TGF-beta receptors and actions

Activation of signalling by the activin receptor complex

Activin exerts its effects by simultaneously binding to two types of p rotein serine/threonine kinase receptors, each type existing in various isoforms. Using the ActR-IB and ActR-IIB receptor isoforms, we have investigated the mechanism of activin receptor activation. ActR-IIB are phosphoproteins with demonstrable affinity for each other. However, activin addition strongly promotes an interaction between these two proteins. Activin binds directly to ActR-IIB, and this complex associates with ActR-IB, which does not bind ligand on its own. In the resulting complex, ActR-IB becomes hyperphosphorylated, and this requires the kinase activity of ActR-IIB. Mutation of conserved serines and threonines in the GS domain, a region just upstream of the kinase domain in ActR-IB, abrogates both phosphorylation and signal propagation, suggesting that this domain contains phosphorylation sites required for signalling. ActR-IB activation can be mimicked by mutation of Thr-206 to aspartic acid, which yields a construct, ActR-IB(T206D), that signals in the absence of ligand. Furthermore, the signalling activity of this mutant construct is undisturbed by overexpression of a dominant negative kinase-defective ActR-IIB construct, indicating that ActR-IB(T206D) can signal independently of ActR-IIB. The evidence suggests that ActR-IIB acts as a primary activin receptor and ActR-IB acts as a downstream transducer of activin signals.

Identification of important regions in the cytoplasmic juxtamembrane domain of type I receptor that separate signaling pathways of transforming growth factor-beta

Proteins in the transforming growth factor-beta (TGF-beta) superfamily exert their effects by forming heteromeric complexes of their type I and type II serine/threonine kinase receptors. The type I and type II receptors form distinct subgroups in the serine/threonine kinase receptor family based on the sequences of the kinase domains and the presence of a highly conserved region called the GS domain (or type I box) located just N-terminal to the kinase domain in the type I receptors. Recent studies have revealed that upon TGF-beta binding several serine and threonine residues in the GS domain of TGF-beta type I receptor (T beta R-I) are phosphorylated by TGF-beta type II receptor (T beta R-II) and that the phosphorylation of GS domain is essential for TGF-beta signaling. Here we investigated the role of cytoplasmic juxtamembrane region located between the transmembrane domain and the GS domain of T beta R-I by mutational analyses using mutant mink lung epithelial cells, which lack endogenous T beta R-I. Upon transfection, wild-type T beta R-I restored the TGF-beta signals for growth inhibition and production of plasminogen activator inhibitor-1 (PAI-1) and fibronectin. A deletion mutant, T beta R-I/JD1(delta 150-181), which lacks the juxtamembrane region preceding the GS domain, bound TGF-beta in concert with T beta R-II and transduced a signal leading to production of PAI-I but not growth inhibition. Recombinant receptors with mutations that change serine 172 to alanine (S172A) or threonine 176 to valine (T176V) were similar to wild-type T beta R-I in their abilities to bind TGF-beta, formed complexes with T beta R-II, and transduced a signal for PAI-1 and fibronectin. Similar to T beta R-I/JD1 (delta 150-181), however, these missence mutant receptors were impaired to mediate a growth inhibitory signal. These observations indicate that serine 172 and threonine 176 of T beta R-I are dispensable for extracellular matrix protein production but essential to the growth inhibition by TGF-beta.

Acquired TGF beta 1 sensitivity and TGF beta 1 expression in cell lines established from a single small cell lung cancer patient during clinical progression

Three small cell lung cancer cell lines established from a single patient during longitudinal follow-up were examined for in vitro expression of TGF beta and TGF beta receptors, i.e. the components of an autocrine loop. GLC 14 was established prior to treatment, GLC 16 on relapse after chemotherapy and GLC 19 on recurrence after radiotherapy. TGF beta was detected by ELISA and TGF beta receptors by chemical crosslinking to radiolabelled TGF beta 1. Furthermore, TGF beta and TGF beta receptor mRNAs were detected by northern blot analysis. Expression of type II TGF beta receptor mRNA and protein was found in GLC 16 and GLC 19. These cell lines were also growth inhibited by exogenously administrated TGF beta 1. TGF beta 1 mRNA and protein in its latent form was only expressed in the radiotherapy-resistant cell line, GLC 19. The results indicate that disease progression in this patient was paralleled by a gain in sensitivity to the growth inhibition by TGF beta 1 due to type II TGF beta receptor, and a gain of latent TGF beta 1 protein. Lack of type II receptor expression in GLC 14, which was also resistant to growth inhibition by exogenous TGF beta 1, was not due to gross structural changes in the type II receptor gene, as examined by Southern blotting. Also, the type I receptor could not be detected by ligand binding assay in this cell line, despite expression of mRNA for this receptor. This agrees with previous findings that type I receptor cannot bind TGF beta 1 without co-expression of the type II receptor.

Feedback inhibitors in normal and tumor tissues

Negative feedback represents the principal mechanism for regulating growth in biological systems. Over the past 20 years, our understanding of the role played by inhibitory factors governing this process has advanced considerably. This is particularly well illustrated in the field of experimental hematology with the recognition of hemopoietic progenitor cell proliferation inhibitors, an expanding group of unrelated peptides that act to limit proliferation in hemopoietic precursor cells. The characterization and subsequent production of these molecules by chemical synthesis or recombinant DNA technology has enabled investigators to explore their role in normal hemopoiesis and define a potential role in clinical medicine. A number of inhibitory factors, including macrophage inflammatory protein-1 alpha (MIP-1 alpha) and the tetrapeptide AcSDKP appear to share a relative specificity to hemopoietic progenitor cell subsets. Others, such as interferon and tumor necrosis factor, have a more complex action and their hemopoietic effects are likely to be indirect and nonspecific. In addition to the role of inhibitors in normal steady state, it has become increasingly evident that loss of sensitivity to the normal feedback inhibitory signals may be of central importance in carcinogenesis and tumor promotion. This presumably represents a developmental strategy that allows the neoplastic cell to maintain a growth advantage over its normal cell counterpart. The underlying mechanisms that terminate in inhibitor-resistance are yet to be elucidated, but in some instances they may be associated with aberrant tumor suppressor gene function.

Expression and Regulation of Transforming Growth Factor B1 in Cultured Normal and Neoplastic Rat Pituitary Cells

Pituitary prolactin (PRL) cell gene expression and proliferation are regulated by hormones and growth factors. Transforming growth factor beta (TGFB) and blast growth factor (bFGF) have been implicated in the regulation of antenor pituitary function. To study the roles of TGFB and bFGF in anterior pituitary cell function, we analyzed normal and neoplastic pituitary cells in serum-free media. The various isoforms of TGFB and TGFB receptor types I, II, and III were also analyzed by reverse transcription-polymerase chain reaction (RT-PCR) in pituitary cells. Transforming growth factor beta 1 (TGFB1) stimulated PRL expression and PRL cell proliferation in normal pituitary. TGFB1 stimulated PRL expression, but inhibited proliferation in the growth hormone (GH) and PRL-producing GH(3) cells. Estradiol 17 B (E(2)) and bFGE stimulated PRL gene expression in normal pituitary and GH(3) cells, whereas E(2) inhibited and bFGF stimulated TGFB1 mRNA levels in normal pituitary PRL cells, but not in GH(3) cells. Both normal pituitary and GH(3) cells expressed the mRNAs for TGFB1, TGFB2, and TGFB3 isoforms and for TGFB receptors I, II, and III. These results indicate that there is a relative loss of regulatory control by growth factors in neoplastic GH(3) cells compared to normal pituitary PRL cells.

Enhancement of the haemodynamic effects of NG-monomethyl-L-arginine by transforming growth factor-beta 1 in conscious, normal, but not endotoxaemic, rats

1. Male, Long Evans rats (350-450 g) were chronically instrumented for the measurement of regional haemodynamics, and the effects of TGF-beta 1 (25 micrograms kg-1 i.v. bolus) were assessed during infusion of saline (n = 9) or lipopolysaccharide (LPS, 150 micrograms kg-1 h-1; n = 12). In the same animals, responses to NG-monomethyl-L-arginine (L-NMMA 10 mg kg-1 bolus; 10 mg kg-1 h-1 infusion) were determined 18 h after administration of TGF-beta 1. In a separate experiment, the effects of the endothelin antagonist, SB 209670 (10 micrograms kg-1 min-1) on responses to TGF-beta 1 and to L-NMMA subsequently, were determined. 2. In the absence of LPS, TGF-beta 1 had slow-onset bradycardic and pressor effects accompanied by mesenteric and hindquarters, but not renal, vasoconstriction. Eighteen hours after TGF-beta 1, these effects had gone, but the bradycardic, pressor, and mesenteric vasoconstrictor responses to L-NMMA were enhanced. The haemodynamic changes following TGF-beta 1, and the augmentation of the subsequent responses to L-NMMA, were inhibited by SB 209670. These results are consistent with TGF-beta 1 stimulating the synthesis and release of endothelin, and an involvement of the latter in responses to L-NMMA. 3. The pressor effects of TGF-beta 1 were similar in LPS-infused and saline-infused animals, but in the former group the mesenteric vasoconstriction was enhanced and the hindquarters vasoconstriction diminished. Since, in the absence of TGF-beta 1, LPS-infused animals showed a developing hindquarters vasodilatation and mesenteric vasoconstriction, it is feasible that, in the presence of TGF-beta 1 and LPS together, the haemodynamic profile represented an amalgam of the individual effects of the two interventions, rather than a specific effect of TGF-beta 1 on the haemodynamic sequelae of endotoxaemia. 4. In the presence of LPS, haemodynamic responses to L-NMMA were suppressed, and TGF-beta 1 generally did not affect this suppression. A possible explanation of this observation is that LPS increased circulating endothelin levels, and thus resulted in desensitization to the effects of endothelin released following administration of L-NMMA.

Cartilage morphogenesis: role of bone and cartilage morphogenetic proteins, homeobox genes and extracellular matrix

Cartilage morphogenesis is one of the central topics in skeletal development. Cartilage geometry determines the future architecture of bones, joints and associated ligaments and tendons. Recent progress in this area has come from purification, cloning and expression of genes encoding bone and cartilage morphogenetic proteins (BMPs and CDMPs). BMPs initiate de novo cartilage and bone differentiation. BMPs are a family of pleiotropic signals for progenitor cell migration by chemotaxis, proliferation, and differentiation. Very recently another class of related morphogenetic proteins, CDMPs have been isolated and cloned. CDMPs may be critical for mesenchymal condensation prior to overt cartilage differentiation, the first step in morphogenesis of both cartilage and bone. The cartilage morphogenetic cascade is a cellular and molecular continuum driven by regulatory signalling molecules such as BMPs and CDMPs and their receptors, homeobox genes, transcription factors, and finally the synthesis and supramolecular assembly of structural macromolecules of the extracellular matrix. BMPs and CDMPs bind to heparin, heparan sulfates, and collagens I and IV. Thus there is a symbiosis of regulatory and structural macromolecules in the morphogenesis of cartilage. An avalanche of recent advances from seemingly disparate areas bodes well for the complete elucidation of the molecular basis of morphogenesis of cartilage, the architectural blue-print for the skeleton.

Identification of a human type II receptor for bone morphogenetic protein-4 that forms differential heteromeric complexes with bone morphogenetic protein type I receptors

Bone morphogenetic proteins (BMPs) comprise the largest subfamily of TGF-beta-related ligands and are known to bind to type I and type II receptor serine/threonine kinases. Although several mammalian BMP type I receptors have been identified, the mammalian BMP type II receptors have remained elusive. We have isolated a cDNA encoding a novel transmembrane serine/threonine kinase from human skin fibroblasts which we demonstrate here to be a type II receptor that binds BMP-4. This receptor (BRK-3) is distantly related to other known type II receptors and is distinguished from them by an extremely long carboxyl-terminal sequence following the intracellular kinase domain. The BRK-3 gene is widely expressed in a variety of adult tissues. When expressed alone in COS cells, BRK-3 specifically binds BMP-4, but cross-linking of BMP-4 to BRK-3 is undetectable in the absence of either the BRK-1 or BRK-2 BMP type I receptors. Cotransfection of BRK-2 with BRK-3 greatly enhanced affinity labeling of BMP-4 to the type I receptor, in contrast to the affinity labeling pattern observed with the BRK-1 + BRK-3 heteromeric complex. Furthermore, a subpopulation of super-high affinity binding sites is formed in COS cells upon cotransfection only of BRK-2 + BRK-3, suggesting that the different heteromeric BMP receptor complexes have different signaling potential.

Irradiation induces up-regulation of E9 protein (CD105) in human vascular endothelial cells

The use of MAb E-9 raised against tissue-cultured endothelial cells (EC) has shown marked heterogeneity in vascular EC lining the blood vessels of normal and tumour tissues. MAb E-9 is human EC-specific and the protein recognized by it is a homodimer with a molecular mass of 97 kDa. The E-9 protein is resistant to treatment by 3 mM sodium periodate, but is sensitive to 10% trichloroacetic acid and 70% ethanol. E-9 protein has been assigned to a new cluster, CD105, and mapped to human chromosome 9q3. It has approximately 70% homology with type-III cell-surface receptor for transforming growth factor beta (TGF-beta). Recently CD105 has been reported to be the gene in patients with hereditary haemorrhagic telangiectasia. We have examined the effects of radiation on its expression in normal human umbilical-vein endothelial cells (HUVEC) and brain-tumour-derived endothelial cells (BTEC). Irradiation induced dose- and time-dependent up-regulation in the expression of the E-9 protein on the plasma membranes of EC, and also resulted in greater increase in the expression of the E-9 protein in semi-confluent (proliferating) as compared with confluent (non-proliferating) EC. It may well be that, following radiotherapy in cancer patients, E-9 protein is also up-regulated. The presence of increased amounts of E-9 protein in EC makes it an attractive target in the control of angiogenesis, especially after radiotherapy in cancer patients. The time scale involved in the up-regulation of E-9 protein following irradiation has led us to suggest that it may be a secondary event, the primary being the production and release of mitogenic factors (such as basic fibroblast growth factor) from irradiated EC.

Down-regulation of gamma interferon, tumor necrosis factor type I, interleukin 1 (IL-1) type I, IL-3, IL-4, and transforming growth factor beta type I receptors at the local site during the acute phase of Shigella infection

An immunohistochemical technique was used to examine whether there was a colocalization of cytokine-specific receptors with cytokine-expressing cells. We have previously shown that there is extensive cytokine production and secretion in the rectal mucosa in shigellosis (interleukin 1 alpha [IL-1 alpha], IL-1 beta, IL-1ra, IL-4, IL-6, IL-8, IL-10, tumor necrosis factor alpha [TNF-alpha], TNF-beta, gamma interferon, granulocyte-macrophage colony-stimulating factor, and transforming growth factor beta [TGF-beta]) (R. Raqib, A. A. Lindberg, B. Wretlind, P. K. Bardhan, U. Andersson, and J. Andersson, Infect. Immun. 63:289-296, 1995; R. Raqib, B. Wretlind, J. Andersson, and A. A. Lindberg, J. Infect. Dis. 171:376-384, 1995). Kinetics for receptor expression was compared with that for cytokine synthesis in the inflamed rectal mucosa from Shigella-infected patients during acute (2 to 6 days after onset of diarrhea) and convalescent (30 to 40 days after onset) stages. Quantification of receptor expression was assessed by computer-assisted analysis of video microscopic images. A selective down-regulation of the receptors for gamma interferon, tumor necrosis factor (TNF receptor [TNFR] type I), IL-1 (IL-1 receptor [IL-1R] types I and type II), IL-3, IL-4, and TGF-beta (TGF-beta receptor type I) was observed at the onset of the disease, with a gradual reappearance during the convalescent stage. However, IL-2R, IL-6R, granulocyte-macrophage colony-stimulating factor receptor, TNFR type II, and TGF-beta receptor type II showed no change in expression during the study period and were comparable to controls. Cytokine receptors were predominantly located to the epithelial layer of the mucosal surface and crypts, with variable expression patterns in the lamina propria. A time-dependent kinetic curve was seen for the soluble IL-2R (sIL-2R), sIL-6R, and sTNFR types I and type II shed in stool at the acute stage similar to that observed for cytokine secretion in stool but at four- to six-times-lower concentration. In contrast, soluble receptor levels in plasma were 100-fold higher than the cytokine levels. The results suggest a dissociation in immune regulation between cytokine production and cytokine receptor expression. The down-regulation of the receptors in acute shigellosis was probably a consequence of cytokine-induced internalization and shedding of the receptors during signal transduction as well as due to programmed regulatory roles played by cytokines and the bacterial antigens.

Human type II receptor for bone morphogenic proteins (BMPs): extension of the two-kinase receptor model to the BMPs

Bone morphogenic proteins (BMPs) are universal regulators of animal development. We report the identification and cloning of the BMP type II receptor (BMPR-II), a missing component of this receptor system in vertebrates. BMPR-II is a transmembrane serine/threonine kinase that binds BMP-2 and BMP-7 in association with multiple type I receptors, including BMPR-IA/Brk1, BMPR-IB, and ActR-I, which is also an activin type I receptor. Cloning of BMPR-II resulted from a strong interaction of its cytoplasmic domain with diverse transforming growth factor beta family type I receptor cytoplasmic domains in a yeast two-hybrid system. In mammalian cells, however, the interaction of BMPR-II is restricted to BMP type I receptors and is ligand dependent. BMPR-II binds BMP-2 and -7 on its own, but binding is enhanced by coexpression of type I BMP receptors. BMP-2 and BMP-7 can induce a transcriptional response when added to cells coexpressing ActR-I and BMPR-II but not to cells expressing either receptor alone. The kinase activity of both receptors is essential for signaling. Thus, despite their ability to bind to type I and II receptors receptors separately, BMPs appear to require the cooperation of these two receptors for optimal binding and for signal transduction. The combinatorial nature of these receptors and their capacity to crosstalk with the activin receptor system may underlie the multifunctional nature of their ligands.

Peripheral expression and biological activities of GDNF, a new neurotrophic factor for avian and mammalian peripheral neurons

Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic polypeptide, distantly related to transforming growth factor-beta (TGF-beta), originally isolated by virtue of its ability to induce dopamine uptake and cell survival in cultures of embryonic ventral midbrain dopaminergic neurons, and more recently shown to be a potent neurotrophic factor for motorneurons. The biological activities and distribution of this molecule outside the central nervous system are presently unknown. We report here on the mRNA expression, biological activities and initial receptor binding characterization of GDNF and a shorter spliced variant termed GDNF beta in different organs and peripheral neurons of the developing rat. Both GDNF mRNA forms were found to be most highly expressed in developing skin, whisker pad, kidney, stomach and testis. Lower expression was also detected in developing skeletal muscle, ovary, lung, and adrenal gland. Developing spinal cord, superior cervical ganglion (SCG) and dorsal root ganglion (DRG) also expressed low levels of GDNF mRNA. Two days after nerve transection, GDNF mRNA levels increased dramatically in the sciatic nerve. Overall, GDNF mRNA expression was significantly higher in peripheral organs than in neuronal tissues. Expression of either GDNF mRNA isoform in insect cells resulted in the production of indistinguishable mature GDNF polypeptides. Purified recombinant GDNF promoted neurite outgrowth and survival of embryonic chick sympathetic neurons. GDNF produced robust bundle-like, fasciculated outgrowth from chick sympathetic ganglion explants. Although GDNF displayed only low activity on survival of newborn rat SCG neurons, this protein was found to increase the expression of vasoactive intestinal peptide and preprotachykinin-A mRNAs in cultured SCG neurons. GDNF also promoted survival of about half of the neurons in embryonic chick nodose ganglion and a small subpopulation of embryonic sensory neurons in chick dorsal root and rat trigeminal ganglia. Embryonic chick sympathetic neurons expressed receptors for GDNF with Kd 1-5 x 10(-9) M, as measured by saturation and displacement binding assays. Our findings indicate GDNF is a new neurotrophic factor for developing peripheral neurons and suggest possible non-neuronal roles for GDNF in the developing reproductive system.

The Drosophila schnurri gene acts in the Dpp/TGF beta signaling pathway and encodes a transcription factor homologous to the human MBP family

Decapentaplegic (dpp), a TGF beta-related ligand, plays a key role in Drosophila development. Although dpp receptors have been isolated, the downstream components of the signaling pathway remain to be identified. We have cloned the schnurri (shn) gene and show that it encodes a putative zinc finger transcription factor homologous to the human major histocompatibility complex-binding proteins 1 and 2. Mutations in shn affect multiple events that require dpp signaling as well as the transcription of dpp-responsive genes. Genetic interactions and the strikingly similar phenotypes of mutations in shn and the dpp receptors encoded by thick veins and punt suggest that shn plays a downstream role in dpp signaling.

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.

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.

Drosophila Dpp signaling is mediated by the punt gene product: a dual ligand-binding type II receptor of the TGF beta receptor family

Signaling by TGF beta-related factors requires ligand-induced association between type I and type II transmembrane serine/threonine kinases. In Drosophila, the saxophone (sax) and thick veins (tkv) genes encode type I receptors that mediate signaling by decapentaplegic (dpp), a member of the bone morphogenetic protein (BMP) subgroup of TGF beta-type factors. In this report, we demonstrate that the Drosophila punt gene encodes atr-II, a previously described type II receptor that on its own is able to bind activin but not BMP2, a vertebrate ortholog of dpp. Mutations in punt produce phenotypes similar to those exhibited by tkv, sax, and dpp mutants. Furthermore, punt will bind BMP2 in concert with tkv or sax, forming complexes with these receptors. We suggest that punt functions as a type II receptors for dpp and propose that BMP signaling in vertebrates may also involve sharing of type II receptors by diverse ligands.

Disruption of transforming growth factor beta signaling by a mutation that prevents transphosphorylation within the receptor complex

T beta R-II (transforming growth factor beta [TGF-beta] type II receptor) is a transmembrane serine/threonine kinase that acts as the primary TGF-beta receptor. Ligand binding to T beta R-II leads to the recruitment and phosphorylation of T beta R-I, a distantly related transmembrane kinase that acts as a downstream signaling component. T beta R-I phosphorylation by T beta R-II is shown here to be essential for signaling. A mutant T beta R-II that binds ligand but lacks signaling activity was identified. This mutant was identified by screening with a TGF-beta-inducible vector a series of mink lung epithelial cell clones that have normal TGF-beta binding activity but have lost antiproliferative and transcriptional responses to TGF-beta. When transiently cotransfected with T beta R-II, one of these cell lines, S-21, recovered TGF-beta responsiveness. cDNA cloning and sequencing of T beta R-II from S-21 cells revealed a point mutation that changes proline 525 to leucine in kinase subdomain XI. A recombinant receptor containing this mutation, T beta R-II(P525L), is similar to wild-type T beta R-II in its abilities to bind ligand, support ligand binding to T beta R-I, and form a complex with T beta R-I in vivo. T beta R-II(P525L) has autophosphorylating activity in vitro and in vivo; however, unlike the wild-type receptor, it fails to phosphorylate an associated T beta R-I. These results suggest that T beta R-II(P525L) is a catalytically active receptor that cannot recognize T beta R-I as a substrate. The close link between T beta R-I transphosphorylation and signaling activity argues that transphosphorylation is essential for signal propagation via T beta R-I.

Mammalian antiproliferative signals and their targets

The effect of mitogens on the mammalian cell cycle is opposed by the action of antimitogens, such as TGF-beta, cAMP agonists, and various antiproliferative drugs. The recent identification of TGF-beta receptors that initiate antimitogenic signals and of cell cycle kinase inhibitors that respond to these signals has provided new insights into this process. The evidence argues that mitogenic and antimitogenic signals confront each other by regulating in opposite ways the activity of cyclin-dependent kinases that control cell commitment to DNA replication.

The Janus protein tyrosine kinase family and its role in cytokine signaling

During the past 2 years, research from quite divergent areas has converged to provide the first insights into the mechanisms by which cytokines that utilize receptors of the cytokine receptor superfamily function. On the one hand, the obscure Jak family of cytoplasmic protein tyrosine kinases was independently implicated in IFN and hematopoietic growth factor signaling. Recent studies have expanded these initial observations to demonstrate that Jaks are critical to the functioning of all the receptors of the cytokine receptor superfamily. A variety of questions remain to be explored regarding the structure and function of Jaks and their interaction with receptors. It will also be important to pursue additional approaches to determine if the Jaks are necessary for various biological responses, particularly for mitogenic responses. The second major area of convergence has been the demonstration that members of the Stat family of transcription factors, initially identified in IFN-regulated gene expression, are generally involved in cytokine signaling. Clearly, a number of Stat-like activities remain to be cloned and it can be anticipated that the family contains additional members. Although a variety of genes are known to be regulated by the Stats association with IFN responses, much less is known concerning the genes regulated by the new Stats in cytokine signaling. Of particular importance is information relating to their potential contribution to mitogenic responses. From a biochemical standpoint, the Stats represent a remarkable family of proteins with regard to the ability of the modification of a single tyrosine residue to so dramatically affect cellular localization and DNA binding activity. Studies to identify the domains involved, and associated proteins that might contribute to either property, will be of considerable interest. More generally, it can hypothesized that Jaks and Stats, if important for proliferation and differentiation, may be the targets for malignant transformation. Although none of the genes map to chromosomal breakpoints that have been implicated in transformation, gain of function mutations is a likely mechanism that needs to be explored. Similarly, the Jak-Stat pathway would appear to be an excellent target for the development of drugs that affect a variety of cytokine functions.

Transforming growth factor beta 2 stimulates acute and chronic activation of the mitogen-activated protein kinase cascade in rat renal mesangial cells

Exposure of rat glomerular mesangial cells to transforming growth factor beta 2 (TGF beta 2) stimulates a biphasic mitogen-activated protein kinase (MAP kinase) activation. A rapid increase in activity (maximal at 10 min) is followed by a second persistent level of activity which steadily increases over 24 h. The second peak of MAP kinase activity is markedly attenuated by the protein synthesis inhibitor cycloheximide and consequently is paralleled by a pronounced de-novo synthesis of p42 and p44 MAP kinase as measured by immunoprecipitation of [35S]methionine-labeled mesangial cells. In addition, an increased de-novo synthesis of MAP kinase kinase (MEK), the upstream activator of MAP kinase, is observed in response to TGF beta 2 stimulation. We propose that TGF beta-induced activation and de-novo synthesis of MAP kinases and MEK is important for the multifunctional actions of this cytokine in mesangial cells and its role in disease states characterized by excessive fibrosis.

Bone and cartilage differentiation

Recent progress in the study of regulation of bone and cartilage differentiation has come from the isolation, cloning, and expression of genes encoding bone morphogenetic proteins (BMPs). BMPs initiate cartilage and bone formation in a sequential cascade. Their pleiotropic effects on chemotaxis, mitosis, and differentiation are based on concentration-dependent thresholds. The existence of multiple members of the BMP family raises issues concerning functional redundancy. Current work in progress in different laboratories has revealed that BMP-2 or BMP-4 gene knockout by homologous recombination results, surprisingly, in embryonic lethality. Cartilage and bone differentiation during endochondral development involves a continuum of steps: initiation, promotion, maintenance, modeling, and termination. The signaling factors for initiation and maintenance are being defined at the molecular level, and future studies will focus on the gene regulation of initial signaling molecules such as BMPs. Critical progress in the determination of the role of BMPs in bone development has been accomplished by systematic study of skeletal mutations such as short ear and brachypodism in mice. The accelerating pace of advance in this area augurs well for the resolution of the molecular basis of morphogenesis of bone and cartilage.

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.

Characterization and relationship of Dpp receptors encoded by the saxophone and thick veins genes in Drosophila

The dpp/BMP family of TGF beta-related factors controls numerous events in pattern formation and morphogenesis. How these polypeptide signals are received and transduced by target cells is largely unknown. We combine molecular and genetic approaches to establish that the Drosophila saxophone (sax) gene encodes a dpp receptor. We compare the structural properties and expression patterns of sax with a second dpp receptor encoded by the thick veins (tkv) gene. While the sax gene is expressed ubiquitously, tkv is expressed in a highly localized and dynamic pattern during development. Some, but not all, of the tkv expression pattern parallels that of dpp. Ubiquitous expression of a tkv transgene rescues both tkv and sax loss-of-function mutations. Thus, there is at least partial functional overlap of the sax and tkv receptors in vivo. We consider these observations in terms of possible ligand-receptor interactions during Drosophila development.