Endocrine Abstracts

T3 and T4 rapidly activate intracellular signalling cascades via thyroid hormone receptor (TR)-independent actions, suggesting the existence of a plasma membrane receptor. Recent studies support a role for the RGD recognition site of integrin, a transmembrane glycoprotein, as a cell surface TR. We have demonstrated, using PCR and Western blotting, the expression of integrin α_Vβ₃ mRNA and protein in the transformed human osteosarcoma cell line MG63. Treatment of these cells with T3 (10⁻⁶–10⁻⁹ M) or T4 (10⁻⁷–10⁻⁸ M) for 10 min stimulated extra cellular signal-regulated kinase activity (ERK, a component of the MAPK pathway) as determined by fluorescent immunocytochemistry or an immunocomplex activity assay. Pre-treatment for 30 min with the specific MAPK kinase (MEK) inhibitor, U0126 (10⁻⁶ M), or an anti-integrin α_Vβ₃ antibody inhibited MAPK activation. T3 (10⁻⁸ M) and T4 (10⁻⁷ M) significantly stimulated DNA incorporation into MG63 cells by 2.3±0.66 – fold and 2.1±0.1 – fold respectively. To establish whether transient MAPK activation mediates these effects, MG63 cells were pre-treated for 30 min without or with U0126 (10⁻⁶ M), or an anti-integrin α_Vβ₃ antibody. Both treatments significantly inhibited T3-stimulated proliferation by 60% and T4-stimulated proliferation by 30%. Thus our results suggest that T3 and T4 rapidly stimulate ERK activation in MG63 cells and that one of the functional effects of this ERK activation is increased DNA synthesis and cell proliferation. In addition, this study identifies integrin α_Vβ₃ as a cell surface receptor for T3 and T4 and as the initiation site for T3 and T4-induced activation of MAP kinase and cell proliferation in human osteoblast-like cells.