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 ca n modulate cell attachment, migration, and proliferation. We now show that intact soluble thrombospondin causes a transient [Ca2+](i) increa se in IMR-90 fibroblasts. This [Ca2+](i) increase is mediated partly b y the RGD containing domain of thrombospondin that binds to the integr in alpha v beta 3 as demonstrated by inhibitor studies using anti-alph a v beta 3 antibody and RGD-containing peptides. A non-RGD and non-alp ha v beta 3 component of this [Ca2+](i) increase is mediated by the ca rboxyl-terminal domain of thrombospondin through an unidentified recep tor on fibroblasts as shown by the antibody to the carboxyl terminal o f thrombospondin, C6.7. In addition, the carboxyl terminal derived pep tide, RFYVVMWK, also triggers [Ca2+](i) increase in similar to 35% of fibroblasts. Both EGTA and Ni2+ block the entire [Ca2+](i) increase in dicating that this is due to an influx of extracellular Ca2+. B6H12, a n antibody to the integrin-associated protein, blocks this [Ca2+](i) i ncrease by 50%, suggesting that some of the Ca2+ might be entering thr ough an integrin-associated calcium channel. The current findings demo nstrate that multiple domains on thrombospondin can trigger signal tra nsduction events by increasing [Ca2+](i) through their interactions wi th different cell receptors.