Identification of prolactin-induced transcription factors in the mammary gland

a number of biological processes in vertebrates, including reproduction, growth and development, osmoregulation, metabolism and immunoregulation (Nicoll, 1974). The biological actions of prolactin have been most extensively characterized in the mammary gland, where it has an essential role both in the differentiation of the gland during pregnancy and in the regulation of milk protein gene expression (Houdebine et al., 1985). The molecular cloning of the rat prolactin receptor identified a single gene that encodes predominantly two classes of polypeptide, a long form (591 amino acids) and a short form (291 amino acids) that differ in the size of their intracellular domains (Kelly et al., 1992). Significantly, although both forms have a similar capacity to bind prolactin, only the longer form receptor can stimulate transcription from a prolactin responsive promoter. A mutant form of the rat prolactin receptor, cloned from the prolactin-dependent pre-T lymphoma Nb2 cell line, has a deletion within the cytoplasmic domain and retains the ability to stimulate prolactin-induced gene expression (Kelly et al., 1992). These various forms of the prolactin receptor have provided important experimental tools with which to begin mapping regions of the intracellular domains that are essential for prolactin signalling (Lebrun et al., 1995). Analysis of the prolactin receptor polypeptide sequence revealed that the receptor is a member of the cytokine receptor superfamily (Taniguchi, 1995). This family includes the receptor for growth hormone (GH), to which the prolactin receptor is most closely related, the haematopoietic receptors, interleukin (IL)-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-9, erythropoietin (EPO), granulocyte colony-stimulating factor (G-CSF), granulocyte– macrophage CSF (GM-CSF), leukaemia inhibitory factor (LIF), oncostatin M (OSM), ciliary neurotrophic factor (CNTF) and the more distantly related interferon (IFN) receptors. The extracellular domains of these receptors all have conserved motifs including a tryptophan-serine (WSXWS) sequence and four cysteine residues. The intracellular domains are not well conserved, with the exception of a membrane-proximal cytoplasmic region comprising the box 1 and box 2 motifs, which are required for the mitogenic function of the receptors. In common with the other cytokine receptors, the prolactin receptor has no detectable intrinsic kinase activity, and yet activation by prolactin is associated with tyrosine phosphorylation of both the receptor and other intracellular substrates (Lebrun et al., 1995). Establishing the connections between the activation of the prolactin receptor, tyrosine phosphorylation and the induction of gene expression has only been elucidated in the past 3 years through the convergence of studies on transcriptional regulation of prolactin responsive genes and on cytokine signalling pathways.