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

Lin H, Sun M, Tang H, Lin C, Luidens MK, Mousa SA, Incerpi S, Drusano GL, Davis FB, Davis PJ. L-Thyroxine vs. 3,5,3 '-triiodo-L-thyronine and cell proliferation: activation of mitogen-activated protein kinase and phosphatidylinositol 3-kinase. Am J Physiol Cell Physiol 296: C980-C991, 2009. First published January 21, 2009; doi:10.1152/ajpcell.00305.2008.-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 alpha(v)beta(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-alpha (TR alpha) from cytoplasm to nucleus and accumulation of hypoxia-inducible factor (HIF)-1 alpha mRNA; LY-294002 inhibited these actions. Results of studies involving alpha(v)beta(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 TR alpha trafficking and HIF-1 alpha 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.