,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 ␣ v␤3 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 ␣v␤3 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 ␣v␤3; 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...

Plasma membrane integrin αvβ3 is a cell surface receptor for thyroid hormone at which nongenomic actions are initiated. L-thyroxine (T₄) and 3,3',5-triiodo-L-thyronine (T₃) promote angiogenesis and tumor cell proliferation via the receptor. Tetraiodothyroacetic acid (tetrac), a deaminated T₄ derivative, blocks the nongenomic proliferative and proangiogenic actions of T₄ and T₃. Acting at the integrin independently of T₄ and T₃, tetrac and a novel nanoparticulate formulation of tetrac that acts exclusively at the cell surface have oncologically desirable antiproliferative actions on multiple tumor cell survival pathway genes. These agents also block the angiogenic activity of vascular growth factors. Volume and vascular support of xenografts of human pancreatic, kidney, lung, and breast cancers are downregulated by tetrac formulations. The integrin αvβ3 receptor site for thyroid hormone selectively regulates signal transduction pathways and distinguishes between unmodified tetrac and the nanoparticulate formulation. The receptor also mediates nongenomic thyroid hormone effects on plasma membrane ion transporters and on intracellular protein trafficking.

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A cell surface receptor for thyroid hormone that activates extracellular regulated kinase (ERK) 1/2 has been identified on integrin αvβ3. We have examined the actions of thyroid hormone initiated at the integrin on human NCI-H522 non-small cell lung carcinoma and NCI-H510A small cell lung cancer cells. At a physiologic total hormone concentration (10−7 M), T4 significantly increased proliferating cell nuclear antigen (PCNA) abundance in these cell lines, as did 3, 5, 3′-triiodo-L-thyronine (T3) at a supraphysiologic concentration. Neutralizing antibody to integrin αvβ3 and an integrin-binding Arg-Gly-Asp (RGD) peptide blocked thyroid hormone-induced PCNA expression. Tetraiodothyroacetic acid (tetrac) lacks thyroid hormone function but inhibits binding of T4 and T3 to the integrin receptor; tetrac eliminated thyroid hormone-induced lung cancer cell proliferation and ERK1/2 activation. In these estrogen receptor-α (ERα)-positive lung cancer cells, thyroid hormone (T4>T3) caused phosphorylation of ERα; the specific ERα antagonist ICI 182,780 blocked T4-induced, but not T3-induced ERK1/2 activation, as well as ERα phosphorylation, proliferating-cell nuclear antigen (PCNA) expression and hormone-dependent thymidine uptake by tumor cells. Thus, in ERα-positive human lung cancer cells, the proliferative action of thyroid hormone initiated at the plasma membrane is at least in part mediated by ERα. In summary, thyroid hormone may be one of several endogenous factors capable of supporting proliferation of lung cancer cells. Activity as an inhibitor of lung cancer cell proliferation induced at the integrin receptor makes tetrac a novel anti-proliferative agent.

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