The MEK pathway is required for stimulation of p21(WAF1/CIP1) by transforming growth factor-beta

15-deoxy-Delta12,14 prostaglandin J2 up-regulates Kruppel-like factor 4 expression independently of peroxisome proliferator-activated receptor gamma by activating the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase signal transduction pathway in HT-29 colon cancer cells

15-Deoxy-Delta(12,14) prostaglandin J2 (15d-PGJ2) is a natural ligand for the peroxisome proliferator-activated receptor gamma (PPARgamma) that exhibits antiproliferative activity in colon cancer cells, but its mechanism of action is still poorly understood. In this study, we showed that Krüppel-like factor 4 (KLF4) is one of the downstream effectors of 15d-PGJ2. Treatment of HT-29 cells with 15d-PGJ2 resulted in up-regulation of both KLF4 mRNA and protein expression, and these increases were also observed in other colon cancer cell lines. Down-regulation of KLF4 expression by small interfering RNA (siRNA) targeting KLF4 reduced 15d-PGJ2-mediated G1 phase arrest, suggesting that KLF4-mediated function of 15d-PGJ2. The effect of 15d-PGJ2 on KLF4 expression seems not to involve its nuclear receptor PPARgamma, in that our data show that:1) KLF4 gene promoter does not contain putative PPRE sequence, 2) 15d-PGJ2 rapidly activates extracellular signal-regulated kinase (ERK) and induces KLF4 mRNA expression, 3) KLF4 is induced by 15d-PGJ2 but not by rosiglitazone, a synthetic PPARgamma ligand, and 4) 15d-PGJ2 is unable to stimulate PPAR-dependent promoter activity in the absence of cotransfected PPARgamma. Moreover, 15d-PGJ2-mediated KLF4 mRNA expression was blocked by 2'-amino-3'-methoxyflavone (PD98059) or 1,4-diamino-2,3-dicyano-1,4-bis(methylthio)butadiene (U0126), two ERK kinase MAP inhibitors, whereas the phosphoinositol-3 kinase inhibitors wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) had no such effect. Furthermore, KLF4 induction by 15d-PGJ2 occurred only in signal transducer and activator of transcription 1 (STAT1)-expressing, not in STAT1-knockout cells. Together, these results suggest that 15d-PGJ2-induced growth inhibition of colon cancer cells is mediated, at least in part, through up-regulation of KLF4 expression. This induction is unlikely to be mediated through the PPARgamma receptor but may involve the mitogen-activated protein kinase kinase/ERK pathway and is STAT1-dependent.

Ras/ERK signaling pathway mediates activation of the p21WAF1 gene promoter in vascular smooth muscle cells by platelet-derived growth factor

We previously demonstrated that platelet-derived growth factor (PDGF) induces the cyclin-dependent kinase inhibitor p21/WAF1 promoter in vascular smooth muscle cells (VSMC) via activation of a Sp1 site in VSMC. In this report, the role and relevance of the signaling pathway in the transcriptional regulation of p21WAF1 in VSMC was examined. PDGF stimulated the expression of p21WAF1 in VSMC, as evidenced by Immunoblot and Northern blot analyses. Treatment with PD98059, a specific MEK inhibitor, and the transient expression of VSMC with DN-MEK1 plasmid effectively down-regulated PDGF-induced p21WAF1 expression and promoter activity, respectively. Furthermore, the transactivation of PDGF-stimulated Sp1 was inhibited by treatment with PD98059 and the transient expression of VSMC with the DN-MEK1 plasmid. Finally, the transient transfection of VSMC with a dominant negative Ras (RasN17) suppressed PDGF-induced ERK activity, p21WAF1 expression, and promoter activity. The overexpression of RasN17 also abolished PDGF-stimulated Sp1 activity. In conclusion, the findings herein presented indicate that the activation of the Ras/ERK pathway contributes to the induction of p21WAF1 expression in VSMC. In addition, the transcription factor Sp1 that is involved in the Ras/ERK-mediated control of p21WAF1 regulation in VSMC in response to PDGF has now been identified.

Crosstalk mechanisms between the mitogen-activated protein kinase pathways and Smad signaling downstream of TGF-beta: implications for carcinogenesis

Transforming growth factor-beta (TGF-beta) superfamily members signal via membrane-bound heteromeric serine-threonine kinase receptor complexes. Upon ligand-binding, receptor activation leads to phosphorylation of cytoplasmic protein substrates of the Smad family. Following phosphorylation and oligomerization, the latter move into the nucleus to act as transcription factors to regulate target gene expression. TGF-beta responses are not solely the result of the activation Smad cascade, but are highly cell-type specific and dependent upon interactions of Smad signaling with a variety of other intracellular signaling mechanisms, initiated or not by TGF-beta itself, that may either potentiate, synergize, or antagonize, the rather linear TGF-beta/Smad pathway. These include, (a), regulation of Smad activity by mitogen-activated protein kinases (MAPKs), (b), nuclear interaction of activated Smads with transcriptional cofactors, whether coactivators or corepressors, that may be themselves be regulated by diverse signaling mechanisms, and (c), negative feedback loops exerted by inhibitory Smads, transcriptional targets of the Smad cascade. This review focuses on how MAPKs modulate the outcome of Smad activation by TGF-beta, and how cross-signaling mechanisms between the Smad and MAPK pathways may take place and affect cell fate in the context of carcinogenesis.

Non-Smad TGF-β signals

During the past 10 years, it has been firmly established that Smad pathways are central mediators of signals from the receptors for transforming growth factor β (TGF-β) superfamily members to the nucleus. However, growing biochemical and developmental evidence supports the notion that alternative, non-Smad pathways also participate in TGF-β signalling. Non-Smad signalling proteins have three general mechanisms by which they contribute to physiological responses to TGF-β: (1) non-Smad signalling pathways directly modify (e.g. phosphorylate) the Smads and thus modulate the activity of the central effectors; (2) Smads directly interact and modulate the activity of other signalling proteins (e.g. kinases), thus transmitting signals to other pathways; and (3) the TGF-β receptors directly interact with or phosphorylate non-Smad proteins, thus initiating parallel signalling that cooperates with the Smad pathway in eliciting physiological responses. Thus, non-Smad signal transducers under the control of TGF-β provide quantitative regulation of the signalling pathway, and serve as nodes for crosstalk with other major signalling pathways, such as tyrosine kinase, G-protein-coupled or cytokine receptors.

Induction of p21WAF1 expression protects HT29 colon cancer cells from apoptosis induced by cryoinjury

BACKGROUND

Cryotherapy is a method of in situ destruction of tumors by freeze/thaw mechanisms. Cancer cells located in the peripheral zone of the tumor undergoing cryotherapy can die by apoptosis. We hypothesized that p21WAF1 is involved in the mediation of cryotherapy-induced apoptosis.

METHODS

HT29 cells grown on a plate were subjected to -10 degrees C and returned to 37 degrees C for various periods of time. Cells were analyzed by flow cytometry, Western blot, and reverse transcriptase-polymerase chain reaction for determining cell-cycle distribution, p21WAF1 protein expression, and messenger RNA levels, respectively. The p21WAF1 expression in nude mouse tumor xenografts after cryotherapy was examined by immunofluorescence staining. A series of the p21WAF1 promoter cloned into a luciferase reporter vector were transfected into HT29 cells for identifying the response element to cryoinjury. Antisense oligodeoxynucleotide (ODN) was applied to examine the effect of p21WAF1 expression on cryotherapy-induced apoptosis.

RESULTS

Both protein and messenger RNA of p21WAF1 were induced by cryoinjury in cultured cells and tumor xenografts. Deletion analysis of the p21WAF1 promoter revealed that a region from -121 to -95 base pairs was responsible for the activation and that this activation was p53 independent. HT29 cells arrested at the G1 phase after cryoinjury. The cryotherapy-induced apoptotic rate in HT29 cells was increased in the presence of antisense p21WAF1 ODN in comparison to the random ODN.

CONCLUSIONS

Induction of p21WAF1 increases tumor cell survival and may result in recurrences at treated sites after cryotherapy. Combining antisense ODN targeted against p21WAF1 and cryotherapy may improve clinical outcomes in the treatment of colorectal cancer.

Wnt-7a Up-regulates Matrix Metalloproteinase-12 Expression and Promotes Cell Proliferation in Corneal Epithelial Cells during Wound Healing

Corneal wound repair involves the rapid coverage of a denuded area by residual epithelial cells. During wound healing, there are different cell behaviors in different regions of the epithelium: cell proliferation in the peripheral epithelium and cell migration in the central epithelium. We found that Wnt-7a was rapidly induced in the wounded cornea, promoted the proliferation of corneal epithelial cells, and enhanced wound closure. Matrix metalloproteinase-12 (MMP-12) was detected in the peripheral epithelium, where cell proliferation was enhanced, but was diminished in the migrating central epithelium. Wnt-7a induced the accumulation of beta-catenin and the activation of Rac and beta-catenin, and Rac synergistically induced the transcription of MMP-12. Blocking the function of MMP-12 delayed wound closure induced by Wnt-7a. Our results also suggest that, in addition to the beta-catenin pathway, Wnt-7a might induce a beta-catenin-independent pathway. By regulating the proliferation of corneal epithelial cells, Wnt-7a and MMP-12 appear to contribute to corneal wound healing.

Farnesyl transferase inhibitor (R115777)-induced inhibition of STAT3(Tyr705) phosphorylation in human pancreatic cancer cell lines require extracellular signal-regulated kinases

In this study, we report that R115777, a nonpeptidomimetic farnesyl transferase inhibitor, suppresses the growth of human pancreatic adenocarcinoma cell lines and that this growth inhibition is associated with modulation in the phosphorylation levels of signal transducers and activators of transcription 3 (STAT3) and extracellular signal-regulated kinases (ERK). Treatment of cells with R115777 inhibited the tyrosine phosphorylation of STAT3((Tyr705)), while increasing the serine phosphorylation of STAT3((Ser727)). We found the differential phosphorylation of STAT3 was due to an increased and prolonged activation of ERKs. The biological significance of ERK-mediated inhibition of STAT3((Tyr705)) phosphorylation was further assessed by treating the cells with an inhibitor (PD98059) of mitogen-activated protein kinase kinase (MEK) or by transfecting the cells with a vector that expresses constitutively active MEK-1. Expression of constitutively active MEK-1 caused an increase of ERK activity and inhibited STAT3((Tyr705)) phosphorylation. Conversely, inhibition of ERK activity by PD98059 reversed the R115777-induced inhibition of STAT3((Tyr705)) phosphorylation. R115777 also caused the inhibition of the binding of STAT3 to its consensus binding element. An increase in the activation of ERKs either by overexpressing MEK-1 or treatment of cells with R115777 caused an up-regulation in the levels of a cyclin-dependent kinase (cdk) inhibitor, p21(cip1/waf1). These observations suggest that R115777-induced growth inhibition is partly due to the prolonged activation of ERKs that mediates an inhibition of STAT3((Tyr705)) phosphorylation and an increase in the levels of p21(cip1/waf1) in human pancreatic adenocarcinoma cell lines.

Mechanism of sulforaphane-induced cell cycle arrest and apoptosis in human colon cancer cells

Sulforaphane (SFN) is a natural micronutrient found in cruciferous vegetables that has been shown to possess antitumoral properties in carcinogen-treated rats. In vitro, SFN regulates phase II enzymes, cell cycle, and apoptosis. In the present study, we investigated the relationship between SFN induction of apoptosis and cell cycle arrest in HT29 human colon carcinoma cells. In previously published data, a significant increase in the G2/M phase of the cell cycle has been observed in SFN-treated cells that was associated with increased cyclin B1 protein levels. In the present study, our results show that SFN induced p21 expression. Moreover, preincubation of HT29 cells with roscovitine, a specific cdc2 kinase inhibitor, blocked the G2/M phase accumulation of HT29 cells treated with SFN and abolished its apoptotic effect (22.2 +/- 4 of floating cells in SFN-treated cells vs. 6.55 +/- 2 in cells treated with both SFN and roscovitine). These results suggest that the cdc2 kinase could be a key target for SFN in the regulation of G2/M block and apoptosis. Moreover, in SFN-treated cells the retinoblastoma tumor suppressor protein (Rb) is highly phosphorylated. Inhibition of the cdc2 kinase by roscovitine did not change the phosphorylation status of Rb in SFN-treated cells, suggesting that this cyclin-dependent kinase may not be involved. In our study, we did not observe any significant change in the proteasomal activity between control and SFN-treated cells. Moreover, inhibition of proteasomal activity through the use of MG132 diminished SFN-induced HT29 cell death, suggesting that the apoptotic effect of SFN requires a functional proteasome-dependent degradation system. In summary, we have elucidated part of the mechanism of action of SFN in the concomitant regulation of intestinal cell growth and death.

Role of transforming growth factor Beta in human cancer

Transforming growth factor beta (TGF-beta) is a ubiquitous and essential regulator of cellular and physiologic processes including proliferation, differentiation, migration, cell survival, angiogenesis, and immunosurveillance. Alterations in the TGF-beta signaling pathway, including mutation or deletion of members of the signaling pathway and resistance to TGF-beta-mediated inhibition of proliferation are frequently observed in human cancers. Although these alterations define a tumor suppressor role for the TGF-beta pathway in human cancer, TGF-beta also mediates tumor-promoting effects, either through differential effects on tumor and stromal cells or through a fundamental alteration in the TGF-beta responsiveness of the tumor cells themselves. TGF-beta and members of the TGF-beta signaling pathway are being evaluated as prognostic or predictive markers for cancer patients. Ongoing advances in understanding the TGF-beta signaling pathway will enable targeting of this pathway for the chemoprevention and treatment of human cancers.

Epigallocatechin-3-gallate causes the p21/WAF1-mediated G1-phase arrest of cell cycle and inhibits matrix metalloproteinase-9 expression in TNF-α-induced vascular smooth muscle cells

It has been suggested that epigallocatechin-3-gallate (EGCG), a major catechin found in green tea, plays a role in preventing the progression of atherosclerosis. Although EGCG has anti-atherogenic effects on vascular smooth muscle cells (VSMC), the molecular mechanisms associated with TNF-alpha-induced VSMC are not known with certainty. To determine whether EGCG has the capacity to modulate VSMC responses, cell cycle regulation and MMP-9 expression were examined in TNF-alpha-induced VSMC. Treatment with EGCG, which blocks the cell cycle in the G(1) phase, induced a down-regulation of cyclins and CDKs and an up-regulation in the expression of p21/WAF1, a CDK inhibitor, whereas the up-regulation of p27 by EGCG was not observed. Moreover, treatment with EGCG markedly increased the promoter activity of the p21/WAF1 gene. Immunoblot and deletion analysis results for the p21/WAF1 promoter showed that EGCG induced the expression of p21/WAF1 independent of the p53 pathway. Zymographic and immunoblot analyses showed that EGCG suppressed TNF-alpha-induced MMP-9 expression in a dose-dependent manner. Further experiments demonstrated that EGCG reduced the transcriptional activity of activator protein-1 (AP-1) and nuclear factor kappaB (NF-kappaB), two important nuclear transcription factors that are involved in MMP-9 expression. Collectively, these results suggest that EGCG inhibits G(1) to S-phase cell cycle progress and MMP-9 expression through the transcription factors NF-kappaB and AP-1 in TNF-alpha-induced VSMC.

15-LOX-1 inhibits p21 (Cip/WAF 1) expression by enhancing MEK-ERK 1/2 signaling in colon carcinoma cells

Currently, some controversy exists regarding the precise role of 15-lipoxygenase-1 (15-LOX-1) in colorectal carcinogenesis and other aspects of cancer biology. The aim of this study was to evaluate the effect of 15-LOX-1 on p21 (Cip/WAF 1) expression and growth regulation in human colon carcinoma cells. The effect of 13-S-hydroxyoctadecadienoic acid (HODE), a product of 15-LOX-1, on p21 (Cip/WAF 1) expression was evaluated in Caco-2 cells treated with sodium butyrate (NaBT) and/or nordihydroguaiarectic acid (NDGA), a LOX inhibitor. The effect of transfecting HCT-116 cells with 15-LOX-1 was also examined. NaBT-induced p21 (Cip/WAF 1) expression was enhanced by treatment with NDGA and 13-S-HODE reversed NaBT-induced p21 (Cip/WAF 1) expression in Caco-2 cells. Overexpression of 15-LOX-1 induced extracellular signal-related kinase (ERK) 1/2 phosphorylation, decreased p21 (Cip/WAF 1) expression, and increased HCT-116 cell growth. Treatment with NDGA decreased ERK 1/2 phosphorylation, and increased p21 (Cip/WAF 1) expression in 15-LOX-1 overexpressing HCT-116 cells. Our experimental results support the hypothesis that 15-LOX-1 may have "pro-neoplastic" effects during the development of colorectal cancer.

Smad pathway-specific transcriptional regulation of the cell cycle inhibitor p21WAF1/Cip1

Transforming growth factor-beta (TGF-beta) inhibits epithelial cell growth, in part via transcriptional induction of the cell cycle inhibitor p21(WAF1/Cip1) (p21). We show that bone morphogenetic protein (BMP)-7 induces higher p21 expression than TGF-beta1 in various epithelial cells. Despite this, BMP-7 only weakly suppresses epithelial cell proliferation, as Id2, a cell cycle-promoting factor, becomes concomitantly induced by BMP-7. Signaling studies with all type I receptors of the TGF-beta superfamily show that BMP receptors induce higher p21 expression than TGF-beta/activin receptors. Smad4 is essential for p21 regulation by all receptor pathways. Based on the previously known ability of c-Myc to block p21 expression and epithelial growth arrest in response to TGF-beta1, we demonstrate that ectopic c-Myc expression can abrogate Smad-mediated p21 induction by all TGF-beta and BMP receptors. Furthermore, p21 induction by all receptor pathways can be blocked by the natural inhibitors of the TGF-beta superfamily. Smad7 inhibits all pathways whereas Smad6 selectively inhibits the BMP pathways. The observed pathway specificity reflects the efficiency by which BMP Smads, compared to TGF-beta Smads, transactivate the p21 promoter. In addition, BMP-specific Smads, Smad1, Smad5, and especially Smad8, induce endogenous p21 mRNA and protein levels, while they fail to induce epithelial growth inhibition when compared to TGF-beta receptor-phosphorylated Smads (R-Smads), Smad2 and Smad3. Thus, p21 is a common target of all TGF-beta superfamily pathways. However, the ability of TGF-beta superfamily members to induce cell growth arrest depends on the regulation of additional gene targets.

Cystathionine gamma-lyase overexpression inhibits cell proliferation via a H2S-dependent modulation of ERK1/2 phosphorylation and p21Cip/WAK-1

Cystathionine gamma-lyase (CSE) is a key enzyme in the trans-sulfuration pathway. CSE uses L-cysteine as a substrate to produce hydrogen sulfide (H2S). The CSE/H2S system has been shown to play an important role in regulating cellular functions in different systems. In the present study, we used CSE stably overexpressed HEK-293 cells to explore the effect of the CSE/H2S system on cell growth and proliferation. The overexpression of CSE resulted in increases in CSE mRNA levels, CSE proteins, and intracellular H2S production rates, as well as the inhibition of cell proliferation and DNA synthesis. These effects were accompanied by a sustained ERK activation and up-regulation of the cyclin-dependent kinase inhibitor p21Cip/WAK-1. Blocking the action of ERK with U0126 inhibited the induction of p21Cip/WAK-1, suggesting that ERK activation functions upstream of p21Cip/WAK-1 activation to initiate the CSE overexpression-induced cell growth inhibition. The antiproliferative effect of CSE is likely mediated by endogenously produced H2S because the H2S scavenger methemoglobin (10 microm) significantly decreased the H2S production rate and reversed the antiproliferative effect afforded by CSE. Exogenous H2S (100 microm) also inhibited cell proliferation. However, the other CSE-catalyzed products, ammonium and pyruvate, failed to inhibit cell proliferation. Methemoglobin also abolished the inhibitory effect of exogenous H2S on cell proliferation. Moreover, exogenous H2S induced a sustained ERK and p21Cip/WAK-1 activation. These findings support the hypothesis that endogenously produced H2S may play a fundamental role in cell proliferation and survival.

The bone morphogenetic protein type Ib receptor is a major mediator of glial differentiation and cell survival in adult hippocampal progenitor cell culture

Bone morphogenetic proteins (BMPs) act as growth regulators and inducers of differentiation. They transduce their signal via three different type I receptors, termed activin receptor-like kinase 2 (Alk2), Alk3, or bone morphogenetic protein receptor Ia (BMPRIa) and Alk6 or BMPRIb. Little is known about functional differences between the three type I receptors. Here, we have investigated consequences of constitutively active (ca) and dominant negative (dn) type I receptor overexpression in adult-derived hippocampal progenitor cells (AHPs). The dn receptors have a nonfunctional intracellular but functional extracellular domain. They thus trap BMPs that are endogenously produced by AHPs. We found that effects obtained by overexpression of dnAlk2 and dnAlk6 were similar, suggesting similar ligand binding patterns for these receptors. Thus, cell survival was decreased, glial fibrillary acidic protein (GFAP) expression was reduced, whereas the number of oligodendrocytes increased. No effect on neuronal differentiation was seen. Whereas the expression of Alk2 and Alk3 mRNA remained unchanged, the Alk6 mRNA was induced after impaired BMP signaling. After dnAlk3 overexpression, cell survival and astroglial differentiation increased in parallel to augmented Alk6 receptor signaling. We conclude that endogenous BMPs mediate cell survival, astroglial differentiation and the suppression of oligodendrocytic cell fate mainly via the Alk6 receptor in AHP culture.

Mediation of BMP7 neuroprotection by MAPK and PKC IN rat primary cortical cultures

We have previously demonstrated that pretreatment with bone morphogenetic protein 7 (BMP7), a trophic factor in the TGFbeta superfamily, reduces ischemia-induced brain infarction induced by middle cerebral artery ligation in rats. Since the mitogen-activated protein kinase (MAPK) pathway is involved in many TGFbeta-mediated responses, we examined the interaction of BMP7 and MAPK in primary cultures obtained from the cerebral cortex of E16-17 rat embryos. Lactate dehydrogenase (LDH) in the media was used as an index of cell death. BMP7 did not alter LDH levels at low concentration (1.25 nM), but exhibited increased cellular toxicity at higher concentration (>12.5 nM). BMP7 at the low concentration significantly attenuated H2O2-induced increases in LDH activity and decreases in neuronal density. Pharmacological interactions were used to examine if MAPK was involved in this response. BMP7-induced protection was antagonized by the p42,44 MAPK kinase inhibitors PD98059 and U0125. The p38 MAPK antagonist SB203580, and their inactive analog SB202474, also attenuated BMP7-induced protection, suggesting that the interaction with p38 MAPK is nonspecific. Previous studies have indicated that SB202474 has inhibitory effects on other protein kinases. We found that the protein kinase C inhibitor chelerythrine antagonized BMP7-induced protection against H2O2. Western blot analysis indicated that BMP7 increased phosphorylation of p42,44 MAPK and PKC. Taken together, our data suggest that BMP7 is neuroprotective at low concentrations in primary cortical cell culture. The protective effects of BMP7 may involve the activation of p42,44 MAPK and PKC.

Transcription factor Sp1 mediates p38MAPK-dependent activation of the p21WAF1 gene promoter in vascular smooth muscle cells by pyrrolidine dithiocarbamate

Previously, we demonstrated that pyrrolidine dithiocarbamate (PDTC) induced G1 cell cycle arrest in vascular smooth muscle cells (VSMC) through inducing p21WAF1 expression. It has recently been reported that the transcription factors involved in p21WAF1 activation by certain signaling factors may vary in different cell types. However, little is known concerning the molecular mechanisms by which PDTC induces p21WAF1 gene expression in VSMC. In this report, we demonstrate that PDTC induces the p21WAF1 expression at the mRNA level. This increase in p21WAF1 gene expression was due to p38MAPK-dependent activation of the p21WAF1 promoter by PDTC. Transcription factor Sp1 binding site was identified as the cis-element for the activation of p21WAF1 promoter by PDTC, as determined by deletion and mutation analysis. In addition, gel shift and supershift assays demonstrated that this PDTC-responsive element binds specifically to the transcription factor Sp1. Treatment with SB203580, an inhibitor of the p38MAPK, significantly down-regulated transactivation of PDTC-induced Sp1. Finally, the transient expression of VSMC with dominant negative p38MAPK plasmid suppressed PDTC-stimulated Sp1 activity. In conclusion, we report the novel finding that transcription factor Sp1 that is involved in the p38MAPK-mediated control of p21WAF1 regulation on VSMC in response to PDTC has now been identified.

Endogenous control of cell cycle progression by autocrine transforming growth factor beta in breast cancer cells

Tumor progression due to loss of autocrine negative transforming growth factor-beta (TGF-beta) activity was reported in various cancers of epithelial origin. Estrogen receptor expressing (ER(+)) breast cancer cells are refractory to TGF-beta effects and exhibit malignant behavior due to loss or inadequate expression of TGF-beta receptor type II (RII). The exogenous TGF-beta effects on the modulation of cell cycle machinery were analyzed previously. However, very little is known regarding the endogenous control of cell cycle progression by autocrine TGF-beta. In this study, we have used a tetracycline regulatable RII cDNA expression vector to demonstrate that RII replacement reconstitutes autocrine negative TGF-beta activity in ER(+) breast cancer cells as evidenced by the delayed entry into S phase by the RII transfectants. Reversal of the delayed entry into S phase by the RII transfectants in the presence of tetracycline in addition to the decreased steady state transcription from a promoter containing the TGF-beta responsive element (p3TP-Lux) by TGF-beta neutralizing antibody treatment of the RII transfected cells confirmed that autocrine-negative TGF-beta activity was induced in the transfectants. Histone H1 kinase assays indicated that the delayed entry of RII transfectants into phase was associated with markedly reduced cyclin-dependent kinase (CDK)2 kinase activity. This reduction in kinase activity was due to the induction of CDK inhibitors p21/waf1/cip1 and p27/kip, and their association with CDK2. Tetracycline treatment of RII transfectants led to the suppression of p21/waf1/cip1and p27/kip expression, thus, directly demonstrating induction of CDK inhibitors by autocrine TGF-beta leading to growth control of ER(+) breast cancer cells.

Role of p12(CDK2-AP1) in transforming growth factor-beta1-mediated growth suppression

p12(CDK2-AP1) (p12) is a growth suppressor isolated from normal keratinocytes. Ectopic expression of p12 in squamous carcinoma cells reversed the malignant phenotype of these cells, in part due an ability of p12 to bind to both DNA polymerase alpha/primase and to cyclin-dependent kinase 2 (CDK2), thereby inhibiting their activities. We report in this article that in normal epithelial cells, transforming growth factor beta1 (TGF-beta1) induces p12 expression transcriptionally, which, in turn, mediates the growth inhibitory activity of TGF-beta1. We created inducible p12 antisense HaCaT cell lines [ip12 (-) HaCaT] and showed that selective reduction of cellular p12 resulted in an increase in: (a) CDK2-associated kinase activity; (b) protein retinoblastoma (pRB) phosphorylation; and (c) [(3)H]thymidine incorporation, and partially reversed TGF-beta1-mediated inhibition of CDK2 kinase activity, pRB phosphorylation, and cell proliferation. Furthermore, we generated p12-deficient mouse oral keratinocytes (MOK(p12-/-)) and compared their growth characteristics and response to TGF-beta1 with that of wild-type mouse oral keratinocytes (MOK(WT)). Under normal culture conditions, the number of MOK(p12-/-) in S phase is 2-fold greater than that of MOK(WT). Concomitantly, fewer cells are in G(2) phase in MOK(p12-/-) than that in MOK(WT). Moreover, response to TGF-beta1-mediated growth suppression is compromised in MOK(p12-/-) cells. Mechanistic studies showed that MOK(p12-/-) have increased CDK2 activity and reduced sensitivity to inhibition by TGF-beta1. Collectively our data suggest that p12 plays a role in TGF-beta1-mediated growth suppression by modulating CDK2 activities and pRB phosphorylation.

Smad4-independent regulation of p21/WAF1 by transforming growth factor-beta

The transforming growth factor-beta (TGF-beta)-Smad signaling pathway inhibits the growth of human epithelial cells and plays a role in tumor suppression. The Smad4 gene is mutated or deleted in 50% of pancreatic cancers. In this study, the Smad4-null pancreatic cancer cell line BxPC-3 was transfected with either the Smad4 expression vector or the empty vector and incubated in the presence or absence of TGF-beta. The cells were analysed using a cDNA microarray, which included 2280 named genes to screen for target genes regulated by TGF-beta in either a Smad4-dependent or -independent manner. The microarray and subsequent quantitative RT-PCR analysis demonstrated that the Smad4-independent and -dependent signaling pathways driven by TGF-beta upregulated only one of the 2280 genes, respectively, suggesting that Smad4-independent signaling downstream of TGF-beta might be as widespread as Smad4-dependent signaling. In this study, we demonstrated that the cyclin-dependent kinase inhibitor p21/WAF1, which has been considered the major effector of the Smad-dependent growth inhibitory signal of TGF-beta, is upregulated in a Smad4-independent manner. The upregulation occurs through Smad2/3-dependent transcriptional activation of the p21/WAF1 promoter region. These results suggest a novel mechanism of gene regulation, that is, a novel signal mediator other than Smad4.

Transcriptional regulation by the MAP kinase signaling cascades

A major function of the mitogen-activated protein kinase (MAPK) pathways is to control eukaryotic gene expression programmes in response to extracellular signals. MAPKs directly control gene expression by phosphorylating transcription factors. However, it is becoming clear that transcriptional regulation in response to MAPK signaling is more complex. MAPKs can also target coactivators and corepressors and affect nucleosomal structure by inducing histone modifications. Furthermore, multiple inputs into individual promoters can be elicited by MAPKs by targeting different components of the same coregulatory complex or by triggering different events on the same transcription factor. "Postgenomic approaches" are beginning to impact on our understanding of these gene regulatory networks. In this review, we summarise the current knowledge of MAPK-mediated gene regulation, and focus on how complexities in signaling outcomes are achieved and how this relates to physiological processes.

The cyclin-dependent kinase inhibitor p21CIP/WAF is a positive regulator of insulin-like growth factor I-induced cell proliferation in MCF-7 human breast cancer cells

To study the role of IGF-I receptor signaling on cell cycle events we utilized MCF-7 breast cancer cells. IGF-I at physiological concentrations increased the level of p21CIP/WAF mRNA after 4has well as protein after 8hby 10- and 6-fold, respectively, in MCF-7 cells. This IGF-1 effect was reduced by 50% in MCF-7-derived cells (SX13), which exhibit a 50% reduction in IGF-1R expression, demonstrating that IGF-1 receptor activation was involved in this process. Preincubation with the ERK1/2 inhibitor U0126 significantly reduced the IGF-1 effect on the amount of p21CIP/WAF protein in MCF-7 cells. These results were confirmed by the expression of a dominant negative construct for MEK-1 suggesting that the increase of the abundance of p21CIP/WAF in response to IGF-1 occurs via the ERK1/2 mitogen-activated protein kinase pathway. Using an antisense strategy, we demonstrated that abolition of p21CIP/WAF expression decreased by 2-fold the IGF-1 effect on cell proliferation in MCF-7. This latter result is explained by a delay in G1 to S cell cycle progression due partly to a reduction in the activation of some components of cell cycle including the induction of cyclin D1 expression in response to IGF-1. MCF-7 cells transiently overexpressing p21 showed increased basal and IGF-I-induced thymidine incorporation. Taken together, these results define p21CIP/WAF as a positive regulator in the cell proliferation induced by IGF-1 in MCF-7 cells.

Platelet-derived growth factor (PDGF) receptor-alpha-activated c-Jun NH2-terminal kinase-1 is critical for PDGF-induced p21WAF1/CIP1 promoter activity independent of p53

Platelet-derived growth factor (PDGF) is a potent mitogen for mesenchymal cells. PDGF AA functions as a "competent factor" that stimulates cell cycle entry but requires additional (progression) factors in serum to transit the cell cycle beyond the G1/S checkpoint. Unlike PDGF AA, PDGF B-chain (c-sis) homodimer (PDGF BB) and its viral counterpart v-sis can serve as both competent and progression factors. PDGF BB activates alpha- and beta-receptor subunits (alpha-PDGFR and beta-PDGFR) and induces phenotypic transformation in NIH 3T3 cells, whereas PDGF AA activates alpha-PDGFR only and fails to induce transformation. We showed previously that alpha-PDGFR antagonizes beta-PDGFR-mediated transformation through activation of stress-activated protein kinase-1/c-Jun NH2-terminal kinase-1, whereas both alpha-PDGFR and beta-PDGFR induce mitogenic signals. These studies revealed a striking feature of PDGF signaling; the specificity and the strength of the PDGF growth signal is modulated by alpha-PDGFR-mediated simultaneous activation of growth stimulatory and inhibitory signals, whereas beta-PDGFR mainly induces a growth-promoting signal. Here we demonstrate that PDGF BB activation of beta-PDGFR alone results in more efficient cell cycle transition from G1 to S phase than PDGF BB activation of both alpha-PDGFR and beta-PDGFR. PDGF AA activation of alpha-PDGFR or PDGF BB activation of both alpha- and beta-PDGFRs up-regulates expression of p21WAF1/CIP1, an inhibitor of cell cycle-dependent kinases and a downstream mediator of the tumor suppressor gene product p53. However, beta-PDGFR activation alone fails to induce p21WAF1/CIP1 expression. We also demonstrate that alpha-PDGFR-activated JNK-1 is a critical signaling component for PDGF induction of p21WAF1/CIP1 promoter activity. The ability of PDGF/JNK-1 to induce p21WAF1/CIP1 promoter activity is independent of p53, although the overall p21WAF1/CIP1 promoter activities are greatly reduced in the absence of p53. These results provide a molecular basis for differential regulation of the cell cycle and transformation by alpha- and beta-PDGFRs.

Expression of p21(WAF1/Cip1) through Sp1 sites by histone deacetylase inhibitor apicidin requires PI 3-kinase-PKC epsilon signaling pathway

We previously reported that the activation of p21(WAF1/Cip1) transcription by histone deacetylase inhibitor apicidin was mediated through Sp1 sites and pointed to the possible participation of protein kinase C (PKC). In this study, we investigated the role and identity of the specific isoforms of PKC involved and identified phosphatidylinositol 3-kinase (PI 3-kinase) as an upstream effector in HeLa cells. Using an isoform-specific pharmacological inhibitor of PKC, a PKC epsilon dominant-negative mutant, and antisense oligonucleotide to inhibit PKC epsilon specifically, we found that among PKC isoforms, PKC epsilon was required for the p21(WAF1/Cip1) expression by apicidin. In addition to PKC epsilon, PI 3-kinase appeared to participate in the activation of p21(WAF1/Cip1) promoter by apicidin, since inactivation of PI 3-kinase either by transient expression of dominant-negative mutant of PI 3-kinase or its specific inhibitors, LY294002 and wortmannin, attenuated the activation of p21(WAF1/Cip1) promoter and p21(WAF1/Cip1) protein expression by apicidin. Furthermore, membrane translocation of PKC epsilon in response to apicidin was blocked by the PI 3-kinase inhibitor, indicating the role of PI 3-kinase as an upstream molecule of PKC epsilon in the p21(WAF1/Cip1) promoter activation by apicidin. However, the p21(WAF1/Cip1) expression by apicidin appeared to be independent of the histone hyperacetylation, since apicidin-induced histone hyperacetylation of p21(WAF1/Cip1) promoter region was not affected by inhibition of PI 3-kinase and PKC, suggesting that the chromatin remodeling through the histone hyperacetylation alone might not be sufficient for the expression of p21(WAF1/Cip1) by apicidin. Taken together, these results suggest that the PI 3-kinase-PKC epsilon signaling pathway plays a pivotal role in the expression of the p21(WAF1/Cip1) by apicidin.

Phosphorylation of Nrf2 at Ser40 by protein kinase C in response to antioxidants leads to the release of Nrf2 from INrf2, but is not required for Nrf2 stabilization/accumulation in the nucleus and transcriptional activation of antioxidant response element-mediated NAD(P)H:quinone oxidoreductase-1 gene expression

The antioxidant response element (ARE) and transcription factor Nrf2 regulate basal expression and antioxidant induction of NAD(P)H:quinone oxidoreductase-1 (NQO1) and other detoxifying genes. Under normal conditions, Nrf2 is targeted for proteasomal degradation by INrf2. Oxidative stress causes release of Nrf2 from INrf2. Nrf2 translocates to the nucleus, binds to the ARE, and activates gene expression. In this study, we demonstrate that protein kinase C (PKC) plays a significant role in the regulation of ARE-mediated NQO1 gene expression and induction in response to t-butylhydroquinone. Treatment of HepG2 cells with the PKC inhibitors staurosporine and calphostin C repressed ARE-mediated induction of a luciferase reporter as well as that of the endogenous NQO1 gene. Similar experiments with inhibitors of MEK/ERK, p38, phosphatidylinositol 3-kinase, and tyrosine kinases failed to repress ARE-mediated gene expression. The PKC inhibitor staurosporine blocked the nuclear translocation of Nrf2, suggesting that Nrf2 might be the target for PKC regulation. A Prosite search revealed the presence of seven putative PKC sites in mouse Nrf2. The PKC site at Ser40 is conserved among species and lies in the Neh2 domain, which interacts with INrf2. We demonstrate that phosphorylation of Ser40 is necessary for Nrf2 release from INrf2, but is not required for Nrf2 stabilization/accumulation in the nucleus and transcriptional activation of ARE-mediated NQO1 gene expression. A peptide that competes with endogenous Nrf2 for INrf2 binding was able to induce ARE activity more effectively than t-butylhydroquinone, and Nrf2 that accumulated in the nucleus as a result was not phosphorylated.

Cross-talk between ERK MAP kinase and Smad signaling pathways enhances TGF-beta-dependent responses in human mesangial cells

Transforming growth factor beta (TGF-beta) stimulates renal cell fibrogenesis by a poorly understood mechanism. Previously, we suggested a synergy between TGF-beta1 activated extracellular signal-regulated kinase (ERK) and Smad signaling in collagen production by human glomerular mesangial cells. In a heterologous DNA binding transcription assay, biochemical or dominant-negative ERK blockade reduced TGF-beta1 induced Smad3 activity. Total serine phosphorylation of Smad2/3, but not phosphorylation of the C-terminal SS(P)XS(P) motif, was decreased by pretreatment with the MEK/ERK inhibitors, PD98059 (10 microM) or U0126 (25 microM). This effect was not seen in the mouse mammary epithelial NMuMG cell line, indicating that ERK-dependent activation of Smad2/3 occurs only in certain cell types. TGF-beta stimulated phosphorylation of an expressed Smad3A construct, with a mutated C-terminal SS(P)XS(P) motif, was reduced by a MEK/ERK inhibitor. In contrast, MEK/ERK inhibition did not affect phosphorylation of a Smad3 construct mutated at consensus phosphorylation sites in the linker region (Smad3EPSM). Constitutively active MEK (caMEK) induced alpha2(I) collagen promoter activity, an effect blocked by co-transfected Smad3EPSM, but not Smad3A. The effects of caMEK and TGF-beta1 on collagen promoter activity were additive. These results indicate that ERK-dependent R-Smad linker region phosphorylation enhances collagen I synthesis and imply positive cross talk between the ERK and Smad pathways in human mesangial cells.

p38 MAP kinase modulates Smad-dependent changes in human prostate cell adhesion

Transforming growth factor beta (TGFbeta) regulates cell adhesion, proliferation, and differentiation in a variety of cells. Smad proteins are receptor-activated transcription factors that translocate to the nucleus in response to TGFbeta. We demonstrate here that TGFbeta increases cell adhesion in metastatic PC3-M prostate cancer cells. TGFbeta treatment of PC3-M cells leads to nuclear translocation of R-Smad proteins. We show that Smad proteins are necessary, but not sufficient, for TGFbeta-mediated cell adhesion. After showing that TGFbeta upregulated p38 MAP kinase activity in PC3-M cells, we show that inhibition of p38 MAP kinase partially blocked TGFbeta-mediated increase in cell adhesion, as well as nuclear translocation of Smad3. Finally, we show that Smad3 is phosphorylated by p38 MAP kinase in vitro. These findings implicate crosstalk between the MAP kinase and Smad signaling pathways in TGFbeta's regulation of cell adhesion in human prostate cells. This represents a mechanism by which the pleiotropic effects of TGFbeta may be channeled to modulate cell adhesion.

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

p21Cip1 gene expression is modulated by Egr1: a novel regulatory mechanism involved in the resveratrol antiproliferative effect

Epidemiological observations indicate that resveratrol, a natural antioxidant stilbene, exerts cardioprotective and chemopreventive effects. Moreover, the molecule induces in vitro cell growth inhibition and differentiation. Using human erythroleukemic K562 cells as model system, we demonstrated that resveratrol induces a remarkable gamma-globin synthesis, the erythroid differentiation being linked to impairment of cell proliferation, increased p21Cip1 expression and inhibition of cdk2 activity. The up-regulation of p21Cip1 transcription is prevented by cycloheximide, indicating the requirement of intermediate protein(s), which, in turn, regulate gene expression. The quantitative analysis of some transcription factors involved in the erythroid lineage, namely GATA-1, GATA-2, and Egr1, indicated that resveratrol selectively up-regulates Egr1 by an Erk1/2-dependent mechanism. The presence of an Egr1 consensus sequence in the p21Cip1 promoter suggested the hypothesis that this transcription factor directly regulates the expression of the cdk inhibitor. Transfection studies with deleted gene promoter constructs, as well as EMSA, pull-down, and chromatin immunoprecipitation experiments substantiated this view, demonstrating that Egr1 binds in vitro and in vivo to the identified consensus sequence of the p21Cip1 promoter. Moreover, an Egr1 phosphorothioate antisense hinders p21Cip1 accumulation and the antiproliferative effects of resveratrol. In conclusion, this is the first demonstration that Egr1 controls p21Cip1 expression by directly interacting with a specific sequence on its gene promoter. The identified regulatory mechanism also contributes to the clarification of the complex chemopreventive and antiproliferative properties of resveratrol.

Genealogy, expression, and cellular function of transforming growth factor-beta

The transforming growth factor-beta (TGF-beta) gene superfamily expresses a large set of structurally and functionally related polypeptides. Three TGF-beta isoforms are regulated by specific genes and have been identified in mammals (TGF-beta1, -beta2, and -beta3). All three-protein isoforms are observed abundantly during development and display overlapping and distinct spatial and temporal patterns of expressions. Each isoform plays a distinct role, the nature of which depends on the cell type, its state of differentiation, and growth conditions, and on the other growth factors present. TGF-beta regulates many of the processes common to both tissue repair and disease, including angiogenesis, chemotoxins, fibroblast proliferation and the controlled synthesis, and degradation of matrix proteins, such as collagen and fibronectin. This review will examine the genealogy and mode of actions of TGF-beta on the cell types involved in inflammation and repair, as well as in carcinoma.

Regulation of trespin expression by modulators of cell growth, differentiation, and apoptosis in prostatic epithelial cells

We recently identified a novel rat ov-serpin, Trespin, which inhibits the trypsin-like serine proteinase plasmin and is expressed in several tissues, including prostate. In this report Trespin expression was studied in prostatic cell lines, NRP-152, NRP-154, and DP-153, derived from the Lobund-Wistar rat. Northern blots revealed Trespin mRNA is expressed in NRP-152 and DP-153 basal epithelial cell lines but not in the luminal line, NRP-154. Similarly, Trespin levels drop >30-fold following transdifferentiation of NRP-152 cells toward a luminal variant, further suggesting Trespin expression is specific for basal prostatic epithelial cells. Trespin expression in NRP-152 cells is up-regulated by dexamethasone (Dex) and insulin-like growth factor-I (IGF-I), each of which stimulate growth and prevent differentiation and apoptosis. However, Dex (alone) facilitates loss of Trespin by TGF-beta, yet enhances the ability of LR(3)-IGF-I to reverse such loss, similar to the pattern of apoptosis induced by TGF-beta. Likewise, several apoptosis inducers markedly decrease Trespin mRNA levels. HEK293 cells stably overexpressing Trespin display increased cell proliferation and partial resistance to growth inhibition and phosphorylation of c-Jun induced by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA). Together these data strongly suggest that Trespin has critical functions tied to the regulation of growth, differentiation, and apoptosis of prostatic epithelial cells.

P21Waf1 control of epithelial cell cycle and cell fate

As a broad-acting cyclin-dependent kinase inhibitor, p21(WAF1) occupies a central position in the cell cycle regulation of self-renewing tissues such as oral mucosa and skin. In addition to regulating normal cell cycle progression decisions, p21(WAF1) integrates genotoxic insults into growth arrest and apoptotic signaling pathways that ultimately determine cell fate. As a result of its complex interactions with cell cycle machinery and response to mutagenic agents, p21(WAF1) also has stage-specific roles in epithelial carcinogenesis. Finally, a view is emerging of p21(WAF1) as not merely a cyclin-dependent kinase inhibitor, but also as a direct participant in regulating genes involved in growth arrest, senescence, and aging, thus providing an additional layer of control over matters of the cell cycle. This review discusses these various roles played by p21(WAF1) in cell cycle control, and attempts to relate these to epithelial cell biology, with special emphasis on keratinocytes.

Elucidation of Smad requirement in transforming growth factor-beta type I receptor-induced responses

Transforming growth factor-beta (TGF-beta) elicits cellular effects by activating specific Smad proteins that control the transcription of target genes. Whereas there is growing evidence that there are TGF-beta type I receptor-initiated intracellular pathways that are distinct from the pivotal Smad pathway, their physiological importance in TGF-beta signaling is not well understood. Therefore, we generated TGF-beta type I receptors (also termed ALK5s) with mutations in the L45 loop of the kinase domain, termed ALK5(D266A) and ALK5(3A). These mutants showed retained kinase activity but were unable to activate Smads. Characterization of their signaling properties revealed that the two L45 loop mutants did not mediate Smad-dependent transcriptional responses, TGF-beta-induced growth inhibition, and fibronectin and plasminogen activator-1 production in R4-2 mink lung epithelial cells lacking functional ALK5 protein. Mutation in the L45 loop region did not affect the binding of inhibitory Smads but did abrogate the weak binding of X-linked inhibitor of apoptosis protein and Disabled-2 to ALK5. This suggests that the L45 loop in the kinase domain is important for docking of other binding proteins. Interestingly, JNK MAP kinase activity was found to be activated by the ALK5(3A) mutant in various cell types. In addition, TGF-beta-induced inhibition of cyclin D1 expression and stimulation of PMEPA1 (androgen-regulated prostatic mRNA) expression were found to occur, albeit weakly, in an Smad-independent manner in normal murine mammary gland cells. However, the TGF-beta-induced epithelial to mesenchymal transdifferentiation was found to require an intact L45 loop and is likely to be dependent on the Smad pathways.

Transforming growth factor-beta1 (TGF-beta)-induced apoptosis of prostate cancer cells involves Smad7-dependent activation of p38 by TGF-beta-activated kinase 1 and mitogen-activated protein kinase kinase 3

The inhibitory Smad7, a direct target gene for transforming growth factor-beta (TGF-beta), mediates TGF-beta1-induced apoptosis in several cell types. Herein, we report that apoptosis of human prostate cancer PC-3U cells induced by TGF-beta1 or Smad7 overexpression is caused by a specific activation of the p38 mitogen-activated protein kinase pathway in a TGF-beta-activated kinase 1 (TAK1)- and mitogen-activated protein kinase kinase 3 (MKK3)-dependent manner. Expression of dominant negative p38, dominant negative MKK3, or incubation with the p38 selective inhibitor [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole], prevented TGF-beta1-induced apoptosis. The expression of Smad7 was required for TGF-beta-induced activation of MKK3 and p38 kinases, and endogenous Smad7 was found to interact with phosphorylated p38 in a ligand-dependent manner. Ectopic expression of wild-type TAK1 promoted TGF-beta1-induced phosphorylation of p38 and apoptosis, whereas dominant negative TAK1 reduced TGF-beta1-induced phosphorylation of p38 and apoptosis. Endogenous Smad7 was found to interact with TAK1, and TAK1, MKK3, and p38 were coimmunoprecipitated with Smad7 in transiently transfected COS1 cells. Moreover, ectopically expressed Smad7 enhanced the coimmunoprecipitation of HA-MKK3 and Flag-p38, supporting the notion that Smad7 may act as a scaffolding protein and facilitate TAK1- and MKK3-mediated activation of p38.

A novel transforming growth factor-beta receptor-interacting protein that is also a light chain of the motor protein dynein

The phosphorylated, activated cytoplasmic domains of the transforming growth factor-beta (TGFbeta) receptors were used as probes to screen an expression library that was prepared from a highly TGFbeta-responsive intestinal epithelial cell line. One of the TGFbeta receptor-interacting proteins isolated was identified to be the mammalian homologue of the LC7 family (mLC7) of dynein light chains (DLCs). This 11-kDa cytoplasmic protein interacts with the TGFbeta receptor complex intracellularly and is phosphorylated on serine residues after ligand-receptor engagement. Forced expression of mLC7-1 induces specific TGFbeta responses, including an activation of Jun N-terminal kinase (JNK), a phosphorylation of c-Jun, and an inhibition of cell growth. Furthermore, TGFbeta induces the recruitment of mLC7-1 to the intermediate chain of dynein. A kinase-deficient form of TGFbeta RII prevents both mLC7-1 phosphorylation and interaction with the dynein intermediate chain (DIC). This is the first demonstration of a link between cytoplasmic dynein and a natural growth inhibitory cytokine. Furthermore, our results suggest that TGFbeta pathway components may use a motor protein light chain as a receptor for the recruitment and transport of specific cargo along microtublules.

Cytosolic retention of phosphorylated extracellular signal-regulated kinase and a Rho-associated kinase-mediated signal impair expression of p21(Cip1/Waf1) in phorbol 12-myristate-13- acetate-induced apoptotic cells

In response to treatment with phorbol-12-myristate-13-acetate (PMA), the half-population of erythromyeloblast D2 cells, a cytokine-independent variant of TF-1 cells, displayed adhesion and differentiated into a monocyte/macrophage-like morphology, while the other half-population remained in suspension and underwent apoptosis. Expression of the cell cycle inhibitor p21(Cip1/Waf1) was induced after PMA treatment in the adherent cells but not in the proapoptotic cells. We investigated the mechanism responsible for the impairment of p21(Cip1/Waf1) induction in PMA-induced proapoptotic cells. We demonstrated that in PMA-induced adherent cells, upregulation of p21(Cip1/Waf1) requires the activation and nuclear translocation of phosphorylated extracellular signal-regulated kinase (phospho-ERK). Although ERK was phosphorylated to comparable levels in PMA-induced proapoptotic and adherent cells, nuclear distribution of phospho-ERK was seen only in the adherent, not in the proapoptotic cells. We also found that only PMA-induced proapoptotic cells contained the phosphorylated form of myosin light chain, which is dependent on Rho-associated kinase (ROCK) activation, and that expression of a dominant-active form of ROCK suppressed activation of the p21(Cip1/Waf1) promoter during PMA induction. Finally, we demonstrated that inhibition of ROCK restores nuclear distribution of phospho-ERK and activation of p21(Cip1/Waf1) expression. Based on these findings, we propose that a ROCK-mediated signal is involved in interfering with the process of ERK-mediated p21(Cip1/Waf1) induction in PMA-induced proapoptotic TF-1 and D2 cells.

Modulation of Smad2-mediated signaling by extracellular signal-regulated kinase

Components of the transforming growth factor-beta and mitogen-activated protein kinase pathways interact in controlling cell growth and differentiation. We show that phosphorylation of Smad2, a mediator of the activin/transforming growth factor-beta signal, by activated extracellular signal-regulated kinase 1 (ERK1) increases the amount of Smad2 protein and leads to enhanced transcriptional activity. Epidermal growth factor increased phosphorylation of Smad2 in COS7 cells, and Smad2-dependent transcription in a mink lung epithelial cell line, L17, was enhanced by co-transfection of a constitutively active MEK1. In addition, transfection of Smad2 mutants lacking ERK sites resulted in reduced transcription, whereas mutants that mimicked ERK phosphorylation stimulated transcription. The amount of Smad2 protein was increased by transfection with a constitutively active MEK1 and reduced by co-transfection with the ERK phosphatase, HVH2. The elevation of Smad2 protein levels was because of increased half-life and resulted in increased complex formation with Smad4. A site of ERK-dependent phosphorylation on Smad2 was located to Thr(8), a site that overlaps with the calmodulin binding region. We show that calmodulin inhibits Smad2 phosphorylation by ERK1, and overexpressing calmodulin, or stimulating calmodulin activity with ionomycin, reduces Smad2 levels. These findings suggest that the ERK pathway positively regulates Smad2 signaling by phosphorylating Smad2 and that negative regulation of Smad2 signaling by calmodulin is achieved in part by inhibiting this phosphorylation.

A mechanism in Epstein-Barr virus oncogenesis: inhibition of transforming growth factor-beta 1-mediated induction of MAPK/p21 by LMP1

Epstein-Barr virus (EBV)-infected, gastric epithelial cell line GT38 is resistant to TGF-beta 1-mediated growth inhibition and apoptosis, although TGF-beta 1 partially induces EBV reactivation in the cells. These findings indicate that abnormalities exist in these cells in the TGF-beta 1-mediated signaling pathway, influencing growth inhibition and apoptosis. In order to characterize the steps with abnormalities, we analyzed the TGF-beta 1/MAPK/p21 pathway in the cells. TGF-beta 1 activated MAPK (ERK 1/2) and p21 in the TGF-beta 1-susceptible cell line HSC-39 but not in GT38 cells. GT38 cells had higher constitutive levels of ERK 1/2 phosphorylation and p21 expression than did HSC-39 cells. U0126, a specific inhibitor of MEK, suppressed TGF-beta 1-mediated ERK 1/2 phosphorylation and p21 induction in HSC-39 cells and constitutive ERK 1/2 phosphorylation in GT38 cells. EBV latent membrane protein 1 (LMP1) induced constitutive ERK 1/2 phosphorylation and NF-kappa B activation in LMP1-transfected HSC-39 cells, which then became resistant to TGF-beta 1-mediated growth inhibition, TGF-beta 1-mediated ERK 1/2 phosphorylation, and p21 induction, and proliferated in low-serum medium. These results are consistent with the conclusion that the TGF-beta 1/MAPK/p21 pathway is required for TGF-beta 1-mediated growth inhibition, and that the resistance to TGF in GT38 cells is derived from constitutive MAPK phosphorylation induced by LMP1.

CTGF expression in mesangial cells: involvement of SMADs, MAP kinase, and PKC

BACKGROUND

The induction of excess matrix in renal fibrosis seems to be mediated, at least in part, by the transforming growth factor-beta (TGF-beta)-mediated induction of connective tissue growth factor (CTGF) in mesangial cells.

METHODS

By examining CTGF protein and mRNA expression and promoter activity in the presence or absence of TGF-beta or inhibitors, the signaling pathways controlling basal and TGF-beta-induced CTGF expression in mesangial cells were investigated.

RESULTS

TGF-beta enhances CTGF mRNA and protein expression in mesangial cells. Mutation of a consensus SMAD binding element in the CTGF promoter completely abolished TGF-beta-induced CTGF expression and reduced basal CTGF expression. The previously identified basal control element-1 (BCE-1) site, but not Sp1 contributes to basal CTGF promoter activity. Ras/MEK/ERK, protein kinase C (PKC) and tyrosine kinase activity also contribute to basal and TGF-beta-induced CTGF promoter activity in cultured mesangial cells.

CONCLUSIONS

The TGF-beta-induction of CTGF in mesangial cells requires SMADs and PKC/ras/MEK/ERK pathways. SMADs are involved in basal CTGF expression, which presumably reflects the fact that mesangial cells express TGF-beta endogenously. TGF-beta also induces CTGF through ras/MEK/ERK. Inhibiting ras/MEK/ERK seems not to reduce phosphorylation (that is, activation) of SMADs, suggesting that SMADs, although necessary, are insufficient for the TGF-beta-stimulation of the CTGF promoter through ras/MEK/ERK. Thus, maximal TGF-beta induction of CTGF requires synergy between SMAD and ras/MEK/ERK signaling.

Myostatin inhibits myoblast differentiation by down-regulating MyoD expression

Myostatin, a negative regulator of myogenesis, is shown to function by controlling the proliferation of myoblasts. In this study we show that myostatin is an inhibitor of myoblast differentiation and that this inhibition is mediated through Smad 3. In vitro, increasing concentrations of recombinant mature myostatin reversibly blocked the myogenic differentiation of myoblasts, cultured in low serum media. Western and Northern blot analysis indicated that addition of myostatin to the low serum culture media repressed the levels of MyoD, Myf5, myogenin, and p21 leading to the inhibition of myogenic differentiation. The transient transfection of C(2)C(12) myoblasts with MyoD expressing constructs did not rescue myostatin-inhibited myogenic differentiation. Myostatin signaling specifically induced Smad 3 phosphorylation and increased Smad 3.MyoD association, suggesting that Smad 3 may mediate the myostatin signal by interfering with MyoD activity and expression. Consistent with this, the expression of dominant-negative Smad3 rescued the activity of a MyoD promoter-reporter in C(2)C(12) myoblasts treated with myostatin. Taken together, these results suggest that myostatin inhibits MyoD activity and expression via Smad 3 resulting in the failure of the myoblasts to differentiate into myotubes. Thus we propose that myostatin plays a critical role in myogenic differentiation and that the muscular hyperplasia and hypertrophy seen in animals that lack functional myostatin is because of deregulated proliferation and differentiation of myoblasts.

Silencing of transcription of the human luteinizing hormone receptor gene by histone deacetylase-mSin3A complex

Modification of chromatin structure by histone acetylases and deacetylases is an important mechanism in modulation of eukaryotic gene transcription. The present study investigated regulation of the human luteinizing hormone receptor (hLHR) gene by histone deacetylases. Inhibition of histone deacetylases (HDACs) by trichostatin A (TSA) increased hLHR promoter activity by 40-fold in JAR cells and markedly elevated endogenous hLHR mRNA levels. Acetylated histones H3 and H4 accumulated in TSA-treated cells and associated predominantly with the hLHR promoter. Furthermore, TSA significantly enhanced the recruitment of RNA polymerase II to the promoter. One of the two Sp1 sites essential for basal promoter activity was identified as critical for the TSA effect, but the binding of Sp1/Sp3 to this site remained unchanged in the absence or presence of TSA. A multiprotein complex was recruited to the hLHR promoter via interaction with Sp1 and Sp3, in which HDAC1 and HDAC2 were docked directly to Sp1-bound DNA and indirectly to Sp3-bound DNA through RbAp48, while mSin3A interacted with both HDACs. HDAC1 and HDAC2 were shown to potently repress the hLHR gene transcription, and mSin3A potentiated the inhibition mediated by HDAC1. Our studies have demonstrated that the HDAC-mSin3A complex has an important role in the regulation of hLHR gene transcription by interaction with Sp1/Sp3 and by region-specific changes in histone acetylation and polymerase II recruitment within the hLHR promoter.

Involvement of both extracellular signal-regulated kinase and c-jun N-terminal kinase pathways in the 12-O-tetradecanoylphorbol-13-acetate-induced upregulation of p21(Cip1) in colon cancer cells

Protein kinase C (PKC), a family of serine-threonine kinases, has been implicated in the regulation of colon tumorigenesis. However, the specific isoform of PKC involved in this process is not clear. In the present study, we found that treatment of the cultured human colon cancer cell line COLO-205 with a PKC agonist, 12-O-tetradecanoylphorbol-13-acetate (TPA), resulted in cell-cycle arrest at the G(0)/G(1) phase, decrease in cell number, PKCgamma isoform translocation, and upregulation of p21(Cip1) protein. Pretreatment of the cells with a PKC inhibitor, staurosporine, prevented the TPA-induced upregulation of p21(Cip1) protein. Based on the findings of the present study including that (a) both extracellular signal-regulated kinase (ERK) and c-jun N-terminal kinase (JNK) were activated in the TPA-treated COLO-205 cells, (b) pretreatment with the mitogen-activated protein kinase kinase inhibitor PD98059 but not with the p38 mitogen-activated protein kinase inhibitor SB203580 blocked the TPA-induced p21(Cip1) in COLO-205 cells, and (c) transient transfection of the COLO-205 cells with dominant negative ERK or JNK plasmid significantly suppressed the TPA-induced p21(Cip1) protein induction, we conclude that both the ERK and JNK pathways are involved in the TPA-induced upregulation of p21(Cip1) protein in the COLO-205 cells.

Drm/Gremlin transcriptionally activates p21(Cip1) via a novel mechanism and inhibits neoplastic transformation

Drm/Gremlin, a member of the Dan family of BMP antagonists, is known to function in early embryonic development, but is also expressed in a tissue-specific fashion in adults and is significantly downregulated in transformed cells. In this report, we demonstrate that overexpression of Drm in the tumor-derived cell lines Daoy (primitive neuroectodermal) and Saos-2 (osteoblastic), either under ecdysone-inducible or constitutive promoters, significantly inhibits tumorigenesis. Furthermore, Drm overexpression in these cells increases the level of p21(Cip1) protein and reduces the level of phosphorylated p42/44 MAP kinase. Finally, our data indicate that Drm can induce p21(Cip1) transcriptionally via a novel pathway that is independent of p53 and the p38 and p42/44 MAP kinases. These results provide evidence that Drm can function as a novel transformation suppressor and suggest that this may occur through its affect on the levels of p21(Cip1) and phosphorylated p42/44 MAPK.

Hepatocyte growth factor inhibits anoikis in head and neck squamous cell carcinoma cells by activation of ERK and Akt signaling independent of NFkappa B

Hepatocyte growth factor (HGF), also known as a scatter factor, regulates a variety of biological activities including cell proliferation, survival, migration, and angiogenesis. Importantly, HGF and its receptor c-Met have been found to be associated with metastasis of human head and neck squamous cell carcinoma (HNSCC). Because anoikis resistance plays an important role in tumor progression and metastasis, here we examined whether HGF suppressed suspension-induced apoptosis (anoikis) in HNSCC cells, and if so, we assessed downstream signaling pathways mediated by HGF. We found that HNSCC cells underwent anoikis upon loss of matrix contact, whereas HGF provided protection against it. HGF-induced anoikis resistance was found to be dependent on both ERK and Akt signaling pathways. The inhibition of either ERK or Akt activation abolished HGF-mediated survival. Furthermore, we found that HGF did not activate NFkappaB transcription in HNSCC cells and that HGF-mediated anoikis resistance was independent of NFkappaB. Taken together, our results suggest that anoikis resistance induced by HGF may also play an important role in the progression and metastasis of HNSCC.

Mechanisms of TGF-beta signaling in regulation of cell growth and differentiation

Transforming growth factor beta (TGF-beta) is a secreted protein that regulates proliferation, differentiation and death of various cell types. All immune cell lineages, including B, T and dendritic cells as well as macrophages, secrete TGF-beta, which negatively regulates their proliferation, differentiation and activation by other cytokines. Thus, TGF-beta is a potent immunosuppressor and perturbation of TGF-beta signaling is linked to autoimmunity, inflammation and cancer. Regulation of cell proliferation and differentiation by TGF-beta is a topic of great basic and clinical importance. We summarize our work on the growth inhibitory pathway downstream of TGF-beta, which is triggered by receptor serine/threonine kinases at the cell surface and downstream effectors of the Smad family. Activated Smads regulate transcription of target genes, including cell cycle inhibitors such as p21, which mediate the anti-proliferative response and partially explain the tumor suppressive action of the TGF-beta pathway. We have described a molecular mechanism of regulation of the p21 gene by Smads and transcription factor Sp1. At late stages of tumor progression, TGF-beta promotes tumorigenesis via suppression of the immune system and changes in cell differentiation of epithelial tumor cells, a phenomenon termed epithelial to mesenchymal transdifferentiation (EMT). We review our work on the role of the Smad pathway in controlling EMT. In conclusion, the molecular pathways that describe the anti-proliferative and transdifferentiating effects of TGF-beta in epithelial cells have been uncovered to great molecular detail; a future challenge will be to test their generality in other systems, including the immune system.

Overview of causes of COPD. New understanding of pathogenesis and mechanisms can guide future therapy

Recent studies have greatly expanded the understanding of pathogenetic mechanisms underlying COPD. Cigarette smoking is the most important cause. Activation of inflammatory mechanisms in the lung leads to tissue damage by a variety of mechanisms. Active proteases and oxidants are believed to play crucial roles. In response to this injury, repair mechanisms are activated. Unfortunately, these repair mechanisms also may be adversely affected by cigarette smoking. Insufficient repair of alveolar structures in response to injury likely leads to emphysema. Repair with scarring likely leads to airflow limitation in chronic bronchitis. Moreover, mediators that drive these processes have systemic effects that contribute to the syndromes characteristic of COPD. Understanding these mechanisms has created novel therapeutic approaches to the management of this disorder.

Troglitazone activates p21Cip/WAF1 through the ERK pathway in HCT15 human colorectal cancer cells

In this study, we identified a new mechanism for the anti-proliferation of HCT15 colorectal cancer cells by troglitazone (TRO). Treating HCT15 cells with 20 microM of TRO transiently increased extracellular signal regulated kinase (ERK) activity within 15 min, and this subsequently induced p21Cip/WAF1 cell cycle regulator and localized in the nucleus. Raf-1 modification and MEK activation also occurred after TRO treatment, and Elk-1-dependent trans-reporter gene expression was concomitantly induced. The induction and nuclear localization of p21Cip/WAF1 by TRO were blocked by PD98059 pre-treatment, which suggested a role for the ERK pathway in p21Cip/WAF1 activation. TRO inhibited BrdU incorporation and no BrdU incorporation was observed in most p21Cip/WAF1-activated cells. Therefore, TRO regulates the proliferation of HCT15 cells at least partly by a mechanism involving the activation of p21Cip/WAF1.

Cross-talk between ERK and p38 MAPK mediates selective suppression of pro-inflammatory cytokines by transforming growth factor-beta

Phagocytosis of apoptotic cells by macrophages results in the production of transforming growth factor-beta (TGF-beta), which plays an important role in induction of an anti-inflammatory phenotype and resolution of inflammation. In this study, we show that TGF-beta prevents pro-inflammatory cytokine production through inhibition of p38 mitogen-activated protein kinase (MAPK) and NF-kappaB. Blockade of extracellular signal-regulated kinase (ERK) signaling by the MEK-1/2 inhibitor PD 98059 reversed the inhibitory effects of TGF-beta, suggesting that cross-talk between MAPKs is essential for this response. Further investigation indicated that TGF-beta activated ERK, which in turn up-regulated MAPK phosphatase-1, thereby inactivating p38 MAPK. On the other hand, TGF-beta maintained or slightly increased production of the CC chemokine MCP-1, which is regulated predominantly by AP-1. Although SB 203580, an inhibitor of p38 MAPK, and dominant-negative p38 MAPK both increased AP-1 transcription, lack of effect of TGF-beta on lipopolysaccharide-stimulated SAPK/JNK phosphorylation along with a demonstrated inhibition of TGF-beta-induced AP-1 activation by dominant-negative Smad3 suggest that TGF-beta-stimulated AP-1 activation was not caused by inhibition of p38 MAPK but rather through the activation of Smads. Our data provide evidence that TGF-beta selectively inhibits inflammatory cytokine production through cross-talk between MAPKs.

Cripto-1 activates nodal- and ALK4-dependent and -independent signaling pathways in mammary epithelial Cells

Cripto-1 (CR-1), an epidermal growth factor-CFC (EGF-CFC) family member, has a demonstrated role in embryogenesis and mammary gland development and is overexpressed in several human tumors. Recently, EGF-CFC proteins were implicated as essential signaling cofactors for Nodal, a transforming growth factor beta family member whose expression has previously been defined as embryo specific. To identify a receptor for CR-1, a human brain cDNA phage display library was screened using CR-1 protein as bait. Phage inserts with identity to ALK4, a type I serine/threonine kinase receptor for Activin, were identified. CR-1 binds to cell surface ALK4 expressed on mammalian epithelial cells in fluorescence-activated cell sorter analysis, as well as by coimmunoprecipitation. Nodal is coexpressed with mouse Cr-1 in the mammary gland, and CR-1 can phosphorylate the transcription factor Smad-2 in EpH-4 mammary epithelial cells only in the presence of Nodal and ALK4. In contrast, CR-1 stimulation of mitogen-activated protein kinase and AKT in these cells is independent of Nodal and ALK4, suggesting that CR-1 may modulate different signaling pathways to mediate its different functional roles.

Suppressor and oncogenic roles of transforming growth factor-beta and its signaling pathways in tumorigenesis

Transforming growth factor-beta (TGF-beta) has been implicated in oncogenesis since the time of its discovery almost 20 years ago. The complex, multifunctional activities of TGF-beta endow it with both tumor suppressor and tumor promoting activities, depending on the stage of carcinogenesis and the responsivity of the tumor cell. Dysregulation or alteration of TGF-beta signaling in tumorigenesis can occur at many different levels, including activation of the ligand, mutation or transcriptional suppression of the receptors, or alteration of downstream signal transduction pathways resulting from mutation or changes in expression patterns of signaling intermediates or from changes in expression of other proteins which modulate signaling. New insights into signaling from the TGF-beta receptors, including the identification of Smad signaling pathways and their interaction with mitogen-activated protein (MAP) kinase pathways, are providing an understanding of the changes involved in the change from tumor suppressor to tumor promoting activities of TGF-beta. It is now appreciated that loss of sensitivity to inhibition of growth by TGF-beta by most tumor cells is not synonymous with complete loss of TGF-beta signaling but rather suggests that tumor cells gain advantage by selective inactivation of the tumor suppressor activities of TGF-beta with retention of its tumor promoting activities, especially those dependent on cross talk with MAP kinase pathways and AP-1.

TGF-beta1 is an autocrine mediator of renal tubular epithelial cell growth and collagen IV production

Recent studies in cultured cells have provided evidence that a variety of pathobiologic stimuli, including high glucose, angiotensin II, and thromboxane A(2), trigger a signaling pathway leading to autocrine induction of TGF-beta1. TGF-beta1 production through this pathway may profoundly affect cell growth, matrix synthesis, and response to injury. This study examines the role of autocrine versus exogenously added TGF-beta1 in cellular proliferation and collagen IV production, critical targets of TGF-beta1 signaling, using renal cells derived from TGF-beta1 knockout (KO) animals or wild-type (WT) controls. Growth of WT and KO cells was assessed by cell counting and [(3)H]thymidine uptake. Basal and TGF-beta1-stimulated collagen production was assessed by Northern and Western blotting; transcriptional activity of the alpha1(IV) collagen gene was assessed by transient transfection analysis. KO cells grew at a faster rate than WT cells carefully matched for plating density and passage number. This increased growth rate was paralleled by increases in [(3)H]thymidine uptake. KO cells expressed lower levels of the cell cycle inhibitors p21 and p27 than WT cells. KO cells failed to express TGF-beta1, as expected. Basal TGF-beta3 mRNA levels were higher in KO cells than in WT cells. WT cells expressed higher basal levels of TGF-beta2 mRNA than KO cells. Basal alpha1(IV) and alpha2(IV) collagen mRNA and protein expression were significantly lower in KO cells than WT cells. Administration of exogenous TGF-beta1 induced collagen IV production in both KO and WT cells. Although basal transcriptional activity of an alpha1(IV) collagen-CAT construct was lower in KO cells than WT cells, administration of exogenous TGF-beta1 was associated with significant increases in transcriptional activity of this construct in both KO and WT cells. These studies provide evidence that autocrine production of TGF-beta1 may play a critical role in regulation of growth and basal collagen IV production by renal tubular epithelial cells.