Human papillomavirus (HPV) types from the beta genus (beta-HPVs) have been implicated in the development of skin cancer. A potentially important aspect of their carcinogenic role is the ability of the E6 protein to degrade the proapoptotic family member Bak, which gives cells the ability to survive UV damage. However, it is unknown if the ability to degrade Bak is limited to certain beta-HPV types or whether E6 expression in keratinocytes affects other proteins important for apoptosis signaling. We tested the abilities of E6 proteins from several representative members of the beta-HPVs to degrade Bak and protect UV-treated keratinocytes from apoptosis. The E6 proteins of the beta-HPV type 5 (HPV5), -8, -20, -22, -38, -76, -92, and -96, as well as the alpha genus HPV HPV16, all degraded Bak or prevented its accumulation following UV treatment but did not degrade Bak constitutively. In addition, when tested using HPV16 E6 (16E6) and 8E6 as representative E6 proteins from the alpha and beta genera, respectively, Bak degradation was dependent on the E3 ubiquitin ligase, E6AP. Other important regulators of apoptotic signaling were examined and found to be unperturbed by the expression of the beta-HPV E6 proteins. Importantly, the expression of beta-HPV E6 proteins protected keratinocytes from apoptosis to the same extent as 16E6-expressing cells. In conclusion, several of the beta-HPV types possess the ability to protect UV-treated keratinocytes from apoptosis by reducing levels of Bak in those cells, thus blocking the intrinsic apoptotic pathway.Human papillomaviruses (HPVs) are small DNA tumor viruses that infect cutaneous and mucosal epithelia and disseminate by replication in terminally differentiated keratinocytes. So far, over 100 different types have been identified and characterized on the basis of DNA sequence analysis (12). Only a small number of these display a strong association with cancer development. Most notably, the high-risk types (16, 18, 31, 33, etc.) of alpha genus HPVs (alpha-HPVs) play a critical role in the development of cervical cancer. These same types have also been implicated in the majority of other anogenital cancers and a subset of head and neck carcinomas (11,43).Recently, the beta-HPVs, which cause cutaneous lesions in humans, have been linked to the development of skin cancers (32). The association between beta-HPVs and skin cancer was first identified in patients with the rare inherited disorder epidermodysplasia verruciformis (EV) (27). These individuals have a predisposition to the early development of disseminated, persistent flat warts and macular lesions following infection with a specific group of about 20 related beta-HPV genotypes, also known as EV types. About half of EV patients develop premalignant skin lesions and squamous cell carcinomas by age 40, primarily in sun-exposed areas (30). DNA from these lesions was found to harbor HPV genomes, suggesting a cocarcinogenic role of beta-HPVs and UV radiation in the early development of EV cancers (32). By use of sensitive P...

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Human papillomaviruses (HPVs) belonging to the Betapapillomavirus genus have recently been implicated in squamous cell carcinomas of the skin, though the mechanisms by which they initiate carcinogenesis are unclear. We show that human foreskin keratinocytes (HFKs) expressing several betapapillomavirus E6 (beta-E6) proteins display life span extension, but not to the extent seen in HFKs expressing HPV type 16 E6 (16E6). Additionally, we demonstrate that beta-E6 proteins can differentially activate telomerase. HFKs expressing 38E6 exhibit significant telomerase activity but to a lesser degree than that observed with 16E6; however, other beta-E6 proteins, including 5E6, 8E6, 20E6, and 22E6, exhibit low or background levels of telomerase activity. Utilizing glutathione S-transferase pull-down and coimmunoprecipitation experiments, the beta-E6 proteins were shown to interact with the cellular proteins E6-associated protein (E6AP) and NFX1-91, two proteins known to be important for telomerase activation by 16E6. Interestingly, the relative strength of the interaction between E6 and E6AP or NFX1-91 was proportionate to the activation of telomerase by each beta-E6 protein. To address the requirement for E6AP in telomerase activation by beta-E6 proteins, we utilized a shRNA to knock down endogenous levels of E6AP. Lysates with decreased levels of E6AP showed a reduced ability to activate telomerase, suggesting that E6AP is a necessary component. These data suggest that complex formation between E6, E6AP, and NFX1-91 is a critical step in mediating telomerase activation, which may be one contributing factor to cellular life span extension during human betapapillomavirus infection.High-risk human papillomaviruses (HPVs) belonging to the Alphapapillomavirus genus are known to be a common cause of cervical cancer through the function of the viral oncoproteins E6 and E7 (10,19,26). More recently, human betapapillomaviruses (beta-HPVs; such as HPV types 5, 8, and 38) have been implicated in skin cancer progression, yet the molecular mechanisms of cancer development are still largely unknown. BetaHPVs were first isolated from patients with the rare genetic disorder epidermodysplasia verruciformis, who suffer from life-long development of benign lesions such as warts but are also at greatly increased risk for development of squamous cell skin carcinomas (6-8). Cancers from patients with epidermodysplasia verruciformis led to the original isolation and identification of HPV type 5 (HPV5) and HPV8, whereas benign lesions from these patients contained dozens of different beta-HPV types (8). It is still unclear which, if any, betaHPVs can be categorized as high risk and what molecular functions are associated with high-risk phenotypes.Other evidence supporting a role for beta-HPVs in skin cancer includes previous reports that expression of HPV5 E6 (5E6) causes an inhibition of the intrinsic apoptotic cascade following UV light treatment (14,15). These studies have shown that Bak, a BH3-containing proapoptotic factor, is degraded by ...