Effects of 5-azacytidine in Syrian golden hamsters: toxicity, tumorigenicity, and differential modulation of bronchial carcinogenesis

Targeting the Epigenome for the Treatment and Prevention of Lung Cancer

Alterations in chromatin structure resulting from aberrant DNA methylation and perturbations of the histone code profoundly influence gene expression during pulmonary carcinogenesis. Recent studies indicate that DNA demethylating agents and histone deacetylase (HDAC) inhibitors synergistically induce gene expression and apoptosis in cultured lung cancer cells, and prevent lung cancer development in animals following exposure to tobacco carcinogens. Preliminary clinical trials have established proof of principle regarding the use of DNA demethylating agents and HDAC inhibitors for enhancing immunogenicity and apoptosis of lung cancer cells, and have revealed the complexities concerning the mechanisms by which chromatin remodeling agents mediate antitumor effects in vivo. These data support additional investigations pertaining to the epigenetics of lung cancer, and the evaluation of chromatin remodeling agents for the treatment and prevention of this disease.

Genomic Surgery for Lung Cancer

An emerging body of literature indicates that reversible alterations in chromatin structure modulate gene expression during malignant transformation. Chromatin structure is regulated in part by DNA methylation and histone acetylation; these independent yet highly interrelated epigenetic processes are influenced by a variety of signal transduction pathways. The present review highlights recent advances regarding cancer epigenetics, focusing on the potential utilization of chromatin remodeling agents to induce apoptosis and enhance the immunogenicity of thoracic malignancies.

Role of the cytosine DNA-methyltransferase and p16INK4a genes in the development of mouse lung tumors

CpG island methylation is an epigenetic modification of DNA associated with the silencing of gene transcription. The p16INK4a (p16) tumor suppressor gene is inactivated in human non-small cell lung cancers (NSCLCs) by either homozygous deletion or aberrant methylation. Inactivation of tumor suppressor genes by methylation has been linked in part to altered activity of the cytosine DNA-methyltransferase (DNA-MTase), the enzyme that catalyzes DNA methylation at CpG sites. The purpose of these studies was to define the role of DNA-MTase and p16 in the development of murine lung cancer. DNA-MTase activity was determined in alveolar type II and Clara cells from A/J and C3H mice that exhibit high and low susceptibility, respectively, for lung tumor formation. Increased DNA-MTase activity leading to an increase in overall DNA methylation was found only in alveolar type II cells, the target for murine adenocarcinomas. Both DNA-MTase and DNA methylation changes were detected 7 days after carcinogen exposure and, thus, were early events in neoplastic evolution. In addition, enzyme activity increased incrementally during lung cancer progression. Expression of p16 was detected in all primary lung tumors from A/J mice; however, levels of expression differed by up to 15-fold between tumors. The apparent low levels of expression seen in approximately half of the tumors was not attributed to methylation of the p16 gene. In contrast to the detection of p16 expression in primary tumors, this gene was deleted in four tumor-derived cell lines induced in the A/J mouse by NNK. The results from these studies indicate that the modulation of DNA-MTase activity was cell specific, segregated with susceptibility, and occurred early in neoplastic evolution. Thus, the marked increase in enzyme activity detected in alveolar type II cells after carcinogen treatment could be a major factor contributing to the high susceptibility for chemical-induced neoplasia associated with the A/J mouse strain. The inactivation of the p16 gene in murine cancers induced by NNK most likely arises as a late event via homozygous deletion.

Increased cytosine DNA-methyltransferase activity is target-cell-specific and an early event in lung cancer

The association between increased DNA-methyltransferase (DNA-MTase) activity and tumor development suggest a fundamental role for this enzyme in the initiation and progression of cancer. A true functional role for DNA-MTase in the neoplastic process would be further substantiated if the target cells affected by the initiating carcinogen exhibit changes in enzyme activity. This hypothesis was addressed by examining DNA-MTase activity in alveolar type II (target) and Clara (nontarget) cells from A/J and C3H mice that exhibit high and low susceptibility, respectively, for lung tumor formation. Increased DNA-MTase activity was found only in the target alveolar type II cells of the susceptible A/J mouse and caused a marked increase in overall DNA methylation in these cells. Both DNA-MTase and DNA methylation changes were detected 7 days after carcinogen exposure and, thus, were early events in neoplastic evolution. Increased gene expression was also detected by RNA in situ hybridization in hypertrophic alveolar type II cells of carcinogen-treated A/J mice, indicating that elevated levels of expression may be a biomarker for premalignancy. Enzyme activity increased incrementally during lung cancer progression and coincided with increased expression of the DNA-MTase activity are strongly associated with neoplastic development and constitute a key step in carcinogenesis. The detection of premalignant lung disease through increased DNA-MTase expression and the possibility of blocking the deleterious effects of this change with specific inhibitors will offer new intervention strategies for lung cancer.

A step section method for full histopathological assessment of carcinogen-affected target tissue during respiratory carcinogenesis

Studies of carcinogenesis that are not limited to overt neoplasms but also involve evaluations of preneoplastic stages require histopathological assessment of the entire carcinogen-affected tissue so that the true nature and sequence of the progressive process can be determined. The customary serial sectioning approach achieves this goal, but at an inordinate logistic cost. In studies of hamster bronchial carcinogenesis, a step section method was compared to a quasi-random approach and to the customary serial section method. The step section method achieved the same diagnostic completeness as serial sectioning, but at a two orders of magnitude reduction in costs.

Clara cell antigen in normal and migratory dysplastic Clara cells, and bronchioloalveolar carcinoma of Syrian hamsters induced by N-nitrosomethyl-n-heptylamine

Histogenetic features of lung tumours were studied in Syrian hamsters that had been induced with 6.8 mg N-nitrosomethyl-n-heptylamine/animal by gavage once a week for 35 weeks. At intervals from experimental week 2 until week 46, pulmonary tissues from hamsters were examined by light and electron microscopy. This report describes early hyperplastic lesions associated with terminal bronchioles and the progression of these lesions to bronchioloalveolar tumours. Using immunohistochemical and ultrastructural colloidal gold labelling techniques, hamster Clara cell antigen was found to be localized in Clara cell granules and smooth endoplasmic reticulum of normal cells, in dysplastic Clara cells migrating through basement membrane defects or from the open end of terminal bronchioles, and in hyperplastic peribronchiolar cell foci. The latter progressed to bronchioloalveolar tumours growing out along alveolar basement membranes in a characteristic lace-like, lepidic pattern. Tumours were composed of secretory (Clara), ciliated, mucous, and undifferentiated cells, as well as trapped, non-neoplastic alveolar type II cells. Hyperplastic neuroendocrine cell foci lining airways were immunoreactive for chromogranin, but these cells did not participate in the pre-neoplastic or neoplastic process. It is suggested that bronchioloalveolar carcinomas in hamsters are derived from bronchiolar secretory (Clara) cells growing along alveolar walls, differentiating into other bronchiolar cell types and entrapping resident alveolar type II cells. Due to the migratory capacity of Clara cells, it is also possible for tumours composed of bronchiolar cells to develop at the lung periphery.

Hamster bronchial carcinogenesis induced by carcinogen-containing sustained release implants placed endobronchially: a clinically relevant model

For several widely appreciated reasons, the Syrian hamster has been the most frequently used experimental animal in investigations of conducting airway carcinogenesis. To develop a model where bronchogenic cancer arises focally at a predetermined site, we used the Laskin-Kuschner self-retaining intrabronchial pellet principle, employing a carcinogen-containing silastic polymer sustained release implant (SRI). The SRI is placed in the right lower lobe bronchus via tracheostomy; when modified, the SRI can be removed without loss of the animal. Special preparation of the SRI implant site after fixation but prior to paraffin embedding allows for full histopathological examination of the carcinogen-affected target tissue. Logistic regression analysis of histological findings provides valid quantitative inter-regimen comparisons of histomorphic classifications suitable for determining modulation of carcinogenesis by external influences. Using this model, we demonstrated that the sequential progression of carcinogenesis (SPC) in hamster bronchus is similar to that which occurs in humans and in dogs, including both the ploidy increases that are progressive during the SPC, and the histological patterns of the induced cancers. We have shown genetic variation in susceptibility to carcinogenesis among inbred hamster strains, and have assessed effects of Bacillus Calmette-Guerin (BCG) immunostimulation on the SPC. Time/dose response studies were performed, as were comparisons between four polycyclic aromatic hydrocarbon carcinogens. Systemic administration of 5-azacytidine (AZC) soon after SRI placement was found to inhibit the SPC, to alter the ploidy changes during the SPC and in the eventual cancers, and to affect the degree of differentiation of the cancers. Studies using removable SRIs have assessed the duration of carcinogen exposure required to induce a neoplastic transformation that proceeds to cancer without further carcinogen exposure. Serial syngeneic transplantation of cancers arising in inbred animals has shown that the degree of tumor differentiation is affected by the extent of host immunocompetence, and has also led to development of models for study of the processes of metastasis.

Lung cancer model for study of the metastatic process

In our hamster model of focal, chemically induced nonsmall cell lung cancer (NSCLC), we studied metastases in autochthonous hamster hosts (n = 300) and in syngeneic hamster and nude mice recipients (n = 230) of serial tumor transplants. Metastases in autochthonous hosts and transplant recipients occurred in regional lymph nodes, liver, and adrenals. In autochthonous host hamsters no metastases were noted from microinvasive (n = 112) or visible cancer less than 3.0 mm in diameter (n = 66); the incidence of metastasis was 8.2% (4/49) from 3- to 10-mm cancers and 22% (16/73) from cancers 10 mm in diameter or larger (p less than 0.05). Serial transplants were used to evaluate the metastatic propensity of 20 primary and six metastatic NSCLCs. Six primary NSCLCs that metastasized in the autochthonous host and six metastatic NSCLCs all metastasized promptly in recipients. This expression of metastatic potential was significantly different (p less than 0.05) from 14 primary cancers without autochthonous host metastases. Eight of the 14 caused no metastases in recipients, even after 5 to 11 tumor growth cycles; metastases occurred from the other six primary NSCLC after 3 to 12 tumor growth cycles in transplant recipients. Primary hamster NSCLCs metastasize in the autochthonous host with a frequency and a distribution pattern similar to human NSCLCs. A new model to study serially the cellular changes that govern the process of metastasis in NSCLC has been developed.