Selective modification of recombinant bovine placental lactogen by site-directed mutagenesis at its C terminus

Radioimmunoassay of bovine placental lactogen using recombinant and native preparations: determination of fetal concentrations across gestation

Concentrations of bovine placental lactogen (bPL) were determined in fetal plasma samples by twelve double-antibody competitive radioimmunoassay systems (RIA I-XII) based on either recombinant bPL (non-glycosylated) or native bPL (glycosylated). Both preparations were used as standard and tracer, and for primary antisera production. The minimum detection limit measured by these RIA varied from 0.02 to 0.6 ng bPL mL(-1). The coefficients of correlation of different bPL RIA systems were up to 90% (P < 0.0001) when each RIA was tested against the average values of all twelve RIA systems. All developed RIA were used to investigate the incidence of different bPL isoforms in bovine fetal serum samples (n = 71). Fetal concentrations ranged from 11.8 to 35.7 ng mL(-1) at the third month and from 1.1 to 13.5 ng mL(-1) at the ninth month of gestation. They tended to decrease with advancing gestation. In general, those RIA systems that used recombinant bPL as the standard measured higher values than those using the native bPL preparation. These differences decreased toward the end of gestation (P < 0.05), suggesting a lower rate of glycosylation. Our results provide evidence of different glycosylated isoforms of bPL in fetal serum at different gestation periods.

Ovine placental lactogen-induced heterodimerization of ovine growth hormone and prolactin receptors in living cells is demonstrated by fluorescence resonance energy transfer microscopy and leads to prolonged phosphorylation of signal transducer and activator of transcription (STAT)1 and STAT3

HEK-293T cells transiently transfected with ovine (o) GH receptor (GHR) and prolactin receptor (PRLR) constructs respectively tagged downstream with cyan or yellow fluorescent proteins were used to study ovine placental lactogen (oPL)-stimulated heterodimerization by fluorescence resonance energy transfer (FRET) microscopy. The oPL-stimulated transient heterodimerization of GHR and PRLR had a peak occurring 2.5-3 min after oPL application, whereas oGH or oPRL had no effect at all. The results indicate none or only little dimerization occurring before the hormonal stimulation. The effect of heterodimerization was studied by comparing activation of Janus kinase 2, signal transducer and activator of transcription (STAT)1, STAT3, STAT5, and MAPK in Chinese hamster ovary cells stably transfected with chimeric genes encoding receptors consisting of cytosolic and transmembrane parts of oGHR and oPRLR, extracellular domains of human granulocyte and macrophage colony-stimulating factor (hGM-CSF) receptor alpha or beta, and cells transfected with the two forms (alpha or beta) of PRLR and GHR. Functionality of those proteins was verified by hGM-CSF-induced phosphorylation of both intracellular PRLR and GHR domains and hGM-CSF-induced heterodimerization was documented by chimeric receptor coimmunoprecipitation. Homodimerization or heterodimerization of PRLRs and GHRs had no differential effect on activation of STAT5 and MAPK. However, heterodimerization resulted in a prolonged phosphorylation of STAT1 and in particular STAT3, suggesting that the heterodimerization of alpha-oGHR and beta-oPRLR is able to transduce a signal, which is distinct from that occurring on homodimeric associations.

Mechanism of ruminant placental lactogen action: molecular and in vivo studies

Ruminant placental lactogens (PLs) are structurally related to prolactins (PRLs) and growth hormones (GHs) and are secreted by placentae. Ruminant PLs are unusual in their capacity to bind and activate PRL and GH receptors (Rs) from other species. The present minireview summarizes several works showing that unlike in heterologous species (rat, rabbit, human), in homologous (ruminant) species, PLs act by activating PRLRs or by heterodimerizing GHRs and PRLRs, and suggests that this may be the main mechanism of PL action in vivo. Mutations impairing the ability of ovine (o)PL or bovine (b)PL to form complexes with PRLRs (but not with GHRs) do not cause loss of biological activity, because the transient existence of the homodimeric complex is still sufficient to initiate the signal transduction; however, mutants do lose their ability to activate homologous PRLRs. To explain this difference, we proposed a novel term-minimal time of homodimer persistence-which assumes that to initiate the signal transduction, a "minimal time" of homodimer existence is required for transphosphorylation of associated JAK2s. In interactions between ruminant PLs and homologous PRLRs, this minimal time is met through the interaction with homologous PRLRs, which has a shorter half-life than with heterologous PRLRs. Thus oPL or bPL are active in cells possessing both homologous and heterologous PRLRs. Mutations of PLs decrease the affinity, shortening the "time of homodimer persistence." In heterologous interactions, the minimal time is still sufficient to initiate the biological activity, whereas in homologous interactions, which in any case are weaker, further destabilization of the complex shortens its persistence below the minimal time, causing loss of biological activity.

Functional heterodimerization of prolactin and growth hormone receptors by ovine placental lactogen

Although homo- or heterodimerization are common mechanisms for activation of cytokine receptors, cross-talk between two distinct receptors in this superfamily has been never shown. Here we show a physiologically relevant example indicating that such an interaction does occurs, thus raising the hypothesis that heterodimerization between distinct cytokine receptors may be a novel mechanism contributing to the diversity of cytokine signaling. These findings were documented using both surface plasmon resonance and gel filtration experiments and show that ovine placental lactogen (PL) heterodimerizes the extracellular domains (ECDs) of ruminant growth hormone receptor (GHR) and prolactin receptor (PRLR). We also show that PL or PL analogues that exhibit little or no activity in cells transfected with PRLRs and no activity in cells transfected with ovine GHRs exhibit largely enhanced activity in cells cotransfected with both PRLRs and GHRs. Furthermore, chimeric receptors consisting of cytosolic and transmembrane part of ovine GHR or ovine PRLR and ECDs of human granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR) alpha or beta were constructed. Upon transfection into Chinese hamster ovary cells along with reporter luciferase gene and stimulation by GM-CSF, a significant increase in luciferase activity occurred when GM-CSFR-alpha-PRLR and GM-CSFR-beta-GHR or GM-CSFR-alpha-GHR and GM-CSRR-beta-PRLR were cotransfected. In conclusion, we show that ovine PL is capable of functional heterodimerization of GHR and PRLR and that when their cytosolic parts, coupled to the ECD of GM-CSF receptors, are heterodimerized by GM-CSF, they are capable of transducing biological signal.

Ruminant placental lactogens act as antagonists to homologous growth hormone receptors and as agonists to human or rabbit growth hormone receptors

Growth hormone receptor (GHR)-mediated activity of ruminant placental lactogens (PLs) and ovine (o) GH was compared, using cells transfected with full size human (h), rabbit (rb), and oGHRs. All three PLs acted as agonists in heterologous bioassays, whereas in homologous bioassays in cells transfected with oGHRs they antagonized the oGH activity. Despite these differences, oGH and PLs bound with similar affinity to the oGHR extracellular domain (oGHR-ECD), indicating that the binding occurs through hormone site I. Gel filtration of complexes between oPL and oGHR-ECD showed a 1:1 stoichiometry, confirming this conclusion. The oPL T185D and bPL T188D, which exhibited weak biological activity mediated through GHRs, behaved as site I antagonists, whereas oPL G130R and bPL G133R formed a 1:1 complex with GHR-ECDs and bound to h/rb/oGHR-ECDs with affinity similar to that of wild-type oPL. They had no agonistic activity in all models transfected with h/rb and oGHRs, but were antagonistic to all of them. In conclusion, ruminant PLs antagonize the activity of oGH in homologous systems, because they cannot homodimerize oGHRs, whereas in heterologous systems they act as agonists. The structural analysis hints that minor differences in the sequence of the GHR-ECDs may account for this difference. Since the initial step in the activity transduced through cytokine/hemapoietic receptors family is receptor homodimerization or heterodimerization, we suggest that the question of homologous versus heterologous interactions should be reexamined.

Preparation of recombinant gilthead seabream (Sparus aurata) growth hormone and its use for stimulation of larvae growth by oral administration

Gilthead seabream (Sparus aurata) growth hormone (gsGH) cDNA coding for the mature protein was cloned in a pGEM-T vector and then transferred into prokaryotic expression vector pET-8 and expressed in E. coli BL21 (DE3) cells upon induction with IPTG. The expressed protein, contained within the inclusion-body pellet, was solubilized in 4.5 M urea, refolded at pH 11.3 in the presence of catalytic amounts of cysteine, and purified to over 98% purity, as evidenced by SDS-PAGE. Gel-filtration on a Superdex column under nondenaturing conditions and partial amino acid N-terminal sequence showed the purified protein to be a monomeric alanyl-gsGH. Over 90% pure bacterial beta-lactamase was copurified as a by-product. Binding assays of the [125I]gsGH to gs liver microsomal fraction resulted in high specific binding characterized by a Kd = 1.93 nM. Recombinant gsGH, like ovine placental lactogen, exhibited growth-stimulating activity when applied orally to S. aurata larvae or intraperitoneally to juvenile fish.

Novel recombinant analogues of bovine placental lactogen. G133K and G133R provide a tool to understand the difference between the action of prolactin and growth hormone receptors

Two new analogues of bovine placental lactogen (bPL), bPL(G133K) and bPL(G133R), were expressed in Escherichia coli, refolded, and purified to a native form. Binding experiments, which are likely to represent the binding to site 1 only, to intact FDC-P1 cells transfected with rabbit (rb) growth hormone receptor (GHR) or with human (h) GHR, to Nb2 rat lymphoma cells, or to rabbit mammary gland membranes prolactin receptor (PRLR), revealed only small or no reduction in binding capacity. The complex formation between these analogues and receptor extracellular domains (R-ECD) of various hormones was determined by gel filtration. Wild type bPL yielded 1:2 complex with hGHR-ECD, rat PRLR-ECD, and rbPRLR-ECD, whereas both analogues formed only 1:1 complexes with all R-ECDs tested. Real time kinetics experiments demonstrated that the ability of the analogues to form homodimeric complexes was compromised in both PRLR- and GHR-ECDs. The biological activity transduced through lactogenic receptors in in vitro bioassays in rabbit mammary gland acini culture and in Nb2 cells was almost fully retained, whereas the activity transduced through somatogenic receptors in FDC-P1 cells transfected with rbGHRs or with hGHRs was abolished. Both analogues exhibited antagonistic activity in the latter cells. To explain the discrepancy between the effect of the mutation on the signal transduced by PLR versus GHRs we suggest that: 1) the mutation impairs the ability of site 2 of bPL to form a stable homodimeric complex with both lactogenic and somatogenic receptors by a drastic shortening of the half-life of 2:1 complex; 2) the transient existence of the homodimeric complex is still sufficient to initiate the signal transduced through lactogenic receptors but not through somatogenic receptors; and 3) one possible reason for this difference is that JAK2, which serves as a mediator of both receptors, is already associated with lactogenic receptors prior to hormone binding-induced receptor dimerization, whereas in somatogenic receptors the JAK2 receptor association occurs subsequently to receptor dimerization.

Site-directed mutagenesis of recombinant bovine placental lactogen at lysine-73 leads to selective attenuation of its somatogenic activity

Bovine placental lactogen (bPL) is capable of binding and transducing biological activity via somatogenic and lactogenic receptors. To modify this capability, three analogs, bPL(K73D), bPL(K73F) and bPL(K73A), mutated at position 73, and corresponding to R64 in human GH (hGH), were produced in Escherichia coli. Circular dichroic spectrum analyses indicated proper refolding in all cases. Biological activity of these analogs was tested in vitro. In a lactogenic-receptor-mediated Nb2 rat lymphoma cell bioassay, bPL and its analogs acted similarly. In another lactogenic bioassay that measures beta-casein synthesis by HC-11 mouse mammary-gland cells, the analogs were 30-40% as potent as bPL. In contrast, somatogenic receptor-mediated bioactivity in FDC-P1 cells transfected with either rabbit (rb) or hGH receptor (R) was almost completely abolished in these analogs. In receptor binding assays, the effect was more conspicuous and the mutations affected not only somatogenic but also lactogenic binding. Binding to rat (r) and rabbit PRL receptor extracellular domains (ECDs) or membrane-embedded receptors was only slightly changed, except for bPL (K73D), which displayed very low affinity. In somatogenic binding assays to intact IM-9 human lymphocytes, hGHR-ECD or bovine liver membranes, bPL (K73D) did not bind at all, and bPL(K73F) or bPL(K73A) binding was drastically reduced. Binding experiments performed in real time using a BIAcore apparatus revealed that the decreased binding could be mainly attributed to increased k(off) rather than decreased k(on) values. The complex with hGHR-ECD revealed a 2:1 stoichiometry with bPL, bPL(K73F) and bPL(K73A), although the complex with these analogs was less stable than with bPL, whereas bPL(K73D) scarcely assembled a 1:1 complex. In contrast, bPL and the three analogs formed stable 1:2 complexes with rPRL-ECD. These results suggest that position 73 in bPL is more important for somatogenic than lactogenic properties and concurs with results from other groups, which have shown that R64, the analogous amino acid in hGH holds the same differential importance with respect to somatogenic binding.

Selective modification at the N-terminal region of human growth hormone that shows antagonistic activity

A new analogue of recombinant human growth hormone (hGH), hGH des(1-6,14) was expressed in Escherichia coli, refolded and purified to homogeneity. The mutation decreased the hormone's ability to bind lactogenic and somatogenic receptors through its site 1, and almost completely abolished its ability to bind these receptors through site 2, as evidenced by both binding and gel-filtration experiments. More specifically, the binding to prolactin receptors (PRLRs) from various species or their soluble recombinant extracellular domains (ECDs) was decreased 1.5-4-fold, whereas the binding to hGH receptor (hGHR) was decreased 10-85-fold. These changes caused an almost total loss of hormone agonistic activity in several in vitro bioassays and subsequently, the hGH des(1-6,14) analogue acquired antagonistic properties. This antagonistic activity was dependent upon modification of site 1. In those cases in which the binding was reduced only slightly, e.g. binding to rabbit PRLRs, hGH des(1-6,14) acted as a strong antagonist, whereas in others in which the binding of site 1 was reduced to a higher degree, such as other PRLRs and, in particular, hGHR, the antagonistic activity was correspondingly weaker. Circular dichroism spectra of the analogue suggested that these changes do not result from a decrease in overall alpha-helix content, but rather from minor local structural modifications at the N-terminus.

Recombinant analogues of prolactin, growth hormone, and placental lactogen: correlations between physical structure, binding characteristics, and activity

The availability of recombinant growth hormones, prolactins, placental lactogens and a few soluble extracellular domains of their receptors have extended our ability to study the interaction of somatogenic and lactogenic hormones with their receptors. Modifications of their respective cDNAs have enabled the preparation of sufficient amounts of the corresponding proteins. The present review summarizes two aspects of these interactions: (a) the relationship between binding, the apparent ability to dimerize the receptors and biological activities in vitro and in vivo; and (b) the effect of mutations on selective changes in the ability of human growth hormone and bovine placental lactogen to interact with somatogenic and lactogenic receptors. In view of this summary, strategies for preparing a second generation of biologically relevant recombinant hormones are discussed.