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

DBL-1, a TGF-β, is essential for Caenorhabditis elegans aversive olfactory learning

The TGF-β superfamily is conserved throughout metazoan, and its members play essential roles in development and disease. TGF-β has also been implicated in adult neural plasticity. However, the underlying mechanisms are not well understood. Here we report that DBL-1, a Caenorhabditis elegans TGF-β homolog known to control body morphology and immunity, is essential for aversive olfactory learning of potentially harmful bacteria food. We show that DBL-1 generated by the AVA command interneurons, which are critical for sensorimotor responses, regulates aversive olfactory learning, and that the activity of the type I TGF-β receptor SMA-6 in the hypodermis is needed during adulthood to generate olfactory plasticity. These spatial and temporal mechanisms are critical for the DBL-1 signaling to achieve its diverse functions in development and adult neural plasticity. Interestingly, aversive training decreases AVA calcium response, leading to an increase in the DBL-1 signal secreted from AVA, revealing an experience-dependent change that can underlie the role of TGF-β signaling in mediating plasticity.

Thyroid Hormone Increases TGF-beta1 in Cardiomyocytes Cultures Independently of Angiotensin II Type 1 and Type 2 Receptors

TH-induced cardiac hypertrophy in vivo is accompanied by increased cardiac Transforming Growth Factor-beta1 (TGF-beta1) levels, which is mediated by Angiotensin II type 1 receptors (AT1R) and type 2 receptors (AT2R). However, the possible involvement of this factor in TH-induced cardiac hypertrophy is unknown. In this study we evaluated whether TH is able to modulate TGF-beta1 in isolated cardiac, as well as the possible contribution of AT1R and AT2R in this response. The cardiac fibroblasts treated with T(3) did not show alteration on TGF-beta1 expression. However, cardiomyocytes treated with T(3) presented an increase in TGF-beta1 expression, as well as an increase in protein synthesis. The AT1R blockade prevented the T(3)-induced cardiomyocyte hypertrophy, while the AT2R blockage attenuated this response. The T(3)-induced increase on TGF-beta1 expression in cardiomyocytes was not changed by the use of AT1R and AT2R blockers. These results indicate that Angiotensin II receptors are not implicated in T(3)-induced increase on TGF-beta expression and suggest that the trophic effects exerted by T(3) on cardiomyocytes are not dependent on the higher TGF-beta1 levels, since the AT1R and AT2R blockers were able to attenuate the T(3)-induced cardiomyocyte hypertrophy but were not able to attenuate the increase on TGF-beta1 levels promoted by T(3).

Angiotensin type 1 (AT1) and type 2 (AT2) receptors mediate the increase in TGF-β1 in thyroid hormone-induced cardiac hypertrophy

Increased thyroid hormone (TH) levels are known to induce cardiac hypertrophy. Some studies have provided evidence for a functional link between angiotensin II (ANG II) and transforming growth factor beta1 (TGF-beta1) in the heart, both being able to also induce cardiac hypertrophy. However, the contribution of this growth factor activated directly by TH or indirectly by ANG II in cardiac hypertrophy development remains unknown. To analyze the possible role of TGF-beta1 in cardiac hypertrophy induced by TH and also to evaluate if the TGF-beta1 effect is mediated by ANG II receptors, we employed Wistar rats separated into control, hypothyroid (hypo) and hyperthyroid (T4 - 10) groups combined or not with ANG II receptor blockers (losartan or PD123319). Serum levels of T3 and T4, systolic pressure and heart rate confirmed the thyroid state of the groups. The T4 - 10 group presented a significant increase in cardiac TGF-beta1 levels; however, TGF-beta1 levels in the hypo group did not change in relation to the control. Inhibition of the increase in cardiac TGF-beta1 levels was observed in the groups treated with T4 in association with losartan or PD123319 when compared to the T4 - 10 group. These results demonstrate for the first time the TH-modulated induction of cardiac TGF-beta1 in cardiac hypertrophy, and that this effect is mediated by ANG II receptors.

Identification of intracellular pathways through which TGF-beta1 upregulates PAI-1 expression in endothelial cells

Upregulation of plasminogen activator inhibitor type 1 (PAI-1) expression is a critical mechanism through which transforming growth factor-beta1 (TGF-beta1) accelerates intimal growth. The aim of this study was to identify signaling pathways through which TGF-beta1 upregulates PAI-1 expression in endothelial cells (EC) and test interventions for blocking these pathways. We transduced cultured bovine EC with an adenoviral vector containing the PAI-1 promoter fused to a beta-galactosidase reporter gene. We used these cells, along with vectors expressing potential modifiers of TGF-beta1 signaling and pharmacologic antagonists of mitogen-activated protein kinase (MAPK) pathways to identify key mediators of basal and TGF-beta1-regulated PAI-1 expression. Basal activity of the PAI-1 promoter was directly correlated with Ras activation and was blocked by a dominant negative (DN) type I TGF-beta receptor. TGF-beta1-stimulated activity of the PAI-1 promoter did not require Ras activation, and was lessened or eliminated by expression of either DN type I or type II TGF-beta receptors and by inhibition of either of two MAPKs: MEK and p38. Our results suggest unanticipated pathways of TGF-beta1 signaling in EC and point to new strategies to limit TGF-beta1-induced vascular disease.

Bone morphogenetic protein receptor-II mutation Arg491Trp causes malignant phenotype of familial primary pulmonary hypertension

A four-generation pedigree of familial primary pulmonary hypertension (FPPH) with 14 alive members was collected. In the family, three of the 14 alive familial members were diagnosed as FPPH. Mutations in bone morphogenetic protein receptor-II (BMPR-II) gene were screened by using sequencing analysis. A C-to-T transition at position 1471 in exon 11 of the BMPR-II gene was identified, resulting in an Arg491Trp mutation. We confirmed segregation of the mutation within the family and excluded the presence of the mutations in a panel of 240 chromosomes from normal individuals. No mutations were found in BMPR-II gene in other 10 patients with sporadic primary pulmonary hypertension. The Arg491Trp mutation is located in the kinase domain and predicted to disturb the kinase activity of BMPR-II. Total 7 familial members died at age 8-45 years with various symptoms, indicating other genetic or environmental modifiers involved in the modification of the clinical phenotype.

TGFβ2 mediates the effects of Hedgehog on hypertrophic differentiation and PTHrP expression

The development of endochondral bones requires the coordination of signals from several cell types within the cartilage rudiment. A signaling cascade involving Indian hedgehog (Ihh) and parathyroid hormone related peptide (PTHrP) has been described in which hypertrophic differentiation is limited by a signal secreted from chondrocytes as they become committed to hypertrophy. In this negative-feedback loop, Ihh inhibits hypertrophic differentiation by regulating the expression of Pthrp, which in turn acts directly on chondrocytes in the growth plate that express the PTH/PTHrP receptor. Previously, we have shown that PTHrP also acts downstream of transforming growth factor β (TGFβ) in a common signaling cascade to regulate hypertrophic differentiation in embryonic mouse metatarsal organ cultures. As members of the TGFβ superfamily have been shown to mediate the effects of Hedgehog in several developmental systems, we proposed a model where TGFβ acts downstream of Ihh and upstream of PTHrP in a cascade of signals that regulate hypertrophic differentiation in the growth plate. This report tests the hypothesis that TGFβ signaling is required for the effects of Hedgehog on hypertrophic differentiation and expression of Pthrp. We show that Sonic hedgehog (Shh), a functional substitute for Ihh, stimulates expression of Tgfb2 and Tgfb3 mRNA in the perichondrium of embryonic mouse metatarsal bones grown in organ cultures and that TGFβ signaling in the perichondrium is required for inhibition of differentiation and regulation of Pthrp expression by Shh. The effects of Shh are specifically dependent on TGFβ2, as cultures from Tgfb3-null embryos respond to Shh but cultures from Tgfb2-null embryos do not. Taken together, these data suggest that TGFβ2 acts as a signal relay between Ihh and PTHrP in the regulation of cartilage hypertrophic differentiation.

Consequences of altered TGF-beta expression and responsiveness in breast cancer: evidence for autocrine and paracrine effects

To characterize the impact of increased production of TGF-beta in a xenograft model of human breast cancer, TGF-beta-responsive MDA-231 cells were genetically modified by stable transfection so as to increase their production of active TGF-beta1. Compared with control cells, cells that produced increased amounts of TGF-beta proliferated in vitro more slowly. In vivo, however, tumors derived from these cells exhibited increased proliferation and grew at an accelerated pace. To evaluate the role of autocrine TGF-beta signaling, cells were also transfected with a dominant-negative truncated type II TGF-beta receptor (TbetaRII). Disruption of autocrine TGF-beta signaling in the TGF-beta-overexpressing cells reduced their in vivo growth rate. Co-inoculation of Matrigel with the TGF-beta-overexpressing cells expressing the truncated TbetaRII compensated for their diminished in vivo growth capacity, compared with the TGF-beta-overexpressing cells with an intact autocrine loop. Tissue invasion by the tumor was a distinctive feature of the TGF-beta-overexpressing cells, whether or not the autocrine loop was intact. Furthermore, tumors derived from TGF-beta-overexpressing cells, irrespective of the status of the autocrine TGF-beta-signaling pathway, had a higher incidence of lung metastasis. Consistent with the suggestion that TGF-beta's enhancement of invasion and metastasis is paracrine-based, we observed no significant differences among the cell clones in an in vitro invasion assay. Thus, in this experimental model system in vitro assays of cell proliferation and invasion do not accurately reflect in vivo observations, perhaps due to autocrine and paracrine effects of TGF-beta that influence the important in vivo-based phenomena of tumor growth, invasion, and metastasis.

Requirement for TGFβ receptor signaling during terminal lens fiber differentiation

Several families of growth factors have been identified as regulators of cell fate in the developing lens. Members of the fibroblast growth factor family are potent inducers of lens fiber differentiation. Members of the transforming growth factor β (TGFβ) family, particularly bone morphogenetic proteins, have also been implicated in various stages of lens and ocular development, including lens induction and lens placode formation. However, at later stages of lens development, TGFβ family members have been shown to induce pathological changes in lens epithelial cells similar to those seen in forms of human subcapsular cataract. Previous studies have shown that type I and type II TGFβ receptors, in addition to being expressed in the epithelium, are also expressed in patterns consistent with a role in lens fiber differentiation. In this study we have investigated the consequences of disrupting TGFβ signaling during lens fiber differentiation by using the mouse αΑ-crystallin promoter to overexpress mutant (kinase deficient), dominant-negative forms of either type I or type II TGFβ receptors in the lens fibers of transgenic mice. Mice expressing these transgenes had pronounced bilateral nuclear cataracts. The phenotype was characterized by attenuated lens fiber elongation in the cortex and disruption of fiber differentiation, culminating in fiber cell apoptosis and degeneration in the lens nucleus. Inhibition of TGFβ signaling resulted in altered expression patterns of the fiber-specific proteins, α-crystallin, filensin, phakinin and MIP. In addition, in an in vitro assay of cell migration, explanted lens cells from transgenic mice showed impaired migration on laminin and a lack of actin filament assembly, compared with cells from wild-type mice. These results indicate that TGFβ signaling is a key event during fiber differentiation and is required for completion of terminal differentiation.

Expression of transforming growth factor-beta isoforms and their receptors in chronic tendinosis

Chronic tendon lesions are degenerative conditions and may represent a failure to repair or remodel the extracellular matrix after repeated micro-injury. Since TGF-beta is strongly associated with tissue repair, we investigated the expression of TGF-beta isoforms (beta1, beta2 and beta3) and their 2 signalling receptors (TGF-betaRI and TGF-betaRII) in normal and pathological Achilles tendons. In all tissues, all 3 TGF-beta isoforms and the 2 receptors were present at sites of blood vessels. Cells in the matrix showed no staining for TGF-beta1 or beta3, while TGF-beta2 was associated with cells throughout the normal cadaver tendon. Tissue from tendons with pathological lesions showed an increase in cell numbers and percentage TGF-beta2 expression. TGF-betaRII showed a wide distribution in cells throughout the tissue sections. As with TGF-beta2, there was an increase in the number of cells expressing TGF-betaRII in pathological tissue. TGF-betaRI was restricted to blood vessels and was absent from the fibrillar matrix. We conclude that despite the presence and upregulation of TGF-beta2, TGF-beta signalling is not propagated due to the lack of TGF-betaRI. This might explain why chronic tendon lesions fail to resolve and suggests that the addition of exogenous TGF-beta will have little effect on chronic tendinopathy.

Truncated activin type I receptor Alk4 isoforms are dominant negative receptors inhibiting activin signaling

Activin, a member of the transforming growth factor beta (TGFbeta) superfamily of cytokines, inhibits cell proliferation in a variety of cell types. The functions of activin are mediated by type I and type II serine/threonine kinase receptors. The main type I receptor mediating activin signaling in human cells is ActRIB, also called Alk4. We have previously reported that several truncated Alk4 receptor isoforms are exclusively expressed in human pituitary tumors, and that the majority of such tumors did not exhibit activin-induced growth arrest in culture. We therefore studied the function of these truncated receptor isoforms. Transient expression of these truncated receptors inhibited activin-activated transcription from an activin-responsive reporter construct, 3TPLux. When each of these truncated Alk4 receptors was stably transfected into K562 cells, activin-induced expression of an endogenous gene, junB, was blocked, indicating that inhibition of gene expression also occurred at the chromosomal level. Furthermore, activin administration failed to cause growth inhibition and an increase of the G1 population in these cells. Coimmunoprecipitation experiments showed that the truncated Alk4 receptors formed complexes with type II activin receptors, but were not phosphorylated. These data indicate that the truncated activin type I receptors, predominantly expressed in human pituitary adenomas, function as dominant negative receptors to interfere with wild-type receptor function and block the antiproliferative effect of activin. This may contribute to uncontrolled pituitary cell growth and the development of human pituitary tumors.

Effects of diabetes and hypertension on glomerular transforming growth factor-beta receptor expression

BACKGROUND

Several studies have suggested that transforming growth factor-beta1 (TGF-beta1) is an important determinant of diabetic glomerular injury. TGF-beta1 forms a heteromeric complex with two cellular receptor subtypes, designated TGF-beta RII and TGF-beta RI, but the effects of diabetes mellitus on glomerular TGF-beta receptor expression have not been completely elucidated. We first compared the effect of experimental type I diabetes mellitus and uninephrectomy on glomerular TGF-beta receptor expression in spontaneously hypertensive rats (SHRs), and then sought to determine whether changes in TGF-beta receptor expression were strain specific by studying normotensive Wistar-Kyoto (WKY) rats.

METHODS

Five groups of male SHRs were studied. The first group received streptozotocin (60 mg/kg IV) and was studied after one week. The second group received streptozotocin and was studied after two weeks. The third group received streptozotocin (60 mg/kg IV) but received insulin to maintain euglycemia. The fourth group of age-matched SHRs served as the control group, while a fifth group of SHRs underwent uninephrectomy. Four groups of male WKY rats were also studied. The first group of WKY rats served as the age-matched control group. The second group of WKY rats received streptozotocin, while a third group of WKY rats underwent uninephrectomy. The fourth group underwent uninephrectomy and received streptozotocin. At each time point, glomeruli were isolated for protein extraction, and the protein was subjected to Western blot analysis of TGF-beta RII and TGF-beta RI expression.

RESULTS

Basal expression of both TGF-beta receptors per microgram of glomerular protein was similar in normotensive WKY rats and hypertensive SHRs. Hyperglycemia (blood glucose level, 17.8 +/- 2.9 mmol/L) led to an early twofold increase in TGF-beta RII protein expression and a fourfold increase in TGF-beta RI protein expression in the glomeruli of hypertensive diabetic SHRs compared with euglycemic SHRs (blood glucose level, 5.8 +/- 0.8 mmol/L), which was sustained after two weeks. Insulin treatment (blood glucose level, 5. 2 +/- 0.9 mmol/L) normalized both TGF-beta RII and TGF-beta RI expression in the glomeruli of SHRs that received streptozotocin. Glomerular capillary hypertension in the uninephrectomized SHRs led to a twofold increase in glomerular TGF-beta RII protein expression, but did not reproduce the effect of diabetes mellitus on TGF-beta RI expression. In contrast to the findings in SHRs, neither hyperglycemia (blood glucose level, 15.5 +/- 2.1 mmol/L), uninephrectomy, nor hyperglycemia (blood glucose level, 16.8 +/- 3.0 mmol/L) and uninephrectomy altered TGF-beta receptor expression in the glomeruli of normotensive WKY rats.

CONCLUSION

These studies support the hypothesis that hemodynamic factors and metabolic factors influence glomerular TGF-beta receptor in vivo in the SHRs.

Familial primary pulmonary hypertension (gene PPH1) is caused by mutations in the bone morphogenetic protein receptor-II gene

Familial primary pulmonary hypertension is a rare autosomal dominant disorder that has reduced penetrance and that has been mapped to a 3-cM region on chromosome 2q33 (locus PPH1). The phenotype is characterized by monoclonal plexiform lesions of proliferating endothelial cells in pulmonary arterioles. These lesions lead to elevated pulmonary-artery pressures, right-ventricular failure, and death. Although primary pulmonary hypertension is rare, cases secondary to known etiologies are more common and include those associated with the appetite-suppressant drugs, including phentermine-fenfluramine. We genotyped 35 multiplex families with the disorder, using 27 microsatellite markers; we constructed disease haplotypes; and we looked for evidence of haplotype sharing across families, using the program TRANSMIT. Suggestive evidence of sharing was observed with markers GGAA19e07 and D2S307, and three nearby candidate genes were examined by denaturing high-performance liquid chromatography on individuals from 19 families. One of these genes (BMPR2), which encodes bone morphogenetic protein receptor type II, was found to contain five mutations that predict premature termination of the protein product and two missense mutations. These mutations were not observed in 196 control chromosomes. These findings indicate that the bone morphogenetic protein-signaling pathway is defective in patients with primary pulmonary hypertension and may implicate the pathway in the nonfamilial forms of the disease.

Characterization of a model of hydrocephalus in transgenic mice

OBJECT

The purpose of this study was to elucidate the pathophysiological characteristics of hydrocephalus in a new transgenic model of mice created to overproduce the cytokine transforming growth factor-beta1 (TGFbeta1) in the central nervous system (CNS).

METHODS

Galbreath and colleagues generated transgenic mice that overexpressed TGFbeta1 in the CNS in an effort to examine the role of this cytokine in the response of astrocytes to injury. Unexpectedly, the animals developed severe hydrocephalus and died. The authors have perpetuated this transgenic colony to serve as a model of congenital hydrocephalus, breeding asymptomatic carrier males that are heterozygous for the transgene with wild-type females. One hundred twelve (49.6%) of 226 mice developed clinical manifestations of hydrocephalus, characterized by dorsal doming of the calvaria, spasticity, limb tremors, ataxia, and, ultimately, death. The presence of the TGFbeta1 transgene was determined by performing polymerase chain reaction (PCR) analysis of sample tail slices. Animals with the hydrocephalic phenotype consistently carried the transgene, although some animals with the transgene did not develop hydrocephalus. Animals without the transgene did not develop hydrocephalus. Alterations in brain structure were characterized using magnetic resonance (MR) imaging, gross and light microscopic analysis, and immunocytochemical studies. Magnetic resonance imaging readily distinguished hydrocephalic animals from nonhydrocephalic controls and demonstrated an obstruction at the outlets of the fourth ventricle. Gross and light microscopic examination confirmed the MR findings. The results of immunofluorescent staining of brain tissue slices revealed the presence of the TGFbeta1 cytokine and its receptor preferentially in the meninges and subarachnoid space in both hydrocephalic and control mice. Reverse transcriptase-PCR analysis demonstrated tissue-specific expression of the TGFbeta1, gene in the brains of transgenic mice, and enzyme-linked immunosorbent assay confirmed overexpression of the TGFbeta1 cytokine in brain, cerebrospinal fluid, and plasma.

CONCLUSIONS

The transgenic murine model provides a reproducible representation of congenital hydrocephalus. The authors hypothesize that overexpression of TGFbeta1 in the CNS causes hydrocephalus by altering the environment of the extracellular matrix and interfering with the circulation of cerebrospinal fluid. A model of hydrocephalus in which the genetic basis is known should be useful for evaluating hypotheses regarding the pathogenesis of this disorder and should also help in the search for new treatment strategies.

Inhibition of TGF-beta receptor signaling in osteoblasts leads to decreased bone remodeling and increased trabecular bone mass

Transforming growth factor-beta (TGF-beta) is abundant in bone matrix and has been shown to regulate the activity of osteoblasts and osteoclasts in vitro. To explore the role of endogenous TGF-(beta) in osteoblast function in vivo, we have inhibited osteoblastic responsiveness to TGF-beta in transgenic mice by expressing a cytoplasmically truncated type II TGF-beta receptor from the osteocalcin promoter. These transgenic mice develop an age-dependent increase in trabecular bone mass, which progresses up to the age of 6 months, due to an imbalance between bone formation and resorption during bone remodeling. Since the rate of osteoblastic bone formation was not altered, their increased trabecular bone mass is likely due to decreased bone resorption by osteoclasts. Accordingly, direct evidence of reduced osteoclast activity was found in transgenic mouse skulls, which had less cavitation and fewer mature osteoclasts relative to skulls of wild-type mice. These bone remodeling defects resulted in altered biomechanical properties. The femurs of transgenic mice were tougher, and their vertebral bodies were stiffer and stronger than those of wild-type mice. Lastly, osteocyte density was decreased in transgenic mice, suggesting that TGF-beta signaling in osteoblasts is required for normal osteoblast differentiation in vivo. Our results demonstrate that endogenous TGF-beta acts directly on osteoblasts to regulate bone remodeling, structure and biomechanical properties.

Inhibition of transforming growth factor-beta type II receptor signaling accelerates tooth formation in mouse first branchial arch explants

Members of the transforming growth factor-beta (TGF-beta) superfamily signal through their cognate receptors to determine cell phenotypes during embryogenesis. Our previous studies on the regulation of first branchial arch morphogenesis have identified critical components of a hierarchy of different TGF-beta isoforms and their possible functions in regulating tooth and cartilage formation during mandibular morphogenesis. Here we tested the hypothesis that TGF-beta type II receptor (TGF-beta IIR) is a critical component in the TGF-beta signaling pathway regulating tooth formation. To establish the precise location of TGF-beta ligand and its cognate receptor, we first performed detailed analyses of the localization of both TGF-beta2 and TGF-beta IIR during initiation and subsequent morphogenesis of developing embryonic mouse tooth organs. A possible autocrine functional role for TGF-beta and its cognate receptor (TGF-beta IIR) was inferred due to the temporal and spatial localization patterns during the early inductive stages of tooth morphogenesis. Second, loss of function of TGF-beta IIR in a mandibular explant culture model resulted in the acceleration of tooth formation to the cap stage while the mandibular explants in the control group only showed bud stage tooth formation. In addition, there was a significant increase in odontogenic epithelial cell proliferation following TGF-beta IIR abrogation. These results demonstrate, for the first time, that abrogation of the TGF-beta IIR stimulates embryonic tooth morphogenesis in culture and reverses the negative regulation of endogenous TGF-beta signaling upon enamel organ epithelial cell proliferation.

Development of TGF-beta resistance during malignant progression

Transforming growth factor-beta (TGF-beta) is the prototypical multifunctional cytokine, participating in the regulation of vital cellular activities such as proliferation and differentiation as well as a number of basic physiological functions. The effects of TGF-beta are critically dependent on the expression and distribution of a family of TGF-beta receptors, the TGF-beta types I, II, and III. It is now known that a wide variety of human pathology can be caused by aberrant expression and function of these receptors. The coding sequence of the type II receptor (RII) appears to render it uniquely susceptible to DNA replication errors in the course of normal cell division. By virtue of its key role in the regulation of cell proliferation, TGF-beta RII should be considered as a tumor suppressor gene. High levels of mutation in the TGF-beta RII gene have been observed in a wide range of primarily epithelial malignancies, including colon and gastric cancer. It appears likely that mutation of the TGF-beta RII gene may be a very critical step in the pathway of carcinogenesis.

Physical and functional interactions between type I transforming growth factor beta receptors and Balpha, a WD-40 repeat subunit of phosphatase 2A

We have previously shown that a WD-40 repeat protein, TRIP-1, associates with the type II transforming growth factor beta (TGF-beta) receptor. In this report, we show that another WD-40 repeat protein, the Balpha subunit of protein phosphatase 2A, associates with the cytoplasmic domain of type I TGF-beta receptors. This association depends on the kinase activity of the type I receptor, is increased by coexpression of the type II receptor, which is known to phosphorylate and activate the type I receptor, and allows the type I receptor to phosphorylate Balpha. Furthermore, Balpha enhances the growth inhibition activity of TGF-beta in a receptor-dependent manner. Because Balpha has been characterized as a regulator of phosphatase 2A activity, our observations suggest possible functional interactions between the TGF-beta receptor complex and the regulation of protein phosphatase 2A.

Expression of transforming growth factor-beta receptor type II and tumorigenicity in human breast adenocarcinoma MCF-7 cells

To analyze transforming growth factor-beta (TGF-beta) response during MCF-7 cell progression, early passage (MCF-7E, < 200 passage) and late passage (MCF-7L, > 500 passage) cells were compared. MCF-7E cells showed an IC50 of approximately 10 ng/ml of TGF-beta1, whereas MCF-7L cells were insensitive. MCF-7E cells contained approximately threefold higher levels of TGF-beta receptor type II (TbetaRII) mRNA than MCF-7L, but their TbetaRI levels were similar. MCF-7E parental cells showed higher TbetaRII promoter activity than MCF-7L cells, which could be attributed to changes in Sp1 nuclear protein levels. Receptor cross-linking studies indicated that the cell surface receptor levels parallel mRNA levels in both cell lines. Limiting dilution clones of MCF-7E cells were established to determine the heterogeneity of TbetaRII expression in this cell line, and they showed varying degrees of TbetaRII expression. Fibronectin was induced at higher levels in cells expressing higher TbetaRII levels. All three TGF-beta isoforms were detected in limiting dilution clones and parental cells, but TGF-beta1 was more abundant relative to TGF-beta2 or 3, and no correlation between TGF-beta isoform profile with TGF-beta sensitivity was found. MCF-7L cells were tumorigenic and formed xenografts rapidly and progressively, whereas MCF-7E parental and limiting dilution clonal cells showed transient tumor formation followed by regression. These results indicate that decreased TbetaRII transcription in breast cancer cells leads to a loss of TbetaRII expression, resulting in cellular resistance to TGF-beta which contributes to escape from negative growth regulation and tumor progression.

Probing the role of homomeric and heteromeric receptor interactions in TGF-beta signaling using small molecule dimerizers

BACKGROUND

Transforming growth factor Beta (TGF-Beta) arrests many cell types in the G1 phase of the cell and upregulates plasminogen activator inhibitor 1 (PAI-1). The type 1 (TGF-Beta RI) an II (TGF-Beta RII) TGF-Beta receptors mediate these and other effects of TGF-Beta on target cells. TGF-Beta initially binds to TGF-Beta RII and subsequently TGF-Beta RI is recruited to form a heteromeric complex. TGF-Beta RI phosphorylates the downstream effectors Smad2 and Smad3, leading to their translocation into the nucleus. Here, we explored the role of receptor oligomerization in TGF-Beta signaling.

RESULTS

We constructed fusion proteins containing receptor cytoplasmic tails linked to binding domains for small-molecule dimerizers. In COS-1 cells, recruitment of a soluble TGF-Beta RII tail to a myristoylated TGF-Beta RI tail promoted Smad2 nuclear translocation. In mink lung cells, homo-oligomerization of a myristoylated TGF-Beta Ri tail in presence of a myristoylated TGF-Beta RII tail activated the PAI-1 promoter. Oligomerization of an acidic mutant of the TGF-Beta RI tail in absence of TGF-Beta RII activated the PAI-A promoter and inhibited the growth of mink lung cells.

CONCLUSIONS

Non-toxic, small molecules designed to oligomerize cytoplasmic tails of TGF-Beta receptors at the plasma membrane can activate TGF-Beta signaling. Although TGF-Beta normally signals through two receptors that are both necessary for signaling, in one small-molecule system, a dimerizer activates signaling through a single type of receptor that is sufficient to induce TGF-Beta signaling. These methods of activating TGF-Beta signaling could be extended to signaling pathways of other TGF-Beta superfamily members such as activin and the bone morphogenetic proteins.

A novel missense mutation and frameshift mutations in the type II receptor of transforming growth factor-beta gene in sporadic colon cancer with microsatellite instability

Microsatellite instability of DNA samples of 79 sporadic colon cancer patients were analyzed. These samples were also screened to search mutations in the repeat sequences in the gene for the type II receptor of transforming growth factor-beta (TGF-beta RII) using polymerase chain reaction (PCR), electrophoresis with urea gel, and PCR-single strand conformation polymorphism (PCR-SSCP) method. The incidence of microsatellite instability, defined as severe replication error phenotype (RER) with microsatellite alterations in more than three loci, was 6%. Deletion and insertion of an A residue in the (A)10 region, which cause frameshift mutation, were found in four samples and their incidence in the samples with microsatellite instability was 80%. A novel nucleotide substitution of T for G at 1918, which causes missense mutation of arginine to leucine at codon 528, was found in a sample with microsatellite instability. The mutation at 1918 was in highly conservative amino acid residue.

Opposite effect of stable transfection of bioactive transforming growth factor-beta 1 (TGF beta 1) versus exogenous TGF beta 1 treatment on expression of 92-kDa type IV collagenase in mouse skin squamous cell carcinoma CH72 cells

We have previously shown that transforming growth factor-beta 1 (TGF beta 1) mRNA is consistently overexpressed in squamous cell carcinomas relative to normal mouse skin. Here we show that 92-kDa type IV collagenase (matrix metalloproteinase) (MMP-9) mRNA was likewise progressively overexpressed during mouse skin carcinogenesis. To determine if overexpression of MMP-9 and TGF beta 1 are linked, we stably transfected a bioactive TGF beta 1 into a mouse skin squamous cell carcinoma cell line (CH72), which resulted in about twofold to three-fold higher levels of secreted active TGF beta 1. Active TGF beta 1-transfected cells grew only slightly, but not significantly, more slowly in vitro and in vivo than vector-only transfectants. Two clones overexpressing active TGF beta 1 secreted much reduced levels of MMP-9 activity, as determined by zymogram analyses. However, treatment of these clones with 40 pM exogenous TGF beta 1 for 48 h enhanced secretion of MMP-9 activity. Constitutive mRNA expression of MMP-9 was reduced twofold to 70-fold in five untreated active TGF beta 1-transfected clones relative to the other transfectants. In contrast, treatment with 40 pM exogenous TGF beta 1 induced MMP-9 mRNA expression in a time-dependent fashion, from twofold to fourfold after 4 h to a maximum of 12- to 19-fold after 24-48 h. Induction of MMP-9 mRNA was dose dependent at TGF beta 1 concentrations of 4-400 pM. Thus, stable transfection of bioactive TGF beta 1 downregulated whereas exogenous TGF beta 1 treatment upregulated MMP-9 activity and expression. Treatment of transfectants with a neutralizing TGF beta 1 antibody slightly downregulated constitutive MMP-9 mRNA (20-30%) but completely blocked induction by exogenous TGF beta 1. Thus, the effect of TGF beta 1 transfection was not due to secreted TGF beta 1 but may have been a secondary effect.

Serine/threonine kinase receptors and ligands

Serine/threonine receptors transduce signals for the TGF-beta family, several members of which, such as decapentaplegic and bone morphogenetic proteins, are involved in early patterning of the embryo. The gene encoding the anti-Müllerian hormone (AMH) receptor has recently been cloned; gene targeting produces the same effects as targeting of the AMH gene itself. Another divergent member of the TGF-beta family, GDNF, signals through Ret, a tyrosine kinase receptor; binding to Ret requires the cooperation of GDNFR-alpha. The signal transduction pathway of serine/threonine receptors is now being intensively studied; the immunophilin FKBP-12 and MAD proteins are known to be involved.

Characterization of functional domains within Smad4/DPC4

Smad proteins are a family of highly conserved, intracellular proteins that signal cellular responses downstream of transforming growth factor-beta (TGF-beta) family serine/threonine kinase receptors. One of these molecules, Smad4, originally identified as the candidate tumor suppressor gene dpc-4, reconstitutes TGF-beta- and activin-dependent transcriptional responses in Smad4 null cell lines and interacts in a ligand-dependent manner with other Smad family members in both TGF-beta, activin, and bone morphogenetic protein-2/-4 pathways. Here, we used an assay based on the restoration of ligand-dependent transcriptional responses in a Smad4 null cell line to characterize functional domain structures within Smad4. We showed that restoration of TGF-beta-induced transcriptional responses by Smad4 was inhibited by co-transfection with a kinase dead TGF-beta type II receptor and that constitutive activation was blocked with TGF-beta neutralizing antibodies, confirming the essential role of Smad4 in TGF-beta signaling. Using a series of Smad4 mutation, deletion, and Smad1/Smad4 chimera constructs we identified a 47-amino acid deletion within the middle-linker region of Smad4 that is essential for the mediation of signaling responses. In addition, we showed that the NH2-terminal domain of Smad4 augments ligand-dependent activation associated with the middle-linker region, indicating that there is a distinct ligand-response domain within the N terminus of this molecule.

Biosynthesis of the type I and type II TGF-beta receptors. Implications for complex formation

The TGF-beta type I and type II receptors (TbetaRI and TbetaRII) are signaling receptors that form heteromeric cell surface complexes with the TGF-betas as one of the earliest events in the cellular response to these multifunctional growth factors. Using TGF-beta-responsive mink lung epithelial cells (Mv1Lu), we have determined the half-lives of the endoplasmic reticulum (ER) and mature forms of these receptors. In metabolically labeled cells, approximately 90% of newly synthesized type II receptor undergoes modification of N-linked sugars in the Golgi, with a half-life of 30-35 min; the Golgi-processed form of the receptor has a relatively short metabolic half-life of 2.5 h. In contrast, only 50% of pulse-labeled type I receptor is converted to the Golgi-processed and therefore endoglycosidase H-resistant form, and the endoglycosidase H-sensitive ER form has a half-life of 2.8-3 h. Addition of 100 pM TGF-beta1 causes the Golgi-processed type II receptor to become less stable, with a half-life of 1.7 h, and also destabilizes the Golgi-processed type I receptor. TGF-beta1 binding and cross-linking experiments on cells treated with tunicamycin for various times confirm different ER to cell surface processing times for TbetaRI and TbetaRII. Our results, which suggest that stable complexes between type I and II TGF-beta receptors do not form until the proteins reach a post-ER compartment (presumably the cell surface), have important implications for our understanding of complex formation and receptor regulation.

Bone morphogenetic protein-2 inhibits terminal differentiation of myogenic cells by suppressing the transcriptional activity of MyoD and myogenin

Bone morphogenetic protein (BMP) is a family of cytokines that induce ectopic bone formation when implanted into muscular tissues. We reported that BMP-2 inhibits the terminal differentiation of C2C12 myoblasts and converts them into osteoblast lineage cells (Katagiri, T., Yamaguchi, A., Komaki, M., Abe, E., Takahashi, N., Ikeda, T., Rosen, V., Wozney, J. M., Fujisawa-Sehara, A., and Suda, T. (1994) J. Cell Biol. 127, 1755-1766). In the present study, we examined the molecular mechanism of the inhibitory effect of BMP-2 on terminal differentiation of myogenic cells. When either MyoD or myogenin cDNA was introduced into C3H10T1/2 (10T1/2) cells with a muscle-specific CAT reporter containing four copies of the right E-box of muscle creatine kinase (MCK) enhancer, the CAT activity was dose-dependently suppressed by BMP-2. Furthermore, BMP-2 inhibited the terminal differentiation of these subclonal 10T1/2 cells that stably expressed MyoD or myogenin into mature myotubes that expressed myosin heavy chain and troponin T. The differentiation of a subclone of the MyoD-transfected NIH3T3 cells into mature muscle cells was also inhibited by BMP-2. BMP-2 induced alkaline phosphatase activity in 10T1/2-derived, but not in NIH3T3-derived MyoD-transfected cells. These cells constitutively expressed exogenous MyoD and myogenin, which were localized exclusively in the nuclei irrespective of the presence and the absence of BMP-2. However, these cells failed to express the mRNAs of endogenous myogenic factors and MCK when cultured with BMP-2. In the electrophoresis mobility shift assay using nuclear extracts of the myogenic cells, MyoD and myogenin bound to the right E-box in the enhancer region of the MCK gene even in the presence of BMP-2. These results suggest that BMP-2 inhibits the terminal differentiation of myogenic cells by suppressing the transcriptional activity of the myogenic factors.

Signal transduction by members of the transforming growth factor-beta superfamily

Transforming growth factor-beta (TGF beta) superfamily members exert their diverse biological effects through their interaction with heteromeric receptor complexes of transmembrane serine/threonine kinases. Both components of the receptor complex, known as receptor I and receptor II are essential for signal transduction. The composition of these complexes can vary significantly due to the promiscuous nature of the ligands and the receptors, and this diversity of interactions can yield a variety of biological responses. Several receptor interacting proteins and potential mediators of signal transduction have now been identified. Recent advances, particularly in our understanding of the function of Mothers against dpp-related (MADR) proteins, are providing new insights into how the TGF beta superfamily signals its diverse biological activities.

Regulation of differentiation by TGF-beta

Recent experiments in neural, skeletal, endothelial, and hematopoietic tissues have provided new insights into the way members of the transforming growth factor-beta (TGF-beta) superfamily regulate cellular differentiation. TGF-betas regulate the fate of multipotential stem cells instructively (in the neural crest) by regulating the expression or function of tissue-specific transcription factors, as well as selectively (in the mesenchyme) by regulating the expression of required growth factors and their receptors. During skeletal development, TGF-betas have unique functions and act sequentially to modulate chondrocyte and osteoblast differentiation. Responsiveness to TGF-betas changes as cells differentiate and evidence now suggests that changes in TGF-beta receptor profile may account for some of these differences. Drosophila and transgenic mouse models are now providing useful insights into mechanisms of TGF-beta action in vivo.

FKBP-12 recognition is dispensable for signal generation by type I transforming growth factor-beta receptors

The FK506-binding protein, FKBP12, is a putative target of type I receptors for transforming growth factor-beta (TbetaR-I). As the FK506 motif that competes with TbetaR-I for FKBP12 resembles an invariant Leu-Pro dipeptide in TbetaR-I, we replaced Leu193 and Pro194 with Ala, along with mutations across the Gly/Ser box. L193A, P194A, and L193A/P194A do not alter TbetaR-I function; T204D partially activates, independent of ligand; L193A/P194A/T204D was an even more potent constitutive mutation. Association with FKBP12 in a yeast two-hybrid assay was disrupted by P194A, L193A/P194A, and L193A/P194A/T204D, but not L193A or T204D alone. Thus, FKBP12 recognition is dispensable for TGFbeta signaling.

Structure and expression of the promoter for the R4/ALK5 human type I transforming growth factor-beta receptor: regulation by TGF-beta

The type I transforming growth factor-beta (TGF-beta) receptors are serine/threonine kinases that are essential for the action of TGF-beta. In this paper, we describe the molecular cloning and expression of the R4/ALK5 human type I TGF-beta receptor promoter. DNA sequence analysis indicates that the promoter lacks a TATA and CAAT box but is highly GC-rich and contains putative Sp1 binding sites. The transcriptional start site is approx. 232 base pairs upstream of the AUG start codon. In human lung fibroblasts, TGF-beta induced a 3-fold increase in steady-state level for type I receptor mRNA. Exposure of cells transfected with a 618 bp promoter fragment to TGF-beta 1 up-regulated transcriptional activity indicating that a TGF-beta response element is contained within this region.

A dominant inhibitory mutant of the type II transforming growth factor beta receptor in the malignant progression of a cutaneous T-cell lymphoma

In many cancers, inactivating mutations in both alleles of the transforming growth factor beta (TGF-beta) type 11 receptor (TbetaRII) gene occur and correlate with loss of sensitivity to TGF-beta. Here we describe a novel mechanism for loss of sensitivity to growth inhibition by TGF-beta in tumor development. Mac-1 cells, isolated from the blood of a patient with an indolent form of cutaneous T-cell lymphoma, express wild-type TbetaRII and are sensitive to TGF-beta. Mac-2A cells, clonally related to Mac-1 and isolated from a skin nodule of the same patient at a later, clinically aggressive stage of lymphoma, are resistant to TGF-beta. They express both the wild-type TbetaRII and a receptor with a single point mutation (Asp-404-Gly [D404G]) in the kinase domain (D404G-->TbetaRII); no TbetaRI or TbetaRII is found on the plasma membrane, suggesting that D404G-TbetaRII dominantly inhibits the function of the wild-type receptor by inhibiting its appearance on the plasma membrane. Indeed, inducible expression, under control of a tetracycline-regulated promoter, of D404G-TbetaRII in TGF-beta- sensitive Mac-1 cells as well as in Hep3B hepatoma cells results in resistance to TGF-beta and disappearance of cell surface TbetaRI and TbetaRII. Overexpression of wild-type TbetaRII in Mac-2A cells restores cell surface TbetaRI and TbetaRH and sensitivity to TGF-beta. The ability of the D404G-TbetaRH to dominantly inhibit function of wild-type TGF-beta receptors represents a new mechanism for loss of sensitivity to the growth-inhibitory functions of TGF-beta in tumor development.

Serum response factor mediates AP-1-dependent induction of the skeletal alpha-actin promoter in ventricular myocytes

"Fetal" gene transcription, including activation of the skeletal alpha-actin (SkA) promoter, is provoked in cardiac myocytes by mechanical stress and trophic ligands. Induction of the promoter by transforming growth factor beta or norepinephrine requires serum response factor (SRF) and TEF-1; expression is inhibited by YY1. We and others postulated that immediate-early transcription factors might couple trophic signals to this fetal program. However, multiple Fos/Jun proteins exist, and the exact relationship between control by Fos/Jun versus SRF, TEF-1, and YY1 is unexplained. We therefore cotransfected ventricular myocytes with Fos, Jun, or JunB, and SkA reporter genes. SkA transcription was augmented by Jun, Fos/Jun, Fos/JunB, and Jun/JunB; Fos and JunB alone were neutral or inhibitory. Mutation of the SRF site, SRE1, impaired activation by Jun; YY1, TEF-1, and Sp1 sites were dispensable. SRE1 conferred Jun activation to a heterologous promoter, as did the c-fos SRE. Deletions of DNA binding, dimerization, or trans-activation domains of Jun and SRF abolished activation by Jun and synergy with SRF. Neither direct binding of Fos/Jun to SREs, nor physical interaction between Fos/Jun and SRF, was detected in mobility-shift assays. Thus, AP-1 factors activate a hypertrophy-associated gene via SRF, without detectable binding to the promoter or to SRF.

Signal transduction and TGF-beta superfamily receptors

The TGF-beta superfamily includes a large number of related growth and differentiation factors expressed in virtually all phyla. Superfamily members bind to specific cell surface receptors that activate signal transduction mechanisms to elicit their effects. Candidate receptors fall into two primary groups, termed type I and type II receptors. Both types are serine/threonine kinases. Upon activation by the appropriate ligand, type I and type II receptors physically interact to form hetero-oligomers and subsequently activate intracellular signaling cascades, ultimately regulating gene transcription and expression. In addition, TGF-beta binds to a third receptor class, type III, a membrane-anchored proteoglycan lacking the kinase activity typical of signal transducing molecules. Type III receptors appear to regulate ligand availability to type I and type II receptors. Although a number of transduction mechanisms may be available to TGF-beta superfamily members, evidence gathered through the use of specific kinase and G-protein inhibitors and through assays measuring activation and levels of signaling intermediates suggests that at least one signaling pathway interacts with Ras and Raf proteins via a G-protein intermediate. Raf begins the cytoplasmic kinase cascade that leads to gene regulation. The myriad responses regulated by TGF-beta superfamily members makes the understanding of signal transduction mechanisms utilized by these proteins of great interest to a wide range of biological disciplines.

Loss of responsiveness to transforming growth factor beta (TGFbeta) is tightly linked to tumorigenicity in a model of thyroid tumour progression

It has been suggested that an important step in the progression of some epithelial tumours is the loss of responsiveness to the growth-inhibitory effects of transforming growth factor beta (TGFbeta). Here we describe the use of a model of thyroid tumorigenesis to investigate this question. Seven genetically closely related epithelial cell lines were derived following infection of primary cultures of rat thyroid epithelium with retroviral vectors encoding mutant ras. A strong negative correlation (p < 0.001) was found between the responsiveness of the lines to TGFbeta growth inhibition in vitro and their tumorigenicity in nude mice. Whereas TGFbeta-unresponsive and TGFbeta-stimulated lines formed rapidly growing, poorly differentiated tumours at all injection sites, cells that retained a partial inhibitory response formed much more slowly growing tumours, which showed a high degree of glandular differentiation. A line which retained full inhibition by TGFbeta formed slowly growing tumours at only 30% of injection sites, and cells explanted from these tumours subsequently showed a much reduced TGFbeta response in vitro. Our data using thyroid cells thus greatly strengthen the suggestion from previous studies that loss of growth inhibition by TGFbeta is associated with malignant progression of epithelial tumours. We also present an experimental model of papillary thyroid cancer which may prove useful in identifying the molecular changes involved in progression to the anaplastic form of the disease.

Insensitivity to anti-müllerian hormone due to a mutation in the human anti-müllerian hormone receptor

Anti-Müllerian hormone (AMH) and its receptor are involved in the regression of Müllerian ducts in male fetuses. We have now cloned and mapped the human AMH receptor gene and provide genetic proof that it is required for AMH signalling, by identifying a mutation in the AMH receptor in a patient with persistent Müllerian duct syndrome. The mutation destroys the invariant dinucleotide at the 5' end of the second intron, generating two abnormal mRNAs, one missing the second exon, required for ligand binding, and the other incorporating the first 12 bases of the second intron. The similar phenotypes observed in AMH-deficient and AMH receptor-deficient individuals indicate that the AMH signalling machinery is remarkably simple, consisting of one ligand and one type II receptor.

Transforming growth factor-beta (TGF-beta)-induced down-regulation of cyclin A expression requires a functional TGF-beta receptor complex. Characterization of chimeric and truncated type I and type II receptors

Transforming growth factor-beta (TGF-beta) inhibits the proliferation of epithelial cells by altering the expression or function of various components of the cell cycle machinery. Expression of one of these components, cyclin A, is inhibited by TGF-beta treatment. We have identified a 760-base pair fragment of the human cyclin A gene promoter that is sufficient to confer TGF-beta responsiveness. Using this promoter fragment, we have developed a cyclin A-based luciferase reporter assay that quantitates the growth inhibitory effect of TGF-beta in transient transfection assays. This assay was used to determine which domains of the type I (RI) and type II (RII) receptors were required for the antiproliferative effect of TGF-beta. In parallel, the functionality of chimeric receptors, between RI and RII (RI-RII or RII-RI), was tested for TGF-beta effect on gene expression using a reporter assay based on the plasminogen activator inhibitor type 1 (PAI-1) promoter. We found that TGF-beta-induced inhibition of cyclin A expression was absent in RI or RII-deficient Mv1Lu cells and that this response was restored by expression of wild-type type I or type II receptors in these cells. Furthermore, expression of a single chimeric receptor, either RI-RII or RII-RI, did not confer cyclin A regulation by TGF-beta. However, expression of two reciprocal chimeras (RI-RII and RII-RI) resulted in growth inhibition, similarly to wild-type receptors. In addition, chimeric receptors as well as mutant receptors with a deleted cytoplasmic domain and kinase-negative receptors inhibited TGF-beta responsiveness in the cyclin A reporter assay in a dominant negative fashion. Finally, in both receptor types, the juxtamembrane domain preceding the kinase domain was essential for receptor function but the cytoplasmic tail was dispensable. Our results suggest that a functional TGF-beta receptor complex is required for TGF-beta-dependent down-regulation of cyclin A gene expression and illustrate the identical receptor requirements for TGF-beta-induced growth inhibition and gene expression.