Integrin alpha(V)beta(3) is a heterodimeric plasma membrane protein whose several extracellular matrix protein ligands contain an RGD recognition sequence. This study identifies integrin alpha(V)beta(3) as a cell surface receptor for thyroid hormone [L-T(4) (T(4))] and as the initiation site for T(4)-induced activation of intracellular signaling cascades. Integrin alpha(V)beta(3) dissociably binds radiolabeled T(4) with high affinity, and this binding is displaced by tetraiodothyroacetic acid, alpha(V)beta(3) antibodies, and an integrin RGD recognition site peptide. CV-1 cells lack nuclear thyroid hormone receptor, but express plasma membrane alpha(V)beta(3); treatment of these cells with physiological concentrations of T(4) activates the MAPK pathway, an effect inhibited by tetraiodothyroacetic acid, RGD peptide, and alpha(V)beta(3) antibodies. Inhibitors of T(4) binding to the integrin also block the MAPK-mediated proangiogenic action of T(4). T(4)-induced phosphorylation of MAPK is inhibited by small interfering RNA knockdown of alpha(V) and beta(3). These findings suggest that T(4) binds to alpha(V)beta(3) near the RGD recognition site and show that hormone-binding to alpha(V)beta(3) has physiological consequences.

,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.

The stilbene resveratrol (RV) initiates p53-dependent apoptosis via plasma membrane integrin aVb3 in human cancer cells. A thyroid hormone (L-thyroxine, T 4 ) membrane receptor also exists on aVb3. Stilbene and T 4 signals are both transduced by extracellular-regulated kinases 1 and 2 (ERK1/2); however, T 4 promotes cell proliferation in cancer cells, whereas RV is proapoptotic. Thyroid hormone has been shown to interfere with RV-induced apoptosis. However, the mechanisms involved are not fully understood. In this study, we examined the mechanism whereby T 4 inhibits RV-induced apoptosis in glioma cells. RV activated conventional protein kinase C and ERK1/2 and caused nuclear localization of cyclooxygenase-2 (COX-2), consequent p53 phosphorylation and apoptosis. RV-induced ERK1/2 activation is involved in not only COX-2 expression but also nuclear COX-2 accumulation. NS-398, a COX-2 inhibitor, did not affect ERK1/2 activation, but reduced the nuclear abundance of COX-2 protein and the formation of complexes of nuclear COX-2 and activated ERK1/2 that are required for p53-dependent apoptosis in RVtreated cells. T 4 inhibited RV-induced nuclear COX-2 and cytosolic pro-apoptotic protein, BcLx-s, accumulation. Furthermore, T 4 inhibited RV-induced apoptosis by interfering with the interaction of nuclear COX-2 and ERK1/2. This effect of T 4 was prevented by tetraiodothyroacetic acid (tetrac), an inhibitor of the binding of thyroid hormone to its integrin receptor. Tetrac did not, in the absence of T 4 , affect induction of apoptosis by RV. Thus, the receptor sites on aVb3 for RV and thyroid hormone are discrete and activate ERK1/2-dependent downstream effects on apoptosis that are distinctive.