Opposite regulation of the HPV 20-URR and HPV 27-URR promoters by ultraviolet irradiation and cytokines

The Role of Beta HPV Types and HPV-Associated Inflammatory Processes in Cutaneous Squamous Cell Carcinoma

Cutaneous squamous cell carcinoma (cSCC) is a common form of skin cancer with a complex but not fully understood pathogenesis. Recent research suggests the role of beta human papillomavirus (HPV) types and HPV-associated inflammatory processes in cSCC development. Beta HPV types are components of the normal flora; however, under the influence of certain cofactors, the virus may trigger a malignant process. Dysregulation of the immune system (chronic inflammation and immunosuppression), environmental factors (ultraviolet radiation), and genetic factors are the most important cofactors involved in beta HPV-related carcinogenesis. In addition, the oncoproteins E6 and E7 of beta HPV types differ biochemically from their counterparts in the structure of alpha HPV types, resulting in different mechanisms of action in carcinogenesis. The aim of our manuscript is to present an updated point of view on the involvement of beta HPV types in cSCC pathogenesis.

Human Papillomaviruses and Skin Cancer

Human papillomaviruses (HPVs) infect squamous epithelia and can induce hyperproliferative lesions. More than 220 different HPV types have been characterized and classified into five different genera. While mucosal high-risk HPVs have a well-established causal role in anogenital carcinogenesis, the biology of cutaneous HPVs is less well understood.From patients with the rare genetic disorder epidermodysplasia verruciformis (EV) and animal models, evidence is accumulating that cutaneous PV of genus β synergize with ultraviolet (UV) radiation in the development of cutaneous squamous cell carcinoma (cSCC). In 2009, the International Agency for Research on Cancer (IARC) classified the genus β-HPV types 5 and 8 as "possible carcinogenic" biological agents (group 2B) in EV disease. Epidemiological and biological studies indicate that genus β-PV infection may also play a role in UV-mediated skin carcinogenesis in non-EV patients. However, they rather act at early stages of carcinogenesis and become dispensable for the maintenance of the malignant phenotype, compatible with a "hit-and-run" mechanism.This chapter will give an overview on genus β-PV infections and discuss similarities and differences of cutaneous and genus α mucosal high-risk HPV in epithelial carcinogenesis.

Apoptosis and in situ and invasive squamous cell carcinoma in sun-exposed sites

In vitro evidence indicates that the E6 protein of human papillomavirus (HPV) targets Bak, a proapoptotic protein, expression of which is enhanced in epidermal keratinocytes in response to ultraviolet B radiation. Given this, our aim was to ascertain Bak expression and prevalence of beta-HPV (β-HPV) in cutaneous squamous cell carcinoma (SCC) from sun-exposed sites to test our hypothesis that the virus plays a role in the neoplastic process by suppressing UV-induced apoptosis. This retrospective study included 30 cases of cutaneous SCC and 30 cases of SCC in situ (SCCIS) from sun-exposed sites. Immunohistochemical staining for Bak protein was performed on all, and β-HPV subtyping on 10 randomly selected cases from each group, using a broad-spectrum polymerase chain reaction-reverse hybridization assay. A semiquantitative scoring system for immunohistochemical expression of Bak was used based on the percentage positivity of the cells. Of cases studied, 30 of 30 (100%) of SCCIS and SCC (mean score 4.2 and 4.6, respectively, demonstrated immunopositivity, albeit to varying degrees, with Bak. Of the selected cases studied with reverse hybridization assay, 7 of 10 (70%) of SCCIS and 3 of 10 (30%) of SCC had β-HPV with HPV-5 being the most common subtype detected. Enhanced Bak immunoexpression confirms the presence of UV-induced apoptosis in both in situ as well as invasive epithelial malignancies, although the lack of differences in expression of Bak between both groups studied suggests that its relevance in disease progression is minimal. Expression of Bak in 100% of HPV-containing lesions from sun-exposed sites suggests that the virus does not abrogate UV-induced apoptosis.

Longitudinal study of seroprevalence and serostability of 34 human papillomavirus types in European organ transplant recipients

Organ transplant recipients (OTR) are at increased risk of cutaneous squamous cell carcinoma, which may be related to reactivation of human papillomavirus (HPV) infections. Measurement of change in HPV antibodies after transplantation would help to explore this hypothesis. We measured antibodies to 34 HPV types on up to six occasions over 18 months in 441 OTRs from five European countries. At baseline (mean 24 days after transplantation), 80% of all OTRs were seropositive to at least one HPV type. The beta HPV genus had the highest seroprevalence (45%). For most HPV genera baseline seroprevalence peaked between 40 and 59 years old. Most OTRs retained their serostatus over time and antibody levels were stable. Seroprevalence in immunosuppressed OTRs is stable in the 18 months immediately after transplantation. Thus there is no short-term evidence that immunosuppression leads to new or reactivated skin infection with HPV sufficient to induce antibodies.

The role of inflammation in HPV carcinogenesis

The role of inflammation in human papillomavirus (HPV) infection and disease is complex since it involves responses capable of preventing initial infections, clearing those ongoing as well as promoting persistence and progression of associated lesions. Avoiding the immune response has been considered a key aspect of HPV persistence which is the main factor leading to HPV-related neoplasia. HPVs have evolved different ways of targeting immune signaling pathways. Moreover, host inflammatory response may promote lesion progression and affect tumor fate by diverse mechanisms including the direct participation of inflammatory cells. In this review, we discuss the interplay between HPV oncogenic proteins and an array of inflammatory responses that ultimately may lead to cancer.

Papillomaviruses in the causation of human cancers - a brief historical account

Approximately 35 years ago a role of human papillomaviruses (HPV) in cervical cancer has been postulated. Today it is well established that this very heterogeneous virus family harbours important human carcinogens, causing not only the vast majority of cervical, but also a substantial proportion of other anogenital and head and neck cancers. In addition, specific types have been linked to certain cutaneous cancers. In females, HPV infections on a global scale account for more than 50% of infection-linked cancers, in males for barely 5%. Vaccines against the high risk HPV types 16 and 18 represent the first preventive vaccines directly developed to protect against a major human cancer (cervical carcinoma). This review will cover some of the historical aspects of papillomavirus research; it tries briefly to analyze the present state of linking HPV to human cancers and will discuss some emerging developments.

Human papillomavirus type spectrum in normal skin of individuals with or without a history of frequent sun exposure

Cutaneous human papillomavirus (HPV) has been widely detected in healthy skin. Previous studies have found that UV radiation can activate several HPV types, and a possible role for cutaneous HPV in the development of non-melanoma skin cancer has been suggested. This study investigated the prevalence and type-spectrum of cutaneous HPV in relation to UV radiation by studying forehead skin swab samples from 50 healthy males frequently exposed to the sun and 50 healthy males who were not frequently exposed to the sun. A questionnaire including ethnic background of the participants, history of cancers and a self-assessment of sun-exposure was also conducted and analysed. PCR with the FAP primer pair was carried out to detect HPV DNA in samples. HPV prevalence was higher in individuals who spent more time outdoors and in individuals with a history of skin cancers (P=0.044 and P=0.04, respectively). Furthermore, individuals wearing sunglasses as a means of sun protection had a lower prevalence of HPV (P=0.018). Interestingly, HPV-76 was only detected in the group without frequent sun-exposure (P=0.001). These results suggest that increased UV radiation exposure may be a factor leading to a difference in prevalence of cutaneous HPV types.

Altered expression of UVB-induced cytokines in human papillomavirus-immortalized epithelial cells

Keratinocytes can be induced to produce cytokines by exogenous stimuli, such as UVB, and dysregulation of this production has been described in various skin diseases, including cancer. In this study, we compared the effect of UVB on the secretion of several cytokines involved in inflammation by human keratinocytes immortalized or not with human papillomavirus (HPV)16 or HPV38 at the mRNA and protein levels. We show that expression of the HPV E6/E7 oncoproteins influences not only the basal cytokine secretion profile of keratinocytes, but also its modulation upon UVB irradiation. In particular, UVB upregulates interleukin (IL)-6, IL-8 and transforming growth factor (TGF)-beta in HPV-immortalized cells to a higher extent than in control keratinocytes. Moreover, expression of other pro-inflammatory molecules such as S100A8/9 and interferon (IFN)-kappa was downregulated in HPV-immortalized cells. These data support the functional similarity between HPV16 and 38, and suggest an active role of these viruses in modulation of the inflammatory process.

Beta-papillomaviruses and psoriasis: an intra-patient comparison of human papillomavirus carriage in skin and hair

BACKGROUND

Human papillomaviruses (HPVs) of the beta genus (beta-PV), especially HPV5 and HPV36, are proposed to play a pathogenic role in psoriasis, but many previous studies have failed to control for potential confounders, including treatment.

OBJECTIVES

To re-examine the relationship between beta-PV and psoriasis addressing limitations present in previous studies and analyse intra-patient concordance for carriage of HPV.

METHODS

Plucked eyebrow hairs and forearm skin scrapes were collected from 20 newly diagnosed, previously untreated adult patients with psoriasis and 23 normal controls. A combination of type-specific and degenerate polymerase chain reaction methods was used to achieve comprehensive HPV DNA detection.

RESULTS

The prevalence of HPV in hair and skin from psoriasis patients was higher than in controls (83.3% vs. 46.7%, respectively, P < 0.03 corrected for age and clustering). HPV5 or HPV36 were not over-represented. The profile of diverse beta-PV types was comparable in the two groups. Intra-patient concordance for HPV DNA at separate sites was high (P < 0.00001).

CONCLUSIONS

Our data do not support a specific causal role for HPV5 or HPV36 in psoriasis, but suggest that psoriatic skin may be more permissive for viral presence than normal skin. High intra-patient concordance for specific HPV types at separate sites, together with the ubiquity of HPV DNA in normal human skin, suggests that an individual becomes colonized with a particular beta-PV profile presumably to the exclusion of other types. To what extent this HPV profile is then causal in the subsequent development of hyperproliferative skin disease is unknown.

Cutaneous human papillomaviruses found in sun-exposed skin: Beta-papillomavirus species 2 predominates in squamous cell carcinoma

BACKGROUND

A spectrum of cutaneous human papillomaviruses (HPVs) is detectable in nonmelanoma skin cancers, as well as in healthy skin, but the significance that the presence of these types of HPV DNA has for the pathogenesis of skin cancer remains unclear.

METHODS

We studied 349 nonimmunosuppressed patients with skin lesions (82 with squamous cell carcinomas, 126 with basal cell carcinomas, 49 with actinic keratoses, and 92 with benign lesions). After superficial skin had been removed by tape, paired biopsy samples--from the lesion and from healthy skin from the same patient--were tested for HPV DNA. Risk factors for HPV DNA were analyzed in multivariate models.

RESULTS

Overall, 12% of healthy skin samples were positive for HPV DNA, compared with 26% of benign lesions, 22% of actinic keratoses, 18% of basal cell carcinomas, and 26% of squamous cell carcinomas. HPV DNA was associated with sites extensively exposed to the sun, both for the lesions (odds ratio [OR], 4.45 [95% confidence interval {CI}, 2.44-8.11]) and for the healthy skin samples (OR, 3.65 [95% CI 1.79-7.44]). HPV types of Beta-papillomavirus species 2 predominate in squamous cell carcinomas (OR, 4.40 [95% CI, 1.92-10.06]), whereas HPV types of Beta-papillomavirus species 1 are primarily found in benign lesions (OR, 3.47 [95% CI, 1.72-6.99]).

CONCLUSIONS

Cutaneous HPV types are primarily detected at sites extensively exposed to the sun. HPV types of Beta-papillomavirus species 2, but not of species 1, are associated with squamous cell carcinoma.

E6/E7 expression of human papillomavirus type 20 (HPV-20) and HPV-27 influences proliferation and differentiation of the skin in UV-irradiated SKH-hr1 transgenic mice

The functional role of UV irradiation, in combination with the E6 and E7 proteins of the cutaneous human papillomavirus (HPV) types in the malignant conversion of benign papillomatous lesions, has not been elucidated. Transgenic SKH-hr1 hairless mice expressing HPV-20 and HPV-27 E6 and E7 proteins in the suprabasal compartment were generated and exposed to chronic UV irradiation. Histological and immunohistochemical examination of skin samples revealed enhanced proliferation of the epidermal layers and papilloma formation in both transgenic strains in comparison to what was observed with nontransgenic mice. Squamous cell carcinoma developed in the HPV-20 E6/E7 transgenic line as well as in the HPV-27 E6/E7 transgenic line. Several weeks after cessation of UV-B exposure, enhanced proliferation, as measured by BrdU incorporation, was maintained only in HPV-20 transgenic skin. Keratin 6 expression was increased in the transgenic mice throughout all cell layers. Expression of the differentiation markers involucrin and loricrin was reduced and disturbed. p63alpha expression was differentially regulated with high levels of cytoplasmic expression in clusters of cells in the granular layer of the skin in the transgenic lines 20 weeks after cessation of UV-B exposure, in contrast to uninterrupted staining in the nontransgenic lines. p53 was expressed in clusters of cells in nontransgenic and HPV-27 transgenic mice, in contrast to an even distribution in a higher number of cells in HPV-20 transgenic animals.

TAp63alpha indirectly regulates a cutaneous HPV promoter through complex formation with Jun family members

The p63alpha isoforms of the p53 family have been demonstrated to play a crucial role in the development and differentiation of the skin. We show that expression of the TAp63alpha isoform leads to an upregulation of the cutaneous papillomavirus HPV 20 promoter, which is increased at least three-fold when c-Jun is co-expressed, in contrast to a minimal increase in activity in the presence of c-Jun alone. Co-expression of TAp63alpha with JunB or JunD, respectively, and in combination, leads to a reduction in the viral promoter activation measured by the expression of TAp63alpha alone. JunB and JunD also inhibits the additive effect exerted on the TAp63alpha activation by c-Jun. Co-immunoprecipitation assays demonstrate a complex formation of c-Jun, JunB and JunD with TAp63alpha through the SAM domain mediating protein-protein interactions, which is characteristic for p63alpha. Co-expression of p53 mutant R248W not only downregulates the differential modulation of the viral promoter by TAp63alpha alone and in the presence of the Jun family members, but leads to a reduction in the protein levels of the overexpressed c-Jun, JunB, JunD, as well as TAp63alpha. This model system provides insight into yet unknown pathways through which TAp63alpha and Jun may cooperate in the pathogenesis of HPV associated cutaneous lesions.

The Effect of Ultraviolet Radiation on Human Viral Infections

Exposure to UV radiation is recognized to suppress cell-mediated immunity and therefore could adversely affect the course of a viral infection. Rodent models of viral infection confirm this possibility but the situation in human subjects is not so clear, apart from two exceptions. These are herpes simplex, in which sunlight exposure can cause reactivation, and certain papillomavirus types in which sunlight exposure can lead to the development of nonmelanoma skin cancer. In both cases, there are UV response elements in the viral genomes that alter the normal interactions between the viruses and the host following exposure, and UV-induced effects on the immune response occur in addition. These complex mechanisms are discussed, and the situation regarding UV radiation and viral exanthems plus other viruses, including the retroviruses, summarized. Finally viral vaccination is considered in the context of UV exposure and the importance of the host's genetic background emphasized. Further research is required to evaluate whether sunlight can significantly affect the resistance to common viral infections and vaccines.

Human papillomaviruses and skin cancer

Cutaneous epidermodysplasia verruciformis (EV)-associated human papillomaviruses (HPVs) are found frequently in skin cancers especially in immunosuppressed people. They are also detectable in the normal skin and hair follicles of a proportion of individuals who have no immune defect. The available evidence to link HPVs causally with skin carcinogenesis is not conclusive, but includes epidemiological, molecular and immunological studies.

High prevalence of cutaneous human papillomavirus DNA on the top of skin tumors but not in "Stripped" biopsies from the same tumors

Genomes of human papillomaviruses (HPV) are common in biopsies from non-melanoma skin cancers but are also found on healthy skin and it is possible that HPV positivity in tumor biopsies by PCR may merely reflect contamination of the lesion surface. To investigate this issue, 229 immunocompetent patients were tested for HPV DNA in swab samples collected on top of skin tumors and in biopsies of the same tumors, obtained after stripping with tape to remove superficial layers. HPV DNA was detected on top of 69% (159 of 229) of the lesions, and in 12% (28 of 229) of the stripped biopsies (p<0.001). The difference was seen for all four types of tumors studied. Seborrheic keratosis had 79% (34 of 43) HPV positivity on top of lesions versus 19% (eight of 43) in biopsies; actinic keratosis had 83% (38 of 46) HPV positivity on top versus 11% (five of 46) in biopsies; basal cell carcinoma had 63% (69 of 109) on top versus 8% (nine of 109) in biopsies and squamous cell carcinoma had 58% (18 of 31) on top versus 19% (six of 31) in biopsies. HPV DNA is common in superficial layers of lesions, but is not necessarily present throughout tumors.

Chapter 8: Human papillomavirus and skin cancer

A high prevalence of human papillomavirus (HPV) DNA, particularly in squamous cell skin carcinoma of immunosuppressed but also of immunocompetent patients, has renewed great interest in a possible etiologic role of HPV in nonmelanoma skin cancer. It is difficult, however, to interpret these findings against a background of low-level infections with multiple HPV types from supergroup B (HPV4-related and epidermodysplasia verruciformis [EV] HPV), probably acquired by everyone early in and throughout life. Thus far, no high-risk HPV types have been identified. Because of the low copy numbers of HPV DNA in skin cancers, probably not every tumor cell contains a viral genome, which is compatible with cutaneous HPV being possibly important for tumor initiation and progression, but not for maintenance of the malignant phenotype. The question with regard to high-risk types should, therefore, be readdressed in case-control studies on the basis of serology, which can reveal viral activities over years. The viruses lingering in all people are apparently activated by sunlight (UV) exposure, by immunosuppression, and by hyperproliferation of the epithelium (psoriasis) and/or in the specific genetic background of the host (EV). It is intriguing that most of these factors are established risk factors in skin carcinogenesis. The weak transforming activity of cutaneous HPV in vitro compared with the transforming activity of genital HPV may explain the need for activators and synergistic factors. The antiapoptotic activities of E6 proteins of cutaneous HPV could be relevant to oncogenesis in the interplay with UV exposure. Prospective studies should determine the kinetics of HPV activation relative to tumor development.

UVB irradiation reduces the half-life and transactivation potential of the human papillomavirus 16 E2 protein

Human papillomaviruses (HPV) are causative agents of human cancers including those of the cervix and also of the head and neck; HPV16 is the most commonly found type in these diseases. The viral E2 protein regulates transcription from the viral genome by interacting with DNA-binding sequences in the HPV transcriptional control region; it also regulates replication by interacting with and recruiting the HPV replication factor E1 to the viral origin. Therefore, E2 is essential for the viral life cycle. The E2 protein interacts with several proteins involved in the cellular response to DNA damage including p53, TopBP1, and PARP. We therefore set out to establish whether DNA-damaging agents can regulate E2 activity. Here we show that UVB irradiation downregulates transcriptional activity of both HPV16 and HPV8 E2, while hydroxyurea and etoposide do not. This downregulation of E2 activity is independent of p53 function as it occurs in p53 wild type and null cell types as well as in the presence of functional HPV16 E6 that degrades p53. Using stable cell lines expressing E2 we show that this downregulation of E2 function by UVB is due to a reduction of the E2 protein half-life. The identification of the pathway(s) through which UVB downregulates E2 transcriptional activity and protein levels will present a novel target for the treatment of HPV-related diseases.

Epidermal tissue regeneration and stromal interaction in HaCaT cells is initiated by TGF-alpha

The human keratinocyte cell line HaCaT expresses essentially all epidermal differentiation markers but exhibits deficiencies in tissue organization as surface transplants in nude mice and even more so in organotypic co-cultures with fibroblasts. Whereas tissue differentiation by normal keratinocytes (NEKs) is regulated by stromal interactions, this mechanism is impaired in HaCaT cells. This regulatory process is initiated by interleukin-1 (IL-1) release in keratinocytes, which induces expression of keratinocyte growth factor (KGF/FGF-7) and granulocyte macrophage-colony stimulating factor (GM-CSF) in fibroblasts. Production and release of IL-1 is very low and, consequently, expression of the fibroblast-derived growth factors KGF/FGF-7 and GM-CSF is absent in HaCaT-fibroblast co-cultures. However, addition of KGF and GMCSF, respectively, is inefficient to improve stratification and differentiation by HaCaT cells due to the low expression of their cognate receptors. More importantly, expression and release of the autocrine keratinocyte growth factor TGF-alpha is dramatically decreased in HaCaT cells. Addition of TGF- alpha or EGF stimulated HaCaT cell proliferation but, even more effectively, suppressed apoptosis, thus facilitating the formation of a regularly stratified epithelium. Furthermore, TGF-alpha enhanced the expression of the receptors for KGF and GM-CSF so that addition of these growth factors, or of their inducer IL-1, further improved epidermal tissue differentiation leading to in vitro skin equivalents comparable with cultures of NEKs. Thus, supplementing TGF-alpha normalized epidermal tissue regeneration by immortal HaCaT keratinocytes and their interaction with stromal cells so that regular skin equivalents are produced as standardized in vitro models.

The role of p53 in the immunobiology of cutaneous squamous cell carcinoma

Cutaneous squamous cell carcinoma is typically characterized by the over-expression of the tumour suppressor protein p53. Considerable evidence suggests that immune competence is important in the control of cutaneous SCC. We discuss the immunobiology of p53 and its relevance to cutaneous SCC, including the potential interaction with human papillomavirus.

Mucosally-derived HPV-40 can infect both human genital foreskin and cutaneous hand skin tissues grafted into athymic mice

HPV-40 is a rare HPV type that has been detected only in genital mucosal tissues. This HPV type is very closely related to HPV-7, which has a predominantly cutaneous tissue tropism. We have shown, previously, that an isolate of HPV-40 (described here as HPV-40(Hershey) or HPV-40(H)) productively infected genital tissues. In this study, HPV-40(H) was tested for productive infection of cutaneous tissue. Fetal hand skin fragments were incubated with infectious HPV-40(H) and implanted subrenally into athymic mice. After 120 days, xenografts showed morphological changes consistent with HPV-40(H) infection and were HPV-40 DNA in situ positive and capsid antigen positive. The results demonstrated that hand skin can support HPV-40(H) infection thereby indicating that this viral type has the capacity to infect both genital mucosal and cutaneous tissues.

The enhanced IL-18 production by UVB irradiation requires ROI and AP-1 signaling in human keratinocyte cell line (HaCaT)

Based on our recent observation that enhanced IL-18 expression positively correlates with malignant skin tumors, such as SCC and melanoma, we examined the possible role of UVB, known to be associated with skin cancer development, in the enhancement of IL-18 production using primary human epidermal keratinocytes and human keratinocyte cell line HaCaT. After cells were exposed to UVB irradiation in vitro, IL-18 production was examined by Northern blot analysis and ELISA, and it was found that IL-18 production is enhanced by UVB irradiation in a dose- and time-dependent manner. In addition, we confirmed that it is functionally active form of IL-18 using the inhibitor of caspase-1. The effect of UVB irradiation was blocked by antioxidant, N-acetyl-L-cysteine (NAC), which suggested the involvement of reactive oxygen intermediates (ROI) in the signal transduction of UVB irradiation-enhanced IL-18 synthesis. We also found that UVB irradiation increased AP-1 binding activity by using EMSA with AP-1-specific oligonucleotide. Furthermore, inhibitors of UVB-induced AP-1 activity, such as PD98059, blocked enhanced IL-18 production, indicating that AP-1 activation is required for UVB-induced IL-18 production. Taken together, our results suggest that UVB irradiation-enhanced IL-18 production is selectively mediated through the generation of ROI and the activation of AP-1.

Intracellular redox regulation by a cystine derivative suppresses UV-induced NF-kappa B activation

Nuclear factor (NF)-kappa B pathways are influenced by the intracellular reduction-oxidation (redox) balance. While NF-kappa B is activated through inhibitor (I)-kappa B degradation by oxidative stress, its DNA binding is accelerated in the reduced state. We found that N,N'-diacetyl-L-cystine dimethylester (DACDM) suppressed the UVB-induced NF-kappa B binding activity at a much lower concentration (50-100 microM) than N-acetyl-L-cysteine (NAC, 10-30 mM). While NAC suppressed the I-kappa B degradation but not the DNA binding, DACDM prevented the activated NF-kappa B from binding DNA, without influencing the I-kappa B degradation. These properties of DACDM make it possible to effectively regulate the intracellular redox balance.

Human papillomaviruses and non-melanoma skin cancer

Epidemiological and experimental studies have overwhelmingly confirmed human papillomaviruses as important causal agents in anogenital carcinogenesis. A role for human papillomaviruses has also been proposed in a diverse range of other malignancies, and particular interest has focused on non-melanoma skin cancer, the commonest malignancy in fair-skinned populations worldwide. Although the evidence for this is considerably less convincing than for anogenital cancer, important epidemiological and functional data have emerged over the past year that have furthered our understanding of the possible contribution of human papillomaviruses to skin cancer. Epidemiological human papillomavirus DNA detection studies have shown associations with non-melanoma skin cancer, but have also emphasized the ubiquity of epidermodysplasia verruciformis human papillomavirus types in normal skin, hair follicles and benign hyperproliferative disorders, as have seroepidemiological approaches. Functional investigations have demonstrated mechanistically relevant interactions between the virus and ultraviolet radiation, host cytokines and cellular proteins including p53 and the pro-apoptotic protein Bak. Taken together, these data have advanced our understanding of the contribution of human papillomaviruses to malignant transformation in cutaneous keratinocytes, but further research is required before a causal association between human papillomaviruses and skin cancer is reliably confirmed.