Insulin-like Growth Factor Binding Protein-3 is Present in the Alpha-granules of Platelets

The lysis of the alpha-granules of platelets has been shown to release insulin-like growth factors (IGFs). One of the major regulators of the activity of IGFs is the IGF binding proteins (IGFBPs), but IGFBPs were thought to be absent from platelet alpha-granules. Human platelets were isolated, and the contents of the alpha-granules were released by thrombin digestion. IGFBP-3 was identified in the lysate by RIA and a ligand-binding Western blot; no other IGFBPs were observed. Platelet IGFBP-3 was present as the characteristic glycosylated doublet at 45 and 43 kilodaltons on ligand-binding Western blot, but was not associated with the acid-labile subunit in the alpha-granule. Thus, the IGFs in platelet alpha-granules coexist with IGFBP-3, which can play a major role in regulating and targeting the actions of IGFs when released by lysis.