Prolactin-like protein-B: heterologous expression and characterization of placental and decidual species

Prolactin-like protein-B (PLP-B) is a member of a family of proteins expressed by the rat placenta and/or decidua with characteristics similar to prolactin (PRL). In this report, we present the heterologous expression and characterization of PLP-B. Recombinant PLP-B heterologously expressed in Chinese hamster ovary cells exhibited similar immunoreactive and electrophoretic characteristics with PLP-B produced by rat placental and decidual tissues. N-terminal sequencing verified the identity and purity of the recombinant PLP-B species and the site of cleavage of the signal peptide from the mature secreted PLP-B species. Polyclonal antibodies were generated to the recombinant PLP-B and used for Western blot and immunocytochemical analyses. Recombinant and native PLP-B migrated as a doublet at 30-31 kDa in SDS-PAGE under reducing conditions. Treatment of recombinant and native PLP-B with N-glycanase accelerated their electrophoretic mobility, indicative of their glycoprotein nature. PLP-B was localized exclusively to decidual cells in the developing deciduum and spongiotrophoblast cells in the placental junctional zone. The level of PLP-B protein expression dramatically declined prior to parturition. Potential PRL-like biological actions of PLP-B were also investigated. PLP-B bound weakly to ovarian and liver PRL receptors and did not stimulate the proliferation of lactogen-dependent Nb2 lymphoma cells. In conclusion, recombinant PLP-B possesses characteristics similar to native decidual and placental PLP-B and may represent a hormone/cytokine that has important modulatory actions during the establishment of pregnancy and the initiation of parturition.