Glycosylation of ovine prolactin during cell-free biosynthesis

Changes in levels of immunoreactive prolactin isoforms during a reproductive cycle in turkey hens

Changes in the ratio between immunoreactive isoforms of prolactin using Western blotting and in the total prolactin content using radioimmunoassay were measured in pituitary glands from turkey hens at different physiological stages. The type of glycosylation (N- or O-linked carbohydrates) was determined using endoglycosidase digestion (N-glycosidase F, O-glycosidase, and neuraminidase). Low levels of prolactin were observed in pituitary glands from sexually immature, out-of-lay, and molting hens. Higher levels were present during the egg-laying period and the highest levels were detected in hens which expressed incubation behavior. Two immunoreactive bands of apparent molecular weights of 24 and 27 kDa were visualized on Western blots, corresponding to the nonglycosylated and glycosylated forms of prolactin, respectively. In pituitary glands from incubating turkey hens, about 70% of the prolactin was glycosylated (27-kDa isoforms), whereas about 60% was glycosylated in immature and in hens during the first egg-laying period. In pituitaries from out-of-lay and molting hens the percentage of glycosylated prolactin was 38 and 33%, respectively. Thus, higher percentages of glycosylated isoforms (27 kDa) were associated with high levels of total prolactin and lower percentages were associated with low levels of prolactin content in the pituitary gland. Digestion of the isoforms with N-glycosidase F resulted in a single band with an apparent molecular weight of 24 kDa. Partial deglycosylation was achieved using neuraminidase, whereas digestion with O-glycosidase had no apparent effect on the isoforms. Thus it appears that the glycosylated isoforms of prolactin have N-linked carbohydrates containing sialic acid.

Isolation and biochemical properties of four forms of glycosylated porcine prolactin

Four isoforms of glycosylated prolactin (G-pPRL) were isolated from porcine pituitary glands by affinity chromatography and concanavalin A-Sepharose, based upon differences in their affinity for the lectin. Structural analysis indicated differences in the carbohydrate units of the four G-pPRLs. N-glycanase treatment cleaved the oligosaccharide from the G-pPRLs, establishing N-linked glycosylation. The binding of G-pPRLs to receptors from lactating rabbit mammary glands was only 3-8% that of nonglycosylated pPRL (NG-pPRL). The immunological crossreactivity of the G-pPRLs varied from 36 to 65% that of NG-pPRL. When tested in the pigeon crop sac bioassay, G-pPRLs were only 11-40% as active as NG-pPRL. The metabolic clearance rate of one of the G-pPRLs was slower and another faster than that of NG-pPRL. We conclude that there are several forms of G-PRL of variable immuno- and bio-potencies in the porcine pituitary, and that the current radioimmunoassay for the hormone does not measure the actual bioactivity.

Phosphorylated variant of bovine prolactin

Bovine pituitary explants and cell cultures were incubated with [32P]orthophosphate. Extracts were prepared from the explants and analyzed by sodium dodecyl sulfate-containing acrylamide gel electrophoresis and autoradiography revealing a phosphoprotein that co-migrated with authentic bovine prolactin. Clonal antibodies to bovine prolactin were produced, purified and used to prepare affinity columns. Extracts of [32P]orthophosphate-labeled explants and cells or media were applied to prolactin affinity columns and a radiolabeled protein was eluted with a pH 2.8 wash. The eluted protein was identified as prolactin by co-migration with standard on gel electrophoresis and by amino acid analysis. Treatment of immunoaffinity-purified pituitary prolactin with alkaline phosphatase reduced the phosphate associated with prolactin in a time-dependent manner, indicating a covalent phosphate linkage. Autoradiography of gels revealed prolactin from explants, cells and their associated media to be a phosphoprotein. A phosphorylated variant of bovine prolactin is synthesized and secreted in both explant and cell cultures.

Cloning and nucleotide sequence of ovine prolactin cDNA

A cDNA expression library was constructed in the lambda gt 11 phage vector using ovine (o) pituitary mRNA. The clone, pOP1, carrying a 934-bp insert contains an open reading frame beginning with the first nucleotide (nt) and ending with the stop codon TAA at nt position 781. Two potential translation start codons (ATGs) are present in the 5' region of this cDNA. Translation initiation could occur at the 5' proximal ATG at nt position 61. The nucleotide sequence around this ATG (TCCATGG), resembles the optimum sequence context for translation initiation by the eukaryotic ribosomes, as defined by mutational analysis [Kozak, Cell 44 (1986) 283-292)], with its substitution of the A at -3 of the consensus sequence by a T residue in this clone. Translation initiated at this codon could potentially code for the entire pre-prolactin (pre-PRL) molecule. The 3'-untranslated region is 154 nt long and contains a polyadenylation signal AATAAA. The deduced amino acid sequence agrees in totality with the published amino acid sequence of the mature hormone. The present study reports on the nucleotide sequence of o-PRL mRNA and the deduced amino acid sequence in the signal peptide of the hormone.

Monoclonal antibodies specific for non-glycosylated porcine prolactin and for pituitary porcine prolactin

Glycosylated and non-glycosylated forms of porcine pituitary prolactin were prepared using Concanavalin A Sepharose chromatography. Anti-prolactin monoclonal antibodies were screened for their ability to distinguish these two forms. One monoclonal antibody (17D9) exhibited high affinity binding for the non-glycosylated form of porcine prolactin, but little or no affinity for the glycosylated form. Using this antibody in conjunction with other monoclonals which equally recognize both forms, we developed immunoassays which can be used to determine the amount of the glycosylated vs. non-glycosylated prolactin in serum or other tissue samples.

Glycosylated prolactin in the murine pituitary: detection by a novel assay and alteration of concentrations by physiological and pharmacological stimuli

In both rat and mouse pituitary extracts, we detected concanavalin A-binding prolactin immunoreactivity by a lectin-binding radioimmunoassay developed recently. The activity increased in response to estradiol benzoate treatment and lactation, stimuli that augment prolactin secretion, and decreased in response to acute nursing and perphenzine administration, stimuli that cause massive release of prolactin. Western blot analysis revealed a prolactin-immunoreactive band 2,000-3,000 greater in Mr than the main prolactin band that bound to 125I-labeled concanavalin A. These results suggest the existence in the murine adenohypophysis of a glycosylated form of prolactin, which seems to be released under certain physiological states.

Detection of multiple PRL- and GH-like proteins in human pituitary by Western blot analysis

Surgically removed normal and tumorous pituitary tissues from a prolactinoma patient were analyzed by Western blot techniques for PRL and GH variants. Criteria for identification were the Rf of the bands within the gel, immunologic crossreactivity to specific antisera, and structural verification by tyrosine peptide-mapping of individual bands from the gel. The authors found the tumor-tissue to be characterized by the presence of a PRL band greater in concentration than in the normal tissue and the virtual absence of a GH band. Immunoblotting of the electrophoretically resolved proteins form both types of tissues revealed several new bands crossreactive with human PRL and GH antibodies. Some of the new bands were of Mr greater than the monomeric PRL and GH and others were of lower Mr. Relative of two of the low mobility Mr PRL-immunoreactive bands designated as 16K and 8K corresponded to the Rf of the two fragments of cleaved PRL, which suggested that cleaved PRL occurred naturally in the human pituitary gland. The most conspicuous of the new PRL-immunoreactive bands, a 25,000 Mr protein migrating slightly behind PRL, displayed strong crossreactivity to hPRL antibodies and was present in greater concentration in the prolactinoma tissue than in the normal tissue. These properties suggested that it was related to hPRL and perhaps represented its glycosylated variant. However, its tyrosine peptide map did not resemble that of hPRL. Thus, it is not clear whether it represented G-hPRL or a new pituitary protein that cross-reacts with hPRL antibodies. In addition, two other bands of low Mr, designated as unknown 1 and unknown 2, reacted with hPRL antibodies. Immunostaining with hGH antibodies revealed the 20K-hGH variant, the F1 fragment of cleaved hGH, and a pair of new bands immediately behind GH that could represent glycosylated hGH--possibly a product of Seeburg's variant hGH gene. Both PRL and GH antibodies elicited numerous bands of high Mr by the technique employed, far more than ever observed by Sephadex chromatography. The nature of the high Mr bands remains unknown. Further characterization of these new PRL- and GH-immunoreactive proteins might help in the understanding of the multiple physiologic functions of PRL and GH in man.

Detection and enzymatic deglycosylation of a glycosylated variant of prolactin in human plasma

Immunoperoxidase electrophoresis was applied to the plasma of a patient showing a high level of prolactin (PRL) secreted by a pituitary adenoma. Two PRL monomers were detected with an anti-hPRL antiserum: a major 22 kDa form and a minor 25 kDa form. Concanavalin A-Sepharose 4B chromatography revealed that the 25 kDa form was a glycosylated variant of PRL. Incubation of this variant with endoglycosidase F led to its transformation into the 22 kDa form.

Cloning and sequence analysis of cDNA for mouse prolactin

The present study was undertaken to find out whether or not sexual dimorphism in biological activities and amino acid compositions of mouse prolactin might be due to heterogeneity in mRNA for mouse prolactin Cloned cDNAs for mouse prolactin were first isolated from a mouse pituitary cDNA library by hybridization with a rat prolactin cDNA. Then, one clone of about 140 positive clones obtained from 2000 transformants was subjected to nucleotide sequence analysis and verified to contain a nearly full length of cDNA sequence coding for mouse prolactin precursor. The deduced complete amino acid sequence indicates that the precursor molecule consists of 31 amino acids as the signal peptide and 197 amino acids of prolactin, in which two amino acids were found to be different from the amino acid sequence previously published elsewhere. S1 nuclease mapping analysis using male and female pituitary RNAs indicates that mouse preprolactin is encoded by two mRNAs in both sexes. The two mRNAs differ from each other based upon the deletion of three nucleotides in the coding region for the signal peptide determined by the nucleotide sequence analysis in other cDNA clones. In the present study, no sexual difference was revealed in murine prolactin mRNA.

Specific and reversible degranulation of pituitary mammotrophs

When dissociated anterior pituitary cells were cultured over a layer of gelatin, electron microscopic observation showed the mammotrophs in the culture to be essentially degranulated and to have massively dilated rough endoplasmic reticulum (RER) and Golgi apparatus (GA), while there were no discernible effects on any of the other cell types. PRL synthesis in the gelatin-cultured cells occurred at twice the rate of that in the control cells and the gelatin-cultured cells contained 3% of the normal amount of PRL. Transfer of the cells to a more traditional culture situation reversed most of the morphological changes within 24 h. The mammotrophs were responsive to thyrotropin-releasing hormone (10(-7) M) both immediately after gelatin culture (3-fold control) and after reversal (5-fold control). As placement of the gelatin-cultured cells into traditional culture synchronizes the onset of granulation, we conclude that mammotrophs cultured in this fashion will be useful in the further study of PRL synthesis, packaging, storage and degradation.