Thyroid hormone mediated changes in gene expression can be initiated by cytosolic action of the thyroid hormone receptor β through the phosphatidylinositol 3-kinase pathway

Thyroid hormone (TH) action is mediated principally through binding of the hormone ligand, 3,3,5-triiodothyronine (T(3)), to TH receptors (TRs). This hormone-receptor interaction recruits other proteins to form complexes that regulate gene expression by binding to DNA sequences in the promoter of target genes. We recently described an extranuclear mechanism of TH action that consists of the association of TH-liganded TRβ with p85α [regulatory subunit of phosphatidylinositol 3-kinase (PI3K)] in the cytosol and subsequent activation of the PI3K, generating phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P3]. This initiates the activation of a signaling cascade by phosphorylation of Akt, mammalian target of rapamycin (mTOR) and its substrate p70(S6K), leading to the stimulation of ZAKI-4α synthesis, a calcineurin inhibitor. Furthermore, we found that this same mechanism leads to induction of the transcription factor hypoxia-inducible factor (HIF-1α), and its target genes, glucose transporter (GLUT)1, platelet-type phosphofructokinase (PFKP), and monocarboxylate transporter (MCT) 4. These genes are of special interest, because their products have important roles in cellular glucose metabolism, from glucose uptake (GLUT1) to glycolysis (PFKP) and lactate export (MCT4). These results demonstrate that the TH-TRβ complex can exert a non-genomic action in the cytosol leading to changes in gene expression by direct (HIF-1α) and indirect (ZAKI-4α, GLUT1, PFKP) means.