TGF-beta signaling in murine embryonic palate cells involves phosphorylation of the CREB transcription factor

Serum Induces the Subunit-Specific Activation of NF-κB in Proliferating Human Cardiac Stem Cells

Cardiovascular diseases (CVDs) are often linked to ageing and are the major cause of death worldwide. The declined proliferation of adult stem cells in the heart often impedes its regenerative potential. Thus, an investigation of the proliferative potential of adult human cardiac stem cells (hCSCs) might be of great interest for improving cell-based treatments of cardiovascular diseases. The application of human blood serum was already shown to enhance hCSC proliferation and reduce senescence. Here, the underlying signalling pathways of serum-mediated hCSC proliferation were studied. We are the first to demonstrate the involvement of the transcription factor NF-κB in the serum-mediated proliferative response of hCSCs by utilizing the NF-κB inhibitor pyrrolidine dithiocarbamate (PDTC). RNA-Sequencing (RNA-Seq) revealed ATF6B, COX5B, and TNFRSF14 as potential targets of NF-κB that are involved in serum-induced hCSC proliferation.

Protein Kinase A Inhibitor H89 Attenuates Experimental Proliferative Vitreoretinopathy

PURPOSE

This study aimed to explore the role of the protein kinase A (PKA) pathway in proliferative vitreoretinopathy (PVR) and the effect of the PKA inhibitor H89 on experimental PVR.

METHODS

Epiretinal membranes (ERMs) were acquired from PVR patients and analyzed by frozen-section immunofluorescence. An in vivo model was developed by intravitreal injecting rat eyes with ARPE-19 cells and platelet-rich plasma, and changes in eye structures and vision function were observed. An in vitro epithelial-mesenchymal transition (EMT) cell model was established by stimulating ARPE-19 cells with transforming growth factor (TGF)-β. Alterations in EMT-related genes and cell function were detected. Mechanistically, PKA activation and activity were explored to assess the relationship between TGF-β1 stimulation and the PKA pathway. The effect of H89 on the TGF-β-Smad2/3 pathway was detected. RNA sequencing was used to analyze gene expression profile changes after H89 treatment.

RESULTS

PKA was activated in human PVR membranes. In vivo, H89 treatment protected against structural changes in the retina and prevented decreases in electroretinogram b-wave amplitudes. In vitro, H89 treatment inhibited EMT-related gene alterations and partially reversed the functions of the cells. TGF-β-induced PKA activation was blocked by H89 pretreatment. H89 did not affect the phosphorylation or nuclear translocation of regulatory Smad2/3 but increased the expression of inhibitory Smad6.

CONCLUSIONS

PKA pathway activation is involved in PVR pathogenesis, and the PKA inhibitor H89 can effectively inhibit PVR, both in vivo and in vitro. Furthermore, the protective effect of H89 is related to an increase in inhibitory Smad6.

TGF-β1 stimulates mouse macrophages to express APRIL through Smad and p38MAPK/CREB pathways

A proliferation-inducing ligand (APRIL), a new TNF family member, supports B-cell survival and tumor cell proliferation. APRIL is secreted as a soluble protein by macrophages, dendritic cells and activated T cells. However, factors involved in regulation of APRIL expression are as yet unknown. In this study, we investigated the effect of TGF-β1 on APRIL expression in P388D1, a mouse macrophage cell line. TGF-β1 induced APRIL mRNA expression in a time- and dose-dependent manner. One nanogram per milliliter of TGF-β1 was optimal and APRIL transcripts appeared as early as 3 h after stimulation. Based on our studies, which included overexpression of Smad3, DN-Smad3, and sh-Smad3, we found that Smad3 mediates APRIL transcription at least partially. Further, experiments using inhibitors revealed that p38MAPK and CREB are also involved in TGF-β1-induced APRIL expression. These results suggest that TGF-β1, through Smad3 and p38MAPK/CREB signaling pathways, stimulates APRIL expression in macrophages.

Neural crest-specific loss of Prkar1a causes perinatal lethality resulting from defects in intramembranous ossification

The cranial neural crest (CNC) undergoes complex molecular and morphological changes during embryogenesis in order to form the vertebrate skull, and nearly three quarters of all birth defects result from defects in craniofacial development. The molecular events leading to CNC differentiation have been extensively studied; however, the role of the cAMP-dependent protein kinase [protein kinase A (PKA)] during craniofacial development has only been described in palate formation. Here, we provide evidence that strict PKA regulation in postmigratory CNC cells is essential during craniofacial bone development. Selective inactivation of Prkar1a, a regulatory subunit of the PKA holoenzyme, in the CNC results in perinatal lethality caused by dysmorphic craniofacial development and subsequent asphyxiation. Additionally, aberrant differentiation of CNC mesenchymal cells results in anomalous intramembranous ossification characterized by formation of cartilaginous islands in some areas and osteolysis of bony trabeculae with fibrous connective tissue stabilization in others. Genetic interaction studies revealed that genetic reduction of the PKA catalytic subunit C(alpha) was able to rescue the phenotype, whereas reduction in Cbeta had no effect. Overall, these observations provide evidence of the essential role of proper regulation of PKA during the ossification of the bones of the skull. This knowledge may have implications for the understanding and treatment of craniofacial birth defects.

Transforming growth factor-beta1 causes pulmonary microvascular endothelial cell apoptosis via ALK5

We have previously shown that transforming growth factor (TGF)-beta1 protected against main pulmonary artery endothelial cell (PAEC) apoptosis induced by serum deprivation and VEGF receptor blockade through a mechanism associated with ALK5-mediated Bcl-2 upregulation. In the current study, we investigated the effect of TGF-beta1 on pulmonary microvascular endothelial cell (PMVEC) apoptosis. We found that, in contrast to the results seen in conduit PAEC, TGF-beta1 caused apoptosis of PMVEC, an effect that was also dependent on ALK5 activity. We noted that non-SMAD signaling pathways did not play a role in TGF-beta1-induced apoptosis. Both SMAD2 and SMAD1/5 were activated upon exposure to TGF-beta1. TGF-beta1-induced activation of SMAD2, but not SMAD1/5, was abolished by ALK5 inhibition, an effect that associated with prevention of TGF-beta1-induced apoptosis. These results suggest that SMAD2 is important in TGF-beta1-induced apoptosis of PMVEC. While caspase-12 activity was not altered, caspase-8 was activated by TGF-beta1, an effect that correlated with a reduction of cFLIP protein levels. Additionally, TGF-beta1 decreased Bcl-2 protein levels and induced cytochrome c cytosolic redistribution. These results suggest that TGF-beta1 caused apoptosis of PMVEC likely through both caspase-8-dependent extrinsic pathway and mitochondria-mediated intrinsic pathway. We noted that inhibition of ALK5 attenuated serum deprivation-induced apoptosis, an effect that correlated with increased expression and activation of CREB and its potential target genes, Bcl-2 and cFLIP. These results suggest that CREB may be important in mediating apoptosis resistance of PMVEC upon ALK5 inhibition perhaps through upregulation of Bcl-2 and cFLIP. Finally, we noted that SMAD1/5 were activated upon ALK5 inhibition in the presence of low levels of TGF-beta1, an effect associated with enhanced endothelial proliferation. We speculate that imbalance of ALK1 and ALK5 may contribute to the development of pulmonary artery hypertension.

IL-4-induced AID expression and its relevance to IgA class switch recombination

Activation-induced cytidine deaminase (AID) is an inducible gene that plays a critical role in Ig class switch recombination and somatic hypermutation in B cells. We explored the mechanisms by which IL-4 induces AID expression in mouse B cells. IL-4 increased AID expression and over-expression of Stat6 further augmented IL-4-induced promoter activity. The involvement of Stat6 in the promoter activity was confirmed using ChIP assays and site-directed mutagenesis. Treatment with H89, a PKA inhibitor, markedly decreased IL-4-induced AID expression, and over-expression of CREB enhanced it. These results indicate that Stat6 and PKA/CREB are involved in IL-4-induced AID expression. The relevance of these signal transducing molecules was verified using the TGFbeta1-induced IgA isotype switching model. Our results indicate that IL-4, through Stat6 and PKA/CREB, induces AID expression leading to Ig isotype switching event.

Expression profiling of transforming growth factor beta superfamily genes in developing orofacial tissue

BACKGROUND

Numerous signaling molecules have been shown to participate in the dynamic process of orofacial development. Among these signal mediators, members of the transforming growth factor beta (TGFbeta) superfamily have been shown to play critical roles. Developing orofacial tissue expresses TGFbeta and bone morphogenetic protein (BMP) mRNAs, their protein isoforms and TGFbeta- and BMP-specific receptors. All these molecules display unique temporospatial patterns of expression in embryonic orofacial tissue, suggesting functional roles in orofacial development. For example, the TGFbetas and BMPs regulate maxillary mesenchymal cell proliferation and extracellular matrix synthesis. This is particularly noteworthy in that perturbation of either process results in orofacial clefting. Although the cellular and phenotypic effects of the TGFbeta superfamily of growth factors on embryonic orofacial tissue have been extensively studied, the specific genes that function as effectors of these cytokines in orofacial development have not been well defined.

METHODS

In the present study, oligonucleotide-based microarray technology was utilized to provide a comprehensive analysis of the expression of the panoply of genes related to the TGFbeta superfamily, as well as those encoding diverse groups of proteins functionally associated with this superfamily, during orofacial ontogenesis.

RESULTS

Of the 7000 genes whose expression was detected in the developing orofacial region, 249 have been identified that encode proteins related to the TGFbeta superfamily. Expression of some (27) of these genes was temporally regulated. In addition, several candidate genes, whose precise role in orofacial development is still unknown, were also identified. Examples of genes constituting this cluster include: TGFbeta1-induced antiapoptotic factor-1 and -2, TGFbeta-induced factor 2, TGFbeta1 induced transcript-1 and -4, TGFbeta-inducible early growth response 1, follistatin-like 1, follistatin-like 3, transmembrane protein with EGF-like and two follistatin-like domains (Tmeff)-1 and -2, nodal modulator 1, various isoforms of signal transducers and activators of transcription (Stat), notch, and growth and differentiation factors.

CONCLUSIONS

Elucidation of the precise physiological roles of these proteins in orofacial ontogenesis should provide unique insights into the intricacies of the TGFbeta superfamily signal transduction pathways utilized during orofacial development.

Erk1/2 signaling is required for Tgf-β2–induced suture closure

Transforming growth factors beta (Tgf-betas) act by means of Smad signaling pathways and may also interact with the mitogen-activated protein kinase pathway. The hypothesis was tested that Erk1/2 signaling is required for Tgf-beta2-induced suture closure, by culturing embryonic mouse calvariae in the presence of Tgf-beta2 with or without Erk1/2 inhibitor PD98059 (PD). Suture widths were measured daily, and microdissected sutures and bones were homogenized and protein analyzed by Western blots. Tgf-beta2 induced narrowing of the sutures after 72 hr, an effect inhibited by treatment with PD. Erk1/2 and Egf but not Smad2/3 protein expression was up-regulated by Tgf-beta2 calvarial tissues at 72 hr. PD inhibited endogenous and Tgf-beta2-stimulated Erk1/2 protein as well as Tgf-beta2-stimulated Egf, but increased Smad2/3 protein expression. In tissues harvested 0, 15, and 30 min after exposure to Tgf-beta2, Erk1/2 phosphorylation was up-regulated after 15 min, an effect abrogated by the simultaneous addition of PD. In summary, Tgf-beta2 stimulated Erk1/2 phosphorylation and induced Egf and Erk1/2 expression, associated with suture closure after 72 hr. Blocking Erk1/2 activity with PD inhibited these effects but increased Smad2/3 expression. We postulate that Tgf-beta2 regulates suture closure directly by means of phosphorylation of Erk1/2 and indirectly by up-regulating Erk1/2, a substrate for Fgf receptor signaling required for Fgf induction of premature suture obliteration.

Developmental gene expression profiling of mammalian, fetal orofacial tissue

BACKGROUND

The embryonic orofacial region is an excellent developmental paradigm that has revealed the centrality of numerous genes encoding proteins with diverse and important biological functions in embryonic growth and morphogenesis. DNA microarray technology presents an efficient means of acquiring novel and valuable information regarding the expression, regulation, and function of a panoply of genes involved in mammalian orofacial development.

METHODS

To identify differentially expressed genes during mammalian orofacial ontogenesis, the transcript profiles of GD-12, GD-13, and GD-14 murine orofacial tissue were compared utilizing GeneChip arrays from Affymetrix. Changes in gene expression were verified by TaqMan quantitative real-time PCR. Cluster analysis of the microarray data was done with the GeneCluster 2.0 Data Mining Tool and the GeneSpring software.

RESULTS

Expression of >50% of the approximately 12,000 genes and expressed sequence tags examined in this study was detected in GD-12, GD-13, and GD-14 murine orofacial tissues and the expression of several hundred genes was up- and downregulated in the developing orofacial tissue from GD-12 to GD-13, as well as from GD-13 to GD-14. Such differential gene expression represents changes in the expression of genes encoding growth factors and signaling molecules; transcription factors; and proteins involved in epithelial-mesenchymal interactions, extracellular matrix synthesis, cell adhesion, proliferation, differentiation, and apoptosis. Following cluster analysis of the microarray data, eight distinct patterns of gene expression during murine orofacial ontogenesis were selected for graphic presentation of gene expression patterns.

CONCLUSIONS

This gene expression profiling study identifies a number of potentially unique developmental participants and serves as a valuable aid in deciphering the complex molecular mechanisms crucial for mammalian orofacial development.

Tyrosine phosphorylation-dependent PI 3 kinase/Akt signal transduction regulates TGFbeta-induced fibronectin expression in mesangial cells

TGFbeta stimulates expression of fibronectin in various cells, including mesangial cells. The mechanism by which TGFbeta exerts this effect is not fully understood. We investigated the involvement of tyrosine phosphorylation and the phosphatidylinositol (PI) 3 kinase/Akt signalling pathway in this process. TGFbeta increased tyrosine phosphorylation, resulting in activation of PI 3 kinase in mesangial cells. Inhibition of tyrosine phosphorylation blocked TGFbeta-induced fibronectin expression. Inhibition of PI 3 kinase activity also prevented fibronectin expression induced by TGFbeta. Furthermore, expression of constitutively active PI 3 kinase by adenovirus-mediated gene transfer increased fibronectin expression similar to TGFbeta. TGFbeta stimulated Akt serine threonine kinase in a tyrosine kinase- and PI 3 kinase-dependent manner. Inhibition of TGFbeta-induced Akt activity by adenovirus-mediated expression of a dominant-negative mutant of Akt abolished expression of fibronectin. Dominant-negative PI 3 kinase or dominant-negative Akt inhibited TGFbeta-induced fibronectin transcription. In contrast, and similarly to TGFbeta, expression of constitutively active PI 3 kinase or constitutively active Akt increased transcription of fibronectin, confirming a prominent role of these kinases in expression of fibronectin. These data provide the evidence that activation of TGFbeta receptor serine threonine kinase stimulates the PI 3 kinase/Akt pathway in a tyrosine phosphorylation-dependent manner and define a role for the same signal transduction pathway in TGFbeta-induced fibronectin expression.

An address to young research workers: Inconsequences and blindness to the facts and unpublished observations

The critical remembrance of my research activity displays aspects of inconsequence and mental rigidity. Particular probative examples are called to mind. Unpublished observations concerning immortalized dental cells are summarized. As a consequence, if I could start again, I would improve several aspects of my behavior and attitude of mind.

Nuclear convergence of the TGFbeta and cAMP signal transduction pathways in murine embryonic palate mesenchymal cells

Transforming growth factors beta (TGFbeta) and cyclic AMP (cAMP) both participate in growth and differentiation of the developing mammalian secondary palate and elicit similar biological responses. Cross-talk between these two signal transduction pathways in cells derived from the embryonic palate has been demonstrated previously. In the present study, we have examined nuclear convergence of these signalling pathways at the level of transcriptional complex formation. Biotinylated oligonucleotides encoding a consensus Smad binding element (SBE), or a cyclic AMP response element (CRE), were mixed with cell extracts from murine embryonic palate mesenchymal (MEPM) cells that were treated with either TGFbeta or forskolin. Protein-oligonucleotide complexes were precipitated with streptavidin-agarose, and analysed by Western blotting to identify proteins in the complex bound to each consensus oligonucleotide. TGFbeta treatment of MEPM cells increased the levels of phosphorylated Smad2, phosphorylated cAMP response element binding protein (CREB), and the coactivator, CREB binding protein (CBP), that were part of a complex bound to the SBE. Treatment of cells with forskolin, a stimulator of adenylate cyclase, increased the amount of phosphorylated CREB and CBP, but not the amount of phosphorylated Smad2 bound in a complex to the SBE. Additionally, the presence of the co-repressors, c-Ski and SnoN, was demonstrated as part of a complex bound to the SBE (but not the CRE). Amounts of c-Ski and SnoN found in the SBE-containing complex increased in response to either TGFbeta or forskolin. These results demonstrate that phosphorylated CREB forms a complex with the co-activator CBP, phosphorylated Smad2 and the co-repressors c-Ski and SnoN on a consensus SBE. This suggests cooperative regulation of genes with SBE-containing promoters by the cAMP and TGFbeta signalling pathways in the developing palate.

Phosphatase regulation of gene expression during development of the palate

In mammalian cells, including those of the embryonic palate, the level of phosphorylation of cellular proteins at any given time reflects the activities of protein kinases and protein phosphatases. Both protein phosphatase-1 (PP-1) and PP-2A inhibit cAMP-mediated increases in transcription by dephosphorylating CREB at ser-133. Western blot analysis indicated that protein phosphatase 1 (PP-1) was expressed constitutively in palatal tissue during its development. Expression of PP-2A was regulated developmentally with maximal expression on gestational day (gd) 14. Densitometric scanning revealed a 30% increase in expression from gd 13 to gd 14. Virtually all phosphatase activity in the tissue extracts could be inhibited by 5 microM okadaic acid, demonstrating that PP-1 and PP-2A account for all detectable ser/thr protein phosphatase activity present in the developing palate. Moreover, no significant differences in PP-1 and PP-2A activities were observed during the period of palate development. Treatment of primary cultures of murine embryonic palate mesenchymal (MEPM) cells with forskolin (20 microM) to elevate intracellular cAMP levels, resulted in a time-dependent increase in CREB ser-133 phosphorylation and a corresponding time dependent decrease in PP-1 and PP-2A levels. Moreover, treatment of MEPM cells with okadaic acid resulted in a dramatic increase in basal CREB ser-133 phosphorylation. This suggests that PP-1 activity may contribute to transcriptional regulation of CREB and that PP-1 and PP-2A are regulated differentially by cAMP. Treatment of MEPM cells with TGF beta 1 (1 ng/ml) under conditions of TGF beta-induced CREB phosphorylation resulted in no effect on the expression of either PP-1 or PP-2A proteins and no significant alterations in total basal protein phosphatase activity. These results demonstrate that transcriptional regulation of CREB in embryonic palatal issue is dependent on the coordinate activity of specific kinases and phosphatases.

Synergistic teratogenic effects induced by retinoids in mice by coadministration of a RARalpha- or RARgamma-selective agonist with a RXR-selective agonist

To study the interaction of retinoid-induced limb defects and cleft palate on day 11 of gestation, a RXR-selective agonist (AGN191701, an arylpropenyl-thiophene-carboxylic acid derivative, 20 mg/kg orally) was coadministered with a RARalpha-agonist (Am580, an arylcarboxamidobenzoic acid derivative, 5 mg/kg orally) to NMRI mice. AGN191701 was neither fetotoxic nor teratogenic at the dose used but potentiated Am580-induced limb defects and cleft palate and prevented Am580-induced fetal weight retardation. These results suggest that Am580-induced limb defects and probably cleft palate on day 11 of gestation may be mediated via RARalpha-RXR heterodimerization, particularly in the absence of toxicokinetic interactions. AGN191701 was also coadministered with a RARgamma-agonist (CD437, an adamantyl-hydroxyphenyl naphthoic acid derivative, 15 mg/kg orally) on days 8 and 11 of gestation to investigate which CD437-induced defects are mediated via RARgamma-RXR heterodimerization. On day 8 of gestation, AGN191701 potentiated CD437-induced embryolethality, exencephaly, spina bifida aperta, cleft palate, and tail defects, as well as visceral and skeletal defects, but not micrognathia. On day 11 of gestation, the incidence of CD437-induced cleft palate and limb defects was also potentiated when coadministered with the RXR agonist. These results suggest that synergistic teratogenic effects can be induced by coadministration of two receptor-selective retinoids, indicating the importance of RARalpha-RXR and RARgamma-RXR heterodimers in producing structural defects during organogenesis.

Efficient TGF-beta induction of the Smad7 gene requires cooperation between AP-1, Sp1, and Smad proteins on the mouse Smad7 promoter

Sma- and Mad-related protein 7 (Smad7) is an antagonist of transforming growth factor-beta (TGF-beta) signaling, which has been shown to be induced by TGF-beta itself and also by other stimuli. In an effort to understand the molecular mechanisms underlying the transcriptional regulation of the Smad7 gene by TGF-beta, we cloned and functionally characterized a mouse genomic DNA fragment encompassing the mouse Smad7 proximal promoter. This region was found to contain a CpG island and to be devoid of a classical TATA box. Cloned upstream of a promoter-lacking luciferase reporter gene, this region conferred robust TGF-beta-induced transcription. Point mutations in a palindromic Smad binding element, abolished TGF-beta inducibility completely. Through the use of electrophoretic mobility shift assays, we showed the presence of Smad2, Smad3, and Smad4 in complexes binding to the Smad binding element. Interestingly, we also found that point mutation and/or deletion of binding sites for the transcription factors activator protein-1 and Sp1 led to an attenuation of the basal promoter activity, as well as of the TGF-beta-mediated induction of Smad7. Taken together, our data imply that Smads, together with activator protein-1 and Sp1 transcription factors, are essential for efficient Smad7 promoter activity.

Identification of genes expressed in the epithelium of porcine oviduct containing early embryos at various stages of development

As a first step toward elucidation of the action of factors secreted by the epithelium of oviduct, differential display reverse transcription-polymerase chain reaction (DDRT-PCR) was used in this study to identify transcripts of such oviductal factors in gilts carrying various stages of early embryo development post hormone-induced ovulation. A total of 13 differentially expressed transcripts were identified between 50 and 120 hr post-hCG injection (between 1- and 8-cell embryonic stages). Twelve of these transcripts were found to be initially expressed at 96 hr post-hCG injection (at 4-cell embryonic stage) and beyond. Three of such genes were shown by sequence analysis to be the porcine transforming growth factor-alpha, the porcine transforming growth factor-beta-binding protein II and a porcine astral natriuretic factor receptor-like transcript. Only one differentially expressed gene was detected between 50-60 and 85 hr post-hCG injection, and this gene turned out to be the porcine follicle-stimulating hormone receptor. The remaining eight transcripts detected by DDRT-PCR were novel. Moreover, most of these newly expressed genes were found to be turned on at a time coincidental with that of the 4-cell block of porcine embryos cultured in vitro. Our results demonstrate that DDRT-PCR is a feasible approach for rapid identification of genes that are differentially expressed in oviductal epithelium. Some of the genes thus identified may be important for unhindered development of embryos in the oviduct.

Transforming growth factor beta regulates clusterin gene expression via modulation of transcription factor c-Fos

Transforming growth factor-beta (TGFbeta) induces gene expression of the glycoprotein clusterin in a variety of cell types via a consensus AP-1 binding site. Here, we demonstrate, by supershift analysis, that JunB, JunD, Fra1, Fra2, and c-Fos bound to AP-1 but that prior treatment of the cells with TGFbeta reduced dramatically c-Fos binding, suggesting that c-Fos might be playing a negative regulatory role in clusterin gene expression. Transient cotransfection assays in mink lung epithelial (CCL64) cells, using a human c-Fos expressing plasmid together with a clusterin promoter/reporter construct or the artificial TGFbeta-inducible reporter construct 3TPLux, revealed that c-Fos was indeed repressive for TGFbeta-induced promoter transactivation. Further, we demonstrate that in stable c-Fos-overexpressing cell lines, TGFbeta induction of endogenous clusterin mRNA, as well as clusterin promoter transactivation are blocked. Co-transfection with c-Fos deletion constructs revealed that the C-terminal region, including the homologue box 2 motif and the extreme C-terminal serine phosphorylation sites (Ser362 and Ser374) are required for repression of clusterin and 3TPLux transactivation. TGFbeta treatment of CCL64 cells resulted in the induction of c-Fos mRNA but caused no alternation in total c-Fos protein levels. The results suggest that the c-Fos represses clusterin gene expression, maintaining a low basal level in the absence of TGFbeta, and that TGFbeta, presumably through its effects on c-Fos protein synthesis and/or stability, abrogates the repression of c-Fos, thereby resulting in gene expression.

Selective modulation of MAP kinase in embryonic palate cells

Murine embryonic palate mesenchyme (MEPM) cells are responsive to a number of endogenous factors found in the local embryonic tissue environment. Recently, it was shown that activation of the cyclic AMP (cAMP) or the transforming growth factor beta (TGFbeta) signal transduction pathways modulates the proliferative response of MEPM cells to epidermal growth factor (EGF). Since the mitogen-activated protein kinase (MAPK) cascade is a signal transduction pathway that mediates cellular responsiveness to EGF, we examined the possibility that several signaling pathways which abrogate EGF-stimulated proliferation do so via the p42/p44 MAPK signaling pathway. We demonstrate that EGF stimulates MAPK phosphorylation and activity in MEPM cells maximally at 5 minutes. Tyrosine phosphorylation and activation of MAPK was unaffected by treatment of MEPM cells with TGFbeta or cholera toxin. Similarly, TGFbeta altered neither EGF-induced MAPK tyrosine phosphorylation nor activity. However, the calcium ionophore, A23187, significantly increased MAPK phosphorylation which was further increased in the presence of EGF, although calcium mobilization reduced EGF-induced proliferation. Despite the increase in phosphorylation, we could not demonstrate induction of MAPK activity by A23187. Like EGF, phorbol ester, under conditions which activate PKC isozymes in MEPM cells, increased MAPK phosphorylation and activity but was also growth inhibitory to MEPM cells. The MEK inhibitor, PD098059, only partially abrogated EGF-induced phosphorylation. Likewise, depletion of PKC isozymes partially abrogated EGF-induced MAPK phosphorylation. Inhibition of both MEK and PKC isozymes resulted in a marked decrease in MAPK activity, confirming that EGF uses multiple pathways to stimulate MAPK activity. These data indicate that the MAPK cascade does not mediate signal transduction of several agents that inhibit growth in MEPM cells, and that there is a dissociation of the proliferative response and MAP kinase activation. Furthermore, other signaling pathways known to play significant roles in differentiation of palatal tissue converge with the MAPK cascade and may use this pathway in the regulation of alternative cellular processes.

Transforming growth factor-beta1 stimulates protein kinase A in mesangial cells

We recently demonstrated that transforming growth factor-beta (TGF-beta) stimulates phosphorylation of the type I inositol 1,4, 5-trisphosphate receptor (Sharma, K., Wang, L., Zhu, Y., Bokkala, S., and Joseph, S. (1997) J. Biol. Chem. 272, 14617-14623), possibly via protein kinase A (PKA) activation in murine mesangial cells. In the present study, we evaluated whether TGF-beta stimulates PKA activation. Utilizing a specific PKA kinase assay, we found that TGF-beta increases PKA activity by 3-fold within 15 min of TGF-beta1 treatment, and the enhanced kinase activity was completely reversed by the inhibitory peptide for PKA (PKI; 1 microM). In mesangial cells transfected with a PKI expression vector, enhanced PKA activity could not be demonstrated with TGF-beta1 treatment. TGF-beta1 was also found to stimulate translocation of the alpha-catalytic subunit of PKA to the nucleus by Western analysis of nuclear protein as well as by confocal microscopy. TGF-beta1-mediated phosphorylation of cAMP response element-binding protein was completely reversed by H-89 (3 microM), a specific inhibitor of PKA. Stimulation of fibronectin mRNA by TGF-beta1 was also attenuated in cells overexpressing PKI. We thus conclude that TGF-beta stimulates the PKA signaling pathway in mesangial cells and that PKA activation contributes to TGF-beta stimulation of cAMP response element-binding protein phosphorylation and fibronectin expression.

The neuron-glia signal beta neuregulin induces sustained CREB phosphorylation on Ser-133 in cultured rat Schwann cells

beta neuregulins (also called NDF, GGF, ARIA, and heregulins) are neuron-derived molecules that are likely to be responsible for Schwann cell precursor survival, proliferation, and maturation in vivo and in vitro. Although the receptors to which beta neuregulins bind have been defined, little is known about the transcription factors these important ligands activate. Using antibodies, quantitative imaging methods and Western blotting, we show that beta neuregulin induces a high level of phosphorylation of the transcription factor cyclic AMP response element binding protein (CREB) on Ser-133 in cultured rat Schwann cells and that the phosphorylation is prolonged over several hours. In contrast, neurotrophins, CNTF, FGF-2, EGF, and TGF beta induce little or no phosphorylation of CREB despite the fact that receptors for these factors are present on Schwann cells. As expected CREB phosphorylation was detected following cAMP elevation, and it was also induced by elevation of cytoplasmic Ca2+, endothelin 1, and PDGF-BB. The signal was lower than that seen in response to beta neuregulin, and transient, unlike the sustained CREB activation induced by beta neuregulin. Our results suggest that the sustained phosphorylation of CREB on Ser-133 may contribute to the broad spectrum of effects that beta neuregulins have on cells of the Schwann cell lineage and that the CREB pathway may be important for transduction of neuregulin signals in Schwann cells.

Cross-talk between signaling pathways in murine embryonic palate cells: effect of TGF beta and cAMP on EGF-induced DNA synthesis

Signaling pathways utilized by EGF, cAMP, and TGF beta have been demonstrated to play critical roles in normal palate development. Stimulation of these pathways has been shown in palate cells and numerous other systems to affect cell growth. Because proper regulation of cell growth is critical to palate development, we speculate that fine regulation of palatal cell growth may be accomplished through crosstalk between these signaling pathways. We therefore set out to determine the effects of cAMP and TGF beta on EGF-induced cell proliferation in murine embryonic palate cells. We found that both TGF beta and cAMP inhibited the proliferative response of cells to treatment with EGF, whereas H89, a serine/ threonine protein kinase inhibitor with selectivity towards cAMP-dependent protein kinase, increased the cells' proliferative response to EGF. Genestein, a selective inhibitor of tyrosine kinases, at high doses abrogated the cells' proliferative response to EGF, confirming that EGF's ability to induce cell proliferation is critically dependent upon tyrosine kinase activity. Lower doses of genestein, however, actually enhanced cellular response to EGF. The data suggest that both the TGF beta- and cAMP-mediated signaling pathways may be involved in modulation of the effects of EGF on palate cell growth in vivo.