH2(H3K27me3甲基转移酶)siRNA转染组;染砷时分为对照、砷处理、砷+阴性转染、砷+JMJD3siRNA转染、砷+EZH2siRNA转染组。未染砷时siRNA和转染试剂按照100 pmol∶7.5μl比例转染细胞6 h[正常组加入与转染试剂同体积的磷酸盐缓冲液(PBS)],再换培养基培养,共48 h;染砷时同上述方法转染和培养细胞24 h,染砷各组再加入终浓度为30μmol/L的亚砷酸钠(NaAsO2)处理24 h(对照组加入与NaAsO2同体积的PBS处理24 h)。采用实时无标记细胞分析(RTCA)技术动态观察染砷时细胞增殖情况;流式细胞术检测染砷时细胞的凋亡情况;蛋白免疫印迹(Western blot)法检测未染砷时和染砷时细胞JMJD3、EZH2、H3K27me3、BCL2蛋白表达水平;染色质免疫共沉淀技术检测染砷时细胞BCL2基因启动子区H3K27me3的富集水平。结果对照、砷处理、砷+阴性转染、砷+JMJD3siRNA转染、砷+EZH2siRNA转染组细胞增殖率分别为(100.00±10.43)%、(12.19±3.37)%、(31.86±1.95)%、(24.58±3.64)%、(11.53±1.11)%,细胞凋亡率分别为(1.15±0.04)%、(13.06±1.33)%、(17.39±0.22)%、(23.90±1.66)%、(15.07±0.88)%,组间比较差异均有统计学意义(F=146.50、194.30,P均<0.001)。JMJD3siRNA转染组H3K27me3蛋白表达水平均高于正常、转染试剂、阴性转染组,EZH2siRNA转染组则相反(P均<0.05);JMJD3siRNA转染组BCL2蛋白表达水平均低于正常、转染试剂、阴性转染组,EZH2siRNA转染组则相反(P均<0.05),而在正常、转染试剂、阴性转染组之间,H3K27me3、BCL2蛋白表达水平均未见明显改变(P均>0.05)。在对照、砷处理、砷+阴性转染、砷+JMJD3siRNA转染、砷+EZH2siRNA转染组之间,JMJD3、EZH2、H3K27me3、BCL2蛋白表达水平比较,差异均有统计学意义(F=26.56、7.82、9.81、31.19,P均<0.05)。与对照组比较,砷处理组JMJD3、EZH2蛋白表达水平均较低,H3K27me3蛋白表达水平较高,同时BCL2蛋白表达水平较低(P均<0.05);与砷+阴性转染组比较,砷+JMJD3siRNA转染组JMJD3蛋白表达水平较低,砷+EZH2siRNA转染组EZH2蛋白表达水平较低,砷+JMJD3siRNA转染组H3K27me3蛋白表达水平较高,同时BCL2蛋白表达水平较低,而砷+EZH2siRNA转染组则相反(P均<0.05)。与对照组比较,砷处理组细胞BCL2基因启动子区(CHIP1、CHIP2)H3K27me3的富集水平均较高(P均<0.05)。结论砷通过增加BCL2基因启动子区H3K27me3的富集水平,进而抑制BCL2的表达,促进肝细胞凋亡。

Objective To investigate the role of modification level of lysine trimethylation at position 27 of histone 3(H3K27me3)on expression of anti-apoptotic protein B lymphocyte tumor-2 gene(BCL2)during arsenic-induced hepatocyte apoptosis.Methods Rat liver BRL-3A cells were cultured in vitro.According to the arsenic treatment factor,the experiment was divided into two parts,in the first part arsenic was not added,the experiment was divided into normal,transfection reagent,negative transfection,H3K27me3 specific demethylase(JMJD3)small interfering RNA(siRNA)transfection and H3K27me3 methyltransferase(EZH2)siRNA transfection groups.In the second part arsenic was added,the experiment was divided into control,arsenic treatment,arsenic+negative transfection,arsenic+JMJD3siRNA transfection and arsenic+EZH2siRNA transfection groups.When arsenic was not added,the corresponding siRNA and transfection reagent was used to transfect cells at a ratio of 100 pmol:7.5μl for 6 h[the normal group was treated with phosphate buffer solution(PBS)of the same volume as transfection reagent],then the medium was changed and the cells were incubated for a total of 48 h.After 24 h of treatment with the above transfection and culture method in arsenic added group,a final concentration of 30μmol/L sodium arsenite(NaAsO2)was added and the cells were incubated for 24 h(the control group was treated with PBS with the same volume of NaAsO2 for 24 h).Real-time cell analysis(RTCA)was used to measure the proliferation of BRL-3A cells in arsenic added group.Apoptosis of BRL-3A cells was analyzed by flow cytometry in arsenic added group.Western blotting was used to detect JMJD3,EZH2,H3K27me3 and BCL2 in no-arsenic and arsenic-added BRL-3A cells.The modification levels of H3K27me3 in BCL2 gene promoter regions were detected by chromatin immunoprecipitation of the cells exposed to arsenic.Results There were statistically significant differences of the proliferation rates[control,arsenic treatment,arsenic+negative transfection,arsenic+JMJD3siRNA transfection and arsenic+EZH2siRNA transfection groups:(100.00±10.43)%,(12.19±3.37)%,(31.86±1.95)%,(24.58±3.64)%,(11.53±1.11)%]and the apoptosis rates[(1.15±0.04)%,(13.06±1.33)%,(17.39±0.22)%,(23.90±1.66)%,(15.07±0.88)%]between groups(F=146.50,194.30,P<0.001),correspondingly.The protein expression level of H3K27me3 in JMJD3siRNA transfection group was higher than that of normal,transfection reagent and negative transfection groups,while EZH2siRNA transfection group had an opposite result(P<0.05).The protein expression level of BCL2 in JMJD3siRNA transfection group was lower than that of normal,transfection reagent and negative transfection groups,while EZH2siRNA transfection group had an opposite result(P<0.05).The protein expression levels of H3K27me3 and BCL2 were not statistically significant differences between normal,transfection reagent and negative transfection groups(P>0.05).The protein expression levels of JMJD3,EZH2,H3K27me3 and BCL2 among control,arsenic treatment,arsenic+negative transfection,arsenic+JMJD3siRNA transfection and arsenic+EZH2siRNA transfection groups were compared,and the differences were statistically significant(F=26.56,7.82,9.81,31.19,P<0.05).Compared with control group,the protein expression levels of JMJD3 and EZH2 in arsenic treatment group were significantly reduced(P<0.05),and the protein expression level of H3K27me3 was higher(P<0.05),meanwhile the protein expression level of BCL2 was lower(P<0.05).Compared with arsenic+negative transfection group,the protein expression level of JMJD3 was significantly reduced in arsenic+JMJD3siRNA group,and the protein expression level of EZH2 was significantly reduced in arsenic+EZH2siRNA group(P<0.05).In addition,arsenic+JMJD3siRNA increased the level of H3K27me3 modification while reducing the protein expression of BCL2,while arsenic+EZH2siRNA had an opposite result(P<0.05).Compared with control group,the enrichment levels of H3K27me3 in BCL2 gene promoter regions(CHIP1 and CHIP2)in arsenic treatment group were significantly higher(P<0.05).Conclusion Arsenic may inhibit the expression of BCL2 by increasing the enrichment level of H3K27me3 in the promoter regions of BCL2 gene,and promoting hepatocyte apoptosis.