A third prolactin-like protein expressed by the developing rat placenta: complementary deoxyribonucleic acid sequence and partial structure of the gene.

We have previously reported on the isolation and characterization of two PRL-related cDNA clones, rat placental lactogen II and rat PRL-like protein A which are among the more abundant mRNAs expressed int he late term rat placenta. In this paper we report the isolation and characterization of cDNA clones of a third abundant placental protein, the predicted amino acid sequence of which shows a 44% homology to rPRL. This protein which we have called rPRL-like protein B (rPLP-B) is different from, but related to, the other PRL related proteins which have been identified in the rat, mouse, and bovine placentas. Two rPLP-B mRNA transcripts of 0.9 and 1.2 kilobases are strongly expressed in essentially equal amounts from day 14 of pregnancy until term. Nucleotide sequence and hybrid select translation data predict one secreted, potentially glycosylated protein of approximately mol wt 27,000. Hybridization and primer extension studies show that the two transcripts differ in their 5'-untranslated regions. One of the cDNA clones isolated represents a portion of the unprocessed rPLP-B mRNA. All intron/exon boundaries in this clone are of the same splice class and occur in identical locations within the coding region as in the rPRL gene.

Isolation and characterization of a complementary DNA (galanin) clone from estrogen-induced pituitary tumor messenger RNA.

The administration of high levels of estrogen is a well established method for producing prolactin-secreting pituitary tumors in rodents but the mechanism of tumor induction is not clear. In this paper we describe a cDNA clone (pEIC) which has been isolated from an estrogen-induced pituitary tumor cDNA library. The mRNA transcript corresponding to the pEIC clone is 0.9 kilobase in length and is not detectable in normal pituitaries but is expressed as early as 3 h after estrogen stimulation. Nucleotide sequence analysis of two 700-base pair recombinant clones shows that they encode a 124-amino acid protein which is 70% identical to the porcine galanin precursor. The sequence of 29 amino acid residues coded for by the pEIC cDNA clone is 88% identical with porcine galanin with only three amino acid substitutions near the C terminus. This extensive homology suggests that the pEIC cDNA clone codes for rat galanin or a protein belonging to the galanin gene family. These results provide the first evidence of a physiological regulator (estrogen) of the expression of the galanin gene. They also imply that galanin is secreted by prolactin-secreting tumors. Because intracerebroventricular injection of galanin can stimulate prolactin secretion and galanin inhibits hypothalamic dopamine release, it is conceivable that galanin may play a role in the induction of prolactin-secreting tumors.

Isolation of a novel prolactin-like cDNA clone from developing rat placenta.

The mid- to late-term rat placenta produces several moderately abundant proteins in a specific temporal manner, one of which we have identified as rat placental lactogen II (rPLII) (Duckworth, M. L., Kirk, K. L., and Friesen, H. G. (1986) J. Biol. Chem. 261, 10871-10878). In this paper, we describe the isolation and characterization of cDNA clones of another of these proteins, called rat prolactin-like protein A (rPLP-A) because of its homology with the prolactins. The single mRNA transcript corresponding to rPLP-A is 1 kilo-base in length and first appears at Day 14 of pregnancy, 2 days later than rPLII mRNA, and then increases and remains at high levels until term. The mRNA hybridizing to rPLP-A cDNA clones translates in vitro to a protein of 25,000 daltons which is processed by dog pancreatic microsomes to 27,000 daltons. The amino acid sequence deduced from the nucleotide sequence of the rPLP-A clones suggests that rPLP-A is a secreted protein of 196 amino acids with two potential glycosylation signals. The sequence is more than 40% homologous at the amino acid level to rat and human prolactins and rPLII. This study provides clear evidence for the existence of considerable amounts of undescribed prolactin-like proteins in late term rat placenta.