Attenuation of serum inducibility of immediate early genes by oncoproteins in tyrosine kinase signaling pathways

Cross-Phosphorylation and Interaction between Src/FAK and MAPKAP5/PRAK in Early Focal Adhesions Controls Cell Motility

P38-regulated and activated kinase (PRAK/MAPKAPK5) is a serine/threonine kinase which lies downstream of the p38 and ERK3/4 MAP kinase pathways. PRAK plays diverse roles in the processes of cell growth, nutrient starvation response, programmed cell death, senescence and motility. PRAK has been shown to both promote and inhibit cell motility in different contexts. The pro-motility functions of PRAK are attributed mainly to cytoskeletal rearrangement occurring downstream of its phosphorylated substrate HSP27; however, it was recently shown that PRAK is required for motility in endothelial cells upstream of Focal adhesion kinase (FAK). Along with Src, FAK functions as a mediator of motility signaling through the phosphorylation of substrates in focal adhesions. Here, we show that PRAK, initially identified as a FAK substrate in an in situ/ kinase overlay assay, is a Src substrate, the phosphorylation of which directs PRAK to focal adhesions. Focal adhesion localization of PRAK was not found to affect cell motility, however transient over expression of PRAK inhibited motility in HeLa cells. This effect requires PRAK kinase activity and proceeds through an impairment of FAK activation via phosphorylation on Y861. Our studies demonstrate for the first time that PRAK is regulated by tyrosine phosphorylation, localizes to focal adhesions, and interacts physically with and can phosphorylate FAK/Src. Further we provide a novel mechanism for the inhibition of motility downstream of PRAK.

Ataxin-2 modulates the levels of Grb2 and SRC but not ras signaling

Ataxin-2 (ATXN2) is implicated mainly in mRNA processing. Some ATXN2 associates with receptor tyrosine kinases (RTK), inhibiting their endocytic internalization through interaction of proline-rich domains (PRD) in ATXN2 with SH3 motifs in Src. Gain of function of ATXN2 leads to neuronal atrophy in the diseases spinocerebellar ataxia type 2 (SCA2) and amyotrophic lateral sclerosis (ALS). Conversely, ATXN2 knockout (KO) mice show hypertrophy and insulin resistance. To elucidate the influence of ATXN2 on trophic regulation, we surveyed interactions of ATXN2 with SH3 motifs from numerous proteins and observed a novel interaction with Grb2. Direct binding in glutathione S-transferase (GST) pull-down assays and coimmunoprecipitation of the endogenous proteins indicated a physiologically relevant association. In SCA2 patient fibroblasts, Grb2 more than Src protein levels were diminished, with an upregulation of both transcripts suggesting enhanced protein turnover. In KO mouse embryonal fibroblasts (MEF), the protein levels of Grb2 and Src were decreased. ATXN2 absence by itself was insufficient to significantly change Grb2-dependent signaling for endogenous Ras levels, Ras-GTP levels, and kinetics as well as MEK1 phosphorylation, suggesting that other factors compensate for proliferation control. In KO tissue with postmitotic neurons, a significant decrease of Src protein levels is prominent rather than Grb2. ATXN2 mutations modulate the levels of several components of the RTK endocytosis complex and may thus contribute to alter cell proliferation as well as translation and growth.

Suppression of Egr-1 transcription through targeting of the serum response factor by oncogenic H-Ras

The transcription factor Egr-1 functions as a key regulator in cellular growth, differentiation, and apoptosis. The loss of Egr-1 expression is closely associated with tumor development, although the molecular mechanism behind the suppression of Egr-1 is largely unknown. In this report, we show that growth factor-induced transcriptional activation of Egr-1 gene is downregulated by chronic expression of oncogenic H-Ras in NIH3T3 fibroblasts. Our results demonstrate that phosphoinositide 3-kinase (PI3K) signaling is necessary for oncogenic H-Ras-mediated reduction of Egr-1 gene expression. Aberrant activation of PI3K signaling by oncogenic Ras decreased the level of serum response factor (SRF) protein through the acceleration of proteolysis, which resulted in decreased SRF binding to the serum response element (SRE) sites within the Egr-1 promoter, leading to the suppression of Egr-1 transcription. Inhibition of PI3K signaling restored the downregulation of SRF and Egr-1 expression caused by oncogenic Ras. Our findings suggest a novel signaling mechanism by which prolonged activation of oncogenic H-Ras can trigger the loss of tumor suppressor Egr-1 through the PI3K pathway in NIH3T3 fibroblast model cell lines.

Downregulation of c-fos gene transcription in cells transformed by E1A and cHa-ras oncogenes: a role of sustained activation of MAP/ERK kinase cascade and of inactive chromatin structure at c-fos promoter

REF cells transformed by oncogenes E1A and cHa-ras reveal high and constitutive DNA-binding activity of AP-1 factor lacking in c-Fos protein. Consistently, the transcription of c-fos gene has been found to be downregulated. To elucidate the mechanisms of c-fos downregulation in E1A+cHa-ras transformants, we studied the levels of activity of ERK, JNK/SAPK and p38 kinases and phosphorylation state of Elk-1 transcription factor involved in regulation of c-fos gene. Using two approaches, Western blot analysis with phospho-specific antibodies to MAP kinases and in vitro kinase assay with specific substrates, we show here that ectopic expression of E1A and ras oncogenes leads to a sustained activation of ERK and p38 kinases, whereas JNK/SAPK kinase activity is similar to that in non-transformed REF52 cells. Due to sustained activity of the MAP kinase cascades, Elk-1 transcription factor is being phosphorylated even in serum-starved E1A+cHa-ras cells; moreover, serum does not additionally increase phosphorylation of Elk-1, which is predominant TCF protein bound to SRE region of c-fos gene promoter in these cells. Although the amount of ternary complexes SRE/SRF/TCF estimated by EMSA was similar both in serum-starved and serum-stimulated transformed cells, serum addition still caused a modest activation of c-fos gene transcription at the level of 20% to normal REF cells. In attempt to determine how serum caused the stimulatory effect, we found that PD98059, an inhibitor of MEK/ERK kinase cascade, completely suppressed serum-induced c-fos transcription both in REF and E1A+cHa-ras cells, implicating the ERK as primary kinase for c-fos transcription in these cells. In contrast, SB203580, an inhibitor of p38 kinase, augmented noticeably serum-stimulated transcription of c-fos gene in REF cells, implying the involvement of p38 kinase in negative regulation of c-fos. Furthermore, sodium butyrate, an inhibitor of histone deacetylase activity, was capable of activating c-fos transcription both in serum-stimulated and even in serum-starved E1A+cHa-ras cells. Conversely, serum-starved REF cells fail to respond to sodium butyrate treatment by c-fos activation confirming necessity of prior Elk-1 phosphorylation. Taken together, these data suggest that downregulation of c-fos in E1A+cHa-ras cells seems to occur due to a maintenance of a refractory state that arises in normal REF cells after serum-stimulation. The refractory state of c-fos in E1A+cHa-ras cells is likely a consequence of Ras-induced sustained activation of MAPK (ERK) cascade and persistent phosphorylation of TCF (Elk-1) bound to SRE. Combination of these events eventually does contribute to formation of an inactive chromatin structure at c-fos promoter mediated through recruitment of histone deacetylase activity.

Increased activity of c-Src and Csk in fibroblasts transformed by v-src oncogene

When c-Src and v-Src were immunoprecipitated together from hamster fibroblasts transformed by Rous sarcoma virus containing v-src oncogene, the total Src activity was almost threefold higher compared to c-Src activity in the control cells. The activity of v-Src immunoprecipitated separately, however, accounting for only 40% of the total Src activity, indicating that c-Src is activated upon transformation. An increased activity of Csk was also found in RSV-transformed cells. It decreased upon serum stimulation in parallel with an increase in Src kinase activity. In nontransformed cells, serum stimulation induced an enhanced Csk activity, but no changes in c-Src activity were observed. This may suggest that Csk may have more functions in hamster fibroblasts, in addition to its inhibitory effect on c-Src.

Attenuation of haptoglobin gene expression by TGFbeta requires the MAP kinase pathway

In addition to important roles in the regulation of cell growth and cell restitution, both pro- and anti-inflammatory effects have been ascribed to TGFbeta in intestinal epithelial cells. However, the mechanisms involved in TGFbeta-dependent anti-inflammatory activities remain to be determined. In the rat intestinal epithelial cell line IEC-6, TGFbeta attenuated the glucocorticoid-dependent increases in mRNA levels of the acute phase protein gene haptoglobin, and of C/EBP isoforms beta and delta. Supershift assays demonstrated a TGFbeta-mediated decrease in the binding of C/EBP isoforms beta and delta to the haptoA and haptoC C/EBP DNA-binding sites from the haptoglobin promoter. Mutations of both HaptoA and HaptoC sites abolished the glucocorticoid-dependent activation and the TGFbeta-mediated attenuation of the haptoglobin promoter, as assessed by transient transfection assays. TGFbeta induced p42/p44 MAP kinase activities. Treatment with the MEK 1/2 inhibitor PD 98059 abolished TGFbeta attenuation. These results suggest that C/EBP isoforms are involved both in the glucocorticoid-dependent induction and in the TGFbeta-mediated attenuation of haptoglobin expression. Furthermore, p42/p44 MAP kinases may function in a TGFbeta-dependent signaling pathway leading to attenuation of haptoglobin expression.

Constitutive Activation of the JAK2/STAT5 Signal Transduction Pathway Correlates With Growth Factor Independence of Megakaryocytic Leukemic Cell Lines

The factor-independent Dami/HEL and Meg-01 and factor-dependent Mo7e leukemic cell lines were used as models to investigate JAK/STAT signal transduction pathways in leukemic cell proliferation. Although Dami/HEL and Meg-01 cell proliferation in vitro was independent of and unresponsive to exogenous cytokines including granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), IL-6, thrombopoietin (TPO), and tumor necrosis factor- (TNF-), the growth of Mo7e cells was dependent on hematopoietic growth factors. When these cell lines were cultured in medium without cytokines, a constitutively activated STAT-like DNA-binding factor was detected in nuclear extracts from both Dami/HEL and Meg-01 cells. However, the STAT-like factor was not detectable in untreated Mo7e cells, but was activated transiently in Mo7e cells in response to cytokine treatments. The constitutively activated and cytokine-induced STAT-like DNA-binding factor in these three cell lines was identified as STAT5 by oligonucleotide competition gel mobility assays and by specific anti-STAT antibody gel supershift assays. Constitutive activation of JAK2 also was detected in the factor-independent cell lines, but not in Mo7e cells without cytokine exposure. Meg-01 cells express a p185 BCR/ABL oncogene, which may be responsible for the constitutive activation of STAT5. Dami/HEL cells do not express the BCR/ABL oncogene, but increased constitutive phosphorylation of Raf-1 oncoprotein was detected. In cytokine bioassays using growth factor-dependent Mo7e and TF-1 cells as targets, conditioned media from Dami/HEL and Meg-01 cells did not show stimulatory effects on cell proliferation. Our results indicate that the constitutive activation of JAK2/STAT5 correlates with the factor-independent growth of Dami/HEL and Meg-01 cells. The constitutive activation of JAK2/STAT5 in Dami/HEL cells is triggered by a mechanism other than autocrine cytokines or the BCR/ABL oncoprotein.

Constitutive Activation of the JAK2/STAT5 Signal Transduction Pathway Correlates With Growth Factor Independence of Megakaryocytic Leukemic Cell Lines

The factor-independent Dami/HEL and Meg-01 and factor-dependent Mo7e leukemic cell lines were used as models to investigate JAK/STAT signal transduction pathways in leukemic cell proliferation. Although Dami/HEL and Meg-01 cell proliferation in vitro was independent of and unresponsive to exogenous cytokines including granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), IL-6, thrombopoietin (TPO), and tumor necrosis factor- (TNF-), the growth of Mo7e cells was dependent on hematopoietic growth factors. When these cell lines were cultured in medium without cytokines, a constitutively activated STAT-like DNA-binding factor was detected in nuclear extracts from both Dami/HEL and Meg-01 cells. However, the STAT-like factor was not detectable in untreated Mo7e cells, but was activated transiently in Mo7e cells in response to cytokine treatments. The constitutively activated and cytokine-induced STAT-like DNA-binding factor in these three cell lines was identified as STAT5 by oligonucleotide competition gel mobility assays and by specific anti-STAT antibody gel supershift assays. Constitutive activation of JAK2 also was detected in the factor-independent cell lines, but not in Mo7e cells without cytokine exposure. Meg-01 cells express a p185 BCR/ABL oncogene, which may be responsible for the constitutive activation of STAT5. Dami/HEL cells do not express the BCR/ABL oncogene, but increased constitutive phosphorylation of Raf-1 oncoprotein was detected. In cytokine bioassays using growth factor-dependent Mo7e and TF-1 cells as targets, conditioned media from Dami/HEL and Meg-01 cells did not show stimulatory effects on cell proliferation. Our results indicate that the constitutive activation of JAK2/STAT5 correlates with the factor-independent growth of Dami/HEL and Meg-01 cells. The constitutive activation of JAK2/STAT5 in Dami/HEL cells is triggered by a mechanism other than autocrine cytokines or the BCR/ABL oncoprotein.

Attenuated expression of the serum responsive T1 gene in ras transformed fibroblasts due to the inhibition of c-fos gene activity

The T1 gene encodes a protein, which shares homology with the IL-1 receptors. In fibroblasts, T1 is induced by growth factors and in response to the onset of oncogene expression. The c-fos gene is transiently activated in these situations and was shown to be the major mediator of T1 gene induction. In contrast, the sustained expression of a ras oncogene in NIH3T3 cells resulted in the downregulation of basal T1 gene activity and the attenuation of T1 gene induction in response to mitogenic signals. Likewise, the immediate early genes encoding c-Fos, FosB, and Fra-2 are repressed in these cells. T1 gene repression could be overcome by the forced expression of c-fos in ras transformed fibroblasts. Thus, the lack of c-fos gene expression is the likely cause for ras mediated T1 gene repression. Fra-1, in contrast to the other three members of the Fos family, is permanently synthesized in high amounts in ras transformed NIH3T3 fibroblasts. We show that AP-1, which is abundant in these cells throughout the whole cell cycle, consists predominantly of Fra-1/c-Jun and Fra1/JunD heterodimers. We provide evidence that Fra1/c-Jun heterodimers are responsible for the repression of c-fos gene induction following serum stimulation. The introduction of a dominant negative version of c-Jun into ras transformed fibroblasts was able to rescue c-fos gene induction in response to serum stimulation, further demonstrating that AP-1 is indeed involved in c-fos gene repression. We conclude that oncogenic ras mediates the activation of the fra-1 gene which results in elevated AP-1 activity throughout the cell cycle. Fra-1 containing AP-1 complexes repress the c-fos and possibly other immediate early genes thereby preventing the induction of certain delayed early genes such as the T1 gene in response to mitogenic stimulation.

The ras oncogene inhibits growth factor inducibility of early response genes, and promotes selectively expression of NGFI-A in a PC12 cell line

Expression of oncogenic Ras in UR61 cells (a PC12 subclone) results in neuronal differentiation. We have observed that the oncoprotein selectively increased the levels of NGFI-A transcripts, but was unable to induce NGFI-B or c-fos transcripts. In contrast, nerve growth factor (NGF) elicited a strong induction of the three immediate early genes (IEGs). Thus, activation of Ras alone is sufficient for the induction of NGFI-A by NGF, whereas an additional pathway(s), besides Ras, is required for the stimulation of NGFI-B and c-fos gene expression. These results show that the acquisition of a neuronal phenotype does not correlate with induction of IEG expression. Additionally, Ras markedly reduces the response of the three genes to NGF and to other growth factors. This attenuation could reflect a negative regulatory mechanism acting on signalling pathways normally stimulated by growth factor receptors.

E1A + cHa-ras transformed rat embryo fibroblast cells are characterized by high and constitutive DNA binding activities of AP-1 dimers with significantly altered composition

Transcription factors of the AP-1/ATF family, including c-Fos, c-Jun, and ATF-2, play an important role in the regulation of cell proliferation and differentiation, and changes in their levels and/or activities may contribute to oncogenesis. We analyzed the alterations of AP-1/ATF transcription factors upon immortalization and transformation in a panel of cell lines derived from rat embryo fibroblast (REF) cells. The tumorigenic E1A + cHa-ras cells are characterized by high and constitutive DNA binding activities of AP-1, in contrast to nontransformed cells and the E1A cells. The expression of c-fos and c-jun genes was affected differently by the oncogenic transformation. By using antibodies to c-Jun and c-Fos proteins in electrophoretic mobility shift assays (EMSA), we showed that E1A + cHa-ras transformants did not contain c-Fos under any condition of cell cultivation and growth factor stimulation, whereas c-Jun was constitutively upregulated. In the absence of c-fos gene expression, c-Fos protein appears to be replaced by proteins of Fos family (Fra-1) and ATF family (ATF-2 and ATFa). To determine the possible mechanisms of c-fos downregulation in E1A + cHa-ras transformants we have obtained populations of geneticin-resistant clones containing integrated reporter construct -711fos-CAT and its mutants in serum-responsive element (SRE) and cAMP-responsive element (CRE). Data obtained show that the mutations within the SRE lead to a manifold activation of fos-CAT expression. This allows to suggest that c-fos downregulation in E1A + cHa-ras transformants is provided by a negative control mediated through the SRE regulatory region. The profound differences in regulation and composition of transcription factors of the AP-1 family probably play a pivotal role in the transformation of REF cells by E1A and cHa-ras oncogenes.

Repression of mitogen-activated protein kinases ERK1/ERK2 activity by a protein tyrosine phosphatase in rat fibroblasts transformed by upstream oncoproteins

The observation that mitogen-activated protein (MAP) kinases ERK1 and ERK2 are constitutively activated in a number of oncogene-transformed cell lines has led to the hypothesis that prolonged activation of these enzymes is required for the transformation process. To investigate this question, we have examined the regulation of the ERK pathway in Rat1 fibroblasts transformed with activated c-Raf-1 (Raf22W), v-Ha-Ras, and v-Src. Expression of these oncoproteins had no effect on the enzymatic activity of ERK1 and ERK2 in either serum-starved or exponentially growing cells. Moreover, the stimulatory effect of serum on ERK1/ERK2 activity was substantially reduced or abrogated in these cells; this impairment was associated with a strong attenuation of c-fos gene induction. In contrast, expression of Raf22w, v-Ha-Ras, or v-Src resulted in the constitutive activation of the upstream kinases MEK1 and MEK2. Treatment of the cells with vanadate completely restored the activation of ERK1/ERK2 in oncogene-transformed cells, suggesting the involvement of a vanadate-sensitive tyrosine phosphatase. Northern blot analysis of VH1-like dual-specificity MAP kinase phosphatases did not reveal any significant difference in the mRNA expression pattern of these genes between parental and transformed Rat1 cells. Phosphoamino acid analysis indicated that ERK1 is phosphorylated on threonine, but not on tyrosine, in oncogene-transformed cells and that vanadate treatment restores tyrosine phosphorylation. We conclude from these results that ERK1/ERK2 activity is repressed by a single-specificity tyrosine phosphatase in oncogene-transformed rat fibroblasts.

The role of endothelins in the paracrine control of the secretion and growth of the adrenal cortex

Endothelins (ETs) are a family of vasoactive peptides (ET-1, ET-2, and ET-3) mainly secreted by vascular endothelium and widely distributed in the various body systems, where they play major autocrine/paracrine regulatory functions, acting via two subtypes of receptors (ETA and ETB): Adrenal cortex synthesizes and releases ETS and expresses both ETA and ETB. Zona glomerulosa possesses both ETA and ETB, whereas zona fasciculata/reticularis is almost exclusively provided with ETB. ETS exert a strong mineralocorticoid and a less intense glucocorticoid secretagogue action, mainly via ETB receptors. ETS also appear to enhance the growth and steroidogenic capacity of zona glomerulosa and to stimulate its proliferative activity. This trophic action of ETS is likely to be mediated mainly by ETA receptors. The intraadrenal release of ETS undergoes a multiple regulation, with the rise in blood flow rate and the local release of nitric oxide being the main stimulatory factors. Data are also available that indicate that ETS may also have a role in the pathophysiology of primary aldosteronism caused by adrenal adenomas and carcinomas.

CD44 protein levels and its biological activity are regulated in Balb/c 3T3 fibroblasts by serum factors and by transformation with the ras but not with the sis oncogene

CD44s (standard isoform) levels and hyaluronan-binding activity were investigated in Balb/c 3T3 cells and their derivatives transformed with ras or sis oncogenes as a function of serum concentration in the medium. 3T3 cells contained low levels of CD44 and did not bind hyaluronan when grown in medium containing 0.5 or 10% serum. In 5% serum, however, the cells had much higher levels of CD44 and were able to bind hyaluronan. CD44 levels also increased in 3T3 cells restimulated with either 5 or 10% serum after prior maintenance in low serum. In cells restimulated with 5% serum, high levels of CD44 were sustained for at least 72 hr. In cells restimulated with 10% serum, however, the increase in CD44 levels reverted by 48 hr. Transformation of 3T3 cells with ras (but not with sis) oncogene rendered CD44 levels insensitive to serum modulation: ras-transformed cells contained high levels of CD44 and bound hyaluronan at all serum concentrations and at all time points tested. Sis-transformed cells behaved like 3T3 cells in these modulatory changes. Platelet-derived growth factor (PDGF), when supplementing 0.5% serum, mimicked the effects of serum on the levels and hyaluronan-binding capacity of CD44 in 3T3 cells and the CD44-upregulating activity of serum was neutralized by incubation with anti-PDGF antibodies. These data demonstrate that serum factors, specifically PDGF, mediate regulation of CD44 levels in BAlb/c 3T3 cells and that transformation of 3T3 cells by ras renders CD44 expression insensitive to the modulating effects of serum in vitro. These results correlate with the metastatic capacity of these cells in vivo.

Prostaglandin E2 induces Egr-1 mRNA in MC3T3-E1 osteoblastic cells by a protein kinase C-dependent pathway

Prostaglandin E2 (PGE2) plays an important role in the regulation of osteoblast metabolism. However, the nuclear signal transduction mechanisms involved in the actions of PGE2 have not been clearly defined. One mechanism may involve induction of immediate early genes such as the transcription factor Egr-1. In the present study, we examined the effects of PGE2 on induction of Egr-1 mRNA in MC3T3-E1 osteoblasts. Time course studies with 2 microM PGE2 showed maximal induction of Egr-1 mRNA at 30 min. In cells pretreated with cycloheximide (CHX), induction of Egr-1 mRNA reached a maximum at 60 min and remained elevated for at least 240 min. Preincubation with CHX was associated with superinduction of Egr-1. Inhibition of protein kinase C activity by pretreatment with 1 microM chelerythrine chloride or by prolonged stimulation with 50 ng/ml tetradecanoyl phorbol acetate (TPA) attenuated the induction of Egr-1 mRNA by 2 microM PGE2. These data indicate that in MC3T3-E1 cells, PGE2 increase Egr-1 mRNA levels via a protein kinase C-dependent pathway.

Epidermal growth factor induces Egr-1 messenger RNA and protein in mouse osteoblastic cells

The nuclear signaling events activated when epidermal growth factor (EGF) interacts with osteoblasts to produce effects on growth and differentiation are not clearly understood, and may include induction of immediate early genes such as Egr-1, a zinc finger transcription factor. In the present study, Northern analyses were performed to define the effects of EGF on the expression of Egr-1 mRNA in MC3T3-E1 mouse osteoblastic cells. Following treatment of quiescent, subconfluent MC3T3-E1 cells with 0.1-100 ng/ml EGF for various periods, maximal induction of Egr-1 mRNA occurred when cells were treated for 30-60 minutes with 1-10 ng/ml EGF. Inhibition of protein kinase C activity by pretreatment with 1 microM chelerythrine chloride or by prolonged stimulation with 50 ng/ml tetradecanoyl phorbol acetate (TPA) partially diminished the induction of Egr-1 by EGF. Using an immunohistochemical approach, 10 ng/ml EGF was observed to induce Egr-1 protein within 30-60 minutes and this induction was localized to the nucleus. These observations indicate that EGF induces Egr-1 mRNA and protein via protein kinase C and other signaling pathways, and that Egr-1 may be part of the regulatory network mediating the actions of EGF on growth and differentiation of osteoblasts.

Differential regulation of primary response gene expression in skeletal muscle cells through multiple signal transduction pathways

One of the earliest cellular responses to growth factors is the rapid induction of primary response genes. One group of such genes was originally isolated as tetradecanoyl phorbol acetate (TPA) inducible sequences (TIS genes) from mouse 3T3 cells. Proteins encoded by the TIS genes include two transcription factors: TIS8 (also known as egr1/NGFIA/zif268) and TIS1 (also known as NGFIB/nur77/N10). We have examined the inducibility of these two genes in a skeletal muscle cell line in response to agents that have been reported to block muscle differentiation. We report here that basic fibroblast growth factor (bFGF) induced the expression of both TIS1 and TIS8 in mouse C2C12cells. Both genes were also inducible by TPA while forskolin which activates the cAMP-dependent pathway induced TIS1 but not TIS8. Down-regulation of protein kinase C (PKC) activity by TPA pretreatment repressed the bFGF induction of TIS1 but had little effect on the bFGF-stimulated expression of TIS8. Moreover, while both TPA and bFGF stimulated the hyperphosphorylation of c-RAF and the activity of MAP kinase, TPA pretreatment failed to block RAF phosphorylation or the stimulation of MAP kinase activity by bFGF. Induction of the two TIS genes in skeletal myoblasts therefore appeared to be dependent to different extents on the activation of protein kinase A (PKA), PKC and MAP kinase.

Alterations in cellular differentiation, mitogenesis, cytoskeleton and growth characteristics during Syrian hamster embryo cell multistep in vitro transformation

In vitro Syrian hamster embryo (SHE) cell transformation is a neoplastic process that proceeds through several identifiable consecutive stages including in vitro morphological transformation (mt), acquisition of immortality (I+), acquisition of tumorigenicity (T+) and tumor-derived cells (I'TD). Eight transformed lineages consisting of cells at the mt, I+, T+ and I'TD stages were assayed for alterations in general markers of cell differentiation, mitogenic signaling pathways, cytoskeleton and cellular growth in 3D matrix. Alterations in cellular differentiation markers included a decrease in H19 gene expression and placental alkaline phosphatase enzymatic activity at the mt stage in all lineages examined with a complete absence of H19 gene expression and placental alkaline phosphatase enzymatic activity by the I'TD stage in a majority of transformed lineages. Changes in mitogenic signaling pathways included the production of autocrine growth factors and alterations in growth factor-induced immediate early gene expression by the I'TD stage of transformation in the majority of transformed lineages investigated. By the I'TD stage of transformation in most lineages, changes in both the cytoskeleton (including a decrease in tropomyosin-I gene expression) and the Matrigel growth characteristics of SHE cells were observed.

Mediation of suppression of c-fos transcription in rasT24-transformed rat cells by a cis-acting repressor element

Prolonged expression of activated ras mutants resulted in both neoplastic transformation and suppression of serum-induced c-fos expression in Rat1 fibroblasts. Expression of other serum-inducible genes, including c-jun and beta-actin, was not suppressed in ras-transformed Rat1 cells, indicating that these effects are specific for c-fos and that growth-factor signal transduction pathways remain essentially intact. Run-on transcription studies indicated that c-fos transcription was blocked at the level of initiation in these cells. Transient transfection studies using 360 bp from the wild-type c-fos promoter as well as a series of mutated c-fos promoter fragments linked to the chloramphenicol acetyltransferase gene indicated that repression of c-fos was mediated by approximately 49 bp immediately upstream of the dyad symmetry element (DSE). Deletion of this region, referred to as the upstream repressor region (URR), restored serum inducibility to the c-fos promoter in ras-transformed cells. In contrast, suppression of c-fos transcription was not affected by either deletion of 240 bp between the DSE and the TATA element or by base-substitution mutations that inactive the ternary complex factor and fos-AP-1-like binding sites. In addition, in vitro competition studies indicated that ras-transformed cells express one or more repressor factors that interact with as-yet-unidentified elements within the c-fos promoter (possibly the URR) and block serum induction of c-fos. These findings suggest that prolonged expression of activated ras results in the activation of one or more as-yet-unidentified proteins that suppress transcription of the c-fos gene by interacting with the URR.