Targeting the genetic landscape of oral potentially malignant disorders has the potential as a preventative strategy in oral cancer

This study reviews the molecular landscape of oral potentially malignant disorders (OPMD). We examine the impact of tumour heterogeneity, the spectrum of driver mutations (TP53, CDKN2A, TERT, NOTCH1, AJUBA, PIK3CA, CASP8) and gene transcription on tumour progression. We comment on how some of these mutations impact cellular senescence, field cancerization and cancer stem cells. We propose that OPMD can be monitored more closely and more dynamically through the use of liquid biopsies using an appropriate biomarker of transformation. We describe new gene interactions through the use of a systems biology approach and we highlight some of the first studies to identify functional genes using CRISPR-Cas9 technology. We believe that this information has translational implications for the use of re-purposed existing drugs and/or new drug development. Further, we argue that the use of digital technology encompassing clinical and laboratory-based data will create relevant datasets for machine learning/artificial intelligence. We believe that therapeutic intervention at an early molecular premalignant stage should be an important preventative strategy to inhibit the development of oral squamous cell carcinoma and that this approach is applicable to other aerodigestive tract cancers.

Fibroblast activation and senescence in oral cancer

There is now compelling evidence that the tumour stroma plays an important role in the pathogenesis of cancers of epithelial origin. The pre-eminent cell type of the stroma is carcinoma-associated fibroblasts. These cells demonstrate remarkable heterogeneity with activation and senescence being common stress responses. In this review, we summarise the part that these cells play in cancer, particularly oral cancer, and present evidence to show that activation and senescence reflect a unified programme of fibroblast differentiation. We report advances concerning the senescent fibroblast metabolome, mechanisms of gene regulation in these cells and ways in which epithelial cell adhesion is dysregulated by the fibroblast secretome. We suggest that the identification of fibroblast stress responses may be a valuable diagnostic tool in the determination of tumour behaviour and patient outcome. Further, the fact that stromal fibroblasts are a genetically stable diploid cell population suggests that they may be ideal therapeutic targets and early work in this context is encouraging.

Senescent cancer-associated fibroblasts secrete active MMP-2 that promotes keratinocyte dis-cohesion and invasion

Background:

Previous studies have demonstrated that senescent cancer-associated fibroblasts (CAFs) derived from genetically unstable oral squamous cell carcinomas (GU-OSCC), unlike non-senescent CAFs from genetically stable carcinomas (GS-OSCC), promoted keratinocyte invasion in vitro in a paracrine manner. The mechanism by which this occurs is unclear.

Methods:

Previous work to characterise the senescent-associated secretory phenotype (SASP) has used antibody arrays, technology that is limited by the availability of suitable antibodies. To extend this work in an unbiased manner, we used 2D gel electrophoresis and mass spectroscopy for protein identification. Matrix metalloproteinases (MMPs) were investigated by gelatin zymography and western blotting. Neutralising antibodies were used to block key molecules in the functional assays of keratinocyte adhesion and invasion.

Results:

Among a variety of proteins that were differentially expressed between CAFs from GU-OSCC and GS-OSCC, MMP-2 was a major constituent of senescent CAF-CM derived from GU-OSCC. The presence of active MMP-2 was confirmed by gelatine zymography. MMP-2 derived from senescent CAF-CM induced keratinocyte dis-cohesion and epithelial invasion into collagen gels in a TGF-β-dependent manner.

Conclusions:

Senescent CAFs from GU-OSCC promote a more aggressive oral cancer phenotype by production of active MMP-2, disruption of epithelial adhesion and induction of keratinocyte invasion.

Association of increased telomere lengths in limited scleroderma, with a lack of age-related telomere erosion

OBJECTIVES

Telomere erosion, a feature of biological ageing, is implicated in a wide range of diseases. Its impact on autoimmune diseases remains unclear although autoantibodies against many telomere nucleoprotein components are prevalent in these diseases. We aimed to assess if telomere biology was abnormal in a cohort of patients with limited cutaneous systemic sclerosis (lcSSc).

METHODS

Telomere lengths in peripheral blood leucocytes (PBL) were determined using Southern blotting methods in a cohort of lcSSc subjects (n=43; age range 37-80 years) and a control population (n=107; age range 21-65 years).

RESULTS

Telomere lengths in lcSSc subjects were longer than controls (p<0.001), did not show age-related telomere erosion and differed significantly from age-matched controls only after 50 years of age (p<0.001).

CONCLUSIONS

This is the first report of maintenance of telomere lengths in an autoimmune disease state. These data indicate aberrant telomere biology and irregular biological ageing from the fifth decade of life. These findings provide insight into compromised DNA damage repair in lcSSc. Whether these observations indicate a causal or consequential relationship requires further investigation. This in turn, may provide potential novel targets for therapeutic intervention.

Upregulation of HIF-1alpha in malignant transformation of oral submucous fibrosis

BACKGROUND

Oral submucous fibrosis (OSF) is a precancerous condition showing extensive fibrosis of the submucosa and affects most parts of the oral cavity, including pharynx and upper third of the oesophagus. The molecules involved in the biological pathways of the fibrotic process appeared to be either down- or upregulated at different stages of the disease. Despite the precancerous nature, malignant transformation of the epithelium in the background of fibrosis has not been studied in detail. HIF-1alpha is a known transcription factor that is induced by hypoxia.

AIMS

To test the hypothesis that hypoxia plays a role in malignant transformation and progression of OSF.

MATERIALS AND METHODS

We used both formalin-fixed and frozen samples of OSF and normal mucosa to investigate the relationship between HIF-1alpha and epithelial dysplasia using immunohistochemistry and RT-PCR.

CONCLUSIONS

Our data indicate that HIF-1alpha is upregulated at both protein and mRNA levels in OSF and the correlation with epithelial dysplasia is statistically significant (P < 0.001). We propose that HIF-1alpha may play a role in malignant transformation of OSF. Further, over-expression of HIF-1alpha may contribute to the progression of fibrosis. It may be possible to use HIF-1alpha as a marker for malignant transformation of OSF.

Human squamous cell carcinomas lose a mortality gene from chromosome 6q14.3 to q15

Normal human keratinocytes possess a finite replicative lifespan. Most advanced squamous cell carcinomas (SCCs), however, are immortal, a phenotype that is associated with p53 and INK4A dysfunction, high levels of telomerase and loss of heterozygosity (LOH) at several genetic loci, suggestive of the dysfunction of other mortality genes. We show here that human chromosome 6 specifically reduces the proliferation or viability of a human SCC line, BICR31, possessing LOH across the chromosome. This was determined by an 88% reduction in colony yield (P<0.001), following the reintroduction of an intact normal chromosome 6 by monochromosome transfer. Deletion analysis of immortal segregants using polymorphic markers revealed the loss of a 2.9 Mbp interval, centred on marker D6S1045 at 6q14.3-q15, in 6/19 segregants. Crucially, allelic losses of this region were not identified in control hybrids constructed between chromosome 6 and the BICR6 SCC cell line that is heterozygous for chromosome 6 and which showed no reduction in colony formation relative to the control chromosome transfers. This indicates that the minimally deleted region at D6S1045 is not the result of fragile sites, a recombination hot spot, or a feature of the monochromosome transfer technique. LOH of D6S1045 was found in 2/9 immortal SCC lines and was part of a minimally deleted region of line BICR19. Furthermore, allelic imbalance, consistent with LOH, was detected in 3/17 advanced SCCs of the tongue. These results suggest the existence of a suppressor of SCC immortality and tumour development at chromosome 6q14.3-q15, which is important to a subset of human SCCs.

Human fibroblast replicative senescence can occur in the absence of extensive cell division and short telomeres

Ectopic expression of telomerase blocks both telomeric attrition and senescence, suggesting that telomeric attrition is a mitotic counting mechanism that culminates in replicative senescence. By holding human fibroblast cultures confluent for up to 12 weeks at a time, we confirmed previous observations and showed that telomeric attrition requires cell division and also, that senescence occurs at a constant average telomere length, not at a constant time point. However, on resuming cell division, these long-term confluent (LTC) cultures completed 15-25 fewer mean population doublings (MPDs) than the controls prior to senescence. These lost divisions were mainly accounted for by slow cell turnover of the LTC cultures and by permanent cell cycle exit of 94% of the LTC cells, which resulted in many cell divisions being unmeasured by the MPD method. In the LTC cultures, p27(KIP1) accumulated and pRb became under-phosphorylated and under-expressed. Also, coincident with permanent cell cycle exit and before 1 MPD was completed, the LTC cultures upregulated the cell cycle inhibitors p21(WAF) and p16(INK4A) but not p14(ARF) and developed other markers of senescence. We then tested the relationship between cell cycle re-entry and the cell cycle-inhibitory proteins following subculture of the LTC cultures. In these cultures, the downregulation of p27(KIP1) and the phosphorylation of pRb preceded the complete resumption of normal proliferation rate, which was accompanied by the down-regulation of p16(INK4A). Our results show that most normal human fibroblasts can accumulate p16(INK4A), p21(WAF) and p27(KIP1) and senesce by cell division-independent mechanism(s). Furthermore, this form of senescence likely requires p16(INK4A) and perhaps p27(KIP1).

Detection of functional PTEN lipid phosphatase protein and enzyme activity in squamous cell carcinomas of the head and neck, despite loss of heterozygosity at this locus

The human tumour suppressor gene PTEN located at 10q23 is mutated in a variety of tumour types particularly metastatic cases and in the germline of some individuals with Cowdens cancer predisposition syndrome. We have assessed the status of PTEN and associated pathways in cell lines derived from 19 squamous cell carcinomas of the head and neck. Loss of heterozygosity is evident at, or close to the PTEN gene in 5 cases, however there were no mutations in the remaining alleles. Furthermore by Western analysis PTEN protein levels are normal in all of these SCC-HN tumours and cell lines. To assess the possibility that PTEN may be inactivated by another mechanism, we characterized lipid phosphatase levels and from a specific PIP3 biochemical assay it is clear that PTEN is functionally active in all 19 human SCCs. Our data strongly suggest the possibility that a tumour suppressor gene associated with development of SCC-HN, other than PTEN, is located in this chromosomal region. This gene does not appear to be MXI-1, which has been implicated in some other human tumour types. PTEN is an important negative regulator of PI3Kinase, of which subunit alpha is frequently amplified in SCC-HN. To examine the possibility that PI3K is upregulated by amplification in this tumour set we assessed the phosphorylation status of Akt, a downstream target of PI3K. In all cases there is no detectable increase in Akt phosphorylation. Therefore there is no detectable defect in the PI3K pathway in SCC-HN suggesting that the reason for 3q26.3 over-representation may be due to genes other than PI3K110alpha.

Decreased expression of TGF-beta cell surface receptors during progression of human oral squamous cell carcinoma

This study examined the immunocytochemical expression of the transforming growth factor-beta (TGF-beta) isoforms TGF-beta1, TGF-beta2, and TGF-beta3, together with the TGF-beta cell surface receptors TbetaR-I and TbetaR-II, in patient-matched tissue pairs of normal human oral epithelium, primary squamous cell carcinomas, and metastatic lymph node tumour deposits. There were no significant differences in the intensity of TGF-beta isoform specific staining between the normal oral epithelium, the primary tumours, and the lymph node metastases. By contrast, there was significantly less TbetaR-II in the metastases than in the primary tumour and between the primary tumour and the normal oral epithelium. Similar trends were evident with TbetaR-I, but not at a statistically significant level. This study also examined the structure of TbetaR-I and TbetaR-II in normal human oral keratinocytes in vitro and in 14 human oral carcinoma cell lines with known responses to TGF-beta1. No structural abnormalities of TbetaR-II were present in the normal keratinocytes or in 13 of 14 malignant cell lines; in one line, there were both normal and mutant forms of TbetaR-II, the latter being in the form of a frameshift mutation with the insertion of a single adenine base (bases 709-718, codons 125-128), predicting a truncated receptor having no kinase domain. No defects were present in TbetaR-I. The structures of TbetaR-I and TbetaR-II did not correlate with growth inhibition by TGF-beta1. The data suggest that decreased expression of TGF-beta receptors, rather than structural defects of these genes, may be important in oral epithelial tumour progression. In order to examine the functional significance of a specific decrease in TbetaR-II expression, a dominant-negative TbetaR-II construct (dnTbetaR-II) was transfected into a human oral carcinoma cell line with a normal TGF-beta receptor profile and known to be markedly inhibited by TGF-beta1. In those clones that overexpressed the dnTbetaR-II, growth inhibition and Smad binding activity were decreased, whilst the regulation of Fra-1 and collagenase-1 remained unchanged following treatment with TGF-beta1. The results demonstrate that a decrease in TbetaR-II relative to TbetaR-I leads to selective gene regulation with loss of growth inhibition but continued transcription of AP-1-dependent genes that are involved in the regulation of the extracellular matrix.

Decreased expression of TGF-beta cell surface receptors during progression of human oral squamous cell carcinoma

This study examined the immunocytochemical expression of the transforming growth factor-beta (TGF-beta) isoforms TGF-beta1, TGF-beta2, and TGF-beta3, together with the TGF-beta cell surface receptors TbetaR-I and TbetaR-II, in patient-matched tissue pairs of normal human oral epithelium, primary squamous cell carcinomas, and metastatic lymph node tumour deposits. There were no significant differences in the intensity of TGF-beta isoform specific staining between the normal oral epithelium, the primary tumours, and the lymph node metastases. By contrast, there was significantly less TbetaR-II in the metastases than in the primary tumour and between the primary tumour and the normal oral epithelium. Similar trends were evident with TbetaR-I, but not at a statistically significant level. This study also examined the structure of TbetaR-I and TbetaR-II in normal human oral keratinocytes in vitro and in 14 human oral carcinoma cell lines with known responses to TGF-beta1. No structural abnormalities of TbetaR-II were present in the normal keratinocytes or in 13 of 14 malignant cell lines; in one line, there were both normal and mutant forms of TbetaR-II, the latter being in the form of a frameshift mutation with the insertion of a single adenine base (bases 709-718, codons 125-128), predicting a truncated receptor having no kinase domain. No defects were present in TbetaR-I. The structures of TbetaR-I and TbetaR-II did not correlate with growth inhibition by TGF-beta1. The data suggest that decreased expression of TGF-beta receptors, rather than structural defects of these genes, may be important in oral epithelial tumour progression. In order to examine the functional significance of a specific decrease in TbetaR-II expression, a dominant-negative TbetaR-II construct (dnTbetaR-II) was transfected into a human oral carcinoma cell line with a normal TGF-beta receptor profile and known to be markedly inhibited by TGF-beta1. In those clones that overexpressed the dnTbetaR-II, growth inhibition and Smad binding activity were decreased, whilst the regulation of Fra-1 and collagenase-1 remained unchanged following treatment with TGF-beta1. The results demonstrate that a decrease in TbetaR-II relative to TbetaR-I leads to selective gene regulation with loss of growth inhibition but continued transcription of AP-1-dependent genes that are involved in the regulation of the extracellular matrix.

Early genetic and functional events in the pathogenesis of oral cancer

Oral squamous cell carcinoma (SCC) is a major world health problem, but the changes leading to the development of malignancy remain essentially unknown. Early changes are thought to include the loss of tumour suppressor genes on chromosomes 3p, 9p, and 17p. Although what genes are involved on chromosome 3 remains speculative, p16 (9p21) and p53 (17p13) are inactivated in a high proportion of oral dysplastic lesions and carcinomas. SCC-derived cell lines are immortal, have decreased growth requirements in vitro, and show a variable capacity to form tumours in athymic mice. Normal oral keratinocytes and cells from potentially malignant lesions invariably senesce at early culture passage, have strict growth requirements in vitro, and are nontumorigenic in vivo. By contrast to normal oral keratinocytes, cells from potentially malignant lesions are defective in their capacity to terminally differentiate in suspension culture. Loss of cellular senescence and gain of the immortal phenotype is associated with inactivation of p16 and p53.

E-cadherin at the cell periphery is a determinant of keratinocyte differentiation in vitro

The origin of the signal for keratinocyte differentiation is still unknown. Here, we show that Ca(2+)- and density-induced translocation of E-cadherin, but not P-cadherin, is accompanied by induction of differentiation-specific proteins in cultured keratinocytes. Antibodies that artificially cluster cell-surface E-cadherin in low extracellular Ca(2+) also induce differentiation-specific proteins, implicating E-cadherin as a determinant of keratinocyte differentiation in vitro.

Replicative senescence as a barrier to human cancer

There is evidence that one critically short telomere may be recognized as DNA damage and, as a consequence, induce a p53/p21WAF- and p16INK4A-dependent G1 cell cycle checkpoint to cause senescence. Additionally, senescence via a p53- and p16(INK4A)-dependent mechanism can be induced by the over- or under-stimulation of certain signalling pathways that are involved in cancer. Central to this alternative senescence mechanism is the p14ARF protein, which connects oncogene activation, but not DNA damage, to p53 activation and senescence. We find that immortal keratinocytes almost invariably have dysfunctional p53 and p16 and have high levels of telomerase, but very often express a wild-type p14(ARF). Furthermore, when normal keratinocytes senesce they show a striking elevation of p16 protein, but not of p14(ARF) or its downstream targets p53 and p21(WAF). These results suggest that p16, rather than p14(ARF), is the more important gene in human keratinocyte senescence, but do not exclude a co-operative role for p14(ARF), perhaps in the induction of senescence by activated oncogenes in neoplasia. Regardless of mechanism, these results suggest that replicative senescence acts as a barrier to human cancer development.

The catalytic activity of the Src family kinases is required to disrupt cadherin-dependent cell-cell contacts

Despite the importance of epithelial cell contacts in determining cell behavior, we still lack a detailed understanding of the assembly and disassembly of intercellular contacts. Here we examined the role of the catalytic activity of the Src family kinases at epithelial cell contacts in vitro. Like E- and P-cadherin, Ca(2+) treatment of normal and tumor-derived human keratinocytes resulted in c-Yes (and c-Src and Fyn), as well as their putative substrate p120(CTN), being recruited to cell-cell contacts. A tyrosine kinase inhibitor with selectivity against the Src family kinases, PD162531, and a dominant-inhibitory c-Src protein that interferes with the catalytic function of the endogenous Src kinases induced cell-cell contact and E-cadherin redistribution, even in low Ca(2+), which does not normally support stable cell-cell adhesion. Time-lapse microscopy demonstrated that Src kinase inhibition induced stabilization of transiently formed intercellular contacts in low Ca(2+). Furthermore, a combination of E- and P-cadherin-specific antibodies suppressed cell-cell contact, indicating cadherin involvement. As a consequence of contact stabilization, normal cells were unable to dissociate from an epithelial sheet formed at high density and repair a wound in vitro, although individual cells were still motile. Thus, cadherin-dependent contacts can be stabilized both by high Ca(2+) and by inhibiting Src activity in low (0.03 mM) Ca(2+) in vitro.

Increased dosage and amplification of the focal adhesion kinase gene in human cancer cells

Focal adhesion kinase (pp125FAK) is present at sites of cell/extracellular matrix adhesion and has been implicated in the control of cell behaviour. In particular, as a key component of integrin-stimulated signal transduction pathways, pp125FAK is involved in cellular processes such as spreading, motility, growth and survival. In addition, a number of reports have indicated that pp125FAK may be up-regulated in human tumour cells of diverse origin, and consequently, a role has been proposed for pp125FAK in the development of invasive cancers. However, to date the mechanisms that lead to elevated pp125FAK expression in tumour cells have not been determined. Here we used in situ hybridization to confirm chromosome 8q as the genomic location of the human fak gene and report that elevation of pp125FAK protein in cell lines derived from invasive squamous cell carcinomas is accompanied by gains in copy number of the fak gene in all cases examined. In addition, we observed increased fak copy number in frozen sections of squamous cell carcinomas. Furthermore, increased dosage of the fak gene was also observed in many cell lines derived from human tumours of lung, breast and colon, including two cell lines Calu3 and HT29, in which fak was amplified. In addition, in an in vitro model for human colon cancer progression there was a copy number gain of the fak gene during conversion from adenoma to carcinoma, which was associated with increased pp125FAK protein expression. Thus, we show for the first time that many cell lines derived from invasive epithelial tumours have increased dosage of the fak gene, which may contribute to the elevated protein expression commonly observed. Although other genes near the fak locus are co-amplified or increased in copy number, including the proto-oncogene c-myc, the biological properties of pp125FAK in controlling the growth, survival and invasiveness of tumour cells, suggest that it may contribute to the selection pressure for maintaining increased dosage of the region of chromosome 8q that encodes these genes.

Role of the alternative INK4A proteins in human keratinocyte senescence: evidence for the specific inactivation of p16INK4A upon immortalization

The INK4A locus on human chromosome 9p21 encodes two genes that have been implicated in replicative senescence and tumor suppression, p16INK4A and p14ARF. In contrast to p16INK4A, which is up-regulated to high levels, we were unable to detect p14ARF protein in senescent human keratinocytes. Also, p53, an established target of p14ARF, did not increase, suggesting that p14ARF is not instrumental in human keratinocyte senescence. In neoplastic keratinocyte cultures, p16INK4A inactivation was invariably associated with the immortal phenotype, and there was evidence for the inactivation of p16INK4A, independent of p14ARF, in 6 of 10 lines that lacked large homozygous deletions. In contrast, we failed to detect exon 1beta mutations or p16INK4A-independent deletions. These results emphasize the previously proposed role for p16INK4A in human keratinocyte senescence but do not rule out a supporting role for p14ARF inactivation.

Genetic and functional analyses exclude mortality factor 4 (MORF4) as a keratinocyte senescence gene

Approximately 50% of immortal human keratinocyte lines show loss of heterozygosity of chromosome region 4q33-q34, and the reintroduction of chromosome 4 into one such line, BICR 6, causes proliferation arrest and features of replicative senescence. Recently, a candidate gene, mortality factor 4 (MORF4), was identified in this region and sequenced in 21 immortal keratinocyte lines. There were no mutations or deletions, and two of the seven lines that showed loss of heterozygosity at 4q33-q34 were heterozygous for MORF4 itself. Furthermore, the transfer of a chromosomal segment containing the entire MORF4 gene did not mimic the senescence effect of chromosome 4 in BICR 6. These results suggest that the inactivation of MORF4 is not required for human keratinocyte immortality.

Analysis of the CAVEOLIN-1 gene at human chromosome 7q31.1 in primary tumours and tumour-derived cell lines

We identified CAVEOLIN-1 as a candidate for a tumour suppressor gene mapping to human chromosome 7q31.1. A number of studies suggest that caveolin could function as a tumour suppressor. Expression of caveolin, and in turn the number of caveolae within a cell, are inversely correlated with the transforming ability of numerous oncoproteins, including H-ras, v-abl, and bcr-abl, and caveolin is a major transformation-dependent substrate of v-src. Heterologous expression of caveolin has been shown to abrogate anchorage-independent growth and induce apoptosis in transformed fibroblasts and also to suppress anchorage-independent growth in human mammary carcinoma cells. We have analysed the status and expression of the human CAVEOLIN-1 gene in primary tumours and tumour-derived cell lines. We found no evidence for mutation of CAVEOLIN-1 in human cancers. Additionally, we found that while the first two exons of CAVEOLIN-1 are associated with a CpG island, this is not methylated in either primary tumours or in tumour-derived cell lines in which Caveolin-1 expression is low or undetectable. The level of expression of Caveolin-1 does not correlate with loss of heterozygosity at the CAVEOLIN-1 locus in these same cell lines. Contrary to other published studies, we have shown that CAVEOLIN-1 is not expressed in normal breast ductal epithelial cells in vivo. CAVEOLIN-1 is however highly expressed in breast myoepithelial cells and its expression is retained in tumours derived from breast myoepithelium. Together our data refute a role for CAVEOLIN-1 as a breast tumour suppressor gene in vivo.

Identification of a gene that reverses the immortal phenotype of a subset of cells and is a member of a novel family of transcription factor-like genes

Based on the dominance of cellular senescence over immortality, immortal human cell lines have been assigned to four complementation groups for indefinite division. Human chromosomes carrying senescence genes have been identified, including chromosome 4. We report the cloning and identification of a gene, mortality factor 4 (MORF 4), which induces a senescent-like phenotype in immortal cell lines assigned to complementation group B with concomitant changes in two markers for senescence. MORF 4 is a member of a novel family of genes with transcription factor-like motifs. We present here the sequences of the seven family members, their chromosomal locations, and a partial characterization of the three members that are expressed. Elucidation of the mechanism of action of these genes should enhance our understanding of growth regulation and cellular aging.

Telomerase repressor sequences on chromosome 3 and induction of permanent growth arrest in human breast cancer cells

BACKGROUND

Activation of the enzyme telomerase, which has been associated with cellular immortality, may constitute a key step in the development of human cancer. Telomerase is repressed in most normal human somatic cells. This study was conducted, using a genetic complementation approach, with the aim of identifying and mapping the genes responsible for repressing telomerase and, simultaneously, to establish the effect of experimentally induced telomerase repression on human tumor cell growth.

METHODS

Individual human chromosomes isolated from normal diploid cells and tagged with bacterial antibiotic resistance genes (for later selection) were introduced into cells of the human breast carcinoma cell line 21NT by means of microcell transfer. Selected hybrid clones were screened for telomerase activity by use of the polymerase chain reaction-based telomere repeat amplification protocol (TRAP) assay, and the proliferative fate of the hybrid clones was determined. Regions of the introduced chromosomes associated with telomerase repression were mapped using segregant hybrids and a deletion analysis that employed microsatellite DNA markers.

RESULTS

Strong repression of telomerase was observed following transfer of human chromosome 3 into 21NT cells but not after transfer of chromosomes 8, 12, or 20. The vast majority of hybrid clones with repressed telomerase entered permanent growth arrest after 10-18 population doublings. Deletion analysis of nonrepressed segregant monochromosome 3 hybrids indicated two regions on the short arm of chromosome 3 (3p21.3-p22 and 3p12-21.1) where telomerase regulator genes may be located.

CONCLUSIONS

Telomerase in human breast cancer cells is efficiently repressed by a gene or genes on normal human chromosome 3p, and this repression is associated with permanent growth arrest of the tumor cells.

Cloning and characterization of human and mouse telomerase RNA gene promoter sequences

Variation in telomerase activity is correlated with cellular senescence and tumour progression. However, although the enzymatic activity of telomerase has been well studied, very little is known about how expression of telomerase genes is regulated in mammalian cells. We have therefore cloned the promoter regions of the human (hTR), and mouse, (terc), telomerase RNA genes in order to identify the regulatory elements controlling telomerase RNA gene transcription. 1.76 kb encompassing the hTR gene promoter region was sequenced, as was 4 kb encompassing the terc promoter. No significant sequence similarity could be detected in comparisons between human and mouse 5'-regions, flanking the transcribed sequences. However, both the human and mouse telomerase RNA genes are within CpG islands and may therefore be under the regulation of DNA methylation. Transient expression of hTR-reporter gene constructs in HeLa and GM847 cells identified the elements responsible for promoter activity are contained in a 231 bp region upstream of the transcriptional start site. Transient expression of terc-reporter gene constructs in Swiss3T3 and A9 cells identified the elements responsible for promoter activity are contained in a 73 bp region upstream of the transcriptional start site. These studies have implications for novel transcription targeted cancer therapies.

Evidence for the inactivation of multiple replicative lifespan genes in immortal human squamous cell carcinoma keratinocytes

Human keratinocyte immortality is genetically recessive to the normal phenotype of limited replicative lifespan and appears to require the dysfunction of p53 and the cyclin D-Cdk inhibitor p16. In order to test for the inactivation of other candidate replicative lifespan genes in the immortal cells of human tumors, we developed a series of mortal and immortal keratinocyte cultures derived from neoplastic lesions of the head and neck which were amenable to molecular genetic analysis by the loss of heterozygosity (LOH) technique. The results indicate that keratinocyte immortalization in head and neck squamous cell carcinoma (SCC-HN) development involves the inactivation of at least two further pathways to senescence and four in all. Chromosomes 1, 4 and 7 carry genes representing immortality complementation groups C, B and D respectively and immortal keratinocytes showed LOH at either 4q32-q34 between D4S1554 and D4S171 (group B) or 7q31 (group D) but never 1q25 (group C). These results tentatively suggest that the genes responsible for the immortality complementation groups encode proteins on the same pathway to senescence. In addition, all of the immortal keratinocyte lines possessed high levels of telomerase activity and a suppressor of telomerase activity has been mapped to the short arm of chromosome 3p. Five out of eight lines showed LOH at 3p21.2-p21.3, a region which may carry a gene capable of suppressing SCC-HN telomerase. However, alternative mechanisms of telomerase reactivation were also suggested by our results. None of the above genetic alterations were seen in seven senescent neoplastic keratinocyte cultures. Other loci harbouring antiproliferative genes implicated in replicative lifespan showed few or no alterations and any alterations seen were additional to those described above.

The genetic basis of human keratinocyte immortalisation in squamous cell carcinoma development: the role of telomerase reactivation

Normal human keratinocytes have a finite replicative lifespan which culminates in senescence. Chromosomal telomere length may act as a mediator of replicative senescence, signalling cell cycle arrest in G1 when one or more telomeres become too short. Telomeric attrition in normal keratinocytes may be due to inadequate levels of telomerase activity and possibly also to oxidative damage. In advanced squamous cell carcinoma replicative senescence breaks down to yield immortal variants, in which several dominantly acting genes are functionally compromised, including p53 and the cyclin D-Cdk4/6 inhibitor CDKN2A/p16. The increased activity of both of these proteins would be expected to contribute to the G1 arrest in senescence and we have shown that levels of p16 are dramatically increased in senescent keratinocytes. In addition, two other genes which control a cell cycle G1 checkpoint independently of p53 and pRb appear dysfunctional. These genes are uncloned but map to chromosome 4q and 7q31.1 and appear to represent senescence complementation groups B and D, respectively. In immortal neoplastic keratinocytes, telomerase is strongly upregulated and there is evidence for a suppressor of the enzyme on the short arm of chromosome 3 mapping to 3p21.2-p21.3. We have also mapped the human telomerase RNA gene to 3q26.3 and found it to be overrepresented or amplified in a proportion of squamous cell tumours and cell lines. These observations may explain why isochromosome 3q is so common in human squamous carcinoma. None of these genetic alterations are seen in carcinomas which senesce and suggest that multiple genetic alterations are required for keratinocyte immortality.

Amplification, increased dosage and in situ expression of the telomerase RNA gene in human cancer

Telomere length is maintained by the enzyme, telomerase, which has been linked to cellular immortality and tumour progression. However, the reasons for the high levels of telomerase found in human tumours are unknown. We have mapped the human telomerase RNA gene, (hTR), to chromosome 3q26.3 and show the hTR gene to be amplified in four carcinomas, (2/33 cervix, 1/31 head and neck, 1/9 lung). In addition, increased copy numbers of the hTR locus was also observed in 97% of tumours. By in situ hybridisation, the histological distribution of high levels of hTR expression could be demonstrated in a lung tumour and its metastasis with hTR amplification. These results are the first report of genetic alterations involving a known component of telomerase in human cancer. Indeed, it is also the first report of the amplification of a specific locus within the chromosome 3q region frequently subject to copy number gains in human tumours. In addition, we also show for the first time the histological distribution of the RNA component of telomerase in human tumours.

Dysregulation of autocrine TGF-beta isoform production and ligand responses in human tumour-derived and Ha-ras-transfected keratinocytes and fibroblasts

This study examined the autocrine production of TGF-beta 1, -beta 2 and -beta 3 in culture supernatants from tumour-derived (H series, n = 7; BICR series, n = 5), Ha-ras-transfected (n = 4) and normal (n = 2) human keratinocytes using a sandwich enzyme-linked immunosorbent assay (ELISA). Detection limits were 39.0 pg ml-1 for TGF-beta 1, 78.0 pg ml-1 for TGF-beta 2 and 1.9 ng ml-1 for TGF-beta 3. Tumour-derived oral keratinocytes predominantly produced less TGF-beta 1 than normal oral epithelial cells; the expression of endogenous TGF-beta 2 was variable. In keratinocytes containing mutant Ha-ras, TGF-beta 1 production was enhanced and TGF-beta 2 was undetectable. TGF-beta 3 mRNA was detected by reverse transcription-polymerase chain reaction (RT-PCR) but the protein was not detected in conditioned media, most probably because of the low detection limits of the ELISA for this isoform. Neutralisation experiments indicated that the latent TGF-beta peptide was secreted in keratinocyte conditioned medium. Seven tumour-derived keratinocyte cell lines (H series) and fibroblasts separated from normal (n = 1) and tumour-derived (n = 2) keratinocyte cultures were examined for their response to exogenous TGF-beta 1, -beta 2 and -beta 3. Six of seven tumour-derived keratinocyte cell lines were inhibited by TGF-beta 1 and TGF-beta 2 (-beta 1 > -beta 2); one cell line was refractory to both TGF-beta 1 and TGF-beta 2. Keratinocytes were inhibited (4 of 7), stimulated (1 of 7) or failed to respond (2 of 7) to TGF-beta 3, TGF-beta 1, -beta 2 and -beta 3 stimulated both normal and tumour-associated fibroblasts, but the tumour-associated fibroblasts showed less response to the ligands than their normal counterparts following prolonged treatment with each isoform. The results demonstrate variable autocrine production of TGF-beta isoforms by malignant keratinocytes, with loss of TGF-beta 1 generally associated with the tumour-derived phenotype and modification of endogenous isoform production dependent on the genetic background of the tumour cells. Further, the variable response of the tumour-derived keratinocytes and contiguous fibroblasts to the TGF-beta isoforms suggests that dysregulation of TGF-beta autocrine and paracrine networks are common characteristics of squamous epithelial malignancy.

Sensitivity to transforming growth factor beta 1-induced growth arrest is common in human squamous cell carcinoma cell lines: c-MYC down-regulation and p21waf1 induction are important early events

Transforming growth factor beta 1 (TGF-beta 1) is a potent inhibitor of keratinocyte proliferation and a potential tumor suppressor of squamous cell carcinomas (SCCs). TGF-beta 1 exerts its antiproliferative effects by inhibiting key transitions required for progression from G1 to the S phase of the cell cycle, exemplified by a rapid reduction of c-MYC and inhibition of the G1 cyclin/cyclin-dependent kinases by induction of their inhibitors p21waf1, p27kip1, and p15INK4B. A significant majority of a new series of human SCC cell lines were found to be as sensitive as primary human epidermal keratinocytes to TGF-beta 1 growth inhibition. Only a minority of cell lines derived from late-stage tumors were resistant. An early and rapid increase in p21waf1 and reduction in c-MYC protein levels were important concomitants for TGF-beta 1 growth inhibition; these changes occurred exclusively in each of the sensitive cell lines. Expression of p15INK4B was found to be neither necessary nor sufficient for TGF-beta 1 growth arrest in the sensitive and resistant cell lines, respectively. TGF-beta 1 induced alterations in other cell cycle regulatory molecules, cyclin-dependent kinase 4, cyclin D1, pRB, and p27Kip1, occurred late and were dispensable in some of the sensitive cell lines. Expression of exogenous mycER fusion protein in one of the sensitive cell lines did not render the cells resistant to TGF-beta 1-induced growth arrest nor prevent p21waf1 induction or down-regulation of both c-MYC and mycER proteins. However, in TGF-beta 1-resistant subclones of sensitive mycER-expressing cells, p21waf1 was not induced, whereas both c-MYC and mycER protein levels decreased following TGF-beta 1 treatment. We conclude that TGF-beta 1 activates multiple cell cycle inhibitory pathways dependent upon p21waf1 induction and c-MYC degradation and that it does not function as a tumor suppressor in the majority of SCCs.

Association of CDKN2A/p16INK4A with human head and neck keratinocyte replicative senescence: relationship of dysfunction to immortality and neoplasia

We have previously suggested that a gene mapping to chromosome 9p21 could contribute to replicative senescence and suppress cullular immortality in squamous neoplasia. Two candidate genes, the cyclin D1/cyclindependent kinase inhibitors CDKN2A/p16INK4A (p16) and CDKN2B/p15INK4B (p15) have now been identified in this region and we show here that p16 is upregulated when normal human keratinocytes undergo replicative senescence but not when they undergo differentiation. Furthermore, all of 19 immortal neoplastic keratinocyte head and neck lines, including nine showing loss of heterozygosity (LOH) at 9p21, showed undetectable p16 expression, whereas five of six senscent neoplastic cultures showed normal levels of expression. The retinoblastoma protein (pRb) appeared functional in all the cell lines and cultures examined. The mechanism of p16 inactivation appeared to be transcriptional silencing in 10 of 18 lines and homozygous deletions in the rest. Treatment of two of the immortal cell lines which had transcriptionally silent wild type p16 genes with 5aza-2deoxycytidine resulted in the re-expression of p16, thus implicating DNA methylation as one mechanism of transcriptional silencing in the immortal SCC-HN lines. We observed no cases of p16 point mutation. In contrast, the p15 gene was rarely transcriptionally silent and was not deleted in any of the cell lines which showed p16 deletions. Our results show that p16 dysfunction correlates strongly with keratinocyte immortalisation but less strongly with the stage of tumour progression. P16 dysfunction was not related to the neoplastic state or the length of time spent in vitro. The results also suggest that p16 but not p15 is involved in the keratinocyte replicative senescence programme. However, two neoplastic cell cultures which lacked p16 expression were still mortal, suggesting that the loss of p16 is a necessary but insufficient condition for human keratinocyte immortality.

Cellular immortality: a late event in the progression of human squamous cell carcinoma of the head and neck associated with p53 alteration and a high frequency of allele loss

Many human tumors contain variant cells that, unlike their normal counterparts, possess indefinite proliferative potential in vitro. However, little is known of the relevance of these immortal cells to human carcinomas in vivo. To investigate immortality in a human tumor system, we established cultures from different stages of head and neck squamous carcinoma (SCC-HN). All the neoplastic cultures were transformed because they showed very low cornification in surface or suspension culture and were partially or completely resistant to suspension-induced death. Immortal variants were not detected in premalignant erythroplakia cultures, but their frequency increased with tumor progression, indicating that immortality is a late event in carcinogenesis. Some late-stage carcinomas still produced senescent cultures, but, significantly, all recurrent tumors were immortal. Immortal but not senescent carcinoma cultures were associated with p53 dysfunction and a high frequency of allele loss, indicative of tumor suppressor gene inactivation. These results show that there are at least two classes of human SCC-HN that are phenotypically and genotypically distinct and that the pathological stage of a given tumor is not necessarily indicative of the kind of cells it contains.

Loss of heterozygosity of chromosome 9p21 is associated with the immortal phenotype of neoplastic human head and neck keratinocytes

Human chromosomes 1,4,6, and 9 harbor genes which induce cellular senescence in vitro but a role for their inactivation in human tumors is not established. To investigate this we searched for loss of heterozygosity (LOH) on these chromosomes in keratinocyte cultures obtained from different stages of human squamous cell carcinoma progression. There was consistent LOH between markers D9S171 and D9S157 in 9 of 9 (100%) informative immortal cultures and in one line which entered crisis, but 0 of 7 informative senescent cultures showed LOH. These results suggest that inactivation of a gene at 9p21 is important but insufficient for human squamous cell carcinoma keratinocyte immortalization.

The p53 status of cultured human premalignant oral keratinocytes

Around 60% of oral squamous cell carcinomas (SCCs) have been shown to harbour p53 mutations, and other studies have demonstrated mutant p53 genes in normal and dysplastic squamous epithelium adjacent to these SCCs. In line with these earlier studies we show here that DOK, a keratinocyte cell line derived from a dysplasia, displays elevated levels of p53 protein and harbours a 12 bp in-frame deletion of the p53 gene spanning codons 188-191. In contrast, the coding region of the p53 gene was normal in a series of six benign recurrent laryngeal papillomas and a series of four premalignant oral erythroplakia biopsies and their cell cultures. All but one of these lesions were free of malignancy at the time of biopsy, in contrast to the premalignant lesions studied by previous investigators, but keratinocytes cultured from these lesions all displayed a partially transformed phenotype that was less pronounced than that of DOK. Since three out of four of the erythroplakia patients developed SCC within 1 year of biopsy, these lesions were by definition premalignant. The availability of strains of partially transformed keratinocytes from premalignant erythroplakias which possess normal p53 genes should enable us to test the role of mutant p53 in the progression of erythroplakia to SCC. The premalignant tissues and cultures were also tested for the presence of human papillomavirus (HPV), which is known to inactivate p53 function in some cases. Only the benign papillomas were shown to contain high levels of either HPV 6 or HPV 11 E6 DNA, but not both, and none of the samples contained detectable levels of HPV 16, HPV 18 or HPV 33 E6 DNA or L1 DNA of several other HPV types. There was therefore no evidence to suggest that p53 was being inactivated by a highly oncogenic HPV in these samples.

Maintenance of identical p53 mutations throughout progression of squamous cell carcinomas of the tongue

The occurrence of mutations within the coding sequence of the p53 tumour suppressor gene is now well documented for squamous cell carcinomas of the head and neck region. However, evidence that these mutations are required for the maintenance and progression of squamous tumours is still formally lacking. To test this we have examined whether p53 mutations detected in primary squamous cell carcinomas of the tongue are also detected in the corresponding lymph node metastases. Three different p53 mutations were detected in each of three primary tongue squamous cell carcinomas (SCC), and in each case the same mutation was detected in a lymph node metastasis excised from the same patient. Although the sample number is small, the chance of obtaining the same p53 mutation independently in both the primary and metastatic tumour of each patient is at least 10(-4), therefore the results indicate that keratinocytes harbouring these p53 mutations possess a selective advantage throughout SCC progression.

Assignment of two human epidermal squamous cell carcinomas cell lines to more than one complementation group for the immortal phenotype

Two human cell lines derived from squamous cell carcinomas (SCCs) of the epidermis, SCC-12 clone F and SCC-13 clone Y, were made to be independent of the Swiss 3T3 feeder layer to perform somatic-cell genetic experiments. We fused these SCC lines with normal human fibroblasts, and all resulting hybrids senesced after completing 12-17 population doublings, suggesting that in part, immortalization of the keratinocyte during SCC development results from the loss of gene function. We also tested whether these two SCC lines mapped to known complementation groups for immortality by fusing them with representatives of groups A (GM847), B (HeLa), and C (143B), but most of these hybrids were indistinguishable from those derived from homotypic crosses set up as immortal hybrid controls. As reported by others, fusions of cell lines from different complementation groups-143B (group C) x HeLa (group B) or GM847 (group A) x Hela (group B)--resulted in predominantly senescent hybrids. Our results confirmed and extended previous observations by others that the phenomenon of senescence is dominant to that of immortality, but they did not allow us to assign either of the SCC lines we studied to a complementation group for immortality.

Hemopoietic stem cell inhibitor (SCI/MIP-1 alpha) also inhibits clonogenic epidermal keratinocyte proliferation

The maintenance and regulation of continuously renewing tissues is ultimately controlled at the level of stem-cell proliferation. We have recently identified a reversible inhibitor of hemopoietic stem-cell proliferation (stem-cell inhibitor [SCI]), which is identical to the macrophage inflammatory protein, MIP-1 alpha, a 69-amino-acid heparin-binding cytokine. To test the cell/tissue specificity of the inhibition of proliferation by SCI/MIP-1 alpha, we have investigated its activity on epidermal keratinocytes, the principal cell type of another continuously renewing tissue. Here we show that SCI/MIP-1 alpha inhibits the proliferation of epidermal keratinocytes in vitro and that the MIP-1 alpha mRNA is present in epidermal Langerhans cells but not in keratinocytes. This suggests an important growth regulatory function for SCI/MIP-1 alpha in keratopoiesis, as well as hemopoiesis, and may also indicate a novel role for the epidermal Langerhans cell. As SCI/MIP-1 alpha can inhibit the proliferation of embryologically distinct precursor cells, this raises the possibility that it may also function in a number of other tissues.

Gene mutations and increased levels of p53 protein in human squamous cell carcinomas and their cell lines

Using immunocytochemical and Western blotting techniques we have demonstrated the presence of abnormally high levels of p53 protein in 8/24 (33%) of human squamous cell carcinomas (SCC) and 9/18 (50%) of SCC cell lines. There was a correlation between the immunocytochemical results obtained with eight SCC samples and their corresponding cell lines. Direct sequencing of PCR-amplified, reverse transcribed, p53 mRNA confirmed the expression of point mutations in six of the positive cell lines and detected in-frame deletions in two others. We also detected two stop mutations and three out-of-frame deletions in five lines which did not express elevated levels of p53 protein. Several of the mutations found in SCC of the tongue (3/7) were in a region (codons 144-166) previously identified as being a p53 mutational hot spot in non-small cell lung tumours (Mitsudomi et al., 1992). In 11/13 cases only the mutant alleles were expressed suggesting loss or reduced expression of the wild type alleles in these cases. Six of the mutations were also detected in the SCCs from which the lines were derived, strongly suggesting that the mutations occurred, and were selected, in vivo. The 12th mutation GTG-->GGG (valine-->glycine) at codon 216 was expressed in line SCC-12 clone B along with an apparently normal p53 allele and is to our knowledge a novel mutation. Line BICR-19 also expressed a normal p53 allele in addition to one where exon 10 was deleted. Additionally 15 of the SCC lines (including all of those which did not show elevated p53 protein levels) were screened for the presence of human papillomavirus types 16 and 18 and were found to be negative. These results are discussed in relation to the pathogenesis of SCC and the immortalisation of human keratinocytes in vitro.

Elevated levels of the p53 tumour suppressor protein in the basal layer of recurrent laryngeal papillomas

Using immunocytochemical techniques, recurrent laryngeal papillomas from six adult patients were studied. The p53 tumour suppressor gene protein was detected in the basal epithelial cells in papillomas from all six patients. None was detected in normal epithelium. Elevated levels of p53 protein are usually indicative of its inactivation either by point mutation or by complexing with tumour virus proteins.

Epidermal keratinocyte stem cells: their maintenance and regulation

The epidermis is a stratified epithelium consisting of interfollicular regions and appendages (hair follicles, sweat glands, sebaceous glands). The dominant cell type (the keratinocyte) is arranged in groups of cells termed epidermal proliferative units (EPUs), and one centrally-located clonogenic stem cell is ultimately responsible for replacing the remainder of the cells in the unit. Evidence is reviewed which indicates that the epidermal Langerhans' cell (ELC), and the cells comprising the dermis, may modify the keratinocyte microenvironment to create stem cell 'niches' and cellular diversity within the basal layer.

In vivo and in vitro effects of v-fos and EJ-Ha-ras oncogene expression in murine epidermal keratinocytes

Primary neonatal Balb/c keratinocyte (NEK) cultures grown, using 3T3 feeder cell support in high calcium, serum supplemented medium, were transfected with EJ-Ha-ras or v-fos DNA sequences and pSV2 neo. Several neo resistant clones were isolated and several established cell lines expressing the transfected gene products derived. Two of these lines, Ras 8 and Fos 1, have been examined in detail with respect to their self renewal capacity and differentiation potential in vitro and in vivo. In vitro, both lines (when compared to normal NEK) have an extended probably immortal phenotype, enhanced colony forming efficiency (a measure of in vitro self renewal capacity) and a reduction in growth factor and serum dependence. When grafted onto syngeneic recipients neither cell line is tumourigenic. Instead a histologically abnormal epithelium with no stratum corneum and with features specific to the oncogene expressed is formed. The extent of the histological atypia correlates with the in vitro alterations in cytoskeletal peptides as revealed by 2D PAGE. However despite the gross histological abnormality there is no alteration in the in vivo self renewal capacity (measured as the number of grafted cells required for epidermal reformation) between normal NEK and the Ras 8 or Fos 1 lines; in each case a minimum of 10(5) cells/1.14 cm2 is required before a full thickness epithelium forms.

Metabolically inactive 3T3 cells can substitute for marrow stromal cells to promote the proliferation and development of multipotent haemopoietic stem cells

When highly enriched multipotential spleen colony forming cells (CFU-S) obtained following fluorescence activated cell sorting (FACS-CFU-S) are cultured on marrow stromal cells, they undergo proliferation and development to produce mature haemopoietic cells (Spooncer et al., Nature, 316:62-64, 1985). We now show that FACS-CFU-S behave in a similar way when cultured on monolayers of 3T3 cells, indicating that the 3T3 cells can supply at least part of the environment which is representative of marrow stromal cells and provide, therefore, a system for studying stromal cell: haemopoietic cell interactions. We also demonstrate that IL-3-dependent multipotential stem cell lines (FDCP-Mix), but not a variety of other "committed" IL-3-dependent cell lines, resemble FACS-CFU-S in terms of their ability to proliferate and differentiate when cultured on 3T3 cells in the absence of IL-3. In this system, attachment of the FDCP-Mix to the 3T3 cells is critical for the subsequent maintenance of viability and stimulation of development of the cells. When the FDCP-Mix cells are physically separated from the 3T3 cells, they die and their death cannot be prevented by using 3T3-cell-conditioned medium. The extracellular matrix generated by 3T3 cells is not sufficient for promoting attachment or viability of the FDCP-Mix cells, indicating the importance of integral membrane components. However, attachment and development of FDCP-Mix cells occurs on 3T3 cells that have been lightly fixed with glutaraldehyde indicating that active metabolism is not essential for the effects promoted by the 3T3 cells. We suggest that the ability of FACS-CFU-S and FDCP-Mix cells to respond to 3T3 cells involves specific ligand/receptor interactions.

The effect of donor age on the proliferative response of human and mouse keratinocytes to phorbol, 12-myristate, 13-acetate

Epidermal keratinocytes grow clonally when provided with a 3T3 feeder layer and medium supplemented with 20% foetal bovine serum, hydrocortisone and cholera toxin. In this culture system the proliferative response of freshly isolated human and mouse keratinocytes to a short exposure (24 h) of the tumour promoter phorbol, 12-myristate, 13-acetate (PMA) was dependent on the donor age but was independent of the species or the biopsy site. Human keratinocytes from early (16-18 week) foetal donors displayed a strong proliferative response to PMA as determined by a 5- to 7-fold increase in colony number and an increase in the average colony size. In contrast, adult and juvenile keratinocytes of all ages from both mice and humans displayed an inhibition of colony forming efficiency and a reduction in colony size. When continuously passaged in culture human foetal keratinocytes gradually changed the pattern of their response to PMA so that they were inhibited from growing rather than being stimulated and after 21 days (three passages) their response was quantitatively similar to juvenile keratinocytes. Co-culture of juvenile keratinocytes with irradiated foetal keratinocytes did not alter their response to PMA so that the observed proliferative response of foetal keratinocytes to PMA is not readily explained by the autosecretion of mitogens or other regulatory molecules by these cells. Late foetal (17 days gestation), neonatal and post-neonatal (5-10 days old) mouse keratinocytes were also inhibited from growing by PMA but the magnitude of the effect was directly related to the age of the mouse and was in all cases less than that observed with adult mice. The relationship of these results to the mechanism of action of phorbol esters in epidermis is discussed.

Phospholipase C mimics the differential effects of phorbol-12-myristate-13-acetate on the colony formation and cornification of cultured normal and transformed human keratinocytes

Phorbol ester tumour promoters like phorbol-12-myristate-13-acetate (PMA) and serum-derived factors inhibit growth and induce terminal differentiation in normal human and mouse keratinocytes but have a much reduced effect on their transformed counterparts. These observations may be relevant to potency of PMA and wounding as tumour promoters in mouse epidermis. Since some serum factors produced during wounding are thought to exert their effects through the production of diacylglycerol (DAG-the proposed physiological ligand for the phorbol ester receptor) from phospholipids by the activation of phospholipase C (PC) we have compared the effects of PC with PMA in cultures of normal and transformed human keratinocytes. The addition of PC from Clostridium perfringens (0.1-3.0 units/ml) to the culture medium of normal human keratinocytes produced similar morphological changes to PMA and also mimicked the effects of the phorbol ester on cloning efficiency and cornified envelope formation. Most importantly PC, like PMA, had a very weak effect on the human squamous cell carcinoma lines SCC-12B and SCC-15. All the effects of PC were abolished by boiling the enzyme. These results are discussed in relation to the proposed role of serum factors in tumour promotion by deep skin wounding and their mechanistic relationship to phorbol esters.

The radiosensitivity of cultured human and mouse keratinocytes

Clonogenic survival assays after gamma-radiation in vitro were performed on freshly isolated and subcultured keratinocytes from mouse skin, mouse tongue and human skin. Survival curves were constructed by fitting the data to a multi-target model of cell survival. When subcultured, keratinocytes from all sites produced survival curves which showed a reduced shoulder region and an increased D0 when compared with their freshly isolated counterparts. Freshly isolated human skin keratinocytes were more radiosensitive than mouse keratinocytes from either skin or tongue.

Defective responses of transformed keratinocytes to terminal differentiation stimuli. Their role in epidermal tumour promotion by phorbol esters and by deep skin wounding

Epidermal tumourigenesis can be achieved in rodents by the application of a single subthreshold dose of a carcinogen (initiation) followed by repeated applications of a tumour promoter such as 12-0-tetradecanoyl phorbol, 13-acetate (TPA). TPA induces terminal differentiation in the majority of epidermal keratinocytes in vitro. However, transformed keratinocytes respond weakly to this terminal differentiation signal, and it is suggested that this property allows initiated cells and their progeny to obtain a selective advantage over their normal counterparts during promotion of papilloma formation by TPA. New data are reviewed which suggest that a putative wound hormone TGF-beta has similar differential effects on normal and transformed epithelial cells to those of TPA. It is proposed that the release of TGF-beta from platelets following deep skin wounding may be an explanation as to why wounding is a promoting stimulus but milder forms of epidermal injury are not. Weakly promoting hyperplasiogenic agents are also discussed within the context of a selection theory of tumour promotion.

Responses of human cervical keratinocytes in vitro to tumour promoters and diethylstilboestrol

We have compared the responses of normal human cervical keratinocytes (HCE) to diethylstilboestrol (DES), and the promoting agents, phorbol-12-myristate-13-acetate (PMA) and mezerein using the loss of cloning efficiency as a measure of terminal differentiation in vitro. Dose-response studies showed that normal HCE are growth inhibited by chronic exposure to DES at concentrations greater than or equal to 2.5 X 10(-5) M, to PMA at concentrations greater than 10(-8) M and mezerein at concentrations greater than 10(-9) M. Compared to acetone controls, promoter or DES-treated cells exhibited a 10- to 12-fold increase in cornified-envelope formation. Normal HCE exhibit a heterogeneous response to PMA in that 85-90% of colony-forming cells lose their colony-forming ability after a 24-h exposure to 10(-6) M PMA. The PMA-resistant subpopulation, PMAR, remains constant and is not reduced even after 96 h chronic exposure to PMA. In contrast, the colony-forming ability of normal HCE is almost totally suppressed after 24 h exposure to 10(-6) M mezerein. After 24 h incubation with 5 X 10(-5) M DES, 20% of normal HCE are capable of colony formation but this resistant fraction is eliminated after 96 h chronic exposure. Cornified-envelope formation was negligible in malignant cervical keratinocytes grown in the presence of DES or promotors and these cells were characterised by a very large PMAR fraction - 85 - 90% of cells retained colony-forming ability after exposure to 10(-6) M PMA for 24 h. Furthermore, 90-100% of malignant cervical keratinocytes retained their colony-forming capacity after exposure to 10(-6) M mezerein. However, colony-forming ability declined steadily in the presence of 5 X 10(-5) M DES and after 96 h only a tiny fraction, 1% of malignant cervical keratinocytes could form colonies on replating. The mechanisms by which DES inhibits growth and induces cornified-envelope formation in HCE would appear to be distinct from those activated by PMA and mezerein.

Alteration of the extracellular matrix of cultured human keratinocytes by transformation and during differentiation

We have investigated the production of 3 extracellular matrix proteins (fibronectin, laminin and entactin) and glycosaminoglycans (GAGs) by keratinocytes derived from human squamous-cell carcinomas (SCCs). All the SCC lines retained the ability to synthesize fibronectin, laminin and entactin, and to incorporate them into an extracellular matrix. In some of the SCC lines fibronectin production was higher than in normal keratinocytes, and in most lines laminin production was equal to or higher than that seen in normal keratinocyte strains. All the tumorigenic SCC lines produced less entactin than normal keratinocytes. Five out of 7 SCC lines showed a shift in GAG production compared with normal keratinocytes, so that in these lines heparan sulphate was the major GAG as opposed to hyaluronic acid in the normal keratinocytes. As these changes could have been a consequence of the reduced differentiation found in the SCC lines, we fractionated normal keratinocytes into 4 size classes corresponding to different stages of differentiation, using Percoll gradient centrifugation. It was found that the ability to produce these extracellular matrix glycoproteins and GAGs was lost as keratinocytes terminally differentiated. However, this did not explain the matrix changes seen in the SCC lines, since the undifferentiated normal keratinocytes produced a normal pattern of extracellular matrix components. Nonetheless, the loss of extracellular matrix production may well explain the reduction in substratum adhesiveness which occurs as keratinocytes terminally differentiate.