THROMBOSPONDIN MEDIATES CALCIUM MOBILIZATION IN FIBROBLASTS VIA ITS ARG-GLY-ASP AND CARBOXYL-TERMINAL DOMAINS

Thrombospondin is a matrix glycoprotein found in various cells that can modulate cell attachment, migration, and proliferation. We now show that intact soluble thrombospondin causes a transient [Ca2+](i) increase in IMR-90 fibroblasts. This [Ca2+](i) increase is mediated partly by the RGD containing domain of thrombospondin that binds to the integrin alpha v beta 3 as demonstrated by inhibitor studies using anti-alpha v beta 3 antibody and RGD-containing peptides. A non-RGD and non-alpha v beta 3 component of this [Ca2+](i) increase is mediated by the carboxyl-terminal domain of thrombospondin through an unidentified receptor on fibroblasts as shown by the antibody to the carboxyl terminal of thrombospondin, C6.7. In addition, the carboxyl terminal derived peptide, RFYVVMWK, also triggers [Ca2+](i) increase in similar to 35% of fibroblasts. Both EGTA and Ni2+ block the entire [Ca2+](i) increase indicating that this is due to an influx of extracellular Ca2+. B6H12, an antibody to the integrin-associated protein, blocks this [Ca2+](i) increase by 50%, suggesting that some of the Ca2+ might be entering through an integrin-associated calcium channel. The current findings demonstrate that multiple domains on thrombospondin can trigger signal transduction events by increasing [Ca2+](i) through their interactions with different cell receptors.