RHOA AND THE FUNCTION OF PLATELET INTEGRIN ALPHA(IIB)BETA(3)

Integrins respond to ''inside-out'' signals, which enable them to bind adhesive ligands, and ligand binding initiates ''outside-in'' signals that mediate anchorage-dependent cellular responses. RhoA is a GTPase that regulates certain actin rearrangements and transcriptional event s. It has also been implicated in integrin signaling, but the exact re lationship is not understood. To examine this further, platelets were incubated with C3 exoenzyme to adenine diphosphate (ADP)-ribosylate an d inactivate RhoA, and the function of integrin alpha(IIb)beta(3) was studied. Despite inactivation of greater than or equal to 90% of RhoA, platelets exhibited normal inside-out signaling, as monitored by agon ist-induced binding of a fibrinogen-mimetic anti-alpha(IIb)beta(3) ant ibody and normal fibrinogen-dependent aggregation. On the other hand, RhoA inactivation decreased the adhesion of agonist-stimulated platele ts to fibrinogen (P < .04) and the formation of vinculin-rich focal ad hesions in platelets that did adhere (P < .001). These effects were se lective because fibrin clot retraction, a response also dependent on a lpha(IIb)beta(3) and actin contractility, was unaffected by C3, as was the content of F-actin in resting or agonist-stimulated platelets. Si milar results were obtained in a Chinese hamster ovary (CHO) cell mode l system of alpha(IIb)beta(3): C3 exoenzyme (or overexpression of domi nant-negative N19RhoA) failed to influence integrin activation state, but it blocked the formation of focal adhesions in cells spread on fib rinogen. These studies establish that RhoA plays a highly selective ro le in alpha(IIb)beta(3) signaling, and they identify a subset of respo nses to integrin ligation that may be uniquely dependent on the actin rearrangements regulated by this GTPase. (C) 1998 by The American Soci ety of Hematology.