Inactivation of p73 Protein by Cervical Cancer-inducing HPV E6 Protein Novel p53-independent function of E6

Namkoong, Sung Eun; Kim, Seung Jo; Kim, Eun Joo; Um, Soo Jong; Park, Jong Sup

Abstract

OBJECTIVE
Human papillomavirus (HPV) is strongly implicated as a causative agent in the etiology of cervical cancer. Of its gene products, E6 and E7 oncoproteins play major roles by inactivation of cellular p53 and pRb tumor suppressor proteins, respectively. However, it has been recently suggested that p53 and/or pRb-independent functions of E6 and E7 are involved in cervical carcinogenesis. The purpose of this study is to identify novel a cellular target, p73, of E6 and to determine how E6 inactivates p73 function,
METHODS
The interaction between E6 and p73 were identified by the yeast two-hybrid assay in vivo and the GST pull-down assay in vitro. The function of the interaction was determined by transient transfections using p21 promoter-CAT reporter plasmid. The molecular mechanism underlying the functional significance of the interaction was further assessed by in vivo and in vitro protein degradation assays, and gel mobility shift assays.
RESULTS
Yeast two-hybrid and GST pull-down assays indicate a physical interaction between p73 and either HPV-16 or HPV-11 E6 proteins in vivo and in vitro, respectively. Transactivation domain (amino acid residues 1-49) is found to be absolutely required for this interaction. Transient co-expression of E6 significantly inhibits the p73-mediated activation of p21WAF1 promoter in a p53-defective C33A cell line. Using Ga14-p73 fusion protein, we demonstrate that E6 inhibition of p73 transactivation function is independent of sequence-specific DNA binding, which is confirmed by direct electrophoretic mobility shift assay. Moreover, E6 inhibits p73 function by interfering with the activity of the amino-terminal activation domain. The protein degradation assays in vivo and in vitro indicate that p73, unlike p53, is not susceptible to E6-dependent proteolysis.
CONCLUSION
Throughout this study, we identified p73 as a novel cellular target of HPV-E6 protein and found that E6 binds p73 through the amino-terminal transactivation domain, and inhibits its transactivation function independent of the protein degradation and DNA binding. These overall results, consequently, suggest that in addition to the inactivation of p53, the functional interference of p73 by HPV-E6 may, at least in part, contribute to E6-mediated cellular transformation.