宫颈癌细胞系中HPV16 E6、E7及E6/E7基因对STK31基因甲基化状态及表达的影响

摘要

背景与目的：丝氨酸/苏氨酸蛋白激酶31(serine/threonine kinases 31，STK31)基因在人类多种癌症中扮演重要角色，且STK31基因的表达受其启动子及第一外显子区甲基化状态的影响；病毒感染与肿瘤组织中某些抑癌基因启动子区高甲基化有关。本研究旨在探讨宫颈癌细胞系中HPV16 E6、E7及E6/E7癌基因对STK31基因甲基化状态及表达的影响，以及不同种类甲基转移酶(DNA methyltransferases，DNMTs)基因在STK31基因甲基化中的潜在作用。方法：构建外源性HPV16 E6、E7以及E6/E7基因共表达慢病毒，分别感染人乳头瘤病毒(human papillomavirus，HPV)阴性宫颈癌细胞系HT-3及C33A，获得稳定转染的细胞系；采用亚硫酸氢盐基因组测序法(bisulfite genomic sequencing，BGS)和甲基化特异性PCR (methylation-specific PCR，MSP)检测3种HPV阳性宫颈癌细胞系HeLa、SiHa和CasKi以及HPV阴性宫颈癌细胞系HT-3和C33A转染前后STK31基因的甲基化状态；RT-PCR及蛋白[质]印迹法(Western blot)检测上述宫颈癌细胞系中STK31基因的表达以及DNMT1、DNMT2、DNMT3a、DNMT3b和DNMT3L基因在HPV16转染前后宫颈癌细胞系及HPV阳性、HPV阴性宫颈癌组织中的表达情况。结果：外源性HPV16 E6、E7以及E6/E7基因可在HPV阴性宫颈癌细胞系中稳定表达。3种HPV阳性细胞系HeLa、SiHa和CasKi中STK31基因呈低甲基化状态，STK31 mRNA及蛋白质表达阳性；2种HPV阴性细胞系HT-3、C33A中STK31基因则表现为高甲基化状态，STK31 mRNA及蛋白质表达缺失；与未感染慢病毒HT-3和C33A细胞系比较，外源性HPV16 E7以及E6/E7表达的HT-3和C33A细胞系STK31基因甲基化程度降低，其mRNA及蛋白质重新表达。DNMT1、DNMT3a和DNMT3b基因在HT-3E6/E7和C33AE6/E7细胞系中mRNA水平分别高于HT-3空载细胞系和C33A空载细胞系，差异有统计学意义(P<0.001)。DNMT1、DNMT3a和DN-MT3b基因的mRNA水平在HPV16阳性宫颈癌组织中的表达高于其在HPV阴性宫颈癌组织中的表达，差异有统计学意义(t=5.997，P<0.001；t=6.743，P<0.001；t=7.926，P<0.001)。DNMT2在HT-3E6/E7和C33AE6/E7细胞系中mRNA表达水平分别低于HT-3空载细胞系和C33A空载细胞系，差异有统计学意义(t=7.451，P<0.001；t=2.451， P<0.05)；DNMT2基因转录水平在HPV16阳性宫颈癌组织中低于HPV阴性宫颈癌组织(t=9.134，P<0.001)。DN-MT3L mRNA表达水平在宫颈癌细胞系转染前后及HPV阴阳性宫颈癌组织中的差异无统计学意义(P>0.05)。结论：HPV感染可导致STK31基因启动子及第1外显子区甲基化状态降低，低甲基化状态促进该基因表达。STK31基因的表达受其启动子及第1外显子区甲基化状态的调控。HPV16 E7、E6/E7基因可能通过影响DNMT2的表达参与调控癌基因STK31基因启动子及第1外显子区甲基化状态。%Background and purpose:Studies have proved that the serine/threonine kinases 31 (STK31) gene plays important roles in human cancers. TheSTK31 gene expression was demonstrated to be regulated by the methylation status of its promoter/exon1 region. Viral infection was revealed to be associated with the hypermethylation of some tumor suppressor genes in some tumor samples. The purposes of this paper were to study the roles ofHPV16E6,E7, orE6/E7 oncogenes in methylation status and expression of theSTK31 gene, and potential effects of DNA methyltransferases (DNMTs) onSTK31 gene methylation status.Methods:Ectopically-expressed HPV16 E6, E7, or E6/E7 cells were estab-lished by transfectingHPV16E6,E7, orE6/E7 oncogenes with lentivirus vectors into HPV-negative cervical cancer cell lines HT-3 and C33A. Bisulfite genomic sequencing PCR (BGS) combined with TA clone and MSP (methylation-specific PCR) were used to analyze methylation status of theSTK31 gene promoter/exon1 region in HPV-positive cervical cancer cell lines (HeLa, SiHa, CaSki), HPV-negative cervical carcinoma cell lines (C33A, HT-3) and the transfected cells. The mRNA and protein expression of STK31, DNMT1, DNMT2, DNMT3a, DNMT3b and DNMT3L were detected by RT-PCR and Western blot.Results:Transfection efficiency was tested by Western blot, which showed that the transfected cells successfully expressed E6, E7, or E6/E7 proteins, respectively. TheSTK31 gene promoter/exon1 was hypomethylated in HPV-positive cell lines HeLa, SiHa and CasKi resulting in detection of mRNA and protein expression.STK31 gene promoter/exon1 showed hypermethylation leading to silenced expression in the two HPV-negative cervical cancer cells HT-3 and C33A. Compared with primary HT-3 and C33A cells, the methylation status ofSTK31 promoter/exon1 was down-regulated that led to expression of STK31 in the ectopically-expressed HPV16 E7 and E6/E7 cells. Expressions of DNMT1,DNMT3a andDNMT3b genes at the level of transcription were higher in C33AE6/E7 and HT-3E6/E7 cells than those in C33A-vector and HT-3 vector cells, respectively (P<0.001). mRNA levels of DNMT1, DNMT3a and DNMT3b were higher in HPV16-positive cervical cancer tissues than those in HPV-negative cervical cancer tissues, respectively (t=5.997,P<0.001;t=6.743,P<0.001;t=7.926,P<0.001). DNMT2 mRNA level was lower in C33AE6/E7 and HT-3E6/E7 cells than those in C33A-vector and HT-3 vector cells, respectively (t=7.451,P<0.001;t=2.451,P<0.05). mRNA level of DNMT2 gene was lower in HPV16-positive cervical cancer tissues than in HPV-negative cervical cancer tissues (t=9.134, P<0.001). There was no statistically significant difference in expression levels of DNMT3L mRNA between cervical cancer cell lines before and after transfection, or HPV16-positive and HPV-negative cervical cancer tissues, respectively (P>0.05, data not shown).Conclusion:HPV infection leads to the down-regulated methylation status ofSTK31 promoter/exon1 that results in the expression of STK31.STK31 gene expression is regulated by methylation status of its promoter/exon1 region. HPV16E7 andE6/E7 oncogenes may influence the methylation status ofSTK31 gene promoter/exon1 region by regulating the expression of DNMT2.