L-thyroxine vs. 3,5,3′-triiodo-L-thyronine and cell proliferation: Activation of mitogen-activated protein kinase and phosphatidylinositol 3-kinase

3,5,3′-Triiodo-L-thyronine (T ₃), but not L-thyroxine (T ₄), activated Src kinase and, downstream, phosphatidylinositol 3-kinase (PI3-kinase) by means of an α _vβ ₃ integrin receptor on human glioblastoma U-87 MG cells. Although both T ₃ and T ₄ stimulated extracellular signal-regulated kinase (ERK) 1/2, activated ERK1/2 did not contribute to T ₃-induced Src kinase or PI3-kinase activation, and an inhibitor of PI3-kinase, LY-294002, did not block activation of ERK1/2 by physiological concentrations of T ₃ and T ₄. Thus the PI3-kinase, Src kinase, and ERK1/2 signaling cascades are parallel pathways in T ₃-treated U-87 MG cells. T ₃ and T ₄ both caused proliferation of U-87 MG cells; these effects were blocked by the ERK1/2 inhibitor PD-98059 but not by LY-294002. Small-interfering RNA knockdown of PI3-kinase confirmed that PI3-kinase was not involved in the proliferative action of T ₃ on U-87 MG cells. PI3-kinase-dependent actions of T ₃ in these cells included shuttling of nuclear thyroid hormone receptor-α (TRα) from cytoplasm to nucleus and accumulation of hypoxia-inducible factor (HIF)-1α mRNA; LY-294002 inhibited these actions. Results of studies involving α _vβ ₃ receptor antagonists tetraiodothyroacetic acid (tetrac) and Arg-Gly-Asp (RGD) peptide, together with mathematical modeling of the kinetics of displacement of radiolabeled T ₃ from the integrin by unlabeled T ₃ and by unlabeled T ₄, are consistent with the presence of two iodothyronine receptor domains on the integrin. A model proposes that one site binds T ₃ exclusively, activates PI3-kinase via Src kinase, and stimulates TRα trafficking and HIF-1α gene expression. Tetrac and RGD peptide both inhibit T ₃ action at this site. The second site binds T ₄ and T ₃, and, via this receptor, the iodothyronines stimulate ERK1/2-dependent tumor cell proliferation. T ₃ action here is inhibited by tetrac alone, but the effect of T ₄ is blocked by both tetrac and the RGD peptide.