Skin Hyperproliferation and Susceptibility to Chemical Carcinogenesis in Transgenic Mice Expressing E6 and E7 of Human Papillomavirus Type 38

Abstract

The oncoproteins E6 and E7 of human papillomavirus type 38 (HPV38) display several transforming activities in vitro, including immortalization of primary human keratinocytes. To evaluate the oncogenic activities of the viral proteins in an in vivo model, we generated transgenic mice expressing HPV38 E6 and E7 under the control of the bovine homologue of the human keratin 10 (K10) promoter. Two distinct lines of HPV38 E6/E7-expressing transgenic mice that express the viral genes at different levels were obtained. In both lines, HPV38 E6 and E7 induced cellular proliferation, hyperplasia, and dysplasia in the epidermis. The rate of occurrence of these events was proportional to the levels of HPV38 E6 and E7 expression in the two transgenic lines. Exposure of the epidermis of nontransgenic mice to UV led to p21 WAF1 accumulation and cell cycle arrest. In contrast, keratinocytes from transgenic mice continued to proliferate and were not positive for p21 WAF1, indicating that cell cycle checkpoints are altered in keratinocytes expressing the viral genes. Although the HPV38 E6/E7-expressing transgenic mice did not develop spontaneous tumors during their life span, two-stage carcinogen treatment led to a high incidence of papillomas, keratoacanthomas, and squamous-cell carcinomas in HPV38 mice compared with nontransgenic animals. Together, these data show that HPV38 E6 and E7 display transforming properties in vivo, providing further support for the role of HPV38 in carcinogenesis.Nonmelanoma skin cancers (NMSC), basal-cell carcinoma (BCC), and squamous-cell carcinoma (SCC) are the most common malignancies occurring worldwide. Several findings from experimental systems in vitro and in vivo have shown that UV plays a direct role in skin carcinogenesis (3). In addition, a subgroup of the epitheliotropic human papillomaviruses may cooperate with UV in the development of NMSC (28). Individuals with an autosomal recessive disorder termed epidermodysplasia verruciformis (EV) are susceptible to infection by specific HPV types that belong to the genotype beta branch of the HPV phylogenetic tree (7). Individuals with EV develop confluent flat warts that, in approximately 30% of cases, progress to SCC on sun-exposed areas (11,14,20,27). The involvement of an infective agent in skin carcinogenesis is supported by the fact that immunosuppressed organ transplant recipients have a 50-to 100-fold higher risk of developing NMSC than the general population (4, 16, 32, 33). In addition, independent studies have reported that DNA of cutaneous HPV types can be detected in SCC and BCC of immunocompetent individuals, suggesting their role in skin carcinogenesis in the general population (28). The potential carcinogenic role of the EV HPV types is consistent with the fact that other members of the papillomavirus family, i.e., the mucosal high-risk types, are associated with anogenital cancers (36) and a subset of head and neck cancers (17).The products of two early genes, E6 and E7, are the major oncoproteins of the high-risk mucosal HPV typ...

Specific betapapillomaviruses associated with squamous cell carcinoma of the skin inhibit UVB-induced apoptosis of primary human keratinocytes

“…We did observe a reduction in Bak expression in cells expressing HPV38 and especially HPV5 E6E7, which is in agreement with increased Bak degradation as described by others (Dong et al, 2005;. Unlike , no distinct differences in Bak expression levels were observed after UVB irradiation between the HPV E6E7-transduced PHKs and the vector control.…”

“…E6 and E7 of several bPV types (HPV5, HPV15, HPV17, HPV20 and HPV38) influence both growth and differentiation of PHKs in organotypic raft cultures (Boxman et al, 2001). Furthermore, invasive SCC was found to develop spontaneously in mice transgenic for the complete early region of HPV8 (Schaper et al, 2005), and after chemical carcinogen application in HPV38 E6E7-transgenic mice (Dong et al, 2005). Recently, a study showed that E6/E7 expression in UV-irradiated HPV20-transgenic mice affected proliferation and differentiation of the skin and promoted SCC development (Michel et al, 2006).…”

Loss of p53 or p73 in human papillomavirus type 38 E6 and E7 transgenic mice partially restores the UV-activated cell cycle checkpoints

“…Surprisingly, we observed that BrdU incorporation is strongly decreased in skin keratinocytes of p53 À/À HPV38 E6/E7 Tg mice upon UV treatment (Figure 1b) compared to p53 þ / þ HPV38 Tg mice, suggesting that p53 is required for the lack of UV response in cells expressing HPV38 E6 and E7. To evaluate whether the p53-regulated cyclin-dependent kinase inhibitor p21 WAF1 is involved in this process, we next performed immunohistochemical analysis on the mouse skin as previously described (Dong et al, 2005). As expected, UV-irradiated skin of normal mice was strongly positive for p21 WAF1 (Figure 1c).…”

“…In addition, in contrast to normal mice, HPV38 E6/E7 Tg mice do not respond to ultraviolet (UV) irradiation. For instance, no accumulation of the product of the p53-regulated gene p21 WAF1 and consequent inhibition of cellular proliferation were observed in the epidermis of the Tg animals (Dong et al, 2005).…”

“…The beta HPV types were first isolated in skin cancer-prone patients suffering from a rare autosomal recessive genetic disorder called epidermodysplasia verruciformis, but it is now clear that they are commonly found in the skin of healthy individuals (Pfister, 2003). Characterization of the biological properties of the products of the early genes E6 and E7 from different beta HPV types in in vitro and in vivo model has highlighted the potential carcinogenicity of these viruses (Jackson et al, 2000;Caldeira et al, 2003;Akgul et al, 2005;Dong et al, 2005;Schaper et al, 2005;Accardi et al, 2006;Michel et al, 2006). We showed that expression of E6 and E7 from the beta HPV38 in primary human keratinocytes results in inactivation of the tumour suppressor p53 and pRb and immortalization (Caldeira et al, 2003;Accardi et al, 2006).…”