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 approximately 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.