,5,3Ј-Triiodo-L-thyronine (T3), but not L-thyroxine (T4), activated Src kinase and, downstream, phosphatidylinositol 3-kinase (PI3-kinase) by means of an ␣ v3 integrin receptor on human glioblastoma U-87 MG cells. Although both T 3 and T4 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 T4. Thus the PI3-kinase, Src kinase, and ERK1/2 signaling cascades are parallel pathways in T 3-treated U-87 MG cells. T3 and T4 both caused proliferation of U-87 MG cells; these effects were blocked by the ERK1/2 inhibitor PD-98059 but not by LY-294002. Smallinterfering 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-␣ (TR␣) from cytoplasm to nucleus and accumulation of hypoxia-inducible factor (HIF)-1␣ mRNA; LY-294002 inhibited these actions. Results of studies involving ␣v3 receptor antagonists tetraiodothyroacetic acid (tetrac) and Arg-Gly-Asp (RGD) peptide, together with mathematical modeling of the kinetics of displacement of radiolabeled T3 from the integrin by unlabeled T3 and by unlabeled T4, are consistent with the presence of two iodothyronine receptor domains on the integrin. A model proposes that one site binds T3 exclusively, activates PI3-kinase via Src kinase, and stimulates TR␣ trafficking and HIF-1␣ gene expression. Tetrac and RGD peptide both inhibit T3 action at this site. The second site binds T4 and T3, and, via this receptor, the iodothyronines stimulate ERK1/2-dependent tumor cell proliferation. T3 action here is inhibited by tetrac alone, but the effect of T4 is blocked by both tetrac and the RGD peptide. thyroid hormone; phosphatidylinositol 3-kinase; extracellular signal-regulated kinase 1/2; integrin ␣v3; glioblastoma cells; Src kinase; mitogenactivated protein kinase; intracellular hormone receptor trafficking ACTIONS OF THYROID HORMONE [L-thyroxine (T 4 ); 3,5,3Ј-triiodo-L-thyronine (T 3 )] that are independent of ligand binding to nuclear thyroid hormone receptors are called nongenomic actions. Nongenomic effects of thyroid hormone are initiated outside the cell nucleus but may culminate in complex cellular events that are nucleus mediated (7, 8, 26 -29). Initiation of nongenomic actions includes a plasma membrane receptor for T 4 and T 3 on integrin ␣ v  3 that is linked to mitogen-activated protein kinase [extracellular signal-regulated kinase (ERK) 1/2] for transduction of the hormone signal (3) and nuclear receptors residing in the cytosol of unstimulated cells, such as thyroid hormone receptor (TR) 1 (28).The phosphatidylinositol 3-kinase (PI3-kinase)/protein kinase B (Akt) pathway is an important regulator of cellular growth, metabolism, and survival (13, 19). Studies of Storey et al. (38) indi...