| Summary |
In the hamster, serum total lactogenic activity increases during the latter half of gestation (Days 8-16). On Days 10 and 12 a substantial amount of lactogenic activity cannot be attributed to prolactin and hamster placental lactogen-II (haPL-II); therefore, the presence of a molecule similar to placental lactogen-I (PL-I) as found in the rat and mouse has been hypothesized for the hamster. The objectives for this study were to obtain the nucleotide sequence of this placental molecule, determine the levels of expression throughout gestation, and establish which cells synthesize and secrete this molecule. Preliminary Northern analysis indicated that a mouse PL-I cDNA did not hybridize to total hamster placental RNA. Subsequently, a 21 bp oligonucleotide derived from a region of nucleotide homology for mPL-I and rPL-I was used in 3' RACE methodology. A 444 bp cDNA fragment that had nucleotide sequence similarity with members of the prolactingrowth hormone gene family was generated. This cDNA fragment was utilized to screen a Day 16 placental bacteriophage cDNA library, and a clone containing the entire coding region was identified and sequenced. The molecule had 77% nucleotide sequence homology with mouse proliferin-related protein (mPRP) and somewhat less homology (-60%) with hamster, rat and mouse prolactin or placental lactogens. As a result, this molecule will be tentatively referred to as hamster proliferin-related protein (haPRP). The deduced amino acid sequence of this molecule contained a 15 residue signal sequence and a 219 residue peptide with a calculated molecular weight of 25,477 daltons. The peptide shared 58% amino acid sequence identity with mPRP. Temporal expression of haPRP mRNA during the latter half of gestation was evaluated by Northern and slot blot analysis using the 444 bp cDNA fragment as a hybridization probe. A l-kb transcript was first detected on Day 9. Expression increased between Days 9 and 10, remained high on Day 11, and decreased on Day 12. Low levels of expression were detected on Day 14, but the l-kb transcript was not detected on Day 15. Interestingly, a larger (-1.2 kb) transcript was detected on Day 15. Immunocytochemistry, as well as preliminary in situ hybridization studies, indicate that cytotrophoblast cells of the trophospongium are responsible for production of haPRP. The nucleotide sequence and site of synthesis of the identified molecule suggest that it may be the hamster homologue of mouse proliferin-related protein. Although the temporal expression of this molecule parallels previously detected serum lactogenic activity in pregnant hamsters, further study is required to determine if it has lactogenic activity. |
