Preview

PDF, English - main document Download (3MB) | Terms of use

Abstract

Ultraviolet (UV) irradiation and UVB in particular, is the major environmental risk factor for the development of skin cancer. Furthermore, an increasing body of evidence supports a role for cutaneous human papillomaviruses (HPVs) in combination with UV irradiation in the development of cutaneous squamous cell carcinoma. Moreover, cutaneous HPV E6 proteins have been described to indirectly influence pathways controlled by p53, to inhibit UV-induced apoptosis and to prolong the life span of keratinocytes, the natural host of all HPVs. The present study identified the LIM protein Ajuba as a novel interaction partner of the E6 protein of the beta2 HPV23, the gamma1 HPV4 and the gamma11 HPV148. These E6 proteins were shown to directly interact with Ajuba both in vitro and in vivo as well as to co-localise with Ajuba in the cytoplasm. Furthermore, the E6 proteins of HPV23 and HPV148 showed a coincidental decrease in protein expression together with their interaction partner Ajuba upon DNA damage induction and siRNA-mediated Ajuba knockdown. This knockdown also revealed that the p53 protein was involved in the regulation of Ajuba and HPV E6 protein levels. On the other hand, Ajuba co-accumulated following overexpression of the cutaneous E6 proteins, further pointing to a close interaction of both proteins in vivo. A trimeric complex formation between p53, Ajuba and HPV148 E6 was additionally discovered, which did not take place with HPV23 E6. Moreover, p53 was shown to co-localise and to interact directly with HPV148 E6 in vitro and in vivo, even in the absence of Ajuba. A functional luciferase reporter assay revealed a strong repression of p53 activity in the presence of HPV148 E6, demonstrating an impairment of the transactivation activity of p53. This observation could however not be reported for HPV23 E6. In summary, the present study identified the LIM protein Ajuba as a novel interaction partner of cutaneous E6 proteins. Additionally, it demonstrated for the first time that the p53 protein not only binds to cutaneous HPV E6 proteins in vitro, but also interacts and co-localises with HPV148 E6 in vivo, subsequently repressing p53 activity. These findings suggest a novel regulatory mechanism elicited by cutaneous E6 proteins and further strengthen the hypothesis that cutaneous HPVs can act as co-factors in the development of skin cancer.