Isolation and characterization of phosphorylated bovine prolactin

Molecular form identification of anterior pituitary gland-secreted prolactin in chicken

Endocrine changes during bird reproduction are well documented. Prolactin (PRL) exhibits a strong relationship between incubation and broody behavior. The molecular forms of PRL in the anterior pituitary gland during the reproductive cycle have already been previously identified but not those in the secreted form. To identify the molecular forms of secreted PRL during the reproductive cycle, we thus monitored the physiological status and incubation behavior of 10 Silkie hens by a video recording system over 1-2 years. Nine out of ten mature hens exhibited incubation behavior multiple times during the experiment. Ten hens demonstrated two interesting features. In a typical clutch, hens spent 10-15 min in the nest to lay an egg. Once they spent over 1 h in the nest, the nest occupancy increased incrementally. This shift in the nest occupancy occurred 7-10 days before the incubation onset and was highly repeatable. Based on the behavior of the hens, we cultured the anterior pituitary gland during four stages (premature non-laying, laying, trans, and incubation) with physiological PRL-releasing factor, vasoactive intestinal peptide (VIP). Based on our two-dimensional protein analysis, glycosylated PRL (G-PRL) displayed several isoforms with varying isoelectric points (pI), whereas we could detect one primary signal for non-glycosylated PRL (NG-PRL). However, 3-4 NG-PRL isoforms were detected in the anterior pituitary gland. These results suggested that secreted PRL, especially from the trans and incubation stages, contains various isoforms and it is post-translationally glycosylated and phosphorylated.

Estrogen induces phosphorylation of prolactin through p21-activated kinase 2 activation in the mouse pituitary gland

A phosphorylated prolactin is a kind of modified prolactin, which is produced through phosphorylation of native prolactin (PRL) by p21-activated kinase 2 (PAK2) in secretory granules in lactotrophs. Phosphorylated prolactin is involved in the regulation of the estrous cycle and in apoptosis in cancer cells, which seem to have important physiological and pathological roles, respectively. In previous research, it has been reported that estrogen induced the phosphorylation of prolactin in the mouse pituitary gland. However, the relationship between estrogen and PAK2 in the production of phosphorylated PRL has not been clarified yet. In order to examine whether PAK2 is involved in PRL phosphorylation by estrogen, we analyzed PAK2 protein levels in mice and phosphorylated prolactin levels in mouse pituitary cells by western blot analysis. The ratio of phosphorylated PAK2/total PAK2 was increased in estrogen implanted mice, but PAK2 protein and gene expression levels were decreased. In addition, the ratio of phosphorylated prolactin/non-phosphorylated prolactin was decreased in primary pituitary cells with introduced siPAK2. These findings suggest that estrogen could induce the phosphorylation of PRL through PAK2 activation. Therefore, this study contributes to better understanding of the mechanism of phosphorylated PRL production in physiological and pathological conditions associated with estrogen.

Effects of an S84E Mutation of Bovine Growth Hormone in Transgenic Mice

The ability of mutant bovine growth hormones (bGH) to serve as either agonist or antagonist has been demonstrated in transgenic mice. We have prepared two transgenic strains of FVB/N mice, one expressing wild-type bGH and a second with a glutamic acid mutation at serine 84 in helix 2. Comparison of their phenotypes to those of nontransgenic littermates indicates that wild-type bGH induces a previously described phenotype for hyper-somatotrophic mice. In contrast, the replacement of the side chain hydroxyl at serine 84 with acetic acid produced a phenotype that expressed bGH at appreciable concentrations, but failed to elicit the phenotype observed with either an agonist or an antagonist of bGH. These results indicate that serine 84 is crucial for the activity of bGH despite this site being distal to the receptor binding surfaces.

Molecular mechanisms of prolactin and its receptor

Prolactin and the prolactin receptors are members of a family of hormone/receptor pairs which include GH, erythropoietin, and other ligand/receptor pairs. The mechanisms of these ligand/receptor pairs have broad similarities, including general structures, ligand/receptor stoichiometries, and activation of several common signaling pathways. But significant variations in the structural and mechanistic details are present among these hormones and their type 1 receptors. The prolactin receptor is particularly interesting because it can be activated by three sequence-diverse human hormones: prolactin, GH, and placental lactogen. This system offers a unique opportunity to compare the detailed molecular mechanisms of these related hormone/receptor pairs. This review critically evaluates selected literature that informs these mechanisms, compares the mechanisms of the three lactogenic hormones, compares the mechanism with those of other class 1 ligand/receptor pairs, and identifies information that will be required to resolve mechanistic ambiguities. The literature describes distinct mechanistic differences between the three lactogenic hormones and their interaction with the prolactin receptor and describes more significant differences between the mechanisms by which other related ligands interact with and activate their receptors.

Efficient secretion of small proteins in mammalian cells relies on Sec62-dependent posttranslational translocation

Mammalian cells secrete a large number of small proteins, but their mode of translocation into the endoplasmic reticulum is not fully understood. Cotranslational translocation was expected to be inefficient due to the small time window for signal sequence recognition by the signal recognition particle (SRP). Impairing the SRP pathway and reducing cellular levels of the translocon component Sec62 by RNA interference, we found an alternate, Sec62-dependent translocation path in mammalian cells required for the efficient translocation of small proteins with N-terminal signal sequences. The Sec62-dependent translocation occurs posttranslationally via the Sec61 translocon and requires ATP. We classified preproteins into three groups: 1) those that comprise ≤100 amino acids are strongly dependent on Sec62 for efficient translocation; 2) those in the size range of 120-160 amino acids use the SRP pathway, albeit inefficiently, and therefore rely on Sec62 for efficient translocation; and 3) those larger than 160 amino acids depend on the SRP pathway to preserve a transient translocation competence independent of Sec62. Thus, unlike in yeast, the Sec62-dependent translocation pathway in mammalian cells serves mainly as a fail-safe mechanism to ensure efficient secretion of small proteins and provides cells with an opportunity to regulate secretion of small proteins independent of the SRP pathway.

Changes in post-translational modifications of prolactin during development and reproductive cycles in the chicken

Changes in proportion of glycosylated prolactin in the anterior pituitary glands of chickens were assessed using one- and two-dimensional western blotting analysis during the perihatch stage of embryos and reproductive cycles. Multiple isoforms of prolactin were detected by one-dimensional analysis and glycosylated (G) and non-glycosylated (NG) isoforms were identified by N-glycosidase and neuraminidase treatment. Increases of ratio of G to NG isoforms were observed in both embryonic stages and reproductive cycles by the one-dimensional analysis. Although a similar tendency of increase of proportion of G prolactin was obtained, different values of proportion were observed between one-dimensional and two-dimensional analysis. Since two-dimensional analysis may better resolve isoforms differing slightly in molecular size of G prolactin, the results from two-dimensional analysis may reflect the actual proportion of prolactin isoforms. Furthermore, isoforms differing in isoelectric points were detected after N-glycosidase and neuraminidase treatment. These results indicate that prolactin may also be additionally post-translationally modified such as by phosphorylation. Thus function and biological activity of prolactin were, at least in part, regulated by post-translational modification in the various physiological stages.

Paradigm-shifters: phosphorylated prolactin and short prolactin receptors

Since the discovery of physiologically-regulated prolactin (PRL) phosphorylation, one focus of the laboratory has been an examination of the different functions of the unmodified and phosphorylated hormone. In the mammary gland, unmodified PRL promotes growth activities, whereas phosphorylated or pseudophosphorylated PRL antagonizes this while also being a superior agonist for changes that favor differentiation. Phosphorylated PRL also increases expression of the short forms of the PRL receptor. These short forms of the receptor have functions beyond the accepted dominant negative and in mammary epithelial cells are capable of generating an intracellular signal leading to increased tight junction formation and beta-casein expression.

S179D prolactin: antagonistic agony!

The aims of this review are three-fold: first, to collate what is known about the production and activities of phosphorylated prolactin (PRL), the latter largely, but not exclusively, as illustrated through the use of the molecular mimic, S179D PRL; second, to apply this and related knowledge to produce an updated model of prolactin-receptor interactions that may apply to other members of this cytokine super-family; and third, to promote a shift in the current paradigm for the development of clinically important growth antagonists. This third aim explains the title since, based on results with S179D PRL, it is proposed that agents which signal to antagonistic ends may be better therapeutics than pure antagonists-hence antagonistic agony. Since S179D PRL is not a pure antagonist, we have proposed the term selective prolactin receptor modulator (SPeRM) for this and like molecules.

Current methods for phosphoprotein isolation and enrichment

The phosphorylation of proteins is a central paradigm of signal transduction. The substitution of neutral hydroxyl groups of serine, threonine and tyrosine with a negatively charged phosphate group alters the physicochemical and immunogenic properties of the protein, which then can be used to isolate these isoforms. In the last decades several different techniques were applied, attempting to selectively enrich protein populations with this post-translational modification. This review aims to give an overview on the arsenal of available methods to extract phosphoproteins focusing on chromatographic approaches.

The effect of estrogen on phosphorylation of prolactin in the mouse pituitary gland

Several studies have indicated that prolactin (PRL) assumes oligomeric, proteolytically cleaved, phosphorylated and glycosylated forms. Phosphorylated PRL (PPRL) is considered to be the most important posttranslationally modified form in the rat. In the present study, we examined whether or not PRL is present in the mouse pituitary gland in the phosphorylated form. Mouse pituitary PRL was digested with acid phosphatase, resolved by two-dimensional gel electrophoresis, stained with Coomassie brilliant blue, and then immunoblotted against the anti-PRL, anti-phosphoserine and anti-phosphothreonine antibodies. We also examined whether PRL is phosphorylated by protein kinases and semi-quantified the ratios of PPRL to PRL in the pituitary gland. The results indicated that three types of PRL are present in the pituitary glands of both male and female mice. One was non-phosphorylated (isoform 1), and the other two were immunoreactive to anti-phosphoserine (isoform 2) and/or anti-phosphothreonine (isoform 3) antibodies. The ratio between isoforms 2 and 1 of the 30-day-old female mice was higher than that of the 20-day-old female mice. However, the ratios among the three isoforms in the male pituitary glands did not differ with age. The ratio of PPRL to isoform 1 was obviously reduced after ovariectomy (OVX), and it recovered with estrogen replacement. These results suggest that estrogen influences PRL phosphorylation in female mice.

Immunoglobulin G subclasses and prolactin (PRL) isoforms in macroprolactinemia due to anti-PRL autoantibodies

Although macroprolactinemia due to antiprolactin (anti-PRL) autoantibodies is not uncommon among hyperprolactinemic patients, the pathogenesis of such macroprolactinemia is still unknown. We examined IgG subclasses of anti-PRL autoantibodies by enzyme immunoassay, and PRL phosphorylation and isoforms by Western blotting, mass spectrometry, and two-dimensional electrophoresis in six patients with anti-PRL autoantibodies and in 29 controls. PRL-specific IgG subclasses in patients with anti-PRL autoantibodies were heterogeneous, but five of six patients showed IgG4 predominance, which is known to be produced by chronic antigen stimulation. Western blot and mass spectrometric analyses revealed that human pituitary PRL was phosphorylated at serine 194 and serine 163, whereas serine 163 in serum PRL was dephosphorylated. On two-dimensional electrophoresis, serum PRL mainly consisted of isoform with isoelectric point (pI) 6.58 in control hyperprolactinemic patients, whereas acidic isoforms (pIs 6.43 and 6.29) were also observed in patients with anti-PRL autoantibodies. Our data first demonstrate that human pituitary PRL is serine phosphorylated and partially dephosphorylated in serum, and suggest that the acidic isoforms may give rise to chronic antigen stimulation in patients with anti-PRL autoantibodies.

Functional relationship of serine 90 phosphorylation and the surrounding putative salt bridge in bovine prolactin

The biological activity of bovine prolactin (PRL) is reduced by in vivo phosphorylation of serine 90 (S90) that is located within a putative N+4 salt bridge (R89 and D93). We substituted hydrophobic, polar, or acidic residues for S90 and/or replaced members of the putative R89/D93 salt bridge to determine if a functional relationship between the putative salt bridge and the phosphorylation could be observed. At position 90 the bulk of the residue was the most important factor in modulating biological activity in either the rat Nb2 cell bioassay or PRL receptor binding. Charge played a smaller role. Replacement of either partner of the salt bridge reduced both biological and binding activities indicating the presence of a salt bridge at this position. The combination of replacing a salt bridge member and substituting glutamic acid at S90 produced greater than additive changes in our experimental endpoints, indicating a functional coupling between the salt bridge and phosphorylation site. We interpret the data to indicate that either in vivo phosphorylation or specific mutations that destabilize the salt bridge impairs biological activity.

The tertiary structure and backbone dynamics of human prolactin

Human prolactin is a 199-residue (23 kDa) protein closely related to growth hormone and placental lactogen with properties and functions resembling both a hormone and a cytokine. As a traditional hormone, prolactin is produced by lactotrophic cells in the pituitary and secreted into the bloodstream where it acts distally to regulate reproduction and promote lactation. Pituitary cells store prolactin in secretory granules organized around large prolactin aggregates, which are produced within the trans layer of the Golgi complex. Extrapituitary prolactin is synthesized by a wide variety of cells but is not stored in secretory granules. Extrapituitary prolactin displays immunomodulatory activities and acts as a growth factor for cancers of the breast, prostate and tissues of the female reproductive system. We have determined the tertiary structure of human prolactin using three-dimensional (3D) and four-dimensional (4D) heteronuclear NMR spectroscopy. As expected, prolactin adopts an "up-up-down-down" four-helical bundle topology and resembles other members of the family of hematopoietic cytokines. Prolactin displays three discrete structural differences from growth hormone: (1) a structured N-terminal loop in contact with the first helix, (2) a missing mini-helix in the loop between the first and second helices, and (3) a shorter loop between the second and third helices lacking the perpendicular mini-helix observed in growth hormone. Residues necessary for functional binding to the prolactin receptor are clustered on the prolactin surface in a position similar to growth hormone. The backbone dynamics of prolactin were investigated by analysis of 15N NMR relaxation phenomena and demonstrated a rigid four-helical bundle with relatively mobile interconnecting loops. Comparison of global macromolecular tumbling at 0.1mM and 1.0mM prolactin revealed reversible oligomerization, which was correlated to dynamic light scattering experiments. The existence of a reversible oligomerization reaction in solution provides insight into previous results describing the in vitro and in vivo aggregation properties of human prolactin.

S179D-human PRL, a pseudophosphorylated human PRL analog, is an agonist and not an antagonist

For many years, our group has been involved in the development of human PRL antagonists. In two recent publications, S179D-human PRL, a human PRL analog designed to mimic a putative S179-phosphorylated human PRL, was reported to be a highly potent antagonist of human PRL-induced proliferation and signaling in rat Nb2 cells. We prepared this analog with the aim of testing it in various bioassays involving the homologous, human PRL receptor. In our hands, S179D- human PRL was able to stimulate 1) the proliferation of rat Nb2 cells and of human mammary tumor epithelial cells (T-47D), 2) transcriptional activation of the lactogenic hormone response element-luciferase reporter gene, and 3) activation of the Janus kinase/signal transducer and activator of transcription and MAPK pathways. Using the previously characterized antagonist G129R-human PRL as a control, we failed to observe any evidence for antagonism of S179D-human PRL toward any of the human PRL-induced effects analyzed, including cell proliferation, transcriptional activation, and signaling. In conclusion, our data argue that S179D-human PRL is an agonist displaying slightly reduced affinity and activity due to local alteration of receptor binding site 1, and that the antagonistic properties previously attributed to S179D-human PRL cannot be confirmed in any of the assays analyzed in this study.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Stanniocalcin 1 and 2 are secreted as phosphoproteins from human fibrosarcoma cells

Stanniocalcin 1 (STC1) and stanniocalcin 2 (STC2) are two recently identified mammalian peptide hormones. STC1 plays a role in calcium and phosphate homoeostasis, while the role of STC2 is unknown. We examined a human fibrosarcoma cell line, HT1080, that has high steady-state STC1 and STC2 mRNA levels, to determine whether these proteins are secreted. Following incubation of HT1080 cells with (32)P, labelled STC1 and STC2 were found to be secreted into the medium. STC1 was phosphorylated in vitro by protein kinase C (PKC). In vitro and in vivo phosphorylation both occurred exclusively on serine and the phosphopeptide maps were similar, suggesting that PKC might be the in vivo kinase. STC2 was phosphorylated in vitro by casein kinase II (CK2), in vitro and in vivo phosphorylation were exclusively on serine and the phosphopeptide maps were indistinguishable. Phosphorylation of STC2 in intact cells resulted from the action of an ecto-protein kinase, since exogenous STC2 was phosphorylated by HT1080 cells and no phosphorylated STC2 was detectable inside the cells. The ectokinase activity was abolished by heparin and GTP could substitute for ATP as the phosphate donor, indicative of an ecto-CK2-like activity. The in vitro CK2 phosphorylation site was shown by matrix-assisted laser-desorption ionization-time-of-flight MS to be a single serine located between Ser-285 and Ser-298 in the C-terminal region of STC2. This is the first report of the secretion of STC1 or STC2 from mammalian cells. We conclude that these human fibrosarcoma cells express both STC1 and STC2 as secreted phosphoproteins in vivo, with STC2 being phosphorylated by an ecto-CK2-like enzyme.

Proteolysis of human prolactin: resistance to cathepsin D and formation of a nonangiostatic, C-terminal 16K fragment by thrombin

The N-terminal 16K fragments of rat and human PRLs possess angiostatic activity. 16K PRL has also been detected in vivo in both humans and rats. Based on an in vitro study, cathepsin D, an acid protease, has been implicated in the generation of rat 16K PRL. However, the proteolytic cleavage of human PRL has not been demonstrated. Our objective was to identify an enzyme that is capable of forming an angiostatic human 16K PRL. To confirm the angiostatic action of rat 16K PRL, the fragment was generated by incubating 23K PRL with rat mammary microsomal fraction at pH 3.2. Upon incubation with human umbilical vein endothelial cells (HUVEC), rat 16K PRL, but not 23K PRL, inhibited basal- and basic fibroblast growth factor-stimulated cell proliferation. Intact rat and human PRLs were then incubated with cathepsin D or acidified microsomal pellets of MCF-7 human breast cancer cells. Analysis by SDS-PAGE showed cleavage of rat, but not human, PRL. Next, hormones were incubated with thrombin at pH 7.4. As shown by SDS-PAGE, digestion of both human and rat PRL by thrombin resulted in the formation of 16K fragments. PRL contained within human amniotic fluid was also cleaved by thrombin. Enzyme specificity was supported by prevention of cleavage by the thrombin inhibitor hirudin. When tested with HUVEC, the human 16K PRL was devoid of angiostatic activity. The activity of this fragment in the Nb2 lymphoma bioassay was 10- to 15-fold lower than that of 23K PRL. Mass spectrometry revealed that the fragment has a mass of 16,878.30+/-15.8 Daltons. Subsequent N-terminal sequencing showed that the thrombin cleavage occurred between amino acid residues 53 (Lys) and 54 (Ala), resulting in the formation of a C-terminal, not an N-terminal, 16K fragment. We conclude that, unlike rat PRL, human PRL is resistant to cleavage by cathepsin D. Thrombin at a physiological pH can generate a C-terminal 16K fragment of human PRL that is not angiostatic and retains little mitogenic activity. We suggest that the precise nature of endogenous 16K PRL fragments that are present in human tissues and body fluids should be carefully examined.

GH kinase activity in bovine anterior pituitary subcellular fractions

Growth hormone (GH) and prolactin (PRL) share significant structural homology. We have previously characterized the phosphorylation of bovine PRL and wish to determine whether a similar kinase activity phosphorylates bovine GH. Phosphorylation of bovine GH was performed using [alpha-32P]ATP labeling of subcellular fractions. Bovine GH phosphorylation was dependent on Zn2+ or Cu2+ with apparent Km's of 0.9 and 1.0 mM, respectively, and a pH maxima of 7.0. The apparent Km's of bovine GH kinase activity for exogenous bovine GH and ATP were 30 microM and 376 microM, respectively. Exogenous bovine PRL served as a competitive substrate, increasing the apparent Km for bovine GH by threefold compared to the Km determined without exogenous bovine PRL. We conclude: 1) in vitro phosphorylation of bovine GH occurs under conditions that are consistent with those found in anterior pituitary cells, and 2) a similar kinase activity phosphorylates both bovine PRL and GH.

Prolactin kinase activity in bovine anterior pituitary sub-cellular fractions

Bovine anterior pituitary cells phosphorylate prolactin (PRL). We describe the phosphorylation of endogenous and exogenous bPRL in highly enriched subcellular fractions of bovine anterior pituitary using [gamma-32P]-ATP. 32P-labeling of endogenous and exogenous bPRL occurred in all subcellular membrane fractions, but most significantly in the fraction enriched for secretory granules. Zn2+ (0.8 mM), Cu2+ (0.8 mM), and Mn2+ (9.8 mM) increased bPRL phosphorylation by 268, 214, and 154%, respectively, relative to basal phosphorylation with no added cations. Neither Mg2+ (10 mM) nor Ca2+ (0.9 mM) increased bPRL phosphorylation above basal levels. Phosphorylation was dependent on the concentration of Zn2+ with an apparent Km of 570 microM. bPRL phosphorylation occurred over a wide pH range of 5.9-8.3, with the greatest activity at pH of 6.7 or greater. Phosphorylation of bPRL was time-dependent. The apparent Kms of the bPRL kinase for exogenous bPRL and ATP were 15.3 and 267 microM, respectively. bPRL incorporation of 32P was unaffected by the presence of calcium and calmodulin, cAMP, phosphotidylserine and diolein, or spermine. From these results we conclude that in vitro phosphorylation of bPRL occurs under physiological conditions that would be found in pituitary cells.

Phosphorylation of bovine prolactin eliminates luteotropic activity in the rat

Phosphorylated and nonphosphorylated prolactins were isolated from bovine pituitaries and their luteotropic activity determined in female rats. Three groups of rats in day 1 of diestrus were treated i.p. twice daily for three days with 0.25 mg of either prolactin preparation or vehicle. Rats were sacrificed each day of treatment. Serum progesterone concentrations of the groups receiving vehicle or phosphorylated prolactin were similar and the vaginal cytology of these animals indicated that phosphorylated bovine prolactin (bPRL) treatment had not prolonged diestrus. Treatment with nonphosphorylated bPRL significantly increased serum progesterone concentration and the vaginal cytology indicated a diestrus prolonged for up to 4 days. Nonphosphorylated and phosphorylated bPRLs were cleared from the blood at similar rates after i.p. injection. In vitro receptor binding studies demonstrated that phosphorylated bPRL did not bind the ovarian prolactin receptor. Nonphosphorylated, but not phosphorylated, bPRL competed with radiolabeled bovine hormone for occupancy of rat ovarian prolactin receptors. These data are the first to test the activities of phosphorylated bPRL in vivo and indicate; 1) nonphosphorylated bPRL is luteotropic, 2) phosphorylated bPRL is neither luteotropic nor a prolactin receptor agonist or antagonist and 3) phosphorylated bPRL is not dephosphorylated in vivo rapidly enough to provide sufficient biologically-active bPRL to maintain luteal function.

Development of recombinant human prolactin receptor antagonists by molecular mimicry of the phosphorylated hormone

Previous studies have demonstrated that naturally phosphorylated PRL antagonizes the growth-promoting effects of unmodified PRL in two different PRL-responsive cell lines. In this study our aim was to produce a molecular mimic of phosphorylated PRL by substituting a fairly bulky, negatively charged amino acid (glutamate or aspartate) for the normally phosphorylated serine [serine 179 in human PRL (hPRL)]. In addition, because of the marked effect of phosphorylation on biological activity, we investigated the importance of the unmodified serine in the growth-promoting activity of PRL. hPRL complementary DNA was obtained from the American Type Culture Collection and subcloned into pT7-SCII after site-directed mutagenesis using the deoxyuridine approach. Proteins were expressed in Escherichia coli BL21 (DE3) and were primarily found in inclusion bodies. Agonist and antagonist activities of each serine 179 mutant were assessed using the Nb2 bioassay. Compared with standard hPRL, the recombinant wild-type was more active in the Nb2 assay, attesting to both the absence, or low level, of endotoxin contamination in preparations from these cells and the appropriate folding of the molecule. The aspartate and glutamate mutants had no intrinsic agonist activity, but both antagonized the growth-promoting activity of wild-type PRL, with the aspartate mutant proving to be a very effective antagonist. Two hundred picograms per ml of the aspartate mutant negated 75% of the growth response to 400 pg/ml wild-type PRL. When serine 179 was mutated to alanine or valine, mutant PRLs with 0% and 14% of the biological activity of wild-type PRL, respectively, were produced. These results demonstrate 1) that molecular mimicry of the phosphorylated hormone does produce a PRL antagonist, and 2) that the serine at position 179 is crucial to the growth-promoting activity of PRL. The aspartate mutant can now be used to study many aspects of the physiology of PRL.

Mutation of serine 90 to glutamic acid mimics phosphorylation of bovine prolactin

Phosphorylated prolactin has been identified and isolated from bovine pituitaries. The biological activity of this phosphoprotein is severely reduced in comparison with nonphosphorylated prolactin. The sites of phosphorylation are serines 26, 34, and 90, and the stoichiometry is 1:1:10, respectively. In this report, the phosphoserine residues have been individually replaced with glutamic acid in recombinant methionyl bovine prolactins in order to mimic phosphorylation at each site. Substitution of glutamic acid for serine at positions 26, 34, and 90 reduced protein helical contents by 10, 6, and 14%, respectively. UV absorbances for S26E and S34E bovine prolactins were blue-shifted, similar to the biological isolates of phosphorylated bovine prolactin, but the biological activities of the S26E and S34E mutants (ED50 values of 16.3 and 18.8 pM, respectively) were similar to that of wild-type prolactin (ED50 value of 18.6 pM) in the Nb2 rat lymphoma assay. S90E bovine prolactin had the greatest reduction in helical content but showed similar UV and fluorescent spectra to the wild-type bovine prolactin. The biological activity of S90E bovine prolactin (ED50 value of 672 pM) was reduced to an activity similar to that of phosphorylated bovine prolactin. The data indicate that the phosphorylation of serine 90 is responsible for the reduction in biological activity.

Biological activity of phosphorylated and dephosphorylated bovine prolactin

Phosphorylation is a mechanism by which cells regulate structure and function of proteins. We have previously demonstrated in vivo synthesis and secretion of phosphorylated bovine prolactin (bPRL) from the pituitary, and have isolated and partially characterized the phosphorylated bPRL. In order to investigate the structure/function role of phosphorylation on the biological activity of bPRL, we compared the activities of nonphosphorylated and phosphorylated bPRL isolated from pituitaries, with bPRL provided by the NIDDK (NIDDK-bPRL) to stimulate Nb2 cell proliferation. Nonphosphorylated bPRL has activity similar to, although slightly lower than that of NIDDK bPRL (ED50 = 7.03 pM and 22.8 pM, respectively). The activity of phosphorylated bPRL is significantly reduced (ED50 = 1066 pM). Using Nb2 lymphoma cell homogenate, NIDDK and nonphosphorylated bPRLs are equally effective in competitive receptor binding assays (Kd = 0.252 and 0.269 nM, respectively). Phosphorylated bPRL does not compete for the PRL receptor at concentrations up to 65 nM. Following enzymatic removal of the phosphate group using alkaline phosphatase, there is an increase in the biological activity of phosphorylated bPRL (ED50 = 73.3 pM) while the activity of nonphosphorylated BPRL remained unchanged following enzyme treatment (21.4 pM). We conclude that (1) structural changes induced by phosphorylation of bPRL are responsible for loss of bioactivity, (2) dephosphorylation of phosphorylated bPRL restores biological activity, and (3) the reduction in biological activity of phosphorylated bPRL is mediated by a decrease in receptor binding.

Phosphorylated and nonphosphorylated Prolactin isoforms

Specific posttranslational modifications o f monomer PRL produce a number of hormone variants. This review describes the current state of knowledge for the nonphosphorylated and phosphorylated 24-kD monomer forms. Of particular interest and significance for our future understanding of PRL endocrinology are the studies showing their differential release in response to different physiologic signals and their different activities in target tissues. In at least two target tissues, the monophosphorylated variant seems to act as an antagonist to the unmodified hormone.

Preparative purification of phosphorylated and nonphosphorylated bovine prolactins

Extracts from 12 pituitaries were applied to preparative immunoaffinity chromatography columns containing between 500 mg and 800 mg of clonal antiprolactin IgG1. Extracts were cycled through the column and subsequently washed from the affinity matrix. Total prolactin was eluted from the affinity matrix by a 0.1 M glycine, pH 2.8. The total prolactin was neutralized and dialyzed into 20 mM Bis-Tris (pH 7.0) and applied to a weak anion exchange column. Nonphosphorylated prolactin was eluted with 0.5 M NaCl in Bis-Tris buffer. Phosphorylated prolactin was eluted with 20% acetic acid. Yields averaged 19.6 mg and 11.7 mg for native and phosphorylated prolactin (n = 14 runs), respectively. The molecular weights of the prolactins were similar to prolactin standards. The amino acid compositions of the two prolactin variants were similar. Phosphate was associated predominantly with the phosphorylated prolactin. Native prolactin was biologically active in the Nb2 rat lymphoma bioassay, while the phosphorylated prolactin was without activity in this bioassay.