Nanoscale Dynamics of Streptococcal Adhesion to AGE-Modified Collagen.

The adhesion of initial colonizers such as Streptococcus mutans to collagen is critical for dentinal and root caries progression. One of the most described pathological and aging-associated changes in collagen-including dentinal collagen-is the generation of advanced glycation end-products (AGEs) such as methylglyoxal (MGO)-derived AGEs. Despite previous reports suggesting that AGEs alter bacterial adhesion to collagen, the biophysics driving oral streptococcal attachment to MGO-modified collagen remains largely understudied. Thus, the aim of this work was to unravel the dynamics of the initial adhesion of S. mutans to type I collagen in the presence and absence of MGO-derived AGEs by employing bacterial cell force spectroscopy with atomic force microscopy (AFM). Type I collagen gels were treated with 10 mM MGO to induce AGE formation, which was characterized with microscopy and enzyme-linked immunosorbent assay. Subsequently, AFM cantilevers were functionalized with living S. mutans UA 159 or Streptococcus sanguinis SK 36 cells and probed against collagen surfaces to obtain force curves displaying bacterial attachment in real time, from which the adhesion force, number of events, Poisson analysis, and contour and rupture lengths for each individual detachment event were computed. Furthermore, in silico computer simulation docking studies between the relevant S. mutans UA 159 collagen-binding protein SpaP and collagen were computed, in the presence and absence of MGO. Overall, results showed that MGO modification increased both the number and adhesion force of single-unbinding events between S. mutans and collagen, without altering the contour or rupture lengths. Both experimental and in silico simulations suggest that this effect is due to increased specific and nonspecific forces and interactions between S. mutans UA 159 and MGO-modified collagen substrates. In summary, these results suggest that collagen alterations due to aging and glycation may play a role in early bacterial adherence to oral tissues, associated with conditions such as aging or chronic hyperglycemia, among others.

Cigarette smoke mediates epigenetic repression of miR-217 during esophageal adenocarcinogenesis.

Although microRNAs (miRs) have been implicated in the pathogenesis of various human malignancies, limited information is available regarding mechanisms by which these noncoding RNAs contribute to initiation and progression of tobacco-induced esophageal cancers. In this study, array and quantitative reverse transcriptase-PCR techniques were used to examine miR expression in immortalized esophageal epithelia (IEE) and esophageal adenocarcinoma (EAC) cells cultured in normal media with or without cigarette smoke condensate (CSC). Under relevant exposure conditions, CSC significantly decreased miR-217 expression in these cells. Endogenous levels of miR-217 expression in cultured EAC cells (EACC)/primary EACs were significantly lower than those observed in IEE/ paired normal esophageal tissues. RNA crosslink immunoprecipitation, quantitative reverse transcriptase-PCR (qRT-PCR) and immunoblot experiments demonstrated direct interaction of miR-217 with kallikrein 7 (KLK7), encoding a putative oncogene not previously implicated in EAC. Repression of miR-217 correlated with increased levels of KLK7 in primary EACs, particularly those from smokers. Chromatin and methylated DNA immunoprecipitation experiments demonstrated that CSC-mediated repression of miR-217 coincided with DNMT3b-dependent hypermethylation and decreased occupancy of nuclear factor 1 within the miR-217 genomic locus. Deoxyazacytidine induced miR-217 expression and downregulated KLK7 in EACC; deoxyazacytidine also attenuated CSC-mediated miR-217 repression and upregulation of KLK7 in IEE and EACC. Overexpression of miR-217 significantly decreased, whereas overexpression of KLK7 increased proliferation, invasion and tumorigenicity of EACC. Collectively, these data demonstrate that epigenetic repression of miR-217 contributes to the pathogenesis of EAC via upregulation of KLK7 and suggest that restoration of miR-217 expression may be a novel treatment strategy for these malignancies.

Downregulation of splicing factor SRSF3 induces p53ß, an alternatively spliced isoform of p53 that promotes cellular senescence.

Most human pre-mRNA transcripts are alternatively spliced, but the significance and fine-tuning of alternative splicing in different biological processes is only starting to be understood. SRSF3 (SRp20) is a member of a highly conserved family of splicing factors that have critical roles in key biological processes, including tumor progression. Here, we show that SRSF3 regulates cellular senescence, a p53-mediated process to suppress tumorigenesis, through TP53 alternative splicing. Downregulation of SRSF3 was observed in normal human fibroblasts undergoing replicative senescence, and was associated with the upregulation of p53ß, an alternatively spliced isoform of p53 that promotes p53-mediated senescence. Knockdown of SRSF3 by short interfering RNA (siRNA) in early-passage fibroblasts induced senescence, which was associated with elevated expression of p53ß at mRNA and protein levels. Knockdown of p53 partially rescued SRSF3-knockdown-induced senescence, suggesting that SRSF3 acts on p53-mediated cellular senescence. RNA pulldown assays demonstrated that SRSF3 binds to an alternatively spliced exon uniquely included in p53ß mRNA through the consensus SRSF3-binding sequences. RNA crosslinking and immunoprecipitation assays (CLIP) also showed that SRSF3 in vivo binds to endogenous p53 pre-mRNA at the region containing the p53ß-unique exon. Splicing assays using a transfected TP53 minigene in combination with siRNA knockdown of SRSF3 showed that SRSF3 functions to inhibit the inclusion of the p53ß-unique exon in splicing of p53 pre-mRNA. These data suggest that downregulation of SRSF3 represents an endogenous mechanism for cellular senescence that directly regulates the TP53 alternative splicing to generate p53ß. This study uncovers the role for general splicing machinery in tumorigenesis, and suggests that SRSF3 is a direct regulator of p53.

APAF-1 is a transcriptional target of p53 in DNA damage-induced apoptosis.

The expression of genes involved in p53-mediated apoptosis was studied using cDNA microarray after treating isogenic cell lines with either ionizing radiation or doxorubicin. Most of the known p53 transcriptional activation target genes clustered in a functional category defined by early and p53-dependent induction, regardless of the type of stress. Apoptotic protease activating factor-1 (APAF-1) emerged from this analysis as a novel p53 target gene. Genomic sequences upstream of the APAF-1 transcription start site contain a classic p53-responsive element that bound to p53. Consistently, p53 directly induced APAF-1 gene expression. Furthermore, DNA damage-mediated induction of APAF-1 mRNA and protein expression, accompanied by apoptosis, were strictly dependent on wild-type p53 function. These data are consistent with the hypothesis that APAF-1 is an essential downstream effector of p53-mediated apoptosis.

Functional interaction of p53 and BLM DNA helicase in apoptosis.

The Bloom syndrome (BS) protein, BLM, is a member of the RecQ DNA helicase family that also includes the Werner syndrome protein, WRN. Inherited mutations in these proteins are associated with cancer predisposition of these patients. We recently discovered that cells from Werner syndrome patients displayed a deficiency in p53-mediated apoptosis and WRN binds to p53. Here, we report that analogous to WRN, BLM also binds to p53 in vivo and in vitro, and the C-terminal domain of p53 is responsible for the interaction. p53-mediated apoptosis is defective in BS fibroblasts and can be rescued by expression of the normal BLM gene. Moreover, lymphoblastoid cell lines (LCLs) derived from BS donors are resistant to both gamma-radiation and doxorubicin-induced cell killing, and sensitivity can be restored by the stable expression of normal BLM. In contrast, BS cells have a normal Fas-mediated apoptosis, and in response to DNA damage normal accumulation of p53, normal induction of p53 responsive genes, and normal G(1)-S and G(2)-M cell cycle arrest. BLM localizes to nuclear foci referred to as PML nuclear bodies (NBs). Cells from Li-Fraumeni syndrome patients carrying p53 germline mutations and LCLs lacking a functional p53 have a decreased accumulation of BLM in NBs, whereas isogenic lines with functional p53 exhibit normal accumulation. Certain BLM mutants (C1055S or Delta133-237) that have a reduced ability to localize to the NBs when expressed in normal cells can impair the localization of wild type BLM to NBs and block p53-mediated apoptosis, suggesting a dominant-negative effect. Taken together, our results indicate both a novel mechanism of p53 function by which p53 mediates nuclear trafficking of BLM to NBs and the cooperation of p53 and BLM to induce apoptosis.

p53-mediated apoptosis and genomic instability diseases.

Mutations in several DExH-containing DNA helicases, including XPD, XPB, WRN, and BLM, are associated with rare familial cancer syndromes characterized by genomic instability and cancer susceptibility. Known cellular activities of these helicases include DNA replication, repair, recombination, and/or transcription. The p53 tumor suppressor is a regulator of cellular responses to stress, and is biochemically involved in the induction of cell-cycle arrest, apoptosis and DNA repair, all of which contribute to maintenance of genomic integrity. Physical and functional interactions of p53 with DExH-containing DNA helicases have been described. We propose that such interactions could be compromised in inherited disorders and contribute to their cancer susceptibility. In particular, the role of DNA helicases in p53-mediated apoptotic pathways is reviewed.

Drug-induced apoptosis is delayed and reduced in XPD lymphoblastoid cell lines: possible role of TFIIH in p53-mediated apoptotic cell death.

The tumor suppressor gene product p53 can bind to and inhibit the helicase activity of the multisubunit transcription-repair factor TFIIH. We previously reported that p53-mediated apoptosis is attenuated in primary human fibroblasts from individuals with Xeroderma Pigmentosum (XP) that harbor mutations in the TFIIH DNA helicases XPD or XPB. In this study we show that apoptosis is reduced and delayed in three XPD lymphoblastoid cell lines (LCLs), but not in an XPD heterozygote LCL, after exposure to doxorubicin, a DNA-damaging agent and topoisomerase II inhibitor frequently used in cancer therapy. Apoptosis was assessed by quantitation of Annexin V binding to exposed phosphatidylserine residues and by caspase-mediated cleavage of Poly(ADP)Ribose Polymerase (PARP). Apoptosis induced by doxorubicin was suppressed in LCLs retrovirally transduced with the Human Papillomavirus 16 E6 oncoprotein, consistent with the hypothesis that this is a p53-dependent process. PARP cleavage was not delayed in XPD LCLs in response to anti-Fas (CD95) antibody-mediated apoptosis, thus, the defect in the apoptotic pathway in these cells lies upstream of caspase activation. Similar changes in the expression of apoptosis-effector genes, p53, and p53-responsive genes p21Cip1/WAF-1/Sid1 (p21), gadd45, bcl-2 and bax were observed in normal and XPD LCLs after treatment with doxorubicin, indicating that delayed apoptosis was not a consequence of defective transcription of these genes. Thus, our studies provide further support to the hypothesis that XPD and p53 can functionally interact in a p53-mediated apoptotic pathway.

p53-mediated apoptosis is attenuated in Werner syndrome cells.

The WRN DNA helicase is a member of the DExH-containing DNA helicase superfamily that includes XPB, XPD, and BLM. Mutations in WRN are found in patients with the premature aging and cancer susceptibility syndrome known as Werner syndrome (WS). p53 binds to the WRN protein in vivo and in vitro through its carboxyl terminus. WS fibroblasts have an attenuated p53- mediated apoptotic response, and this deficiency can be rescued by expression of wild-type WRN. These data support the hypothesis that p53 can induce apoptosis through the modulation of specific DExH-containing DNA helicases and may have implications for the cancer predisposition observed in WS patients.

ras activity and cyclin D1 expression: an essential mechanism of mouse skin tumor development.

ras is a family of small GTP-binding proteins that transduce signals from tyrosine-kinase receptors to the nucleus and thus play a role in the regulation of cell proliferation and differentiation. Several lines of evidence have shown that the cell-cycle machinery, specifically the circuit cyclin D1/cyclin-dependent kinase (cdk) 4 and 6-p16-pRb, lies downstream of ras. Point mutations that activate the ras protein and its downstream cascade have been observed in human and experimental tumors. In particular, ras mutations have been well characterized in the mouse skin two-stage carcinogenesis model, and a large body of literature has indicated that initiation with the genotoxic carcinogen 7,12-dimethylbenz[a]anthracene induces a specific point mutation in Ha-ras gene in this model. In the last few years, several studies have shown a correlation between ras activation and alterations in the expression of cyclin D1 as well as other cell cycle-regulated proteins, but the actual role of these alterations in tumor development had not been determined until a recent study provided genetic and biochemical evidence that cyclin D1 is a critical target of oncogenic ras in mouse skin carcinogenesis. Here we review these results, including the evidence that cyclin D1 has a role as a downstream mediator of ras activity during tumor development. We propose a model in which cyclin D1 has a unique growth-promoting role in tumor development but does not act as an oncogene independently of ras activity.

Reduced skin tumor development in cyclin D1-deficient mice highlights the oncogenic ras pathway in vivo.

Cyclin D1 is part of a cell cycle control node consistently deregulated in most human cancers. However, studies with cyclin D1-null mice indicate that it is dispensable for normal mouse development as well as cell growth in culture. Here, we provide evidence that ras-mediated tumorigenesis depends on signaling pathways that act preferentially through cyclin D1. Cyclin D1 expression and the activity of its associated kinase are up-regulated in keratinocytes in response to oncogenic ras. Furthermore, cyclin D1 deficiency results in up to an 80% decrease in the development of squamous tumors generated through either grafting of retroviral ras-transduced keratinocytes, phorbol ester treatment of ras transgenic mice, or two-stage carcinogenesis.

Synchronized proliferation induced by 12-O-tetradecanoylphorbol-13-acetate treatment of mouse skin: an in vivo model for cell cycle regulation.

Much of what is known about the mammalian cell cycle comes from studies using established cell lines in culture. In this study, cell cycle-regulatory events were analyzed in vivo after treatment of mouse epidermis with 12-O-tetradecanoylphorbol-13-acetate. A synchronized wave of basal keratinocyte proliferation occurred; over 80% of the cells were in S phase 15 h after treatment. c-myc protein expression was induced, and p57Kip2 protein levels dropped early after stimulation. Before S phase, cyclin D1 and cyclin-dependent kinase (CDK) 6 levels increased, and expression of cyclins E and A was induced. Rb was phosphorylated in late G1, and this correlates with the formation of cyclin D1/CDK4 and cyclin D1/CDK6 complexes. At the end of S phase, the p57Kip2 and p21Cip1 protein levels increased. These findings demonstrate that stimulation of basal epidermal cells by 12-O-tetradecanoylphorbol-13-acetate results in several classic cell cycle events and suggests that p57Kip2 plays a key role in regulating proliferation in the epidermis.

Up-regulation of vascular endothelial growth factor/vascular permeability factor in mouse skin carcinogenesis correlates with malignant progression state and activated H-ras expression levels.

Angiogenesis is a crucial process for tumor growth and metastasis regulated by the balance of positive and negative factors. Vascular endothelial growth factor (VEGF/VPF) is a specific mitogen for endothelial cells that has been shown to be overexpressed in a variety of tumors and other inflammatory diseases. To analyze the implication of VEGF/VPF during tumorigenesis, we have studied its expression at different stages of tumor development using the mouse skin carcinogenesis model. VEGF/VPF mRNA was induced in skin in vivo after 12-O-tetradecanoylphorbol-13-acetate treatment. Constitutive up-regulation of VEGF/VPF at the mRNA and protein levels was also observed in premalignant papillomas and, at a higher level, in squamous carcinomas, suggesting a correlation between VEGF/VPF expression and tumor progression. A direct positive correlation between VEGF/VPF mRNA expression and the level of activated H-ras gene was found in a series of cell lines representing different stages of epidermal tumor development. Consequently, a clone of one of these cell lines, HaCa4, which has lost most of its v-ras expression, down-regulated VEGF mRNA expression concomitantly with its metastatic potential. Direct evidence of H-ras involvement in VEGF induction was obtained when an immortalized mouse keratinocyte cell line transduced with a retrovirus carrying v-H-ras showed highly increased VEGF/VPF mRNA levels. These data show that in mouse skin carcinogenesis, the VEGF/VPF angiogenic stimulus occurs early during premalignant papilloma development and further increases at later stages. Moreover, we demonstrate that increasing the activated H-ras dose, a phenomenon that takes place sequentially throughout mouse skin tumor development, may play an additional role by facilitating malignant in vivo progression through the modulation of VEGF/VPF-mediated angiogenesis.

Expression of G1 cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors in androgen-induced prostate proliferation in castrated rats.

Androgen induces prostate cell proliferation in the castrated rat. We hypothesized that G1 cyclins, cyclin-dependent kinases (cdk), and cdk inhibitors mediate this cellular response to mitogenic signals. In this study, induction of cyclins D1, D2, D3, E, and cdks 2, 4, and 6 expression was observed at various time points during testosterone replacement in the ventral prostate of castrated rats. The induction followed prostate epithelium proliferation, which peaked at 48 h and decreased at 120 h during the treatment. The study of cyclin/cdk complex formation revealed that more cyclin D1/cdk4 and cyclin D1/cdk6 complexes were formed at 48 h than at 120 h of treatment, but cyclin D1/cdk2 complexes remained the same. Furthermore, both hyperphosphorylated and hypophosphorylated forms of Rb were detected at 48 h, but only the hypophosphorylated form was detected at 120 h of treatment. p21Cip1, which was very abundant in the ventral prostate of castrated and intact rats, was not detected when the prostate started proliferation and increased gradually as proliferation decreased during the androgen treatment. Meanwhile, p27Kip1 dramatically increased after androgen treatment, and the induction levels were less at the peak of prostate proliferation and higher when proliferation was low. The results presented here suggest that expression of G1 cyclins and their related kinases and kinase inhibitors are well regulated after androgen replacement in the ventral prostate of castrated rats. The cooperation between these cell cycle regulators leads to a well-controlled prostate regeneration.

Expression of cyclin D1 in epithelial tissues of transgenic mice results in epidermal hyperproliferation and severe thymic hyperplasia.

To study the involvement of cyclin D1 in epithelial growth and differentiation and its putative role as an oncogene in skin, transgenic mice were developed carrying the human cyclin D1 gene driven by a bovine keratin 5 promoter. As expected, all squamous epithelia including skin, oral mucosa, trachea, vaginal epithelium, and the epithelial compartment of the thymus expressed aberrant levels of cyclin D1. The rate of epidermal proliferation increased dramatically in transgenic mice, which also showed basal cell hyperplasia. However, epidermal differentiation was unaffected, as shown by normal growth arrest of newborn primary keratinocytes in response to high extracellular calcium. Moreover, an unexpected phenotype was observed in the thymus. Transgenic mice developed a severe thymic hyperplasia that caused premature death due to cardio-respiratory failure within 4 months of age. By 14 weeks, the thymi of transgenic mice increased in weight up to 40-fold, representing 10% of total body weight. The hyperplastic thymi had normal histology revealing a well-differentiated cortex and medulla, which supported an apparently normal T-cell developmental program based on the distribution of thymocyte subsets. These results suggest that proliferation and differentiation of epithelial cells are under independent genetic controls in these organs and that cyclin D1 can modulate epithelial proliferation without altering the initiation of differentiation programs. No spontaneous development of epithelial tumors or thymic lymphomas was perceived in transgenic mice during their first 8 months of life, although they continue under observation. This model provides in vivo evidence of the action of cyclin D1 as a pure mediator of proliferation in epithelial cells.

Positive immunohistochemical staining of p53 and cyclin D in advanced mouse skin tumors, but not in precancerous lesions produced by benzo[a]pyrene.

Squamous cell carcinomas (SCCs) of the mouse skin, as well as several types of preinvasive carcinoma precursor lesions, were produced by complete carcinogenesis protocols with benzo[a]pyrene (B[a]P). Groups of mice were studied histologically at several time points. Tumors and precursor lesions were systematically counted on microscope slides. The main feature of tumor development using this ubiquitous human carcinogen was the sequential appearance of in situ flat lesions with progressive degrees of dysplasia. These changes, preceding the development of SCCs, were observed 20 weeks after beginning the carcinogen treatments. At this time point, in situ lesions outnumbered SCC approximately 10:1 at the higher total carcinogen dose examined. Ten weeks later, this ratio was approximately 1:1. With the lower total carcinogen dose protocol, progression was delayed since at 27 weeks preinvasive lesions outnumbered SCCs approximately 8:1. In addition to the in situ lesions, papillomas and keratoacanthomas were noted with the high B[a]P dose protocol, but tended to disappear at the end of the experiment, also indicating their probable role as SCC precursors. A study of histochemical markers showed that gamma-glutamyltranspeptidase (GGT) and keratin 13, although good markers of malignant changes in early papillomas produced by two-stage carcinogenesis protocols, were mainly negative in dysplastic lesions produced by complete carcinogenesis with B[a]P. Immunohistochemical detection of p53 showed that 50% of SCCs were positively stained, whereas only 3% of in situ lesions were p53 immunoreactive. Similarly, 62% of SCCs were immunohistochemically positive for cyclin D, but no precursor lesions were positive. Molecular analysis of the tumors showed the absence of H-ras mutations. No amplification of the cyclin-D-1 gene was detected in eight SCCs examined. Collectively, these findings indicate that preinvasive in situ lesions are frequent during early stages of carcinogenesis when B[a]P is used in a complete carcinogenesis protocol. Although the absence of p53 immunoreactivity in this mouse model differs from the observed changes in human premalignant squamous lesions, the sequence of morphological changes and the final incidence of p53 and cyclin D staining abnormalities are very similar to the well-known alterations that take place during human squamous carcinogenesis.

Early overexpression of cyclin D1 protein in mouse skin carcinogenesis.

Abnormal expression of cell-cycle regulatory proteins, particularly cyclin D1, has been described in human cancers. However, there are few reports of this kind in experimental carcinogenesis models, which provide a framework to analyze the importance of those alterations in early cancer development. Previous studies from our laboratory showed that cyclin D1 mRNA was overexpressed in skin tumors generated in SENCAR mice by a two-stage carcinogenesis protocol. In the study presented here, immunoprecipitation of fresh tumor samples confirmed the overexpression of cyclin D1 protein. We also developed an immunohistochemical technique to determine which cells in the lesions overexpressed the cyclin and the timing of deregulation during cancer development. Surprisingly, we found that all premalignant lesions, including small incipient papillomas, overexpressed cyclin D1, whereas normal and hyperproliferative skin were negative. Nuclear immunostaining was detected only in the proliferative compartments of the tumors and showed an apparent cell-cycle-related variation. These results provide evidence for a role of cyclin D1 overexpression in mouse skin carcinogenesis and support the use of this model as an alternative to in vitro studies to help understand the involvement of cyclin deregulation in cancer development.

Induction of cyclin D1 overexpression by activated ras.

Activated ras genes are known to alter control of cell proliferation. This is consistent with the fact that ras proteins are a key component of the biochemical pathway triggered by ligand-bound cell surface receptors that are tyrosine kinases. Although an important part of the ras signaling pathway has been recently uncovered, the molecular target(s) that mediates the effects of ras on cell cycle control remains unknown. Cyclins and cyclin-dependent kinases are key molecules in the control of cell cycle. Cyclin D1, in particular, is a critical target for proliferative signals in G1 and it has been shown that ectopic overexpression of this cyclin can significantly alter cell cycle regulation. Here we report that activated ras induces significant overexpression of cyclin D1 in epithelial cells derived from normal rat intestine and mouse mammary gland. A definitive causal role for activated ras in this overexpression is demonstrated by using intestinal cells transfected with an inducible ras expression vector. Treatment of the ras-transformed intestinal clones with anti-sense cyclin D1 oligonucleotides reduces their rate of cell proliferation indicating that the increment in cyclin D1 expression induced by activated ras is instrumental in the higher rate of cell proliferation conferred by the ras oncogene to the IEC cells. Based on these results we propose that, at least in certain cell types, cyclin D1 can be one of the mediators of the transforming action of activated ras.

Overexpression of cyclin D1 in mouse skin carcinogenesis.

Recent studies have provided evidence suggesting that disruption of cyclin function may play a critical role in tumorigenesis. Cyclin D1, a putative G1 cyclin previously isolated in human parathyroid adenomas (designated PRAD1) and mouse macrophages (designated Cyl1), has been implicated in various neoplasias including breast and squamous cell carcinomas (SCC). The role of cyclin altered regulation in the different stages of tumor progression has not been studied in a well defined animal model system. In the study presented here, Cyl1 was mapped to the distal end of mouse chromosome 7 and found to be dramatically overexpressed in skin SCC. In premalignant stages of tumor development, early papillomas showed basal Cyl1 transcript levels, whereas over-expression was observed in most advanced papillomas. These findings suggest that altered expression of cyclin D1 plays a critical role in mouse skin carcinogenesis and may be related to the acquisition of autonomous growth by papillomas. Further studies on the role of cyclin D1 in the mouse model system should prove valuable for understanding the multistep basis of tumor progression.

Low frequency of codon 61 Ha-ras mutations and lack of keratin 13 expression in 7,12-dimethylbenz[a]-anthracene-induced hamster skin tumors.

Alterations in the pattern of keratin expression are a common feature of skin-tumor development. In this study, we investigated whether the loss of epidermal keratin 1 (K1) and its replacement by mucosal keratin 13 (K13) is unique to mouse skin tumors induced by 7,12-dimethylbenz[a]anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA), since it has been reported that human epidermal tumors do not exhibit aberrant expression of K13. With that purpose, we analyzed the keratin profiles of 16 DMBA-induced hamster skin tumors using monospecific antibodies against K1 and K13. Although all the tumors expressed K1, they also showed an overall tendency towards loss of this keratin; furthermore, none of the tumors expressed K13. Previous studies have suggested that the induction of K13 in mouse skin is related to the mutation of the Ha-ras gene by the initiating agent DMBA, a mutation consistently found in murine DMBA/TPA-induced tumors and rarely found in human skin tumors. Therefore, we also evaluated the tumors for the presence of codon-61 mutations by direct sequencing of DNA extracted from paraffin-embedded tissue sections. Only three tumors showed an A-->T transversion in the second nucleotide of Ha-ras codon 61. However, presence of the mutation did not correlate with K1 staining. Although hamster skin tumors were induced by the same initiator as were mouse skin tumors, hamster skin tumors did not show the same keratin profile. Moreover, their immunohistochemical expression of K1 and K13 and their codon 61 sequences resembled that of their human counterparts. These results suggest that the aberrant expression of K13 may be unique to murine skin. Furthermore, although codon 61 Ha-ras mutation appears to be related to keratin alterations in the mouse model, this mutation is not sufficient to produce the same biochemical changes in other species.