The Human Papillomavirus E6 Oncogene Represses a Cell Adhesion Pathway and Disrupts Focal Adhesion through Degradation of TAp63β upon Transformation

Abstract

Cervical carcinomas result from cellular transformation by the human papillomavirus (HPV) E6 and E7 oncogenes which are constitutively expressed in cancer cells. The E6 oncogene degrades p53 thereby modulating a large set of p53 target genes as shown previously in the cervical carcinoma cell line HeLa. Here we show that the TAp63β isoform of the p63 transcription factor is also a target of E6. The p63 gene plays an essential role in skin homeostasis and is expressed as at least six isoforms. One of these isoforms, ΔNp63α, has been found overexpressed in squamous cell carcinomas and is shown here to be constitutively expressed in Caski cells associated with HPV16. We therefore explored the role of p63 in these cells by performing microarray analyses after repression of endogenous E6/E7 expression. Upon repression of the oncogenes, a large set of p53 target genes was found activated together with many p63 target genes related to cell adhesion. However, through siRNA silencing and ectopic expression of various p63 isoforms we demonstrated that TAp63β is involved in activation of this cell adhesion pathway instead of the constitutively expressed ΔNp63α and β. Furthermore, we showed in cotransfection experiments, combined with E6AP siRNA silencing, that E6 induces an accelerated degradation of TAp63β although not through the E6AP ubiquitin ligase used for degradation of p53. Repression of E6 transcription also induces stabilization of endogenous TAp63β in cervical carcinoma cells that lead to an increased concentration of focal adhesions at the cell surface. Consequently, TAp63β is the only p63 isoform suppressed by E6 in cervical carcinoma as demonstrated previously for p53. Down-modulation of focal adhesions through disruption of TAp63β therefore appears as a novel E6-dependent pathway in transformation. These findings identify a major physiological role for TAp63β in anchorage independent growth that might represent a new critical pathway in human carcinogenesis.

Clinicopathology Significance of p53 and p63 Expression in Indonesian Cervical Squamous Cell Carcinomas

“…Recently, Khalifa et al, 2011 observed in cervical carcinoma cells, repression of E6/E7 stabilizes the p53 transcription factor leading to activation of a large group of cellular p53 target genes. This evidence show that repression of E6/E7 also induces transcriptional activation of an additional large set of genes involved in cell adhesion including previously described p63 target genes.…”

Aberrant methylation-mediated silencing of microRNAs contributes to HPV-induced anchorage independence

“…Cell fusion studies have shown that anchorage independence of hrHPV-transformed cells relies on a recessive event, suggesting that inactivation of tumor suppressor genes is involved in bypassing anoikis [2]. In support of this, functional loss of tumor suppressive host genes Caveolin-1, SOCS1, TAp63β, LKB1, CADM1, MAL, hsa-miR-34c-3p/5p and hsa-miR-203 was shown to affect anchorage independent growth of HPV-transformed cells [7–14]. …”

Individual and Complementary Effects of Human Papillomavirus Oncogenes on Epithelial Cell Proliferation and Differentiation

“…Upon induction of differentiation, in synchrony with HPV genome amplification and late gene expression, levels of p63 rapidly decrease [Mighty and . Interestingly it has been reported that E6 specifically targets the TAp63β isoform for degradation, with subsequent disruption of focal adhesions, suggesting a novel pathway involved in HPV-induced cell transformation [Ben Khalifa et al, 2011]. E6-mediated p53 degradation has also been reported to downregulate miR-203 levels [McKenna et al, 2010], which may also enhance p63 expression and cell proliferation.…”