Previously, we demonstrated that a placental hormone, PRL-like protein E, stimulates megakaryocyte growth and differentiation. We now find that PRL-like protein E and a second placental hormone, PRL-like protein F (PLP-F), bind the same receptor. PLP-F, which is produced later in pregnancy, might therefore act as either an agonist or antagonist of PRL-like protein E. To resolve this question, we produced recombinant PLP-F in mammalian cell cultures, purified the secreted glycoprotein hormone, and determined its activity in primary mouse bone marrow cultures. PLP-F induces megakaryocyte differentiation and megakaryocyte progenitor growth in a dose-dependent manner, with significant activity detected at a concentration as low as 50 ng/ml. PLP-F in maternal serum reaches at least 1 μg/ml on gestational d 14.5, and thus the biological activity of PLP-F is detected at physiological concentrations. These results show that PRL-like proteins E and F have the same stimulatory effects on megakaryocyte growth and differentiation, and therefore represent gestation stage-specific agonists.
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