The Prolactin/Growth Hormone Receptor Family

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Abstract
I. Introduction PROLACTIN (PRL), growth hormone (GH), and placental lactogen (PL), or chorionic somatomamotropin (CS) form a family of polypeptide hormones, which, based on amino acid sequence homology, are reported to have arisen by duplication of an ancestral gene (1). Determination of the structure of the genes encoding PRL, GH, and PL has confirmed this theory. PRL and GH are produced by cells of the anterior pituitary gland and are found in all vertebrates, whereas PL or CS is synthesized by the placenta. The initial step in the mechanism of action of PRL and GH is the binding of the respective ligand to a cell surface receptor. Hormone receptors were originally defined as molecules with low capacity and high affinity and specificity for a ligand, the binding of which resulted in a biological response. Many tissues that are not known targets for these hormones contain measurable binding. A number of studies that have appeared in the literature have described PRL binding, without any direct association with a biological response. The fact that ligand binding occurs suggests, but does not necessarily imply, receptor function. The most likely explanation is that PRL and GH cause effects in these tissues that have not yet been determined.

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