Interaction of the Human Papillomavirus E6 Oncoprotein with Sorting Nexin 27 Modulates Endocytic Cargo Transport Pathways

A subset of high-risk Human Papillomaviruses (HPVs) are the causative agents of a large number of human cancers, of which cervical is the most common. Two viral oncoproteins, E6 and E7, contribute directly towards the development and maintenance of malignancy. A characteristic feature of the E6 oncoproteins from cancer-causing HPV types is the presence of a PDZ binding motif (PBM) at its C-terminus, which confers interaction with cellular proteins harbouring PDZ domains. Here we show that this motif allows E6 interaction with Sorting Nexin 27 (SNX27), an essential component of endosomal recycling pathways. This interaction is highly conserved across E6 proteins from multiple high-risk HPV types and is mediated by a classical PBM-PDZ interaction but unlike many E6 targets, SNX27 is not targeted for degradation by E6. Rather, in HPV-18 positive cell lines the association of SNX27 with components of the retromer complex and the endocytic transport machinery is altered in an E6 PBM-dependent manner. Analysis of a SNX27 cargo, the glucose transporter GLUT1, reveals an E6-dependent maintenance of GLUT1 expression and alteration in its association with components of the endocytic transport machinery. Furthermore, knockdown of E6 in HPV-18 positive cervical cancer cells phenocopies the loss of SNX27, both in terms of GLUT1 expression levels and its vesicular localization, with a concomitant marked reduction in glucose uptake, whilst loss of SNX27 results in slower cell proliferation in low nutrient conditions. These results demonstrate that E6 interaction with SNX27 can alter the recycling of cargo molecules, one consequence of which is modulation of nutrient availability in HPV transformed tumour cells.     

Complex formation of human papillomavirus E7 proteins with the retinoblastoma tumor suppressor gene product.

The E7 proteins encoded by the human papillomaviruses (HPVs) associated with anogenital lesions share significant amino acid sequence homology. The E7 proteins of these different HPVs were assessed for their ability to form complexes with the retinoblastoma tumor suppressor gene product (p105-RB). Similar to the E7 protein of HPV-16, the E7 proteins of HPV-18, HBV-6b and HPV-11 were found to associate with p105-RB in vitro. The E7 proteins of HPV types associated with a high risk of malignant progression (HPV-16 and HPV-18) formed complexes with p105-RB with equal affinities. The E7 proteins encoded by HPV types 6b and 11, which are associated with clinical lesions with a lower risk for progression, bound to p105-RB with lower affinities. The E7 protein of the bovine papillomavirus type 1 (BPV-1), which does not share structural similarity in the amino terminal region with the HPV E7 proteins, was unable to form a detectable complex with p105-RB. The amino acid sequences of the HPV-16 E7 protein involved in complex formation with p105-RB in vitro have been mapped. Only a portion of the sequences that are conserved between the HPV E7 proteins and AdE1A were necessary for association with p105-RB. Furthermore, the HPV-16 E7-p105-RB complex was detected in an HPV-16-transformed human keratinocyte cell line.     

In vitro biological activities of the E6 and E7 genes vary among human papillomaviruses of different oncogenic potential.

Human papillomavirus type 16 (HPV-16) and HPV-18 are often detected in cervical carcinomas, while HPV-6, although frequently present in benign genital lesions, is only rarely present in cancers of the cervix. Therefore, infections with HPV-16 and HPV-18 are considered high risk and infection with HPV-6 is considered low risk. We found, by using a heterologous promoter system, that expression of the E7 transforming protein differs between high- and low-risk HPVs. In high-risk HPV-16, E7 is expressed from constructs containing the complete upstream E6 open reading frame. In contrast, HPV-6 E7 was efficiently translated only when E6 was deleted. By using clones in which the coding regions of HPV-6, HPV-16, and HPV-18 E7s were preceded by identical leader sequences, we found that the ability of the E7 gene products to induce anchorage-independent growth in rodent fibroblasts correlated directly with the oncogenic association of the HPV types. By using an immortalization assay of normal human keratinocytes that requires complementation of E6 and E7, we found that both E6 and E7 of HPV-18 could complement the corresponding gene from HPV-16. However, neither E6 nor E7 from HPV-6 was able to substitute for the corresponding gene of HPV-16 or HPV-18. Our results suggest that multiple factors, including lower intrinsic biological activity of E6 and E7 and differences in the regulation of their expression, account for the low activity of HPV-6, in comparison with HPV-16 and HPV-18, in in vitro assays. These same factors may, in part, account for the apparent difference in oncogenic potential between these viruses.     

Induction of apoptosis in cervical carcinoma cells by peptide aptamers that bind to the HPV-16 E7 oncoprotein.

Human papillomaviruses (HPV) of the high-risk type are causally involved in human tumors, in particular cervical carcinoma. Expression of the viral oncogenes E6 and E7 is maintained in HPV-positive tumors, and it was shown that E6 and E7 of HPV-16 can immortalize human keratinocytes, the natural host cells of the virus. Expression of the viral genes is also required for maintenance of the transformed phenotype. The oncogenic activity of the E6 and E7 oncoproteins is mediated by their physical and functional interaction with cellular regulatory proteins. To knock out the function of the E7 protein in living cells, we have developed peptide aptamers with high specific binding activity for the E7 protein of HPV-16. We show here that E7-binding peptide aptamers induce programmed cell death (apoptosis) in E7-expressing cells, whereas E7-negative cells are not affected. Furthermore, E7-binding peptide aptamers induce apoptosis in HPV-16-positive tumor cells derived from cervical carcinoma. The data suggest that E7-binding peptide aptamers may be useful tools to specifically eliminate HPV-positive tumors.     

E5 proteins of human papillomavirus types 11 and 16 transactivate the c-fos promoter through the NF1 binding element.

Human papillomavirus type 11 (HPV-11) and HPV-16 contain an E5 gene that can induce c-fos gene expression in mouse fibroblasts. This study investigated the human c-fos promoter characteristics by mapping the c-fos promoter sequence with several deletion and point mutants that confer responsiveness to E5 of HPV-11 or HPV-16. The mutant studies show that NF1 binding sequences within the c-fos promoter were crucial for the induction of the c-fos gene by E5, and the gel shift assay study suggested that E5 of both HPV-11 and HPV-16 is associated, perhaps indirectly, with this NF1 element in the transactivation of the human c-fos promoter. Using an inducible system, we demonstrate that increased induction of the HPV-11 E5 gene in cells led to increased transactivation of the NF1 element. In addition, the transactivating activity of a series of HPV-11 E5 mutants on the NF1 element had a strong correlation with their respective transforming activities.     

The Invasive Capacity of HPV Transformed Cells Requires the hDlg-Dependent Enhancement of SGEF/RhoG Activity

A major target of the HPV E6 oncoprotein is the human Discs Large (hDlg) tumour suppressor, although how this interaction contributes to HPV-induced malignancy is still unclear. Using a proteomic approach we show that a strong interacting partner of hDlg is the RhoG-specific guanine nucleotide exchange factor SGEF. The interaction between hDlg1 and SGEF involves both PDZ and SH3 domain recognition, and directly contributes to the regulation of SGEF''s cellular localization and activity. Consistent with this, hDlg is a strong enhancer of RhoG activity, which occurs in an SGEF-dependent manner. We also show that HPV-18 E6 can interact indirectly with SGEF in a manner that is dependent upon the presence of hDlg and PDZ binding capacity. In HPV transformed cells, E6 maintains a high level of RhoG activity, and this is dependent upon the presence of hDlg and SGEF, which are found in complex with E6. Furthermore, we show that E6, hDlg and SGEF each directly contributes to the invasive capacity of HPV-16 and HPV-18 transformed tumour cells. These studies demonstrate that hDlg has a distinct oncogenic function in the context of HPV induced malignancy, one of the outcomes of which is increased RhoG activity and increased invasive capacity.     

Interaction of c-Myc with the pRb-related protein p107 results in inhibition of c-Myc-mediated transactivation.

The product of the c-myc proto-oncogene, c-Myc, is a sequence-specific DNA binding protein with an N-terminal transactivation domain and a C-terminal DNA binding domain. Several lines of evidence indicate that c-Myc activity is essential for normal cell cycle progression. Since the abundance of c-Myc during the cell cycle is constant, c-Myc's activity may be regulated at a post-translational level. We have shown previously that the N-terminus of c-Myc can form a specific complex with the product of the retinoblastoma gene, pRb, in vitro. These data suggested a model in which pRb, or pRb-related proteins, regulate c-Myc activity through direct binding. We show here that the pRb-related protein p107, but not pRb itself, forms a specific complex with the N-terminal transactivation domain of c-Myc in vivo. Binding of p107 to c-Myc causes a significant inhibition of c-Myc transactivation. Expression of c-Myc releases cells from a p107-induced growth arrest, but not from pRb-induced growth arrest. Our data suggest that p107 can control c-Myc activity through direct binding to the transactivation domain and that c-Myc is a target for p107-mediated growth suppression.     

Protein intrinsic disorder and human papillomaviruses: increased amount of disorder in E6 and E7 oncoproteins from high risk HPVs

It is recognized now that many functional proteins or their long segments are devoid of stable secondary and/or tertiary structure and exist instead as very dynamic ensembles of conformations. They are known by different names including natively unfolded, intrinsically disordered, intrinsically unstructured, rheomorphic, pliable, and different combinations thereof. Many important functions and activities have been associated with these intrinsically disordered proteins (IDPs), including molecular recognition, signaling, and regulation. It is also believed that disorder of these proteins allows function to be readily modified through phosphorylation, acetylation, ubiquitination, hydroxylation, and proteolysis. Bioinformatics analysis revealed that IDPs comprise a large fraction of different proteomes. Furthermore, it is established that the intrinsic disorder is relatively abundant among cancer-related and other disease-related proteins and IDPs play a number of key roles in oncogenesis. There are more than 100 different types of human papillomaviruses (HPVs), which are the causative agents of benign papillomas/warts, and cofactors in the development of carcinomas of the genital tract, head and neck, and epidermis. With respect to their association with cancer, HPVs are grouped into two classes, known as low (e.g., HPV-6 and HPV-11) and high-risk (e.g., HPV-16 and HPV-18) types. The entire proteome of HPV includes six nonstructural proteins [E1, E2, E4, E5, E6, and E7 (the latter two are known to function as oncoproteins in the high-risk HPVs)] and two structural proteins (L1 and L2). To understand whether intrinsic disorder plays a role in the oncogenic potential of different HPV types, we have performed a detailed bioinformatics analysis of proteomes of high-risk and low-risk HPVs with the major focus on E6 and E7 oncoproteins. The results of this analysis are consistent with the conclusion that high-risk HPVs are characterized by the increased amount of intrinsic disorder in transforming proteins E6 and E7.     

Conditionally activated E7 proteins of high-risk and low-risk human papillomaviruses induce S phase in postmitotic, differentiated human keratinocytes

The productive program of human papillomaviruses (HPVs) in epithelia is tightly linked to squamous differentiation. The E7 proteins of high-risk HPV genotypes efficiently inactivate the pRB family of proteins that control the cell cycle, triggering S phase in suprabasal keratinocytes. This ability has until now not been demonstrated for the low-risk HPV-6 or HPV-11 E7 proteins. An inducible system in which HPV-16 E7 is fused to the ligand binding domain of the human estrogen receptor (ER) was described by Smith-McCune et al. (K. Smith-McCune, D. Kalman, C. Robbins, S. Shivakumar, L. Yuschenkoff, and J. M. Bishop, Proc. Natl. Acad. Sci. USA 96:6999-7004, 1999). In the absence of hormone, E7ER is cytoplasmic, and upon addition of 17beta-estradiol, it translocates to the nucleus. Using organotypic epithelial raft cultures developed from primary human keratinocytes, we show that 16E7ER promotes either S-phase reentry or p21cip1 accumulation in differentiated keratinocytes in a stochastic manner as early as 6 h postinduction with 17beta-estradiol. A vector expressing the ER moiety alone had no effect. These observations prove unequivocally that the E7 protein drives S-phase reentry in postmitotic, differentiated keratinocytes rather than preventing S-phase exit while the cells ascend through the epithelium. HPV-11 E7ER and, much less efficiently, HPV-6 E7ER also promoted S-phase reentry by differentiated cells upon exposure to 17beta-estradiol. S-phase induction required the consensus pRB binding motif. We propose that the elevated nuclear levels of the low-risk HPV E7 protein afforded by the inducible system account for the positive results. These observations are entirely consistent with the fact that low-risk HPV genotypes replicate in the differentiated strata in patient specimens, as do the high-risk HPVs.