The EVER proteins as a natural barrier against papillomaviruses: a new insight into the pathogenesis of human papillomavirus infections

Abstract

Infections by human papillomaviruses (HPVs) are the most frequently occurring sexually transmitted diseases. The crucial role of genital oncogenic HPV in cervical carcinoma development is now well established. In contrast, the role of cutaneous HPV in skin cancer development remains a matter of debate. Cutaneous beta-HPV strains show an amazing ubiquity. The fact that a few oncogenic genotypes cause cancers in patients suffering from epidermodysplasia verruciformis is in sharp contrast to the unapparent course of infection in the general population. Our recent investigations revealed that a natural barrier exists in humans, which protects them against infection with these papillomaviruses. A central role in the function of this HPV-specific barrier is played by a complex of the zinc-transporting proteins EVER1, EVER2, and ZnT-1, which maintain cellular zinc homeostasis. Apparently, the deregulation of the cellular zinc balance emerges as an important step in the life cycles not only of cutaneous but also of genital HPVs, although the latter viruses have developed a mechanism by which they can break the barrier and impose a zinc imbalance. Herein, we present a previously unpublished list of the cellular partners of EVER proteins, which points to future directions concerning investigations of the mechanisms of action of the EVER/ZnT-1 complex. We also present a general overview of the pathogenesis of HPV infections, taking into account the latest discoveries regarding the role of cellular zinc homeostasis in the HPV life cycle. We propose a potential model for the mechanism of function of the anti-HPV barrier.

FIG. 1.

Genetic organization of the HPV16 (A) and HPV8 (B) genomes. The main functions of the early (E) and late (L) genes are mentioned. Promoters (P97 and P670) and early (AE) and late (AL) polyadenylation sites are indicated for HPV16. Early (P175) and late (P7535) sites are mentioned for HPV8. A schematic representation of the LCR is shown at the top of the figure, comprising an enhancer region, the origin of viral replication, and the E6/E7 promoter, as well as some binding sites for viral (E2) or cellular (YY1, NF1, AP1, GR/PR, CEBP, TEF1, TEF2, OCT-1, Sp1, and TFIID) transcription factors. Specific conserved motifs (M33 and M29) among the beta-HPVs are shown in the LCR of HPV8.

FIG. 2.

The HPV life cycle. (Adapted from reference with permission of the publisher.)

FIG. 3.

Patient suffering from epidermodysplasia verruciformis. Note the presence of a squamous cell carcinoma on the forehead.

FIG. 4.

Cellular proteins interacting with EVER1, EVER2, and ZnT-1. HGNC, HUGO Gene Nomenclature Committee.

FIG. 5.

Cytoscape software subgraph illustration of the EVER1 and EVER2 interactome data sets filtered for the overrepresentation of GO attributes (GO terms) using the Bingo plug-in (194). The node color is proportional to the P value for GO term enrichment. Clusters overlaid by color are related to the functional categories listed in Fig. 4.

FIG. 6.

Model proposed for the limitation of expression of HPV in keratinocytes. (A) In normal keratinocytes, the activation of AP-1 transcription factor family synthesis by Zn²⁺ is thwarted by the EVER1/EVER2/ZnT-1 complex. Infection of keratinocytes by HPV16 allows the synthesis of the HPV16 E5 protein, which alleviates the inhibition of the EVER/ZnT-1 complex and facilitates the expression of the transcription factors necessary for the expression and replication of the viral genome. (B) The lack of the E5 protein encoded by the genome of beta-HPV does not allow the vegetative replication of these viruses and protects the general population. (C) In epidermodysplasia verruciformis, a mutation in the EVER1 or EVER2 gene would block the formation of the EVER/ZnT-1 complex and would allow the expression of AP-1 transcription factors and viral replication.