Determination of type I receptor specificity by the type II receptors for TGF-beta or activin

The novel type I serine-threonine kinase receptor Alk8 binds TGF-beta in the presence of TGF-betaRII

The TGF-beta superfamily consists of an array of ligands including BMP, TGF-beta, activin, and nodal subfamilies. The extensive range of biological effects elicited by TGF-beta family signaling is due in part to the large numbers and promiscuity of types I and II TGF-beta family member receptors. Alk8 is a novel type I TGF-beta family member receptor first identified in zebrafish [Dev. Dyn. 211 (4) (1998) 352], which participates in BMP signaling pathways [Development 128 (6) (2001) 849; Development 128 (6) (2001) 859; Mech. Dev. 100 (2) (2001) 275; J. Dent. Res. 80 (11) (2001) 1968]. Here we report that Alk8 also forms active signaling complexes with TGF-beta in the presence of TGF-betaRII. These results expand the signaling repertoire of zAlk8 by demonstrating an ability to participate in two distinct TGF-beta subfamily signaling pathways.

Structural determinants of binding and specificity in transforming growth factor-receptor interactions

Transforming growth factor (TGF-beta) protein families are cytokines that occur as a large number of homologous proteins. Three major subgroups of these proteins with marked specificities for their receptors have been found-TGF-beta, activin/inhibin, and bone morphogenic protein. Although structural information is available for some members of the TGF-beta family of ligands and receptors, very little is known about the way these growth factors interact with the extracellular domains of their cell surface receptors, especially the type II receptor. In addition, the elements that are the determinants of binding and specificity of the ligands are poorly understood. The structure of the extracellular domain of the receptor is a three-finger fold similar to some toxin structures. Amino acid exchanges between multiply aligned homologous sequences of type II receptors point to a residue at the surface, specifically finger 1, as the determinant of ligand specificity and complex formation. The "knuckle" epitope of ligands was predicted to be the surface that interacts with the type II receptor. The residues on strands beta2, beta3, beta7, beta8 and the loop region joining beta2 and beta3 and joining beta7 and beta8 of the ligands were identified as determinants of binding and specificity. These results are supported by studies on the docking of the type II receptor to the ligand dimer-type I receptor complex.

The type I serine/threonine kinase receptor Alk8/Lost-a-fin is required for Bmp2b/7 signal transduction during dorsoventral patterning of the zebrafish embryo

Ventral specification of mesoderm and ectoderm depends on signaling by members of the bone morphogenetic protein (Bmp) family. Bmp signals are transmitted by a complex of type I and type II serine/threonine kinase transmembrane receptors. Here, we show that Alk8, a novel member of the Alk1 subgroup of type I receptors, is disrupted in zebrafish lost-a-fin (laf) mutants. Two alk8/laf null alleles are described. In laf(tm110), a conserved extracellular cysteine residue is replaced by an arginine, while in laf(m100), Alk8 is prematurely terminated directly after the transmembrane domain. The zygotic effect of both mutations leads to dorsalization of intermediate strength. A much stronger dorsalization, similar to that of bmp2b/swirl and bmp7/snailhouse mutants, however, is obtained by inhibiting both maternally and zygotically supplied alk8 gene products with morpholino antisense oligonucleotides. The phenotype of laf mutants and alk8 morphants can be rescued by injected mRNA encoding Alk8 or the Bmp-regulated transcription factor Smad5, but not by mRNA encoding Bmp2b or Bmp7. Conversely, injected mRNA encoding a constitutively active version of Alk8 can rescue the strong dorsalization of bmp2b/swirl and bmp7/snailhouse mutants, whereas smad5/somitabun mutant embryos do not respond. Altogether, the data suggest that Alk8 acts as a Bmp2b/7 receptor upstream of Smad5.

Smad proteins function as co-modulators for MEF2 transcriptional regulatory proteins

An emerging theme in transforming growth factor-ss (TGF-ss) signalling is the association of the Smad proteins with diverse groups of transcriptional regulatory proteins. Several Smad cofactors have been identified to date but the diversity of TGF-ss effects on gene transcription suggests that interactions with other co-regulators must occur. In these studies we addressed the possible interaction of Smad proteins with the myocyte enhancer-binding factor 2 (MEF2) transcriptional regulators. Our studies indicate that Smad2 and 4 (Smad2/4) complexes cooperate with MEF2 regulatory proteins in a GAL4-based one-hybrid reporter gene assay. We have also observed in vivo interactions between Smad2 and MEF2A using co-immunoprecipitation assays. This interaction is confirmed by glutathione S:-transferase pull-down analysis. Immunofluorescence studies in C2C12 myotubes show that Smad2 and MEF2A co-localise in the nucleus of multinuclear myotubes during differentiation. Interestingly, phospho-acceptor site mutations of MEF2 that render it unresponsive to p38 MAP kinase signalling abrogate the cooperativity with the Smads suggesting that p38 MAP Kinase-catalysed phosphorylation of MEF2 is a prerequisite for the Smad-MEF2 interaction. Thus, the association between Smad2 and MEF2A may subserve a physical link between TGF-ss signalling and a diverse array of genes controlled by the MEF2 cis element.

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.

TGFbeta1 represses proliferation of pancreatic carcinoma cells which correlates with Smad4-independent inhibition of ERK activation

Transforming growth factor beta (TGFbeta) is a tumor suppressor acting as inhibitor of cell cycle progression of epithelial cells. We show that treatment of the pancreatic carcinoma cell lines PANC-1 and BxPC-3 with TGFbeta1 inhibits both growth factor-induced activation of the extracellular signal-regulated kinase 2 (ERK2) and translocation of the kinase to the nucleus. TGFbeta1 causes a concentration-dependent reduction of cell proliferation in both cell lines. By measuring ERK activation, we can show that TGFbeta1 is able to repress ERK activation induced by mitogenic stimuli such as EGF. This inhibitory effect of TGFbeta1 is not mediated by suppression of Ras or c-Raf-1 activation, but mediated by TGFbeta1-induced activation of a serine-threonine phosphatase, as demonstrated by inhibition of phosphatases by treatment with okadaic acid. Results obtained in the Smad4-deficient pancreatic carcinoma cell line BxPC-3, demonstrate that TGFbeta1-induced growth inhibition is mediated by a Smad4-independent prevention of ERK2 activation. In contrast to the effects of TGFbeta1 on epithelial cells, mesenchymal NIH3T3 fibroblasts exhibit elevated ERK2 activation and increased cell proliferation in response to TGFbeta1 treatment. Smad4-independent phosphatase-mediated inhibition of mitogen-activated ERK2 represents a novel effector pathway contributing to suppression of epithelial pancreatic carcinoma cell proliferation by TGFbeta1, in addition to the well-known Smad-induced tumor suppressor activity of TGFbeta. Oncogene (2000) 19, 4531 - 4541.

Induction of cardiac fibrosis by transforming growth factor-beta(1)

The role of transforming growth factor-beta(1) (TGF-beta(1)) in the production and deposition of collagens and in the induction of gene expression in the myocardium in relation to the development of myocardial fibrosis will be discussed. Very low expression of TGF-beta(1) and collagen type I and III mRNA is seen in the normal rat heart. Both expressions are markedly increased in the infarcted heart and the levels of TGF-beta(1) mRNA precedes increases in mRNA levels for extracellular matrix (ECM) proteins, suggesting a possible role of TGF-beta(1) in remodeling processes in the myocardium. The TGF-beta(1) expression is normally only transient since continuous TGF-beta(1) overexpression seems to promote nonadaptive cardiac hypertrophy and myocardial fibrosis. In vitro, TGF-beta(1) induces an increase in collagen production and secretion and enhances the abundance of mRNA levels for collagen type I and III in rat cardiac fibroblasts in culture. TGF-beta(1) also stimulates in vivo the expression of ECM proteins and in vivo gene transfer of TGF-beta(1) can induce myocardial fibrosis. Increased myocardial TGF-beta(1) and ECM protein mRNA are found in myocardial fibrosis induced by angiotensin II infusion, by noradrenaline treatment, by isoprenaline infusion, and by long-term blockade of NO synthesis. In vivo antagonism of TGF-beta(1) by neutralizing anti-TGF-beta(1) antibodies or by proteoglycans prevents the increase in gene expression of ECM proteins and inhibits myocardial fibrosis, suggesting that the increases in matrix protein production and fibrosis are mediated by TGF-beta(1).

Activin betaC and betaE genes are not essential for mouse liver growth, differentiation, and regeneration

The liver is an essential organ that produces several serum proteins, stores vital nutrients, and detoxifies many carcinogenic and xenobiotic compounds. Various growth factors positively regulate liver growth, but only a few negative regulators are known. Among the latter are the transforming growth factor beta (TGF-beta) superfamily members TGF-beta1 and activin A. To study the function of novel activin family members, we have cloned and generated mice deficient in the activin betaC and betaE genes. Expression analyses demonstrated that these novel genes are liver specific in adult mice. Here, we show by RNase protection that activin betaC transcripts are present in the liver beginning at embryonic day 11.5 (E11.5) whereas activin betaE expression is detected starting from E17.5. Gene targeting in embryonic stem cells was used to generate mice with null mutations in either the individual activin betaC and betaE genes or both genes. In contrast to the structurally related activin betaA and betaB subunits, which are necessary for embryonic development and pituitary follicle-stimulating hormone homeostasis, mice deficient in activin betaC and betaE were viable, survived to adulthood, and demonstrated no reproductive abnormalities. Although activin betaC and betaE mRNAs are abundantly expressed in the liver of wild-type mice, the single and double mutants did not show any defects in liver development and function. Furthermore, in the homozygous mutant mice, liver regeneration after >70% partial hepatectomy was comparable to that in wild-type mice. Our results suggest that activin betaC and betaE are not essential for either embryonic development or liver function.

Transient expression of activin betaA mRNA on osteoprogenitor cells in rat bone regeneration after drill-hole injury

We investigated the expression of activin betaA on osteoprogenitor cells in the regenerating bone and bone marrow of the rat femur after drill-hole injury, by immunocytochemistry and in situ hybridization. The periosteum and endosteum adjacent to the wound region showed marked thickening at day 3 and abundant osteoprogenitor cells, which were immunoreactive for proliferating cell nuclear antigen and showed positive reactions for alkaline phosphatase activity, and existed in the inner layer of the periosteum as well as in the endosteum. During the same period, these osteoprogenitor cells began to exhibit activin betaA immunoreactivity and mRNA expression. However, the latter expression gradually reduced the intensity as the cells started to express osteocalcin mRNA during their differentiation to osteoblasts participating in the periosteal and medullary bone formation from day 5. Immunoreactivity for activin type IB and II receptors was also found on activin betaA-immunoreactive cells between days 3 and 7. The above findings suggest that proliferating osteoprogenitor cells, before their transformation to osteoblasts, transiently produce and release activin A, which may play crucial roles in bone and bone marrow regeneration in a receptor-mediated, autocrine and paracrine fashion.

Smad7 selectively interferes with different pathways of activin signaling and inhibits erythroid leukemia cell differentiation

Smad family proteins are essential for transforming growth factor β (TGF-β) signal mediation downstream of a heteromeric complex of the type I and type II receptor serine/threonine kinases. A distant family member, Smad7, is expressed in most mammalian tissues and cells and prevents TGF-β signaling. In this study, we examined the physiologic role of Smad7 in mediating the effects of activin, a member of the TGF-β superfamily of peptides that functions in a number of processes, including blood-cell development. We report here that Smad7 expression is specifically absent in particular hematopoietic cells that respond to activin by differentiating into the erythroid lineage and that ectopic production of Smad7 causes mouse erythroid leukemia (F5-5) cells to become resistant to activin induction of erythroid differentiation. When coexpressed with type I activin receptor ActR-I or ActR-IB in concert with type II receptor ActR-II, Smad7 efficiently reduced an early transcriptional response mediated by ActR-I but had only a minimal effect on the response mediated by ActR-IB. In the presence of Smad7, overexpression of an activated form of ActR-IB, but not of an activated form of ActR-I, induced F5-5 cells to differentiate. These results suggest that Smad7 selectively interferes with the ActR-I pathway in activin signal transduction. The findings also indicate the existence of a novel activity of Smad7 that inhibits erythroid differentiation by blocking intracellular signaling of activin.

Smad7 selectively interferes with different pathways of activin signaling and inhibits erythroid leukemia cell differentiation

Smad family proteins are essential for transforming growth factor β (TGF-β) signal mediation downstream of a heteromeric complex of the type I and type II receptor serine/threonine kinases. A distant family member, Smad7, is expressed in most mammalian tissues and cells and prevents TGF-β signaling. In this study, we examined the physiologic role of Smad7 in mediating the effects of activin, a member of the TGF-β superfamily of peptides that functions in a number of processes, including blood-cell development. We report here that Smad7 expression is specifically absent in particular hematopoietic cells that respond to activin by differentiating into the erythroid lineage and that ectopic production of Smad7 causes mouse erythroid leukemia (F5-5) cells to become resistant to activin induction of erythroid differentiation. When coexpressed with type I activin receptor ActR-I or ActR-IB in concert with type II receptor ActR-II, Smad7 efficiently reduced an early transcriptional response mediated by ActR-I but had only a minimal effect on the response mediated by ActR-IB. In the presence of Smad7, overexpression of an activated form of ActR-IB, but not of an activated form of ActR-I, induced F5-5 cells to differentiate. These results suggest that Smad7 selectively interferes with the ActR-I pathway in activin signal transduction. The findings also indicate the existence of a novel activity of Smad7 that inhibits erythroid differentiation by blocking intracellular signaling of activin.

Activin receptor-like kinase 2 can mediate atrioventricular cushion transformation

Epithelial-mesenchymal transformation in the atrioventricular (AV) cushion of the tubular heart is a critical step in the formation of the valves and membranous septa. Transforming growth factor beta (TGFbeta) ligands are a primary signal of this transformation. To investigate the expression and function of specific Type I TGFbeta receptors during AV cushion transformation, we cloned and characterized the chicken homologues of two mammalian activin receptor-like kinases (ALK), ALK2 and ALK5, and generated specific, polyclonal antibodies against the extracellular binding domains of each. Both the chicken ALK2 (ChALK2) and the chicken ALK5 (ChALK5) cDNAs encode proteins that bind TGFbeta1 in the presence of the Type II TGFbeta receptor. However, as expected, only ChALK5 stimulated the TGFbeta-responsive PAI-1 promoter. These data establish that ChALK2 and ChALK5 are the chicken homologues of the mammalian receptors ALK2 and ALK5. Both ChALK2 and ChALK5 are expressed by AV endocardial cells. AV cushion explants harvested from stage 13-18 embryos were incubated with antisera to ChALK2 or ChALK5. Anti-ChALK2 antisera inhibited mesenchyme formation by 34-50% while neutralizing anti-ChALK5 antisera were without effect. These data demonstrate that ChALK2 can mediate transformation in the AV cushion.

Activins and their receptors in female reproduction

Activins are growth and differentiation factors belonging to the transforming growth factor-β superfamily. They are dimeric proteins consisting of two inhibin β subunits. The structure of activins is highly conserved during vertebrate evolution. Activins signal through type I and type II receptor proteins, both of which are serine/threonine kinases. Subsequently, downstream signals such as Smad proteins are phosphorylated. Activins and their receptors are present in many tissues of mammals and lower vertebrates where they function as autocrine and (or) paracrine regulators of a variety of physiological processes, including reproduction. In the hypothalamus, activins are thought to stimulate the release of gonadotropin-releasing hormone. In the pituitary, activins increase follicle-stimulating hormone secretion and up-regulate gonadotropin-releasing hormone receptor expression. In the ovaries of vertebrates, activins are expressed predominantly in the follicular layer of the oocyte where they regulate processes such as folliculogenesis, steroid hormone production, and oocyte maturation. During pregnancy, activin-A is also involved in the regulation of placental functions. This review provides a brief overview of activins and their receptors, including their structures, expression, and functions in the female reproductive axis as well as in the placenta. Special effort is made to compare activins and their receptors in different vertebrates.

Key words: activins, activin receptors, reproductive axis, placenta.

Activin A-induced HepG2 liver cell apoptosis: involvement of activin receptors and smad proteins

A balance between cell proliferation and apoptosis is important for regulating normal liver function. Proteins of the transforming growth factor-beta superfamily are known to be important mediators of apoptosis in the liver. In this study we demonstrate that activin A potently induces apoptotic cell death in a hepatoma cell line, HepG2 cells. To determine the roles of activin receptors and downstream signaling proteins in activin A-induced apoptosis in these cells, the activin signaling pathway was analyzed using the transcription of an activin-responsive reporter gene, p3TP-Lux, as an assay. Although individual activin receptors had little effect on transcriptional activity, coexpression of an activin type I receptor and a type II receptor significantly increased both basal and activin-induced transcriptional activation, with the combination ofreceptors IB and IIB being the most potent. Similarly, expression of individual Smad proteins had only a modest effect on reporter gene activity, but the combination of Smad2 and Smad4 strongly stimulated transcription. Activin signaling induced a rapid relocation of Smad2 to the nucleus, as determined using a green fluorescence protein-Smad2 fusion protein. In contrast, green fluorescence protein-Smad4 remained localized to the cytoplasm unless it was coexpressed with Smad2. In agreement with the transcriptional response assays, overexpression or suppression of activin signaling components in HepG2 cells altered apoptosis. Overexpression of receptors IB and IIB or Smad proteins 2 and 4 stimulated apoptosis, whereas dominant negative mutant forms of the activin type IIB receptor or Smad2 blocked activin-stimulated apoptosis. These studies suggest that signaling from the cell surface to the nucleus through Smad proteins is a required component of the activin A-induced cell death process in liver cells.

The ActR-I activin receptor protein is expressed in notochord, lens placode and pituitary primordium cells in the mouse embryo

ActR-I is a type I serine/threonine kinase receptor which has been shown to bind activin and bone morphogenetic proteins (BMPs). To study the function of ActR-I, we have generated novel monoclonal antibodies that specifically recognize the extracellular domain of mouse ActR-I. We examined the level of ActR-I protein during mouse development by immunohistochemistry. We found that in the embryonic body, ActR-I protein first appears in a restricted part of the primitive streak region and is present throughout the length of notochord. Furthermore, ActR-I protein is expressed in the facial sensory organ primordia, including eye area, otic vesicle and olfactory placode, which all contain invaginating ectoderm. In addition, ActR-I is produced in pituitary primordium (Rathke's pouch), mammary buds and the epithelial layer of branchial arches. Interestingly, in the lens placodes and in early Rathke's pouch, ActR-I protein is transiently localized at the apical surface of the epithelial cells, indicating the presence of an apical-basal asymmetry in these cells.

Isolation of recombinant BMP receptor IA ectodomain and its 2:1 complex with BMP-2

Bone morphogenetic protein-2 (BMP-2) is a member of the transforming growth factor beta superfamily which induces bone formation and regeneration, and important steps during early embryonic development. BMP-2 signals via oligomerization of type I and type II serine/threonine kinase receptors. We report here expression of the extracellular domain of the human type IA receptor for BMP-2 (BMPR-IA) in Escherichia coli. This soluble form of BMPR-IA (sBMPR-IA) was purified employing a BMP-2 affinity column. Gel filtration experiments and analysis of gel filtration fractions by polyacrylamide electrophoresis and densitometry reveal that BMP-2 forms a defined 1:2 complex with sBMPR-IA that can be purified and hopefully used for crystallization studies.

Extracellular domain of the transforming growth factor-beta receptor negatively regulates ligand-independent receptor activation

We have previously proposed that transforming growth factor (TGF)-beta receptor activation occurs via a relative rotation between the receptors. This model suggests that in the absence of the ligand the receptor extracellular domain negatively regulates the activation of the receptor complex. To investigate this proposition, four TGF-beta type I and II receptor extracellular/transmembrane-cytoplasmic and extracellular-transmembrane/cytoplasmic chimeras, TbetaRII-I-I and TbetaRI-II-II as well as TbetaRII-II-I and TbetaRI-I-II, and two extracellular domain truncated receptors TbetaRI-STC and TbetaRII-STC were generated. In either mutant mink lung R1B (lacking functional type I receptor) or DR26 (where the type II receptor is nonfunctional) cells, coexpression of two chimeric receptors, which are complementary in extracellular and cytoplasmic domains, transduced TGF-beta induced signaling, as measured by the transcriptional activation of a p3TP-Lux reporter gene. Coexpression of this type of chimeric receptor with a wild-type receptor containing the opposite cytoplasmic domain exhibited a varied level of constitutive activity depending on the particular combination of the extracellular domains. In general, the type I-type I extracellular domain combination gave higher constitutive activity than the type I-type II or type II-type II combinations. Furthermore, coexpression of the extracellular domain truncated receptor with any receptor containing the opposite cytoplasmic domain always resulted in ligand independent receptor signaling. Immunoprecipitation studies showed that the formation of the receptor complexes paralleled the ligand independent activation of p3TP-Lux. Our results support the conclusion that the TGF-beta receptor extracellular domain plays a negative regulatory role in receptor activation in the absence of ligand.

Reduction of bleomycin induced lung fibrosis by transforming growth factor beta soluble receptor in hamsters

BACKGROUND

Transforming growth factor beta (TGF-beta) is a key mediator of collagen synthesis in the development of lung fibrosis. It has previously been shown that the administration of TGF-beta antibody and TGF-beta binding proteoglycan, decorin, reduced bleomycin (BL) induced lung fibrosis in animals. The present study was carried out to investigate whether intratracheal instillation of TGF-beta soluble receptor (TR) would minimise the BL induced lung fibrosis in hamsters.

METHODS

The effect of a recombinant TR (TGFbetaRII) on the lung collagen accumulation was evaluated in a BL hamster model of pulmonary fibrosis. Animals were divided into four groups and intratracheally injected with saline or BL at 6.5 U/4 ml/kg followed by intratracheal instillation of phosphate buffered saline (PBS) or 4 nmol TR in 0.3 ml twice a week. Twenty days after the first intratracheal instillation the hamsters were killed for bronchoalveolar lavage (BAL) fluid, biochemical, and histopathological analyses.

RESULTS

Treatment of hamsters with TR after intratracheal instillation of BL significantly reduced BL induced lung fibrosis as shown by decreases in the lung hydroxyproline level and prolyl hydroxylase activity, although they were still significantly higher than those of the saline control. Histopathological examination showed a considerable decrease in BL induced fibrotic lesions by TR treatment. However, TR did not prevent the BL induced increases in total cells and protein in the BAL fluid.

CONCLUSIONS

These results suggest that TR has antifibrotic potential in vivo and may be useful in the treatment of fibrotic diseases where increased TGF-beta is associated with excess collagen accumulation.

Control of digit formation by activin signalling

Major advances in the genetics of vertebrate limb development have been obtained in recent years. However, the nature of the signals which trigger differentiation of the mesoderm to form the limb skeleton remains elusive. Previously, we have obtained evidence for a role of TGFbeta2 in digit formation. Here, we show that activins A and B and/or AB are also signals involved in digit skeletogenesis. activin betaA gene expression correlates with the initiation of digit chondrogenesis while activin betaB is expressed coincidently with the formation of the last phalanx of each digit. Exogenous administration of activins A, B or AB into the interdigital regions induces the formation of extra digits. follistatin, a natural antagonist of activins, is expressed, under the control of activin, peripherally to the digit chondrogenic aggregates marking the prospective tendinous blastemas. Exogenous application of follistatin blocks physiological and activin-induced digit formation. Evidence for a close interaction between activins and other signalling molecules, such as BMPs and FGFs, operating at the distal tip of the limb at these stages is also provided. Chondrogenesis by activins is mediated by BMPs through the regulation of the BMP receptor bmpR-1b and in turn activin expression is upregulated by BMP signalling. In addition, AER hyperactivity secondary to Wnt3A misexpression or local administration of FGFs, inhibits activin expression. In correlation with the restricted expression of activins in the course of digit formation, neither activin nor follistatin treatment affects the development of the skeletal components of the stylopod or zeugopod indicating that the formation of the limb skeleton is regulated by segment-specific chondrogenic signals.

Transforming growth factor-beta (TGF-beta) type I and type II receptors are both required for TGF-beta-mediated extracellular matrix production in lung fibroblasts

Transforming growth factor-beta (TGF-beta) regulates a variety of cellular activities including cell growth, differentiation and extracellular matrix production. The TGF-beta type I and type II serine/threonine kinase receptors (TbetaRI and TbetaRII) have been identified as signal-transducing TGF-beta receptors. This study was undertaken to examine the role of the type I and type II receptors in TGF-beta-induced extracellular matrix production of lung fibroblasts. We constructed expression plasmids containing truncated derivatives of TbetaRI and TbetaRII that lacked the cytoplasmic serine/threonine kinase domain (TbetaRI deltaK and TbetaRII deltaK), and transfected them into lung fibroblasts. TbetaRII deltaK expressed by lung fibroblasts was able to bind 125I-TGF-beta1, whereas TbetaRI deltaK was unable to bind ligand when expressed alone. Co-expression with TbetaRII was required for binding and cross-linking of TGF-beta1 to TbetaRI deltaK. Lung fibroblasts upregulate tenascin and fibronectin production when treated with TGF-beta1. The kinase-defective deletions of both TbetaRI and TbetaRII were dominant-acting inhibitors of TGF-beta signal transduction. Expression of either TbetaRI deltaK or TbetaRII deltaK alone was sufficient to block TGF-beta-induced tenascin and fibronectin production of lung fibroblasts. The results indicate that both TbetaRI and TbetaRII were required for TGF-beta signaling in regulation of extracellular matrix production by lung fibroblasts.

Overexpression of a kinase-deficient transforming growth factor-beta type II receptor in mouse mammary stroma results in increased epithelial branching

Members of the transforming growth factor-beta (TGF-beta) superfamily signal through heteromeric type I and type II serine/threonine kinase receptors. Transgenic mice that overexpress a dominant-negative mutation of the TGF-beta type II receptor (DNIIR) under the control of a metallothionein-derived promoter (MT-DNIIR) were used to determine the role of endogenous TGF-betas in the developing mammary gland. The expression of the dominant-negative receptor was induced with zinc and was primarily localized to the stroma underlying the ductal epithelium in the mammary glands of virgin transgenic mice from two separate mouse lines. In MT-DNIIR virgin females treated with zinc, there was an increase in lateral branching of the ductal epithelium. We tested the hypothesis that expression of the dominant-negative receptor may alter expression of genes that are expressed in the stroma and regulated by TGF-betas, potentially resulting in the increased lateral branching seen in the MT-DNIIR mammary glands. The expression of hepatocyte growth factor mRNA was increased in mammary glands from transgenic animals relative to the wild-type controls, suggesting that this factor may play a role in TGF-beta-mediated regulation of lateral branching. Loss of responsiveness to TGF-betas in the mammary stroma resulted in increased branching in mammary epithelium, suggesting that TGF-betas play an important role in the stromal-epithelial interactions required for branching morphogenesis.

Smad1 recognition and activation by the ALK1 group of transforming growth factor-beta family receptors

Two structural elements, the L45 loop on the kinase domain of the transforming growth factor-beta (TGF-beta) family type I receptors and the L3 loop on the MH2 domain of Smad proteins, determine the specificity of the interactions between these receptors and Smad proteins. The L45 sequence of the TGF-beta type I receptor (TbetaR-I) specifies Smad2 interaction, whereas the related L45 sequence of the bone morphogenetic protein (BMP) type I receptor (BMPR-I) specifies Smad1 interactions. Here we report that members of a third receptor group, which includes ALK1 and ALK2 from vertebrates and Saxophone from Drosophila, specifically phosphorylate and activate Smad1 even though the L45 sequence of this group is very divergent from that of BMPR-I. We investigated the structural elements that determine the specific recognition of Smad1 by ALK1 and ALK2. In addition to the receptor L45 loop and the Smad1 L3 loop, the specificity of this recognition requires the alpha-helix 1 of Smad1. The alpha-helix 1 is a conserved structural element located in the vicinity of the L3 loop on the surface of the Smad MH2 domain. Thus, Smad1 recognizes two distinct groups of receptors, the BMPR-I group and the ALK1 group, through different L45 sequences on the receptor kinase domain and a differential use of two surface structures on the Smad1 MH2 domain.

Endoglin is an accessory protein that interacts with the signaling receptor complex of multiple members of the transforming growth factor-beta superfamily

Endoglin (CD105) is a transmembrane glycoprotein that binds transforming growth factor (TGF)-beta1 and -beta3, and coprecipitates with the Ser/Thr kinase signaling receptor complex by affinity labeling of endothelial and leukemic cells. The present study shows that in addition to TGF-beta1 and -beta3, endoglin interacts with activin-A, bone morphogenetic protein (BMP)-7, and BMP-2 but requires coexpression of the respective ligand binding kinase receptor for this association. Endoglin cannot bind ligands on its own and does not alter binding to the kinase receptors. It binds TGF-beta1 and -beta3 by associating with the TGF-beta type II receptor and interacts with activin-A and BMP-7 via activin type II receptors, ActRII and ActRIIB, regardless of which type I receptor partner is coexpressed. However, endoglin binds BMP-2 by interacting with the ligand binding type I receptors, ALK3 and ALK6. The formation of heteromeric signaling complexes was not altered by the presence of endoglin, although it was coprecipitated with these complexes. Endoglin did not interact with BMP-7 through complexes containing the BMP type II receptor, demonstrating specificity of its action. Our data suggest that endoglin is an accessory protein of multiple kinase receptor complexes of the TGF-beta superfamily.

Roles of pathway-specific and inhibitory Smads in activin receptor signaling

Activins and other members of the transforming growth factor-beta-like superfamily of growth factors transduce their signals by interacting with two types of receptor serine/threonine kinases. The Smad proteins, a new family of intracellular mediators are involved in the signaling pathways of these receptors, but the initial stages of their activation as well as their specific functions remain to be defined. We report here that the pathway-specific Smad2 and 3 can form a complex with the activin receptor in a ligand-dependent manner. This complex formation is rapid but also transient. Indeed, soon after their association with the activin receptor, Smad2 and Smad3 are released into the cytoplasm where they interact with the common partner Smad4. These Smad complexes then mediate activin-induced transcription. Finally, we show that the inhibitory Smad7 can prevent the association of the two pathway-specific Smads with the activin receptor complex, thereby blocking the activin signal.

Downstream factors in transforming growth factor-beta family signaling

The recently identified family of Smad proteins has given insight in the understanding of how members of the transforming growth factor-beta (TGF-beta) family relay their signal to the nucleus. Besides Smad proteins, G proteins and MAPKs are also involved in the downstream signaling of TGF-beta family members. The identification of elements that function downstream in the TGF-beta signaling pathway and the fact that these downstream players can interact with the signaling cascade of other growth factors, may give insight into the diverse biological responses evoked by the TGF-beta family members.

Smad7 is an activin-inducible inhibitor of activin-induced growth arrest and apoptosis in mouse B cells

Members of the transforming growth factor-beta (TGF-beta) family, which includes the activins, relay signals from serine/threonine kinase receptors in membrane to nucleus via intracellular Sma- and Mad-related (Smad) proteins. Inhibitory Smad proteins were found to prevent the interaction between the serine/threonine kinase receptors and pathway-restricted Smad proteins. Smad7 was identified as a TGF-beta-inducible antagonist of TGF-beta signaling, and it may participate in a negative feedback loop to control TGF-beta signaling. Here we demonstrate that the mRNA expression of Smad7 is induced by activin A in mouse B cell hybridoma HS-72 cells, which undergo growth arrest and apoptosis upon exposure to activin A. The ectopic expression of mouse Smad7 in HS-72 cells suppressed the activin A-induced cell cycle arrest in the G1 phase by abolishing the activin A-induced expression of p21(CIP1/WAF1) and hypophosphorylation of retinoblastoma protein. Furthermore, Smad7 expression suppressed activin A-induced apoptosis in HS-72 cells. Thus, our data indicate that Smad7 is an activin A-inducible antagonist of activin A-induced growth arrest and apoptosis of B lineage cells.

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.

Regulation of cellular and system function by activin

Activin is an important molecule that regulates hormonogenesis, cellular homeostasis (divide or die pathways), and differentiation programs (developmentally and in adult cells). The cellular mechanisms that integrate an activin signal into a physiological response include a binary receptor complex and tandem serine threonine kinases, intracellular signal mediators, and nuclear transcription factors. Activin antagonists (inhibins) and bioneutralizing binding proteins (follistatins) act as gating molecules to ensure accurate delivery of activin signals to cellular machinery. Correct execution of an activin cue intracellularly permits actions as fundamental as embryonic mesoderm development, neuronal survival, hematopoietic function, and reproductive cyclicity. Absent or incorrect activin signaling results in phenotypes as catastrophic as embryonic lethality, tumor formation, and infertility. The general ways in which a cell senses and responds to an activin signal will be reviewed in the first part of this paper. The role of this ligand in reproductive function will also be examined as a specific example of activin activity.

The type I activin receptor ActRIB is required for egg cylinder organization and gastrulation in the mouse

ActRIB is a type I transmembrane serine/threonine kinase receptor that has been shown to form heteromeric complexes with the type II activin receptors to mediate activin signal. To investigate the function of ActRIB in mammalian development, we generated ActRIB-deficient ES cell lines and mice by gene targeting. Analysis of the ActRIB-/- embryos showed that the epiblast and the extraembryonic ectoderm were disorganized, resulting in disruption and developmental arrest of the egg cylinder before gastrulation. To assess the function of ActRIB in mesoderm formation and gastrulation, chimera analysis was conducted. We found that ActRIB-/- ES cells injected into wild-type blastocysts were able to contribute to the mesoderm in chimeric embryos, suggesting that ActRIB is not required for mesoderm formation. Primitive streak formation, however, was impaired in chimeras when ActRIB-/- cells contributed highly to the epiblast. Further, chimeras generated by injection of wild-type ES cells into ActRIB-/- blastocysts formed relatively normal extraembryonic tissues, but the embryo proper developed poorly probably resulting from severe gastrulation defect. These results provide genetic evidence that ActRIB functions in both epiblast and extraembryonic cells to mediate signals that are required for egg cylinder organization and gastrulation.

Immunohistochemical localization and mRNA expression of activin, inhibin, follistatin, and activin receptor in bovine cumulus-oocyte complexes during in vitro maturation

The aim of this study was to investigate whether bovine cumulus-oocyte complexes (COCs) synthesize activin A, inhibin, and follistatin and whether they contain activin receptor during in vitro maturation. Therefore, COCs obtained from small and medium-sized follicles were cultured in M-199 supplemented with 10% fetal calf serum (FCS) and gonadotropins for 24 hr. At 0, 6, 12, and 24 hr after the onset of culture, COCs were removed for immunohistochemical staining to detect the expression of activin A, inhibin, follistatin, and activin receptor type II proteins. At 0 and 24 hr, COCs were removed and prepared for reverse-transcriptase polymerase chain reaction (RT-PCR) to assess the presence of mRNA of these proteins. It appeared that cumulus cells and oocytes express activin, follistatin, and activin receptor proteins as well as their mRNA. While expression of inhibin mRNA was found exclusively in cumulus cells, the inhibin protein was present in cumulus cells and oocytes. Immunohistochemical study both in cumulus cells and in oocytes often showed a moderate and strong staining intensity for activin and follistatin, respectively. Activin staining underwent little or no change during culture except at 24 hr of maturation, where about 60% of the oocytes showed no staining. Follistatin immunoreactivity remained strong in the majority of COCs. At the onset of culture, a spotlike inhibin staining was observed in the oocyte, which increased after 12 hr and was absent at the end of culture. Activin receptor immunoreactivity in cumulus cell membranes and oolemma increased during oocyte maturation to maximum values at the end of culture in most of the COCs. It is concluded that the consistent presence of activin and the increase in activin receptor in cumulus cells and oocytes during in vitro maturation indicate a paracrine and/or autocrine action for activin on bovine oocyte maturation. This action may be modulated by inhibin and/or follistatin.

Inhibins, activins, and follistatins: expression of mRNAs and cellular localization in tissues from men with benign prostatic hyperplasia

BACKGROUND

The transforming growth factor beta (TGF beta) superfamily of growth factors includes activins and inhibins, which have been shown to be present in the rat ventral prostate, and human prostate tumor cell lines, although their localization in benign prostatic hyperplasia (BPH) tissue is currently unknown.

METHODS

BPH tissues were obtained at surgery, and the mRNA expression for the inhibin alpha, beta A, beta B subunits, the putative activin beta C subunit, the activin type II receptor (ActRII), and the activin binding protein, follistatin, was determined by reverse transcription polymerase chain reaction (RT-PCR) and Southern blot analysis. Antibodies specific for alpha, beta A, beta B, activin A, and follistatin were used to determine the localization of these proteins in BPH tissue specimens.

RESULTS

Southern blot analysis confirmed that mRNA for ActRII, beta C subunit, and follistatin was present in all biopsy samples assayed. However, alpha, beta A, and beta B subunit mRNA expression was variable between patient samples. Immunohistochemistry demonstrated the predominant localization of beta A, beta B, and activin A proteins to the epithelium of BPH tissues. No immunoreactivity for the inhibin alpha subunit was detected; follistatin immunoreactivity was localized to the fibroblastic stroma.

CONCLUSIONS

The compartmentalization of activin subunit proteins to the epithelium, and of follistatin to the stroma, suggests that a paracrine interaction occurs between the activin ligands and follistatin-binding proteins in BPH tissue.

TGF-beta signal transduction

The transforming growth factor beta (TGF-beta) family of growth factors control the development and homeostasis of most tissues in metazoan organisms. Work over the past few years has led to the elucidation of a TGF-beta signal transduction network. This network involves receptor serine/threonine kinases at the cell surface and their substrates, the SMAD proteins, which move into the nucleus, where they activate target gene transcription in association with DNA-binding partners. Distinct repertoires of receptors, SMAD proteins, and DNA-binding partners seemingly underlie, in a cell-specific manner, the multifunctional nature of TGF-beta and related factors. Mutations in these pathways are the cause of various forms of human cancer and developmental disorders.

Smad8 mediates the signaling of the ALK-2 [corrected] receptor serine kinase

Smad proteins are critical intracellular mediators of signaling by growth and differentiation factors of the transforming growth factor beta superfamily. We have isolated a member of the Smad family, Smad8, from a rat brain cDNA library and biochemically and functionally characterized its ability to transduce signals from serine kinase receptors. In Xenopus embryo, Smad8 is able to transcriptionally activate a subset of mesoderm target genes similar to those induced by the receptor serine kinase, activin receptor-like kinase (ALK)-2. Smad8 can be specifically phosphorylated by a constitutively active ALK-2 but not the related receptor serine kinase, ALK-4. In response to signaling from ALK-2, Smad8 associates with a common regulatory molecule, Smad4, and this association leads to a synergistic effect on gene transcription. Furthermore, Smad8 is able to rescue the expression of mesoderm genes blocked by truncated ALK-2 in the embryo. These results indicate that Smad8 can function as a downstream signaling mediator of ALK-2.

Transforming growth factor-beta receptor type I and type II expression during murine hair follicle development and cycling

Although the TGF-beta family of growth factors probably regulates skin and hair follicle development, its exact role is still quite ill-defined. Here, we characterize the correlative expression pattern of the interdependent high affinity receptor proteins for TGF-beta1 and TGF-beta3, TGF-beta receptor type I (TGF-betaRI) and TGF-beta receptor type II (TGF-betaRII), during hair follicle development and cycling in C57BL/6 mice. During neonatal follicle development, TGF-betaRII immunoreactivity is confined to epithelial cells. Focal epidermal TGF-betaRII expression is seen even before actual hair placode formation. In contrast to the TGF-betaRII immunoreactivity in the outer root sheath, precortical hair matrix and inner root sheath cells were TGF-betaRII negative during hair bulb morphogenesis. TGF-betaRI (Alk-5) immunoreactivity largely overlapped the TGF-betaRII expression pattern, but was more widespread. During hair follicle cycling in adolescent mice, TGF-betaRII immunoreactivity was restricted to follicles, and was strikingly hair cycle dependent (maximal immunoreactivity: anagen VI and early catagen). Again, TGF-betaRI (Alk-5) immunoreactivity co-localized with TGF-betaRII immunoreactivity, but was more extensive. Reverse transcriptase polymerase chain reaction analysis of TGF-betaRII mRNA confirmed peak transcript levels in back skin with most hair follicles in the anagen VI-catagen transformation. mRNA levels of TGF-betaRI (Alk-5) did not vary significantly during the hair cycle, whereas those of TGF-betaRI (threonine-serine kinase 7 L) declined during early anagen, and were maximal during the anagen-catagen transition. This provides a basis for defining the choreography of TGF-beta-related signalling during hair follicle morphogenesis and cycling, introduces intraepidermal TGF-betaRII immunoreactivity as a marker for imminent follicle development, and supports the concept that both TGF-betaRII and TGF-betaRI stimulation is involved in, but not restricted to, the control of catagen induction.

Expression of ALK-1, a type 1 serine/threonine kinase receptor, coincides with sites of vasculogenesis and angiogenesis in early mouse development

ALK-1 is a type I serine/threonine kinase receptor for members of the TGF-beta superfamily of growth factors; its endogenous ligand is not known. In this study, we have analyzed the temporal and spatial expression pattern of ALK-1 mRNA in mouse embryos from the one-cell zygote until 12.5 dpc using RT-PCR and in situ hybridization. ALK-1 mRNA was first detected in the embryo at 6.5 dpc. From 7.5-8.5 dpc expression was highest at sites of vasculogenesis in both the embryonic and extraembryonic part of the conceptus, in trophoblast giant cells, and in the endothelial lining of the blood vessels in the decidua. From 9.5-12.5 dpc, ALK-1 was found to be expressed in several different tissues and organs, but was highest in blood vessels, mesenchyme of the lung, submucosal layer of the stomach and intestines, and at specific sites of epithelial-mesenchymal interactions. Its expression pattern suggests that ALK-1 is a type I receptor for TGF-beta1 in the developing mouse.

Function of the type V transforming growth factor beta receptor in transforming growth factor beta-induced growth inhibition of mink lung epithelial cells

The type V transforming growth factor beta (TGF-beta) is a 400-kDa nonproteoglycan membrane protein that co-expresses with the type I, type II, and type III TGF-beta receptors in most cell types. The type V TGF-beta receptor exhibits a Ser/Thr-specific protein kinase activity with distinct substrate specificity (Liu, Q., Huang, S. S., and Huang, J. (1994) J. Biol. Chem. 269, 9221-9226). In mink lung epithelial cells, the type V TGF-beta receptor was found to form heterocomplexes with the type I TGF-beta receptor by immunoprecipitation with antiserum to the type V TGF-beta receptor after 125I-TGF-beta affinity labeling or Trans35S-label metabolic labeling of the cells. The kinase activity of the type V TGF-beta receptor was stimulated after treatment of mink lung epithelial cells with TGF-beta. TGF-beta stimulation resulted in the growth inhibition of wild-type mink lung epithelial cells and to a lesser extent of the type I and type II TGF-beta receptor-defective mutants, although higher concentrations of TGF-beta were required for the growth inhibition of these mutants. TGF-beta was unable to induce growth inhibition in human colorectal carcinoma cells lacking the type V TGF-beta receptor but expressing the type I and type II TGF-beta receptors. These results suggest that the type V TGF-beta receptor can mediate the TGF-beta-induced growth inhibitory response in the absence of the type I or type II TGF-beta receptor. These results also support the hypothesis that loss of the type V TGF-beta receptor may contribute to the malignancy of certain carcinoma cells.

The type II transforming growth factor-beta receptor autophosphorylates not only on serine and threonine but also on tyrosine residues

The type I and type II receptors for transforming growth factor-beta (TGF-beta) are structurally related transmembrane serine/threonine kinases, which are able to physically interact with each other at the cell surface. To help define the initial events in TGF-beta signaling, we characterized the kinase activity of the type II TGF-beta receptor. A recombinant cytoplasmic domain of the receptor was purified from Escherichia coli and baculovirus-infected insect cells. Anti-phosphotyrosine Western blotting demonstrated that the type II receptor kinase can autophosphorylate on tyrosine. Following an in vitro kinase reaction, the autophosphorylation of the cytoplasmic domain and phosphorylation of exogenous substrate was shown by phosphoamino acid analysis to occur not only on serine and threonine but also on tyrosine. The dual kinase specificity of the receptor was also demonstrated using immunoprecipitated receptors expressed in mammalian cells and in vivo 32P labeling showed phosphorylation of the receptor on serine and tyrosine. In addition, the kinase activity of the cytoplasmic domain was inhibited by the tyrosine kinase inhibitor tyrphostin. Tryptic mapping and amino acid sequencing of in vitro autophosphorylated type II receptor cytoplasmic domain allowed the localization of the sites of tyrosine phosphorylation to positions 259, 336, and 424. Replacement of all three tyrosines with phenylalanines strongly inhibited the kinase activity of the receptor, suggesting that tyrosine autophosphorylation may play an autoregulatory role for the kinase activity of this receptor. These results demonstrate that the type II TGF-beta receptor can function as a dual specificity kinase and suggest a role for tyrosine autophosphorylation in TGF-beta receptor signaling.

Activin and inhibin have antagonistic effects on ligand-dependent heteromerization of the type I and type II activin receptors and human erythroid differentiation

Activins and inhibins belong to the transforming growth factor beta (TGF-beta)-like superfamily and exert their effects on a broad range of cellular targets by modulating cell differentiation and proliferation. Members of this family interact with two structurally related classes of receptors (type I and type II), both containing a serine/threonine kinase domain. When expressed alone, the type II but not the type I activin receptor can bind activin. However, the presence of a type I receptor is required for signaling. For TGF-beta1, ligand binding to the type II receptor results in the recruitment and transphosphorylation of the type I receptor. Transient overexpression of the two types of activin receptor results in ligand-independent receptor heteromerization and activation. Nevertheless, activin addition to the transfected cells increased complex formation between the two receptors, suggesting a mechanism of action similar to that observed for the TGF-beta receptor. In the present study, we generated a stable cell line, overexpressing the two types of activin receptor upon induction, in the human erythroleukemia cell line K562. We demonstrate here that activin specifically induces heteromer formation between the type I and type II receptors in a time-dependent manner. Using this stable line, we analyzed the effects of activin and inhibin on human erythroid differentiation. Our results indicate that activin signal transduction mediated through its type I and type II receptors results in an increase in the hemoglobin content of the cells and limits their proliferation. Finally, using cell lines that can be induced to overexpress ActRII and ActRIB or ActRIB only, we show that the inhibin antagonistic effects on activin-induced biological responses are mediated through a competition for the type II activin receptor but also require the presence of an inhibin-specific binding component.

Differential effects on Xenopus development of interference with type IIA and type IIB activin receptors

One candidate for a mesoderm-inducing factor in early amphibian development is activin, a member of the TGF beta family. Overexpression of a truncated form of an activin receptor Type IIB abolishes activin responsiveness and mesoderm formation in vivo. The Xenopus Type IIA activin receptor XSTK9 differs from the Type IIB receptor by 43 and 25% in extracellular and intracellular domains respectively, suggesting the possibility of different functions in vivo. In this paper, we compare the Type IIA receptor with the Type IIB to test such a possibility. Simple overexpression of the wild-type receptors reveals minimal differences, but experiments with dominant negative mutants of each receptor show qualitatively distinct effects. We show that while truncated (kinase domain-deleted) Type IIB receptors cause axial defects as previously described, truncated type IIA receptors cause formation of secondary axes, similar to those seen by overexpression of truncated receptors for BMP-4, another TGF beta family member. Furthermore, in animal cap assays, truncated type IIB receptors inhibit induction of all mesodermal markers tested, while truncated type IIA receptors suppress induction only of ventral markers; the anterior/dorsal marker goosecoid is virtually unaffected. The suppression of ventral development by the type IIA truncated receptor suggests either that the truncated Type IIA receptor interferes with ventral BMP pathways, or that activin signaling through the Type IIA receptor is necessary for ventral patterning.

Activin receptor mRNA expression by neurons of the avian ciliary ganglion

Previous studies have suggested that activin may serve as a neurodifferentiation factor regulating somatostatin expression in neurons of the avian ciliary ganglion (CG). As one aspect of examining the role of activin in CG development, we inquired whether any of the known activin receptors are expressed by developing CG neurons in vivo. In addition, we examined whether activin A mRNA is expressed in the choroid layer and iris of the chicken eye. Oligonucleotide primers were designed for the chicken activin receptor type IIA (cActR-IIA), type IIB (cActR-IIB), and activin A. In reverse-transcription-polymerase chain reaction (rtPCR), an appropriately sized product was amplified from CG cDNA using primers to the cActR-IIA but not the cActR-IIB. Sequencing confirmed the identity of the PCR product as a fragment of the cActR-IIA. It thus appears that mRNA for the type IIA but not the type IIB activin receptor is expressed in the chicken CG. An antisense strand digoxigenin-labeled riboprobe complimentary to a 358-bp portion of the cActR-IIA kinase region hybridized to cells within cryostat sections of embryonic CG. From E6.5-E18, hybridization of this probe appears to be specific for cells with a neuronal morphology. Using rtPCR with activin A-specific primers we detected activin mRNA in the choroid layer of E14 and E19 eyes, and from the iris at E14. Our results are consistent with a role for activin as a neurodifferentiation factor in vivo, and imply that within the CG, the cActR-IIA is specifically expressed by neurons, and that activin A is expressed in the targets of these neurons.

Cyclin-dependent kinase regulation during G1 phase and cell cycle regulation by TGF-beta

The aim of this review is to provide insight into the molecular mechanisms by which transforming growth factor-beta (TGF-beta) modulates cell cycle progression in different cell types. Particular attention is focused on the differences between these mechanisms in cells of epithelial origin and in mesenchymally derived cells. This is important because many transformed epithelial cells lose responsiveness to the growth-inhibitory effects of TGF-beta, thus generating a more fibroblast-like phenotype. Loss of negative growth control, including a lack of response to growth-inhibitory factors, is a common feature of many tumor cells. G1 phase cyclin-dependent kinases (cdks) and their inhibitors (ckis) are central to the pathways that regulate commitment to cellular division in response to positive as well as negative growth effectors. Many checkpoints are deregulated in oncogenesis, and this is often due to alterations in cyclin-cdk complexes. The loss of R-point regulation, in particular, can allow cell growth and division to proceed autonomously of external signals. This may occur due to either the aberrant expression of positive regulators, such as the cyclins and cdks, or the loss of negative regulators, such as the ckis. Beginning with a survey of the role of the cdks in the mammalian cell cycle, the review examines how cdk activity is modulated by cyclin binding, phosphorylation, and ckis, including the Ink4 proteins and the closely related inhibitors p21Cip1 and p27Kip1. Particular attention is paid to the role of p27Kip1 and p21Cip1 in the mechanisms of TGF-beta-induced suppression or stimulation of the cell cycle and how these mechanisms contrast between epithelial cells and cells of mesenchymal origin. Other aspects of TGF-beta signal transduction are discussed, including its effects on cyclin and cdk expression in various cell types, and the downstream targets of cdks and their modulation by TGF-beta and other growth factors are also discussed. These include proteins of the retinoblastoma family, and the related modulation of the transcriptional activity of the E2F family members. Finally, the role of cell cycle regulatory proteins in oncogenesis is review in view of the findings described here.

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.

Integrated processes responsible for soft tissue healing

Wounded soft tissues undergo repair through a complex series of interrelated events that involve both physical and chemical activities. These processes are currently undergoing extensive investigation as efforts are directed toward achieving augmented and accelerated healing. Early wound-healing research focused on expanding traditional histologic descriptions of tissue healing by attempting to characterize the environment and biologic mediators responsible for healing. These initial studies successfully identified a number of agents and physiochemical factors present in healing wounds, but their precise roles and importance remain largely unknown. This review article summarizes the current literature on soft tissue healing. An effort has been made to correlate the activities of the major growth factors and cytokines with the individual reparative processes including the inflammatory response, hemostasis, fibroplasia, angiogenesis, and remodeling. Explanations and characteristics of growth factor function as well as brief descriptions of several major factors and their spectrum of activity are also provided.

Immediate postnatal rat heart development modified by abdominal aortic banding: analysis of gene expression

Proliferative growth of the ventricular myocyte (cardiomyocyte) is primarily limited to embryonic, fetal and very early neonatal periods of heart development. In contrast, cardiomyocyte maturation, as evidenced by cellular hypertrophy, is a long-term process that can occupy the bulk of the life-span of the mature organism. As the newborn myocyte undergoes a 'transition' from proliferative to hypertrophic growth, ventricular remodeling of the non-myocyte compartment is characterized by increased extracellular matrix (ECM) formation and coronary capillary angiogenesis. A role for ventricular-derived growth factors (GFs) in these inter-related processes are examined in an animal model of altered heart development produced by neonatal aortic banding. The suprarenal abdominal aorta of five day old rat pups were banded (B), sham operated (S), or untreated (C) and ventricular tissue (left ventricular free wall and septum) obtained at 7-, 14-, and 21-days post-intervention. Using Northern blot RNA hybridizations, expression of growth factors (GFs) and/or GF-receptors (GFR's) temporally associated with heart development were evaluated. Transcript levels for TGF-beta 1, IGF-II, and their associated cell surface receptors were increased in B animals. Concomitant changes in extracellular matrix (ECM) genes (as evaluated by Collagens Type I, III, and IV) were also increased in B animals. In addition, transcript levels for the vascular morphogenesis and remodeling-related protein SPARC (Secreted Protein, Acidic and Rich in Cysteine) was also elevated in the B animals. In several instances, S animals demonstrated changes in steady state transcript levels for genes which may influence myocyte maturation during the postnatal period. This suggests that normal autocrine/paracrine growth regulatory stimuli and responses can be modified (by surgical intervention and/or abdominal aortic banding) and these perturbations in gene expression may be related to previously documented changes in myocyte cell number, vascular composition, and ventricular architecture of the banded, neonatal heart. Future studies using this model will provide an opportunity to evaluate and possibly identify the stimuli and signal transduction machinery that regulate the final phases of myocyte proliferation, stimulate capillary formation and ECM deposition, and orchestrate the transition to hypertrophic growth during heart development.

Identification of type I and type II serine/threonine kinase receptors for growth/differentiation factor-5

Growth/differentiation factor-5 (GDF-5) is a member of the bone morphogenetic protein (BMP) family, which plays an important role in bone development in vivo. Mutations in the GDF-5 gene result in brachypodism in mice and Hunter-Thompson type chondrodysplasia in human. BMPs transduce their effects through binding to two different types of serine/threonine kinase receptors, type I and type II. However, binding abilities appear to be different among the members of the BMP family. BMP-4 binds to two different type I receptors, BMP receptors type IA (BMPR-IA) and type IB (BMPR-IB), and a type II receptor, BMP receptor type II (BMPR-II). In addition to these receptors, osteogenic protein-1 (OP-1, also known as BMP-7) binds to activin type I receptor (ActR-I) as well as activin type II receptors (ActR-II and ActR-IIB). Here we investigate the binding and signaling properties of GDF-5 through type I and type II receptors. GDF-5 induced alkaline phosphatase activity in a rat osteoprogenitor-like cell line, ROB-C26. 125I-GDF-5 bound to BMPR-IB and BMPR-II but not to BMPR-IA in ROB-C26 cells and other nontransfected cell lines. Analysis using COS-1 cells transfected with the receptor cDNAs revealed that GDF-5 bound to BMPR-IB but not to the other type I receptors when expressed alone. When COS-1 cells were transfected with type II receptor cDNAs, GDF-5 bound to ActR-II, ActR-IIB, and BMPR-II but not to transforming growth factor-beta type II receptor. In the presence of type II receptors, GDF-5 bound to different sets of type I receptors, but the binding was most efficient to BMPR-IB compared with the other type I receptors. Moreover, a transcriptional activation signal was efficiently transduced by BMPR-IB in the presence of BMPR-II or ActR-II after stimulation by GDF-5. These results suggest that BMPR-IB mediates certain signals for GDF-5 after forming the heteromeric complex with BMPR-II or ActR-II.