Receptor proteins for nongenomic actions of thyroid hormone

Genomic actions of thyroid hormone require the intranuclear binding by nuclear thyroid hormone receptors (TRs) of 3,5,3’-triiodo-L-thyronine (T₃). Nongenomic actions of the hormone have been described that are initiated at the plasma membrane, in cytoplasm or in the mitochondrion. These are complex processes associated with maintenance of the cytoskeleton, control of cell respiration, cell proliferation—including tumor cell proliferation and angiogenesis—and nervous system function. We briefly review here the nature of the proteins which are now appreciated to initiate nongenomic actions of the hormone when they bind T3 or L-thyroxine (T₄). These receptor proteins for nongenomic effects include truncated isoforms of TRα, cytoplasmic intact TRβ, certain cytoplasmic enzymes and a structural protein of the plasma membrane, integrin αvβ3.