The Human Papillomavirus Type 8 E2 Protein Suppresses β4-Integrin Expression in Primary Human Keratinocytes

Abstract

Human papillomaviruses (HPVs) infect keratinocytes of skin and mucosa. Homeostasis of these constantly renewing, stratified epithelia is maintained by balanced keratinocyte proliferation and terminal differentiation. Instructions from the extracellular matrix engaging integrins strongly regulate these keratinocyte functions. The papillomavirus life cycle parallels the differentiation program of stratified epithelia, and viral progeny is produced only in terminally differentiating keratinocytes. Whereas papillomavirus oncoproteins can inhibit keratinocyte differentiation, the viral transcription factor E2 seems to counterbalance the impact of oncoproteins. In this study we show that high expression of HPV type 8 (HPV8) E2 in cultured primary keratinocytes leads to strong down-regulation of ␤4-integrin expression levels, partial reduction of ␤1-integrin, and detachment of transfected keratinocytes from underlying structures. Unlike HPV18 E2-expressing keratinocytes, HPV8 E2 transfectants did not primarily undergo apoptosis. HPV8 E2 partially suppressed ␤4-integrin promoter activity by binding to a specific E2 binding site leading to displacement of at least one cellular DNA binding factor. To our knowledge, we show for the first time that specific E2 binding contributes to regulation of a cellular promoter. In vivo, decreased ␤4-integrin expression is associated with detachment of keratinocytes from the underlying basement membrane and their egress from the basal to suprabasal layers. In papillomavirus disease, ␤4-integrin down-regulation in keratinocytes with higher E2 expression may push virally infected cells into the transit-amplifying compartment and ensure their commitment to the differentiation process required for virus replication.Human papillomaviruses (HPVs) infect keratinocytes of skin or mucosa, leading to the induction of proliferative lesions. They play a key role in anogenital cancer, head and neck cancer, and squamous cell skin carcinomas arising in patients suffering from epidermodysplasia verruciformis (EV), a rare genetic disease. Recently, it has been shown also in immunocompetent individuals that seroreactivity to the cutaneous high-risk EV-associated HPV type 8 (HPV8) is correlated with a significantly higher risk for nonmelanoma skin cancer (11,26).HPV infection targets basal keratinocytes in stratified epithelia. A balanced keratinocyte proliferation rate and terminal differentiation maintain homeostasis of these constantly renewing tissues. Both proliferation and differentiation are strongly regulated by instructive signals from the underlying extracellular matrix. These signals are conveyed to the cells by integrins. Three major keratinocyte integrins, ␣2␤1, ␣3␤1, and ␣6␤4, have been defined (summarized in references 2, 45, and 48). While the ␣2␤1 and ␣3␤1 integrins are localized to focal contacts at apicolateral surfaces of basal keratinocytes, ␣6␤4 integrins are unique and atypical in that they do not localize to focal contacts like most other integrins (in particular ␤1-integrins). Ra...

Development of Skin Tumors in Mice Transgenic for Early Genes of Human Papillomavirus Type 8

“…The selective advantage of the frequent loss of E2 in cervical cancers induced by HPV16 or 18 has been explained by E2-mediated suppression of the viral E6/7 promotor, which is not relevant in transgenic mice, and by the induction of senescence or apoptosis (38)(39)(40). Interestingly, HPV8 E2 expressing keratinocytes do not undergo apoptosis (41), which may suggest that it has at least no inhibitory effect on carcinogenesis. Further studies are under way to test whether the increased levels of E2 may promote transformation.…”

Functional Divergence of Kaposi's Sarcoma-Associated Herpesvirus and Related Gamma-2 Herpesvirus Thymidine Kinases: Novel Cytoplasmic Phosphoproteins That Alter Cellular Morphology and Disrupt Adhesion

“…Notably, the papillomavirus E2 proteins are transcription factors that effectively antagonize the ability of E6 and E7 to block senescence (19), although the precise mechanisms are unknown. Recent studies showed that when expressed alone in cell lines or primary keratinocytes, certain E2 proteins directly downregulate ␤4-integrin (and to some extent ␤1-integrin) expression (36). In vivo, decreased ␤4-integrin expression is associated with terminal differentiation of stratified epithelia and is manifest by a loss of epithelial cell adhesion, altered cell morphology, and egress from basal to suprabasal layers (11,17), suggesting that E2 causes senescence by a similar mechanism.…”

“…The recent discovery that human papillomavirus type 8 E2 protein produces similar changes in cellular morphology when expressed in epithelial cells may provide new insights into KSHV TK functions as well (36). Notably, the papillomavirus E2 proteins are transcription factors that effectively antagonize the ability of E6 and E7 to block senescence (19), although the precise mechanisms are unknown.…”

Papillomavirus E2 Protein Induces Expression of the Matrix Metalloproteinase-9 via the Extracellular Signal-Regulated Kinase/Activator Protein-1 Signaling Pathway

“…binding in the promoter region and displacement of cellular factors (36). In MMP-9, however, binding of E2 to the promoter region is not required for induction, which is mediated predominantly through the presence of an AP-1 site in close proximity to the transcription start site (31,34).…”