Cloning of a type I TGF-beta receptor and its effect on TGF-beta binding to the type II receptor

Bone morphogenetic proteins inhibit adipocyte differentiation by bone marrow stromal cells

The bone morphogenetic proteins were originally identified based on their ability to induce ectopic bone formation in vivo and have since been identified as members of the transforming growth factor-beta gene superfamily. It has been well established that the bone morphogenetic cytokines enhance osteogenic activity in bone marrow stromal cells in vitro. Recent reports have described how bone morphogenetic proteins inhibited myogenic differentiation of bone marrow stromal cells in vitro. In vivo, bone marrow stromal cells differentiate along the related adipogenic pathway with advancing age. The current work reports the inhibitory effects of the bone morphorphogenetic proteins on adipogenesis in a multipotent murine bone marrow stromal cell line, BMS2. When exposed to bone morphogenetic protein-2, the pre-adipocyte BMS2 cells exhibited the expected induction of the osteogenic-related enzyme, alkaline phosphatase. Following induction of the BMS2 cells with adipogenic agonists, adipocyte differentiation was assessed by morphologic, enzymatic, and mRNA markers. Flow cytometric analysis combined with staining by the lipophilic fluorescent dye, Nile red, was used to quantitate the extent of lipid accumulation within the BMS2 cells. By this morphologic criteria, the bone morphogenetic proteins inhibited adipogenesis at concentrations of 50 to 500 ng/ml. This correlated with decreased levels of adipocyte specific enzymes and mRNAs. The BMS2 pre-adipocytes constitutively expressed mRNA encoding bone morphogenetic protein-4 and this was inhibited by adipogenic agonists. Together, these findings demonstrate that bone morphogenetic proteins act as adipogenic antagonists. This supports the hypothesis that adipogenesis and osteogenesis in the bone marrow microenvironment are reciprocally regulated.

Expression of transforming growth factor type III receptor in vascular endothelial cells increases their responsiveness to transforming growth factor beta 2

Bovine aortic endothelial cells (BAECs) express both type I and type II receptors for transforming growth factor beta (TGF beta). These cells respond to TGF beta 1 but are relatively refractory to another isoform of TGF beta, termed TGF beta 2. TGF beta s are thought to signal through receptor complexes composed of type I and/or type II receptors, both of which appear to be functional serine-threonine kinases. The TGF beta type III receptor, on the other hand, does not seem to have any direct signaling capacity. We have now stably transfected BAECs with the type III receptor cDNA. These cells displayed surface expression of the type III receptor protein, as determined by cross-linking with iodinated TGF beta 1 and immunoprecipitation with antibodies to the type III receptor protein. Transfected BAECs exhibit increased responsiveness to TGF beta 2 by several different criteria including an increase in plasminogen activator inhibitor-1 protein and inhibition of migration and proliferation. Thus, the type III receptor protein may play a role in presenting TGF beta 2 to the type II receptor and increase responsiveness to TGF beta 2 to a level comparable to that of TGF beta 1.

Colon cancer cells that are not growth inhibited by TGF-beta lack functional type I and type II TGF-beta receptors

OBJECTIVE

The authors determined the molecular mechanisms for the failure of transforming growth factor-beta (TGF-beta) to inhibit the growth of SW1116 and SW48 colon cancer cell lines.

BACKGROUND

Transforming growth factor-beta is a bifunctional regulator of cell growth that typically stimulates proliferation of mesenchymal cells, but inhibits proliferation of normal epithelial cells. In the colon, TGF-beta appears to arrest proliferation of enterocytes as they leave the intestinal crypt and move to the villus tip. Transforming growth factor-beta actions are mediated by binding to heteromeric complexes of type I and type II TGF-beta receptors. Loss of TGF-beta responsiveness may contribute to uncontrolled cell growth and cancer.

METHODS

The effects of TGF-beta 1 on DNA synthesis were measured by incorporation of tritiated thymidine into DNA of cultures of moderately differentiated adenocarcinoma (SW48) and poorly differentiated adenocarcinoma (SW1116) colon cell lines and a mink lung epithelial cell line (CCL-64). The effects of TGF-beta on the expression of c-myc, TGF-alpha, and TGF-beta in SW48 cells, SW1116 cells, and CCL-64 cells (c-myc only) were measured by Northern blot analysis. Expression of TGF-beta receptors in the cell lines was measured using competitive binding assays, receptor affinity labelling techniques, and reverse transcriptase-polymerase chain reaction.

RESULTS

Incubation with TGF-beta 1 (50 ng/mL) did not decrease serum-stimulated uptake of [3H]-thymidine into actively growing cultures of SW48 or SW1116 cells, but suppressed DNA synthesis of actively growing CCL-64 cells by 90%. Similarly, incubation with TGF-beta 1 (12 ng/mL) for 4 hours did not substantially alter the mRNA levels of c-myc, TGF-alpha, and TGF-beta 1 in either colon tumor cell line, although levels of c-myc mRNA in CCL-64 cells were reduced by TGF-beta 1 treatment. Competitive displacement of [125I]-TGF-beta 1 binding detected high levels (16,500 TGF-beta receptors per cell) of specific, high-affinity (200 pmol/L half-displacement) TGF-beta receptors on CCL-64 cells. In marked contrast, very low levels of TGF-beta 1 binding to SW1116 cells (250 receptors per cell) and SW48 cells (260 receptors per cell) were detected. Autoradiograms of CCL-64 cells affinity labelled with [125I]TGF-beta 1 revealed the presence of type I, type II, and type III TGF-beta receptors. No TGF-beta receptors were identified on SW1116 cells, and only very low levels of the nonsignaling type III TGF-beta receptors were detected on SW48 cells. Reverse transcriptase-polymerase chain reaction amplification detected mRNAs for type I, type II, and type III TGF-beta receptors in CCL-64 cells. SW48 cells, and SW1116 cells.

CONCLUSIONS

These results suggest that the lack of growth inhibition by TGF-beta in SW48 and SW1116 colon cancer cells may be caused by a lack of expression of functional TGF-beta receptors.

Cytokines regulate the cellular phenotype of developing neural lineage species

The patterns and mechanisms of action of inductive signals that orchestrate neural lineage commitment and differentiation in the mammalian brain are incompletely understood. To examine these developmental issues, we have utilized several culture systems including conditionally immortalized cell lines, subventricular zone progenitor cells and primary neuronal cultures. A neural stem and progenitor cell line (MK31) was established from murine embryonic hippocampus by retroviral transduction of temperature-sensitive alleles of the simian virus 40 large tumor antigen. At the non-permissive temperature for antigen expression (39 degrees C) in serum-free media, the neural stem cells give rise to a series of increasingly mature neuronal progenitor and differentiated cellular forms under the influence of a subset of hematolymphopoietic cytokines (interleukins 5, 7, 9 and 11), when individually co-applied with transforming growth factor alpha, after pretreatment with basic fibroblast growth factor. These cellular forms elaborated a series of progressively more mature neurofilament proteins, a sequential pattern of ligand-gated channels, and inward currents and generation of action potentials with mature physiological properties. Because the factors regulating the development of central nervous system astrocytes have been so difficult to define, we have chosen to focus, in this manuscript, on the elaboration of this cell type. At 39 degrees C, application of a subfamily of bone morphogenetic proteins of the transforming growth factor beta superfamily of growth factors sanctioned the selective expression of astrocytic progenitor cells and mature astrocytes, as defined by sequential elaboration of the Yb subunit of glutathione-S-transferase and glial fibrillary acidic protein. These lineage-specific cytokine inductive relationships were verified using subventricular zone neural progenitor cells generated by the application of epidermal growth factor, alone or in combination with basic fibroblast growth factor, to dissociated cellular cultures derived from early embryonic murine brain, a normal non-transformed developmental population. Finally, application of a different series of cytokines from five distinct factor classes (basic fibroblast growth factor, platelet-derived growth factor-AA, insulin-like growth factor 1, neurotrophin 3 and representative gp130 receptor subunit-related ligands) caused the elaboration of oligodendroglial progenitor species and post-mitotic oligodendrocytes, defined by progressive morphological maturation and the expression of increasingly advanced oligodendroglial and oligodendrocyte lineage markers. In addition, seven different gp130-associated neuropoietic (ciliary neurotrophic factor, leukemia inhibitory factor, oncostatin-M) and hematopoietic (interleukins 6, 11, 12, granulocyte-colony stimulating factor) cytokines exhibited differential trophic effects on oligodendroglial lineage maturation and factor class interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Independent changes in type I and type II receptors for transforming growth factor beta induced by bone morphogenetic protein 2 parallel expression of the osteoblast phenotype

Transforming growth factor beta (TGF-beta), a potent regulator of bone formation, has bifunctional effects on osteoblast replication and biochemical activity that appear differentiation dependent. We now show that cell surface binding sites for TGF-beta vary markedly among fibroblasts, bone-derived cells, and highly differentiated osteosarcoma cultures from fetal rats. Expression of betaglycan and type II receptors decline relative to type I receptor expression in parallel with an increase in osteoblast-like activity, predicting that the ratio among various TGF-beta binding sites could influence how its signals are perceived. Bone morphogenetic protein 2 (BMP-2), which induces osteoblast function, does not alter TGF-beta binding or biochemical activity in fibroblasts and has only small effects in less differentiated bone cells. In contrast, BMP-2 rapidly reduces TGF-beta binding to betaglycan and type II receptors in osteoblast-enriched primary cell cultures and increases its relative binding to type I receptors in these cells and in ROS 17/2.8 cultures. Pretreatment with BMP-2 diminishes TGF-beta-induced DNA synthesis in osteoblast-enriched cultures but synergistically enhances its stimulatory effects on either collagen synthesis or alkaline phosphatase activity, depending on the present state of bone cell differentiation. Therefore, BMP-2 shifts the TGF-beta binding profile on bone cells in ways that are consistent with progressive expression of osteoblast phenotype, and these changes distinguish the biochemical effects mediated by each receptor. Our observations indicate specific stepwise actions by TGF-beta family members during osteoblast differentiation, developing in part from changes imprinted by BMP-2 on TGF-beta receptor stoichiometry.

Phosphorylation-dependent interaction of the cytoplasmic domains of the type I and type II transforming growth factor-beta receptors

Transforming growth factor-beta (TGF-beta) transduces signals through its type I and type II receptors. Both receptor types have previously been shown to interact in a heteromeric complex in the presence of TGF-beta. We have now characterized these interactions between both receptor types using a combination of yeast two-hybrid interaction assays and coimmunoprecipitation analyses. Our results indicate a direct association between the cytoplasmic domains of the two receptor types. Mutation analysis of these cytoplasmic domains reveals that this direct interaction requires kinase activity and, thus, depends on phosphorylation, probably via a transphosphorylation mechanism. Furthermore, the two receptor types already have an inherent affinity for each other in the absence of TGF-beta, and the heteromeric complex can be detected in coimmunoprecipitations under these conditions. Taken together, our results reveal a novel mechanism of receptor complex formation, whereby two different cytoplasmic domains directly associate with each other. This interaction may play a major role in activation of serine/threonine kinase receptors.

GS domain mutations that constitutively activate T beta R-I, the downstream signaling component in the TGF-beta receptor complex

The TGF-beta type II receptor (T beta R-II) is a transmembrane serine/threonine kinase that, upon ligand binding, recruits and phosphorylates a second transmembrane kinase, T beta R-I, as a requirement for signal transduction. T beta R-I is phosphorylated by T beta R-II in the GS domain, a 30 amino acid region preceding the kinase domain and conserved in type I receptors for other TGF-beta-related factors. The functional role of seven serines and threonines in the T beta R-I GS domain was investigated by mutational analysis. Five of these residues are clustered (TTSGSGSG) in the middle of the GS domain. Mutation of two or more of these residues impairs phosphorylation and signaling activity. Two additional threonines are located near the canonical start of the kinase domain, and their individual mutation to valine strongly inhibits receptor phosphorylation and signaling activity. Replacement of one of these residues, Thr204, with aspartic acid yields a product that has elevated in vitro kinase activity and signals anti-proliferative and transcriptional responses in the absence of ligand and T beta R-II. The identification of constitutively active T beta R-I forms confirms the hypothesis that this kinase acts as a down-stream signaling component in the TGF-beta receptor complex, and its activation by T beta R-II or by mutation is necessary and sufficient for propagation of anti-proliferative and transcriptional responses.

Osteogenic growth peptide regulates proliferation and osteogenic maturation of human and rabbit bone marrow stromal cells

The recently discovered osteogenic growth peptide (OGP) has been shown to regulate proliferation in fibroblastic and osteoblastic cell lines derived from rats and mice and also alkaline phosphatase activity in the latter was found to be affected. In vivo the OGP enhances bone formation and trabecular bone density. The results of the current study indicate that the OGP is also a potent regulator of marrow stromal cells from man and rabbit, as well as rabbit muscle fibroblasts. The main OGP activity in both marrow systems is a marked stimulation of alkaline phosphatase activity and matrix mineralization. In the rabbit-derived cell culture this enhancement is accompanied by a reciprocal inhibition of proliferation. On the other hand, the human cells show a concomitant increase of both parameters. The proliferative effect of the OGP is similar to that of growth hormone (GH) and basic fibroblast growth factor (bFGF). The combined activity of the OGP with GH is smaller than that of each of the polypeptides alone. The OGP and bFGF potentiate each other. Of the three polypeptides tested, OGP is the most potent enhancer of alkaline phosphatase activity and mineralization. bFGF has no influence on these characteristics of osteogenic maturation. The OGP maturational activity is unaffected by either GH or bFGF. These data suggest that the marrow stromal cells serve as targets for the OGP that mediate the OGP-induced increase in osteogenesis. The effect on the human cells implies a role for the OGP in clinical situations where the osteogenic potential of bone marrow is involved.

Beta IG-H3, a novel secretory protein inducible by transforming growth factor-beta, is present in normal skin and promotes the adhesion and spreading of dermal fibroblasts in vitro

We have previously identified a gene, beta ig-h3, which is highly induced in A549 cells (human lung adenocarcinoma) after growth arrest by transforming growth factor-beta. The beta ig-h3 gene encodes a 683-amino-acid secretory protein termed beta IG-H3, and treatment of several cell lines with transforming growth factor-beta results in increased secretion of beta IG-H3 into cell culture supernatants. In this report, we further characterize beta IG-H3 with respect to its synthesis and function. Primary human foreskin fibroblasts grown in monolayer culture produced beta IG-H3 mRNA and secreted beta IG-H3 protein into the growth media. Treatment of these cells with transforming growth factor-beta led to an increase in beta IG-H3 mRNA and protein. Cells grown on three-dimensional scaffolds secreted beta IG-H3 into the extracellular matrix, as judged by immunostaining with anti-beta IG-H3 antibodies. beta IG-H3 was also detected in normal human skin, especially in the papillary dermis. Finally, we show that recombinant beta IG-H3 supported attachment and spreading of dermal fibroblasts, suggesting that beta IG-H3 may function as an extracellular attachment protein in skin.

Inactive type II and type I receptors for TGF beta are dominant inhibitors of TGF beta-dependent transcription

Although transforming growth factor-beta (TGF beta) is implicated in differentiation and disease, proof of in vivo function requires specific inhibitors of the TGF beta cascade. TGF beta binds a family of type I and type II receptors (T beta RI, T beta RII), containing a cytoplasmic serine/threonine kinase domain. We previously reported that kinase-deficient T beta RII (delta kT beta RII) blocks TGF beta-dependent transcription in cardiac myocytes. It is controversial whether both receptors are needed in all cells for gene regulation by TGF beta or whether they mediate distinct subsets of TGF beta-dependent events. To resolve this uncertainty, TGF beta-dependent transcription was investigated in cardiac myocytes versus mink lung epithelial cells. 1) delta kT beta RII inhibits induction of a TGF beta-responsive reporter gene, in both cell backgrounds. 2) Charged-to-alanine mutations of key residues of the T beta RII kinase, including consensus ATP binding and amino acid recognition motifs, are competent for binding but not transcriptional activation. Each inactive receptor inhibits TGF beta-dependent transcription in both cell types. 3) Kinase-deficient T beta RI (delta kT beta RI) likewise impairs TGF beta-dependent transcription, less completely than delta kT beta RII; kinase-deficient activin type I receptor has no effect. 4) TGF beta-binding proteins in cardiac cells and Mv1Lu cells are comparable by affinity labeling and immunoprecipitation; however, Mv1Lu cells express up to 3-fold higher levels of T beta RII and T beta RI. Thus, the model inferred from TGF beta-resistant cell lines (that T beta RII and T beta RI are necessary in tandem for the TGF beta-signaling complex to regulate transcription) is valid for cardiac myocytes, the cell type most prominently affected in TGF beta-deficient animals.

Constitutive overexpression of c-fos protein in rat liver epithelial cells decreases TGF-beta synthesis and increases TGF-beta 1 receptors

We have previously shown that rat liver epithelial cells were more sensitive to TGF-beta 1 when they were transfected with c-fos cDNA. We analyzed the production of TGF-beta and TGF-beta 1 binding proteins in transfected and parental cells. TGF-beta-like activity released in the medium was reduced in c-fos expressing cells. TGF-beta 1 binding sites were more numerous in transfected cells (x3). Cross-linking studies confirmed that c-fos transfected cells showed increased binding of 125I-TGF-beta 1 to membrane binding sites corresponding to type I, II and III receptors. Transfected cells internalized and degraded 125I-TGF-beta 1 more rapidly than parental cells. TGF-beta 1 incubation rapidly down-regulated the receptors. In parental cells, the down-regulation was total, while in transfected cells, a few binding proteins could still be detected. The c-fos cell line is an interesting tool in analysing the mechanism of action of TGF-beta.

Regulation of fibroblast procollagen production. Transforming growth factor-beta 1 induces prostaglandin E2 but not procollagen synthesis via a pertussis toxin-sensitive G-protein

Transforming growth factor-beta 1 (TGF beta 1) initiates a series of signalling events resulting in diverse cellular responses including stimulation of extracellular matrix protein production. In this study we have investigated the role of pertussis toxin-sensitive G-proteins in mediating the effects of TGF beta 1 on fibroblast procollagen metabolism. TGF beta 1 stimulated human fetal lung fibroblast procollagen synthesis and production in a dose-dependent manner which was maximal at 0.5 ng/ml. TGF beta 1 also decreased the proportion of newly synthesized procollagen degraded intracellularly. Pertussis toxin, a G-protein inhibitor, further stimulated TGF beta 1-induced procollagen synthesis and production, but alone it had no effect on fibroblast procollagen metabolism. Addition of indomethacin also potentiated the TGF beta 1-induced increase in procollagen synthesis and production. The effects of pertussis toxin and indomethacin were not additive. Pertussis toxin and indomethacin did not affect the proportion of newly synthesized procollagen degraded intracellularly, either alone or in combination, by control cells. The TGF beta 1-induced decrease in intracellular procollagen degradation was maintained but not further affected by pertussis toxin or indomethacin. TGF beta 1 increased prostaglandin E2 (PGE2) compared with PGE2 production by control cells. Addition of pertussis toxin or indomethacin blocked the TGF beta 1-induced increase in PGE2 production. The TGF beta 1-induced increase in PGE2 preceded the increase in procollagen production. These results demonstrate that TGF beta 1-induced procollagen synthesis by lung fibroblasts is modulated by production of PGE2. Pertussis toxin and indomethacin block the production of PGE2 and enhance the effect of TGF beta 1 on procollagen synthesis. From these data we conclude that the effects of TGF beta 1 on PGE2 production but not procollagen synthesis are mediated via a receptor linked to a pertussis toxin-sensitive G-protein.

Transforming growth factor beta activation of p44mapk in proliferating cultures of epithelial cells

Transforming growth factor-beta (TGF-beta) is a potent growth inhibitor of a variety of epithelial cell types. The primary signaling mechanism involved in mediating this and other cellular effects of TGF-beta is still unknown. We report here that both TGF-beta 1 and TGF-beta 2 resulted in a rapid activation of mitogen-activated protein kinase (MAPK) p44mapk, occurring within 5-10 min of growth factor addition. This effect occurred in exponentially proliferating cultures of intestinal epithelial (IEC) 4-1 cells under conditions in which DNA synthesis was inhibited by 95% to 98%. Furthermore, TGF-beta 2 induced a sustained activation of p44mapk under these conditions, lasting for at least 90 min after initial growth factor treatment. Another TGF-beta-sensitive epithelial cell line (CCL 64) displayed a similar rapid increase in p44mapk activity when treated with TGF-beta 1. In contrast, in IEC 4-6 cells that are resistant to TGF-beta effects on growth and DNA synthesis, TGF-beta 2 treatment did not result in an activation of p44mapk. In contrast to the results in proliferating cultures, treatment of quiescent cultures of IEC 4-1 cells with TGF-beta 2 resulted in no significant change in either DNA synthesis or p44mapk activity within 15 min of TGF-beta addition. In contrast, addition of the growth-stimulatory combination of factors (epidermal growth factor + insulin + transferrin = EIT) to quiescent and proliferating IEC 4-1 cells stimulated DNA synthesis and resulted in a sustained activation of p44mapk. Together, our results suggest an association between activation of p44mapk and both TGF-beta-mediated growth inhibition and EIT-mediated growth stimulation. This suggests that the specificity for the cellular effects of growth factors may not occur at the level of MAPK activation per se, but rather at downstream events that include phosphorylation of distinct transcriptional complexes and activation of a select assortment of genes. With regard to TGF-beta specifically, we have proposed a model to explain how activation of p44mapk may be associated with a growth-inhibitory response.

Transforming growth factor-beta 1 induction of novel extracellular matrix proteins that trigger resistance to tumor necrosis factor cytotoxicity in murine L929 fibroblasts

The molecular basis by which transforming growth factor (TGF)-beta 1 protects certain tumor cells from tumor necrosis factor (TNF) cytotoxicity was investigated. When pretreated, with TGF-beta 1, -beta 2, and -beta 3, murine L929S fibroblasts developed resistance to TNF cytotoxicity. Time course experiments revealed that TGF-beta 1 initially induced both cellular protein-tyrosine phosphorylation and simultaneous secretion of a novel extracellular matrix TNF-resistance triggering (TRT) protein(s), which closely preceded the acquisition of TNF-resistance. TGF-beta 2 and -beta 3 also increased tyrosine phosphorylation. However, both molecules failed to stimulate TRT secretion. The increased levels of phosphorylation, particularly to 9 specific protein tyrosine kinase inhibitor-sensitive cellular proteins, appeared to alter the TNF killing pathway. TGF-beta 1-induced TRT secretion required participation of unknown serum factors. TRT adhered strongly to polystyrene plates and resisted treatment with heat (60 degrees C, 30 min), collagenase, alpha 2-macroglobulin, heparin, antibodies against TGF-beta s, and limited trypsin digestion. Notably, TRT promoted TNF-resistance via activation of tyrosine and serine/threonine kinase functions in L929S. Thus, the molecular pathway involves TGF-beta 1-mediated initiation of a rapid tyrosine phosphorylation of cellular protein substrates (which alters TNF cytotoxic pathway), and a simultaneous secretion of TRT, which in turn signals the cells to maintain the levels of phosphorylation, thereby sustaining the TNF-resistance.

Signaling activity of homologous and heterologous transforming growth factor-beta receptor kinase complexes

Transforming growth factor-beta (TGF-beta) signaling in Mv1Lu lung epithelial cells requires coexpression of TGF-beta receptors I (T beta R-I) and II (T beta R-II), two distantly related transmembrane serine/threonine kinases that form a heteromeric complex upon ligand binding. Here, we examine the formation of TGF-beta receptor homo-oligomers and their possible contribution to signaling. T beta R-I can contact ligand bound to T beta R-II, but not ligand free in the medium, and thus cannot form ligand-induced homo-oligomers. T beta R-II, which binds ligand on its own, formed oligomeric complexes when overexpressed in transfected COS cells. However, these complexes were largely ligand-independent and involved immature receptor protein. Since ligand-induced homo-oligomers could not be obtained with the wild-type TGF-beta receptors, we studied receptor cytoplasmic domain homo-oligomerization by using receptor chimeras. The extracellular domain of T beta R-II was fused to the transmembrane and cytoplasmic domains of T beta R-I, yielding T beta R-II/I, and the extracellular domain of T beta R-I was fused to the transmembrane and cytoplasmic domains of T beta R-II, yielding T beta R-I/II. When contransfected with wild-type receptors and exposed to ligand, T beta R-II/I formed a complex with T beta R-I, and T beta R-I/II formed a complex with T beta R-II, thus yielding complexes with homologous cytoplasmic domains. T beta R-II/I transfected alone or with T beta R-I did not restore TGF-beta responsiveness in T beta R-II-defective cell mutants. Furthermore, T beta R-II/I acted in a dominant negative fashion, inhibiting restoration of TGF-beta responsiveness by a cotransfected T beta R-II in T beta R-II-defective cells and by a cotransfected T beta R-I in T beta R-I-defective cells. Similarly, T beta R-I/II transfected alone or with T beta R-II did not restore TGF-beta responsiveness and acted in a dominant negative fashion against T beta R-I. Together with previous genetic and biochemical evidence, these results suggest that TGF-beta mediates transcriptional and antiproliferative responses through the heteromeric T beta R-I.T beta R-II complex and not through homo-oligomeric T beta R-I or T beta R-II complexes.

Inhibin antagonizes inhibition of liver cell growth by activin by a dominant-negative mechanism

The beta:beta activin homodimer and alpha:beta inhibin heterodimer are mutual antagonists which share a common beta subunit. Recently, it has been shown that, similar to transforming growth factor-beta 1, activin is an inhibitor of hepatocyte DNA synthesis. The activin receptor appears to be an obligatory complex of genetically distinct type I and II transmembrane serine/threonine kinases. Activin type I receptors, SKR1 and SKR2, were first cloned from well differentiated human hepatoma cells (HepG2). This prompted us to investigate the binding of activin and inhibin to receptors from HepG2 cells and the effect of the two ligands on DNA synthesis. Here we show that beta:beta activin binds to the activin type II receptor kinase (ActRII) which induces activin binding to the type I receptor kinase SKR2 to form ActRII.beta:beta.SKR2 complexes in which an activin beta chain occupies each receptor subunit. Inhibin also binds to ActRII through its beta subunit, competes with the binding of activin to ActRII, but fails to form the ActRII.SKR2 complex. No specific binding site for inhibin could be demonstrated in HepG2 cells. Inhibin, which had no activity of its own, antagonized the inhibitory effect of activin on DNA synthesis. The results suggest that inhibin may be a natural antagonist of assembly of the heterodimeric activin receptor complex through a dominant-negative mechanism.

Cloning of a novel type II serine/threonine kinase receptor through interaction with the type I transforming growth factor-beta receptor

The transforming growth factor-beta (TGF-beta) superfamily comprises a number of molecules that are involved in a wide variety of biological processes. Specific receptors for several members of this family have been molecularly identified, forming a new category of transmembrane serine/threonine kinase receptors. The type I and type II receptor interact both physically and functionally, thereby cooperating to generate intracellular signals. The yeast two-hybrid system was used to identify proteins that can interact with the cytoplasmic region of the type I TGF-beta receptor. One of the proteins identified encodes a novel putative serine/threonine kinase receptor. Sequence analysis suggests that this molecule belongs to the type II receptor class. This receptor, however, is distinct from other type II receptors in having an extraordinarily long C-terminal tail region. The pattern of expression in adult tissues is different from that of other known type II receptors; it is highly expressed in heart and liver. In the yeast system, the cytoplasmic regions of different combinations of type I and type II receptors heterodimerize, providing a new cloning strategy for the large number of serine/threonine kinase receptors likely to exist for the many ligands of the TGF-beta superfamily.

Distribution and characterization of specific cellular binding proteins for bone morphogenetic protein-2

Bone morphogenetic proteins (BMPs), which were originally identified by their novel ability to induce de novo cartilage and bone formation in vivo, are multifunctional proteins structurally related to transforming growth facto-beta s, activins, and inhibins. As a first step to elucidate the precise physiological function as well as the action mechanism of BMPs, we have examined the distribution of the specific cellular binding proteins for BMP-2 on a wide variety of cell types. A single class of high affinity-specific binding sites for BMP-2 were identified not only on osteoblastic cells but also on major types of non-hematopoietic cells in a rather ubiquitous fashion (1,200-60,000 receptors/cell, Kd = 35-230 pM); these cells included fibroblasts, keratinocytes, astrocytes, kidney epithelial cells, and tumor cells of bone, muscle, lung, liver, kidney, stomach, colon, prostate, and neuronal tissue. Other growth factors including transforming growth factor-beta 1, activin A, and inhibin A did not compete for the binding of 125I-labeled BMP-2 to the cells. Affinity cross-linking of radiolabeled BMP showed five components with apparent molecular masses of 170, 105, 90, 80, and 70 kDa common to all three fibroblast cell lines analyzed. On the other hand, no specific binding sites for BMP-2 were identified on vascular endothelial cells or on hematopoietic cells including RPMI 1788 and RPMI 8226 (B-lymphocyte lineage), MOLT-3 and MOLT-4 (T-lymphocyte lineage), HL-60 (myeloid lineage), and K-562 (erythroid lineage). These results suggest that major types of cells other than hematopoietic cells and vascular endothelial cells may be potential targets for BMP-2 action.

The soluble exoplasmic domain of the type II transforming growth factor (TGF)-beta receptor. A heterogeneously glycosylated protein with high affinity and selectivity for TGF-beta ligands

The transforming growth factor (TGF)-beta type II receptor is a transmembrane serine/threonine kinase which is essential for all TGF-beta-induced signals. In several cell types TGF-beta 2 is as potent as TGF-beta or TGF-beta 3 in inducing cellular responses, yet TGF-beta 2 does not bind to the majority of expressed type II receptors. Here we characterized the properties of the soluble extracellular domain of the human TGF-beta type II receptor synthesized in COS-7 cells. Like the membrane-attached type II receptor, the soluble receptor contains complex N-linked oligosaccharides as well as additional sialic acid residues that cause it to migrate heterogenously upon SDS-polyacrylamide gel electrophoresis. 125I-TGF-beta 1 binds to and is chemically cross-linked to this protein. Unlabeled TGF-beta 1 inhibits the binding of 125I-TGF-beta 1 with an apparent dissociation constant (Kd) of approximately 200 pM, similar to the apparent Kd (approximately 50 pM) of the cell-surface type II receptor. TGF-beta 3 inhibits the binding of 125I-TGF-beta 1 to the soluble type II receptor with a similar dissociation constant, approximately 500 pM. In contrast, 125I-TGF-beta 2 cannot bind and be chemically cross-linked to the soluble type II receptor, nor does as much as a 125-fold excess of unlabeled TGF-beta 2 inhibit the binding of 125I-TGF-beta 1 to the soluble receptor. This is the first demonstration of the binding affinities of the type II receptor in the absence of the other cell-surface molecules known to bind TGF-beta. Expressed alone in COS-7 cells the type II receptor also cannot bind TGF-beta 2; co-expression of type III receptor enables the type II receptor to bind TGF-beta 2. Thus, the type III receptor or some other component is required for transmission of TGF-beta 2-induced signals by the type II receptor.

GH3 pituitary tumor cells contain heteromeric type I and type II receptor complexes for transforming growth factor beta and activin-A

Transforming growth factors beta (TGF-beta s) and activins induce and inhibins block secretion of follicle-stimulating hormone by rat GH3 pituitary tumor cells. Cheifetz et al. (Cheifetz, S., Ling, N., Guillemin, R., and Massagué, J. (1988) J. Biol. Chem. 263, 17225-17228) reported that GH3 cells express a approximately 50-kDa surface protein, termed the type IV TGF-beta receptor, that directly binds all of these peptide hormones. Here we show that GH3 cells express the previously identified type I and type II receptors for TGF-beta and activin-A. Immunoprecipitation of affinity-labeled surface binding proteins with antisera specific to known receptors demonstrated independent heteromeric complexes of TGF-beta types I and II receptors and of activin types I and II receptors. As judged by ligand-binding and cross-linking analysis, TGF-beta binding to the TGF-beta receptors is not inhibited by activin-A and activin-A binding to its receptors is not inhibited by TGF-beta. Screening of a cDNA library from GH3 cells for potential receptor serine-threonine kinases yielded the known types I and II TGF-beta and activin receptors. The presumed common intracellular signaling pathway for TGF-beta and activin in GH3 cells appears to be mediated by distinct cell-surface receptors.

The role of growth factors in preimplantation development

It has become clear that the mammalian embryo participates in a complex dialogue with the maternal physiology. The language of the dialogue is growth factor signalling. The embryo expresses receptors for insulin, IGFs, GH, EGF and cytokines including LIF, and CSFs; whilst ligands are secreted by the supporting tissues of the oviduct and uterus, and in some cases, the embryo itself. In the preimplantation period when the embryo is travelling to the uterus and passing through its first differentiation, these ligands affect embryonic physiology, apparently in ways that optimise developmental potential and synchronise embryonic and maternal physiologies. It is not yet clear in most cases whether this is by autocrine, paracrine or endocrine mode. In the crucial peri-implantation phase the embryo is preparing to invade the maternal system for which extensive uterine remodelling is necessary. A model is proposed in which a cascade of growth factor activities, orchestrated by the ovarian steroid patterns, choreographs the biochemical players (ECM proteinases and their inhibitors) which initiate this activity.

Characterization of a human hepatoma cell line with acquired resistance to growth inhibition by transforming growth factor beta 1 (TGF-beta 1)

A new cell line (Hep 3B-TR), which is resistant to growth-inhibition by transforming growth factor beta 1 (TGF-beta 1) up to 10 ng/ml (400 pM), was isolated from parental Hep 3B human hepatoma cells, which are sensitive to growth-inhibition by TGF-beta 1. In the presence of TGF-beta 1 (1 to 10 ng/ml), the growth of the parental cell line (Hep 3B-TS) was inhibited by more than 95%. Under the same conditions, the growth rate of the resistant clone (Hep 3B-TR) however, was identical in the presence or absence of TGF-beta 1 and was almost the same as that of the Hep 3B-TS cells in the absence of TGF-beta 1. Affinity crosslinking with 5 pM 125I-labeled TGF-beta 1 showed that the TGF-beta 1 receptors type I (TGF-beta RI) and type II (TGF-beta RII) were not present on the cell surface of the Hep 3B-TR cells, whereas they were present on the sensitive HEP 3B-TS cells. Hep 3B-TS cells had detectable TGF-beta RII mRNA, which was not found in Hep 3B-TR cells. RNA analysis showed different effects on the expression of TGF-beta 1, c-fos, c-myc, and protein disulfide isomerase (PDI) genes in the two cell lines in response to TGF-beta 1 protein. Addition of TGF-beta 1 (1 ng/ml) strongly increased the expression of TGF-beta 1 mRNA in Hep 3B-TS cells, but not in Hep 3B-TR cells.(ABSTRACT TRUNCATED AT 250 WORDS)

TGF-beta induced transdifferentiation of mammary epithelial cells to mesenchymal cells: involvement of type I receptors

The secreted polypeptide transforming growth factor-beta (TGF-beta) exerts its multiple activities through type I and II cell surface receptors. In epithelial cells, activation of the TGF-beta signal transduction pathways leads to inhibition of cell proliferation and an increase in extracellular matrix production. TGF-beta is widely expressed during development and its biological activity has been implicated in epithelial-mesenchymal interactions, e.g., in branching morphogenesis of the lung, kidney, and mammary gland, and in inductive events between mammary epithelium and stroma. In the present study, we investigated the effects of TGF-beta on mouse mammary epithelial cells in vitro. TGF-beta reversibly induced an alteration in the differentiation of normal mammary epithelial NMuMG cells from epithelial to fibroblastic phenotype. The change in cell morphology correlated with (a) decreased expression of the epithelial markers E-cadherin, ZO-1, and desmoplakin I and II; (b) increased expression of mesenchymal markers, such as fibronectin; and (c) a fibroblast-like reorganization of actin fibers. This phenotypic differentiation displays the hallmarks of an epithelial to mesenchymal transdifferentiation event. Since NMuMG cells make high levels of the type I TGF-beta receptor Tsk7L, yet lack expression of the ALK-5/R4 type I receptor which has been reported to mediate TGF-beta responsiveness, we evaluated the role of the Tsk7L receptor in TGF-beta-mediated transdifferentiation. We generated NMuMG cells that stably overexpress a truncated Tsk7L type I receptor that lacks most of the cytoplasmic kinase domain, thus function as a dominant negative mutant. These transfected cells no longer underwent epithelial to mesenchymal morphological change upon exposure to TGF-beta, yet still displayed some TGF-beta-mediated responses. We conclude that TGF-beta has the ability to modulate E-cadherin expression and induce a reversible epithelial to mesenchymal transdifferentiation in epithelial cells. Unlike other transdifferentiating growth factors, such as bFGF and HGF, these changes are accompanied by growth inhibition. Our results also implicate the Tsk7L type I receptor as mediating the TGF-beta-induced epithelial to mesenchymal transition.

Bovine activin receptor type II cDNA: cloning and tissue expression

The cDNA encoding the bovine activin type II receptor has been cloned by reverse transcription-polymerase chain reaction (RT-PCR) amplification of a bovine testicular RNA preparation. Sequence comparisons of the bovine activin type II receptor with its human, mouse and rat homologues show strong evolutionary conservation at the nucleotide level of 94.9%, 93.5%, 92.9% and at the amino acid level of 98.6%, 99.0%, 98.8%, respectively. Bovine activin type II receptor mRNA is widely but not strongly expressed in reproductive tissues, with a major RNA band at 6 kb and minor bands at 5 kb and 3 kb. The differential levels of expression observed in these tissues suggest that levels of bActRII gene expression are regulated. Furthermore, we have observed decreasing levels of the bovine activin type II receptor mRNA with testes maturation.

TGF-beta-receptor-mediated signaling

Transforming growth factor beta (TGF-beta) and its many relatives are thought to play key roles in the control of cell proliferation and differentiation. In particular, the ability of TGF-beta to induce growth arrest in epithelial cells has drawn considerable attention. The recent cloning of TGF-beta receptors, which are considered to be prototypes of a new class of cell-surface receptors, has provided a first insight into how TGF-beta signaling induces a variety of intracellular changes. Furthermore, recent advances in the characterization of the cell-cycle machinery have stimulated studies aimed at understanding how TGF-beta signaling leads to growth arrest in the late G1 phase of the cell cycle.

Expression of activin subunits, activin receptors and follistatin in postimplantation mouse embryos suggests specific developmental functions for different activins

Using in situ hybridization we have studied the localization of the messenger RNAs encoding the inhibin/activin subunits (alpha, beta A, beta B), the activin-binding protein follistatin and activin receptors (IIA, IIB) in mouse embryos during postimplantation development. From 6.5- to 9.5-days post coitum (p.c.) activin beta A and beta B subunit expression was restricted to the decidua, while activin receptor type IIB messages were exclusively detected in the embryo. Expression of activin receptor type IIA was apparent in the embryo as early as 9.5 days p.c. In contrast, follistatin transcripts were present in both the decidua and the embryo at the early postimplantation stages. In particular, the primitive streak region, specific rhombomeres in the developing hindbrain, somites, paraxial mesoderm and parietal endoderm cells attached to the Reichert's membrane showed strong expression of follistatin. In 10.5- and 12.5-day embryos expression of the beta A subunit message was abundant in mesenchymal tissue, in particular in the developing face, the body wall, the heart, precartilage condensations in the limb and in the mesenchyme of structures that show both epithelial and mesenchymal components, including tissues of the embryonic digestive, respiratory and genital tracts. The distribution of beta B transcripts was quite different from that observed for beta A. beta B is strongly expressed in selected regions of the brain, in particular the fore- and hindbrain, and in the spinal cord. Specific hybridization signals were also present in the epithelium of the stomach and oesophagus. Common sites of beta A and beta B expression are blood vessels, intervertebral disc anlagen, mesenchymal condensations in the flank region and the gonad primordium. The latter organ is the only site in the embryo where the alpha subunit is expressed, and thus where inhibit activity may be present. During the period of organogenesis the sites of expression of activin receptors type IIA and IIB messenger RNA (mRNA) generally coincide with or are adjacent to the sites of beta subunit expression. Differences in the expression patterns of the receptor RNAs are the whisker follicles, where type IIA is expressed, and the metanephros and the forebrain where type IIB transcripts are present. Taken together, the present data suggest that follistatin, but not one of the known activin forms (A,B,AB) is involved in early postimplantation development.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of G1 phase cyclin dependent kinases by transforming growth factor beta 1

Transforming growth factor beta 1 (TGF beta 1) inhibits epithelial cell proliferation late in the G1 phase of the cell cycle. We examined the effect of TGF beta 1 on known late G1 cell cycle regulators in an attempt to determine the molecular mechanism of growth inhibition by this physiological inhibitor. The results demonstrate that TGF beta 1 inhibits the late G1 and S phase specific histone H1 kinase activity of p33cdk2. This inhibition is not due to TGF beta 1's effect on p33cdk2 synthesis, but rather due to its negative effects on the late G1 phosphorylation of p33cdk2. It is also shown that TGF beta inhibits both late G1 cyclin A and cyclin E associated histone H1 kinase activities. The inhibitor has no effects on the synthesis of cyclin E but is shown to inhibit the synthesis of cyclin A protein in a cell cycle dependent manner. If TGF beta 1 is added to cells which have progressed further than 8 hours into G1, then it is without inhibitory effect on cyclin A synthesis. These effects of TGF beta 1 on late G1 cell cycle regulators correlate well with its inhibitory effects on cellular growth and suggest that these G1 cyclin dependent kinases might serve as targets for TGF beta 1-mediated growth arrest.

Multiple requirements for the receptor serine/threonine kinase thick veins reveal novel functions of TGF beta homologs during Drosophila embryogenesis

Differentiation of distinct cell types at specific locations within a developing organism depends largely on the ability of cells to communicate. A major class of signalling proteins implicated in cell to cell communication is represented by members of the TGF beta superfamily. A corresponding class of transmembrane serine/threonine kinases has recently been discovered that act as cell surface receptors for ligands of the TGF beta superfamily. The product of the Drosophila gene decapentaplegic (dpp) encodes a TGF beta homolog that plays multiple roles during embryogenesis and the development of imaginal discs. Here we describe the complex expression pattern of thick veins (tkv), which encodes a receptor for dpp. We make use of tkv loss-of-function mutations to examine the consequences of the failure of embryonic cells to respond to dpp and/or other TGF beta homologs. We find that while maternal tkv product allows largely normal dorsoventral pattering of the embryo, zygotic tkv activity is indispensable for dorsal closure of the embryo after germ band retraction. Furthermore, tkv activity is crucial for patterning the visceral mesoderm; in the absence of functional tkv gene product, visceral mesoderm parasegment 7 cells fail to express Ultrabithorax, but instead accumulate Antennapedia protein. The tkv receptor is therefore involved in delimiting the expression domains of homeotic genes in the visceral mesoderm. Interestingly, tkv mutants fail to establish a proper tracheal network. Tracheal braches formed by cells migrating in dorsal or ventral directions are absent in tkv mutants. The requirements for tkv in dorsal closure, visceral mesoderm and trachea development assign novel functions to dpp or a closely related member of the TGF beta superfamily.

Characterization of mutated transforming growth factor-beta s which possess unique biological properties

Transforming growth factor-beta (TGF-beta) is a potent regulator of cell growth and differentiation. On the basis of the crystal structure of TGF-beta 2, we have designed and synthesized two mutant TGF-beta s, TGF-beta 1 (71 Trp) and TGF-beta 1 (delta 69-73). Although both of these molecules inhibited the growth of Mv1Lu mink lung epithelial cells and LS1034 colorectal cancer cells, which are affected equally by TGF-beta 1 and TGF-beta 2, TGF-beta 1 (delta 69-73) was much less potent than TGF-beta 1 or TGF-beta 1 (71 Trp) at inhibiting the growth of LS513 colorectal cancer cells which are growth-inhibited by TGF-beta 1 but not TGF-beta 2. Both TGF-beta 1 (71 Trp) and TGF-beta 1 (delta 69-73) increased levels of mRNAs for fibronectin and plasminogen activator inhibitor with Mv1Lu cells, whereas only TGF-beta 1 (71 Trp) and not TGF-beta 1 (delta 69-73) up-regulated the mRNA level of carcinoembryonic antigen in LS513 cells. The expression level of carcinoembryonic antigen mRNA in LS1034 cells was not altered by either wild-type or mutant TGF-beta s. Receptor labeling experiments demonstrated that TGF-beta 1 (71 Trp) bound with high affinity to the cell-surface receptors of Mv1Lu, LS1034, and LS513 cells while TGF-beta 1 (delta 69-73) bound effectively to the receptors of Mv1Lu and LS1034 cells but much less to the receptors on LS513 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies with a Xenopus BMP receptor suggest that ventral mesoderm-inducing signals override dorsal signals in vivo

We report the isolation of a Xenopus BMP receptor that is expressed maternally in the appropriate location to play a role in mesoderm induction. This receptor binds both BMP-2 and BMP-4 with high affinity. A truncated form of this BMP receptor specifically blocks BMP-4 signaling. Expression of this truncated BMP receptor during embryogenesis converts ventral mesoderm to dorsal mesoderm. Contrary to the popularly held view that ventral is the ground state for all mesoderm, our results suggest that formation of ventral mesoderm requires an active signal and that, in the absence of this ventral signal, dorsal mesoderm is formed.

Effects of transforming growth factor beta 1 on the regulation of osteocalcin synthesis in human MG-63 osteosarcoma cells

Treatment of human MG-63 osteosarcoma cells with human recombinant transforming growth factor beta 1 (TGF-beta 1) was found to inhibit cell proliferation. In addition, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]-induced osteocalcin synthesis was greatly influenced by TGF-beta 1. Dose- and time-dependent inhibition was seen both in medium osteocalcin and the corresponding mRNA concentrations. Furthermore, TGF-beta 1 decreased osteocalcin synthesis modulated negatively by dexamethasone or positively by retinoic acid. The stability of osteocalcin mRNA was not decreased by the TGF-beta 1 treatment, but in vitro transcription assays demonstrated diminished osteocalcin gene transcription caused by the TGF-beta 1 treatment. Binding of vitamin D receptor (VDR) to an oligonucleotide probe containing the osteocalcin vitamin D response element (VDRE) was not influenced by TGF-beta 1, however. Incubation of the cells with the serine/threonine kinase inhibitor H-7 did not block the ability of TGF-beta 1 to decrease osteocalcin synthesis but caused a further inhibition. Also, the 1,25(OH)2D3-induced osteocalcin synthesis was decreased by H-7 treatment, suggesting that phosphorylation as such is involved in the transcriptional activation mechanism of VDR. These results demonstrate that TGF-beta 1 is a strong inhibitor of the synthesis of osteocalcin, a calcium binding protein participating in bone mineralization, by counteracting the stimulatory effects of other hormones on its synthesis. We further suggest that TGF-beta 1 affects the synthesis of osteocalcin at the level of transcription through mechanism(s) different from the serine/threonine kinase pathway.

Genetic changes in the transforming growth factor beta (TGF-beta) type II receptor gene in human gastric cancer cells: correlation with sensitivity to growth inhibition by TGF-beta

We have found several genetic changes in the TGF-beta-type II receptor gene in human gastric cancer cell lines resistant to the growth inhibitory effect of TGF-beta. Southern blot analysis showed deletion of the type II receptor gene in two of eight cell lines and amplification in another two lines. The single cell line we studied that is sensitive to growth inhibition by TGF-beta showed no structural abnormalities of the type II receptor gene. Some of the gastric cancer cells resistant to the growth inhibitory effect of TGF-beta express either truncated or no detectable TGF-beta type II receptor mRNAs, whereas the one that retains responsiveness to the growth inhibitory effect of TGF-beta expresses a full-size type II receptor mRNA. Immunoprecipitation followed by Western blot analysis showed parallel changes in TGF-beta type II receptor expression. Our results suggest that one of the possible mechanisms of escape from autocrine or paracrine growth control by TGF-beta during carcinogenesis could involve genetic changes in the TGF-beta type II receptor gene itself or altered expression of its mRNA.

Characterization and cloning of a receptor for BMP-2 and BMP-4 from NIH 3T3 cells

The bone morphogenetic proteins (BMPs) are a group of transforming growth factor beta (TGF-beta)-related factors whose only receptor identified to date is the product of the daf-4 gene from Caenorhabditis elegans. Mouse embryonic NIH 3T3 fibroblasts display high-affinity 125I-BMP-4 binding sites. Binding assays are not possible with the isoform 125I-BMP-2 unless the positively charged N-terminal sequence is removed to create a modified BMP-2, 125I-DR-BMP-2. Cross-competition experiments reveal that BMP-2 and BMP-4 interact with the same binding sites. Affinity cross-linking assays show that both BMPs interact with cell surface proteins corresponding in size to the type I (57- to 62-kDa) and type II (75- to 82-kDa) receptor components for TGF-beta and activin. Using a PCR approach, we have cloned a cDNA from NIH 3T3 cells which encodes a novel member of the transmembrane serine/threonine kinase family most closely resembling the cloned type I receptors for TGF-beta and activin. Transient expression of this receptor in COS-7 cells leads to an increase in specific 125I-BMP-4 binding and the appearance of a major affinity-labeled product of approximately 64 kDa that can be labeled by either tracer. This receptor has been named BRK-1 in recognition of its ability to bind BMP-2 and BMP-4 and its receptor kinase structure. Although BRK-1 does not require cotransfection of a type II receptor in order to bind ligand in COS cells, complex formation between BRK-1 and the BMP type II receptor DAF-4 can be demonstrated when the two receptors are coexpressed, affinity labeled, and immunoprecipitated with antibodies to either receptor subunit. We conclude that BRK-1 is a putative BMP type I receptor capable of interacting with a known type II receptor for BMPs.

Mechanism of activation of the TGF-beta receptor

Transforming growth factor-beta (TGF-beta) signals by contacting two distantly related transmembrane serine/threonine kinases called receptors I and II. The role of these molecules in signalling has now been determined. TGF-beta binds directly to receptor II, which is a constitutively active kinase. Bound TGF-beta is then recognized by receptor I which is recruited into the complex and becomes phosphorylated by receptor II. Phosphorylation allows receptor I to propagate the signal to downstream substrates. This provides a mechanism by which a cytokine can generate the first step of a signalling cascade.

Phosphorylation of the human-transforming-growth-factor-beta-binding protein endoglin

Endoglin is an homodimeric membrane antigen with capacity to bind transforming growth factor-beta (TGF-beta). Phosphorylation of human endoglin was demonstrated in endothelial cells as well as in mouse fibroblast transfectants expressing two isoforms, L-endoglin or S-endoglin, with distinct cytoplasmic domains. The extent of L-endoglin phosphorylation was found to be 8-fold higher than that of S-endoglin, and phosphopeptide analyses revealed at least three different phosphorylation sites for L-endoglin, whereas S-endoglin produces only one phosphopeptide. The immunoprecipitated L-endoglin was found to be phosphorylated mainly on serine, and, to a minor extent, on threonine, residues. Treatment of the cells with TGF-beta 1 or the protein kinase C inhibitor H-7 resulted in a reduction of the levels of endoglin phosphorylation.

Expression of type II transforming growth factor-beta receptor mRNA in human skin, as revealed by in situ hybridization

We studied the expression of the type II transforming growth factor-beta receptor mRNA in normal and psoriatic human skin in vivo. In situ hybridization analysis showed that its signals were expressed in the epidermal keratinocytes of the basal, the spinous and the granular layer, although no significant signals were observed in the fibroblasts or endothelial cells of the dermis. The follicular epithelium also expressed the type II transforming growth factor-beta receptor mRNA. There was no difference in the pattern of DNA expression between normal and psoriatic skin. These results suggest that the mRNA of the type II transforming growth factor-beta receptor is mainly expressed in the epithelial components of skin and controls the proliferation of the epidermis.

Receptor serine/threonine kinases implicated in the control of Drosophila body pattern by decapentaplegic

Members of the TGF beta superfamily of secreted signaling molecules regulate growth and cellular patterning during development and interact with specific type I and type II membrane receptors possessing a cytoplasmic serine/threonine kinase domain. We describe two members of the type I receptor family in Drosophila and demonstrate that they are encoded by the genes saxophone (sax) and thick veins (tkv). Further, we show that mutations that abolish sax or tkv activity cause phenotypes similar to partial or complete loss of activity, respectively, of the TGF beta homolog decapentaplegic (dpp). We propose that specification of distinct cell fates in response to different concentrations of dpp may be achieved combinatorially by the sax and tkv receptors.

Mapping of the ligand binding domain of the transforming growth factor beta receptor type III by deletion mutagenesis

Transforming growth factor beta (TGF-beta) receptor type III is a membrane-anchored proteoglycan that binds TGF-beta via the core protein. We have determined, by deletion mutagenesis of the receptor type III, the minimal essential region of the extracellular domain that is capable of binding TGF-beta. Nine deletion mutants were produced, six of which are expressed on the cell surface and bind TGF-beta. We find that the shortest of these active mutants, which retains only 253 of the 785 amino acids of the extracellular domain, binds TGF-beta with the same affinity as the full-length receptor. These results indicate that the ligand binding domain lies proximal to the transmembrane domain and is functionally independent from the rest of the extracellular domain. We have determined from the mutants that one of the potential glycosaminoglycan attachment sites in the receptor type III is not utilized. Results from the nonglycosylated mutants confirm that the glycosaminoglycan chains are not required for the folding, targeting, and TGF-beta binding activity of the receptor. Moreover, we present evidence for dimerization and multimerization of the receptor.

The types II and III transforming growth factor-beta receptors form homo-oligomers

Affinity-labeling experiments have detected hetero-oligomers of the types I, II, and III transforming growth factor beta (TGF-beta) receptors which mediate intracellular signaling by TGF-beta, but the oligomeric state of the individual receptor types remains unknown. Here we use two types of experiments to show that a major portion of the receptor types II and III forms homo-oligomers both in the absence and presence of TGF-beta. Both experiments used COS-7 cells co-transfected with combinations of these receptors carrying different epitope tags at their extracellular termini. In immunoprecipitation experiments, radiolabeled TGF-beta was bound and cross-linked to cells co-expressing two differently tagged type II receptors. Sequential immunoprecipitations using anti-epitope monoclonal antibodies showed that type II TGF-beta receptors form homo-oligomers. In cells co-expressing epitope-tagged types II and III receptors, a low level of co-precipitation of the ligand-labeled receptors was observed, indicating that some hetero-oligomers of the types II and III receptors exist in the presence of ligand. Antibody-mediated cross-linking studies based on double-labeling immunofluorescence explored co-patching of the receptors at the cell surface on live cells. In cells co-expressing two differently tagged type II receptors or two differently tagged type III receptors, forcing one receptor into micropatches by IgG induced co-patching of the receptor carrying the other tag, labeled by noncross-linking monovalent Fab'. These studies showed that homo-oligomers of the types II and III receptors exist on the cell surface in the absence or presence of TGF-beta 1 or -beta 2. In cells co-expressing types II and III receptors, the amount of heterocomplexes at the cell surface was too low to be detected in the immunofluorescence co-patching experiments, confirming that hetero-oligomers of the types II and III receptors are minor and probably transient species.