The dog thyroid primary culture system: a model of the regulation of function, growth and differentiation expression by cAMP and other well-defined signaling cascades.

Abstract

Provided adequate culture conditions are found, specialized cells may keep in vitro some differentiated functions they are known to exert in the whole organism. These functions are responsive in vitro to purified hormones, neurotransmitters or growth factors normally secreted by other cell types. These factors are, therefore, assumed, but frequently not proved, to produce analogous effects in vivo, thus participating in the homeostasis of the whole organism. In this way, much of the modern understanding of the regulation of a defined cell within the organism relies on a mental reconstitution, which combines bits of experimental information obtained from various artificial culture systems, within a framework of ‘rules’ dictated by well admitted clinical observations and decades of in vivo animal experimentation. In vitro experimental observations that fit these rules are, therefore, called ‘mechanisms’; the others are disregarded as ‘culture artefacts’. This view is of course too simplistic. Transgenic and knockout animals remind us that in vitro and in vivo mechanisms may differ. On the other hand, previously unsuspected functions or regulations demonstrated in culture systems may eventually prove to have important physiological or pathological relevance. However, the correlation between in vivo and in vitro observations requires an in depth knowledge and understanding of the in vitro systems. For the study of thyroid cell differentiated functions and proliferation, various culture systems have been developed, using either primary cultures or immortal cell lines of different species. Both have advantages and disadvantages. The rat FRTL5 cell line is by far the most frequently used system. It retains most of the features of differentiated follicular thyroid cells, such as thyroidstimulating hormone (TSH)-dependence of growth (but also survival), iodide uptake, thyroglobulin (Tg) and thyroperoxidase (TPO) gene expression. However, the immortality of this cell line is sufficient evidence that it has lost some of the basic mechanisms of cell cycle control. In nude mice, FRTL5 cells develop TSHdependent tumors (1). Moreover this system suffers from instability (2) and clonal variability (3, 4) which explains the opposite results sometimes obtained in different laboratories. The FRTL5 cell line has lost the epidermal growth factor (EGF) control of growth (5), and b-adrenergic cAMP regulation (6), which is re-acquired in one of its variants (4). Because of its simplicity and accessibility, because it allows permanent transfections and genetic experiments and also because of the increasing difficulty in obtaining animals for experimentation, FRTL5 is now the preferred and often only used system in the majority of in vitro studies of thyroid biology (over 600 studies, mainly from the United States, Italy and Japan). Perhaps because of a personal bias, we consider that cells that are not selected by long-term in vitro propagation are less remote from physiology. We have thus always been surprised to observe that the relevance for the thyroid gland of observations from thyrocyte primary cultures is often definitively accepted only once they have been confirmed from FRTL5 cells, rather than conversely (we are all guilty of regarding what exists in our own model system as the only truth!). The confrontation of the different thyroid primary culture systems has pointed out the importance of possible species differences, but also the influence of culture conditions, and the fact that cells have a ‘memory’, which means that their characteristics are not fully stabilized and may evolve depending on their previous in vivo and in vitro story. In the last decade, the amount of information collected from the dog thyroid primary culture system has justified it emerging as the ‘challenger’ of the FRTL5 cell line as an in vitro model of thyroid cell biology. The aim of this review is to summarize the characteristics of this system and its contribution to the regulation of thyroid function, cell proliferation and differentiation. Whereas differences between systems are fully recognized (7), this review is not aimed at drawing up their catalog. We hope that the information on the properties of dog thyrocytes in culture assembled here may help other thyroidologists European Journal of Endocrinology (1997) 137 579–598 ISSN 0804-4643