Abstract
The aim of the present study was to examine whether triiodo-l-thyronine (T(3)) or l-thyroxine (T(4)) rapidly activated the mitogen-activated protein kinase (MAPK) intracellular signalling cascade in osteoblast-like cells and investigate whether this activation was initiated at the integrin alpha(V)beta(3) cell surface receptor. Using PCR and western blotting, the expression of integrin alpha(V)beta(3) mRNA and protein was demonstrated in the human osteoblast-like cell lines MG-63 and SaOS-2. The treatment of MG-63 cells with T(3) (10 nM) or T(4) (100 nM) for 10 min stimulated extracellular signal-regulated kinase activity (ERK, a component of the MAPK pathway) as determined by fluorescent immunocytochemistry and an immunocomplex activity assay (T(3) by 10.7-fold, P<0.01 and T(4) by 10.4-fold, P<0.01 compared with control). T(3) (10 nM) and T(4) (100 nM) also significantly stimulated thymidine incorporation into MG-63 cells by 2.3+/-0.7-fold (P<0.01) and 2.1+/-0.1-fold (P<0.05) respectively. To establish whether transient ERK activation via the integrin alpha(V)beta(3) cell surface receptor mediated these effects, MG-63 cells were pretreated for 30 min with the specific MAPK kinase inhibitor, U0126 (1 microM), or an anti-integrin alpha(V)beta(3)-blocking antibody. Both pretreatments significantly inhibited T(3)- and T(4)-stimulated ERK activation and abolished T(3)-stimulated thymidine incorporation (P<0.01). T(4)-stimulated incorporation was significantly inhibited from 2.1- to 1.3-fold above control (P<0.05). Thus, our results suggest that T(3) and T(4) rapidly stimulate ERK activation in MG-63 cells via integrin alpha(V)beta(3) and that one functional effect of this ERK activation is increased DNA synthesis.