Westrich Joseph A, Warren Cody J, Klausner Michael J, Guo Kejun, Liu Chang-Wei, Santiago Mario L, Pyeon Dohun
Department of Immunology and Microbiology, University of Colorado School of Medicine, Aurora, Colorado, USA.
Division of Infectious Diseases, Department of Medicine, University of Colorado School of Medicine, Aurora, Colorado, USA.
J Virol. 2018 Mar 14;92(7). doi: 10.1128/JVI.01318-17. Print 2018 Apr 1.
APOBEC3 (A3) mutation signatures have been observed in a variety of human cancer genomes, including those of cervical and head and neck cancers caused by human papillomavirus (HPV) infection. However, the driving forces that promote off-target A3 activity remain mostly unclear. Here, we report a mechanism for the dramatic increase of A3A protein levels in HPV-positive keratinocytes. We show that expression of the viral protein E7 from high-risk HPVs, but not E7 from low-risk HPVs, significantly prolongs the cellular half-life of A3A protein in human keratinocytes and HPV-positive cancer cell lines. We have mapped several residues within the cullin 2 (CUL2) binding motif of HPV16 E7 as being important for mediating A3A protein stabilization. Furthermore, we provide direct evidence that both A3A and HPV16 E7 interact with CUL2, suggesting that the E7-CUL2 complex formed during HPV infection may regulate A3A protein levels in the cell. Using an cytidine deaminase assay, we show that E7-stabilized A3A remains catalytically active. Taken together, our findings suggest that the HPV oncoprotein E7 dysregulates endogenous A3A protein levels and thus provides novel mechanistic insight into cellular triggers of A3 mutations in HPV-positive cancers. Human papillomavirus (HPV) is causally associated with over 5% of all human malignancies. Several recent studies have shown that a subset of cancers, including HPV-positive head and neck and cervical cancers, have distinct mutational signatures potentially caused by members of the APOBEC3 cytidine deaminase family. However, the mechanism that induces APOBEC3 activity in cancer cells is poorly understood. Here, we report that the HPV oncoprotein E7 stabilizes the APOBEC3A (A3A) protein in human keratinocytes by inhibiting ubiquitin-dependent protein degradation in a cullin-dependent manner. Interestingly, the HPV E7-stabilized A3A protein maintains its deaminase activity. These findings provide a new insight into cancer mutagenesis enhanced by virus-induced A3A protein stabilization.
在多种人类癌症基因组中都观察到了载脂蛋白B mRNA编辑酶催化多肽样3（APOBEC3，A3）突变特征，包括由人乳头瘤病毒（HPV）感染引起的宫颈癌、头颈癌。然而，促进脱靶A3活性的驱动因素仍大多不明。在此，我们报告了一种导致HPV阳性角质形成细胞中A3A蛋白水平显著增加的机制。我们发现，高危HPV的病毒蛋白E7的表达，而非低危HPV的E7，可显著延长人角质形成细胞和HPV阳性癌细胞系中A3A蛋白的细胞半衰期。我们已确定人乳头瘤病毒16型（HPV16）E7的cullin 2（CUL2）结合基序内的几个残基对介导A3A蛋白稳定很重要。此外，我们提供了直接证据表明A3A和HPV16 E7均与CUL2相互作用，这表明HPV感染期间形成的E7 - CUL2复合物可能调节细胞中的A3A蛋白水平。通过胞苷脱氨酶测定，我们发现E7稳定的A3A仍具有催化活性。综上所述，我们的研究结果表明，HPV癌蛋白E7会失调内源性A3A蛋白水平，从而为HPV阳性癌症中A3突变的细胞触发因素提供了新的机制见解。人乳头瘤病毒（HPV）与超过5%的人类恶性肿瘤存在因果关系。最近的几项研究表明，包括HPV阳性头颈癌和宫颈癌在内的一部分癌症具有可能由APOBEC3胞苷脱氨酶家族成员引起的独特突变特征。然而，癌细胞中诱导APOBEC3活性的机制仍知之甚少。在此，我们报告HPV癌蛋白E7通过以cullin依赖的方式抑制泛素依赖性蛋白降解来稳定人角质形成细胞中的载脂蛋白B mRNA编辑酶催化多肽样3A（A3A）蛋白。有趣的是，HPV E7稳定的A3A蛋白保持其脱氨酶活性。这些发现为病毒诱导的A3A蛋白稳定增强癌症诱变提供了新的见解。
