Growth hormone and adipose differentiation: growth hormone-induced antimitogenic state in 3T3-F442A preadipose cells

Association of growth hormone deficiency with an increased number of preadipocytes in subcutaneous fat

The inhibitory effect of growth hormone (GH) on adipose tissue growth is well known, but the underlying mechanism is not fully understood. In this study, we determined the possibility that GH inhibits adipose tissue growth by inhibiting adipogenesis, the process of formation of adipocytes from stem cells, in the lit/lit mice. The lit/lit mice are GH deficient because of a spontaneous mutation to the GH releasing hormone receptor (ghrhr) gene, and they have more subcutaneous fat despite being smaller than the lit/+ mice at the same age. We found that cells of the stromal vascular fraction (SVF) of subcutaneous fat from the lit/lit mice had greater adipogenic potential than those from the lit/+ mice, as evidenced by forming greater numbers of lipid droplets-containing adipocytes and having greater expression of adipocyte marker genes during induced adipocyte differentiation in culture. However, addition of GH to the culture did not reverse the superior adipogenic potential of subcutaneous SVF from the lit/lit mice. Through florescence-activated cell sorting and quantification of mRNAs of preadipocyte markers, including CD34, CD29, Sca-1, CD24, Pref-1, and PPARγ, we found that subcutaneous SVF from the lit/lit mice contained more preadipocytes than that from the lit/+ mice. These results support the notion that GH inhibits adipose tissue growth in mice at least in part by inhibiting adipogenesis. Furthermore, these results suggest that GH inhibits adipogenesis in mice not by inhibiting the terminal differentiation of preadipocytes into adipocytes, rather by inhibiting the formation of preadipocytes from stem cells or the recruitment of stem cells to the fat depot.

GH action influences adipogenesis of mouse adipose tissue-derived mesenchymal stem cells

GH influences adipocyte differentiation, but both stimulatory and inhibitory effects have been described. Adipose tissue-derived mesenchymal stem cells (AT-MSCs) are multipotent and are able to differentiate into adipocytes, among other cells. Canonical Wnt/β-catenin signaling activation impairs adipogenesis. The aim of the present study was to elucidate the role of GH on AT-MSC adipogenesis using cells isolated from male GH receptor knockout (GHRKO), bovine GH transgenic (bGH) mice, and wild-type littermate control (WT) mice. AT-MSCs from subcutaneous (sc), epididiymal (epi), and mesenteric (mes) AT depots were identified and isolated by flow cytometry (Pdgfrα+ Sca1+ Cd45- Ter119- cells). Their in vitro adipogenic differentiation capacity was determined by cell morphology and real-time RT-PCR. Using identical in vitro conditions, adipogenic differentiation of AT-MSCs was only achieved in the sc depot, and not in epi and mes depots. Notably, we observed an increased differentiation in cells isolated from sc-GHRKO and an impaired differentiation of sc-bGH cells as compared to sc-WT cells. Axin2, a marker of Wnt/β-catenin activation, was increased in mature sc-bGH adipocytes, which suggests that activation of this pathway may be responsible for the decreased adipogenesis. Thus, the present study demonstrates that (i) adipose tissue in mice has a well-defined population of Pdgfrα+ Sca1+ MSCs; (ii) the differentiation capacity of AT-MSCs varies from depot to depot regardless of GH genotype; (iii) the lack of GH action increases adipogenesis in the sc depot; and iv) activation of the Wnt/β-catenin pathway might mediate the GH effect on AT-MSCs. Taken together, the present results suggest that GH diminishes fat mass in part by altering adipogenesis of MSCs.

Minireview: mechanisms of growth hormone-mediated gene regulation

GH exerts a diverse array of physiological actions that include prominent roles in growth and metabolism, with a major contribution via stimulating IGF-1 synthesis. GH achieves its effects by influencing gene expression profiles, and Igf1 is a key transcriptional target of GH signaling in liver and other tissues. This review examines the mechanisms of GH-mediated gene regulation that begin with signal transduction pathways activated downstream of the GH receptor and continue with chromatin events at target genes and additionally encompasses the topics of negative regulation and cross talk with other cellular inputs. The transcription factor, signal transducer and activator of transcription 5b, is regarded as the major signaling pathway by which GH achieves its physiological effects, including in stimulating Igf1 gene transcription in liver. Recent studies exploring the mechanisms of how activated signal transducer and activator of transcription 5b accomplishes this are highlighted, which begin to characterize epigenetic features at regulatory domains of the Igf1 locus. Further research in this field offers promise to better understand the GH-IGF-1 axis in normal physiology and disease and to identify strategies to manipulate the axis to improve human health.

Effect of growth hormone on the differentiation of bovine preadipocytes into adipocytes and the role of the signal transducer and activator of transcription 5b

We evaluated the effect of GH on the differentiation of primary bovine preadipocytes into adipocytes. Bovine preadipocytes, derived from adipose tissue explants, were induced to differentiate into adipocytes in the presence or absence of recombinant bovine GH. The differentiation status of adipocytes was assessed by Oil Red O staining and by measuring the activity of glycerol-3-phosphate dehydrogenase (G3PDH) and the rate of acetate incorporation. Fewer preadipocytes became adipocytes in the presence of GH than in the absence of GH; adipocytes formed in the presence of GH had lower G3PDH activity and lower rate of acetate incorporation than those formed without GH treatment (P < 0.05). These data suggest an inhibitory effect of GH on the differentiation of bovine preadipocytes into adipocytes. Growth hormone decreased the expression of C/EBPα and PPARγ mRNA in bovine adipocytes (P < 0.05). Because C/EBPα and PPARγ are the master regulators of adipocyte differentiation, this data suggests that GH might inhibit the differentiation of bovine preadipocytes into adipocytes by inhibiting the expression of C/EBPα and/or PPARγ. Because the signal transducer and activator of transcription 5 (STAT5) is a major component of signaling from the GH receptor, we next determined the potential role of STAT5 in GH inhibition of bovine adipocyte differentiation. Overexpression of a constitutively active form of STAT5b (STAT5bCA) in bovine preadipocytes through adenoviral transduction mimicked the effects of GH on the formation of lipid-containing adipocytes, G3PDH activity, and acetate incorporation rate. Overexpression of STAT5bCA was associated with decreased expression of C/EBPα mRNA (P < 0.05) but not that of PPARγ mRNA in bovine adipocytes. These results support a role of STAT5b in mediating GH inhibition of C/EBPα expression but not that of PPARγ expression in bovine preadipocytes. Overall, the present study suggests that GH may inhibit adipose growth in cattle in part by inhibiting adipogenesis and that GH inhibits the differentiation of bovine preadipocytes to adipocytes through STAT5b-dependent inhibition of C/EBPα expression and STAT5b-independent inhibition of PPARγ expression.

Evaluation of the biological activity of human growth hormone by enzyme linked immuno-receptor assay (ELIRA) method

The aim of this research was to study the application and effectiveness of Enzyme Linked Immuno Receptor Assay (ELIRA) method for understanding the bioactivity of human Growth Hormone (hGH) in micro-titer plates. For this purpose, rabbit hepatocyte microsomes which contained hGH receptors were used for coating of ELISA micro-titer plates. Then hGH was interacted with coated receptors. Fractions of bounded complexes were identified by antibodies in an Enzyme-based substrate detection system. Different assay conditions such as: buffers, blocking agents, temperatures and times of incubation were analyzed. Our result indicated that, the carbonate coating buffer was not effective in receptor coating in ELIRA. Overnight incubation of hGH and hGH receptors in HEPES assay buffer and BSA blocking resulted in the lower linearity and correlations (R(2) = 0.46 to 0.85). However, 3 h incubation in Tris-HCl assay buffer at 30°C resulted in higher linearity and correlations (R(2) = 0.95 to 0.97). Finally, the coating of microwells by 250 μg/ml of microsome membranes in Tris buffer at 30°C for 3 hr and blocking by skim milk resulted to the best linearity and higher correlation, (R(2) = 0.985) and lower detection limit about 2 ng/ml of bioactive hGH.

Molecular cloning, tissue distribution, and ontogeny of mRNA expression of growth hormone in orange-spotted grouper (Epinephelus coioides)

A full-length cDNA encoding for growth hormone (GH) was cloned from orange-spotted grouper (Epinephelus coioides) pituitary using reverse transcription and rapid amplification of cDNA ends (RACE). The GH precursor cDNA consists of 956 bp in size with a 85 bp 5'-untranslated region and 259 bp 3'-untranslated region. The 612 bp open reading frame encodes a 204 amino acid (aa) protein, which represents the precursor of grouper GH composed of a 17 aa signal peptide followed by a 187 aa mature GH polypeptide. The sequence of grouper GH shares 95% aa sequence homology with GH reported in gilthead sea bream (Sparus aurata), and it also exhibits structural features highly homologous to GH reported in other fish species in the domains representing conserved motifs of GH polypeptides. A single GH transcript of 0.93 kb in size has been detected with Northern blot in the pituitary. Using semi-quantitative PCR approach, dominant PCR products were observed in grouper pituitary, while less PCR products were detected in the brain, spleen, and ovary. The expression of GH mRNA could be detected in 1dph larvae, after that a significant increase in PCR products was found in 5-day-old fish larvae followed by a drop to very low levels in 15-day-old fish larvae. A second rise was then observed in 25-day-old grouper larvae. These findings suggest that in grouper GH mRNA expression can be detected in day 1 post-hatching larvae, and the GH present in eggs and larvae may play a key role in early development of grouper, especially during the process of metamorphosis of fish larva.

Effects of growth hormone and prolactin on adipose tissue development and function

GH and PRL are both implicated in adipose tissue development. Whilst direct effects of GH have been clearly demonstrated, direct effects of PRL have been subject to considerable debate. Recent studies have however provided compelling evidence for PRL receptors on adipocytes and in vitro effects on leptin and lipoprotein lipase activity have been demonstrated. Quantitatively however these effects of PRL are less significant than those of GH and the most pronounced effects of PRL on adipose tissue are indirect, for example, during lactation, when prolactin drives milk synthesis which results in a homeorhetic shift towards lipid mobilization from adipose tissue to support milk production. GH also exhibits such homeorhetic effects, most notably in ruminants, but also clearly has direct, insulin-antagonistic, metabolic effects. The roles of GH and PRL on adipocyte proliferation and differentiation have also been controversial, with GH stimulating adipocyte differentiation in vitro in cell lines whilst stimulating proliferation and inhibiting differentiation of primary cell cultures. Examination of adipose tissue development in PRLRko and GHRko mice has revealed roles for both hormones. PRLRko mice show impaired development of both internal and subcutaneous adipose tissue due to decreased numbers of adipocytes. In contrast, GHRko mice exhibit major decreases in the number of internal (parametrial) adipocytes whereas subcutaneous adipocytes develop almost normally. This leads to major changes in the sites of adipose tissue accretion and bears interesting parallels with the adipose tissue redistribution which occurs in HIV-induced lipodystrophy. Such individuals exhibit a central obesity which can be partially corrected by GH treatment. However, recent studies suggest that this may be a physiological response in which adipose tissue sites containing lymphoid tissue (such as mesenteric) show preservation of adipose tissue perhaps to support augmented immune responses.

Molecular cloning of growth hormone encoding cDNA of Indian major carps by a modified rapid amplification of cDNA ends strategy

A modified rapid amplification of cDNA ends (RACE) strategy has been developed for cloning highly conserved cDNA sequences. Using this modified method, the growth hormone (GH) encoding cDNA sequences of Labeo rohita, Cirrhina mrigala and Catla catla have been cloned, characterized and overexpressed in Escherichia coli. These sequences show 96-98% homology to each other and are about 85% homologous to that of common carp. Besides, an attempt has been made for the first time to describe a 3-D model of the fish GH protein.

Genes induced by growth hormone in a model of adipogenic differentiation

A substantial number of GH regulated genes have been reported in mature hepatocytes, but genes involved in GH-initiated cell differentiation have not yet been identified. Here we have studied a well-characterised model of GH-dependent differentiation, adipogenesis of 3T3-F442A preadipocytes, to identify genes rapidly induced by GH. Using the suppression subtractive hybridisation technique, we have identified eight genes induced within 60 min of GH treatment, and verified these by northern analysis. Six were identifiable as Stat 2, Stat 3, thrombospondin-1, oncostatin M receptor beta chain, a DEAD box RNA helicase, and muscleblind, a developmental transcription factor. Bioinformatic approaches assigned one of the two remaining unknown genes as a novel 436 residue serine/threonine kinase. As each of the identified genes have important developmental roles, they may be important in initiating GH-induced adipogenesis.

The mechanism of effect of growth hormone on preadipocyte and adipocyte function

Growth hormone (GH) is not only the major regulator of postnatal somatic growth but also exerts profound effects on body composition through a combination of anabolic, lipolytic and antinatriuretic actions. GH enhancement of the lipolytic activity of adipose tissue in combination with a reduction of triglyceride accumulation via inhibition of lipoprotein lipase activity appears to be the major mechanism by which GH results in a reduction of the total fat mass. Recently, much progress has been made in understanding the molecular mechanism by which GH affects cellular function. This review provides a brief discourse and summary of the mechanism of effects of GH on preadipocyte/adipocyte function. It is intended to provide a functional understanding of the mechanism of action of GH as it relates to adipogenesis and adipocyte function.

Ternary complex factors Elk-1 and Sap-1a mediate growth hormone-induced transcription of egr-1 (early growth response factor-1) in 3T3-F442A preadipocytes

In our search for transcription factors induced by GH, we have analyzed immediate early gene activation in a model of GH-dependent differentiation. Here we describe the activation of early growth response factor-1 (egr-1) in GH-stimulated 3T3-F442A preadipocytes and the transcription factors responsible for its transactivation. Binding activity of egr-1 in electrophoretic mobility shift assay (EMSA) increased transiently 1 h after GH stimulation, accompanied by a concomitant increase in egr-1 mRNA. egr-1 induction appeared not to be related to proliferation since it was amplified in quiescent preadipocytes at a time when cells were refractive to GH-stimulated DNA synthesis. Truncations of the proximal 1 kb of the egr-1 promoter revealed that a 374-bp region (-624 to -250) contributes about 80% of GH inducibility in 3T3-F442A cells and approximately 90% inducibility in CHO-K1 cells. This region contains three juxtaposed SRE (serum response element)/Ets site pairs known to be important for egr-1 activity in response to exogenous stimuli. Site-specific mutations of individual SRE and Ets sites within this region each reduced GH inducibility of the promoter. Use of these site-specific mutations in EMSA showed that disruption of either Ets or SRE sites abrogated ternary complex formation at the composite sites. DNA binding of ternary complexes, but not binary complexes, in EMSA was rapidly and transiently increased by GH. EMSA supershifts indicated these ternary complexes contained serum response factor (SRF) and the Ets factors Elk-1 and Sap-1a. Coexpression of Sap-1a and Elk-1 resulted in a marked increase in GH induction of egr-1 promoter activity, although transfection with expression vectors for either Ets factor alone did not significantly enhance the GH response. We conclude that GH stimulates transcription of egr-1 primarily through activation of these Ets factors at multiple sites on the promoter and that stabilization of ternary complexes with SRF at these sites maximizes this response.

Production of a biologically active novel goldfish growth hormone in Escherichia coli

Goldfish pituitary contains two types of growth hormones. One with five cysteine residues (type-I) similar to other Cyprinid GHs, and the other with four Cys residues (type-II) similar to those of other fish and tertapod species. Recombinant goldfish type II GH (gfGH-II) was produced in Escherichia coli using the pRSETB expression vector. The gfGH-II was produced fused to a leader sequence, which sequestered into inclusion bodies after expression. The inclusion bodies were solubilized using sodium hydroxide and the fusion protein purified by chelating affinity chromatography. Subsequently, gfGH-II was cleaved and analyzed by Western blotting, using a specific antiserum. For comparison we also produced recombinant common carp GH (cGH) which has 95% similarity to gfGH-II, and tested their growth promoting activity in goldfish. Both forms of GH significantly increased the growth rate of goldfish (P < 0.05), although cGH was found to have a somewhat higher potency than gfGH-II.

Understanding adipocyte differentiation

The adipocyte plays a critical role in energy balance. Adipose tissue growth involves an increase in adipocyte size and the formation of new adipocytes from precursor cells. For the last 20 years, the cellular and molecular mechanisms of adipocyte differentiation have been extensively studied using preadipocyte culture systems. Committed preadipocytes undergo growth arrest and subsequent terminal differentiation into adipocytes. This is accompanied by a dramatic increase in expression of adipocyte genes including adipocyte fatty acid binding protein and lipid-metabolizing enzymes. Characterization of regulatory regions of adipose-specific genes has led to the identification of the transcription factors peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and CCAAT/enhancer binding protein (C/EBP), which play a key role in the complex transcriptional cascade during adipocyte differentiation. Growth and differentiation of preadipocytes is controlled by communication between individual cells or between cells and the extracellular environment. Various hormones and growth factors that affect adipocyte differentiation in a positive or negative manner have been identified. In addition, components involved in cell-cell or cell-matrix interactions such as preadipocyte factor-1 and extracellular matrix proteins are also pivotal in regulating the differentiation process. Identification of these molecules has yielded clues to the biochemical pathways that ultimately result in transcriptional activation via PPAR-gamma and C/EBP. Studies on the regulation of the these transcription factors and the mode of action of various agents that influence adipocyte differentiation will reveal the physiological and pathophysiological mechanisms underlying adipose tissue development.

Growth hormone attenuation of epidermal growth factor-induced mitogenesis

Growth hormone (GH) has previously been reported to influence the adipose conversion of 3T3-F442A murine fibroblasts, partly by causing these cells to exit the cell cycle and to become unresponsive to serum-stimulated mitogenesis. To better understand this process, quiescent fibroblasts were treated with fully stimulatory doses (50 nM) of epidermal growth factor (EGF) in the presence or absence of pituitary human GH (hGH) or the phorbol ester phorbol 12-myristate 13-acetate (PMA), which is known to down-regulate EGF receptor activity. EGF-induced DNA synthesis was attenuated by hGH in a dose-dependent manner with an ED50 of approximately 0.1 nM and a maximally effective dose of 10-30 nM. This effect appeared to be the result of inhibition of DNA synthesis and exclusive of a time shift in the initiation of the S phase of the cell cycle. Additionally, insulin-like growth factor-1 (IGF-1), which can act as an important in vivo mediator of GH, failed to mimic the antimitogenic effects of GH. The ability of hGH to antagonize EGF-stimulated mitogenesis did not appear to be due to the down-regulation of EGF receptor mass or to pronounced changes in EGF-induced tyrosine kinase activity. Furthermore, when GH was administered at various times after EGF addition, GH continued to be effective at inhibiting EGF-induced DNA synthesis for up to 9 hr after EGF treatment. Modulation of EGF-induced cell cycle progression was further evidenced by the ability of GH to promote a marked decrease in the EGF-induced expression of D cyclins. In comparison, PMA inhibited EGF-induced DNA synthesis for up to 18 hr after EGF addition and also down-regulated EGF receptor mass and activity; these observations suggest that the mechanism of GH action is largely distinct from that of PMA. We conclude that GH can selectively and dose-dependently modulate EGF receptor-mediated DNA synthesis exclusive of any rapid or extensive effects on EGF receptor mass or tyrosine kinase activity. Furthermore, the capacity of GH to attenuate EGF-induced mitogenesis, even when administered 9 hr after EGF addition, and the GH modulation of EGF-induced expression of D cyclins, suggest that there are GH-induced effects on systems involved in the transition of these fibroblasts through the G1 phase of the cell cycle. In sum, these data support a specific interaction of this somatotropic hormone/cytokine with EGF in the control of cell cycle progression in 3T3-F442A fibroblasts.

Growth hormone and phorbol esters require specific protein kinase C isoforms to activate mitogen-activated protein kinases in 3T3-F442A cells

Previous studies have shown that the activation of p44 and p42 mitogen-activated protein (MAP) kinases (ERK1 and ERK2) by growth hormone (GH) and phorbol esters, but not by epidermal growth factor, in 3T3-F442A preadipocytes is dependent on protein kinase C (PKC). In the present study two approaches have been taken to determine the PKC isoform dependence of MAP kinase activation in these cells. By immunoblotting with specific antibodies, the cells were found to express PKC-alpha, -gamma,-delta, -epsilon and -zeta. Treatment of cells with 500 nM PMA for 3 h led to the complete depletion of PKC-delta and the partial depletion of PKC-alpha but did not significantly affect the expression of the other PKC isoforms. In parallel, such treatment severely attenuated the ability of GH to activate MAP kinase. The degree of this attenuation was not increased by more prolonged PMA pretreatment, indicating that PKC-delta and perhaps PKC-alpha are important for MAP kinase activation by GH. These experiments further revealed that additional PKC isoforms were required for the full activation of MAP kinases by acute treatment with PMA. A second approach involved the use of anti-sense oligodeoxynucleotides (ODNs) to deplete the individual PKC isoforms selectively. Each of the ODNs used effectively depleted the relevant isoform to undetectable levels and did not affect the expression of the other PKC isoforms. Pretreatment of cells with PKC-delta anti-sense ODN, but not with anti-sense ODN to the other phorbol ester-sensitive isoforms, severely attenuated the activation of MAP kinases by GH. PKC-delta anti-sense ODN also blocked (by approx. 50%) the activation of MAP kinases by PMA. Furthermore a combination of PKC-delta and -epsilon anti-sense ODNs completely blocked the effect of PMA on MAP kinases. Collectively, these results indicate that the novel PKC-delta and -epsilon isoforms can couple to the MAP kinase pathway in 3T3-F442A cells but that the activation of MAP kinases by GH specifically involves PKC-delta.

Prolactin induces growth inhibition and promotes differentiation of CHO cells stably transfected with prolactin receptor complementary DNA

We have characterized a stable and functional transfectant of the rabbit prolactin receptor in Chinese hamster ovary cells, and investigated the action of prolactin (PRL) on the growth and differentiation of this transfectant (clone E32). PRL induced a significant inhibition of E32 cell proliferation. Growth inhibition correlated with gene induction of the molecular marker of ovarian differentiation cholesterol side chain cleavage P450 (P450scc). Both effects were inversely proportional to cell confluence. The limits and potential development of such transfected cellular systems are discussed.

Preparation and comparison of biological properties of recombinant carp (Cyprinus carpio) growth hormone and its Cys-123 to Ala mutant

Carp growth hormone (cGH) cDNA, in which Cys-123 was mutated to Ala, was prepared, transferred to the expression vector, expressed in Escherichia coli and the mutant was purified to homogeneity. The mutation only slightly improved yield of the monomeric fraction, indicating that Cys-123 is not involved in improper refolding. As compared to cGH, the mutant (cGH-C123A) exhibited lower binding affinity toward homologous liver receptors and lower bioactivity in a 3T3-F442A preadipocyte bioassay despite the fact that both hormones exhibited almost identical cross-reactivity with anti-cGH antibodies. These results, along with those of a structural comparison to hGH, suggest that Cys-123 is located in the hydrophobic core of the hormone, and is most likely affecting the conformation of the binding site. Dimeric forms of the hormone and its mutant were less active than their respective monomers. Homologous binding experiments using a carp liver microsomal fraction revealed a single receptor population with Kd = 0.77 nM and Bmax = 241 fmol/mg microsomal protein.

Pituitary factors in blood plasma are necessary for smooth muscle cell proliferation in response to injury in vivo

Intimal thickening in response to vascular injury is inhibited in animals previously subjected to hypophysectomy. We have investigated the nature and cell kinetics of this effect in a balloon catheter model of injury to the rat carotid artery. The ability of injury to stimulate [3H]thymidine labeling 48 hours after injury was almost completely eliminated in hypophysectomized (hypox) compared with control animals (0.1% versus 32.1%). Total DNA content of the developing neointima 14 days after injury was only 30% of the values found in ballooned carotid arteries of normal rats. If hypox rats were treated with recombinant human growth hormone, the proliferative response was not restored. There are two possible general explanations for the reduction of proliferative response in hypox animals: 1) that smooth muscle cells in the hypox animals have lost the ability to respond to the stimulus of injury or 2) that the ability of the smooth muscle cells to respond has not been reduced by prior hypophysectomy, but that the response itself requires the presence of pituitary-dependent factors. Transplantation experiments were performed in vivo to distinguish between these possibilities. Carotid arteries in inbred Lewis rats were excised 1 hour after balloon injury to give platelets the opportunity to adhere. These vessels were then transplanted from hypox into control animals and vice versa. At 48 hours, proliferation of smooth muscle cells in "control-to-hypox" transplants was 0.3% compared with 14.3% in "control-to-control" transplants, whereas vessels from hypox rats increased their indices to 4.8% if transplanted into control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological activity of bovine placental lactogen in 3T3-F442A preadipocytes is mediated through a somatogenic receptor

Bovine placental lactogen (bPL) exhibited antimitogenic differentiation-promoting biological activity in 3T3-F442A preadipocytes. Competitive binding studies and affinity labelling revealed bPL activity to be mediated through a somatogenic type of receptor that recognizes human growth hormone (hGH) and bovine GH, but not ovine prolactin or human PL. The bioactivity of bPL was sixfold lower than that of hGH despite that bPL is binding to the somatogenic receptors with fivefold higher affinity. This discrepancy may result from the relatively low ability of bPL to induce post-receptoral effects such as receptor dimerization.