Histochemical, light and electron microscopic study of polonium-210 induced peripheral tumors in hamster lungs: evidence implicating the Clara cell as the cell of origin

The "Polonium In Vivo" Study: Polonium-210 in Bronchial Lavages of Patients with Suspected Lung Cancer

Few studies have reported on polonium-210, a decay breakdown product of radon-222 and lead-210, in human lungs and there has been no study in patients with suspected lung cancer. The main aim of this “Polonium in vivo” study was to evaluate polonium-210 radioactivity in bronchopulmonary systems of smoker, ex-smoker and never smoker patients with suspected lung cancer. Alpha-spectrometric analyses were performed on bronchial lavage (BL) fluids from two Italian hospitals in 2013–2016. Socio-demographic, smoking, occupational and spirometric characteristics, lung cancer confirmation and histologic type and radon-222 concentration in patients’ homes were collected. Seventy BL samples from never (n = 13), former (n = 35) and current smokers (n = 22) were analyzed; polonium-210 was detected in all samples from current and former smokers and in 54% of samples from never smokers (p < 0.001; median values: 1.20, 1.43 and 0.40 mBq, respectively). Polonium-210 levels were significantly higher in COPD versus no COPD patients (median value: 3.60 vs. 0.97 mBq; p = 0.007); former and current smokers, without and with COPD, had significantly increased polonium-210 levels (p = 0.012); 96% of confirmed versus 69% of non-confirmed lung cancer patients recorded detectable polonium-210 levels (p = 0.018). A polonium-210 detectable activity was measured in BL samples from all current and former smokers. Polonium-210 in the lungs could be the result of lead-210 entrapment, which, with its half-life of 22 years, could provide a continuous emission of alpha radioactivity, even many years after quitting, thus proposing a possible explanation for the onset of lung cancer, particularly in former smokers.

Polonium and lung cancer

The alpha-radioactive polonium 210 (Po-210) is one of the most powerful carcinogenic agents of tobacco smoke and is responsible for the histotype shift of lung cancer from squamous cell type to adenocarcinoma. According to several studies, the principal source of Po-210 is the fertilizers used in tobacco plants, which are rich in polyphosphates containing radio (Ra-226) and its decay products, lead 210 (Pb-210) and Po-210. Tobacco leaves accumulate Pb-210 and Po-210 through their trichomes, and Pb-210 decays into Po-210 over time. With the combustion of the cigarette smoke becomes radioactive and Pb-210 and Po-210 reach the bronchopulmonary apparatus, especially in bifurcations of segmental bronchi. In this place, combined with other agents, it will manifest its carcinogenic activity, especially in patients with compromised mucous-ciliary clearance. Various studies have confirmed that the radiological risk from Po-210 in a smoker of 20 cigarettes per day for a year is equivalent to the one deriving from 300 chest X-rays, with an autonomous oncogenic capability of 4 lung cancers per 10000 smokers. Po-210 can also be found in passive smoke, since part of Po-210 spreads in the surrounding environment during tobacco combustion. Tobacco manufacturers have been aware of the alpha-radioactivity presence in tobacco smoke since the sixties.

In vitro aging of rat lung cells. Downregulation of telomerase activity and continuous decrease of telomere length are not incompatible with malignant transformation

Most normal mammalian somatic cells cultivated in vitro enter replicative senescence after a finite number of divisions, as a consequence of the progressive shortening of telomeres during proliferation that reflects one aspect of organism/cellular aging. The situation appears more complex in rodent cells due to physiological telomerase expression in most somatic normal tissues, great telomere length, and the difficulties of finding suitable in vitro culture conditions. To study in vitro aging of rat lung epithelial cells, we have developed primary culture conditions adapted to rat fresh lung explants and have studied for 1 year (50 passages) the changes in cellular proliferation and mortality, genetic instability, telomerase activity, telomere length, and tumorigenic potential. We have observed an absence of senescence and/or crisis, a transient genetic instability, the persistence of a differentiated Clara cell phenotype, a steady decrease in telomerase activity followed by a low residual activity together with a continuous decrease in telomere length, a constant rate of proliferation, and the acquisition of tumorigenic potential. The bypass of the growth arrest and the acquisition of long-term growth properties could be explained by the loss of p16(INK4a) expression, the ARF/p53 pathway not being altered. In conclusion, these results clearly indicate that, in rat lung epithelial cells, in vitro transformation and acquisition of tumorigenic properties can occur even if the telomere length is still decreasing and telomerase activity remains downregulated.

Immunohistochemical study on cellular origins of rat lung tumors induced by inhalation exposures to plutonium dioxide aerosols as compared to those by X-ray irradiation

Immunohistochemical examinations were performed on rat pulmonary tumors induced by inhalation exposures to 239PuO2 aerosols, or by X-ray-irradiation to identify and compare cellular origins or, in turn, target cells at risk for radiation carcinogenesis. Both plutonium-induced and X-ray-induced pulmonary tumors appeared to occur from the lower respiratory tract epithelium through bronchioles into alveoli, and were histopathologically diagnosed as adenoma, adenocarcinoma, adenosquamous carcinoma, and squamous cell carcinoma. Immunohistochemical staining of neoplastic lesions using rabbit polyclonal antibodies to rat surfactant apoprotein A specific for alveolar type II pneumocytes, and Clara cell antigen specific for nonciliated bronchiolar Clara cells, showed that most of the adenomatous and adenocarcinomatous lesions from plutonium-exposed or X-irradiated rats were positive for either or both antigens, while, in contrast, adenosquamous and squamous lesions were mostly negative for both antigens. Even though there were some differences in the proportions and distributions of immunoreactive cells between plutonium- and X-ray-induced tumors and among neoplastic lesions, the results indicate that radiation-induced pulmonary adenomas and adenocarcinomas mostly originate from either alveolar type II pneumocytes or bronchiolar Clara cells, while adenosquamous and squamous carcinomas may be derived from the other epithelial cell components, or might have lost specific antigenicity during their transforming differentiation.

Induction of lung lesions in female rats following chronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8,-Tetrachlorodibenzo-p-dioxin (TCDD) has been classified as a known human carcinogen, and epidemiologic studies identify the lung as one of the target organs. Few experimental studies have attempted to characterize pulmonary effects of TCDD exposure. In this study, we characterize the induction of lesions in the lung by chronic oral TCDD exposure in diethylnitrosamine (DEN)-initiated or noninitiated female Sprague-Dawley rats. Two or 18 weeks after initiation, rats were treated with TCDD continuously for 14, 30, or 60 weeks by biweekly oral gavage (1,750 ng TCDD/kg) at a dose equivalent to 125 ng/kg body weight per day (controls received corn oil). To assess the time dependence and reversibility of potential changes, some groups included withdrawal periods of 16 or 30 weeks after 30 weeks of TCDD treatment. TCDD treatment alone for 60 weeks caused significant increases in alveolar-bronchiolar (AB) metaplasia. TCDD treatment of DEN-initiated animals for 60 weeks resulted in a significant increase in bronchiolar epithelial hyperplasia. These increases were not observed in animals treated with TCDD for 30 weeks followed by corn oil for 30 weeks, indicating that the development of these lesions required continuous exposure to TCDD. AB hyperplasia increased in an age-dependent manner after DEN initiation but was unaffected by TCDD treatment. Expression of the aromatic hydrocarbon receptor (AHR) and induction of CYP1A1 was observed only in bronchiolar Clara and ciliated cells, indicating that the mechanism of induction of AB metaplasia may be mediated by the AHR. TCDD elimination half-life was monophasic in the lung, and serum and was estimated to be 39.7 days and 44.6 days, respectively, independent of age, tissue TCDD concentration, or body weight. This is the first report to identify the AB region as a target for TCDD-induced metaplastic and proliferative changes after chronic oral exposure.

Squamous metaplasia of bronchiolar cell-derived adenocarcinoma induced by N-nitrosomethyl-n-heptylamine in Syrian hamsters

Morphology and development of experimental bronchiolar lung tumors were studied in Syrian hamsters, using light and electron microscopic techniques. At the age of 9 weeks, 46 hamsters were each given one weekly gavage of 6.8 mg N-nitrosomethyl-n-heptylamine for 35 weeks, and hamsters were examined at intervals from 2 to 46 weeks. The present report describes the progression of adenocarcinomas of bronchiolar cell origin to adenosquamous and squamous cell carcinomas. Squamous metaplasia was commonly noted at the tumor periphery, i.e., zone of growth. In 20 hamsters, 22 adenosquamous and two squamous cell carcinomas (one a large cell carcinoma) were diagnosed by light microscopy. Overt keratinization was infrequent. Squamous cell metaplasia was not a feature of papillary neoplasms but was seen mainly with acinar structures. Ultrastructurally, squamous differentiation (metaplasia) appeared to develop along two different pathways. First, secretory cells were observed with large numbers of intermediate filaments and tonofilaments, with concurrent loss of organelles such as secretory granules and microvilli. Second, squamous metaplasia also appeared to develop from a progeny of tumor cells that failed to mature into secretory cells. Such cells were often present within the basal layer of secretory acini and resembled basal cells of the tracheobronchial tree. These observations were supported by increased expression of cytokeratins, as revealed by immunohistochemical procedures. Immunoelectron microscopic examination localized hamster Clara cell antigen in secretory granules of neoplastic Clara cells, in the cytoplasm between granules, and at the microvillous border. With the onset of squamous differentiation, Clara cell antigen was progressively lost from secretory cells and was only rarely seen in cells with tonofilaments. No labeling was present in squamous cells arising at the base of tumor acini. These results suggest that pulmonary squamous cell carcinomas may develop by direct squamous differentiation of secretory cells or may proceed from undifferentiated tumor cells.

Plutonium-induced proliferative lesions and pulmonary epithelial neoplasms in the rat: immunohistochemical and ultrastructural evidence for their origin from type II pneumocytes

Immunohistochemistry and transmission electron microscopy were used to clarify the cellular origin for plutonium-239-induced pulmonary proliferative (preneoplastic) epithelial lesions and epithelial neoplasms in F344 rats. Examples of each histologic type of proliferative lesion and neoplasm were stained by the avidin-biotin complex immunoperoxidase method using antibodies to rat surfactant apoprotein and Clara cell antigen. Rat surfactant apoprotein immunostaining was detected in type II pneumocytes in sections of normal lung, in the cells of the proliferative lesions classified histologically as alveolar epithelial hyperplasia (51) and mixed foci (alveolar epithelial hyperplasia with fibrosis) (30), and in adenomas (2), adenocarcinomas (3), and adenosquamous carcinomas (2). With the exception of one adenosquamous carcinoma, Clara cell antigen immunostaining was not detected in any of the pulmonary lesions but was detected in nonciliated cuboidal epithelial (Clara) cells in normal bronchioles. The epithelial cells of the proliferative lesions and neoplasms had ultrastructural features consistent with type II pneumocytes, i.e., the presence of cytoplasmic lamellar and multivesicular bodies. The results of these studies indicate that the majority of plutonium-induced proliferative epithelial lesions and neoplasms in the rat originate from alveolar type II pneumocytes.

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.

Cells of origin of primary pulmonary neoplasms in mice: morphologic and histochemical studies

Urethane-induced pulmonary adenomas in mice have two distinct histologic growth patterns--solid and papillary. The development of these tumors between 14 and 56 weeks was investigated in A/J mice. Solid tumor multiplicity remained constant from 14 to 56 weeks, whereas papillary and total tumor multiplicities increased in parallel between 14 and 28 weeks and remained constant through 56 weeks. The simplest explanation of these results is that solid and papillary adenomas arise independently, possibly from different cell types. The cell type of origin of these primary mouse lung tumors was investigated histochemically. Succinate dehydrogenase (SDH) histochemistry readily stained bronchiolar epithelial cells, but alveolar epithelial cells exhibited only slight enzymatic activity. Urethane-induced papillary adenomas exhibited intense SDH staining, whereas solid adenomas stained very lightly. Since Clara cells and type II pneumocytes are the only cells capable of proliferation in the bronchiolar and alveolar epithelia, respectively, the relative SDH activities of these adenomas is consistent with a hypothesis that solid tumors arise from type II pneumocytes and papillary tumors arise from Clara cells.

The genetic basis of susceptibility to lung tumors in mice

This is the first in a series of review articles describing the current state of research on mouse lung tumorigenesis. The system is valuable as a biological model for studying stages of tumor development and the interaction of genetic and environmental factors which dispose towards neoplasia. Additionally, these tumors are analagous to bronchiolo-alveolar cancer in man. Three pulmonary adenoma susceptibility (Pas) genes regulate susceptibility; 1 of these is the proto-oncogene, K-ras2. Candidates for the other 2 genes include the H-2 histocompatibility locus and genes which regulate the basal proliferative rate of the cells from which these tumors arise. Tumor development is favored by a depressed immune system, immature age, and decreased levels of circulating corticosterone.

Pulmonary adenoma and endocrine cell hyperplasia in Syrian golden hamster treated with 4-nitroquinoline 1-oxide

Chronic effects of 4-nitroquinoline 1-oxide (4 NQO) on the lungs of Syrian golden hamsters were studied. 4 NQO was subcutaneously injected weekly for 3 weeks at a dose of 20 mg/kg body weight. The animals were sacrificed at the 65th and 80th experimental weeks. Two cases of pulmonary adenomas were demonstrated in the 10 4 NQO-treated animals at the 80th week, and the tumor cells contained cytoplasmic lamellar inclusion bodies. In a previous study, we reported 4 NQO- induced pulmonary endocrine cell hyperplasias in the 4 NQO-treated hamster after the 20th experimental week (Jpn. J. Cancer Res., 77, 1986). In the present study, 12 pulmonary endocrine cell hyperplasias were recognized in serial sections of the 24 treated animals. The hyperplastic lesions showed positive immunoreactivity to calcitonin. The hyperplastic lesion did not develop to pulmonary endocrine cell neoplasm.

Altered enzyme expression in propylnitrosamine-induced Syrian hamster lung lesions

Focal proliferative and neoplastic lung lesions induced in Syrian hamsters by dihydroxy-di-n-propylnitrosamine (DHPN) were investigated using a combined histochemical, autoradiographic and electron microscopic approach. Expression of elevated glucose-6-phosphate dehydrogenase (G6PD) and gammaglutamyl-transpeptidase (GGT) activities and levels of immunohistochemically demonstrable glutathione S-transferase placental form (GST-P) were evident in epithelial cells of focal proliferative populations and bronchioloalveolar neoplasms. Binding for the GST-C form, normally only weak, became very pronounced in the stromal elements of DHPN-induced lesions. Increased labelling with tritiated thymidine was associated with increase in morphological atypia within the tumours. Although the enzyme phenotype findings were equivocal the presence of lamellar bodies in some cells of focal proliferative and neoplastic lesions suggested an origin from alveolar type II cells. The present results regarding changed enzyme phenotype in lung lesions suggest important similarities at the biochemical level for the process of neoplasia in the different target organs of DHPN in the hamster and indicate that GST-P may be a useful 'marker' for lung neoplasia.

Silica-induced alveolar cell tumors in rats

Min-U-Sil5, a form of alpha quartz, has been shown to induce peripheral lung tumors in rats exposed to the dust by inhalation. The animals were exposed to a nominal particle concentration of 12.4 mg/m3 for 8 hr/day, 4 days/week, for 2 years. The induced tumors were large and peripheral, and, when examined by electron microscopy, were found to be composed predominantly of alveolar type II cells. These cells were found in papillary, acinar, and solid forms of the tumors and were characterized by lamellar inclusion bodies. This is in contrast to the mouse, in which the papillary form was associated with Clara cells and the acinar form was linked with the type II cell. In this study, the Clara cell was a minor component of the tumor mass. No clear risk is established in man linking silica exposure to increased lung tumor rates.

Ultrastructural markers of colonic adenocarcinoma

A mixed population of 96 adenocarcinomas was examined by electron microscopy to establish the presence of organ specific features. This resulted in the identification of fine structural characteristics, occurring consistently in colorectal adenocarcinomas but not in other epithelial tumors. The colorectal "ultrastructural profile" consists of microvilli with dense cores of microfilaments extending as long rootlets into a clear zone of apical cytoplasm, apical electron dense bodies, and abundant glycocalyceal bodies. Of these features, the long rootlets constitute the best morphologic marker for large intestinal type adenocarcinoma. Using these characteristics in another series of 58 adenocarcinomas studied in a double-blind manner, it was possible to distinguish colorectal adenocarcinomas from other carcinomas on ultrastructural grounds alone.