Regulation of the life cycle of HPVs by differentiation and the DNA damage response

Abstract

HPVs are the causative agents of cervical and other anogenital cancers. HPVs infect stratified epithelia and link their productive life cycles to cellular differentiation. Low levels of viral genomes are stably maintained in undifferentiated cells and productive replication or amplification is restricted to differentiated suprabasal cells. Amplification is dependent on the activation of the ATM DNA damage factors that are recruited to viral replication centers and inhibition of this pathway blocks productive replication. The STAT-5 protein appears to play a critical role in mediating activation of the ATM pathway in HPV-positive cells. While HPVs need to activate the DNA damage pathway for replication, cervical cancers contain many genomic alterations suggesting that this pathway is circumvented during progression to malignancy.

Figure 1. Structure of the HPV genome

The linearized HPV31 genome is shown. The replication ori and enhancer are located in the URR. The viral early promoter, later promoter and two polyadenylation sites regulate the expression of three groups of viral genes. E6 and E7 immortalize the host cells and maintain replication competence. E1, E2, E4, E5 and E8 are involved in viral DNA replication, alternation in DNA damage response, suppression of immune response and other functions. L1 and L2 are the two capsid genes. Ori: Origin; URR: Upstream regulatory region.

Figure 2. Life cycle of HPV

HPVs infect keratinocytes in the basal layer of stratified epithelia following exposure through microwounds. (A) A normal uninfected epithelium and (B) a HPV-infected epithelium. Following entry and establishment in the nucleus, viral genomes are replicated in basal cells together with cellular chromosomes. Only a low level of expression of viral genes is observed in infected basal cells and significant levels of viral transcripts are seen in differentiated cells. Upon differentiation, HPV late gene expression and amplification is activated. This is followed by virion assembly, and the newly synthesized virions are shed from the top layers of epithelium.

Figure 3. HPV uses STAT-5 signaling to activate the ATM kinase DNA damage response

STAT-5 has important roles in cell proliferation, apoptosis and differentiation. Normally, the cytoplasmic forms of STAT-5 are phosphorylated upon stimulation with cytokines, such as GM-CSF and IL-6. The activated form of STAT-5 translocates into the nucleus to turn on downstream genes, such as PPARγ. Upon HPV infection, HPV activates STAT-5 in the absence of exogenous cytokines. The phosphorylated STAT-5 induces increased levels of PPARγ in HPV-positive cells, which activates the ATM DNA damage pathway. The ATM DNA damage is necessary for HPV genome amplification. E7 induces activation of ATM, which then turns on downstream effectors, such as BRCA1, CHK2 and p53, leading to cell cycle arrest and HPV amplification. P: Phosphate.