The road to nuclear receptors of thyroid hormone.

BACKGROUND: Early studies on the mechanism of action of thyroid hormone (TH) measured changes in enzyme activities following the addition of l-thyroxine (T4) and 3, 3', 5-triiodothyronine (T3) to tissue extracts and purified enzymes. SCOPE OF REVIEW: As techniques for isolation of mitochondria, ribosomes, nuclei and chromatin, were increasingly refined, it became possible to study complex cellular processes, such as oxidative phosphorylation, protein synthesis, transcription and chromosomal structure. Uncoupling of oxidative phosphorylation and direct action on protein synthesis as mechanisms of action of TH, proposed in the 1950s and 1960s, were found to be untenable as mechanisms of physiological action because of inappropriate experimental conditions. MAJOR CONCLUSIONS: Several findings in the 1960s and 1970s, mainly 1) that near-physiological doses of T3 stimulated transcription measured in vivo or in nuclei isolated from tissues of rats and frog tadpoles, 2) the inhibition of hormone action by inhibitors of transcription and 3) the rapid and almost identical kinetics of accumulation of labelled hormone and RNA synthesis in target cell nuclei, pointed to the cell nucleus as a major site of its action. The application of technologies of recombinant DNA, gene cloning and DNA sequencing in the mid-1980s allowed the identification and understanding of the structure and function of nuclear receptors of TH. GENERAL SIGNIFICANCE: This review traces the road leading to the nuclear receptors of thyroid hormone, thus explaining how the hormone influences gene expression. It also illustrates the importance of how new concepts originate from the progression of technological innovations. This article is part of a Special Issue entitled Thyroid hormone signalling.

Looking for the mechanism of action of thyroid hormone.

The mechanisms of action of thyroid hormone (TH), characterized by multiple physiological activities, proposed over the last 80 years are a reflection of the progression of our knowledge about eukaryotic signalling processes. The cumulative knowledge gained raises the question as to what is so special about the action of this hormone. The discovery in the 1980s that TH receptors belong to the family of nuclear transcription factors that regulate the expression of hormonal target genes was an important milestone. TH receptors are highly organized within the chromatin structure, which itself is modified by several chromosomal and nonchromosomal factors, in the presence and absence of the hormone. Recently, some investigators have suggested that TH acts via both genomic and nongenomic mechanisms and introduced the concept of networking within cellular complexes. While one cannot as yet precisely describe the mechanism of thyroid hormone action, I will attempt here to point out the present thinking and future directions to achieve this goal in the light of the historical background.

A hormone for all seasons.

Thyroid hormone (TH) elicits an extraordinary multiplicity of biochemical, cellular, and physiological responses in the simplest to the most complex organisms. The diverse actions of the biologically active thyroid hormone, L-triiodothyronine (T3), can be divided into two groups: growth and development, and regulation of metabolism. A large number of mechanisms, some mutually contradictory, have been proposed to explain the actions of this hormone. A chronological examination of the mechanisms of action of TH and other hormones reflects the rapid increase in our knowledge of biological regulatory processes. The discovery in the 1980s that the two TH receptors, TR alpha and TR beta, share the same modular structure as other members of the large nuclear receptor multigene family of transcription factors focused attention on transcriptional regulation by TH. TH receptors function together with co-activators and co-repressors in multi-protein complexes, which are organized along with hormonal target genes into the structure of chromatin. Many new facets of TH action are beginning to attract our attention, so that investigators will still be busy attempting to explain the mechanism of its action for decades to come.

Amphibian metamorphosis as a model for the developmental actions of thyroid hormone.

Thyroid hormone (TH) elicits multiple physiological actions in vertebrates from fish to man. These actions can be divided into two broad categories: those where the hormone regulates developmental processes and those that involve actions in the adult organism. Amphibian metamorphosis is a most dramatic example of extensive morphological, biochemical and cellular changes occurring during post-embryonic development, which is obligatorily initiated and sustained by TH. It is, therefore, an ideal model system to understand the action of the hormone. Each tissue of the frog tadpole responds differently to TH, ranging from altered gene expression, morphogenesis, tissue re-structuring and extensive cell death, according to a developmental programme set in place before the thyroid gland begins to secrete the hormone. The key element determining the response to the hormone is the nuclear thyroid hormone receptor (TR). As in most vertebrates, there are two thyroid hormone receptors, TRalpha and TRbeta, which repress transcription in the absence of the ligand and whose concentration in the tissues is directly modulated by the hormone itself. In Xenopus, biochemical and in situ techniques have shown that the amount of TRbeta mRNA and protein are elevated 50-100 times during TH-induced metamorphic climax. This phenomenon of "autoinduction" of receptor is also seen with developmental or inductive processes regulated by other hormones acting through nuclear receptors. It is possible that receptor upregulation may be a pre-requisite for hormonal response. Recent molecular and cell biological studies have suggested that TRs function as multimeric complexes with other nuclear or chromatin proteins, such as co-repressors and co-activators, to regulate the structure of the chromatin, and thereby determine the transcription of the receptor-specified target gene. There is evidence that this may also be so for thyroid hormone regulated transcription during amphibian metamorphosis.

From oxidative phosphorylation to transcription--a postdoctoral adventure.

The period as a postdoctoral fellow is crucial for the establishment of one's scientific research career. I illustrate here its importance based on my own experience. Although luck played a part, moving to the right place at the right time and having generous leaders who allowed me freedom to express unconventional views were most valuable in my venture into two scientific territories that were previously unfamiliar to me. My first encounter with an unknown field led to me challenging the well-established dogma of uncoupling of oxidative phosphorylation as the explanation for hormone action; the second, led to the demonstration of the multiplicity of eukaryotic RNA polymerase. I hope that the events described here will provide some encouragement to young scientists embarking on a research career and also be of interest to others.

Signalling through nuclear receptors.

A century ago, secretions from the pancreas were described as 'hormones', which we now know are secreted from all ductless glands. The development of various technologies has already contributed a great deal -- and will undoubtedly offer more -- to our understanding of their mode of action.

Amphibian metamorphosis: An exquisite model for hormonal regulation of postembryonic development in vertebrates.

Metamorphosis in invertebrates and vertebrates is an ideal model for studying mechanisms of postembryonic development regulated by external signals. Amphibian metamorphosis shares many similarities with mammalian development in the perinatal period. The precocious induction in vivo and in culture of amphibian metamorphosis by exogenous thyroid hormones and its retardation or inhibition by prolactin, have allowed the analysis of such characteristic features of postembryonic development as morphogenesis, tissue remodelling, gene reprogramming and programmed cell death. Recent studies on metamorphosis have revealed the important role played by such processes as auto- and cross-regulation of hormone receptor genes and by cell death or apoptosis, as in the maturation of the central nervous system, tissue restructuring and organolysis.