Growth factor-induced transcription via the serum response element is inhibited by cyclic adenosine 3',5'-monophosphate in MCF-7 breast cancer cells

Systematic analysis of lncRNA-miRNA-mRNA competing endogenous RNA network identifies four-lncRNA signature as a prognostic biomarker for breast cancer

Background

Increasing evidence has underscored the role of long non-coding RNAs (lncRNAs) acting as competing endogenous RNAs (ceRNAs) in the development and progression of tumors. Nevertheless, lncRNA biomarkers in lncRNA-related ceRNA network that can predict the prognosis of breast cancer (BC) are still lacking. The aim of our study was to identify potential lncRNA signatures capable of predicting overall survival (OS) of BC patients.

Methods

The RNA sequencing data and clinical characteristics of BC patients were obtained from the Cancer Genome Atlas database, and differentially expressed lncRNA (DElncRNAs), DEmRNAs, and DEmiRNAs were then identified between BC and normal breast tissue samples. Subsequently, the lncRNA–miRNA–mRNA ceRNA network of BC was established, and the gene oncology enrichment analyses for the DEmRNAs interacting with lncRNAs in the ceRNA network was implemented. Using univariate and multivariate Cox regression analyses, a four-lncRNA signature was developed and used for predicting the survival in BC patients. We applied receiver operating characteristic analysis to assess the performance of our model.

Results

A total of 1061 DElncRNAs, 2150 DEmRNAs, and 82 DEmiRNAs were identified between BC and normal breast tissue samples. A lncRNA–miRNA–mRNA ceRNA network of BC was established, which comprised of 8 DEmiRNAs, 48 DElncRNAs, and 10 DEmRNAs. Further gene oncology enrichment analyses revealed that the DEmRNAs interacting with lncRNAs in the ceRNA network participated in cell leading edge, protease binding, alpha-catenin binding, gamma-catenin binding, and adenylate cyclase binding. A univariate regression analysis of the DElncRNAs revealed 7 lncRNAs (ADAMTS9-AS1, AC061992.1, LINC00536, HOTAIR, AL391421.1, TLR8-AS1 and LINC00491) that were associated with OS of BC patients. A multivariate Cox regression analysis demonstrated that 4 of those lncRNAs (ADAMTS9-AS1, LINC00536, AL391421.1 and LINC00491) had significant prognostic value, and their cumulative risk score indicated that this 4-lncRNA signature independently predicted OS in BC patients. Furthermore, the area under the curve of the 4-lncRNA signature associated with 3-year survival was 0.696.

Conclusions

The current study provides novel insights into the lncRNA-related ceRNA network in BC and the 4 lncRNA biomarkers may be independent prognostic signatures in predicting the survival of BC patients.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1640-2) contains supplementary material, which is available to authorized users.

Breast cancer cells proliferation is regulated by tyrosine phosphatase SHP1 through c-jun N-terminal kinase and cooperative induction of RFX-1 and AP-4 transcription factors

In this study, we show that proliferation of breast cancer cells is suppressed by IGF-1-activated JNK MAPK pathway. The molecular mechanism by which c-jun-NH,-kinase (JNK) activation induces antiproliferative signals in IGF-1-stimulated breast cancer cells remains unknown. Tyrosine phosphatase SHP1 is known to negatively regulate signal transduction pathways activated by cell surface receptors including IGF-1. Moreover, SHP1 transcript and protein levels are increased in epithelial tumors. Therefore, we hypothesized that IGF-activated JNK induces expression of SHP1 in breast cancer cells. To further clarify the role of SHP1 in tumor growth, we correlated the proliferation rates of breast adenocarcinoma cells with SHP1 expression and JNK activation. We show that proliferation of serum- or IGF-1-stimulated breast adenocarcinoma cells is negatively regulated by SHP1 and show for the first time that IGF-1-activated JNK induces SHP1 expression in MCF-7 cells used as experimental model. In an attempt to understand the mechanism by which serum- or IGF-1-activated JNK induces SHP1 expression resulting in suppression of cell proliferation, we reveal for the first time that in serum- or IGF-1-stimulated breast cancer MCF-7 cells, JNK induces SHP1 expression through the binding of AP-4 and RFX-1 transcription factors to the epithelial tissue-specific SHP1 promoter. Overall, we show for the first time that IGF-1-stimulated proliferation of breast adenocarcinoma cells is negatively regulated by SHP1 through activation of JNK.

Neuropeptide Y Y5 receptor promotes cell growth through extracellular signal-regulated kinase signaling and cyclic AMP inhibition in a human breast cancer cell line

Overexpression of neuropeptide Y (NPY) and its receptor system has been reported in various types of cancers. NPY Y5 receptor (Y5R) has been implicated in cell growth and angiogenesis. However, the role of Y5R in breast cancer is unknown. To identify the role of Y5R in breast cancer, we screened several breast cancer cell lines to examine the expression of Y5R and its function in breast cancer. All screened cell lines express both Y1 receptor and Y5R except BT-549, which expresses mainly Y5R. Binding studies showed that NPY, Y5R-selective agonist peptide, and Y5R-selective antagonist (CGP71683A) displaced (125)I-PYY binding in BT-549 cell membranes in a dose-dependent manner. The displacement studies revealed the presence of two binding sites in Y5R with IC(50) values of 29 pmol/L and 531 nmol/L. NPY inhibited forskolin-stimulated cyclic AMP accumulation with an IC(50) value of 52 pmol/L. NPY treatment of BT-549 cells induced extracellular signal-regulated kinase phosphorylation but did not alter intracellular calcium. Y5R activation stimulates BT-549 cell growth, which is inhibited by CGP71683A, pertussis toxin, and extracellular signal-regulated kinase blockade. CGP71683A alone induced cell death in a time- and dose-dependent manner in Y5R-expressing cells. The stimulation of MDA MB-231 cell migration by NPY is inhibited by CGP71683A. Together, our results suggest that Y5R plays an important role in cancer cell growth and migration and could be a novel therapeutic target for breast cancer.

Direct inhibition of ERK1/2 phosphorylation as a possible mechanism for the antiproliferative action of 3,4-diOH-PCB3 in the MCF-7 cell line

Our previously published data showed that 260h of exposure to 3,4-diOH-PCB3 decreased proliferation in the MCF-7 cell line. In the present study, we sought to determine whether this is due to action on the SHBG/cAMP/PKA system, activation of which can inhibit cell proliferation, or to direct inhibition of ERK1/2 phosphorylation. MCF-7 human breast cancer cells were treated for 72h with 4-monochlorobiphenyl (PCB3), 4'-hydroxy-4-monochlorobiphenyl (4-OH-PCB3) or 3'4'-dihydroxy-4-monochlorobiphenyl (3,4-diOH-PCB3) (300nM). After the completion of the treatment, cell proliferation was measured with a BrdU incorporation assay. SHBG, cAMP, PKA and ERK1/2 levels in the cells were determined via ELISA. PCB3 and 4-OH-PCB3 had no effect on extra- or intracellular SHBG levels, while a stimulation of SHBG intra- but not extracellular levels was noted in cells exposed to 3,4-diOH-PCB3. Both, pre- and co-incubation with SHBG decreased the proliferation of 3,4-diOH-PCB3-treated cells. Neither PCB3 nor its metabolite had an effect on the cAMP/PKA pathway. A decrease of both ERK1/2 forms was noted under the influence of 3,4-diOH-PCB3. In conclusion, the data presented clearly showed that the antiproliferative action of 3,4-diOH-PCB3 is not mediated by activation of the SHBG/AMP/PKA pathway, but many other plasma membrane receptors seem to be involved in the non-genomic action of 3,4-diOH-PCB3, and instead is due to direct inhibition of the ERK1/2 system.

Differential effects of muscle fibre length and insulin on muscle-specific mRNA content in isolated mature muscle fibres during long-term culture

The aims of this study were (1) to determine the relationship between muscle fibre cross-sectional area and cytoplasmic density of myonuclei in high- and low-oxidative Xenopus muscle fibres and (2) to test whether insulin and long-term high fibre length caused an increase in the number of myonuclei and in the expression of alpha-skeletal actin and of myogenic regulatory factors (myogenin and MyoD) in these muscle fibres. In high- and low-oxidative muscle fibres from freshly frozen iliofibularis muscles, the number of myonuclei per millimetre fibre length was proportional to muscle fibre cross-sectional area. The in vivo myonuclear density thus seemed to be strictly regulated, suggesting that the induction of hypertrophy required the activation of satellite cells. The effects of muscle fibre length and insulin on myonuclear density and myonuclear mRNA content were investigated on high-oxidative single muscle fibres cultured for 4-5 days. Muscle fibres were kept at a low length (~15% below passive slack length) in culture medium with a high insulin concentration (~6 nmol/l: "high insulin medium") or without insulin, and at a high length (~5% above passive slack length) in high insulin medium. High fibre length and high insulin medium did not change the myonuclear density of isolated muscle fibres during culture. High insulin increased the myonuclear alpha-skeletal actin mRNA content, whereas fibre length had no effect on alpha-skeletal actin mRNA content. After culture at high fibre length in high insulin medium, the myonuclear myogenin mRNA content was 2.5-fold higher than that of fibres cultured at low length in high insulin medium or in medium without insulin. Myonuclear MyoD mRNA content was not affected by fibre length or insulin. These in vitro experiments indicate that high muscle fibre length and insulin enhance muscle gene expression but that other critical factors are required to induce adaptation of muscle fibre size and performance.

Protein kinase A inhibits lysophosphatidic acid induction of serum response factor via alterations in the actin cytoskeleton

Lysophosphatidic acid (LPA; 1-acyl-2-hydroxy-sn-glycero-3-phosphate) is a lipid growth factor that stimulates the proliferation of ovarian cancer cells. Recent studies indicate that elevation of cellular cAMP levels inhibits ovarian epithelial cancer cell growth. In this study, we investigated the effects of elevating cellular cAMP levels on LPA stimulation of OVCAR-3 ovarian cancer cell growth and on LPA stimulation of the serum response factor (SRF) transcription factor. Treatment of OVCAR-3 cells with forskolin and isobutylmethylxanthine (IBMX; 3-Isobutyl-1-methylxanthine) inhibited LPA stimulation of growth. LPA stimulation of SRF-mediated transcription was also inhibited in OVCAR-3 cells that were incubated with forskolin, dibutyryl cyclic AMP (db-cAMP), or paired cAMP analogues (N(6)-mono-tert-butylcarbamoyladenosine-3', 5'-cyclic monophosphate [6-MBC-cAMP] and Sp-5,6-DCl-BIMPS), which selectively activate type II protein kinase A. In contrast, incubation with a cAMP analogue (8-(4-chloro-phenylthio)-2'-O-methyadenosine-3',5'-cyclic monophosphate [8CPT-2Me-cAMP]) that specifically activates the cAMP inducible Rap1 exchange factor, Epac, did not inhibit SRF. Similar results were obtained when HepG2 hepatoma cells, which do not express endogenous LPA receptors, were transfected with a single LPA receptor (LPA(1)). We observed that treatment of OVCAR-3 cells with forskolin greatly reduced both F-actin staining and focal adhesion labeling with anti-paxillin antibodies. Treatment of OVCAR-3 cells with the F-actin stabilizing compound, jasplakinolide, prevented the protein kinase A (PKA)-mediated inhibition of SRF. These results suggest that PKA inhibits LPA stimulation of SRF by promoting the dissolution of F-actin and that this is likely to contribute to the cAMP-mediated inhibition of ovarian cancer cell growth.

Membrane estrogen receptor-alpha levels in MCF-7 breast cancer cells predict cAMP and proliferation responses

Introduction

17β-estradiol (E₂) can rapidly induce cAMP production, but the conditions under which these cAMP levels are best measured and the signaling pathways responsible for the consequent proliferative effects on breast cancer cells are not fully understood. To help resolve these issues, we compared cAMP mechanistic responses in MCF-7 cell lines selected for low (mER^low) and high (mER^high) expression of the membrane form of estrogen receptor (mER)-α, and thus addressed the receptor subform involved in cAMP signaling.

Methods

MCF-7 cells were immunopanned and subsequently separated by fluorescence activated cell sorting into mER^high (mER-α-enriched) and mER^low (mER-α-depleted) populations. Unique (compared with previously reported) incubation conditions at 4°C were found to be optimal for demonstrating E₂-induced cAMP production. Time-dependent and dose-dependent effects of E₂ on cAMP production were determined for both cell subpopulations. The effects of forskolin, 8-CPT cAMP, protein kinase A inhibitor (H-89), and adenylyl cyclase inhibitor (SQ 22,536) on E₂-induced cell proliferation were assessed using the crystal violet assay.

Results

We demonstrated a rapid and transient cAMP increase after 1 pmol/l E₂ stimulation in mER^high cells; at 4°C these responses were much more reliable and robust than at 37°C (the condition most often used). The loss of cAMP at 37°C was not due to export. 3-Isobutyl-1-methylxanthine (IBMX; 1 mmol/l) only partially preserved cAMP, suggesting that multiple phosphodiesterases modulate its level. The accumulated cAMP was consistently much higher in mER^high cells than in mER^low cells, implicating mER-α levels in the process. ICI172,780 blocked the E₂-induced response and 17α-estradiol did not elicit the response, also suggesting activity through an estrogen receptor. E₂ dose-dependent cAMP production, although biphasic in both cell types, was responsive to 50-fold higher E₂ concentrations in mER^high cells. Proliferation of mER^low cells was stimulated over the whole range of E₂concentrations, whereas the number of mER^high cells was greatly decreased at concentrations above 1 nmol/l, suggesting that estrogen over-stimulation can lead to cell death, as has previously been reported, and that mER-α participates. E₂-mediated activation of adenylyl cyclase and downstream participation of protein kinase A were shown to be involved in these responses.

Conclusion

Rapid mER-α-mediated nongenomic signaling cascades generate cAMP and downstream signaling events, which contribute to the regulation of breast cancer cell number.

The role of phosphatidylinositol 3-kinase and the mitogen-activated protein kinases in insulin-like growth factor-I-mediated effects in vascular endothelial cells

Despite an improved understanding of the molecular mechanisms of insulin-like growth factor-I (IGF-I) signaling and the recognition that IGF-I mediates many effects in endothelial cells, some of which may be important for atherosclerosis, little is known about the signal transduction pathways that mediate the effects of IGF-I in endothelial cells. To that end, we examined the signaling pathways activated by IGF-I in endothelial cells and their contribution to IGF-I-stimulated endothelial cell migration and nuclear factor (NF)-kappaB-dependent transcription. Treatment of bovine pulmonary artery endothelial cells (PAEC) with IGF-I activated the mitogen-activated protein kinases extracellular signal-regulated kinase (ERK)1/2 and ERK5. In contrast, IGF-I had no effect on either c-Jun amino-terminal kinase or p38 kinase activity. IGF-I also activated phosphatidylinositol (PI) 3-kinase, as reflected by increased phosphorylation of AKT: There was no evidence of cross-talk between the ERK and PI 3-kinase pathways in PAEC. In PAEC transiently transfected with pTK81-NFkappaB-Luc, which contained four copies of the NF-kappaB DNA binding site 5' to a minimal promoter and the luciferase gene, treatment with 50 ng/ml IGF-I increased luciferase activity 1.8-fold. Inhibition of ERK activity using PD98059 and PI 3-kinase activity with LY 294002 abrogated the induction of NF-kappaB-dependent transcription by IGF-I, suggesting that both pathways contribute to the effect of IGF-I on NF-kappaBdependent transcription. In contrast to the effect of tumor necrosis factor-alpha on NF-kappaB activation, Western blot analyses demonstrated that IGF-I had no effect on IkappaB phosphorylation and degradation or nuclear translocation and DNA binding of NF-kappaB. These data suggest a direct of effect of IGF-I on nuclear NF-kappaB. IGF-I also increased endothelial cell migration approximately 2-fold, as demonstrated using a Boyden chamber apparatus. IGF-I-induced endothelial cell migration was inhibited, in part, by LY 294002 but not PD98059. Together, these studies demonstrate that IGF-I activates multiple signaling pathways in endothelial cells with little evidence for cross-talk between the pathways. Moreover, these pathways appear to mediate both overlapping and distinct effects in that activation of both PI 3-kinase and the ERKs contributed to the stimulation of NF-kappaB-dependent transcription by IGF-I, whereas only PI 3-kinase mediated IGF-I-stimulated endothelial cell migration.

Activation of members of the mitogen-activated protein kinase family by glucose in endothelial cells

To better understand the molecular mechanisms for hyperglycemia-induced proatherogenic changes in endothelial cells, the effect of high glucose on activation of members of the mitogen-activated protein kinase (MAPK) family, including c-Jun NH(2)-terminal kinase (JNK), extracellular signal-regulated kinase (ERK)-1, -2, and -5, and p38 kinase, was examined in bovine pulmonary artery endothelial cells (PAEC). Glucose, fructose, and raffinose induced a concentration-dependent decrease in PAEC growth. Addition of 25 mM glucose, fructose, or raffinose to normal growth medium stimulated an approximately twofold increase in JNK1 activity that was maximal after 24 h, whereas only glucose markedly increased ERK5 activity. Neither ERK1/2 nor p38 kinase activity was increased by glucose, fructose, or raffinose. The antioxidant N-acetylcysteine partially abrogated the glucose-induced increase in ERK5 activity but had no effect on the increase in JNK1 activity. In contrast, azaserine, which prevents increased flux through the hexosamine pathway, decreased glucose-induced JNK1 activity but had no effect on fructose- or raffinose-induced JNK1 activity. Consistent with this finding, glucosamine stimulated a 2.4-fold increase in JNK1 activity and reproduced the inhibitory effect of glucose on PAEC growth. In summary, glucose activates different members of the MAPK family in PAEC via distinct mechanisms. Moreover, the correlation between the ability of different sugars to activate JNK1 and inhibit cell growth suggests that activation of this signaling pathway may contribute to the growth inhibitory effect of glucose in endothelial cells.

Insulin-like growth factor I activates c-Jun N-terminal kinase in MCF-7 breast cancer cells

Insulin-like growth factor I (IGF-I) is an important mediator of breast cancer cell growth, although the signaling pathways important for IGF-I-mediated effects in breast cancer cells are still being elucidated. We had demonstrated previously that increased intracellular cAMP in MCF-7 breast cancer cells inhibited cell growth and IGF-I-induced gene expression, as determined using a reporter gene assay. This effect of cAMP on IGF-I signaling was independent of IGF-I-induced activation of the mitogen-activated protein kinases extracellular signal-regulated kinases 1 and 2 (ERK1 and -2). To determine whether this effect of cAMP may be mediated via another mitogen-activated protein kinase, the ability of IGF-I to activate the c-Jun N-terminal kinases (JNKs) was investigated. Treatment of MCF-7 cells with 100 ng/ml IGF-I increased the level of phosphorylated JNK, as determined by Western blot analysis. JNK phosphorylation was not evident until 15 min after treatment with IGF-I, and peak levels of phosphorylation were present at 30-60 min. This was in contrast to ERK phosphorylation, which was present within 7.5 min of IGF-I treatment. Determination of JNK activity using an immune complex assay demonstrated a 3.3- and 3.5-fold increase in JNK1 and -2 activity, respectively, 30 min after treatment with 100 ng/ml IGF-I. The use of PD98059, which inhibits activation of ERK1 and -2, and LY 294002, an inhibitor of phosphatidylinositol 3-kinase, demonstrated that IGF-I-induced activation of JNK1 is independent of ERK and phosphatidylinositol 3-kinase activation. In contrast, increasing intracellular cAMP with forskolin resulted in abrogation of IGF-I-induced JNK activity. In summary, these data demonstrate that IGF-I activates the JNKs in MCF-7 breast cancer cells and, taken together with the results of our previous study, suggest that JNK may contribute to IGF-I-mediated gene expression and, possibly, cell growth in MCF-7 breast cancer cells.

Interactions between progestins and heregulin (HRG) signaling pathways: HRG acts as mediator of progestins proliferative effects in mouse mammary adenocarcinomas

The present study addressed links between progestin and heregulin (HRG) signaling pathways in mammary tumors. An experimental model of hormonal carcinogenesis, in which the synthetic progestin medroxyprogesterone acetate (MPA) induced mammary adenocarcinomas in female Balb/c mice, was used. MPA induced an in vivo up-regulation of HRG mRNA expression in progestin-dependent (HD) tumor lines. Mammary tumor progression to a progestin-independent (HI) phenotype was accompanied by a high constitutive expression of HRG. The HRG message arose from the tumor epithelial cells. Primary cultures of malignant epithelial cells from a HD tumor line were used to investigate HRG involvement on cell proliferation. HRG induced a potent proliferative effect on these cells and potentiated MPA mitogenic effects. Blocking endogenous HRG synthesis by antisense oligodeoxynucleotides (ASODNs) to HRG mRNA inhibited MPA-induced cell growth, indicating that HRG acts as a mediator of MPA-induced growth. High levels of ErbB-2 and ErbB-3 expression and low ErbB-4 levels were found in HD cells. Treatment of these cells with either MPA or HRG resulted in tyrosine phosphorylation of both ErbB-2 and ErbB-3. Furthermore, both HRG and MPA proliferative effects were abolished when cells were treated with ASODNs to ErbB-2 mRNA, providing evidence for a critical role of ErbB-2 in HRG-induced growth. Finally, blocking type I insulin-like growth factor receptor (IGF-IR) expression with ASODN resulted in the complete inhibition of HRG proliferative effect, demonstrating that a functional IGF-IR is required for HRG mitogenic activity. These results provide the first evidence of interactions between progestins and HRB/ErbB signal transduction pathways in mammary cancer and the first demonstration that IGF-IR is required for HRG proliferative effects.

Diazepam inhibits forskolin-stimulated adenylyl cyclase activity in human tumour cells

Previous studies have shown that the benzodiazepine agonist, diazepam, suppresses adenylyl cyclase activity in rat brain, via a G protein-coupled benzodiazepine receptor. Since diazepam binding sites are also present in diverse non-neuronal tissues including tumour cells, its effects on adenylyl cyclase activity were examined in membranes from human MCF-7 (breast cancer) and M-6 (melanoma) cells. Diazepam caused a biphasic and concentration-dependent inhibition of forskolin-stimulated adenylyl cyclase activity in MCF-7 membranes. The first phase of inhibition, at picomolar to nanomolar drug concentrations (EC50=5.7 x 10(-12)M), is similar to the receptor mediated phase observed in the rat brain. At micromolar concentrations of diazepam (EC50= 1.8 x 10(-4)M), the steep decrease in adenylyl cyclase activity may involve a direct action on the enzyme itself, as detected previously in rat brain membranes. Diazepam-induced suppression of adenylyl cyclase activity was also detected in M-6 membranes. However, in contrast to MCF-7 findings, only micromolar concentrations of diazepam (EC50=5.2 x 10(-4)M) inhibited enzyme activity in M-6 membranes. These findings suggest that G protein-coupled benzodiazepine receptors, which mediate inhibition of the adenylyl cyclase-cAMP pathway in the brain, are also expressed in MCF-7 cells.

Growth hormone-mediated regulation of insulin-like growth factor I promoter activity in C6 glioma cells

The molecular mechanisms by which GH regulates insulin-like growth factor (IGF-I) gene expression remain obscure. One difficulty has been the lack of established GH-responsive cell lines that express the IGF-I gene. To develop such a cell line, we used rat C6 glioma cells which, as determined by RNase protection assay, express the IGF-I gene but not the GH receptor gene. To confer GH responsiveness, C6 cells were cotransfected with vectors that express the GH receptor (pRc/CMV WTrGHR) and Jak2 (pRc/CMV Jak2). GH responsiveness was demonstrated using luciferase reporter genes containing either the Sis-inducible element from the c-fos gene (pTK81-SIE-Luc) or 6 copies of the GH-responsive GAS-like element (GLE) from the rat spi2.1 gene (pSpi-GLE-Luc). The SIE is activated by binding of STAT1 and 3, whereas the GLE binds STAT5. In cells cotransfected with pRc/CMV WTrGHR, pRc/CMV Jak2, and either pTK81-SIE-Luc or pSpi GLE-Luc, treatment with 500 ng/ml GH for 24 h stimulated a 3.1- and 1.7-fold increase in luciferase activity, respectively. These data suggest that in C6 cells cotransfected with pRc/CMV WTrGHR and pRc/CMV Jak2, GH activates STAT1, 3, and 5. To determine whether GH-responsive IGF-I promoter activity could be demonstrated, C6 cells were cotransfected with pRc/CMV WTrGHR, pRc/ CMV Jak2, and an IGF-I-luciferase fusion gene that contained a fragment of the rat IGF-I gene that extended from -412 in the 5'-flanking region of exon 1 to the Met-22 in exon 3. GH stimulated a modest, but reproducible, 1.7-fold increase in luciferase activity in these cells, suggesting that a GH-responsive element is present in this region of the IGF-I gene. To better localize the GH-responsive element, cells were cotransfected with pRc/CMV WTrGHR, pRc/CMV Jak2 plus one of several IGF-I-luciferase fusion genes containing either fragments of one of the two promoters in the IGF-I gene or a fragment of intron 2 that includes a GH-responsive DNase I hypersensitivity site. For all constructs, treatment with GH for 24 h did not stimulate a significant increase in luciferase activity, suggesting that GH-responsive sequences are not located in these specific regions of the IGF-I gene or that GH-directed transcription of the IGF-I gene is mediated via several different regions of the IGF-I gene and the effect of any one of these regions in isolation was not sufficiently robust to be detected in this model system. In summary, transient expression of the GH receptor and Jak2 in C6 cells creates a GH-responsive system that activates STAT1, 3, and 5. Moreover, a fragment of the IGF-I gene that contains exons 1 and 2, a fragment of exon 3, and introns 1 and 2 is GH responsive using this model system.

Crosstalk between protein kinase A and growth factor receptor signaling pathways in arterial smooth muscle

Crosstalk between the cyclic AMP-dependent protein kinase (PKA) and growth factor receptor signaling is one of many emerging concepts of crosstalk in signal transduction. Understanding of PKA crosstalk may have important implications for studies of crosstalk between other, less well known, signaling pathways. This review focuses on PKA crosstalk in arterial smooth muscle. Proliferation and migration of arterial smooth muscle cells (SMCs) contribute to the thickening of the blood vessel wall that occurs in many types of cardiovascular disease. PKA potently inhibits SMC proliferation by antagonizing the major mitogenic signaling pathways induced by growth factors in SMCs. PKA also inhibits growth factor-induced SMC migration. An intricate crosstalk between PKA and the mitogen-activated protein kinase (MAPK/ERK) pathway, the p70 S6 kinase pathway and cyclin-dependent kinases has been described. Further, PKA regulates expression of growth regulatory molecules. The result of PKA activation in SMCs is the potent inhibition of cell cycle traverse and SMC migration. In this review, we discuss recent advances in our understanding of the crosstalk between PKA and signaling pathways induced by growth factor receptors in SMCs, and where relevant, in other cell types in which interesting examples of PKA crosstalk have been described.

Phosphorylation of serine 256 by protein kinase B disrupts transactivation by FKHR and mediates effects of insulin on insulin-like growth factor-binding protein-1 promoter activity through a conserved insulin response sequence

Insulin inhibits the expression of multiple genes in the liver containing an insulin response sequence (IRS) (CAAAA(C/T)AA), and we have reported that protein kinase B (PKB) mediates this effect of insulin. Genetic studies in Caenorhabditis elegans indicate that daf-16, a forkhead/winged-helix transcription factor, is a major target of the insulin receptor-PKB signaling pathway. FKHR, a human homologue of daf-16, contains three PKB sites and is expressed in the liver. Reporter gene studies in HepG2 hepatoma cells show that FKHR stimulates insulin-like growth factor-binding protein-1 promoter activity through an IRS, and introduction of IRSs confers this effect on a heterologous promoter. Insulin disrupts IRS-dependent transactivation by FKHR, and phosphorylation of Ser-256 by PKB is necessary and sufficient to mediate this effect. Antisense studies indicate that FKHR contributes to basal promoter function and is required to mediate effects of insulin and PKB on promoter activity via an IRS. To our knowledge, these results provide the first report that FKHR stimulates promoter activity through an IRS and that phosphorylation of FKHR by PKB mediates effects of insulin on gene expression. Signaling to FKHR-related forkhead proteins via PKB may provide an evolutionarily conserved mechanism by which insulin and related factors regulate gene expression.

Activation of the skeletal alpha-actin promoter during muscle regeneration

Little is known concerning promoter regulation of genes in regenerating skeletal muscles. In young rats, recovery of muscle mass and protein content is complete within 21 days. During the initial 5-10 days of regeneration, mRNA abundance for IGF-I, myogenin and MyoD have been shown to be dramatically increased. The skeletal alpha-actin promoter contains E box and serum response element (SRE) regulatory regions which are directly or indirectly activated by myogenin (or MyoD) and IGF-I proteins, respectively. We hypothesized that the skeletal alpha-actin promoter activity would increase during muscle regeneration, and that this induction would occur before muscle protein content returned to normal. Total protein content and the percentage content of skeletal alpha-actin protein was diminished at 4 and 8 days and re-accumulation had largely occurred by 16 days post-bupivacaine injection. Skeletal alpha-actin mRNA per whole muscle was decreased at day 8, and thereafter returned to control values. During regeneration at day 8, luciferase activity (a reporter of promoter activity) directed by -424 skeletal alpha-actin and -99 skeletal alpha-actin promoter constructs was increased by 700% and 250% respectively; however, at day 16, skeletal alpha-actin promoter activities were similar to control values. Thus, initial activation of the skeletal alpha-actin promoter is associated with regeneration of skeletal muscle, despite not being sustained during the later stages of regrowth. The proximal SRE of the skeletal alpha-actin promoter was not sufficient to confer a regeneration-induced promoter activation, despite increased serum response factor protein binding to this regulatory element in electrophoretic mobility shift assays. Skeletal alpha-actin promoter induction during regeneration is due to a combination of regulatory elements, at least including the SRE and E box.