Platelet-derived growth factor: three isoforms and two receptor types.

Platelet-derived growth factor (PDGF) is a dimeric molecule that occurs as homodimers or heterodimers of related polypeptide chains. Recent data indicate that the isoforms have different functional activities because they bind with different affinities to two distinct receptor types. The activation of at least one of the PDGF receptor types involves receptor dimerization. Furthermore, there are indications that cells respond to PDGF in vivo only when they have been previously stimulated to express the corresponding receptor.