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.