Chemical Carcinogenesis Models for Evaluating Molecular-Targeted Prevention and Treatment of Oral Cancer: Table 1

Impairing squamous differentiation by Klf4 deletion is sufficient to initiate tongue carcinoma development upon K-Ras activation in mice

Oral squamous cell carcinoma (SCC) is among the most prevalent cancers in the world and is characterized by high morbidity and few therapeutic options. Like most cancers, oral SCC arises from a multistep process involving alterations of genes responsible for balancing proliferation and differentiation. Among these, Krϋppel-like factor 4 (Klf4) suppresses cell proliferation and promotes differentiation and thus helps to maintain epithelial homeostasis. However, the prevailing role of Klf4 in maintenance of normal homeostasis in oral epithelium has not been established in vivo. Here, we used an inducible oral-specific mice model to selectively ablate Klf4 in the oral cavity. We generated K14-CreER(Tam)/Klf4 (f/f) mice that survived to adulthood and did not present overt phenotype. However, histologically these mice showed dysplastic lesions, increased cell proliferation and abnormal differentiation in the tongue 4 months after induction, supporting a homeostatic role of Klf4 in the oral epithelia. Furthermore, by breeding these mutants with a transgenic line expressing at endogenous levels K-ras (G12D), we assessed the role of disrupting differentiation gene programs to the carcinogenesis process. The K14-CreER(TAM)/K-ras (G12D)/Klf4 (-) (/-) mice rapidly develop oral SCC in the tongue. Thus, our findings support the emerging notion that activation of differentiating gene programs may represent a barrier preventing carcinogenesis in epithelial cells harboring oncogenic mutations, and thus that molecules acting upstream and downstream of Klf4 may represent components of a novel tumor-suppressive pathway.

Mechanisms of oral carcinogenesis induced by dibenzo[a,l]pyrene: an environmental pollutant and a tobacco smoke constituent

We previously reported that dibenzo[a,l]pyrene (DB[a,l]P), the most potent known environmental carcinogen among polycyclic aromatic hydrocarbons (PAH) congeners, is carcinogenic in the oral tissues of mice. We have now developed a new mouse model which employs the oral application of the fjord region diol epoxide, (±)-anti-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo[a,l]pyrene (DB[a,l]PDE), a metabolite of the tobacco smoke constituent DB[a,l]P, and we show its specific induction of oral squamous cell carcinoma (OSCC) in both tongue and other oral tissues. Groups of B6C3F1 mice (20/group) received 6 or 3 nmol of (±)-anti-DB[a,l]PDE administered into the oral cavity; 3 times per week for 38 weeks. Additional groups received the vehicle alone or were left untreated. Mice were sacrificed 42 weeks after the first carcinogen administration. The high dose induced 74 and 100% OSCC in the tongue and other oral tissues, respectively; the corresponding values at the lower dose were 45 and 89%. Using immunohistochemistry, we showed that DB[a,l]PDE resulted in overexpression of p53 and COX-2 proteins in malignant tissues when compared to normal oral tissues and tongues. Consistent with the carcinogenicity, we demonstrated powerful mutagenicity in cII gene in B6C3F1 (Big Blue) mouse tongue. The mutational profile in lacI reporter gene is similar to those detected in human head and neck cancer, and p53 mutations were observed in mouse oral tumor tissues. Taken together, we conclude that the formation of diol epoxides plays a major role among the mechanisms by which DB[a,l]P exerts its oral mutagenicity and tumorigenicity.

Animal models to study the mutational landscape for oral cavity and oropharyngeal cancers

OBJECTIVES

Cancer is likely caused by alterations in gene structure or expression. Recently, next generation sequencing has documented mutations in 106 head and neck squamous cell cancer genomes, suggesting several new candidate genes. However, it remains difficult to determine which mutations directly contributed to cancer. Here, summarize the animal models which have already validated and may test cancer causing mutations identified by next generation sequencing approaches.

MATERIAL AND METHODS

We reviewed the existing literature on genetically engineered mouse models and next generation sequencing (NGS), as it relates to animal models of squamous cell cancers of the head and neck (HNSCC) in PubMed.

RESULTS

NSG has identified an average of 19 to 130 distinct mutations per HNSCC specimen. While many mutations likely had biological significance, it remains unclear which mutations were essential to, or "drive," carcinogenesis. In contrast, "passenger" mutations also exist that provide no selection advantage. The genes identified by NGS included p53, RAS, Human Papillomavirus oncogenes, as well as novel genes such as NOTCH1, DICER and SYNE1,2. Animal models of HNSCC have already validated some of these common gene mutations identified by NGS.

CONCLUSIONS

The advent of next generation sequencing will provide new leads to the genetic changes occurring in squamous cell cancers of the head and neck. Animal models will enable us to validate these new leads in order to better elucidate the biology of squamous cell cancers of the head and neck.

Urban legends series: oral leukoplakia

To date, the term oral leukoplakia (OL) should be used to recognize 'predominantly white plaques of questionable risk, having excluded (other) known diseases or disorders that carry no increased risk of cancer'. In this review, we addressed four controversial topics regarding oral leukoplakias (OLs): (i) Do tobacco and alcohol cause OLs?, (ii) What percentage of OLs transform into oral squamous cell carcinoma (OSCC)?, (iii) Can we distinguish between premalignant and innocent OLs?, and (iv) Is proliferative verrucous leukoplakia (PVL) a specific entity or just a form of multifocal leukoplakia? Results of extensive literature search suggest that (i) no definitive evidence for direct causal relationship between smoked tobacco and alcohol as causative factors of OLs, (ii and iii) the vast majority of OLs follow a benign course and do not progress into a cancer, and no widely accepted and/or validated clinical and/or biological factors can predict malignant transformation, and (iv) the distinction between multifocal/multiple leukoplakias and PVL in their early presentation is impossible; the temporal clinical progression and the high rate of recurrences and development of cancer of PVL are the most reliable features for diagnosis.

Basal stem cells contribute to squamous cell carcinomas in the oral cavity

The cells of origin of oral cavity squamous cell carcinoma (OCSCC) are unknown. We used a cell lineage tracing approach (adult K14-CreER(TAM); ROSA26 mice transiently treated with tamoxifen) to identify and track normal epithelial stem cells (SCs) in mouse tongues by X-gal staining and to determine if these cells become neoplastically transformed by treatment with a carcinogen, 4-nitroquinoline 1-oxide (4-NQO). Here, we show that in normal tongue epithelia, X-gal(+) cells formed thin columns throughout the entire epithelium 12 weeks after tamoxifen treatment, indicating that the basal layer contains long-lived SCs that produce progeny by asymmetric division to maintain homeostasis. Carcinogen treatment results in a ~10-fold reduction in the total number of X-gal(+) clonal cell populations and horizontal expansion of X-gal(+) clonal cell columns, a pattern consistent with symmetric division of some SCs. Finally, X-gal(+) SCs are present in papillomas and invasive OCSCCs, and these long-lived X-gal(+) SCs are the cells of origin of these tumors. Moreover, the resulting 4-NQO-induced tumors are multiclonal. These findings provide insights into the identity of the initiating cells of oral cancer.

Future directions and treatment strategies for head and neck squamous cell carcinomas

Head and neck cancer is a devastating disease that afflicts many individuals worldwide. Conventional therapies are successful in only a limited subgroup and often leave the patient with disfigurement and long lasting adverse effects on normal physiologic functions. The field is in dire need of new therapies. Oncolytic viral as well as targeted therapies have shown some success in other malignancies and are attractive for the treatment of head and neck cancer. Recently, it has been shown that a subset of head and neck cancers is human papillomavirus (HPV) positive and that this subset of cancers is biologically distinct and more sensitive to chemoradiation therapies although the underlying mechanism is unclear. However, chemoresistance remains a general problem. One candidate mediator of therapeutic response, which is of interest for the targeting of both HPV-positive and -negative tumors is the human DEK proto-oncogene. DEK is upregulated in numerous tumors including head and neck cancers regardless of their HPV status. Depletion of DEK in tumor cells in culture results in sensitivity to genotoxic agents, particularly in rapidly proliferating cells. This suggests that tumors with high DEK protein expression may be correlated with poor clinical response to clastogenic therapies. Targeting molecules such as DEK in combination with new and/or conventional therapies, holds promise for novel future therapeutics for head and neck cancer.

Role of phosphatidylinositol-3-kinase pathway in head and neck squamous cell carcinoma

Activation of the phosphatidylinositol-3-kinase (PI3K) pathway is one of the most frequently observed molecular alterations in many human malignancies, including head and neck squamous cell carcinoma (HNSCC). A growing body of evidence demonstrates the prime importance of the PI3K pathway at each stage of tumorigenesis, that is, tumor initiation, progression, recurrence, and metastasis. Expectedly, targeting the PI3K pathway yields some promising results in both preclinical studies and clinical trials for certain cancer patients. However, there are still many questions that need to be answered, given the complexity of this pathway and the existence of its multiple feedback loops and interactions with other signaling pathways. In this paper, we will summarize recent advances in the understanding of the PI3K pathway role in human malignancies, with an emphasis on HNSCC, and discuss the clinical applications and future direction of this field.

Metformin prevents the development of oral squamous cell carcinomas from carcinogen-induced premalignant lesions

Head and neck squamous cell carcinoma (HNSCC) is a major public health concern. The recent identification of the mTOR complex 1 (mTORC1) signaling pathway as a highly prevalent molecular signature underlying HNSCC pathogenesis has provided the foundation to search for novel therapeutic approaches to prevent and treat HNSCC. Here, we asked whether metformin, the most widely used medication for the treatment of type II diabetes, which acts in part by stimulating the AMP-activated protein kinase (AMPK) signaling pathway thereby reducing mTORC1 activity, may lower the risk of HNSCC development. Indeed, we show that metformin reduces the growth of HNSCC cells and diminishes their mTORC1 activity by both AMPK-dependent and -independent mechanisms. We also optimized an oral-specific carcinogenesis mouse model that results in the accumulation of multiple oral premalignant lesions at the end of the carcinogen exposure, some of which then spontaneously progress into HNSCC. Using this mouse model, we observed that metformin specifically inhibits mTORC1 in the basal proliferating epithelial layer of oral premalignant lesions. Remarkably, metformin prevented the development of HNSCC by reducing significantly the size and number of carcinogen-induced oral tumoral lesions and by preventing their spontaneous conversion to squamous cell carcinomas. Collectively, our data underscore the potential clinical benefits of using metformin as a targeted chemopreventive agent in the control of HNSCC development and progression.

ERK1/2 regulation of CD44 modulates oral cancer aggressiveness

Carcinogen-induced oral cavity squamous cell carcinoma (OSCC) incurs significant morbidity and mortality and constitutes a global health challenge. To gain further insight into this disease, we generated cell line models from 7,12-dimethylbenz(a)anthracene-induced murine primary OSCC capable of tumor formation upon transplantation into immunocompetent wild-type mice. Whereas several cell lines grew rapidly and were capable of metastasis, some grew slowly and did not metastasize. Aggressively growing cell lines displayed ERK1/2 activation, which stimulated expression of CD44, a marker associated with epithelial to mesenchymal transition and putative cancer stem cells. MEK (MAP/ERK kinase) inhibition upstream of ERK1/2 decreased CD44 expression and promoter activity and reduced cell migration and invasion. Conversely, MEK1 activation enhanced CD44 expression and promoter activity, whereas CD44 attenuation reduced in vitro migration and in vivo tumor formation. Extending these findings to freshly resected human OSCC, we confirmed a strict relationship between ERK1/2 phosphorylation and CD44 expression. In summary, our findings identify CD44 as a critical target of ERK1/2 in promoting tumor aggressiveness and offer a preclinical proof-of-concept to target this pathway as a strategy to treat head and neck cancer.

Oral carcinogenesis and oral cancer chemoprevention: a review

Oral cancer is one of the major global threats to public health. The development of oral cancer is a tobacco-related multistep and multifocal process involving field cancerization and carcinogenesis. The rationale for molecular-targeted prevention of oral cancer is promising. Biomarkers of genomic instability, including aneuploidy and allelic imbalance, are possible to measure the cancer risk of oral premalignancies. Understanding of the biology of oral carcinogenesis will yield important advances for detecting high-risk patients, monitoring preventive interventions, and assessing cancer risk and pharmacogenomics. In addition, novel chemopreventive agents based on molecular mechanisms and targets against oral cancers will be derived from studies using appropriate animal carcinogenesis models. New approaches, such as molecular-targeted agents and agent combinations in high-risk oral individuals, are undoubtedly needed to reduce the devastating worldwide consequences of oral malignancy.

Understanding carcinogenesis for fighting oral cancer

Oral cancer is one of the major global threats to public health. Oral cancer development is a tobacco-related multistep and multifocal process involving field cancerization and carcinogenesis. The rationale for molecular-targeted prevention of oral cancer is promising. Biomarkers of genomic instability, including aneuploidy and allelic imbalance, are able to measure the cancer risk of oral premalignancies. Understanding of the biology of oral carcinogenesis will give us important advances for detecting high-risk patients, monitoring preventive interventions, assessing cancer risk, and pharmacogenomics. In addition, novel chemopreventive agents based on molecular mechanisms and targets against oral cancers will be derived from research using appropriate animal carcinogenesis models. New approaches, such as interventions with molecular-targeted agents and agent combinations in high-risk oral individuals, are undoubtedly needed to reduce the devastating worldwide consequences of oral malignancy.

Fenofibrate exhibits a high potential to suppress the formation of squamous cell carcinoma in an oral-specific 4-nitroquinoline 1-oxide/arecoline mouse model

The excessive use of areca nut and/or tobacco may induce the production of free radicals and reactive oxygen species, which affect the lipid contents of the cell membrane and are possibly involved in tumorigenic processes in the oral cavity. The aim of this study was to investigate the therapeutic efficacy of fenofibrate (0.1% or 0.3%, w/w), a ligand of the peroxisome proliferator-activated receptor alpha (PPARα), in a 4-nitroquinoline 1-oxide (4-NQO)/arecoline-induced oral cancer mouse model. The carcinogen, 4-NQO/arecoline, was administrated to C57BL/6JNarl mice for 8weeks followed by fenofibrate treatment for 12 or 20weeks. After 28weeks, changes in serum lipids, the multiplicity of tumor lesions, and tumor sizes were determined together with changes in the immunohistochemical expressions of PPARα, acetyl-coenzyme A carboxylase (ACC), the epidermal growth factor receptor (EGFR), and cyclooxygenase-2 (COX2). The results showed that when compared to the 4-NQO/arecoline only group, 0.3% fenofibrate treatment increased serum total cholesterol, low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C) levels. 0.3% fenofibrate treatment suppressed the incidence rate of tongue lesions, reduced the multiplicity of squamous cell carcinoma (SCC), decreased the tumor size, and increased the immunoreactivity of EGFR and COX2 in oral dysplasia but decreased EGFR and COX2 expressions in SCC. These findings indicated that fenofibrate reduced the tumor incidence rate and suppressed the tumor progression into SCC and that these molecular events might be linked to the EGFR and COX2 regulatory pathways. We suggest that fenofibrate provides a new strategy for preventing oral tumor progression.

Co-treating with arecoline and 4-nitroquinoline 1-oxide to establish a mouse model mimicking oral tumorigenesis

The aim of this study was to establish an effective mouse model of oral cancer and to use this model to identify potential markers of oral tumor progression. C57BL/6JNarl mice were treated with arecoline, 4-nitroquinoline 1-oxide (4-NQO), or both arecoline and 4-NQO in high and low doses for 8 weeks to induce oral tumor. The induced oral lesions were observed for 20 weeks to assess the efficiency of cancer induction and survival rate of the mice. In addition, two target proteins that are frequently overexpressed during tongue cancer tumorigenesis, alphaB-crystallin and Hsp27, were examined by immunohistochemical analysis. In mice exposed to 4-NQO (200 microg/mL) and arecoline (500 microg/mL), the tongue lesions showed evidence of hyperplasia, papilloma, dysplasia, and carcinoma, and the lesions were pathologically similar to those lesions in human oral cancer. The tongue tumor incidence rate was 100% in mice exposed to concomitant 4-NQO (200 microg/mL) and arecoline (500 microg/mL) treatment, 57% in mice exposed to 4-NQO only, and 0% in mice exposed to arecoline only. Immunohistochemical analysis demonstrated that, consistent with human studies, alphaB-crystallin and Hsp27 were upregulated in murine oral tumors. In conclusion, we have established a powerful animal model that enables the study of the promoting effects of arecoline on tongue tumorigenesis. Data subsequently attained from this mouse model support a role for alphaB-crystallin and Hsp27 as clinical markers for tumor progression.

A DNA methyltransferase inhibitor and all-trans retinoic acid reduce oral cavity carcinogenesis induced by the carcinogen 4-nitroquinoline 1-oxide

The transcriptional silencing of some cell cycle inhibitors and tumor suppressors, such as p16 and retinoic acid receptor beta(2), by DNA hypermethylation at CpG islands is commonly found in human oral squamous carcinoma cells. We examined the effects of the DNA methyltransferase inhibitor 5-Aza-2'-deoxycytidine (5-Aza; 0.25 mg/kg body weight), all-trans retinoic acid (RA; given at 100 microg/kg body weight and 1 mg/kg body weight), and the combination of 5-Aza and the low-dose RA on murine oral cavity carcinogenesis induced by the carcinogen 4-nitroquinoline 1-oxide (4-NQO) in a mouse model. All the drug treatments were done for 15 weeks after a 10-week 4-NQO treatment. Mice in all drug treatment groups showed decreases in the average numbers of neoplastic tongue lesions. The combination of 5-Aza and RA effectively attenuated tongue lesion severity. Although all drug treatments limited the increase in the percentage of proliferating cell nuclear antigen-positive cells and the decrease in the percentage of p16-positive cells caused by the 4-NQO treatment in mouse tongue epithelial regions without visible lesions and in the neoplastic tongue lesions, the combination of 5-Aza and RA was the most effective. Collectively, our results show that the combination of a DNA demethylating drug and RA has potential as a strategy to reduce oral cavity cancer in this 4-NQO model.

Overexpression of lecithin:retinol acyltransferase in the epithelial basal layer makes mice more sensitive to oral cavity carcinogenesis induced by a carcinogen

Lecithin:retinol acyltransferase (LRAT) is an enzyme that converts retinol (vitamin A) to retinyl esters. Its expression is often reduced in human cancers, including oral cavity cancers. We investigated the effects of ectopic expression of human lecithin:retinol acyltransferase (LRAT) on murine oral cavity carcinogenesis induced by the carcinogen 4-nitroquinoline 1-oxide (4-NQO). We targeted human LRAT expression specifically to the basal layer of mouse skin and oral cavity epithelia by using a portion of the human cytokeratin 14 (K14) promoter. High levels of human LRAT transgene transcripts were detected in the tongues and skin of adult transgenic positive (TG+) mice, but not in transgenic negative (TG-) mice. The retinyl ester levels in skin of LRAT TG+ mice were 32% +/- 5.4% greater than those in TG- mice, and topical treatment of the back skin with retinol resulted in greater increases in retinyl esters (from 6.9- to 14.3-fold in different TG+ mice) in TG+ mouse skin than in TG- mouse skin (1.3 fold). While carcinogen (4-NQO) treatment induced multifocal precancerous and cancer lesions in the tongues of both TG positive (n=16) and negative mice (n=22), higher percentages of transgenic positive mice (62.5%) developed more severe tongue lesions (grades 3 and 4) than transgenic negative mice (24.8%) after 4-NQO treatment (p < 0.05). Carcinogen treatment also resulted in greater percentages of transgenic positive mouse tongues with hyperplasia (71.4%), dysplasia (85.7%, p < 0.05), and carcinoma (28.6%) than transgenic negative mouse tongues (53.3%, 46.7%, and 20%, respectively). Moreover, we observed higher cyclooxygenase-2 (Cox-2) and lower RARbeta(2) mRNA levels in TG+ mouse tongues as compared to TG- mouse tongues after 4-NQO treatment (p < 0.05). Taken together, these data show that overexpression of human LRAT specifically in oral basal epithelial cells makes these cells more sensitive to carcinogen induced tumorigenesis.