Cervical cancer development is linked to the persistent infection by high-risk mucosal human papillomaviruses (HPVs) types. The E6 and E7 major oncoproteins from this dsDNA virus play a key role in the deregulation of the cell cycle, apoptosis, and adaptive immune surveillance. In this study, we show for the first time that HPV type 16 (HPV16), the most carcinogenic type among the high-risk subgroup, interferes with innate immunity by affecting the expression of TLRs. Infection of human primary keratinocytes with HPV16 E6 and E7 recombinant retroviruses inhibits TLR9 transcription and hence functional loss of TLR9-regulated pathways. Similar findings were achieved in HPV16-positive cancer-derived cell lines and primary cervical cancers, demonstrating that this event occurs also in an in vivo context. Interestingly, E6 and E7 from the low-risk HPV type 6 are unable to down-regulate the TLR9 promoter. In addition, E6 and E7 from the high-risk HPV type 18, which are known to persist less competently in the host than HPV16, have reduced efficiency compared with HPV16 in inhibiting TLR9 transcription. Furthermore, a CpG motif derived from the HPV16 E6 DNA sequence activated TLR9, indicating this virus is able to initiate innate responses via the receptor it later down-regulates. This study reveals a novel mechanism used by HPV16 to suppress the host immune response by deregulating the TLR9 transcript, providing evidence that abolishing innate responses may be a crucial step involved in the carcinogenic events mediated by HPVs.

More than 100 different human papillomavirus (HPV) types have been isolated so far, and they can be sub-grouped in cutaneous or mucosal according to their ability to infect the skin or the mucosa of the genital or upper-respiratory tracts. A sub-group of human mucosal HPVs, referred to as high-risk HPV types, is responsible for approximately 5% of all human cancers, which represents one-third of all the tumours induced by viruses. Epidemiological and biological studies have shown that HPV16 is the most oncogenic type within the high-risk group. Emerging lines of evidence suggest that, in addition to the high-risk mucosal HPV types, certain cutaneous HPVs are involved in skin cancer. HPV-associated cancers are intimately linked to HPV persistence and the accumulation of chromosomal rearrangements. The products of the early genes, E6 and E7, of the high-risk mucosal HPV types play a key role in both events. Indeed, these proteins have developed a number of strategies to evade host immuno-surveillance allowing viral persistence, and to alter cell cycle and apoptosis control, facilitating the accumulation of DNA damage/mutations. Often, the two oncoproteins target the same cellular pathways with different mechanisms, showing a strong synergism in promoting cellular transformation and neutralizing the immune response. Here, we review most of the findings on the biological properties and molecular mechanisms of the oncoproteins E6 and E7 from mucosal and cutaneous HPV types.

Several studies have suggested the involvement of cutaneous human papillomaviruses (HPVs) in the development of nonmelanoma skin cancers. Here we have characterized the in vitro properties of E7 proteins of three cutaneous HPV types, 10, 20, and 38, which are frequently detected in skin specimens. We show that HPV38 E7 is able to inactivate the tumor suppressor pRb and induces loss of G 1 /S transition control, a key event in carcinogenesis. In contrast, HPV10 and HPV20 E7 proteins do not display these in vitro transforming activities. We also show that the two early proteins E6 and E7 of HPV38 are sufficient to corrupt the cell cycle and senescence programs in primary cells, inducing active and long-lasting proliferation of primary human keratinocytes, the natural host cells. Our study shows that E6 and E7 of this cutaneous HPV type have transforming activity in primary human cells, suggesting a role for HPV38 infection in skin carcinogenesis. In further support of such a role, we detected HPV38 DNA in approximately 50% of nonmelanoma skin cancers, but only in 10% of healthy skin specimens (P < 0.001).Nonmelanoma skin cancer is the most frequently occurring malignancy in the Caucasian population (34,38,47). Although these cancers have a good prognosis and are not normally associated with mortality, an increasing incidence of other invasive cancers and cancer mortality following nonmelanoma skin cancers has been reported (17,24,28,29). Several lines of evidence suggest the involvement of an infective agent in the etiology of this condition. Patients suffering from a rare genetic immune suppression termed epidermodysplasia verruciformis and individuals under long-lasting immunosuppression are prone to develop these cancers (21,22,30,37). Epidermodysplasia verruciformis patients are highly susceptible to human papillomavirus (HPV) infections by a specific subgroup of cutaneous HPVs, the so-called epidermodysplasia verruciformis types (e.g., HPV5 and HPV8), that lead to extensive verrucosis of confluent flat warts (22,30,37). In approximately 30% of cases, the HPV lesions develop into multifocal squamous cell carcinomas.Supporting the infectious role of cutaneous HPV types in the tumorigenesis of nonmelanoma skin cancers is the fact that other members of the papillomavirus family are clearly oncogenic (55). Indeed, clinical, epidemiological, and molecular data have demonstrated that mucosal high-risk HPV types (e.g., high-risk HPV16 and HPV18) are the etiological agents of anogenital cancers as well as a subgroup of head and neck cancers (55). The early region of these HPV types encodes two oncoproteins, E6 and E7, which associate with and neutralize the cellular tumor suppressors p53 and retinoblastoma (pRb), respectively (32,36).Independent studies suggest that cutaneous HPV types may also be involved in the development of squamous cell carcinoma and basal cell carcinomas in the general population (6,7,14,43). These indications are based only on studies assessing viral DNA presence in skin tumors by PCR, which ha...

The E6 and E7 of the cutaneous human papillomavirus (HPV) type 38 immortalize primary human keratinocytes, an event normally associated with the inactivation of pathways controlled by the tumour suppressor p53. Here, we show for the first time that HPV38 alters p53 functions. Expression of HPV38 E6 and E7 in human keratinocytes or in the skin of transgenic mice induces stabilization of wild-type p53. This selectively activates the transcription of DNp73, an isoform of the p53-related protein p73, which in turn inhibits the capacity of p53 to induce the transcription of genes involved in growth suppression and apoptosis. DNp73 downregulation by an antisense oligonucleotide leads to transcriptional re-activation of p53-regulated genes and apoptosis. Our findings illustrate a novel mechanism of the alteration of p53 function that is mediated by a cutaneous HPV type and support the role of HPV38 and DNp73 in human carcinogenesis.

The human papillomavirus (HPV) family comprises more than 170 different types that preferentially infect the mucosa of the genitals, upper-respiratory tract, or the skin. The ‘high-risk HPV type’, a sub-group of mucosal HPVs, is the cause of approximately 5% of all human cancers, which corresponds to one-third of all virus-induced tumours. Within the high-risk group, HPV16 is the most oncogenic type, being responsible for approximatively 50% of all worldwide cervical cancers. Many studies suggest that, in addition to the high-risk mucosal HPV types, certain cutaneous HPVs also have a role in the development of non-melanoma skin cancer (NMSC).Functional studies on the HPV early gene products showed that E6 and E7 play a key role in carcinogenesis. These two proteins use multiple mechanisms to evade host immune surveillance, allowing viral persistence, and to deregulate cell cycle and apoptosis control, thus facilitating the accumulation of DNA damage and ultimately cellular transformation.The demonstration that high-risk HPV types are the etiological agents of cervical cancer allowed the implementation in the clinical routine of novel screening strategies for cervical lesions, as well as the development of a very efficient prophylactic vaccine. Because of these remarkable achievements, there is no doubt that in the coming decades we will witness a dramatic reduction of cervical cancer incidence worldwide.

For the p53 Special IssueFunctional loss of the tumor suppressor p53 by alterations in its TP53 gene is a frequent event in cancers of different anatomical regions. Cervical cancer is strongly linked to infection by high-risk human papillomavirus (HPV) types. The viral oncoprotein E6 has the ability to associate with and neutralize the function of p53. E6 interacts with a 100-kDa cellular protein, termed E6 associated protein (E6AP; also called ubiquitin-protein ligase E3A or UBE3A), which functions as an ubiquitin protein ligase. The dimeric complex then binds p53 and E6AP catalyzes multi-ubiquitination and degradation of p53. The ability to promote p53 degradation is an exclusive property of E6 from the high-risk HPV types. Indeed, the low-risk E6 proteins lack this activity, although they can bind p53. Consistent with the E6 function of the high-risk HPV types, the majority of cervical cancer cells have a wild-type p53 gene, but the protein levels are strongly decreased. Several independent studies have shown that in a small percentage of cervical tumors the p53 gene is mutated. However, this event appears to be unrelated to the presence or absence of HPV infection and the nature of the tumor. Hum Mutat 21:307-312,

Felis catus papillomavirus type 2 (FcaPV2) DNA is found in feline cutaneous squamous cell carcinomas (SCCs); however, its biological properties are still uncharacterized. In this study, we successfully expressed FcaPV2 E6 and E7 putative oncogenes in feline epithelial cells and demonstrated that FcaPV2 E6 binds to p53, impairing its protein level. In addition, E6 and E7 inhibited ultraviolet B (UVB)-triggered accumulation of p53, p21 and pro-apoptotic markers such as Cleaved Caspase3, Bax and Bak, suggesting a synergistic action of the virus with UV exposure in tumour pathogenesis. Furthermore, FcaPV2 E7 bound to feline pRb and impaired pRb levels, resulting in upregulation of the downstream pro-proliferative genes Cyclin A and Cdc2. Importantly, we demonstrated mRNA expression of FcaPV2 E2, E6 and E7 in feline SCC samples, strengthening the hypothesis of a causative role in the development of feline SCC.

Cutaneous beta human papillomavirus (HPV) types are suspected to be involved, together with ultraviolet (UV) radiation, in the development of non-melanoma skin cancer (NMSC). Studies in in vitro and in vivo experimental models have highlighted the transforming properties of beta HPV E6 and E7 oncoproteins. However, epidemiological findings indicate that beta HPV types may be required only at an initial stage of carcinogenesis, and may become dispensable after full establishment of NMSC. Here, we further investigate the potential role of beta HPVs in NMSC using a Cre-loxP-based transgenic (Tg) mouse model that expresses beta HPV38 E6 and E7 oncogenes in the basal layer of the skin epidermis and is highly susceptible to UV-induced carcinogenesis. Using whole-exome sequencing, we show that, in contrast to WT animals, when exposed to chronic UV irradiation K14 HPV38 E6/E7 Tg mice accumulate a large number of UV-induced DNA mutations, which increase proportionally with the severity of the skin lesions. The mutation pattern detected in the Tg skin lesions closely resembles that detected in human NMSC, with the highest mutation rate in p53 and Notch genes. Using the Cre-lox recombination system, we observed that deletion of the viral oncogenes after development of UV-induced skin lesions did not affect the tumour growth. Together, these findings support the concept that beta HPV types act only at an initial stage of carcinogenesis, by potentiating the deleterious effects of UV radiation.