Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UMLS:C0019204 (hepatocellular carcinoma)
 71,386 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The cell cycle inhibitor p21/WAF1/Cip1 is expressed in many cell types and is regulated by p53-dependent and p53-independent mechanisms. p21 is an important regulator of hepatocyte cell cycle, differentiation, and liver development, but little is known about the regulation of its synthesis in hepatocytes. We report herein that the p21 gene is constitutively expressed in human hepatoma HepG2 cells. Deletion analysis of the p21 promoter showed that it contains a distal (positions -2,300/-210) and a proximal (positions -124 to -61) region that act synergistically to achieve high levels of constitutive expression. The proximal region that consists of multiple Sp1 binding sites is essential for constitutive p21 promoter activity in hepatocytes. This region also mediates the transcriptional activation of the p21 promoter by members of the Smad family of proteins, which play important role in the transduction of extracellular signals such as transforming growth factor beta, activin, etc. Constitutive expression of p21 was severely reduced by a C-terminally truncated form of Smad4 that was shown previously to block signaling through Smads. Smad3/4 and to a much lesser extent Smad2/4 caused high levels of transcriptional activation of the p21 promoter. Transactivation was compromised by N- or C-terminally truncated forms of Smad3. By using Gal4-Sp1 fusion proteins, we show that Smad proteins can activate gene transcription via functional interactions with the ubiquitous factor Sp1. These data demonstrate that Smad proteins and Sp1 participate in the constitutive or inducible expression of the p21 gene in hepatic cells.
 ...
PMID:Regulation of the human p21/WAF1/Cip1 promoter in hepatic cells by functional interactions between Sp1 and Smad family members. 961 81

The transforming growth factor-beta (TGF-beta) family of cytokines mediates multiple biological effects by regulating the expression of target genes. The Smad family proteins function as intracellular signal transducers downstream of the receptors to transmit the TGF-beta signal from cell membrane to nucleus. The mechanisms by which Smads mediate transcriptional activation of target genes is largely unknown. Here we report the identification of a novel TGF-beta-responsive element in the human type 1 plasminogen activator inhibitor promoter that is required for mediating strong transcriptional activation of this gene by TGF-beta. Smad3 and Smad4 are incorporated into a TGF-beta-inducible complex formed on this element following TGF-beta stimulation of human hepatoma cells. Both Smad3 and Smad4 bind directly to this TGF-beta-responsive element through their conserved MH1 domains. These results indicate that Smad3 and Smad4 mediate TGF-beta signaling by directly interacting with a specific response element in a physiological target gene.
 ...
PMID:Smad4/DPC4 and Smad3 mediate transforming growth factor-beta (TGF-beta) signaling through direct binding to a novel TGF-beta-responsive element in the human plasminogen activator inhibitor-1 promoter. 979 26

Smad7 is a regulatory Smad protein that is able to antagonize signal transduction by transforming growth factor-beta (TGF-beta) and activin receptors. To characterize the regulation of Smad7 at the transcriptional level, we isolated the promoter region of the mouse Smad7 gene. When the Smad7 promoter luciferase reporter gene (-408 and +112 bp) was expressed in human hepatoma (HepG2) cells, its transcriptional activity was increased following TGF-beta or activin treatment. In addition, this region of the Smad7 promoter was stimulated by ectopic expression of Smad3 as well as constitutively active TGF-beta and activin receptors, indicating that Smad7 transcription was modulated by the signaling downstream those two receptors. A gel mobility shift assay indicated that a DNA fragment spanning -408 to -126 base pairs (bp) was able to directly bind purified Smad4. Furthermore, a consensus Smad3-Smad4 binding element (SBE) was discovered in this region of the promoter with a palindromic sequence of GTCTAGAC. A 33-bp Smad7 promoter fragment containing this SBE was able to bind Smad3 and Smad4. In human embryonic kidney 293 cells, the expression of constitutively active TGF-beta type I receptor was able to induce the formation of a Smad3- and Smad4-containing nuclear protein complex that bound the SBE. In HepG2 cells, TGF-beta1 treatment could induce the formation of an endogenous SBE-binding complex. Taken together, these data provided the first evidence that Smad7 transcription is regulated by TGF-beta and activin signaling through direct binding of Smad3 and Smad4 to the Smad7 promoter.
 ...
PMID:Regulation of Smad7 promoter by direct association with Smad3 and Smad4. 1055 22

The serum concentration of transforming growth factor beta (TGF-beta) is elevated as tumors progress in hepatocellular carcinoma (HCC) patients. In this study, we examined whether modulation of tumor-derived TGF-beta signal transduction contributes to malignant progression. We investigated the production of TGF-beta1, the biological effects of TGF-beta and neutralizing antibody on HCC cells, activation of Smad 2, Smad 3, and Smad 4, induction of antagonistic Smads (Smad 6 and Smad 7), and promoter activities of two target genes, plasminogen activator inhibitor type 1 (PAI-1) and p15INK4B. In human cell lines HCC-M and HCC-T, TGF-beta accelerates their proliferation. Smad 2 was activated constitutively by an autocrine mechanism, because in the absence of exogenous TGF-beta, a high level of Smad 2 phosphorylation, induction of PAI-1 transcripts, and nuclear localization of Smad 2 were observed. This constitutive activation of Smad 2 was, at least in part, attributable to the lack of induction of antagonistic Smads by TGF-beta. However, Smads activated by tumor-derived TGF-beta constantly suppressed p151NK4B expression. In addition, 3 of 10 human HCC tissues showed nuclear localization of Smad 2 and low mRNA levels of p15INK4B and antagonistic Smads but a high level of PAI-1. Our observations suggest that this constant suppression of the p15INK4B gene could be involved in the malignant progression of HCC.
 ...
PMID:Autocrine stimulatory mechanism by transforming growth factor beta in human hepatocellular carcinoma. 1072 5

Activation of transforming growth factor-beta (TGF-beta) and activin receptors leads to phosphorylation of Sma- and Mad-related protein 2 (Smad2) and Smad3, which function as transcription factors to regulate gene expression. Smad7 is a regulatory protein which is able to inhibit TGF-beta and activin signalling in a negative-feedback loop, mediated by a direct regulation by Smad3 and Smad4 via a Smad-binding element (SBE) in the Smad7 promoter. Interestingly, we found that the Smad7 promoter was also regulated by nuclear factor kappaB (NF-kappaB), a transcription factor which plays an important role in inflammation and the immune response. Expression of NF-kappaB p65 subunit was able to inhibit the Smad7 promoter activity, and this inhibition could be reversed by co-expression of IkappaB, an inhibitor of NF-kappaB. In addition, the inhibitory activity of p65 was observed in a minimal promoter that contained only the Smad7 SBE and a TATA box, without any consensus NF-kappaB binding site. This inhibitory effect appeared to be common to other TGF-beta- and activin-responsive promoters, since p65 also inhibited the forkhead-activin-signal-transducer-2-mediated activation of a Xenopus Mix.2 promoter, as well as the Smad3-mediated activation of 3TP-lux which contains PMA-responsive elements and a plasminogen-activator-inhibitor-1 promoter. Activation of endogenous NF-kappaB by tumour necrosis factor-alpha (TNF-alpha) was also able to inhibit the Smad7 promoter in human embryonic kidney 293 cells. In human hepatoma HepG2 cells, TNF-alpha was able to inhibit TGF-beta- and activin-mediated transcriptional activation. Furthermore, overexpression of the transcription co-activator p300 could abrogate the inhibitory effect of NF-kappaB on the Smad7 promoter. Taken together, these data have indicated a novel mode of crosstalk between the Smad and the NF-kappaB signalling cascades at the transcriptional level by competing for a limiting pool of transcription co-activators.
 ...
PMID:Repression of transforming-growth-factor-beta-mediated transcription by nuclear factor kappaB. 1083 91

Transforming growth factor-beta (TGF-beta) inhibits cell cycle progression, in part through up-regulation of gene expression of the p21(WAF1/Cip1) (p21) cell cycle inhibitor. Previously we have reported that the intracellular effectors of TGF-beta, Smad3 and Smad4, functionally cooperate with Sp1 to activate the human p21 promoter in hepatoma HepG2 cells. In this study we show that Smad3 and Smad4 when overexpressed in HaCaT keratinocytes lead to activation of the p21 promoter. Activation requires the binding sites for the ubiquitous transcription factor Sp1 on the proximal promoter. Induction of the endogenous HaCaT p21 gene by TGF-beta1 is further enhanced after overexpression of Smad3 and Smad4, whereas dominant negative mutants of Smad3 and Smad4 and the inhibitory Smad7 all inhibit p21 induction by TGF-beta1 in a dose-dependent manner. We show that Sp1 expressed in the Sp1-deficient Drosophila SL-2 cells binds to the proximal p21 promoter sequences, whereas Smad proteins do not. In support of this finding, we show that DNA-binding domain mutants of Smad3 and Smad4 are capable of transactivating the p21 promoter as efficiently as wild type Smads. Co-expression of Smad3 with Smad4 and Sp1 in SL-2 cells or co-incubation of phosphorylated Smad3, Smad4, and Sp1 in vitro results in enhanced binding of Sp1 to the p21 proximal promoter sequences. We demonstrate that Sp1 physically and directly interacts with Smad2, Smad3, and weakly with Smad4 via their amino-terminal (Mad-Homology 1) domain. Finally, by using GAL4 fusion proteins we show that the glutamine-rich sequences in the transactivation domain of Sp1 contribute to the cooperativity with Smad proteins. In conclusion, Smad proteins play important roles in regulation of the p21 gene by TGF-beta, and the functional cooperation of Smad proteins with Sp1 involves the physical interaction of these two types of transcription factors.
 ...
PMID:Role of Smad proteins and transcription factor Sp1 in p21(Waf1/Cip1) regulation by transforming growth factor-beta. 1087 24

Activin A, a member of the transforming growth factor beta (TGFbeta) superfamily, blocks interleukin (IL)-6 biological functions. The molecular basis of the influence of this TGFbeta signaling on the IL-6 receptor triggered cascade is unknown. We studied IL-6-induced secretion of the acute phase protein haptoglobin by hepatoma cells. Overexpression of the C/EBPbeta gene, a downstream effector in the IL-6 pathway, activated transcription from the haptoglobin promoter. This was abolished by either a constitutively active form of activin A type IB receptor (CAactRIB) or by a combination of Smad3 and Smad4. Similarly, Smads abolished transcriptional activation by co-stimulation with IL-6 and STAT3. The transcription co-activator p300 partially overcame the suppressive effect of Smads. Electrophoretic mobility shift assays indicated that C/EBPbeta binding to haptoglobin promoter DNA was reduced by over-expression of CAactRIB and Smad4. We thus show that Smad proteins operate as transcription inhibitors on target genes of the IL-6 induced pathway. The effect of Smads is exerted on components of the transcription activation complex and may also involve interference with DNA binding. This study thus depicts molecular sites of interaction between the TGFbeta superfamily and the IL-6 signaling cascades.
 ...
PMID:Smad proteins suppress CCAAT/enhancer-binding protein (C/EBP) beta- and STAT3-mediated transcriptional activation of the haptoglobin promoter. 1133 Dec 73

Transforming growth factor beta (TGF-beta) induces apoptosis in a variety of cells. We have previously shown that TGF-beta 1 rapidly induces apoptosis in the FaO rat hepatoma cell line. We have now studied the effect of TGF-beta 1 on the expression of different members of the Bcl-2 family in these cells. We observed no detectable changes in the steady-state levels of Bcl-2, Bcl-X(L), and Bax. However, TGF-beta 1 induced caspase-dependent cleavage of BAD at its N terminus to generate a 15-kDa truncated protein. Overexpression of the 15-kDa truncated BAD protein enhanced TGF-beta 1-induced apoptosis, whereas a mutant BAD resistant to caspase 3 cleavage blocked TGF-beta 1-induced apoptosis. Overexpression of Smad3 dramatically enhanced TGF-beta 1-induced cleavage of BAD and apoptosis, whereas antisense Smad3 blocked TGF-beta 1-induced apoptosis and BAD cleavage. These results suggest that TGF-beta 1 induces apoptosis through the cleavage of BAD in a Smad3-dependent mechanism.
 ...
PMID:Transforming growth factor beta 1 induces apoptosis through cleavage of BAD in a Smad3-dependent mechanism in FaO hepatoma cells. 1183 4

Transforming growth factor (TGF)-beta and activin A inhibit the growth and induce cell death of parenchymal liver cells. Smad proteins have recently been identified as intracellular signaling mediators and modulators of TGF-beta family members. This study assessed the role of Smad proteins during the action of TGF-beta and activin A on liver cells using a well-differentiated human hepatoma cell line, Hep3B cells. To study the role of Smad proteins in the anti-proliferative, apoptosis-inducing, activities of TGF-beta and activin A in liver cells, we stably transfected dominant negative Smad2-3SA or Smad3-3SA mutants in Hep3B cells. Transfection of Smad2-3SA or Smad3-3SA abrogated both TGF-beta-induced and activin A-induced growth inhibition and apoptosis of Hep3B cells. Down regulation of Bcl-xL expression by TGF-beta was both Smad2 and Smad3 dependent. We also demonstrate that transfection of Smad7, an intracellular antagonist of Smad signaling, inhibited both TGF-beta- and activin A-induced apoptosis and growth inhibition of these cells. These results suggest that Smad proteins positively and negatively mediate TGF-beta-induced and activin A-induced apoptosis and growth inhibition of liver cells.
 ...
PMID:Involvement of Smad proteins in TGF-beta and activin A-induced apoptosis and growth inhibition of liver cells. 1207 17

Transforming growth factor beta (TGF-beta) is a potent inhibitor of cell proliferation and the loss of responsiveness to TGF-beta may contribute to the development of human cancers. In hepatocellular carcinomas, the potential role of TGF-beta signaling as a tumor suppressor pathway can be illustrated by the presence of mutations in genes encoding TGF-beta receptors or downstream components of this signaling such as Smad2. Although Smad2 is mutated in hepatocellular carcinomas, the alteration of TGF-beta signaling with respect to tumor progression remains to be established. Using the HepG2 hepatoma cells, we showed here that expression of Smad2.Q407R, a missense mutation found in human hepatocellular carcinoma, was less effective than expression of wild-type Smad2 in enhancing the ability of TGF-beta to induce transcription from the Mix.2 promoter. This effect was specifically associated with a decrease in the steady-state level of Smad2.Q407R, presumably because of an enhancement of its ubiquitination and degradation through the proteasome machinery. More importantly, we found that the unstability of Smad2.Q407R was reversed when this mutant undergoes homo-oligomerization with wild-type Smad2 or hetero-oligomerization with Smad3 within the cells. Therefore, our findings allowed us to propose a novel mechanism for suppression of the deleterious effect of a tumor-derived mutation of Smad2, which loss may lead to dysregulated cell proliferation during tumorigenesis.
 ...
PMID:Evidence for a role of Smad3 and Smad2 in stabilization of the tumor-derived mutant Smad2.Q407R. 1270 Feb 38


1 2 3 4 5 6 7 8 9 Next >>