Coexpression of the sis and myc proto-oncogenes in developing human placenta suggests autocrine control of trophoblast growth.

First trimester human placentas actively express the sis proto-oncogene, the structural gene for the B chain of platelet-derived growth factor (PDGF). Using the in situ hybridization technique, the 4.2 kb c-sis transcript has been localized to the cytotrophoblastic component, especially the highly proliferative and invasive cytotrophoblastic shell, paralleling the distribution of c-myc transcripts in early placenta. Explants of first trimester placenta release significant levels of PDGF-like activity into the medium under apparent developmental control. Moreover, cultured trophoblasts display abundant high-affinity PDGF receptors and respond to exogenous authentic PDGF by an activation of the c-myc gene and DNA synthesis. The developing human placenta may therefore represent a case of autocrine growth regulation in a normal tissue, in which cells bearing receptors for a growth factor can also synthesize and respond to that factor.