Human papillomavirus type 16 E5 protein as a therapeutic target

Abstract

Cervical cancer is a progressive disease with an onset of one to two decades on average. During the productive replication stage, the Human papillomavirus (HPV) genome is maintained episomally in the infected cervical epithelium and early gene products, including E5, are expressed. Therefore, E5 has a potential to contribute to the HPV-associated carcinogenic process. In invasive malignancies, the HPV genomes are commonly integrated into the host genome, and E6 and E7 genes remain intact. However, the E5 is lost or, if present, under-expressed as compared with the E6 and E7 proteins. This suggests that E5 may play a critical role in the genesis of cervical cancer but less of a role in its persistence or progression. In the initiation of neoplasia and the premalignant stage, there are fewer malignant cells than in the invasive malignancies. Moreover, cells in the invasive malignant stage are found to have a very low level of MHC class I and II, which could hamper the presentation of the antigen and lead to a decreased immune response. Since the E5 protein is likely to play a role during the early tumorigenesis stage, a therapeutic vaccine to target and eliminate the E5-expressing cells may be a good strategy to prevent premalignant lesions from progressing toward invasive cervical cancers. This paper provides an overview of HPV-induced cervical carcinogenesis and strategies for designing prophylactic and therapeutic vaccines to prevent and cure the cervical cancer. In particular, focus will be on the rationale of targeting the E5 protein to develop therapeutic vaccines.

Fig. 1

Schematic diagram for linear HPV genome structure. The early (E) and late (L) genes, as well as the LCR, are shown. The poly A+ signals (PA) for the early and late mRNA are shown.

Fig. 2

HPV E5 protein facilitates EGFR recycling mediated by E5-ATPase binding. In normal cells, low pH in the endosome degrades the activated EGF receptor, thereby attenuating signaling. The vacuolar ATPase maintains the low pH required for receptor degradation. In the E5-transformed cells, the E5 protein binds directly to the vacuolar ATPase and inhibits its activity. This impairs endosome acidification and EGF receptor degradation, resulting in increased recycling of the receptor to the cell surface and enhanced receptor signaling. Therefore, cell proliferation is induced. (Modified from DiMaio D and Mattoon D, Mechanisms of cell transformation by papillomavirus E5 proteins. Oncogene 2001;20:7866-73.)

Fig. 3

Infection of HPV to target cell and early gene expression in basal layer. In the virion productive stage, the early proteins E1 and E2 are expressed. In the more distal layer, the E5, E6, and E7 proteins are expressed. These proteins promote cell proliferation and delay differentiation. As infected cells differentiate into squamous cells, the E4 protein and the late proteins L1 and L2 are expressed. The viral capsid is shed into the genital tract within desquamated epithelial cells. In the malignant stage, the HPV oncogenes are linearized randomly and integrated into the host cell genome. At that time, the host cell is induced into the premalignant stage by the E5 protein. Viral DNA integration leads to uncontrolled viral oncoprotein (E6, E7) production and host cell transformation.

Fig. 4

Host immune response to early HPV infection. The HPV infects the basal/suprabasal keratinocyte. The infected cell is proliferated and transformed by the E5 protein. This cell is in the premalignant stage. The E5 protein is presented by MHC class I and class II. CD4⁺ T-cells recognize the presented E5 antigenic epitope and are activated. The activated CD4⁺ T-cells activate CD8⁺ T-cells resulting in differentiation to the E5-specific CTL. This CTL specifically kills the E5 presenting premalignant cells.