Thrombospondin mediates calcium mobilization in fibroblasts via its Arg- Gly-Asp and carboxyl-terminal domains

Abstract

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 αvβ3 as demonstrated by inhibitor studies using anti-αvβ3 antibody and RGD- containing peptides. A non-RGD and non-αvβ3 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 ~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.