Induction of p16 during immortalization by HPV 16 and 18 and not during malignant transformation

The p16 (MTS1) tumour-suppressor gene is a cyclin-dependent kinase (cdk) inhibitor that decelerates the cell cycle by inactivating the cdks that phosphorylate the retinoblastoma tumour-suppressor gene (Rb) protein (pRb). In cervical cancers, pRb is inactivated by the HPV E7 oncoprotein or by mutations. The hypothesis of earlier reports was that the disruption of the p16/cdk-cyclin/Rb cascade is essential for malignant cervical transformation/carcinogenesis. We previously established in vitro model systems of cervical cancer representing four steps of oncogenic progression initiated by the two most common oncogenic HPVs in ectocervical and endocervical epithelial cells. This report used these systems to investigate the role of p16 in cervical cancers. A dramatic enhancement of the p16 RNA level was observed after immortalization by HPV 16 or 18. Furthermore, the p16 protein was newly observed following immortalization. However, no further changes were found for RNA or protein levels after serum selection or malignant transformation. For three cervical carcinoma cell lines, similar high levels of p16 expression were seen. Point mutations or homozygous deletions of p16 were not observed in the in vitro systems or in clinical specimens. These results suggest that the inactivation of the p16/cdk-cyclin/Rb cascade does not occur during malignant transformation but occurs during the immortalization by HPV in HPV-harbouring premalignant lesions, the in situ equivalent of immortalized cells. Also suggested is that p16 has no role in the specific malignant transformation step from immortal premalignant lesions during the carcinogenesis of HPV-initiated cervical cancers.

Up-regulation of hormone response of human papillomavirus type 16 expression and increased DNA-protein binding by consensus mutations of viral glucocorticoid response elements

Human papillomaviruses (HPVs) and steroid hormones are linked to the development of cervical cancer. Studies from our laboratory and others showed that the steroid glucocorticoid and progesterone hormones activated the expression of HPV type 16. This activation was attributed to the specific interaction of the glucocorticoid receptor (GR) with the three glucocorticoid response elements (GREs) in the HPV16 regulatory region. In the present study, we first examined the glucocorticoid response mediated through the GREs, using GRE consensus (GREc) mutations and expression assays from a heterologous basal promoter. Both single and triple HPV16 GREc constructs increased expression in the presence of the dexamethasone glucocorticoid in HeLa cervical carcinoma cells and primary baby rat kidney epithelial cells, in comparison with the triple wild-type GREs. Further, the hormone increased significantly the expression of the viral E6-E7 oncogene mRNA from intact HPV in primary human ectocervical cells in in situ hybridization assays. Three in vitro assays of DNA-protein interaction with oligonucleotides and HeLa cell extracts showed a higher binding of protein to two of the HPV16 GREcs than to the wild-type GREs. This applied especially to the GRE containing an overlapping NF1 half site, that also had a greater differential induction by dexamethasone of expression in vivo. The NF1 site was mutated in the GREc that also was bound by unique, lower-mobility complexes in electrophoretic mobility shift assays. UV-crosslinking assays confirmed the increased binding and showed binding by a 96-kDa protein, probably the GR. Our results show an important role of glucocorticoids in HPV16 expression. The direct action through the HPV16 GREs is suggested to be mediated by the hormone-activated GR in association with other factors.

Human JC virus nuclear factor 1 binding motifs and large tumor antigen region rquired for transactivation of late promoter

The nuclear factor 1 (NF-1) motifs, NF-1 II/III, In the two 98-bp repeats of the transcription-regulatory region of JC virus (JCV), have a critical role in brain-specific transcription from the JCV early promoter-enhancer. In this study, the role of these motifs in transactivation of the JCV late promoter-enhancer (JCVL) was examined in differentiating glial P19 embryonal carcinoma cells. The expression of papovaviral large tumor antigen (T-Ag) in the glial cells was shown by double immunofluorescence assays. By using site-directed mutagenesis and in vivo assays, the two wild-type NF-1 II/III sites, but not the third site, were found to be essential for the transactivation of JCVL by JCV T-Ag. In vitro transcription assays confirmed this specific transactivation and the transactivation was abolished by T-Ag antibody. In electrophoretic mobility shift assays, expression of JCV T-Ag increased the binding of a factor(s) to the 98-bp repeat. T-Ag antibody abolished the increase of binding. Binding assays with oligonucleotides of NF-1 11/111 motifs showed that the increased binding specifically required the wild-type NF-1 II/III sequences and confirmed the requirement of T-Ag. To determine the region of T-Ag necessary for transactivation Of JCVL, the coding sequences were mutated. The amino-terminal region of JCV Ag in amino acids 1-437 was essentially required for efficient transactivation. These results indicated that transactivation of JCVL and increased binding require a factor(s) found specifically in glial cells, the JCV NF-1 II/III sites, and the T-Ag amino-terminal region.

Evidence for a mechanism of demyelination by human JC virus: negative transcriptional regulation of RNA and protein levels from myelin basic protein gene by large tumor antigen in human glioblastoma cells

Human JC virus (JCV) is a neurotropic human polyomavirus that was found in the plaques and oligodendroglial cells of the brains of patients with the fatal demyelinating disease, progressive multifocal leukoencephalopathy (PML). Transgenic mice expressing JCV large tumor (T)-antigen from integrated DNA showed dysmyelination in the central nervous system. However, the role of T-antigen from episomal DNA in the demyelination in PML remains unclear. In this report, we examined the effect of episomally expressed JCV T-antigen on the expression of myelin basic protein (MBP) in U-87 MG human glioblastoma cells to study the mechanism of demyelination. Expression assays of the MBP promoter in U-87 MG detected a 2.5-fold reduction in cells expressing intact T-antigen. Next, U-87 MG expressing T-antigen were examined by RNase protection assays for mRNA accumulation from the endogenous MBP promoter. Also, the expression of the MBP promoter plasmid was determined using in vitro transcription assays with extracts from T-antigen expressing cells. Both assays found a similar down-regulation of the MBP promoter by T-antigen, confirming that negative regulation occurred at the transcriptional level for the endogenous and exogenous MBP promoters. Furthermore, in situ immunofluorescence assays and quantitative Western blot analysis provided convincing evidence of a similar reduction in the level of MBP produced from the functional endogenous gene in U-87 MG glioblastoma cells expressing T-antigen. Thus, we provide evidence for the role of T-antigen in a transcriptional control mechanism for the demyelination that is caused by JCV in PML patients.

Glial and muscle embryonal carcinoma cell-specific independent regulation of expression of human JC virus early promoter by cyclic AMP response elements and adjacent nuclear factor 1 binding sites

The human polyoma JC virus (JCV) is a glial cell-specific virus and is the etiological agent for the terminal AIDS-associated brain disease, progressive multifocal leukoencephalopathy (PML). JCV contains several binding sites for transcriptional factors that are important for activity in glial cells, including cyclic AMP (cAMP) response elements (CREs) which are four nucleotides from nuclear factor 1 (NF1) sites within the two 98 bp repeat regions. We studied the combined role of cAMP and NF1 in regulating the expression of the JCV early promoter-enhancer (JCVE) in differentiating glial and muscle P19 embryonal carcinoma cells. JCVE expression remained several-fold higher in the presence of cAMP in glial cells, irrespective of whether the relatively strong activity of JCVE was greatly reduced by NF1 site mutations. In contrast, cAMP had no effect in muscle cells, independent of whether the modest activity of JCVE was two-fold higher due to NF1 site mutations. The in vivo effects were confirmed with in vitro transcription assays using glial cell extracts, competitors of CRE, and the NF1 site, and single repeat JCVE region with mutations in the NF1 II/ III binding sites as templates. The in vitro results also indicated that the effects were due to the CREs of JCV, rather than to the indirect effects of cAMP. Overall, the results indicated that NF1 and cAMP have independent, different, tissue-specific, and direct effects in the regulation of JCVE. These effects may contribute the neurotropic PML-inducing pattern of expression of JCVE.

Identification of two novel cellular genes associated with multistage carcinogenesis of human endocervical cells by mRNA differential display

Carcinogenesis originating from cervical cells has been recognized as a multistage process in which human papillomavirus (HPV) infection and cofactors, such as cigarette smoke, are frequently involved in the development of malignant cancer. However, the molecular mechanism underlying this process remains poorly understood. To identify the cellular genes involved in multistage cervical oncogenesis, we used the mRNA differential display method to analyze primary human endocervical cells (HEN), HPV 16-immortalized cells (HEN-16) and cigarette smoke condensate (CSC)-transformed cells (HEN-16T). Two cDNAs--PA4 and PA9--have been identified and isolated after comparing 8000 cDNAs from HEN, HEN-16 and HEN-16T. Northern blot analysis showed that PA4 was expressed 2- to 3-fold higher in HEN-16 and HEN-16T than in HEN, whereas PA9 was expressed uniquely in HEN. Moreover, the same patterns of PA4 and PA9 expression were found for a second line of HPV 16-immortalized endocervical cells and the corresponding transformed cells. An analysis of cDNA sequences showed that PA4 and PA9 had no homology to nucleotide sequences in Genbank. We suggest that immortalization, but not tumorigenesis, up-regulated a new oncogene, PA4, and down-regulated a new tumor suppressor gene, PA9. These results demonstrated the utility of the human endocervical cell model system and the mRNA differential display method for identifying the genes that may be involved in multistage cervical carcinogenesis.

Malignant transformation of human ectocervical cells immortalized by HPV 18: in vitro model of carcinogenesis by cigarette smoke

In addition to the established role of human papillomaviruses (HPVs) in cervical cancer, smoking has been suggested to be an important cofactor. Previously, primary human ectocervical cells immortalized by HPV types 16 and 18 DNA did not form tumors on nude mice. Here, we derived a new line of HPV 18-immortalized ectocervical cells (HEC-18-1), which was also non-tumorigenic. To examine the role of cigarette smoking in the progression of cervical cancer initiated by HPV 18, we adapted these cells to growth in serum and high calcium and treated the cells with cigarette smoke condensate until tumorigenic cells (HEC-18-1C) were produced. Moderate and late passage serum-adapted untreated HEC-18-1 (HEC-18-1S) remained non-tumorigenic. A typical HEC-18-1C tumor was an invasive squamous cell carcinoma, from which we established a clonal line of cells (HEC-18-1CT). Although the physical state of HPV 18 was not affected by malignant transformation and the gene expression of HPV 18 was not affected by malignant transformation and the gene expression of HPV 18 was affected little, the differentiation of the epithelium derived in organotypic (raft) culture from HEC-18-1CT was altered dramatically. Moderate and late passage HEC-18-1 and HEC-18-1S were reconstructed into mild dysplasia in organotypic (raft) culture. On the other hand, the moderate passage malignantly transformed HEC-18-1CT displayed severe dysplasia/carcinoma in situ in raft culture. We describe here the first direct evidence of the role of cigarette smoke in the progression of HPV-initiated carcinogenesis using an in vitro model system.

Malignant transformation of HPV 16-immortalized human endocervical cells by cigarette smoke condensate and characterization of multistage carcinogenesis

A number of epidemiological studies indicate that cigarette smokers are at increased risk of developing cervical cancer. However, convincing biological evidence is lacking. This report examines the biological and cellular role of human papillomavirus (HPV) type 16 and cigarette smoke in multistage cervical carcinogenesis. Two lines of HPV 16-immortalized human endocervical cells (HEN-16 and HEN-16-2) generated from primary cells (HEN) were treated with cigarette smoke condensate (CSC). CSC-treated, but not untreated, HEN-16 and HEN-16-2 formed tumors that were invasive squamous cell carcinomas in nude mice. The tumors were used to initiate 2 tumor lines of cells (HEN-16T and HEN-16-2T, respectively). Cells of both tumor lines, compared with HEN, HEN-16 and HEN-16-2, featured: (a) tumorigenicity, (b) distinct morphologies in monolayer and organotypic (raft) cultures, (c) faster growth in serum plus high calcium levels after immortalization and after transformation, (d) higher saturation density and (e) anchorage-independent growth. Our results provide unique direct in vitro evidence that cigarette smoke causes cancer in HPV-containing cervices.

Resistance to retinoic acid and altered cytokeratin expression of human papillomavirus type 16-immortalized endocervical cells after tumorigenesis

Human papillomaviruses (HPVs) and cigarette smoking are epidemiologically associated with cervical cancer. We recently found that HEN-16 and HEN-16-2 HPV type 16-immortalized endocervical cells form tumors after treatment with cigarette smoke condensate and derived 2 tumor cell line cultures, HEN-16T and HEN-16-2T, respectively. Here, we examine the molecular pathologic effect of tumorigenesis. HEN-16T and HEN-16-2T exhibit unchanged status and expression of integrated HPV 16 DNA. However, the expression of the cytokeratin CK7 and CK13 endocervical cell markers is more homogeneous in monolayer and organotypic raft cultures after tumorigenesis. For the effect of retinoic acid on monolayers for growth inhibition, HEN-16T were significantly less sensitive than the normal and immortalized non-tumorigenic cells. HEN-16-2T were completely resistant. Moreover, the rafts from both tumorigenic cell line cultures were resistant to retinoic acid and continued to display thick rafts and homogeneous severe dysplasia/carcinoma in situ. In contrast, the non-malignant HEN-16 and HEN-16-2 rafts were thinner, and treatment with retinoic acid blocked the formation of severe dysplasia, reconstructing an epithelium resembling that of the normal endocervix. Our results support the significance of non-viral factors in the mechanism by which cigarette smoking induces tumorigenesis in the late stages of HPV-initiated progression to cervical cancer. Importantly, our data indicate that the sensitivity to retinoic acid of the HPV-containing endocervical cells is lost following tumorigenesis in vitro and possibly in women.

Human JC virus cAMP response elements functional for enhanced glial cell expression in differentiating embryonal carcinoma cells

Human JC virus (JCV) is glial cell-specific for growth and expression. This specificity is attributed to the cis-acting sequences of the two 98-bp tandem repeats in the JCV regulatory region. JCV causes brain lesions, especially in AIDS patients. To study the expression of JCV in glial cells, the role of both repeat region TGAGCTCA sequences, which are homologous to the classical TGAGCTCA cAMP response element (CRE), was examined. The effect of the CRE on expression of the JCV early promoter (JCVE) in response to cAMP was studied with undifferentiated, glial and muscle P19 embryonal carcinoma cells. The results showed a threefold increase in response to cAMP only in the glial cells in which JCV is efficiently expressed. The direct in vivo role of the JCV CRE was confirmed by site-directed mutagenesis. Additionally, a CRE oligonucleotide was induced by cAMP in vivo, and in in vitro transcription assays with glial cell extracts. The early promoter of human BK virus containing nonhomologous CRE sequences was previously shown not to be glial cell-specific and failed to respond to cAMP in glial P19 cells in this study. Mobility shift assays showed the cAMP-induced in vitro interaction of glial cell protein(s) with the CRE oligonucleotide. Southwestern blot and uv crosslinking experiments identified an approximately 43-kDa protein interacting with the JCV CRE oligonucleotide. The results indicate that the in vivo expression of JCVE is specifically increased in response to cAMP only in glial cells and JCV CRE in vitro protein complexes are only detected in response to cAMP for glial cell extracts.

Splice sites of human papillomavirus type 16 E6 gene or heterologous gene required for transformation by E7 and accumulation of E7 RNA

Transformation of primary baby rat kidney cells by the human papillomavirus type 16 (HPV 16) E7 gene and efficient accumulation of E7 RNA have been shown by this laboratory to depend on the integrity of the nucleotide position (nt) 880 splice donor site. Here, the splice sites within the HPV 16 E6 open reading frame (ORF) and the sites of the SV40 splicing unit were examined for an ability to provide this requirement. Constructs containing the HPV 16 E6 sites and the SV40 splice site sequences were used for transformation and RNase protection assays. E6 splice sites supported a low level of transformation, in assays for complete HPV 16 early region constructs containing loss-of-function mutations of the nt 880 site. Using constructs with wild-type E6 or SV40 splice sites showed that both splice sites could substitute similarly for the requirement in cis of the nt 880 site for transformation. HPV 16 E6 mutated splice site and SV40 splice site in reverse, nonfunctional orientation relative to the promoter, were not transformation competent. The HPV 16 E7 RNA levels for the E6 splice site constructs correlated closely with the transformation frequency. The SV40 splice sites were required for E7 transcript accumulation. The results showed E6 splice site function and evidence for enhanced exon skipping from E6 splice donor site to acceptor sites 3' of the E7 ORF. This was shown with constructs containing loss-of-function mutations of the nt 880 site. These results confirmed the function of the splice sites by the transformation competent constructs and suggested lower transformation frequency than for wild type was due to skipping of the E7 exon. These patterns of transcripts may have a role in the regulation of gene expression during progression to malignancy. The combined results revealed that the general presence of a functional splice donor site was absolutely required for transformation by HPV 16 E7 and accumulation of E7 RNA.

Alterations in physical state and expression of human papillomavirus type 18 DNA following crisis and establishment of immortalized ectocervical cells

Integration of episomal human papillomavirus (HPV) DNA in infected cervical lesions during malignant progression is frequently observed, but the importance of integration is poorly understood. We have studied immortalization by HPV-18 of human cervical cells as an in vitro model system. Here, the status and expression of HPV-18 DNA in precrisis ectocervical keratinocytes was compared with that in the same cells after crisis and establishment of immortalization. Southern blots revealed, and two-dimensional gel analysis confirmed, that the precrisis culture contained more than 100 copies/cell of episomal HPV-18 DNA and no detectable integrated viral DNA. In contrast, the postcrisis cells contained a low copy number of only integrated viral genome. The Northern blot patterns of E6-E7 and E2/E4 RNA expression were also different. Analysis of RNA by RT-PCR indicated that neither culture expressed the unspliced HPV-18 E6 oncogene present in tumor cell lines and that the precrisis, but not postcrisis, culture expressed the full-length E2 repressor. The two cultures displayed a similar keratinocyte morphology in vitro and a similar low grade dysplasia in vivo and both were non-tumorigenic. These results suggest that, although insufficient for complete malignant conversion, viral DNA integration during crisis is associated with the establishment of an immortalized phenotype in which HPV-18 DNA is integrated and HPV-18 RNA expression is altered.

Prevention of carcinoma in situ of human papillomavirus type 16-immortalized human endocervical cells by retinoic acid in organotypic raft culture

OBJECTIVE

To determine the effect of retinoic acid on the development of severe dysplasia or carcinoma in situ from endocervical cells containing human papillomavirus (HPV) type 16.

METHODS

Two independent lines of HPV 16-immortalized endocervical cells were reconstructed into two squamous epithelial tissues using the organotypic raft culture system to examine the differentiated phenotype. The effect of retinoic acid on dysplastic morphology of differentiation of the epithelia was examined by light microscopy of stained sections and electron microscopy. The endocervical cell type cytokeratin expression pattern was determined by indirect immunofluorescence using specific monoclonal antibodies. Ribonucleic acid expression of the HPV 16 E7 oncogene was examined by in situ hybridization.

RESULTS

Untreated HPV 16-immortalized endocervical cells were reconstructed into squamous dysplastic lesions resembling carcinoma in situ observed in women. Retinoic acid-treated rafts formed epithelia composed of two to three cell layers of columnar-like cells resembling simple epithelium of the endocervix. Electron microscopy and cytokeratin expression patterns confirmed the histology of a differentiated endocervical phenotype after treatment with retinoic acid. Expression of HPV 16 E7 was modestly lower in treated epithelia, preferentially in basal cells.

CONCLUSION

Retinoic acid prevents the histology and cytokeratin differentiation markers of carcinoma in situ of HPV 16-immortalized endocervical cells. Because the epithelia closely mimic HPV 16-containing severe dysplasias and native endocervical epithelium in women, this immortalized endocervical cell-raft system may be useful as a model to assess the efficacy of agents such as retinoic acid for preventing progression of these lesions to malignant cervical carcinoma.

Differential regulation by c-jun and c-fos protooncogenes of hormone response from composite glucocorticoid response element in human papilloma virus type 16 regulatory region

Glucocorticoid hormones positively regulate human papilloma virus (HPV) type 16 gene expression, and we have previously shown that this regulation is through three glucocorticoid response elements (GREs). The GRE at nucleotide 7640 is a composite GRE (cGRE) containing an overlapping activator protein-1 (AP-1) motif for the c-jun homodimer and c-jun/c-fos heterodimer. This report examined the effects of c-jun and/or c-fos AP-1 protooncogenes and the glucocorticoid hormone dexamethasone on expression of the HPV 16 cGRE in AP-1-deficient P19 embryonal carcinoma cells. The activity of the full-length HPV 16 enhancer was progressively increased with increasing levels of c-jun. The hormone induced an additional response. For the c-jun/c-fos heterodimer, the response to hormone was progressively diminished. Site-specific mutations of the cGRE revealed that the regulation by AP-1 and hormone required both GRE and the AP-1 motif. An enhancer fragment containing the cGRE and excluding the two simple GREs gave similar results. Two disruption mutations of the AP-1 site confirmed the requirement of this site for hormone response. A cGRE oligonucleotide construct substantiated the effect of c-jun for response to hormone. For heterodimer, activity and hormone response were both also progressively increased. The results reveal a unique cross-talk between the distinct AP-1- and hormone-signaling pathways, suggesting the involvement of a complex interaction of c-jun and c-fos and glucocorticoid hormone receptor with the HPV 16 cGRE, resulting in novel control patterns for regulating viral expression.

Role of steroid hormones in potentiating transformation of cervical cells by human papillomaviruses

Human papillomaviruses (HPVs) are etiologically involved in cervical neoplasia, and epidemiological evidence suggests that steroid hormones can increase the risk of this cancer in HPV-infected women. Steroids can interact with hormone-response elements in the viral long control region, enhancing HPV transcription and resulting in transformation of cervical cells. Subsequent malignant progression may involve virus-induced chromosomal instability, facilitating viral DNA integration and deregulation of gene expression.

Human papillomavirus 18-immortalized endocervical cells with in vitro cytokeratin expression characteristics of adenocarcinoma

OBJECTIVE

To determine whether human papillomavirus (HPV) 18 has a role in the development of adenocarcinoma from human endo- or ectocervical cells.

METHODS

Secondary cultures of human endo- and ectocervical cells were assayed for immortalization by HPV 18 DNA using lipofection. The effects of immortalization on the patterns of cytokeratin expression were determined by indirect immunofluorescence using monoclonal antibodies. The differentiation phenotype of the immortalized cells was investigated by a modified in vivo implantation system.

RESULTS

Both endo- and ectocervical cells were immortalized by HPV 18. The immortalized cells contained integrated HPV 18 DNA and expressed E6-E7 RNA. The immortalized endocervical cells had a cytokeratin phenotype characteristic of adenocarcinoma, whereas the immortalized ectocervical cells retained a distinct cytokeratin expression pattern of normal parental cells. In an in vivo implantation system, endocervical cells formed a lesion resembling severe dysplasia or carcinoma in situ, whereas ectocervical cells developed into a lesion resembling mild dysplasia. Both cell lines were nontumorigenic in nude mice.

CONCLUSION

Both endo- and ectocervical cells are targets for immortalization by HPV 18. Based on cytokeratin expression patterns, immortalized endocervical cells, but not ectocervical cells, may be useful as a model for premalignant lesions that progress into adenocarcinoma.

In vitro and in vivo analysis of cellular origin of cervical squamous metaplasia

We have previously shown that cultured normal human endocervical cells (HENs) form epithelium resembling squamous metaplasia in vivo. To analyze the cellular origin of squamous metaplasia, the cytokeratin and mucin expression and morphological features of HENs in monolayer cultures and in implants beneath the skin of nude mice were examined. Primary HENs had two distinct morphological phenotypes in vitro pleomorphic epithelial cells and keratinocytelike cells. Using a panel of monoclonal antibodies for various cytokeratins (CKs), we observed that the pleomorphic cells, which were the primary outgrowths, expressed CK7 and CK18 and produced mucin, suggesting their origin to be the mucosecretory columnar cells (CCs) of the endocervix. Keratinocytelike cells were observed in proximity of the CC-like cells after a few days of HEN culture. Interestingly, these cells were homogeneously negative for CK7 expression, as for native reserve cells (RCs), and homogeneously positive for CK13 expression with the antibody that is specific for RCs. During early passages, the culture consisted mostly of the RC-like keratinocytelike cells, and in the late passages, the CC-like cells were predominant. HEN implants in nude mice morphologically formed epithelia similar to immature squamous metaplasia and showed variable CK18 expression. Moreover, they showed homogeneous CK13 expression throughout all layers and expressed mucin and CK7 in the suprabasal cells. The possibility that the HEN culture was originally a mixed population of CCs and RCs, that we failed to detect, cannot be eliminated. Our results support the more likely view that the endocervical simple epithelia, which form squamous metaplasia, are bipotential cells and undergo differentiation readily and reversibly to give rise to CC-like and RC-like cells in culture.

Glucocorticoid requirement for growth of human papillomavirus 16-transformed primary rat kidney epithelial cells: correlation of development of hormone resistance with viral RNA expression and processing

The establishment of transformation of primary baby rat kidney epithelial cells by human papillomavirus type 16 DNA requires glucocorticoid hormones (Pater et al., Nature (Lond.), 335: 832-835, 1988). In this report we provide evidence that growth of transformed baby rat kidney cells in culture also requires glucocorticoids. However, transformed cells for which growth does not require hormone readily arise after a brief period of crisis, if cultured without added hormone. No reduction of glucocorticoid receptor was evident in non-hormone-requiring cells. The expression of human papillomavirus 16 RNA in these cells was analyzed by Northern blot, primer extension, and RNase protection analysis. Cells that do not require hormone had greatly reduced levels of transcripts initiated from the viral P97 promoter. However, there is evidence for compensating alterations to allow more efficient expression of E7 mRNA, since the growth of these cells is correlated with altered patterns of viral RNA expression and processing.

Multiple human papillomavirus type 16 glucocorticoid response elements functional for transformation, transient expression, and DNA-protein interactions

We have previously shown that human papillomavirus type 16 (HPV-16) can efficiently transform primary baby rat kidney cells in the presence of the steroid hormones progesterone and the glucocorticoid dexamethasone. To study this effect of hormone, different combinations of the previously identified glucocorticoid response element (GRE) at nucleotide 7640 of HPV-16 and the other two GREs that we have recently identified, at nucleotides 7385 and 7474, were mutated. The previously described GRE and the other two GREs were shown to be functional for the induction of transformation by dexamethasone. In addition, transient assays in cervical HeLa cells demonstrated the functional importance of the three individual GREs. Assays for in vitro interaction demonstrated the specific binding of a 97-kDa protein, the glucocorticoid receptor, to both recently identified HPV-16 GREs.

In vivo cytokeratin-expression pattern of stratified squamous epithelium from human papillomavirus-type-16-immortalized ectocervical and foreskin keratinocytes

The association between human papillomavirus (HPV) type 16 and cervical squamous-cell carcinoma has been well documented, and HPV 16 DNA has been shown to immortalize human genital keratinocytes in vitro. Using a panel of cytokeratin(CK)-specific antibodies, we examined the CK expression pattern, an important characteristic of epithelia, of the stratified squamous epithelium reconstructed in vivo from monolayer cultures of 2 human ectocervical and 3 human foreskin keratinocyte cell lines immortalized by HPV 16 DNA. Whereas the abnormal epithelium formed from these grafts presented certain aspects of mature terminal differentiation, such as morphology and expression of CK10/11, the expression patterns for CK19, and especially CK13, were more obviously abnormal. CK18 expression was not detectable in any of the epithelia formed from the 5 cell lines. In contrast, for an HPV-16-immortalized endocervical cell line and the CaSki cervical-carcinoma cell line there was expression of CK18. Our results indicate that HPV-16-induced immortalization of keratinocytes is associated with disruption of the normal CK expression pattern in stratified squamous epithelium and that expression of particular CKs can be differentially disrupted.

Human papillomavirus type 16 expression in cervical keratinocytes: role of progesterone and glucocorticoid hormones

OBJECTIVE

To determine the role of the steroid hormones, progesterone and glucocorticoids, and the viral hormone response elements, in the episomal expression of human papillomavirus (HPV) type 16 in primary human ectocervical cells.

METHODS

In situ hybridization and mutagenesis were used to assess the requirements of these hormones and the HPV 16 glucocorticoid/progesterone response elements in the induction of HPV 16 expression in ectocervical cells.

RESULTS

The assays detected a marked increase in viral messenger RNA only after treatment of the cells with either of the steroid hormones. This response was inhibited by the anti-progestin RU 486 in a concentration-dependent manner. Mutagenesis of the previously identified hormone response element in the regulatory region of the HPV 16 genome had no effect on hormone-induced HPV gene expression. We have now identified two additional hormone response elements. Different combinations of mutations in the three hormone response elements showed that all three were independently sufficient for the hormone-mediated induction of viral transcription.

CONCLUSIONS

Steroid hormones induce HPV 16 gene expression in cervical keratinocytes directly through three hormone response elements in the regulatory region of the viral genome. The anti-progestin RU 486 inhibits this induction. Because the physical state of HPV DNA in this in vitro system and in premalignant cervical lesions is extrachromosomal, steroid hormones may have a critical role in modulating HPV expression in such lesions.

Human JC virus perfect palindromic nuclear factor 1-binding sequences important for glial cell-specific expression in differentiating embryonal carcinoma cells

The brain cell specificity of the human papovavirus JC virus was examined by site-directed mutagenesis of the nuclear factor 1 (NF1) motifs within the viral regulatory region. The NF1 motif sites, located within the 98-bp tandem repeats that contain 6-bp perfect inverted palindromic sequences, were important for glial cell-specific expression of JC virus in differentiated embryonal carcinoma cells in vivo. The NF1 site on the late side of the repeats was not important, a fact confirmed by in vitro transcription studies. These observations were correlated with in vitro DNase I footprinting and mobility shift assays, which demonstrated specific interactions of factors in glial cell nuclear extracts with NF1 sites.

Squamous metaplasia of normal and carcinoma in situ of HPV 16-immortalized human endocervical cells

The importance of cervical squamous metaplasia and human papillomavirus 16 (HPV 16) infection for cervical carcinoma has been well established. Nearly 87% of the intraepithelial neoplasia of the cervix occur in the transformation zone, which is composed of squamous metaplastic cells with unclear origin. HPV DNA, mostly HPV 16, has been found in 90% of cervical carcinomas, but only limited experimental data are available to discern the role of HPV 16 in this tissue specific oncogenesis. We have initiated in vivo studies of cultured endocervical cells as an experimental model system for development of cervical neoplasia. Using a modified in vivo implantation system, cultured normal endocervical epithelial cells formed epithelium resembling squamous metaplasia, whereas those immortalized by HPV 16 developed into lesions resembling carcinoma in situ. In contrast, their ectocervical counterparts formed well differentiated stratified squamous epithelium and a lesion with mild dysplastic change, respectively. The HPV 16-immortalized cells showed in vivo cytokeratin expression patterns similar to their respective normal counterparts, confirming their different origins. Thus, this study provides direct experimental evidence for the transformation of simple epithelial cells of endocervical origin into stratified squamous metaplasia and indicates the differential susceptibility of endo- and ectocervical epithelial cells for conversion to cancer by HPV 16.

Mitochondrial nucleic acids as internal standards for blot hybridization analyses

A plasmid, designated p72, constructed from human lung carcinoma DNA inserted into the promoterless herpes simplex virus thymidine kinase gene pML-TK-Bgl II vector, hybridizes strongly to human nucleic acids on Southern and Northern blots. The portion of the DNA insert responsible for the strong signal following hybridization to human DNA or RNA is a 167-bp 3' terminal portion of the mitochondrial 16S ribosomal RNA gene. The expression of this gene is constitutive in the several human cell lines that were tested and is unaffected by exposure to cytotoxic chemicals that alter the expression of nuclear genes. This plasmid offers an excellent tool for studies of perturbations of gene expression and for controlling for the variations in sample preparation, loading, and transfer in Southern or Northern analysis of nucleic acids.

Glucocorticoid-dependent transformation by human papillomavirus type 16 E7 coding and 3' noncoding sequences

The establishment of transformation of primary rodent cells by human papillomavirus (HPV) type 16 DNA requires glucocorticoid hormones (Pater et al., Nature 335, 832-835, 1988). Here we provide evidence by mutational analysis that, in the context of the hormone-regulated HPV 16 promoter/enhancer, the only protein coding sequences of HPV 16 required are those of the E7 gene. Moreover, additional sequences adjacent to the 3' end of E7 coding sequences are also essential for the establishment of the transformed phenotype. Splice donor sites, especially an E7 ORF 3' proximal one, are implicated for this cis-acting function, since specific deletion mutations of these splice sites greatly or completely reduced the frequency of transformation and the level of E7 RNA.