Thyroid gland formation from inocula of monodispersed cells: early results on quantitation, function, neoplasia and radiation effects

ICRP Publication 131: Stem Cell Biology with Respect to Carcinogenesis Aspects of Radiological Protection

This report provides a review of stem cells/progenitor cells and their responses to ionising radiation in relation to issues relevant to stochastic effects of radiation that form a major part of the International Commission on Radiological Protection's system of radiological protection. Current information on stem cell characteristics, maintenance and renewal, evolution with age, location in stem cell 'niches', and radiosensitivity to acute and protracted exposures is presented in a series of substantial reviews as annexes concerning haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin, and bone. This foundation of knowledge of stem cells is used in the main text of the report to provide a biological insight into issues such as the linear-no-threshold (LNT) model, cancer risk among tissues, dose-rate effects, and changes in the risk of radiation carcinogenesis by age at exposure and attained age. Knowledge of the biology and associated radiation biology of stem cells and progenitor cells is more developed in tissues that renew fairly rapidly, such as haematopoietic tissue, intestinal mucosa, and epidermis, although all the tissues considered here possess stem cell populations. Important features of stem cell maintenance, renewal, and response are the microenvironmental signals operating in the niche residence, for which a well-defined spatial location has been identified in some tissues. The identity of the target cell for carcinogenesis continues to point to the more primitive stem cell population that is mostly quiescent, and hence able to accumulate the protracted sequence of mutations necessary to result in malignancy. In addition, there is some potential for daughter progenitor cells to be target cells in particular cases, such as in haematopoietic tissue and in skin. Several biological processes could contribute to protecting stem cells from mutation accumulation: (a) accurate DNA repair; (b) rapidly induced death of injured stem cells; (c) retention of the DNA parental template strand during divisions in some tissue systems, so that mutations are passed to the daughter differentiating cells and not retained in the parental cell; and (d) stem cell competition, whereby undamaged stem cells outcompete damaged stem cells for residence in the niche. DNA repair mainly occurs within a few days of irradiation, while stem cell competition requires weeks or many months depending on the tissue type. The aforementioned processes may contribute to the differences in carcinogenic radiation risk values between tissues, and may help to explain why a rapidly replicating tissue such as small intestine is less prone to such risk. The processes also provide a mechanistic insight relevant to the LNT model, and the relative and absolute risk models. The radiobiological knowledge also provides a scientific insight into discussions of the dose and dose-rate effectiveness factor currently used in radiological protection guidelines. In addition, the biological information contributes potential reasons for the age-dependent sensitivity to radiation carcinogenesis, including the effects of in-utero exposure.

Thyroid regeneration: how stem cells play a role?

Many tissues if not all are thought to contain stem cells that are responsible for regeneration and repair of the tissue after injury. Dysregulation of tissue regeneration may result in various pathological conditions, among which cancer is the most extensively studied. Notably, the so-called cancer stem cells or tumor-initiating cells, have been studied in order to understand the mechanisms of carcinogenesis and/or metastasis. However, the nature of cancer stem cells, let alone normal stem/progenitor cells, particularly those of the thyroid remains elusive. There remains a gap in knowledge between adult thyroid stem/progenitor cells and cancer stem cells of the thyroid, and if and/or how they are related to each other. Understanding of the mechanism for thyroid regeneration and mode of participation of normal adult thyroid stem/progenitor cells in this process will hopefully yield a more complete understanding of the nature of thyroid cancer stem cells, and/or help understand the pathogenesis of other thyroid diseases. This review summarizes the current understanding of adult thyroid stem/progenitor cells, with particular emphasis on how they contribute to thyroid regeneration.

Characterization and radiosensitivity at high or low dose rate of four cell lines derived from human thyroid tumors

PURPOSE

This study attempted to establish cell lines derived from human differentiated thyroid tumors, and to characterize and evaluate the radiation dose-rate effect.

METHODS AND MATERIALS

Two cell lines (K1 and K2) were derived from papillary carcinomas, one (K7) from a follicular less-differentiated carcinoma and one (A14) from a follicular adenoma. Cell-survival curves after irradiation were established by the in vitro colony method. Radiation doses were delivered either at a high (45.9 Gy/h) or low dose rate (0.6 Gy/h) by a 60Co source. The data were analyzed according to the linear quadratic and multitarget model of radiation action.

RESULTS

Doubling times were around 24 h. All cell lines were positive for intracellular thyroglobulin. Cyclic adenosine monophosphate (cAMP) response to thyrotropin (TSH) was significant for the cell lines derived from the follicular adenoma and follicular carcinoma. The two cell lines derived from papillary carcinomas were tumorigenic after inoculation into nude mice. After high-dose-rate irradiation, the surviving fraction at 2 Gy (SF2) was not significantly different for the three malignant cell lines K1, K2, and K7, and ranged from 0.39 to 0.42. For the adenoma cell line A14, we found a higher radiosensitivity with a lower SF2 value and a higher alpha parameter. After low-dose-rate irradiation, only one cell line (K2) showed a significant low-dose-rate sparing with a dose reduction factor of 1.35.

CONCLUSION

The major result of our study is the weak effect of the dose rate on the survival of thyroid cell lines.

Modification of expression of the malignant phenotype in radiation-initiated cells

Carcinogenesis is a multistage process consisting minimally of initiation and promotion/progression stages. Radiation and many environmental xenobiotics are potent initiating agents. We have shown that initiation of carcinogenesis in vivo by these agents is a common cellular event. In the irradiated thyroid (5 Gy) at least one in 20 cells is initiated. Initiation by both radiation and chemicals has also been shown to be a common cellular event in the mammary gland. Initiation therefore is most likely not the sole rate-limiting event in the carcinogenic process. The propensity of the initiated cell to express the malignant phenotype is modulated by many factors, including environmental chemicals and physiological and genetic factors. Scopal and abscopal physiological factors can either enhance or suppress the progression of initiated cells to a frank tumour. For example, prolactin enhances the rate of progression of radiation and chemically initiated mammary tumours while glucocorticoids suppress this progression. TSH enhances the progression of radiation-initiated thyroid tumours while a scopal factor associated with unirradiated thyroid cells suppresses progression of this tumour type.

Survival of human normal thyroid cells after X-ray irradiation

Normal thyroid cells from 25 individuals treated surgically for malignant or benign thyroid tumour were cultured in vitro and radiation induced cytotoxicity was studied. The mean lethal dose (Do), quasi-threshold (Dq), and extrapolation number (n) of survival curves of actively dividing thyroid cells assayed by colony formation were estimated to be 92.9 +/- 2.8 cGy (rad), 58.1 +/- 6.9 cGy and 2.0 +/- 0.1, respectively (average for 25 individuals +/- standard error). These results suggest that proliferating human thyroid cells are more sensitive to X-rays than most other nonhaematologic mammalian cells in similar assays. Cell survival was not significantly affected by sex, age, disease or exposure to atomic bomb radiation of the cell donor. However, the number of samples currently available is too small for definite conclusions in this regard.

Radiogenic initiation of thyroid cancer: a common cellular event

A transplantation system for clonogenic cells in rat thyroid was used, which allows quantitative evaluation of both the acute cytotoxicity and the late neoplastic effects of ionizing radiation at the cellular level in vivo. We have obtained direct experimental evidence that radiogenic initiation of neoplasia in vivo is a common cellular event, and that cell number influences the expression of initiation. Ten per cent of those graft sites which had received 26 clonogens surviving 5 Gy developed carcinomas, while 4 per cent of those which received 26 unirradiated clonogens developed carcinomas. By comparison, 26 per cent of the sites that were inoculated with 411 surviving irradiated clonogens developed carcinomas while none of the 38 transplant sites that received 411 unirradiated clonogens developed carcinomas. Total tumour incidence (carcinomas plus adenomas) followed the same pattern.

A critical appraisal of clonogenic survival assays in the evaluation of radiation damage to normal tissues

Assessment of radiation damage to normal tissues in terms of dose-response curves for infinite proliferative potential ("survival curves") does not take into account the decrease with increasing dose of the multiplication rate of the clonogenic and non-clonogenic (radiation-sterilized) cells which may be implicated in the expression of the damage to tissue function or gross appearance. While radiation selectively lowers the chance of successful mitotic divisions, other anticancer agents may in addition interfere with different cellular processes. Comparisons of effectiveness of radiation with that of other modalities should not therefore be limited to analysis of survival curves. Assays of cell "survival" in self-renewing normal tissues in situ often define properties of a non-random sample of the clonogens. The nature of repair associated with the post-irradiation delay in performance of transplantation assays for normal clonogenic cells remains unclear. Dose-response relationships for functional impairment or gross damage to tissue, especially those obtained by irradiation with many fractions, do not necessarily yield to interpretation in terms of clonogenic cell "survival".

Enzymatic and ultrastructural study of lysosomes in rats bearing radiation-induced thyroid follicular carcinoma

Radiation-induced well-differentiated and poorly differentiated follicular thyroid cancers were transplanted into the intrascapular fat pads of male Fisher 144 rats. The tumors grew in the recipient rats and after a time interval were removed and studied along with normal rat thyroids for lysosomal activity and ultrastructural characteristics. Plasma from experimental and control rats was also studied for lysosomal activity. Rats with radiation-induced thyroid carcinoma had a decrease in growth rate compared with normal rats. There was no significant increase in plasma lysosomal enzymes in the experimental rats. Well-differentiated thyroid carcinomatous tissue showed increased total activities of lysosomal enzymes as well as a difference in subcellular distribution compared with normal and poorly differentiated carcinomatous tissue. Electron microscopy of normal and carcinomatous tissue demonstrated the greatest number of lysosomes in the well-differentiated carcinoma and the fewest in the poorly differentiated carcinoma.