3-Iodothyronamine (T₁AM): A New Player on the Thyroid Endocrine Team?

Abstract

3-Iodothyronamine (T₁AM) is an endogenous compound with chemical features that are similar to thyroid hormone. T₁AM has a carbon skeleton identical to that of T₄ and contains a single carbon-iodine bond. Theoretically, T₁AM could be produced from T₄ by enzymatic decarboxylation and deiodination. Recent studies show that T₁AM and higher iodinated thyronamines are subject to similar metabolic processing as iodothyronines such as T₄, suggesting a biological linkage between iodothyronines and iodothyronamines. In addition, single doses of T₁AM administered to rodents induce a hypometabolic state that in certain ways resembles hibernation and is opposite to the effects of excess T₄. This review will discuss the latest developments on this recently discovered thyroid hormone derivative.

3-Iodothyronamine is a biologically active, endogenous derivative of thyroid hormone that may represent a new arm of the thyroid endocrine system.

Thyronamines are chemical derivatives of thyronines, which are generally regarded as the principle chemical form of thyroid hormone. T₄, the major thyronine secreted from the thyroid gland and most abundant thyroid hormone in circulation is widely viewed as a prohormone (Fig. 1) (1). Outer-ring enzymatic deiodination of T₄ gives rise to T₃, a high-affinity ligand for the thyroid hormone receptors, whereas inner ring T₄ deiodination produces rT₃, a T₄ metabolite whose biological role is not entirely clear, despite the fact that equal circulating fractions of T₃ and rT₃ are produced from T₄ in healthy individuals, and substantially more rT₃ than T₃ is produced in disorders that are characterized as nonthyroidal illness (2,3,4). In addition to T₃ and rT₃, most of the other theoretically possible further deiodinated thyronines have been detected in tissue and circulation (5). Thyronines are amino acids, the decarboxylation of which gives rise to a phenethylamine derivative called a thyronamine (Fig. 1). There are several literature reports describing the synthesis and pharmacological evaluation of different thyronamines, varying from one another in iodine content, dating back to the 1930s, but none of these older reports demonstrated that thyronamines were endogenous substances (6,7,8,9,10,11,12,13,14).

Figure 1.

The chemical structures of thyronines and thyronamines. T₄ is converted to both T₃ and rT₃ by the indicated deiodinases. T₁AM, the most studied thyronamine to date, has the same chemical skeleton as T₄ and theoretically can be made from T₄ by enzymatic decarboxylation and deiodination.

In 2004, Scanlan and co-workers (15) described the synthesis and biological properties of 3-iodothyronamine (T₁AM), a novel thyronamine that had not been described previously. Using synthetic T₁AM as a standard in liquid chromatography/tandem mass spectrometry assays, T₁AM was shown to be an endogenous component of biogenic amine extracts from rodent brain, liver, heart, and blood. With subsequent liquid chromatography/tandem mass spectrometry method development, this assay is now suitable for quantitative analysis of endogenous T₁AM and pharmacokinetic analysis of pharmacologically administered T₁AM (16). In terms of its actions, T₁AM was found to be a potent agonist of trace amine-associated receptor 1 (TAAR1), an orphan G protein-coupled receptor (GPCR) that is highly homologous to adrenergic, dopaminergic, and serotonergic GPCRs (17). T₁AM was found to activate rat and mouse TAAR1 with EC₅₀ values of 14 and 112 nm, respectively, and subsequent structure-activity studies have further defined the T₁AM ligand pharmacophore that binds to and activates TAAR1 (18,19,20,21). In vivo, a single ip injection of T₁AM was found to rapidly decrease core body temperature and heart rate (15). In mice, T₁AM induced an approximate 10 C drop in body temperature that peaked about 1 h after injection and returned to normal after 4–6 h, depending on the T₁AM dose. The T₁AM-induced reduction in cardiac performance was found to be a direct effect and independent of the T₁AM-induced hypothermia. In a rat working heart preparation held at 37 C, introduction of T₁AM into the perfusion buffer resulted in large and immediate decreases in both heart rate and systolic aortic pressure, and additional studies on the heart further support the direct actions of T₁AM on this organ (22,23).

These initial pharmacological findings indicated that single-dose T₁AM treatment rapidly induced a hypometabolic state in rodents. The induction of hypothermia was thought to have potential neuroprotective benefit in the case of ischemic injury such as stroke. Indeed, T₁AM treatment was found to reduce infarct volume by 40% in a middle cerebral artery occlusion stroke model in mice, and the degree of neuroprotection afforded by T₁AM was found to correlate well with the magnitude of hypothermia that was induced by T₁AM (24). Along with hypothermia and reduced cardiac function, single-dose T₁AM rapidly induces hyperglycemia in mice (25). Mice that receive an ip injection of T₁AM show 2- to 5-fold increased blood glucose peaking 1 h after the injection and returning to baseline after 3–4 h. The mechanism of this effect appears to involve T₁AM activation of α_2A adrenergic receptors (Adra2a) in β-islet cells of the pancreas, because Adra2a knockout mice show no hyperglycemia after T₁AM treatment. Ligand activation of α-2A receptors leads to a decrease in cAMP through coupling to Gα_i, resulting in inhibition of insulin secretion from β-islet cells. At present, it is not certain whether this effect of T₁AM is direct or whether it is purely pharmacological or has physiological significance. In addition to the GPCRs TAAR1 and α-2A, T₁AM also acts at plasma membrane and vesicular biogenic amine transporters (26). T₁AM is an inhibitor, but not a competitive substrate, of the dopamine and norepinephrine plasma membrane transporters and also an inhibitor of vesicular monoamine transporters that package biogenic amine neurotransmitters into vesicles within neurons. The inhibitory potency of T₁AM against these transporters is in the low micromolar range, which is similar in magnitude to the K_M values of the endogenous substrates of these transporters. Whether or not endogenous T₁AM is present at these levels in synapses or neurons of the central nervous system is presently not known. Interestingly, The T₁AM reuptake inhibition is specific for catecholamines because T₁AM shows no inhibitory activity toward the serotonin reuptake transporter.

The metabolism of thyroid hormone is directly connected to its actions, and thyronamines appear to share many of the metabolic reactions of thyronines. Like thyronines, thyronamines are substrates for sulfotransferase enzymes that catalyze the sulfation of the phenolic hydroxyl shared between the two classes of compounds (27). T₁AM is the most efficiently processed thyronamine substrate using a sulfotransferase preparation from human liver, and the V_max/K_M value for T₁AM compares to that of T₃, suggesting that similar to T₃, sulfation of T₁AM may be an important clearance mechanism for regulating free circulating levels. In addition to sulfation, thyronamines are also converted to thyroacetic acids by an amine oxidase activity (28). Upon incubation with HepG2 cells or human thyroid tissue homogenates, T₁AM is robustly converted to 3-iodothyroacetic acid, and T₃AM is converted into 3,5,3′-triiodothyroacetic acid (Triac), a known active metabolite of T₃. This conversion appears to be very efficient and occurs both in vitro and in vivo; the enzymes responsible for this are currently unknown, but monoamine oxidase or semicarbazide-sensitive amine oxidase are reasonable candidates. The biological purpose behind this conversion is also presently unclear. It is possible that this could be a deactivation mechanism, similar to sulfation, although the biological activities (if any) of 3-iodothyroacetic acid are presently unknown. The robust conversion of T₃AM to Triac is intriguing and suggests the possibility that this pathway, involving the intermediacy of a thyronamine, is responsible, at least in part, for the endogenous production of Triac from T₃.

As mentioned above, enzymatic deiodination plays a major role in the activation of thyroid hormone from the latent form, T₄, to the active form, T₃ (2,3). This activation pathway is mediated by the type I deiodinase (Dio1) and Dio2 isozymes. T₄ is also converted to rT₃ in what may be a deactivation mechanism, and this process is catalyzed by the Dio3 isozyme (4). In addition to iodothyronines, iodothyronamines are substrates for the three deiodinases, Dio1, Dio2, and Dio3 (29). Iodothyronamines have comparable V_max/K_M values compared with the corresponding iodothyronine, indicating that iodothyronamines would be as readily deiodinated as iodothyronines should they exist at comparable concentrations and encounter the appropriate deiodinase isozyme. Interestingly, the isozyme selectivity of iodothyronamine deiodination differs from that of iodothyronines. T₄AM is not a substrate for Dio1 or Dio2 and cannot be deiodinated to T₃AM, whereas T₄AM is a good substrate for Dio3 and is readily deiodinated to rT₃AM. Dio1 and Dio2 can then sequentially deiodinate rT₃AM to provide T₁AM. This isozyme-selective processing suggests a specific biosynthetic pathway for endogenous T₁AM production (Fig. 2) with either T₄AM or rT₃AM serving as entry points into the pathway, and these entry points would originate from decarboxylation of either T₄ or rT₃. Although the logic of this proposed biosynthetic pathway is very attractive, the precise relationship between T₁AM and thyroid hormone is at present poorly understood, and although T₁AM is clearly an endogenous chemical derivative of T₄, there is not direct evidence at present, proving that T₁AM is a T₄ metabolite in vivo. However, viable alternative routes for T₁AM production are not obvious, because biosynthesis from tyrosine would require oxidative coupling of two molecules of tyrosine and aromatic ring iodination, which are specialized biochemical reactions catalyzed by thyroperoxidase during thyroid hormone synthesis; alternative robust enzymatic iodination pathways in vertebrates are not presently known.

Figure 2.

Summary of enzyme-catalyzed thyronamine deiodinase reactions. Unlike T₄, T₄AM is not a substrate for Dio1 and -2 and cannot be deiodinated to T₃AM. However, T₄AM is readily deiodinated to rT₃AM by Dio3, and rT₃AM can be further deiodinated to ultimately provide T₁AM. This suggests a unique biosynthetic deiodination pathway for T₁AM starting from the decarboxylation products of either T₄ or rT₃.

In addition to the hypometabolic effects of T₁AM discussed previously, it was recently discovered that single-dose T₁AM dramatically switches fuel utilization away from carbohydrates and toward lipids (30). Siberian hamsters (Phodopus sungorus), a hibernating rodent species, as well as mice, display a complete shift in respiratory quotient (RQ) from a normal, mixed carbohydrate and lipid value (0.90 for hamsters, 0.83 for mice) to an RQ value of approximately 0.7, indicating that a switch to pure lipid burning has occurred upon single-dose T₁AM administration. Interestingly, this 0.7 RQ persisted in summer-acclimatized Siberian hamsters for at least 24 h after the T₁AM injection, making this fueling shift the effect of longest duration yet to be observed with single-dose T₁AM. The kinetics of this effect differs somewhat from the previously studied T₁AM pharmacology. The onset of the RQ effect was slower than the onset of T₁AM-induced hypothermia, bradycardia, or hyperglycemia; these three effects reach a maximum about 1 h after T₁AM injection, whereas the complete RQ shift is reached 4.5 h after injection. Consistent with the shift in RQ toward lipid utilization, T₁AM-treated Siberian hamsters had measurable urine ketone content that peaked 16 h after injection, again illustrating the extended duration of the T₁AM-induced lipid burning switch compared with hypothermia, bradycardia, or hyperglycemia that typically return to baseline values within 4–6 h after T₁AM administration.

Since its discovery, the pharmacology experiments with T₁AM have exclusively involved single, high-dose T₁AM administration while following biological effects that occur rapidly, generally peaking within 1 h after injection. Interesting biological effects have been observed and reported in these studies, and the full spectrum of T₁AM’s pharmacological effects is unlike any other known drug or endogenous biologically active compound. This early work was guided by a working hypothesis that T₁AM may be a novel neuromodulator/neurotransmitter, possessing biological actions similar to the chemically related biogenic amines dopamine, norepinephrine, and serotonin (15). This neurotransmitter view, put forward originally by Dratman (31) almost three decades before the discovery of T₁AM, has been instructive and useful and may indeed account for the majority of T₁AM’s actions. However, given the strong chemical relationship between T₁AM and thyroid hormone in terms of both structure and enzymatic processing, coupled to the powerful biological actions of T₁AM on metabolism and fueling, prompts one to ask whether T₁AM may also possess hormone-like properties akin to those of T₄/T₃ that involve direct T₁AM-induced changes to target gene expression profiles. Such actions would not be readily revealed from single-dose experiments and require the development of multidose pharmacology strategies along with a better understanding of the generation, distribution, and metabolic processing of endogenous T₁AM. This future work will be critical for understanding the physiological basis of T₁AM as well as identifying pathologies and illness that either involve T₁AM or would benefit from T₁AM therapy.