Roles of prolactin and related members of the prolactin/growth hormone/placental lactogen family in angiogenesis.
Mitogen-regulated proteins (also known as proliferin; mrp/plf) are nonclassical members of the PRL/GH family. They are expressed at high levels during midgestation when they are thought to induce angiogenesis and uterine growth. There are between four and six mrp/plf genes, and three different complementary DNAs have been cloned. Here we identify a fourth mrp/plf gene (mrp4) that we have cloned and characterized. MRP4 is 91% identical in amino acid sequence with the other MRP/PLF proteins but is missing two glycosylation sites that are present in the other forms. Consistent with the loss of two of three glycosylation sites, the expressed form of MRP4 has a lower apparent molecular weight compared with other MRP/PLFs. In vivo, mrp4 is expressed in the placenta and the adult skin. Expression of mrp4 messenger RNA peaks in the placenta on day 12. In the skin, mrp4 expression is specific to the ears and tails of mice. Our results suggest that, as well as having growth and angiogenic effects during pregnancy, the MRP/PLFs may have functions in nonreproductive tissues. Unique among the members of the mrp/plf family for its expression in the hair follicles of the tail and ear, MRP4 is expected to have a singular role in the growth and development of these follicles.