TGF-beta and phorbol esters inhibit mitogenesis utilizing parallel protein kinase C-dependent pathways

The transforming growth factor beta ligand TIG-2 modulates the function of neuromuscular junction and muscle energy metabolism in Caenorhabditis elegans

Deciphering the physiological function of TGF-β (the transforming growth factor beta) family ligands is import for understanding the role of TGF-β in animals' development and aging. Here, we investigate the function of TIG-2, one of the ligands in Caenorhabditis elegans TGF-β family, in animals' behavioral modulation. Our results show that a loss-of-function mutation in tig-2 gene result in slower locomotion speed in the early adulthood and an increased density of cholinergic synapses, but a decreased neurotransmitter release at neuromuscular junctions (NMJs). Further tissue-specific rescue results reveal that neuronal and intestinal TIG-2 are essential for the formation of cholinergic synapses at NMJs. Interestingly, tig-2(ok3416) mutant is characterized with reduced muscle mitochondria content and adenosine triphosphate (ATP) production, although the function of muscle acetylcholine receptors and the morphology muscle fibers in the mutant are comparable to that in wild-type animals. Our result suggests that TIG-2 from different neuron and intestine regulates worm locomotion by modulating synaptogenesis and neurotransmission at NMJs, as well as energy metabolism in postsynaptic muscle cells.

Protein kinase C delta plays a key role in cellular senescence programs of human normal diploid cells

In the present study, we clarified that transforming growth factor beta (TGF-beta) induces cellular senescence in human normal diploid cells, TIG-1, and identified protein kinase Cs (PKCs) as downstream mediators of TGF-beta-induced cellular senescence. Among PKCs, we showed that PKC-delta induced cellular senescence in TIG-1 cells and was activated in replicatively and prematurely senescent TIG-1 cells. The causative role of PKC-delta in cellular senescence programs was demonstrated using a kinase negative PKC-delta and small interfering RNA against PKC-delta. Furthermore, PKC-delta was shown to function in human telomerase reverse transcriptase (hTERT) gene repression. These results indicate that PKC-delta plays a key role in cellular senescence programs, and suggest that the induction of senescence and hTERT repression are coordinately regulated by PKC-delta.

Ligand-specific control of src-suppressed C kinase substrate gene expression

The src-suppressed C-kinase substrate, SSeCKS, is now recognized as a key regulator of cell signaling and cytoskeletal dynamics. However, few ligands that control SSeCKS expression have been identified. We report that platelet-derived growth factor-BB (PDGF-BB), lysophosphatidic acid (LPA), and eicosapentaenoic acid (EPA) potently modulate SSeCKS gene expression in cultured smooth muscle (RASM) cells relative to other bioactive ligands tested. In addition, EPA-dependent regulation of SSeCKS expression correlates with distinct changes in cell morphology and adhesion in RASM cells. Independent evidence that ligand-specific control of SSeCKS expression links to the regulation of cell adhesion and morphology was obtained using ras-transformed fibroblasts, KNRK. Sodium butyrate (NaB) upregulates SSeCKS mRNA and protein expression corresponding to increased cell-spreading and adhesion. In addition, ectopic expression of recombinant SSeCKS recapitulates attributes of NaB-induced morphogenesis in KNRK cells. The data provide novel evidence that SSeCKS functions in PDGF-BB-, LPA-, EPA-, and NaB-mediated cell signaling.

Extracellular superoxide dismutase and glomerular mesangial cells: its production and regulation

Extracellular superoxide dismutase (EC-SOD) is synthesized in mesenchymally derived cells and prevents the oxygen radical-induced injury. We studied whether kidney mesangial cells (MCs) produce EC-SOD and how its production is associated with chemokine secretion. Under unstimulated condition, MCs produced EC-SOD, and its production was correlated positively with cyclic adenosine monophosphate (cAMP), but negatively with interleukin (IL)-6 or IL-8 production. By prednisolone or phorbol myristate acetate treatment, EC-SOD levels were correlated negatively with levels of IL-6 and IL-8. The presence of adenylate cyclase inhibitor 2',3'-dideoxyadenosine lost the prednisolone effect. The stimulation of EC-SOD production might be one of the important effects of prednisolone via cAMP pathway in MCs.

Phorbol esters inhibit fibroblast growth factor-2-stimulated fibroblast proliferation by a p38 MAP kinase dependent pathway

Treatment of fibroblasts with the phorbol ester, 12-O-tetradecanoyl phorbol 13-acetate (TPA), specifically inhibits fibroblast growth factor-2 (FGF-2) induced proliferation. TPA treatment has little or no effect on FGF receptor activation but specifically inhibits the activation of p38 MAPK but not other downstream signaling pathways implicated in cell proliferation. p38 MAPK was recently shown to be required for the FGF-2-stimulated proliferation of fibroblasts. The effect of TPA on both p38 MAPK activation and cell proliferation can be reversed by treatment with the PKC inhibitor Go6983. The TPA-mediated inhibition of p38 MAPK activation requires phosphatase activity and is at least partially mediated by ERKs since it is reduced by treatment with the MEK inhibitor PD98059. In contrast, the FGF-2-stimulated differentiation of PC12 cells, which express the same FGF receptor as Swiss 3T3 fibroblasts, is not affected by TPA treatment, consistent with a lack of involvement of p38 MAPK activity in this process. These data indicate that the effects of TPA treatment on cellular function are not only cell type but also stimulus specific and are dependent upon the distinct pathways activated downstream of each stimulus.

Attenuation of matrix protein secretion by antisense oligodeoxynucleotides to the cyclin kinase inhibitor p21(Waf1/Cip1)

Progressive fibrosis in major organs, including the heart, the kidney and the vascular tree, plays an important role in mediating chronic disease and atherosclerosis. Production of extracellular matrix proteins, in many cases regulated by the growth factor TGF-beta is an essential component of this process. In a parallel manner to TGF-beta, the cyclin kinase inhibitors (CKIs; which are induced by TGF-beta) regulate transit through the cell cycle, and their effect on growth has been shown to be bimodal in the case of vascular smooth muscle (VSM) cells. Using an antisense oligodeoxynucleotide to the CKI p21(Waf1/Cip1), developed in our laboratory and shown to specifically inhibit p21(Waf1/Cip1) protein levels, we asked whether attenuation of the CKI p21(Waf1/Cip1) by transfection of this oligodeoxynucleotide results in the abolition of TGF-beta-mediated growth inhibition and/or diminished matrix protein production and secretion in the presence or absence of TGF-beta. Specific inhibition of p21(Waf1/Cip1) protein with the antisense oligodeoxynucleotide markedly reduces the production and secretion of the matrix proteins fibronectin and laminin, both in the presence and absence of TGF-beta stimulation, in VSM cells as observed by Western blotting of cell lysate and conditioned medium. In addition, TGF-beta-mediated cell growth inhibition, though attenuated by this oligo, is preserved. Due to the relative ease and safety of transfecting antisense oligodeoxynucleotides into VSM, we believe that this work unmasks a potentially powerful technique for inhibition of matrix protein synthesis in VSM and related cell lines, and may lead to new treatment strategies for atherosclerotic as well as other systemic diseases characterized by aberrant matrix protein secretion.

Transforming growth factor-beta 1 regulates Kir2.3 inward rectifier K+ channels via phospholipase C and protein kinase C-delta in reactive astrocytes from adult rat brain

The multifunctional cytokine, transforming growth factor beta(1) (TGF-beta(1)), exerts complex effects on astrocytes with early signaling events being less well characterized than transcriptional mechanisms. We examined the effect of TGF-beta(1) on the 14-pS Kir2.3 inward rectifier K(+) channel in rat primary cultured reactive astrocytes. Immunofluorescence study showed that cells co-expressed TGF-beta(1) receptors 1 and 2, Kir2.3, and glial fibrillary acidic protein (GFAP). Patch clamp study showed that TGF-beta(1) (0.1-100 ng/ml) caused a rapid (<5 min) depolarization because of dose-dependent down-regulation of Kir2.3 channels, which was mimicked by the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (10-500 nm) and which was inhibited by the PKC inhibitor calphostin C (100 nm), by PKC desensitization produced by 3 h of exposure to phorbol 12-myristate 13-acetate (100 nm), and by the PKC-delta isoform-specific inhibitor rottlerin (50 microm). Immunoblot analysis and confocal imaging showed that TGF-beta(1) caused PKC-delta translocation to membrane, and co-immunoprecipitation experiments showed that TGF-beta(1) enhanced association between Kir2.3 and PKC-delta. Additional electrophysiological experiments showed that Kir2.3 channel down-regulation was blocked by the phospholipase C inhibitors, neomycin (100 microm) and D609 (200 microm). Given the commonality of signaling involving PLC-PKC-delta, we speculate that TGF-beta(1)-evoked depolarization may be an early signaling event related to gene transcription in astrocytes.

Deranged transcriptional regulation of cell-volume-sensitive kinase hSGK in diabetic nephropathy

Transforming growth factor beta (TGF-beta) has been shown to participate in the pathophysiology of diabetic complications. As shown most recently, TGF-beta stimulates the expression of a distinct serine/threonine kinase (hSGK) which had previously been cloned as an early gene transcriptionally regulated by cell volume alterations. The present study was performed to elucidate transcription and function of hSGK in diabetic nephropathy. As shown by Northern blotting, an increase of extracellular glucose concentration increased hSGK mRNA levels in cultured cells, an effect qualitatively mimicked by osmotic cell shrinkage or treatment with TGF-beta (2 microgram/liter), phorbol 12,13-didecanoate (1 microM), or the Ca(2+) ionophore ionomycin (1 microM) and blunted by high concentrations of nifedipine (10 and 100 microM). In situ hybridization revealed that hSGK transcription was markedly enhanced in diabetic nephropathy, with particularly high expression in mesangial cells, interstitial cells, and cells in thick ascending limbs of Henle's loop and distal tubules. According to voltage clamp and tracer flux studies in Xenopus oocytes expressing the renal epithelial Na(+) channel ENaC or the mouse thick ascending limb Na(+),K(+),2Cl(-) cotransporter BSC-1, coexpression with hSGK stimulated ENaC and BSC-1 11-fold and 6-fold, respectively, effects reversed by kinase inhibitors staurosporine (1 microM) and chelerythrine (1 microM) and not elicited by inactive hSGK. In conclusion, excessive extracellular glucose concentrations enhance hSGK transcription, which in turn stimulates renal tubular Na(+) transport. These observations disclose an additional element in the pathophysiology of diabetic nephropathy.

Transforming growth factor-beta(1) regulation of surfactant protein B gene expression is mediated by protein kinase-dependent intracellular translocation of thyroid transcription factor-1 and hepatocyte nuclear factor 3

The transforming growth factor-beta (TGF beta) polypeptides control a variety of cellular processes including organogenesis and cellular proliferation and differentiation. In the developing lung, TGF beta(1) treatment inhibits airway branching and expression of the genes for surfactant proteins (SP). Many effects of TGF beta are mediated at the level of gene transcription but there is limited information regarding signaling pathways and target transcription factors. In this study with human pulmonary adenocarcinoma H441 cells, we investigated TGF beta(1) effects on SP-B, a protein which is essential for normal function of pulmonary surfactant. TGF beta(1) (10 ng/ml) reduced SP-B mRNA content in a time-dependent fashion, and transient transfection studies localized responsiveness to the region of the SP-B promoter (-112/-72 bp) containing binding sites for thyroid transcription factor-1 (TTF-1) and hepatocyte nuclear factor 3 (HNF3), transcription factors that are important enhancers of SP gene expression. Using electrophoretic mobility shift assay and immunofluorescence, we demonstrated rapid accumulation of these transcription factors in the cytoplasm and subsequent loss from the nucleus on TGF beta(1) treatment of both adenocarcinoma cells and cultured human fetal lung. TGF beta(1) treatment caused intracellular translocation of protein kinase C and effects of TGF beta(1) were mostly abrogated in the presence of the protein kinase inhibitor calphostin C. We conclude that TGF beta(1), acting via protein phosphorylation, blocks nuclear translocation of TTF-1 and HNF3 which results in down-regulation of the SP-B gene and presumably other pulmonary genes which are transactivated by these factors.

TGF-beta signaling in A549 lung carcinoma cells: lipid second messengers

Transforming growth factor-beta (TGF-beta) is a potent inducer of numerous extracellular matrix components, largely through a transcriptional mechanism. To define the postreceptor signaling pathways used by TGF-beta in the induction of extracellular matrix gene expression, we have utilized the human lung carcinoma cell line, A549, in transfection experiments with the TGF-beta inducible reporter construct, p3TP-Lux. Previous work from this laboratory using pharmacologic agents suggested that a phosphatidylcholine-specific phospholipase C and protein kinase C may be involved in early aspects of TGF-beta signaling. Here we provide evidence that TGF-beta induces a rapid and transient increase in diacylglycerol (DAG) production. When cells transfected with the p3TP-Lux reporter plasmid are simultaneously treated with TGF-beta and a DAG kinase inhibitor, we observed a higher level of luciferase than with TGF-beta alone. We also find elevated levels of phosphocholine in cells following TGF-beta treatment. Further, exogenously added bacterial phosphatidylcholine phospholipase C (PC-PLC) is capable of inducing expression of the p3TP-Lux reporter to the same extent as TGF-beta indicating that the bacterial PC-PLC can mimic the TGF-beta effect. In contrast, neither hexanoyl sphingosine (a ceramide analogue) nor arachadonic acid induce expression of the p3TP-Lux reporter. Measurements with the fluorescent, calcium-sensitive dye, FURA2, indicated that there was no change in intracellular calcium in response to TGF-beta. Furthermore, buffering intracellular calcium with the calcium chelating agent BAPTA/AM failed to block TGF-beta induction of the p3TP-Lux reporter. Thus the TGF-beta signaling pathway appears to involve the production of diacylglycerol but is independent of calcium.

Transforming growth factor-beta1 modulates angiotensin II-induced calcium release in vascular smooth muscle cells from spontaneously hypertensive rats

OBJECTIVES

To investigate the role of transforming growth factor-beta1 (TGF-beta1) on Ca2+-dependent mechanisms elicited by angiotensin II in aortic vascular smooth muscle cells (VSMC) of Wistar- Kyoto (WKY) rats and spontaneously hypertensive rats (SHR).

METHODS

Cai2+ release induced by angiotensin II (1 micromol/ l) was studied in cultured VSMC isolated from the aortas of 6-week-old WKY rats and SHR. Intracellular Ca2+ (Cai2+) was assessed in Fura-2 loaded cells using fluorescent imaging microscopy. Angiotensin II receptors were analysed by binding studies.

RESULTS

Pretreatment of VSMC for 24 h with TGF-beta1 significantly increased angiotensin II-induced Cai2+ mobilization from internal stores in SHR, while Ca2+ influx was not altered. This effect involves tyrosine kinase and is not due to an increase in angiotensin II binding sites, or a change in the affinity of the receptors. By contrast, TGF-beta1 did not modify the response of VSMC from WKY rats to angiotensin II.

CONCLUSIONS

These results help our understanding of the interactions between the pathways activated by TGF-beta1 and the G protein-coupled receptor signalling pathway, and their role in genetic hypertension.

LDL stimulates collagen mRNA synthesis in mesangial cells through induction of PKC and TGF-beta expression

Abnormal lipid accumulation in glomeruli could be implicated in the pathogenesis of glomerulosclerosis. Low-density lipoprotein (LDL) stimulates collagen mRNA expression in cultured human mesangial cells (HMC). To explore the possible molecular mechanisms by which LDL promotes collagen gene expression, we examined the effects of LDL on protein kinase C (PKC) activity and transforming growth factor-beta (TGF-beta) expression in relation to collagen gene regulation in HMC. LDL (200 microg/ml) induced an acute increase in PKC activity, particularly PKC-alpha and -delta, within 15 min, which decreased to control value at 2 h. LDL stimulated TGF-beta1, and alpha1(I) and alpha1(IV) collagen mRNA expression within 30 min of incubation with HMC, and levels remained elevated until hour 4. LDL induced the secretion of TGF-beta by HMC. This TGF-beta was shown by CCL-64 mink lung cell assay to be, in part, bioactive. The stimulatory effects of LDL on collagen gene regulation in HMC were blocked by the inhibition of PKC using GF-109203X (GFX) or the downregulation of PKC using phorbol myristate acetate. Neutralizing antibody to TGF-beta inhibited the increased collagen mRNA expression by HMC exposed to LDL. The downregulation or inhibition of PKC did not affect the stimulatory effect of LDL on TGF-beta mRNA or protein expression. These results suggest that in HMC, LDL stimulates collagen mRNA expression through the rapid activation of PKC-alpha and -delta and transcriptional upregulation of TGF-beta. Thus PKC and TGF-beta may function as independent key signaling intermediaries in the pathway by which LDL upregulates collagen gene expression in HMC.

The in vitro effect of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate on Sertoli cell morphology

The objective of the present study was to examine the effects of the well-known tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) on the morphology of cultured Sertoli cells from immature rats. The cells were cultured for 5 days and the TPA was added at the end of the culture period for 8 h at a concentration of 10-7 M. Viability tests showed that controls as well as TPA-treated cells remained viable during the culture period and no deleterious effects were observed as a result. Application of computerized morphometry at both light and electron microscopic levels revealed that TPA caused important changes in cell morphology in vitro. Statistical analysis of the results indicated that compared to the controls, Sertoli cells treated with TPA exhibited fewer astrocytic-type cytoplasmic extensions and a smaller size. Our results support the conclusion that the tumor promoter TPA, when applied to immature Sertoli cells in vitro, causes significant morphological alterations.

Transforming growth factor-beta 1 and ascorbate regulate proliferation of cultured smooth muscle cells by independent mechanisms

We previously reported that ascorbate (vitamin C) can regulate the growth of cultured vascular smooth muscle cells (VSMC) directly as well as by altering the properties of extracellular matrix (ECM) [Mol Cell Cardiol 1997;29:3293-303]. In the present study we compared the effects of ascorbate and transforming growth factor-beta 1 (TGF-beta1) on VSMC growth in order to determine whether their actions were mediated by similar mechanisms. When VSMC proliferation was stimulated by fetal bovine serum, the addition of TGF-beta1 (20 ng/ml) or ascorbate (1 mM) to the cell culture medium inhibited the cellular incorporation of [3H]thymidine by 19 and 59%, respectively, and by 85% when added together. The cell growth inhibitory effects of TGF-beta1 and ascorbate were partially mediated by changing the growth-regulatory properties of the ECM produced by the cells. Thus, VSMC grew more slowly on ECM deposited by VSMC under treatment with 20 ng/ml TGF-beta1 or 1 mM ascorbate (52 and 46% inhibition, respectively) than on control ECM, and their combination had an additional inhibitory effect (84%). Anti-TGF-beta1 neutralizing antibodies prevented the direct and ECM-mediated effects of TGF-beta1 on VSMC growth, but did not alter the effects of ascorbate. When ECM was pre-incubated with increasing concentrations of TGF-beta1, the growth rate of freshly plated VSMC gradually decreased, indicating that ECM-bound TGF-beta1 retained its biological activity. Comparison of the patterns of TGF-betal binding to ECM produced by VSMC in the presence or absence of ascorbate revealed no significant differences. Extraction of ECM-bound TGF-beta1 by incubation of exposed ECM with plasmin did not affect the ECM-mediated inhibitory effect of ascorbate, as the rate of proliferation of secondary VSMC cultures grown on ascorbate-dependent and independent matrices treated with plasmin were equally increased. These results suggest that the amount of ECM-bound TGF-beta1 was not altered by ascorbate. The secretion of TGF-beta1 into the cell culture medium by VSMC also did not depend on the ascorbate supply. Finally, addition of heparin to the VSMC culture medium during ECM production abolished the ECM-mediated growth inhibitory effects of ascorbate, but did not affect the action of TGF-beta1. Our data demonstrate that the growth inhibitory effects of ascorbate on cultured VSMC are independent of the action of TGF-beta1, and the effects of these two compounds on VSMC growth are additive.

Identification of early growth response gene-1 (Egr-1) as a phorbol myristate acetate-induced gene in lung cancer cells by differential mRNA display

Cellular regulatory genes including transcription factors may play an important role in the induction, maintenance, and progression of lung cancer. These regulatory genes are inducible by various mitogenic stimuli including phorbol myristate acetate (PMA). The differential mRNA display method was used to identify potential early response genes regulated by PMA in non-small cell lung cancer (NSCLC) cell lines. Using this technique, several cDNA fragments were found to be potentially differentially regulated by PMA in the squamous NSCLC cell line NCI-H157. One of these cDNA fragments of approximately 100 bp was determined to be differentially induced by at least 30-fold by PMA by northern blot analysis and to hybridize to a single 3.4 kb mRNA species. This cDNA fragment was cloned, sequenced, and identified to be identical to a portion of the 3'-untranslated region of the human early growth response gene-1 (Egr-1). Using Egr-1 cDNA as a probe, it was demonstrated that PMA induces Egr-1 mRNA expression in at least three other NSCLC cells as well. In addition, PMA caused a transient increase in expression of the Egr-1 transcript reaching a maximum level by 1 h before decreasing in NCI-H157 and three other types of NSCLC cells. Treatment of these NSCLC cells with TGF-beta1 showed a transient increase in Egr-1 mRNA similar to PMA which also reached a maximum level after 1 h. Normal human bronchial epithelial (NHBE) cells also showed a rapid, transient increase in expression of Egr-1 mRNA after treatment with PMA. In contrast, treatment of NHBE cells with TGF-beta1 showed that expression of Egr-1 mRNA increased by 1 h but reached a maximum level only after 6 h. These results indicate that both PMA and TGF-beta1 can induce Egr- mRNA expression in NSCLC cells and NHBE cells; however, while PMA induces Egr-1 mRNA similarly in both cell types, TGF-beta1 induces Egr-1 mRNA expression more rapidly and more transiently in NSCLC cells than in NHBE cells. Our results suggest that Egr-1 may play different roles in response to mitogens in normal and malignant lung cells.

Transforming growth factor-beta1 inhibits membrane association of protein kinase C alpha in a human prostate cancer cell line, PC3

The postreceptor signaling pathway(s) that mediates the effects of transforming growth factor-beta1 (TGF-beta1) is incompletely understood. The present study investigated the involvement of protein kinase C (PKC) in the growth-inhibitory action of TGF-beta1 in PC3, a human prostate cancer cell line. PKC alpha, the only conventional PKC isoform detected in PC3 cells, appeared to be constitutively active based on its presence in both Triton-soluble membrane fraction and cytosol. However, levels of membrane-associated PKC alpha were decreased by a growth-inhibitory dose of TGF-beta1. The response to TGF-beta1 was rapid (within 5 min), time dependent, isoform specific, and occurred without apparent changes in levels of total PKC alpha protein. TGF-beta1 also decreased the levels of membrane-associated PKC activity coincident with its inhibitory effect on PKC alpha's membrane association. Inhibition of PKC activity appeared to be associated with growth inhibition in PC3 cells, because chelerythrine (a specific PKC inhibitor) likewise decreased cell proliferation. Taken together, our data suggest that inhibition of PKC activity, at least in part due to inactivation of PKC alpha, is an early event associated with TGF-beta1 postreceptor signaling that might mediate suppression of cell proliferation.

Protein kinase C is not necessary for transforming growth factor beta-induced growth-arrest in leukemia cell lines

Growth and differentiation of blood cell precursors are regulated by cytokines and hormones by mechanisms which are incompletely understood. Protein kinase C (PKC) isozymes are widely regarded as being important in signal transduction pathways. We have shown that one isozyme, PKC beta, is uniquely important in mediating phorbol ester-induced growth-arrest in the HL-60 myeloid cell line. 1,25-dihydroxyvitamin D3 induces differentiation and growth-arrest in many cells. It upregulates the expression of PKC beta, potentiating the action of phorbol ester. We tested the hypotheses that cytokines, which arrest the growth of hematopoietic cells, do so by activating PKC beta, and that differentiation and growth-arrest induced by 1,25-dihydroxyvitamin D3 is caused by upregulation of PKC beta isozyme gene expression. The influence on growth of combinations of five cytokines (TNF alpha, TGF beta 1, gamma-IFN, IL-1, and G-CSF) and 1,25-dihydroxyvitamin D3 on ten human leukemia cell lines (THP-1, HL-60 S, HL-60 PET, U937, K562, Jurkat, MOLT-4, RPM1 8402, KG-1, and KG-1a) was determined. Four cell lines (THP-1, HL-60 S and PET, and U937) exhibited total growth-arrest when incubated with 1,25-dihydroxyvitamin D3 followed by TGF beta 1. The expression by each cell line of mRNA encoding PKC alpha, beta, and delta, both before and after 24 or 48 h of incubation with 1,25-dihydroxyvitamin D3, was determined. Cell lines sensitive to TGF beta 1 each expressed PKC delta endogenously, or expression was up-regulated with 1,25-dihydroxyvitamin D3. U937 cells underexpressed PKC alpha, and HL-60 PET cells underexpressed PKC beta. These data suggested that PKC delta could be responsible for mediating growth-arrest by TGF beta 1. To test this hypothesis directly, we incubated the cells with two bisindolylmaleimide PKC inhibitors during the addition of 1,25-dihydroxyvitamin D3 and TGF beta 1. Surprisingly, the PKC inhibitors did not block the growth-arrest induced by 1,25-dihydroxyvitamin D3 and TGF beta 1. This experiment strongly suggests that neither growth-arrest induced by TGF beta 1 nor the potentiation of this growth-arrest by 1,25-dihydroxyvitamin D3 is mediated by a PKC isozyme which is inhibitable by the bisindolymaleimides.

TGFbeta1 regulates gene expression of its own converting enzyme furin

TGFbeta1 is known for its potent and diverse biological effects, including immune regulation, and cell growth and differentiation. We have recently shown that TGFbeta1 precursor is processed by human furin COOH-terminal to the R-H-R-R278 cleavage site to generate authentic mature TGFbeta1. In the present study, we demonstrate that steady-state furin mRNA levels are increased in rat synovial cells by 2 and 20 ng/ml TGFbeta1. Stimulation with TGFbeta1 results in a significant increase in furin mRNA levels, starting at 3 h with the peak effect observed at 12 h (2.5-fold increase +/-0.4). TGFbeta1 did not increase furin mRNA stability, and treatment of synovial cells with actinomycin D, before TGFbeta1 addition prevented the increase in fur gene expression, suggesting that the observed regulation occurs at the level of gene transcription. Treatment of synovial and NRK-49F fibroblastic cells with exogenous TGFbeta1 (5 ng/ml) or TGFbeta2 (10 ng/ml) translates into an increase in pro-TGFbeta1 processing as evidenced by the appearance of a 40-kD immunoreactive band corresponding to the TGFbeta1 NH2-terminal pro-region. Furin processing activity stimulated by TGFbeta2 correlates with significant increase in extracellular mature and heat-activable TGFbeta1 as determined by an isoform-specific ELISA assay. Taken together, these results demonstrate for the first time that TGFbeta1 upregulates gene expression of its own converting enzyme, and that this expression is translated into augmented processing of the TGFbeta1 precursor form. Such adaptive responsiveness of the TGFbeta1 convertase may represent an important aspect of TGFbeta1 bioavailibility in TGFbeta1-related processes and pathological conditions.

TGF-beta1-induced cell cycle arrest in renal mesangial cells involves inhibition of cyclin E-cdk 2 activation and retinoblastoma protein phosphorylation

In glomerular disease, transforming growth factor-beta1 (TGF-beta1) has been demonstrated to exert anti-mitogenic and anti-inflammatory as well as fibrogenic effects. To better understand the TGF-beta1 action on glomerular cells at the molecular level, we investigated mechanisms of TGF-beta1-induced growth suppression in primary cultures of rat mesangial cells (MCs). TGF-beta1 (5 ng/ml) markedly inhibited proliferation of MCs incubated with PDGF, endothelin-1, bFGF, serotonin, or EGF, indicating that TGF-beta1 interferes with post-receptor signals of mitogenesis. TGF-beta1 did not affect mitogen-stimulated induction of the immediate early genes, c-fos, c-jun, and Egr-1 in MCs that occurred transiently at 30 to 120 minutes. Time-course studies revealed that TGF-beta1 inhibited DNA synthesis and MC replication when added up to six to eight hours after MC stimulation with PDGF. FACS analysis demonstrated that MCs had reached middle to late G1 phase of cell cycle progression at this timepoint. PDGF stimulation of MCs induced protein expression of the G1 phase cyclin D1 as well as the cyclin-dependent kinases cdk 4 and cdk 2. This was not significantly altered when MCs were coincubated with both, PDGF and TGF-beta1. However, TGF-beta1 prevented PDGF-elicited phosphorylation of the retinoblastoma tumor suppressor (pRb), a negative cell cycle regulator. Moreover, TGF-beta1 significantly reduced cyclin E-associated histone H1 kinase activity in the presence of PDGF. These results indicate that TGF-beta1 inhibits mitogen-stimulated MC growth by causing cell cycle arrest in late G1 phase. While TGF-beta1 does not alter the mitogen-induced expression and abundance of G1 phase cyclin D1 and cdks 4 and 2 in MCs, it inhibits cyclin E-cdk 2 activity, thus preventing mitogen-elicited phosphorylation and inactivation of pRb in G1 phase and transition to S phase.

Dominant-negative SMAD-3 interferes with transcriptional activation by multiple agonists

Smad proteins have recently been identified as mediators of transcriptional activation by members of the transforming growth factor-beta superfamily. To determine if Smads might also be involved in inducing gene transcription in response to other agonists, expression vectors for dominant-negative Smad proteins were constructed. These plasmids were transiently cotransfected with luciferase reporter genes and the effects of various agonists on reporter gene activity evaluated in NIH 3T3 cells. Dominant-negative Smad3, but not other dominant-negative Smads, reduced stimulation of the plasminogen activator inhibitor-1 (PAI-1) and other gene promoters by phorbol ester, cAMP, and platelet-derived growth factor. Activation of the PAI-1 promoter by TGF-beta or prostaglandin F2 alpha, and transactivation by c-Jun or JunB were not inhibited by dominant-negative Smad3, supporting the specificity of this mutant. These results suggest that Smad3, like CREB-binding protein (CBP), may participate in transcriptional activation by multiple agonists.

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.

Mitogenic inhibition by phorbol esters is associated with decreased phosphatidylinositol-3 kinase activation

In contrast to their role as potent tumor promoters, phorbol esters can cause inhibition of cell growth. Because the effect of phorbol esters occurs through activation of protein kinase C (PKC) and because activated PKC is translocated to the membrane placing it in a position to act on the intracellular portion of the growth factor receptor, we asked whether this inhibitory effect is mediated through the action of phorbol 12-myristate 13-acetate (PMA) on receptor association with the signal transfer proteins. When added to rat vascular smooth muscle (VSM) cells concurrently with basic fibroblast growth factor (bFGF), PMA at 100 ng/ml completely inhibits bFGF-stimulated DNA synthesis. Under the same growth-inhibitory conditions of PMA addition, aggregation of phosphatidylinositol 3-kinase (PI3K) to the fibroblast growth factor receptor and tyrosine phosphorylation of the 85-kDa regulatory component of the signal transfer protein PI3K are reduced by 94 and 79%, respectively. PI3K catalytic activity, as measured by conversion of phosphatidylinositol to phosphatidylinositol 3-phosphate, is decreased 88% by PMA addition. This effect is not specific to PI3K, since aggregation of phospholipase C-gamma 1 to the activated bFGF receptor is also decreased by PMA treatment. In addition, the PI3K inhibitor wortmannin markedly attenuates bFGF-stimulated VSM cell growth in a dose-dependent manner. These data suggest that the site of growth inhibition by PMA in VSM cells lies upstream of signal transfer particle aggregation and that such growth arrest may be mediated through inhibition of activation of PI3K.