Characterization of cytoplasmic T3 binding sites by adsorption to hydroxyapatite: effects of drug inhibitors of T3 and relationship to glutathione-S-transferases

Novel thyroid hormones

The field of thyroid hormone signaling has grown more complex in recent years. In particular, it has been suggested that some thyroid hormone derivatives, tentatively named "novel thyroid hormones" or "active thyroid hormone metabolites", may act as independent chemical messengers. They include 3,5-diiodothyronine (T2), 3-iodothyronamine (T1AM), and several iodothyroacetic acids, i.e., 3,5,3',5'-thyroacetic acid (TA4), 3,5,3'-thyroacetic acid (TA3), and 3-thyroacetic acid (TA1). We summarize the present knowledge on these compounds, namely their biosynthetic pathways, endogenous levels, molecular targets, and the functional effects elicited in experimental preparations or intact animals after exogenous administration. Their physiological and pathophysiological role is discussed, and potential therapeutic applications are outlined. The requirements needed to qualify these substances as chemical messengers must still be validated, although promising evidence has been collected. At present, the best candidate to the role of independent chemical messenger appears to be T1AM, and its most interesting effects concern metabolism and brain function. The responses elicited in experimental animals have suggested potential therapeutic applications. TA3 has an established role in thyroid hormone resistance syndromes, and is under investigation in Allen-Herndon-Dudley syndrome. Other potential targets are represented by obesity and dyslipidemia (for T2 and T1AM); dementia and degenerative brain disease (for T1AM and TA1); cancer (for T1AM and TA4). Another intriguing and unexplored question is the potential relevance of these metabolites in the clinical picture of hypothyroidism and in the response to replacement therapy.

Preferential inhibition of cytoplasmic T3 binding is associated with reduced nuclear binding in cultured cells

Previous studies from our laboratory have suggested that the nonsteroidal antiinflammatory drug, diclofenac (DCF), is a more potent competitor for T3 binding sites in cytoplasm than for those in the nucleus. In the present study we have examined the competitive potency for DCF and its effect on nuclear binding of T3 in cultured cells. DCF was a weak competitor for T3 binding sites in cytosol and nuclear extracts prepared from HepG2 cells with a potency of 21 and 295 microM, respectively. When expressed relative to T3, DCF was 135-fold more potent in cytosol than in nuclear extract. In intact cells, T3 was bound by nuclei with an affinity, Kd of 0.22 +/- 0.07 nM whereas in nuclear extract the affinity was 0.60 +/- 0.21 nM. DCF was a competitive inhibitor in both preparations but reduced the apparent affinity 4-fold in intact cells but only 2-fold in nuclear extract. In whole-cell experiments, DCF increased the rate of dissociation of T3 from cells prelabeled with hormone for 30 min. When these prelabeled cells were incubated with DCF, 0.1 mM, cell-associated T3 was significantly lower at 30 and 60 min than in cells reincubated without the drug. These data show that cellular transport mechanisms precede nuclear binding by T3 and suggest that there is a critical role for nonnuclear binding proteins in thyroid hormone action.

Effects of amiodarone on beating rate and Na-K-ATPase activity in cultured neonatal rat heart myocytes

1. The purpose of this study was to examine the possibility that the cellular action of amiodarone is mediated by inhibition of thyroid hormone regulatory functions within the myocardial cell. We measured the rate of cell beating and the activity of Na-K-ATPase in cultured neonatal rat heart myocytes. 2. Amiodarone (0.25 and 1 microgram/ml) reduced beating rate up to 75% within 20 min, and Na-K-ATPase activity up to 40% within 2 hr. No toxic effects were detected in the treated cells. 3. The inhibitory actions of amiodarone on beating rate and Na-K-ATPase activity were the same in myocytes grown in the presence or absence of 3-iodothyronine (T3, 5 nM). 4. These data indicate that amiodarone affects beating rate and Na-K-ATPase activity independently of thyroid hormone. It is suggested that interference of amiodarone with thyroid hormone action is not the only mechanism by which this drug modulates some functions of the myocardial cell.