Insulin and interleukin-1 differentially regulate pp63, an acute phase phosphoprotein in hepatoma cell line

Detection and Characterization of Phosphorylation, Glycosylation, and Fatty Acid Bound to Fetuin A in Human Blood

The hepatokine fetuin A (Fet A) has been associated with diverse pathological states such as insulin resistance, type 2 diabetes, macrovascular disease, and systemic ectopic and vascular calcification. Fet A may also play a role in tumor growth and metastasis. The biological activity of Fet A may be affected by various modifications, including phosphorylation, O- and N-glycosylation and fatty acid binding. We developed an antibody-based assay for the detection of Fet A phosphorylated at serine 312. Fatty acid pattern was determined by gas chromatography. Using the antibody, we found that the phosphorylation was stable in human plasma or serum at room temperature for 8 h. We observed that Fet A is present in several glycosylation forms in human plasma, but the extent of Ser³¹² phosphorylation was not associated with glycosylation. The phosphorylation pattern did not change during an oral glucose tolerance test (0–120 min). We further found that human Fet A binds preferentially saturated fatty acids (>90%) at the expense of mono- and poly-unsaturated fatty acids. Our results indicate that different molecular species of Fet A are present in human plasma and that these different modifications may determine the different biological effects of Fet A.

Phosphorylation status of fetuin-A is critical for inhibition of insulin action and is correlated with obesity and insulin resistance

Fetuin-A (Fet-A), a hepatokine associated with insulin resistance, obesity, and incident type 2 diabetes, is shown to exist in both phosphorylated and dephosphorylated forms in circulation. However, studies on fetuin-A phosphorylation status in insulin-resistant conditions and its functional significance are limited. We demonstrate that serum phosphofetuin-A (Ser312) levels were significantly elevated in high-fat diet-induced obese mice, insulin-resistant Zucker diabetic fatty rats, and in individuals with obesity who are insulin resistant. Unlike serum total fetuin-A, serum phosphofetuin-A was associated with body weight, insulin, and markers of insulin resistance. To characterize potential mechanisms, fetuin-A was purified from Hep3B human hepatoma cells. Hep3B Fet-A was phosphorylated (Ser312) and inhibited insulin-stimulated glucose uptake and glycogen synthesis in L6GLUT4 myoblasts. Furthermore, single (Ser312Ala) and double (Ser312Ala + Ser120Ala) phosphorylation-defective Fet-A mutants were without effect on glucose uptake and glycogen synthesis in L6GLUT4 myoblasts. Together, our studies demonstrate that phosphorylation status of Fet-A (Ser312) is associated with obesity and insulin resistance and raise the possibility that Fet-A phosphorylation may play a role in regulation of insulin action.

Regulation of fetuin A gene expression in the neonatal pig liver

Fetuin A (also known as α2-Heremans-Schmid glycoprotein) is a protein primarily expressed by the liver and secreted into the blood. Previous studies have suggested that plasma concentrations of fetuin A are elevated with impaired growth rate in swine. The present study was designed to examine the relationship of porcine fetuin A with growth rate in the pig and to also elucidate the regulation of fetuin A expression by examining the hormonal and cytokine regulation of fetuin A mRNA abundance in hepatocytes prepared from suckling piglets. Quantitative real-time PCR assay was used to quantify the number of fetuin A mRNA molecules/molecule cyclophilin mRNA. Total RNA was isolated from liver of three different groups of pigs to assess changes in mRNA abundance of fetuin A: normal piglets at day 1, day 7 day 21 or 6 months of age (n=6 for each age); runt and control piglets at day 1 of age (n=4); slow growing and normal growing piglets at 21 days of age (n=8). Following birth, fetuin A gene expression increased from day 1 and 7 of age (P<0.05), and then declined at 21 days of age (P<0.05), with a much greater decline to 6 months of age (P<0.01). Fetuin A mRNA abundance was higher in runt pigs v. their normal birth weight littermates (P<0.05). Similarly, fetuin A gene expression was higher in livers of pigs that were born at a normal weight but that grew much slower than littermates with the same birth weight (P<0.05). Hepatocytes were isolated from preweaned piglets and maintained in serum-free monolayer culture for up to 72 h to permit examination of the influences of hormones, cytokines and redox modifiers on fetuin A mRNA abundance. Fetuin A gene expression was enhanced by glucagon, T3 and resveratrol (P<0.05). Growth hormone, cytokines (interleukin6, tumor necrosis factor-α) and antioxidants (N-acetylcysteine, quercertin) reduced fetuin A mRNA abundance (P<0.05). A role for fetuin A in postnatal development is suggested by the differences in fetuin A mRNA abundance between runt piglets or slow growing piglets and their normal growing sized littermates. The hepatocyte experiments suggest multiple hormones and cytokines may contribute to the regulation of fetuin A during early growth of the pig.

AHSG gene variant is associated with leanness among Swedish men

Alpha(2) Heremans-Schmid glycoprotein (AHSG) is a plasma protein inhibiting the activity of the insulin receptor tyrosine kinase. Ahsg knock-out mice have increased insulin sensitivity and are resistant to diet-induced obesity. We hypothesized that functional variants of the AHSG gene segregating in the human population would reflect variation in body mass index (BMI). We genotyped 356 overweight or obese (BMI: 37.2 [25.0-66.5] kg/m(2)) and 148 lean (BMI: 23.7 [23.4-24.9] kg/m(2)) otherwise healthy Swedish men for three non-synonymous single-nucleotide polymorphisms (SNPs) within exon 6 (rs4917) and exon 7 (rs4918 and Arg299Cys) and one SNP in intron 1 (rs2593813) of the AHSG gene. The G/G genotype for rs2593813 was more common among lean than among obese and overweight individuals (odds ratio = 2.01, P = 0.009), whereas rs2593813 was in strong linkage disequilibrium (|D'| > or = 0.97) with rs4917 and rs4918. Homozygosity for the rs2593813:G-rs4917:Met-rs4918:Ser haplotype conferred an increased risk for leanness (odds ratio=1.90, P = 0.027). rs4917:Met and rs4918:Ser have previously been associated with lower AHSG protein level. A common variant of AHSG, previously associated with a lower AHSG protein level, is thus more common among lean than obese and overweight men, supporting the results from Ahsg knock-out mice, namely, that AHSG modulates body mass.

Cell-specific pituitary gene expression profiles after treatment with leukemia inhibitory factor reveal novel modulators for proopiomelanocortin expression

Leukemia inhibitory factor (LIF) mediates the hypothalamo-pituitary-adrenal stress response. Transgenic mice overexpressing LIF in the developing pituitary have altered pituitary differentiation with expansion of corticotropes, maintenance of Rathke's cleft cysts, and suppression of all other pituitary cell types. Affymetrix GeneChips were used to identify modulators of LIF effects in corticotrope (AtT-20) and somatolactotrope (GH(3)) cells. In addition to genes known to respond to LIF in corticotrope cells [e.g. suppressor of cytokine signaling-3 (SOCS-3), signal transducer and activator of transcription-3, SH2 domain-containing tyrosine phosphatase-1, and proopiomelanocortin (POMC)], corticotrope-specific changes were also observed for genes involved in glycolysis and gluconeogenesis, transcription factors, signaling molecules, and expressed sequence tags. Two transcription factors identified, CCAAT/enhancer-binding protein beta (C/EBPbeta) and glial cell-derived neurotrophic factor (GDNF)-inducible factor (GIF), dose-dependently induced expression of the rat POMC promoter when overexpressed in AtT-20 cells. LIF further induced POMC transcription with C/EBPbeta, but not with GIF. C/EBPbeta also induced expression of the SOCS-3 promoter that was further enhanced by cotreatment with LIF. However, GIF did not affect SOCS-3 expression. These results indicate that C/EBPbeta and GIF are downstream effectors of LIF corticotrope action. LIF also stimulates the expression of inhibitors of its actions, such as SOCS-3 and SH2 domain-containing tyrosine phosphatase-1. alpha(2)-HS-glycoprotein (AHSG)/fetuin, a secreted protein that antagonizes bone TGFbeta/bone morphogenic protein signaling, was induced by LIF in a signal transducer and activator of transcription-3-dependent fashion. Pretreatment with AHSG/fetuin blocked LIF-induced expression of the POMC promoter independently of SOCS-3. Thus, using GeneChips, C/EBPbeta and GIF have been identified as novel mediators and AHSG/fetuin as an inhibitor of LIF action in corticotropes.

Improved insulin sensitivity and resistance to weight gain in mice null for the Ahsg gene

Fetuin inhibits insulin-induced insulin receptor (IR) autophosphorylation and tyrosine kinase activity in vitro, in intact cells, and in vivo. The fetuin gene (AHSG) is located on human chromosome 3q27, recently identified as a susceptibility locus for type 2 diabetes and the metabolic syndrome. Here, we explore insulin signaling, glucose homeostasis, and the effect of a high-fat diet on weight gain, body fat composition, and glucose disposal in mice carrying two null alleles for the gene encoding fetuin, Ahsg (B6, 129-Ahsg(tm1Mbl)). Fetuin knockout (KO) mice demonstrate increased basal and insulin-stimulated phosphorylation of IR and the downstream signaling molecules mitogen-activated protein kinase (MAPK) and Akt in liver and skeletal muscle. Glucose and insulin tolerance tests in fetuin KO mice indicate significantly enhanced glucose clearance and insulin sensitivity. Fetuin KO mice subjected to euglycemic-hyperinsulinemic clamp show augmented sensitivity to insulin, evidenced by increased glucose infusion rate (P = 0.077) and significantly increased skeletal muscle glycogen content (P < 0.05). When fed a high-fat diet, fetuin KO mice are resistant to weight gain, demonstrate significantly decreased body fat, and remain insulin sensitive. These data suggest that fetuin may play a significant role in regulating postprandial glucose disposal, insulin sensitivity, weight gain, and fat accumulation and may be a novel therapeutic target in the treatment of type 2 diabetes, obesity, and other insulin-resistant conditions.

Alpha2-HS glycoprotein: a protein in search of a function

In this issue (pp. 305-310) Jun Ren and Amy J. Davidoff author an article entitled 'Alpha2-HS glycoprotein, a putative inhibitor of tyrosine kinase, prevents glucose toxicity associated with cardiomyocyte dysfunction'. The protein responsible for this biological activity has recently come to the forefront of research on the biological activity of plasma proteins.

alpha 2-HS glycoprotein/fetuin, a transforming growth factor-beta/bone morphogenetic protein antagonist, regulates postnatal bone growth and remodeling

Soluble transforming growth factor-beta (TGF-beta)/bone morphogenetic protein (BMP)-binding proteins are widely distributed in mammalian tissues and control cytokine access to membrane signaling receptors. The serum and bone-resident glycoprotein alpha2-HS-glycoprotein/fetuin (ASHG) binds to TGF-beta/BMP cytokines and blocks TGF-beta1 binding to cell surface receptors. Therefore, we examined bone growth and remodeling phenotypes in ASHG-deficient mice. The skeletal structure of Ahsg(-/-) mice appeared normal at birth, but abnormalities were observed in adult Ahsg(-/-) mice. Maturation of growth plate chondrocytes was impaired, and femurs lengthened more slowly between 3 and 18 months of age in Ahsg(-/-) mice. However, bone formation was increased in Ahsg(-/-) mice as indicated by greater cortical thickness, accelerated trabecular bone remodeling, and increased osteoblast numbers on bone surfaces. The normal age-related increase in cortical thickness and bone mineral density was accelerated in Ahsg(-/-) mice and was associated with increased energy required to fracture. Bone formation in response to implanted BMP cytokine extended further from the implant in Ahsg(-/-) compared with Ahsg(+/+) mice, confirming the interaction between ASHG and TGF-beta/BMP cytokines in vivo. Our results demonstrate that ASHG blocks TGF-beta-dependent signaling in osteoblastic cells, and mice lacking ASHG display growth plate defects, increased bone formation with age, and enhanced cytokine-dependent osteogenesis.

Plasma alpha2-HS glycoprotein concentrations in patients with acute myocardial infarction quantified by a modified ELISA

BACKGROUND

Human alpha2-HS glycoprotein (alpha2-HSG) is synthesized and secreted by the liver into circulation. Plasma concentrations of alpha2-HSG decrease significantly following infection, inflammation and malignancy. Since increased plasma concentrations of C-reactive protein are observed in patients with acute myocardial infarction (AMI), we hypothesized that plasma concentrations of alpha2-HSG would decrease during the initial phase of AMI and begin to increase in the recovery phase.

METHODS

Twenty patients diagnosed with AMI were recruited for the study. A sensitive and specific ELISA was developed to assay alpha2-HSG concentrations in plasma.

RESULTS

In AMI patients, plasma alpha2-HSG concentrations were decreased (281.3+/-25.8 mg/l, ranging from 132 to 489 mg/l on admission) compared to healthy individuals (312.3+/-9.9 mg/l, ranging from 210 to 450 mg/l) (P= 0.142). Interestingly, 40% of AMI patients demonstrated alpha2-HSG concentrations below 200 mg/l compared to none in the healthy control group. During the recovery period, alpha2-HSG concentrations begin to increase, with a mean+/-SEM of 290.1+/-22.1 mg/l. Regression analysis comparing plasma alpha2-HSG concentrations on admission to concentrations on discharge showed a significant positive correlation in matched-pair patient samples (P<0.01, r=0.45).

CONCLUSIONS

We conclude that, in contrast to C-reactive protein, alpha2-HSG functions as a negative acute phase protein in AMI patients. Plasma alpha2-HSG concentrations start to decrease within a few hours after the onset of AMI and return to near normal concentrations during the recovery period (5-7 days after AMI).

Autoantibodies in endometriosis sera recognize a Thomsen-Friedenreich-like carbohydrate antigen

Autoantibody responses to endometrial antigens are a common feature of endometriosis. Antibody responses to a number of serum and tissue antigens such as alpha(2)-Heremans Schmidt glycoprotein (alpha(2)-HSG), transferrin, and carbonic anhydrase have been identified. The nature of the epitopes recognized on these proteins has not been determined. In this study we show that the serum antibody response to alpha(2)-HSG and carbonic anhydrase is against a common carbohydrate epitope which is also expressed on bovine fetuin. Removal of carbohydrate moieties from these antigens resulted in loss of antibody binding. Antibody reactivity with alpha(2)-HSG, fetuin and other antigens was removed by binding with the lectin jacalin. Jacalin specifically binds the Thomsen-Friedenreich antigen (Galbeta1-3GalNAc). Demonstrating that the autoantibodies also reacted with other Thomsen-Friedenreich antigen-bearing proteins, serum IgA1 and haemopexin confirmed an association with this epitope. These antigens have not been previously described as autoantigens in endometriosis and are of interest since they raise the possibility that this autoimmune response may either play a direct role in the disease process or reflect an abnormality of glycosylation in endometriosis. These results may also prove useful in the development of a serum diagnostic test for endometriosis.

Regulation of human monocyte proMMP-9 production by fetuin, an endogenous TGF-beta antagonist

Members of the matrix metalloproteinase family of enzymes degrade specific components of the extracellular matrix. MMP-9 is a type IV/V collagenase necessary for normal osteogenesis and is increased in inflammatory and neoplastic conditions. In such destructive diseases as emphysema and arthritis, and in epithelial cancers, MMP-9 is produced by cells of the monocyte lineage. Fetuin, a prominent serum glycoprotein, binds to and inactivates TGF-beta family members and through this mechanism regulates osteogenesis (Binkert et al., 1999, J Biol Chem 274:28514-28520.). We studied the effects of TGF-beta1 and fetuin on proMMP-9 release by the human monocyte line THP-1. Exogenous TGF-beta1 stimulated proMMP-9 release by THP-1 cells, with half-maximal stimulation at approximately 0.01 ng/ml TGF-beta1. Human fetuin (0.5-2 microM) partially inhibited this stimulation. Human fetuin alone stimulated THP-1 monocyte proMMP-9 release, with half maximal stimulation at approximately 0.25 microM fetuin. Neutralizing antibody specific for TGF-beta1 also stimulated proMMP-9 release, suggesting that endogenously-derived TGF-beta1 has an inhibitory effect. In freshly isolated human peripheral blood monocytes, fetuin stimulated proMMP-9 release with a dose-response curve similar to that observed in THP-1 cells. Human fetuin also activated proMMP-9 present in THP-1 conditioned medium. Taken together, these data suggest that under physiological conditions, fetuin facilitates matrix degradation by monocyte-derived MMP-9, both by opposing the autocrine inhibitory effect of endogenous TGF-beta1 on proMMP-9 release, and by activating proMMP-9 in the pericellular milieu. Conversely, fetuin may limit the stimulation of monocyte proMMP-9 release by high levels of exogenous TGF-beta1. Such modulation could prove important under pathological conditions where TGF-beta1 derived from paracrine sources is abundant, such as in epithelial malignancies.

Positive and negative elements modulate the promoter of the human liver-specific alpha2-HS-glycoprotein gene

The human alpha2-HS-glycoprotein (AHSG) and the 63-kDa rat phosphoprotein (pp63) are homologous plasma proteins that belong to the fetuin family. AHSG and pp63 are involved in important functions such as inhibition of insulin receptor tyrosine kinase activity, inhibition of protease activities, and regulation of calcium metabolism and osteogenesis. Studies of the AHSG proximal promoter performed in vitro in rat and human cells indicate that several NF-1 and C/EBP binding sites exert a positive effect on its transcriptional activity. However, until now, no distal elements have been examined in this gene, in either species. We report that the human AHSG gene promoter acts in a liver-specific manner and is further controlled by three distal, 5'-flanking elements. The negative elements III and I are, respectively, located 5' and 3' of the positive element II. All three elements require the natural context of the human AHSG gene to fully exert their negative or positive effect. Element I harbours a single binding site for NF-1. This nuclear factor thus appears to be able to up- or downregulate the AHSG gene depending on the site it binds to. Elements I, II and possibly III are absent in the rodent Ahsg gene encoding pp63.

Alpha2-Heremans Schmid glycoprotein inhibits insulin-stimulated Elk-1 phosphorylation, but not glucose transport, in rat adipose cells

Alpha2-Heremans Schmid glycoprotein (alpha2-HSG) is a member of the fetuin family of serum proteins whose biological functions are not completely understood. There is a consensus that alpha2-HSG plays a role in the regulation of tissue mineralization. However, one aspect of alpha2-HSG function that remains controversial is its ability to inhibit the insulin receptor tyrosine kinase and the biological actions of insulin. Interestingly, some studies suggest that alpha2-HSG differentially inhibits mitogenic, but not metabolic, actions of insulin. However, these previous studies were not carried out in bona fide insulin target cells. Therefore, in the present study we investigate the effects of alpha2-HSG in the physiologically relevant rat adipose cell. We studied insulin-stimulated translocation of the insulin-responsive glucose transporter GLUT4 in transfected rat adipose cells overexpressing human alpha2-HSG. In addition, we measured insulin-stimulated glucose transport in adipose cells cultured with conditioned medium from the transfected cells as well as in freshly isolated adipose cells treated with purified human alpha2-HSG. Compared with control cells, we were unable to demonstrate any significant effect of alpha2-HSG on insulin-stimulated translocation of GLUT4 or glucose transport. In contrast, we did demonstrate that overexpression of alpha2-HSG in adipose cells inhibits both basal and insulin-stimulated phosphorylation of Elk-1 (a transcription factor phosphorylated and activated by mitogen-activated protein kinase and other related upstream kinases). Interestingly, we did not observe any major effects of alpha2-HSG to inhibit insulin-stimulated phosphorylation of the insulin receptor, insulin receptor substrate-1, -2, or -3, in either transfected or freshly isolated adipose cells. We conclude that alpha2-HSG inhibits insulin-stimulated Elk-1 phosphorylation, but not glucose transport, in adipose cells by a mechanism that may involve effector molecules downstream of insulin receptor substrate proteins.

Cytokine regulation of the acute-phase protein levels in multiple myeloma

BACKGROUND

Interleukin (IL) 6 has an important role in the regulation of acute-phase proteins (APPs) during an acute-phase response. We studied IL-6 and other cytokines to determine if they regulate serum APP levels in the same way under the condition of the aberrant, long-lasting 'acute-phase response' that occurs in patients with chronic inflammation and cancer.

METHODS

Serum levels of nine positive APPs [CRP, SAA, C1-INH, Bf, C5, C8, C9, alpha 1-acidic glycoprotein (AGP) and haptoglobin] and two negative APPs [transferrin and alpha 2-HS glycoprotein (AHSG)] were measured using immunochemical methods in 59 multiple myeloma patients and in 72 healthy control subjects. Serum IL-6 and tumour necrosis factor (TNF) alpha levels were determined by bioassays.

RESULTS

IL-6 was negatively correlated with five out of nine (C1-INH, C8, C9, AGP and haptoglobin) positive APPs but positively correlated with C-reactive protein (CRP). When patients with high and low IL-6 serum concentration were compared, CRP levels were higher, AGP and haptoglobin levels were lower in the high- than in the low-L-6 group, whereas no significant difference between the two groups was found in levels of the other positive and negative APPs. TNF-alpha levels were negatively correlated with transferrin and AHSG levels. No difference in the levels of positive APPs was observed between patients with low and high TNF-alpha serum concentration. By contrast, levels of both transferrin and AHSG were significantly lower in the high- than in the low-TNF-alpha group.

CONCLUSIONS

These findings indicate that, except for regulation of the negative APPs by TNF-alpha, the mechanism of APP regulation is different under the conditions of the short-term and the chronic, long-lasting 'acute-phase reaction'.

Structural and functional analysis of the 5'-transcription control region for the human alpha2-HS glycoprotein gene

The human alpha2-HS glycoprotein (A2HS) and rat phosphoprotein of Mr 63000 (pp63) are homologous plasma proteins and members of the fetuin superfamily. A2HS is involved in important functions such as inhibition of the tyrosine kinase activity of the insulin receptor, regulation of calcium metabolism and osteogenesis as well as protease inhibitory activity. We report an analysis of the 5' transcription control region (4 kb) of the A2HS gene. Its most proximal 300 nt display a very potent transcriptional activity. The latter is likely accounted for by C/EBP and NF1 binding sites that are conserved from the human A2HS gene to the rat pp63 gene. In contrast, these human and rat genes appear to largely diverge beyond their proximal promoter.

Cloning and targeted deletion of the mouse fetuin gene

We proposed that the alpha2-Heremans Schmid glycoprotein/fetuin family of serum proteins inhibits unwanted mineralization. To test this hypothesis in animals, we cloned the mouse fetuin gene and generated mice lacking fetuin. The gene consists of seven exons and six introns. The cystatin-like domains D1 and D2 of mouse fetuin are encoded by three exons each, whereas a single terminal exon encodes the carboxyl-terminal domain D3. The promoter structure is well conserved between rat and mouse fetuin genes within the regions shown to bind transcription factors in the rat system. Expression studies demonstrated that mice homozygous for the gene deletion lacked fetuin protein and that mice heterozygous for the null mutation produced roughly half the amount of fetuin protein produced by wild-type mice. Fetuin-deficient mice were fertile and showed no gross anatomical abnormalities. However, the serum inhibition of apatite formation was compromised in these mice as well as in heterozygotes. In addition, some homozygous fetuin-deficient female ex-breeders developed ectopic microcalcifications in soft tissues. These results corroborate a role for fetuin in serum calcium homeostasis. The fact that generalized ectopic calcification did not occur in fetuin-deficient mice proves that additional inhibitors of phase separation exist in serum.

Effect of cell density and growth factors on matrix GLA protein expression by normal rat kidney cells

The present studies demonstrate that the expression of the vitamin K-dependent matrix Gla protein (MGP) is critically dependent on cell density in culture. Subculture of confluent NRK cells to 1/30 of the confluent cell density causes a 50- to 100-fold decline in MGP expression per cell within two days. MGP expression subsequently increases with increasing cell density and eventually attains a level of expression per cell at five days post-confluence that is over 2,000-fold greater than was seen in the cells two days after the 1 to 30 subculture. These reversible, density-dependent changes in MGP expression are far larger than have been previously reported for other secreted proteins and suggest that the as yet unknown function of MGP requires its expression at high cell density but not at low. We have also observed that human epidermal growth factor (EGF) causes a 20-fold reduction in MGP expression in post-confluent, non-dividing cultures and suggest that the suppression of MGP function at high density may be a prelude to cell migration or division in response to appropriate signals.

Fetuin/alpha2-HS glycoprotein is a transforming growth factor-beta type II receptor mimic and cytokine antagonist

The serum glycoprotein fetuin is expressed during embryogenesis in multiple tissues including limb buds and has been shown to promote bone remodeling and stimulate cell proliferation in vitro. In this report, we demonstrate that fetuin antagonizes the antiproliferative action of transforming growth factor-beta1 (TGF-beta1) in cell cultures. Surface plasmon resonance measurements show that fetuin binds directly to TGF-beta1 and TGF-beta2 and with greater affinity to the TGF-beta-related bone morphogenetic proteins (BMP-2, BMP-4, and BMP-6). In a competitive enzyme-linked immunosorbent assay, fetuin blocked binding of TGF-beta1 to the extracellular domain of TGF-beta receptor type II (TbetaRII), one of the primary TGF-beta-binding receptors. A comparison of fetuin and TbetaRII shows homology in an 18-19-amino acid sequence, which we have designated TGF-beta receptor II homology 1 domain (TRH1). Since the TRH1 sequence is known to form a disulfide loop in fetuin, cyclized TRH1 peptides from both fetuin and TbetaRII were chemically synthesized and tested for cytokine binding activity. Cyclized TRH1 peptide from TbetaRII bound to TGF-beta1 with greater affinity than to BMP-2, while the cyclized TRH1 peptide from fetuin bound preferentially to BMP-2. Finally, fetuin or neutralizing anti-TGF-beta antibodies blocked osteogenesis and deposition of calcium-containing matrix in cultures of dexamethasone-treated rat bone marrow cells. In summary, these experiments define the TRH1 peptide loop as a cytokine-binding domain in both TbetaRII and fetuin and suggest that fetuin is a natural antagonist of TGF-beta and BMP activities.

Isolation and molecular cloning of a novel bone phosphoprotein related in sequence to the cystatin family of thiol protease inhibitors

We describe here the isolation of a novel non-collagenous protein from the acid demineralization extract of bovine cortical bone. This 24-kDa protein is multiply phosphorylated at serine residues in Ser-X-Glu/Ser(P) sequences, a recognition motif for phosphorylation by the secretory pathway protein kinase, and we have termed this protein secreted phosphoprotein 24 (spp24). The cDNA structure of spp24 was determined by sequencing cDNA fragments obtained by reverse transcription-polymerase chain reaction, 3'-rapid amplification of cDNA ends, and screening a lambda gt11 cDNA library. This cDNA sequence predicts a 200-residue initial translation product which consists of a 20-residue signal sequence and the 180-residue mature spp24. Northern blot analysis using the spp24 cDNA showed that spp24 mRNA is in liver and bone but not in heart, lung, kidney, or spleen. A search of existing protein sequences revealed that the N-terminal 107 residues of mature spp24 are related in sequence to the cystatin family of thiol protease inhibitors, which suggests that spp24 could function to modulate the thiol protease activities that are known to be involved in bone turnover. Several of the proteins in the cystatin family that are most closely related to spp24 are not only thiol protease inhibitors but are also precursors to peptides with potent biological activity, peptides such as bradykinin and the neutrophil antibiotic peptides. It is therefore possible that the intact form of spp24 found in bone could also be a precursor to a biologically active peptide, a peptide which could coordinate an aspect of bone turnover.