TGF-β1 calcium signaling in osteoblasts

Transforming growth factor-β1 (TGF-β1) action is known to be initiated by its binding to multiple cell surface receptors containing serine/threonine kinase domains that act to stimulate a cascade of signaling events in a variety of cell types. We have previously shown that TGF-β1 and BMP-2 treatment of primary human osteoblasts (HOBs) enhances cell-substrate adhesion. In this report, we demonstrate that TGF-β1 elicits a rapid, transient, and oscillatory rise in the intracellular Ca²⁺ concentration, [Ca²⁺]_i, that is necessary for enhancement of cell adhesion in HOBs but does not alter the phosphorylation state of Smad proteins. This rise in [Ca²⁺]_i in HOB is not observed in the absence of extracellular calcium or when the cells are treated with the L-type Ca²⁺ channel blocker, nifedipine, but is stimulated upon treatment with the L-type Ca²⁺ channel agonist, Bay K 8644, or under high K⁺ conditions. The rise in [Ca²⁺]_i is severely attenuated after treatment of the cells with thapsigargin, a selective endoplasmic reticulum Ca²⁺ pump inhibitor. TGF-β1 enhancement of HOB adhesion to tissue culture polystyrene is also inhibited in cells treated with nifedipine. These data suggest that intracellular Ca²⁺ signaling is an important second messenger of the TGF-β1 signal transduction pathway in osteoblast function.