Abstract
Mammary tumor cell growth factor(s) has been identified in extracts of platelets from both male and female rats, as well as in extracts prepared from pooled outdated human platelets. When assayed by the growth promotion of MTW9/PL rat mammary tumor cells in culture, platelet extracts alone were able to support growth 50β75% as well as whole serum. The mitogenic activity from crude human platelet lysates was shown to be trypsin sensitive, relatively stable to extremes of pH, labile to heat treatment at 70Β°, nonβdialysable, ammonium sulfate precipitable, not removed by 56Β° charcoal treatment, and of apparent molecular weight of 30,000 to 50,000 daltons as estimated by Gβ100 Sephadex chromatography. The platelet derived mammary growth factor activity was not replaced or potentiated by thrombin or known hormones and growth factors such as prolactin, insulin, 17βΞ²βestradiol, progesterone, hydrocortisone, Lβthyroxine, and mouse epidermal growth factor. The experimental report demonstrates that platelets are a rich source growth factor activity for rat epithelial mammary tumor cells, and that the activity appears to be a polypeptide(s) different from other mitogenic activities known to influence growth of mammary tissue.