Studies on the placental PRL-like molecules and their target cells

胎盘作为一个多功能器官，在维持妊娠和胎儿生长方面发挥作用。大鼠胎盘产生胎盘催乳素（PRL）家族的几个成员，其结构与垂体PRL相似。迄今为止，已克隆了胎盘PRL家族的十多个成员，并且根据其结构和功能将其分为两个亚家族：胎盘催乳素（PL）亚家族和类似蛋白（PLP）亚家族。PRL通过PRL受体（PRL-R）发挥其生物学功能。PL亚家族成员也能够结合PRL-R并刺激Nb 2细胞的增殖。PLP亚家族的一些成员不与PRL-R结合，也不刺激催乳活性，即使它们在结构上与PRL相似，这意味着可能存在PRL家族成员的未知特异性受体。为了进一步了解PRL家族成员生物活性表达的机制，我们首先评估了重组PL-Im的生物活性，通过杆状病毒/昆虫细胞表达系统产生的（recPL-Im）recPL-Im不仅显示出结合PRL-R和表达PRL样活性的能力，如Nb 2细胞增殖测定所证明的，而且在体外也显示出促黄体激素（LH）样活性。然后，我们评估了N-连接的寡糖在recPL-Im的分子作用中的作用，发现PL-Im结合PRL受体的能力与N-糖基化无关，尽管它似乎与Nb 2细胞增殖试验中表达PRL样活性有关。这些结果提出了recPL-Im也可能通过除PRL-R之外的未鉴定的受体系统起作用的可能性。本研究还对大鼠胎盘特异性细胞因子/激素的cDNA进行了克隆，并分离到一个新的PLP亚家族成员PLP-H和一个新的分泌蛋白SSP。

The placenta acts as a multi-function organ in the maintenance of pregnancy and in fetal growth. Rat placenta produces several members of the placental prolactin (PRL) family that show structural similarities to pituitary PRL. To date more than ten members of the placental PRL family have been cloned, and they are categorized into two subfamilies according to their Structure and function; the placental lactogen (PL) subfamily and like protein (PLP) subfamily. PRL exerts its biological function through the PRL receptor (PRL-R). PL subfamily members are also able to bind to the PRL-R and to stimulate the proliferation of Nb2 cells. Some members of PLP sub family do not bind to PRL-R nor do they stimulate lactogenic activity even though they are structura11 similar to PRL, implying that there may be yet unknown specific receptors for the PRL family members. To further understand the mechanisms underlying the expression of biological activities of PRL family members, we first evaluated the biological activity of recombinant PL-Im (recPL-Im) produced by means of a baculovirus/insect cell expression system The recPL-Im showed not only an ability to bind to PRL-R and express PRL-like activity, demonstrated by the Nb2 cell proliferation assay, but also showed luteotropic hormone (LH)-like activity in vitro. We then evaluated the role of N-linked oligosaccharides in the mo1ecular action of recPL-Im and found that the ability of PL-Im to bind PRL receptor is independent of the N-glycosylation although it appeared to be relevant for expressing PRL-like activity in the Nb2 cell proliferation assay. These results raised the possibility that recPL-Im may also act through unidentified receptor system besides the PRL-R. In this study we also performed of cDNAs for rat placenta-specific cytokines/ hormones, and isolated a novel member of PLP subfamily, named PLP-H, as well as a novel secretory protein, SSP.