Involvement of Ras/Raf/AP-1 in BMP-4 signaling during Xenopus embryonic development

Crosstalk between MEIS1 and markers of different cell signaling pathways in esophageal squamous cell carcinoma

The homeobox transcription factor MEIS1 is involved in cell fate decision, stem cells properties, gastrointestinal (GI) tract development, and progression of several malignancies such as esophageal squamous cell carcinoma (ESCC). Increasing evidences suggest the crosstalk between MEIS1 and cell signaling pathways. Therefore, our aim in present study was to investigate the probable linkage of MEIS1 expression with key genes of different cell signaling pathways in ESCC tumorigenesis, and their correlation with clinicopathological feature of the patients. The gene expression profiling of MEIS1 and different cell signaling genes including SALL4, SIZN1, and HEY1 (stemness state, BMP, and NOTCH signaling pathways, respectively) was performed using quantitative real-time reverse transcription polymerase chain reaction (PCR) in fresh tumoral compared to margin normal tissues of 50 treatment-naive ESCC samples. The mRNA expression of MEIS1/SIZN1, SIZN1/HEY1, and SIZN1/SALL4 were significantly associated to each other (P < 0.05). There were remarkable correlations between concomitant mRNA expression of MEIS1 and SIZN1 in tumors with invasion to adventitia, early stages of tumor progression and poorly differentiated tumors. Moreover, expression of MEIS1 and HEY1 was correlated to each other in primary stages of tumor progression and non-invaded tumors. Expression of MEIS1 was significantly associated with SALL4 in poorly differentiated tumors. Our results indicated that correlation between different cell signaling pathway-related genes may lead to esophageal tumorigenesis. It is illustrated that MEIS1 as a HOX gene has a significant correlation with stemness state, BMP, and NOTCH signaling pathways via the SIZN1.

Bone morphogenetic protein-2 and -7 mediate the anabolic function of nucleus pulposus cells with discrete mechanisms

Bone morphogenetic proteins (BMPs) play roles in promoting cell anabolism, especially in extracellular matrix production. The difference between BMP members in their capacity to modulate intervertebral disc cell activity is yet to be defined. BMP-7/OP-1 has been shown to retard disc degeneration. We compared the activity of BMP-7 with that of BMP-2 on nucleus pulposus (NP) cell phenotype and function, and investigated how they differentially affect the gene expression profiles of signaling cascade components in human NP cells under degenerative states. We found that while both BMP-2 and BMP-7 enhanced matrix production of bovine NP cells, BMP-7 is more potent than BMP-2 at various dosages (50-800 ng/ml). BMP-7 exerted a relatively stronger stimulation on sulfated glycosaminoglycan production and proliferation in human NP cells. Degenerated NP cells showed an overall weaker response to the BMPs than non-degenerated cells, and were more sensitive to BMP-7 than BMP-2 stimulation. Compared to BMP-2, BMP-7 not only induced the gene expression of canonical BMP components, but also evoked changes in MAPKs as well as CREB1 and EP300 gene expression in degenerated NP cells, suggesting potential activation of the cAMP dependent protein kinase related pathways. In contrast to BMP-2, BMP-7 concomitantly inhibited the expression of profibrotic genes. We propose that BMP-2 and BMP-7, and likely other BMPs, may operate multifaceted but discrete molecular machineries that give rise to their different capacity in regulating NP cell phenotype. Further investigations into such differential capacity may possibly derive alternative cues important for IVD repair or engineering.

The role of heterodimeric AP-1 protein comprised of JunD and c-Fos proteins in hematopoiesis

Activator protein-1 (AP-1) regulates a wide range of cellular processes including proliferation, differentiation, and apoptosis. As a transcription factor, AP-1 is commonly found as a heterodimer comprised of c-Jun and c-Fos proteins. However, other heterodimers may also be formed. The function of these dimers, specifically the heterodimeric AP-1 comprised of JunD and c-Fos (AP-1(JunD/c-Fos)), has not been elucidated. Here, we identified a function of AP-1(JunD/c-Fos) in Xenopus hematopoiesis. A gain-of-function study performed by overexpressing junD and c-fos and a loss-of-function study using morpholino junD demonstrate a critical role for AP-1(JunD/c-Fos) in hematopoiesis during Xenopus embryogenesis. Additionally, we confirmed that JunD of AP-1(JunD/c-Fos) is required for BMP-4-induced hematopoiesis. We also demonstrated that BMP-4 regulated JunD activity at the transcriptional regulation and post-translational modification levels. Collectively, our findings identify AP-1(JunD/c-Fos) as a novel hematopoietic transcription factor and the requirement of AP-1(JunD/c-Fos) in BMP-4-induced hematopoiesis during Xenopus hematopoiesis.

CCN3 increases BMP-4 expression and bone mineralization in osteoblasts

The nephroblastoma overexpressed (NOV) gene, also called CCN3, regulates differentiation of skeletal mesenchymal cells. Bone morphogenetic proteins (BMPs) play important roles in osteoblast differentiation and bone formation, but the effects of CCN3 on BMP expression and bone formation in cultured osteoblasts are largely unknown. Here we found that CCN3 increased BMP-4 expression and bone nodule formation in cultured osteoblast. Monoclonal antibodies for α5β1 and αvβ5 integrins, and inhibitors of integrin-linked kinase (ILK), p38, and JNK, all inhibited CCN3-induced bone nodule formation and BMP-4 up-regulation of osteoblasts. CCN3 stimulation increased the kinase activity of ILK and phosphorylation of p38 and JNK. Inhibitors of activator protein-1 (AP-1) also suppressed bone nodule formation and BMP-4 expression enhanced by CCN3. Moreover, CCN3-induced c-Jun translocation into the nucleus, and the binding of c-Jun to the AP-1 element on the BMP-4 promoter were both inhibited by specific inhibitors of the ILK, p38, and JNK cascades. Taken together, our results provide evidence that CCN3 enhances BMP-4 expression and bone nodule formation in osteoblasts, and that the integrin receptor, ILK, p38, JNK, and AP-1 signaling pathways may be involved.

The function of heterodimeric AP-1 comprised of c-Jun and c-Fos in activin mediated Spemann organizer gene expression

BACKGROUND

Activator protein-1 (AP-1) is a mediator of BMP or FGF signaling during Xenopus embryogenesis. However, specific role of AP-1 in activin signaling has not been elucidated during vertebrate development.

METHODOLOGY/PRINCIPAL FINDINGS

We provide new evidence showing that overexpression of heterodimeric AP-1 comprised of c-jun and c-fos (AP-1(c-Jun/c-Fos)) induces the expression of BMP-antagonizing organizer genes (noggin, chordin and goosecoid) that were normally expressed by high dose of activin. AP-1(c-Jun/c-Fos) enhanced the promoter activities of organizer genes but reduced that of PV.1, a BMP4-response gene. A loss of function study clearly demonstrated that AP-1(c-Jun/c-Fos) is required for the activin-induced organizer and neural gene expression. Moreover, physical interaction of AP-1(c-Jun/c-Fos) and Smad3 cooperatively enhanced the transcriptional activity of goosecoid via direct binding on this promoter. Interestingly, Smad3 mutants at c-Jun binding site failed in regulation of organizer genes, indicating that these physical interactions are specifically necessary for the expression of organizer genes.

CONCLUSIONS/SIGNIFICANCE

AP-1(c-Jun/c-Fos) plays a specific role in organizer gene expression in downstream of activin signal during early Xenopus embryogenesis.

Histone XH2AX is required for Xenopus anterior neural development: critical role of threonine 16 phosphorylation

A role for histone H2AX, one of the variants of the nucleosome core histone H2A, has been demonstrated in DNA repair, tumor suppression, apoptosis, and cell cycle checkpoint function. However, the physiological function and post-translational modification of histone H2AX during vertebrate development have not been elucidated. Here, we provide evidence showing that Xenopus histone H2AX (XH2AX) has a role in the anterior neural plate for eye field formation during Xenopus embryogenesis. A loss-of-function study clearly demonstrated a critical role of XH2AX in anterior neural specification. Through a differentiation assay with Xenopus animal cap embryonic stem cells, we confirmed that XH2AX is required for the activin-induced anterior neural specification of the ectoderm. Furthermore, we found that Chk1 is an essential kinase to phosphorylate histone XH2AX at Thr(16), which is involved in the biological function of this histone. Taken together, our findings reveal that XH2AX has a specific role in anterior neural formation of Xenopus, which is mediated through phosphorylation of XH2AX at Thr(16) by Chk1.

Human ZCCHC12 activates AP-1 and CREB signaling as a transcriptional co-activator

Mouse zinc finger CCHC domain containing 12 gene (ZCCHC12) has been identified as a transcriptional co-activator of bone morphogenetic protein (BMP) signaling, and human ZCCHC12 was reported to be related to non-syndromic X-linked mental retardation (NS-XLMR). However, the details of how human ZCCHC12 involve in the NS-XLMR still remain unclear. In this study, we identified a novel nuclear localization signal (NLS) in the middle of human ZCCHC12 protein which is responsible for the nuclear localization. Multiple-tissue northern blot analysis indicated that ZCCHC12 is highly expressed in human brain. Furthermore, in situ hybridization showed that ZCCHC12 is specifically expressed in neuroepithelium of forebrain, midbrain, and diencephalon regions of mouse E10.5 embryos. Luciferase reporter assays demonstrated that ZCCHC12 enhanced the transcriptional activities of activator protein 1 (AP-1) and cAMP response element binding protein (CREB) as a coactivator. In conclusion, we identified a new NLS in ZCCHC12 and figured out that ZCCHC12 functions as a transcriptional co-activator of AP-1 and CREB.

Multiple signaling pathways converge to regulate bone-morphogenetic-protein-dependent glial gene expression

A fundamental problem in developmental neuroscience is understanding how extracellular cues link to complex intracellular signaling pathways to drive stage-specific developmental decisions. During the formation of the mammalian peripheral nervous system, bone morphogenetic proteins (BMPs) promote neuronal differentiation. BMPs also maintain the expression of early glial genes such as GFAP, while blocking the acquisition of a mature, myelinating Schwann cell phenotype. We investigated the BMP-activated signaling pathways that contribute to early glial gene expression to address the question of how specific signaling interactions contribute to cell fate decisions in neural crest lineages. Using a neural-crest-derived cell line that exhibits the characteristics of immature Schwann cells, we found that BMP2 promotes GFAP expression using Smad signaling as well as the phosphoinositide-3 kinase (PI3K) and mitogen-activated protein kinase1/2extracellular signal-regulated kinase- (MEK1/2/ERK) pathways. The GFAP promoter does not contain known Smad consensus sites, suggesting that Smads may act indirectly to promote GFAP expression. We provide evidence that this indirect effect may be mediated via induction of immediate early genes and the transcription factor Sp1 by demonstrating that these transcriptional regulators are induced by BMP2 and contribute to GFAP promoter activity. These findings demonstrate new roles for intracellular kinase pathways in mediating the effects of BMPs during the early stages of glial differentiation and suggest that differential contributions by signaling and transcriptional networks may contribute to the range of effects of BMPs on neuronal and glial development during the formation of the peripheral nervous system.

c-Jun N-terminal kinase 1 phosphorylates Myt1 to prevent UVA-induced skin cancer

The c-Jun N-terminal kinase (JNK) signaling pathway is known to mediate both survival and apoptosis of tumor cells. Although JNK1 and JNK2 have been shown to differentially regulate the development of skin cancer, the underlying mechanistic basis remains unclear. Here, we demonstrate that JNK1, but not JNK2, interacts with and phosphorylates Myt1 ex vivo and in vitro. UVA induces substantial apoptosis in JNK wild-type (JNK(+/+)) or JNK2-deficient (JNK2(-/-)) mouse embryonic fibroblasts but has no effect on JNK1-deficient (JNK1(-/-)) cells. In addition, UVA-induced caspase-3 cleavage and DNA fragmentation were suppressed by the knockdown of human Myt1 in skin cancer cells. JNK1 deficiency results in suppressed Myt1 phosphorylation and caspase-3 cleavage in skin exposed to UVA irradiation. In contrast, the absence of JNK2 induces Myt1 phosphorylation and caspase-3 cleavage in skin exposed to UVA. The overexpression of JNK1 with Myt1 promotes cellular apoptosis during the early embryonic development of Xenopus laevis, whereas the presence of JNK2 reduces the phenotype of Myt1-induced apoptotic cell death. Most importantly, JNK1(-/-) mice developed more UVA-induced papillomas than either JNK(+/+) or JNK2(-/-) mice, which was associated with suppressed Myt1 phosphorylation and decreased caspase-3 cleavage. Taken together, these data provide mechanistic insights into the distinct roles of the different JNK isoforms, specifically suggesting that the JNK1-mediated phosphorylation of Myt1 plays an important role in UVA-induced apoptosis and the prevention of skin carcinogenesis.

Interaction between the bone morphogenetic proteins and Ras/MAP-kinase signalling pathways in lung cancer

Bone morphogenetic proteins (BMPs) are an integral component of the TGFβ superfamily, responsible for regulation of cell proliferation, differentiation, migration and programmed cell death in a variety of cell types. The BMPs transduce their signals directly through the SMAD family of proteins but they also have been reported to interact with the MAPK and Erk pathways. Inactivation of the BMP pathway genes has been implicated as important in several cancers. Recent work has shown that BMP3b is epigenetically inactivated in cancer and suggests that BMP6 can be epigenetically inactivated. We investigated whether BMP6 is epigenetically inactivated in cell lines and whether BMP3b and BMP6 are epigenetically inactivated in non-small-cell lung cancer (NSCLC). We also studied the relationship between BMP methylation and k-ras mutation. Here, we demonstrate that the BMP3b and BMP6 genes are common targets of epigenetic inactivation in NSCLC, and that they are significantly more likely to be concurrently inactivated (P=0.009). Furthermore, this coinactivation of BMP3b and BMP6 is significantly associated with mutation of k-ras codon 12 in lung cancer (P=0.003); those with a k-ras mutation were six times more likely to have concurrent methylation of these BMP loci. Hence, these data suggest that concurrent inactivation of the BMP and activation of the Ras signalling pathways are important in lung carcinogenesis.

Functional interactions of Raf and MEK with Jun-N-terminal kinase (JNK) result in a positive feedback loop on the oncogenic Ras signaling pathway

In previous studies we have found that oncogenic (Val 12)-ras-p21 induces Xenopus laevis oocyte maturation that is selectively blocked by two ras-p21 peptides, 35-47, also called PNC-7, that blocks its interaction with raf, and 96-110, also called PNC-2, that blocks its interaction with jun-N-terminal kinase (JNK). Each peptide blocks activation of both JNK and MAP kinase (MAPK or ERK) suggesting interaction between the raf-MEK-ERK and JNK-jun pathways. We further found that dominant negative raf blocks JNK induction of oocyte maturation, again suggesting cross-talk between pathways. In this study, we have undertaken to determine where these points of cross-talk occur. First, we have immunoprecipitated injected Val 12-Ha-ras-p21 from oocytes and found that a complex forms between ras-p21 raf, MEK, MAPK, and JNK. Co-injection of either peptide, but not a control peptide, causes diminished binding of ras-p21, raf, and JNK. Thus, one site of interaction is cooperative binding of Val 12-ras-p21 to raf and JNK. Second, we have injected JNK, c-raf, and MEK into oocytes alone and in the presence of raf and MEK inhibitors and found that JNK activation is independent of the raf-MEK-MAPK pathway but that activated JNK activates raf, allowing for activation of ERK. Furthermore, we have found that constitutively activated MEK activates JNK. We have corroborated these findings in studies with isolated protein components from a human astrocyte (U-251) cell line; that is, JNK phosphorylates raf but not the reverse; MEK phosphorylates JNK but not the reverse. We further have found that JNK does not phosphorylate MAPK and that MAPK does not phosphorylate JNK. The stress-inducing agent, anisomycin, causes activation of JNK, raf, MEK, and ERK in this cell line; activation of JNK is not inhibitable by the MEK inhibitor, U0126, while activation of raf, MEK, and ERK are blocked by this agent. These results suggest that activated JNK can, in turn, activate not only jun but also raf that, in turn, activates MEK that can then cross-activate JNK in a positive feedback loop.

Inhibition of phosphatidylinositol 3-kinase and p70S6 kinase blocks osteogenic protein-1 induction of alkaline phosphatase activity in fetal rat calvaria cells

Published studies reveal that Osteogenic Protein-1 (OP-1) and insulin-like growth factor-I (IGF-I) synergistically stimulate alkaline phosphatase (AP) activity and bone nodule formation in fetal rat calvaria (FRC) cells. In the present study, we examined whether there are interactions between the signal transduction pathways activated by these two growth factors. OP-1 did not significantly affect the levels of IRS-1, IRS-2, the p85alpha subunit of phosphatidylinositol 3-kinase (PI 3-kinase) or the extracellular signal-regulated kinase (ERK)-2, but stimulated ERK-1 protein by twofold. OP-1 also induced phosphorylation of ERK-1 and -2, but not of Akt/protein kinase B (PKB), a protein kinase that is downstream of PI 3-kinase. By comparison, IGF-I increased the levels of the phosphorylated forms of ERK-1 and -2, and Akt/PKB. Inhibition of ERK activation by PD98059 did not significantly alter the stimulation of AP activity by OP-1 or OP-1 in combination with IGF-I. In contrast, inhibition of PI 3-kinase activity by LY294002 blocked the induction of AP activity by OP-1 and OP-1 plus IGF-I. Treatment of cells with rapamycin, an inhibitor of the mammalian target of mTOR, resulted in a 47% and a 53% decrease in the AP activity induced by OP-1 alone and by OP-1 plus IGF-I, respectively. These studies suggest that PI 3-kinase and mTOR contribute to the induction of AP activity by OP-1 and the synergistic effect of OP-1 and IGF-I on AP activity in FRC cells.

Identification of the site of inhibition of mitogenic signaling by oncogenic ras-p21 by a ras effector peptide

We have previously found that a ras switch 1 domain peptide (PNC-7, residues 35-47) selectively blocks oocyte maturation induced by oncogenic (Val 12-containing) ras-p21 protein and also blocks c-raf-induced oocyte maturation. We now find that oncogenic ras-p21 does not inhibit oocyte maturation of a constitutively activated raf protein (raf BXB) that is lacking most of the first 301 amino terminal amino acids, including the major ras binding domain and accessory ras-binding regions. We also find that a dominant negative raf that completely blocks c-raf-induced maturation likewise does not block raf-BXB-induced maturation. We conclude that PNC-7 blocks ras by binding to the amino-terminal domain of raf and that raf BXB must initiate signal transduction in the cytosol.

Stimulation of Smad1 transcriptional activity by Ras-extracellular signal-regulated kinase pathway: a possible mechanism for collagen-dependent osteoblastic differentiation

Signals from bone morphogenetic protein receptors (BMPRs) and cell adhesion to type I collagen are both important for osteoblastic differentiation and functions. BMP signals are mediated mostly by Smad and collagen signals are transduced by integrins to activate focal adhesion kinase (FAK) and its downstream molecules. This study was undertaken to clarify how extracellular matrix collagen signals converge with BMP actions. We show that integrin activation by collagen was involved in BMP signals because disruption of either collagen synthesis or collagen-alpha2beta1-integrin binding inhibited the stimulatory effect of BMP-2 on osteoblastic MC3T3-E1 cells. Downstream signals of collagen-integrin might be FAK-Ras-extracellular signal-regulated kinase (ERK) in osteoblastic cells. We further show that Ras-ERK signals enhanced the transcriptional activity of Smad1 in response to BMP in these cells transiently transfected with expression plasmids for a constitutively active mutant RasV12, a dominant negative mutant RasN17, and an ERK phosphatase CL100. Ras-ERK signals did not augment the transcriptional activity of Smad3 in response to transforming growth factor beta (TGF-beta) receptor activation but that of Smad1 in response to BMPR activation as examined in COS-1 cells. These observations suggest that the Ras-ERK pathway downstream of integrin-FAK is involved in Smad1 signals activated by BMP and provide a possible mechanism for cooperation between intracellular signals activated by integrin and BMPRs in osteoblastic cells.

Drosophila-raf acts to elaborate dorsoventral pattern in the ectoderm of developing embryos

In the early Drosophila embryo the activity of the EGF-receptor (Egfr) is required to instruct cells to adopt a ventral neuroectodermal fate. Using a gain-of-function mutation we showed that D-raf acts to transmit this and other late-acting embryonic Egfr signals. A novel role for D-raf was also identified in lateral cell development using partial loss-of-function D-raf mutations. Thus, we provide evidence that zygotic D-raf acts to specify cell fates in two distinct pathways that generate dorsoventral pattern within the ectoderm. These functional requirements for D-raf activity occur subsequent to its maternal role in organizing the anterioposterior axis. The consequences of eliminating key D-raf regulatory domains and specific serine residues in the transmission of Egfr and lateral epidermal signals were also addressed here.

Transforming growth factor-beta signal transduction in epithelial cells

Transforming growth factor (TGF)-beta is a natural and potent growth inhibitor of a variety of cell types, including epithelial, endothelial, and hematopoietic cells. The ability of TGF-beta to potently inhibit the growth of many solid tumors of epithelial origin, including breast and colon carcinomas, is of particular interest. However, many solid tumor cells become refractory to the growth inhibitory effects of TGF-beta due to defects in TGF-beta signaling pathways. In addition, TGF-beta may stimulate the invasiveness of tumor cells via the paracrine effects of TGF-beta. Accordingly, in order to develop more effective anticancer therapeutics, it is necessary to determine the TGF-beta signal transduction pathways underlying the growth inhibitory effects and other cellular effects of TGF-beta in normal epithelial cells. Thus far, two primary signaling cascades downstream of the TGF-beta receptors have been elucidated, the Sma and mothers against decapentaplegic homologues and the Ras/mitogen-activated protein kinase pathways. The major objective of this review is to summarize TGF-beta signaling in epithelial cells, focusing on recent advances involving the Sma and mothers against decapentaplegic homologues and Ras/mitogen-activated protein kinase pathways. This review is particularly timely in that it provides a comprehensive summary of both signal transduction mechanisms and the cell cycle effects of TGF-beta.

c-Jun (AP-1) activates BMP-4 transcription in Xenopus embryos

Zygotic expression of the BMP-4 gene in Xenopus embryos is regulated by an auto-regulatory loop. Since AP-1 is known as a mediator of auto-regulatory loops both in the case of the Drosophila dpp and the mammalian TGF-beta genes, we have analysed the potential of Xenopus c-Jun (AP-1) as a mediator of BMP-4 expression during Xenopus development. RNA injection experiments revealed that both heteromeric c-Fos/c-Jun and homodimeric c-Jun/c-Jun strongly activate BMP-4 transcription, whereas BMP signaling was found to activate the Xenopus c-Jun gene only at a rather low extent. In addition, the lack of zygotic c-Jun transcripts until the end of gastrulation should exclude a role of AP-1 in the activation and the early expression of BMP-4 during gastrulation in vivo. However, at later stages of Xenopus development, we find a spatial overlap of c-Jun and BMP-4 transcripts which suggests that AP-1 might serve as an additional activatory component for the auto-regulation of BMP-4. Promoter/reporter and gel mobility shift assays demonstrate multiple responsive sites for AP-1 in the 5' flanking region and two in the second intron of the BMP-4 gene. We further demonstrate that AP-1 acts independently of Xvent-2 which has recently been shown to mediate the early expression of BMP-4 in gastrula stage embryos.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

Histone acetylation is a checkpoint in FGF-stimulated mesoderm induction

We have previously demonstrated that the transcription factor, AP-1 (c-jun/c-fos heterodimer), mediates fibroblast growth factor (FGF) signaling during mesoderm induction in Xenopus embryo. In the present studies, we show that histone acetylation is involved in FGF-mediated signaling leading to mesoderm induction. Histone acetylation is a dynamic process regulated by the activities of two histone-modifying enzymes, the histone acetyltransferase(s) and histone deacetylase(s) (HDACs). We found that basal and FGF-regulated activator protein 1 (AP-1) activity in Xenopus embryo is markedly reduced by treatment of trichostatin A (TSA), a specific inhibitor of HDAC. However, activity of another transcription factor, NFkappaB, is enhanced by TSA treatment. AP-1-mediated mesoderm induction in the animal caps is dramatically suppressed by TSA at a dose-dependent manner. This suppression can be rescued by ectopic expression of HDAC3 at early stage. Finally, we found that histone acetylation in animal caps is inhibited by FGF whereas enhanced by TSA (as a control). Therefore, we propose that histone acetylation is a checkpoint for transduction of the FGF/AP-1 signals to induce mesoderm. Published 2000 Wiley-Liss, Inc.

Inhibition of oncogenic and activated wild-type ras-p21 protein-induced oocyte maturation by peptides from the guanine-nucleotide exchange protein, SOS, identified from molecular dynamics calculations. Selective inhibition of oncogenic ras-p21

In the preceding paper we performed molecular dynamics calculations of the average structures of the SOS protein bound to wild-type and oncogenic ras-p21. Based on these calculations, we have identified four major domains of the SOS protein, consisting of residues 631-641, 676-691, 718-729, and 994-1004, which differ in structure between the two complexes. We have now microinjected synthetic peptides corresponding to each of these domains into Xenopus laevis oocytes either together with oncogenic (Val 12)-p21 or into oocytes subsequently incubated with insulin. We find that the first three peptides inhibit both oncogenic and wild-type p21-induced oocyte maturation, while the last peptide much more strongly inhibits oncogenic p21 protein-induced oocyte maturation. These results suggest that each identified SOS region is involved in ras-stimulated signal transduction and that the 994-1004 domain is involved uniquely with oncogenic ras-p21 signaling.

The Raf-1/MEK/ERK pathway regulates the expression of the p21(Cip1/Waf1) gene in chondrocytes

The gene encoding the cyclin-dependent kinase inhibitor p21(Cip1/Waf1) is up-regulated in many differentiating cells, including maturing chondrocytes. Since strict control of chondrocyte proliferation is essential for proper bone formation and since p21 is likely involved in this control, we initiated analyses of the mechanisms regulating expression of p21 in chondrocytes. p21 expression and promoter activity was strongly increased during the differentiation of chondrogenic MCT cells. We have identified a 68-base pair fragment conferring transcriptional up-regulation of the p21 gene in chondrocytes. The activity of this fragment required active Raf-1 in MCT cells as well as in primary mouse chondrocytes. Inhibition of downstream factors of Raf-1 (MEK1/2, ERK1/2, and Ets2) also repressed the activity of the 68-base pair fragment in MCT cells. The chemical MEK1/2 inhibitor PD98059 reduced protein levels of p21 in MCTs and primary mouse chondrocytes. These data suggest that signaling through the Raf-1 pathway is necessary for the optimal expression of p21 in chondrocytes and may play an important role in the control of bone formation.

Activated Raf inhibits avian myogenesis through a MAPK-dependent mechanism

Chronic overexpression of the oncogenic form of Ras is a potent inhibitor of skeletal myogenesis. However, the intracellular signaling pathways that mediate the repressive actions of Ras on myogenic differentiation have yet to be identified. We examined the role of Raf-mediated signaling as a modulator of avian myogenesis. Raf overexpression elicited pronounced effects on both myoblasts and mature myocytes. Most notably, the embryonic chick myoblasts overexpressing a constitutively active form of Raf (RCAS-Raf CAAX or RCAS-Raf BXB) fail to form the large multinucleated myofibers characteristic of myogenic cultures. While residual myofibers were apparent in the RCAS-Raf BXB and RCAS-Raf CAAX infected cultures, these fibers had an atrophic phenotype. The altered morphology is not a result of reinitiation of the myonuclei cell cycle nor is it due to apoptosis. Furthermore, the mononucleated myoblasts misexpressing Raf BXB are differentiation-defective due to overt MAPK activity. Supplementation of the culture media with the MAPK kinase (MEK) inhibitor, PD98059, caused a reversal of the phenotype and allowed the formation of multinucleated myofibers at levels comparable to controls. Our results indicate that the Raf/MEK/MAPK axis is intact in chick myoblasts and that persistent activation of this signaling cascade is inhibitory to myogenesis.

The distribution of BrdU- and TUNEL-positive cells during odontogenesis in mouse lower first molars

This study investigated the minute distribution of both proliferating and non-proliferating cells, and cell death in the developing mouse lower first molars using 5-bromo-2'-deoxyuridine (BrdU) incorporation and the terminal deoxynucleotidyl transferase-mediated deoxyuridine-5'-triphosphate (dUTP)-biotin nick end labeling (TUNEL) double-staining technique. The distribution pattern of the TUNEL-positive cells was more notable than that of the BrdU-positive cells. TUNEL-positive cells were localized in the following six sites: (1) in the most superficial layer of the dental epithelium during the initiation stage, (2) in the dental lamina throughout the period during which tooth germs grow after bud formation, (3) in the dental epithelium in the most anterior part of the antero-posterior axis of the tooth germ after bud formation, (4) in the primary enamel knot from the late bud stage to the late cap stage, (5) in the secondary enamel knots from the late cap stage to the late bell stage, and (6) in the stellate reticulum around the tips of the prospective cusps after the early bell stage. These peculiar distributions of TUNEL-positive cells seemed to have some effect on either the determination of the exact position of the tooth germ in the mandible or on the complicated morphogenesis of the cusps. The distribution of BrdU-negative cells was closely associated with TUNEL-positive cells, which thus suggested cell arrest and the cell death to be essential for the tooth morphogenesis.

Characterization of Xenopus RalB and its involvement in F-actin control during early development

We describe the characterization and a functional analysis in Xenopus development of RalB, a small G protein. RalB RNA and protein are detectable during oogenesis and early development, but the gene is expressed only weakly in adult tissues. The RalB transcripts are processed by poly(A) extension during oocyte maturation and up to the gastrulation stage. Microinjection of wild-type or mutant RalB RNAs was performed in fertilized eggs in order to gain insight into the function of RalB during development. We show that during cleavage stages the activated GTP form of RalB specifically induces a cortical reaction that affects the localization of pigment granules. The use of different drugs suggests that this reaction is dependent on the outer cortical actin array. The relation between F-actin and RalB was shown by confocal analysis. Injection of mRNAs encoding the mutated activated form of RalB leads, at dependent doses, to a blocking of gastrulation or defects in closing of neural folding structures. In contrast, the inactivated form blocks only the closing of neural tube. Altogether, these observations suggest that RalB is part of a regulatory pathway that may affect the blastomere cytoskeleton and take part in early development.

Functional analysis of human Smad1: role of the amino-terminal domain

The signals originating from transforming growth factor beta/activin/bone morphogenetic proteins (BMPs) are transduced by a set of evolutionarily conserved family of Smad proteins which, upon activation, directly translocate to the nucleus where they may activate transcription. Smad proteins of different species contain conserved amino- (N) and carboxy- (C) terminal domains separated by a proline-rich linker. Human, Drosophila, and Xenopus Smad1 all have been shown to mediate the biological effects of BMP-4 in Xenopus embryos. We have investigated the functional domains of human Smad1 (hSmad1) using the Xenopus embryo system. Dorsal injection of hSmad1 RNA into the 4-cell-stage embryos results in embryonic ventralization. Since the C-terminus of Smads has been shown to mediate the transcriptional activity, whereas this activity is masked by the presence of the N-terminus, we tested the effect of a hSmad1 construct lacking the C-terminal domain [hSmad1(N)] in the Xenopus embryo system. Surprisingly, we found that hSmad1(N) not only synergizes with hSmad1 in embryonic ventralization, but induces ventralization by itself. Ectopic expression of a dominant negative BMP receptor (DN-BR) as well as neural inducers noggin and chordin induce neurogenesis in the animal cap, which is inhibited by co-expression of either hSmad1 or hSmad1(N). Ventral expression of DN-BR induces formation of a second body axis at tailbud stage, which is also prevented by hSmad1 and hSmad1(N). It has recently been reported that calmodulin interacts with the N-terminal domain of Smad proteins. We demonstrate that the ventralizing activity of hSmad1 and hSmad1(N) is markedly inhibited by calmodulin. Thus, calmodulin acts as a Smad1 inhibitor. A model is proposed to accomodate these findings.

Cross-talk between the Smad1 and Ras/MEK signaling pathways for TGFbeta

Our previous data demonstrated that Ras activation was necessary and sufficient for transforming growth factor-beta (TGFbeta)-mediated Erk1 activation, and was required for TGFbeta up-regulation of the Cdk inhibitors (CKI's) p27(Kip1) and p21(Cip1) (KM Mulder and SL Morris, J. Biol. Chem., 267, 5029-5031, 1992; MT Hartsough and KM Mulder, J. Biol. Chem., 270, 7117-7124, 1995; MT Hartsough et al., J. Biol. Chem., 271, 22368-22375, 1996 and J Yue et al., Oncogene, 17, 47-55, 1998). Here we examined the role of Ras in TGFbeta-mediated effects on a rat homolog of Smad1 (termed RSmad1). We demonstrate that both TGFbeta and bone morphogenetic protein (BMP) can induce endogenous Smad1 phosphorylation in intestinal epithelial cells (IECs). The combination of transient expression of RSmad1 and TGFbeta treatment had an additive effect on induction of the TGFbeta-responsive reporter 3TP-lux. Either inactivation of Ras by stable, inducible expression of a dominant-negative mutant of Ras (RasN17) or addition of MAP and ERK kinase (MEK) inhibitor PD98059 to cells significantly decreased the ability of both TGFbeta and BMP to induce phosphorylation of endogenous Smad1 in IECs. Moreover, either inactivation of Ras or addition of PD98059 to IEC 4-1 cells inhibited the ability of RSmad1 to regulate 3TP luciferase activity in both the presence and absence of TGFbeta. Collectively, our data indicate that TGFbeta can regulate RSmad1 function in epithelial cells, and that the Ras/MEK pathway is partially required for TGFbeta-mediated regulation of RSmad1.

Cloning and expression of a rat Smad1: regulation by TGFbeta and modulation by the Ras/MEK pathway

A new family of signaling intermediates for TGFbeta superfamily members and other growth factors has recently been identified and termed Smads. It has been suggested that the Smad1 subfamily is regulated primarily by the TGFbeta superfamily member bone morphogenetic protein (BMP). Here we demonstrate that TGFbeta induced phosphorylation of endogenous Smad1 in untransformed IECs and that the RI and RII TGFbeta receptors were detectable in Smad1 immunocomplexes. Expression of a dominant-negative mutant of Ras inhibited the ability of TGFbeta to phosphorylate endogenous Smad1. In a separate series of experiments, we have cloned a rat homologue of the drosophila mad gene (termed RSmad1) by screening an intestinal epithelial cell (IEC) cDNA library. By using an in vitro kinase assay with RSmad1 as the substrate, we demonstrate that the TGFbeta receptor complex can directly phosphorylate RSmad1. We show, further, that a dominant-negative mutant of MEK1 inhibited the ability of RSmad1 to induce the TGFbeta-responsive reporter p3TP-Lux in a human breast cancer cell line. Collectively, our data demonstrate that TGFbeta can regulate Smadl and that the Ras and MEK signaling components are partially required for the ability of TGFbeta to regulate Smad1.

Temporal and spatial expression of c-jun and jun-B proto-oncogenes in pulp cells involved with reparative dentinogenesis after cavity preparation of rat molars

c-jun and jun-B are nuclear proto-oncogenes induced by growth factors such as bone morphogenetic proteins (BMPs). These gene products enhance the expression of many genes, including osteocalcin and collagen types, indicating that c-jun and jun-B play important roles in the cell differentiation process. It is also known that BMPs affect the differentiation of pulp cells to odontoblast-like cells during reparative dentinogenesis, but little is known about the transcriptional regulation of genes in cells associated with reparative dentinogenesis. In this study, we examined the expression of c-jun and jun-B in pulp cells during reparative dentinogenesis after cavity preparation of rat molars by in situ hybridization. In rat tooth germs, c-jun and jun-B were co-expressed in the odontoblastic lineage. In rat adult molars, c-jun was expressed in the odontoblast layer, but the jun-B expression was absent in all pulp cells. After cavity preparation, we found that c-jun and jun-B were coexpressed in pulp cells underneath cavities. During the early phase of reparative dentinogenesis, levels of c-jun and jun-B greatly increased in pulp cells within and around the reparative dentin matrix formed adjacent to the cavity floor. Fourteen days after cavity preparation, c-jun and jun-B were expressed only in pulp cells lining the irregular surface of the thick reparative dentin. These results suggest that c-jun and jun-B may play important roles both in physiological and in reparative dentinogenesis; in particular, the limited distribution of the jun-B expression suggests a specific role of jun-B only in cells involved with the active formation of the dentin matrix during primary and reparative dentinogenesis.

Strict regulation of c-Raf kinase levels is required for early organogenesis of the vertebrate inner ear

Regulation of organogenesis involves a dynamic balance of the mechanisms regulating cell division, differentiation and death. Here we have investigated the pattern of expression of c-Raf kinase in the inner ear during early developmental stages and the consequences of manipulating c-Raf levels by misexpression of c-raf viral vectors in organotypic cultures of otic vesicle explants. We found that otic vesicles expressed c-Raf and its level remained constant during embryonic days 2 and 3 (E2-E3). c-Raf activity was increased in response to insulin like growth factor-I (IGF-I) and the activation by IGF-I of the c-Raf kinase pathway was a requirement to turn on cell proliferation in the otic vesicle. Overexpression of c-raf in E2.5 explants increased the proliferative response to low serum and IGF-I and blocked differentiation induced by retinoic acid. The increase in c-Raf levels also prevented nerve growth factor (NGF)-dependent induction of programmed cell death. Consistent with these results, the expression of a dominant negative c-Raf mutant potentiated retinoic acid action and decreased the rate of cell proliferation. We conclude that a strict control of c-Raf levels is essential for the co-ordination of the biological processes that operate simultaneously during early inner ear development.

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.

Transcriptional regulation of BMP-4 in the Xenopus embryo: analysis of genomic BMP-4 and its promoter

Recent experiments in the Xenopus embryo suggest that proper regulation of BMP-4 signaling is critical to the dorsal ventral specification of both mesoderm and ectoderm. Regulation of BMP-4 signaling is known to occur extracellularly by direct binding with chordin, noggin, and follistatin, and intracellularly through the antagonistic signal interaction with dorsalizing TGF-beta family member activin. However, tight repressional regulation of BMP transcription may also be required to sustain the dorsal and neural status of the induced cells. Here we demonstrate that the dominant negative mutant of the BMP receptor (DN-BR) or the BMP-4 antagonizers, chordin and noggin, negatively regulate BMP-4 transcription in animal cap explants. We suggest that repression of BMP-4 transcription is important in the maintenance of dorsal fate and that continuous input of BMP-4 signaling is required to sustain the expression of BMP-4 transcription in the maintenance of epidermal/ventral fate. Consistent with this postulation, we found that the promoter region of the isolated BMP-4 genomic DNA includes several consensus binding sites for transcriptional regulators functioning under BMP-4 signaling such as GATA binding and ventralizing homeobox genes. In a functional assay we found that the GATA binding and ventral homeobox proteins can positively modulate BMP-4 promoter activity. We also observed that DN-BR decreases BMP-4 promoter activity. This was likely due to a repression of the above-mentioned transcription factors. The significance of these observations to embryonic patterning is discussed.

The RAF family: an expanding network of post-translational controls and protein-protein interactions

Protein kinase RAF is strategically located in the "Ras-MAP-kinase signal transduction pathway", a principle system which transmits signals from growth factor receptors to the nucleus, resulting in cell proliferation. Growth factor responses are mediated in part by activation of Ras, which in turn activates RAF to phosphorylate MEK, its downstream substrate. MEK activates MAP-kinase to influence nuclear events. It is clear, however, that a network of signals other than those carried by Ras plays a role in RAF regulation. These orthogonal influences are mediated by: serine/threonine kinases, tyrosine kinases, and protein-protein interactions. As a further complication to the RAF network, three isoforms of RAF have been established which have divergent N-terminal regulatory domains. Whereas these divergent regulatory domains implicate isoform-specific functions, no clear evidence or hypothesis for distinct functions for individual isoforms has been presented. Recently, "isoform-specific protein interactions" have been identified among numerous proteins interacting with RAF. These studies may serve to delineate independent functions for RAF isoforms.

Mesoderm induction by heterodimeric AP-1 (c-Jun and c-Fos) and its involvement in mesoderm formation through the embryonic fibroblast growth factor/Xbra autocatalytic loop during the early development of Xenopus embryos

We have previously demonstrated the involvement of AP-1/Jun in fibroblast growth factor (FGF) signaling by loss-of-function assay (Dong, Z., Xu, R.-H., Kim, J., Zhan, S.-N., Ma, W.-Y., Colburn, N. H., and Kung, H. (1996) J. Biol. Chem. 271, 9942-9946). Further investigations by gain-of-function are reported in this study. AP-1 transactivation activity was increased by the treatment of animal cap explants with FGF. Ectopic overexpression of two components of AP-1 (c-jun and c-fos together, but not alone) produced posteriorized embryos and induced mesoderm formation in animal cap explants, indicating that both AP-1 heterodimers are required for mesoderm induction. Since Ras/AP-1 functions downstream of FGF signaling, we then tested the involvement of Ras/AP-1 in mesoderm maintenance mediated by embryonic FGF/Xbra using dominant-negative mutants. Mesoderm maintenance mediated by embryonic FGF/Xbra was blocked by dominant-negative mutants of Ras/AP-1, and AP-1 enhanced the expression of Xbra. Further studies demonstrated the inhibition of Ras/AP-1-mediated mesoderm formation by dominant-negative mutants of the FGF receptor and Xbra. These results indicate that Ras/AP-1 and FGF/Xbra signals are involved in the mesoderm maintenance machinery and mesoderm formation through the synergistic action of the diversified signal pathways derived from the FGF/Xbra autocatalytic loop.

Osteogenic protein-1 up-regulation of the collagen X promoter activity is mediated by a MEF-2-like sequence and requires an adjacent AP-1 sequence

Bone morphogenetic proteins induce chondrogenesis and osteogenesis in vivo. To investigate molecular mechanisms involved in chondrocyte induction, we examined the effect of osteogenic protein (OP)-1/bone morphogenetic protein-7 on the collagen X promoter. In rat calvaria-derived chondrogenic C5.18 cells, OP-1 up-regulates collagen X mRNA levels and its promoter activity in a cell type- specific manner. Deletion analysis localizes the OP-1 response region to 33 bp (-310/-278), which confers OP-1 responsiveness to both the minimal homologous and heterologous Rous sarcoma virus promoter. Transforming growth factor-beta2 or activin, which up-regulates the expression of a transforming growth factor-beta-inducible p3TP-Lux construct, has little effect on collagen X mRNA and on this 33-bp region. Mutational analysis shows that both an AP-1 like sequence (-294/-285, TGAATCATCA) and an A/T-rich myocyte enhancer factor (MEF)-2 like sequence (-310/-298, TTAAAAATAAAAA) in the 33-bp region are necessary for the OP-1 effect. Gel shift assays show interaction of distinct nuclear proteins from C5.18 cells with the AP-1-like and the MEF-2-like sequences. OP-1 rapidly induces nuclear protein interaction with the MEF-2-like sequence but not with the AP-1 like sequence. MEF-2-like binding activity induced by OP-1 is distinct from the MEF-2 family proteins present in C2C12 myoblasts, in which OP-1 does not induce collagen X mRNA or up-regulate its promoter activity. In conclusion, we identified a specific response region for OP-1 in the mouse collagen X promoter. Mutational and gel shift analyses suggest that OP-1 induces nuclear protein interaction with an A/T-rich MEF-2 like sequence, distinct from the MEF-2 present in myoblasts, and up-regulates collagen X promoter activity, which also requires an AP-1 like sequence.

Studies on the role of fibroblast growth factor signaling in neurogenesis using conjugated/aged animal caps and dorsal ectoderm-grafted embryos

Basic fibroblast growth factor (bFGF) has been shown to induce neural fate in dissociated animal cap (AC) cells or in AC explants cultured in low calcium and magnesium concentrations. However, long-term disclosure of the cap may cause diffusion of the secreted molecule bone morphogenetic protein 4 (BMP-4), a neural inhibitor present in the AC. This may contribute to the subsequent neurogenesis induced by bFGF. Here we used conjugated and aged blastula AC to avoid diffusion of endogenous molecules from the AC. Unlike noggin, bFGF failed to induce neural tissue in this system. However, it enhanced neuralization elicited by a dominant negative BMP receptor (DN-BR) that inhibits the BMP-4 signaling. Posterior neural markers were turned on by bFGF in AC expressing DN-BR or chordin. Blocking the endogenous FGF signal with a dominant negative FGF receptor (XFD) mainly inhibited development of posterior neural tissue in neuralized ACs. These in vitro studies were confirmed in vivo in embryos grafted with XFD-expressing ACs in the place of neuroectoderm. Expression of some regional neural markers was inhibited, although markers for muscle and posterior notochord were still detectable in the grafted embryos, suggesting that XFD specifically affected neurogenesis but not the dorsal mesoderm. The use of these in vitro and in vivo model systems provides new evidence that FGF, although unable to initiate neurogenesis on its own, is required for neural induction as well as for posteriorization.

Studies on the role of fibroblast growth factor signaling in neurogenesis using conjugated/aged animal caps and dorsal ectoderm-grafted embryos

Basic fibroblast growth factor (bFGF) has been shown to induce neural fate in dissociated animal cap (AC) cells or in AC explants cultured in low calcium and magnesium concentrations. However, long-term disclosure of the cap may cause diffusion of the secreted molecule bone morphogenetic protein 4 (BMP-4), a neural inhibitor present in the AC. This may contribute to the subsequent neurogenesis induced by bFGF. Here we used conjugated and aged blastula AC to avoid diffusion of endogenous molecules from the AC. Unlike noggin, bFGF failed to induce neural tissue in this system. However, it enhanced neuralization elicited by a dominant negative BMP receptor (DN-BR) that inhibits the BMP-4 signaling. Posterior neural markers were turned on by bFGF in AC expressing DN-BR or chordin. Blocking the endogenous FGF signal with a dominant negative FGF receptor (XFD) mainly inhibited development of posterior neural tissue in neuralized ACs. These in vitro studies were confirmed in vivo in embryos grafted with XFD-expressing ACs in the place of neuroectoderm. Expression of some regional neural markers was inhibited, although markers for muscle and posterior notochord were still detectable in the grafted embryos, suggesting that XFD specifically affected neurogenesis but not the dorsal mesoderm. The use of these in vitro and in vivo model systems provides new evidence that FGF, although unable to initiate neurogenesis on its own, is required for neural induction as well as for posteriorization.

Growth factor and cytokine-regulated hyaluronan-binding protein TSG-6 is localized to the injury-induced rat neointima and confers enhanced growth in vascular smooth muscle cells

Hyaluronan (HA) and HA-binding proteins have been implicated in a diverse array of biological processes, including development, tissue repair, and tumor invasion. However, the role of HA and HA-binding proteins in atherosclerosis and restenosis is poorly understood. PS4 (TSG-6) is a HA-binding protein expressed by cultured vascular smooth muscle cells (SMCs) in response to serum and growth factor stimulation. To delineate a possible role for TSG-6 in vascular disease progression, we have characterized its expression in cultured SMCs and in a rat vascular injury model, and we have studied the effect of constitutive overexpression of TSG-6 on SMC behavior. We found that interleukin-1 (IL-1) but not tumor necrosis factor or interleukin-6 was able to stimulate TSG-6 expression in SMCs. The IL-1 pathway could be distinguished from the growth factor pathway by its insensitivity to protein synthesis inhibitors. Furthermore, epidermal growth factor, fibroblast growth factor-1, and transforming growth factor-beta1 were all capable of augmenting maximum IL-1-induced expression of TSG-6. To gain further insight into the function of TSG-6 in SMCs, we examined the effect of constitutive overexpression of TSG-6 on these cells. We found that TSG-6-overexpressing cells grew >50% faster than control cells. Furthermore, this growth advantage became more evident in the absence of serum growth factors, with an average increase in cell number of 118% over control cells after 6 days. Consistent with these in vitro data, we observed intense immunostaining for TSG-6 in proliferating SMCs in the rat neointima after injury, whereas only an occasional cell was positive for TSG-6 in the medial layer and in nonballooned arteries. We conclude that the expression of TSG-6 is tightly controlled by growth factors and cytokines via two distinct pathways in SMCs and that overexpression of TSG-6 confers a growth advantage to these cells.

Xmsx-1 modifies mesodermal tissue pattern along dorsoventral axis in Xenopus laevis embryo

This study analyzes the expression and the function of Xenopus msx-1 (Xmsx-1) in embryos, in relation to the ventralizing activity of bone morphogenetic protein-4 (BMP-4). Expression of Xmsx-1 was increased in UV-treated ventralized embryos and decreased in LiCl-treated dorsalized embryos at the neurula stage (stage 14). Whole-mount in situ hybridization analysis showed that Xmsx-1 is expressed in marginal zone and animal pole areas, laterally and ventrally, but not dorsally, at mid-gastrula (stage 11) and late-gastrula (stage 13) stages. Injection of BMP-4 RNA, but not activin RNA, induced Xmsx-1 expression in the dorsal marginal zone at the early gastrula stage (stage 10+), and introduction of a dominant negative form of BMP-4 receptor RNA suppressed Xmsx-1 expression in animal cap and ventral marginal zone explants at stage 14. Thus, Xmsx-1 is a target gene specifically regulated by BMP-4 signaling. Embryos injected with Xmsx-1 RNA in dorsal blastomeres at the 4-cell stage exhibited a ventralized phenotype, with microcephaly and swollen abdomen. Histological observation and immunostaining revealed that these embryos had a large block of muscle tissue in the dorsal mesodermal area instead of notochord. On the basis of molecular marker analysis, however, the injection of Xmsx-1 RNA did not induce the expression of alpha-globin, nor reduce cardiac alpha-actin in dorsal marginal zone explants. Furthermore, a significant amount of alpha-actin was induced and alpha-globin was turned off in the ventral marginal zone explants injected with Xmsx-1. These results indicated that Xmsx-1 is a target gene of BMP-4 signaling, but possesses a distinct activity on dorsal-ventral patterning of mesodermal tissues.

Localization of MAP kinase activity in early Xenopus embryos: implications for endogenous FGF signaling

We have used a sensitive assay for MAP kinase activity to investigate the role of endogenous fibroblast growth factor (FGF)-activated MAP kinase in early Xenopus embryonic patterning. MAP kinase activity is low during cleavage stages and increases significantly during gastrulation. The temporal profile of this activity correlates well with the expression pattern of Xenopus eFGF. Spatially, MAP kinase activity is lowest in animal pole tissue and higher in vegetal pole cells and the marginal zone. Endogenous MAP kinase activity is FGF receptor-dependent, demonstrating that FGF signaling is active in all three germ layers of the early embryo. This activity is necessary for normal expression of Mix.1, a mesoendodermal marker, in the endoderm as well as in the mesoderm, indicating that MAP kinase plays a functional role in patterning of both of these germ layers. Spatial and temporal changes in MAP kinase activation during gastrulation also suggest a role for FGF signaling in this process. In addition, we find that embryonic wounding during dissection results in significant stimulation of this pathway, providing a possible explanation for earlier observations of effects of surgical manipulation on cell fate in early embryos.

Differential regulation of neurogenesis by the two Xenopus GATA-1 genes

Previously, we have shown that the ventralizing factor bone morphogenetic protein 4 (BMP-4) can inhibit Xenopus neurogenesis. The erythroid transcription factor GATA-1 functions downstream of the BMP-4 signaling pathway and mediates BMP-4-induced erythropoiesis. We have found that similar to BMP-4, GATA-1b inhibits neuralization of Xenopus animal cap (AC) cells. The neural inhibition is not seen with GATA-1a, although both GATA-1a and GATA-1b RNAs are translated at the same efficiency and induce globin expression equally in AC cells. GATA-1b RNA injection into AC cells neither induces expression of Xbra (a general mesoderm marker) nor affects expression of XK81 (epidermal keratin) or BMP-4 and Xvent-1 (two ventral markers). These data suggest that GATA-1b retains the epidermal fate of the AC. Intact GATA-1b protein is required for both inhibition of neurogenesis and induction of globin expression. Our findings indicate that GATA-1b can function in ectoderm to specifically regulate neural inducing mechanisms, apparently related to the expression of chordin, a neuralizing gene. Furthermore, tadpole stage embryos injected with GATA-1b are devoid of all dorsoanterior structures including neural tissue. This report provides evidence that the two transcription factors, derived from a recent genome duplication, share a common biological activity (stimulation of erythropoiesis) while also exhibiting a distinct function (inhibition of neurogenesis).

Antagonistic actions of activin A and BMP-2/4 control dorsal lip-specific activation of the early response gene XFD-1' in Xenopus laevis embryos

Transcription of the early response gene XFD-1' (XFKH1) in the dorsal lip (Spemann organizer) of Xenopus embryos is activated by dorsal mesoderm inducing factors. Promoter studies revealed the presence of an activin A response element (ARE) which is both necessary and sufficient for transcriptional activation of reporter genes in animal cap explants incubated with activin A. Surprisingly, this ARE is also active within vegetal explants in the absence of exogenously added inducers, but an additional inhibitory response element prevents transcription of the XFD-1' gene in the ventral/vegetal region of the embryo in vivo. This element is located upstream of the ARE, it responds to bone morphogenic proteins 2 and 4 (BMP-2/4) triggered signals and it overrides the activating properties of the ARE. Expression patterns of BMP-2 and BMP-4 in the late blastula stage embryo and, especially, their absence from the dorsal blastopore lip may thus control the spatial transcription of the XFD-1' gene. Accordingly, the temporal activation and the spatial restriction of XFD-1' gene activity to the Spemann organizer is regulated by antagonistic actions of two distinct members of the TGF-beta family (activin and BMP) which act on different promoter elements.

The Xvent-2 homeobox gene is part of the BMP-4 signalling pathway controlling [correction of controling] dorsoventral patterning of Xenopus mesoderm

We describe a novel Xenopus homeobox gene, Xvent-2, which together with the previously identified homeobox gene Xvent-1, defines a novel class of homeobox genes. vent genes are related by sequence homology, expression pattern and gain-of-function phenotype. Evidence is presented for a role of Xvent-2 in the BMP-4 pathway involved in dorsoventral patterning of mesoderm. (1) Xvent-2 is expressed in regions that also express BMP-4. (2) Xvent-2 and BMP-4 interact in a positive feedback loop. (3) Xvent-2 ventralizes dorsal mesoderm in a dose-dependent manner resulting in phenoytpes ranging from microcephaly to Bauchstuck pieces, as does BMP-4. (4) Like BMP-4 and gsc, Xvent-2 and gsc are able to interact in a crossregulatory loop to suppress each other. (5) Microinjection of Xvent-2 mRNA can rescue dorsalization by a dominant-negative BMP-4 receptor. The results suggest that Xvent-2 functions in the BMP-4 signalling pathway that antagonizes the Spemann organizer.

Extracellular signal-regulated kinase and the small GTP-binding protein, Rac, contribute to the effects of transforming growth factor-beta1 on gene expression

The kinases and regulatory proteins that convey signals initiated by transforming growth factor-beta (TGF-beta) to the nucleus are poorly characterized. To study the role of the extracellular signal-regulated kinase (ERK) pathway in this process, we transiently transfected NIH 3T3 fibroblasts with TGF-beta-responsive luciferase reporter genes and expression vectors designed to interrupt this kinase cascade. Mitogen-activated protein (MAP) kinase phosphatase-1 and a dominant negative MAP/ERK kinase 1 mutant reduced stimulation of plasminogen activator inhibitor-1 (PAI-1) promoter activity by TGF-beta1 from 11.5- to 4-fold and 4.9-fold, respectively. Similar results were observed with the type I collagen promoters. TGF-beta1 increased ERK1 activity 4.5-fold at 5 min and 3. 1-fold at 3 h, while Jun kinase and p38 activity were not affected. Cotransfection of a dominant negative mutant of the small G protein, Rac, but not dominant negative Ras, Cdc42, or Rho mutants, reduced the effects of TGF-beta1 on the PAI-1 promoter by approximately half. In support of a role for Rac in signaling by TGF-beta, GTP binding to Rac was increased 3.7-fold following exposure of NIH 3T3 cells to TGF-beta1 for 3 min. These findings indicate that TGF-beta1 modulates gene expression partly through ERK and Rac in NIH 3T3 cells.

Bone morphogenetic proteins: multifunctional regulators of vertebrate development

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