Human platelets were extracted with acid-ethanol and platelet-derived TGF-beta was purified from the extract by a two-column procedure using sequential gel filtration in the absenceand then presence of urea. Purified TGF-beta is a protein of 25,000-daltons, and it is comprised of two 12,500-dalton subunits held together by disulfide bonds. The purified factor elicits its biological activity at concentrations less than 4pM. Comparative studies showed that platelets contain 100 times more TGF-beta than do other non-neoplastic tissues. Platelets also contain a peptide growth factor related to EGF. These two new growth factors can interact mechanistically. Incubation of TGF-beta with NRK cells for 6 h results in an increased number of cell surface EGF receptors. IGF-II receptors are not affected. Shorter incubations with TGF-beta show that this peptide can also increase the Kd of the high affinity EGF receptor.
