Quantitative DNA alterations during 5-azacytidine-induced differential modulation of benzo(a)pyrene carcinogenesis in hamster bronchi

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.

Regression of bronchial epithelial cancer in hamsters

For bronchogenic carcinoma, if and when the sequential process of carcinogenesis is reversible is fundamental to chemoprevention research. In our hamster model, focally originating non-small cell lung carcinoma (NSCLC) develops via a reproducible sequential process of carcinogenesis by 180 days after endobronchial sustained-release implants (SRIs) of 10% benzo(a)pyrene. In this study, 114 hamsters received removable 10% benzo(a)pyrene SRIs. Short-term controls were sacrificed in 3 groups at 50, 65, and 80 days after SRI placement. Three experimental groups had SRIs removed at 50, 65, and 80 days after placement, and sacrifice was delayed until 100 to 180 days later. Long-term controls retained SRIs until sacrifice at 180 or 240 days after SRI placement. All long-term controls had NSCLC. Preneoplastic change was more common in 50- and 65-day controls, as compared with hamsters with equal duration of SRI exposure whose sacrifice was delayed until 100 to 180 days after SRI removal (p < 0.05). The 56% incidence of early NSCLC in hamsters sacrificed after 80 days of SRI exposure decreased to 5% in hamsters that had delayed sacrifice after SRI removal after 80 days of exposure. At the 10% benzo(a)pyrene dose used, hamster bronchial epithelium requires more than 80 days of continuous exposure to become irreversibly committed to NSCLC uniformly. Microinvasive NSCLC in hamsters often regresses, and it is not necessarily a precursor of overt invasive cancer. The removable SRI model provides new opportunities to evaluate chemoprevention of NSCLC and the related molecular-genetic control mechanisms.

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.