3-iodothyronamine (T1AM) is an endogenous thyroid hormone metabolite with distinct, acute biological effects that are largely opposite those of thyroid hormone. Administration of T1AM to rodents results in rapid and profound reduction in body temperature, heart rate, and metabolism. Since its discovery only five years ago, T1AM is emerging as a potentially key signaling molecule involved in thyroid hormone endocrinology. The structural similarities between T1AM and monoamine neurotransmitters as well as its parent compound, thyroid hormone, suggest an intriguing role for T1AM as both a neuromodulator and a hormone-like molecule that complements or regulates thyroid hormone action.
The known molecular targets of T1AM include both plasma membrane and intracellular proteins, suggesting that intracellular transport of T 1AM may be an important component of its action, although no uptake mechanism has yet been described. Using various human cell lines, we show that, indeed, cellular uptake of T1AM occurs in multiple cell types and that this process involves specific, saturable, and inhibitable transport mechanisms. These mechanisms are sodium- and chloride-independent, pH-dependent, thyronamine-specific, and do not involve the likely candidate transporters of other monoamines, organic cations, or thyroid hormones. A large-scale RNAi screen targeting the entire SLC superfamily of transporter genes reveals that the transport of T1AM into cells involves multiple transporters and we identify eight transporters that may contribute to the uptake of T1AM in HeLa cells. Moreover, we demonstrate that T1AM is taken up into the nucleus of HepG2 cells, suggesting that T1AM might play a role in transcriptional regulation via nuclear receptors, similar to the mechanism of action of its thyroid hormone precursor.
We also investigate the effect of T1AM on cellular entry of thyroid hormones, which is a prerequisite for their subsequent metabolism and action at nuclear thyroid hormone receptors. Transport inhibition studies reveal that T1AM displays differential inhibition of T3 and T4 cellular uptake by the specific thyroid hormone transporter MCT8 as well as by the multispecific organic anion transporting polypeptides (OATPs) 1A2 and 1C1, but does not affect thyroid hormone transport by OATP1B3. Given that OATP1A2, OATP1C1, and MCT8 are all present in the brain, T 1AM may play an important role in modulating thyroid hormone delivery and activity in specific target regions in the central nervous system.
Finally, we identify α2-Macroglobulin (α 2M) as a potential serum binding protein for T1AM. Serum proteins are involved in the binding, transport, and extracellular storage of a wide variety of endogenous compounds, including thyroid hormones. Examination of the mode of T1AM binding to α2M reveals that T1AM does not covalently bind the protein, in contrast to the reported interactions of other monoamines with α2M. T1AM also does not appear to bind α2M at the same sites as other monoamines, suggesting a distinct mechanism of binding. Moreover, T1 AM binding does not result in the conversion of the native form of α 2M to the activated form, a conformational change that does occur upon binding of α2M to proteases and that is necessary for its clearance from the body. α2M is known for various functions in the body, including its unique role as a pan-protease inhibitor, as well as its potential significance in immune defense and modulation of neurotransmitter metabolism. In addition to the possible role of α2M as a carrier protein for T1AM, the discovery of α2M interaction with T1AM opens another interesting area of investigation into this thyroid hormone derivative and its mechanisms of action in the body.