A role for PKCθ in outside-in αIIbβ3 signaling

Fibrinogen binding to platelets triggers alpha(IIb)beta(3)-dependent outside-in signals that promote actin rearrangements and cell spreading. Studies with chemical inhibitors or activators have implicated protein kinase C (PKC) in alpha(IIb)beta(3) function. However, the role of individual PKC isoforms is poorly understood. Biochemical and genetic approaches were used to determine whether PKC theta is involved in alpha(IIb)beta(3) signaling. PKC theta was constitutively associated with alpha(IIb)beta(3) in human and murine platelets. Fibrinogen binding to alpha(IIb)beta(3) stimulated the association of PKC theta with tyrosine kinases Btk and Syk, and tyrosine phosphorylation of PKC theta, Btk and the actin regulator, Wiskott-Aldrich syndrome protein (WASP). Mouse platelets deficient in PKC theta or Btk failed to spread on fibrinogen. Furthermore, PKC theta was required for phosphorylation of WASP-interacting protein on Ser-488, an event that has been linked to WASP activation of the Arp2/3 complex and actin polymerization in lymphocytes. Neither PKC theta nor Btk were required for agonist-induced inside-out signaling and fibrinogen binding to alpha(IIb)beta(3). Thus, PKC theta is a newly identified, essential member of a dynamic outside-in signaling complex that includes Btk and that couples alpha(IIb)beta(3) to the actin cytoskeleton.