Proliferin secreted by cultured cells binds to mannose 6-phosphate receptors

Proliferin is a prolactin-related glycoprotein secreted by proliferating mouse cell lines and by mouse placenta. In an attempt to identify target sites for proliferin action, we looked for proliferin receptors in murine fetal and maternal tissues during pregnancy using proliferin purified from the conditioned medium of a constructed Chinese hamster ovary cell line carrying amplified copies of proliferin cDNA. Purified proliferin bound to membrane preparations from fetal or maternal liver and from placenta with a Kd of 1 to 2 nM. The amount of proliferin bound per microgram of membrane protein varied markedly during pregnancy; maximal binding to day 16 fetal liver membranes was approximately 25 times that to liver membranes from adult animals. Binding to fetal and maternal receptors was specifically and completely inhibited by mannose 6-phosphate, with half-maximal inhibition at 10 microM. Furthermore, non-glycosylated proliferin did not inhibit the binding of the glycosylated protein. A approximately 300 Kd proliferin receptor was purified from the liver of pregnant mice using a proliferin affinity column and elution with mannose 6-phosphate. This receptor reacted with antibodies directed against the rat cation-independent mannose 6-phosphate receptor. We conclude that 1) proliferin secreted by cultured cell binds to cation-independent mannose 6-phosphate receptors and therefore may be a lysosomal protein or targeted to lysosomes, and 2) the concentration or activity of mannose 6-phosphate receptors in murine fetal and maternal liver and in placenta is regulated during pregnancy.