Human Platelet - Derived Mitogens . II . Subcellular Localization of Insulinlike Growth Factor I to the a - Granule and Release in Response to Thrombin By Kenneth

Platelets contain mitogenic activities for MCF-7 human breast cancer cells when assayed under serum-free chemically defined conditions. Purification from outdated human platelets identified insulinlike growth factor I (IGF-I) as the most potent breast cancer cell mitogen in lysates (Karey KP, Sirbasku DA: see accompanying article, this issue). In this study the release and subcellular localization of IGF-l was investigated. Degranulation of platelets by thrombin treatment caused release of lysosomal enzymes (/ -gIucuronidase and N-acetyl-D-glucosaminidase). a-granule proteins (fl-thromboglobulin and fibrinogen) as well as mitogenic activity for MCF-7 cells and IGF-l as measured by radioimmunoassay (RIA) and radioreceptor assay. Release of mitogenic activity and immunologically identified IGF-l