Activation of p38 mitogen-activated protein kinase during normal mitosis in the developing retina

The p38 member of the mitogen-activated protein kinase superfamily is engaged by phosphorylation in response to environmental stress signals, and may have either permissive or inhibitor roles upon cell proliferation. The cell cycle in the proliferative zone of the retina is tightly controlled and proceeds in synchrony with interkinetic migration of the neuroblast nuclei. We examined the association of p38 kinase activity with the cell cycle in the normal, non-stressed retina of the developing rat, maintained either in vivo or in vitro. Using immunohistochemistry, we show that mitotic profiles in the developing retina are highly enriched for phosphorylated p38. Blockade of p38 activity with the chemical inhibitor SB203580 for 4 h transiently arrested cells at the metaphase-anaphase transition and induced cell death after 20 h. p38 inhibition induced an aberrant mitotic profile, with chromosomes arranged in one side of the cell. The data show that p38 is active during normal mitosis and we suggest that p38 is required for the proper cell cycle progression during metaphase-anaphase transition in retinal neuroblasts.

The constitutive activation of the CEF-4/9E3 chemokine gene depends on C/EBPbeta in v-src transformed chicken embryo fibroblasts

The CEF-4/9E3 chemokine gene is expressed constitutively in chicken embryo fibroblasts (CEF) transformed by the Rous sarcoma virus (RSV). This aberrant induction is controlled at the transcriptional and post-transcriptional levels. Transcriptional activation depends on multiple elements of the CEF-4 promoter composing a Src-responsive-Unit or SRU. The SRU includes a TPA responsive element, a PRDII/kappaB domain and a CAAT box. In this report, we identify C/EBPbeta as a component of the trans-acting factor interacting with the CAAT box of the CEF-4 promoter. In addition, we show that C/EBPbeta binds to a second element located in proximity of the TRE. A mutation of this distal CAAT box impaired the activation of the CEF-4 promoter by pp60(v-src) indicating that this element is also part of the SRU. Using the RCASBP retroviral vector, we expressed a dominant negative mutant of C/EBPbeta (designated Delta184-C/EBPbeta) in RSV-transformed CEF. Delta184-C/EBPbeta decreased the accumulation of the CEF-4 mRNA and activation of the CEF-4 promoter by pp60(v-src). The induction of the Cox-2 gene (CEF-147) was also reduced by Delta184-C/EBPbeta. The effect of the dominant negative mutant was observed within 1 h of the activation of a thermolabile pp60(v-src) suggesting that C/EBPbeta is an early target of v-src transformation. The dominant negative mutant did not inhibit the transformation of CEF by RSV and in fact accentuated the transformed cell phenotype. Therefore, the activation of C/EBPbeta is important for the expression of v-src regulated genes but is not required for the in vitro transformation of CEF by pp60(v-src).

C/EBPbeta (NF-M) is essential for activation of the p20K lipocalin gene in growth-arrested chicken embryo fibroblasts

The p20K gene is induced in conditions of reversible growth arrest in chicken embryo fibroblasts (CEF). This expression is dependent on transcriptional activation and on a region of the promoter designated the quiescence-responsive unit (QRU). In this report, we describe the regulatory elements of the QRU responsible for activation in resting cells and characterize the trans-acting proteins interacting with these elements. We show that the QRU consists of functionally distinct domains including quiescence-specific and weak proliferation-responsive elements. The quiescence responsiveness of the QRU was mapped to two C/EBP binding sites, and the activity of the p20K promoter and its QRU was inhibited by the expression of a dominant negative mutant of C/EBPbeta in nondividing cells. The activation of QRU in response to serum starvation and contact inhibition correlated with the presence of a growth arrest-specific complex in electrophoretic mobility shift assays. This complex was supershifted by antibody for C/EBPbeta. C/EBPbeta accumulated in conditions of contact inhibition as a result of transcriptional activation. Therefore, C/EBPbeta was itself regulated as a growth arrest-specific gene in CEF. Finally, we show that the expression of p20K is regulated by linoleic acid, an essential fatty acid binding to p20K. The addition of linoleic acid to contact-inhibited CEF markedly repressed the synthesis of p20K without inducing mitogenesis. The activity of the QRU was inhibited by linoleic acid or the peroxisome proliferator-activated receptor PPARgamma2 in transient expression assays. Therefore, we have identified C/EBPbeta as a key activator of a growth arrest-specific gene in CEF and implicated an essential fatty acid, linoleic acid, in regulation of the QRU and the p20K lipocalin gene.

Transcriptional and post-transcriptional regulation of kappaB-controlled genes by pp60v-src

The CEF-4/9E3 gene is expressed aberrantly in chicken embryo fibroblasts transformed by the Rous sarcoma virus. This aberrant expression is dependent on transcriptional activation and on the stabilization of the CEF-4 mRNA. The characterization of the CEF-4 promoter indicated that three distinct regulatory elements corresponding to an AP-1 binding site, a PRDII/ kappaB domain and a CAAT box are involved in the activation by pp60v-src. Several v-src responsive genes are controlled by AP-1 and members of the Ets family but few appear to be dependent on NF-kappaB. In this study we characterize the expression of genes regulated by NF-kappaB in normal and RSV-transformed CEF. Run-on transcription analysis indicated that pp60v-src induces the transcription of several genes controlled by NF-kappaB but at different levels. While the transcription of CEF-4 was strongly stimulated, that of NF-kappaB1, c-rel, p53 or IkappaB-alpha was activated more modestly by pp60v-src. In addition the CEF-4 mRNA was the only mRNA species to accumulate significantly in transformed CEF. The ectopic expression of RelA or Rel resulted in the stimulation of the transcription of several known targets of NF-kappaB. However, the mRNA for IkappaB-alpha was the only mRNA species to accumulate considerably in the RelA- or Rel-expressing cells. Hence for most kappaB-controlled genes, transcriptional activation was not sufficient to obtain a significant increase in mRNA expression. Likewise, RelA or Rel enhanced the transcription of the CEF-4 gene without a significant accumulation of the CEF-4 mRNA. However, transformation by v-src caused a massive accumulation of the CEF-4 mRNA but not of other mRNA species in the RelA- and Rel-expressing cells. Transient expression assays, run-on transcription and Northern blotting analyses indicated that the effect of pp60v-src on CEF-4 expression was mediated predominantly at the post-transcriptional level in these cells. Therefore transcriptional and post-transcriptional mechanisms determine the restricted pattern of activation of kappaB-controlled genes in RSV-transformed CEF.

Multiple signaling pathways control the activation of the CEF-4/9E3 cytokine gene by pp60v-src

The CEF-4/9E3 cytokine gene is expressed aberrantly in chicken embryo fibroblasts (CEF) transformed by the Rous sarcoma virus. The expression of CEF-4 is dependent on both transcriptional and post-transcriptional mechanisms of regulation. The characterization of the promoter region indicated that three distinct regulatory elements corresponding to an AP-1 binding site (or TRE), a PRDII/kappaB domain, and a CAAT box are involved in the activation by pp60(v-)src. In this report we investigate the signaling pathways controlling the expression of the TRE and PRDII domain. The expression of a dominant negative mutant of p21(ras) reduced the activity of both elements. In contrast a similar mutant of c-Raf-1 affected modestly the activation dependent on the TRE but not PRDII. The stress-activated protein kinase (SAPK)/Jun N-terminal kinase (JNK) pathway was important for the activity of PRDII and the TRE but was not markedly stimulated by pp60(v-)src. The addition of calphostin C and the inhibition of protein kinase C (PKC) diminished the accumulation of the CEF-4 mRNA and reduced the activity of a TRE-controlled promoter. Likewise, the depletion of PKC by chronic treatment with phorbol esters inhibited the activation of the TRE. Rous sarcoma virus-transformed CEF treated with calphostin C were also flatter, did not display a high degree of criss-crossing, and appeared morphologically normal. Hence PKC was important for the activation of AP-1 and the morphological transformation of CEF. The constitutive expression of CEF-4 was correlated with transformation only when dependent on the TRE. This was not true for PRDII, which was the only element required for the constitutive activation to the CEF-4 promoter in nontransformed cells treated chronically with phorbol esters.

Activation of human immunodeficiency virus 1 gene expression by the src oncoprotein

Several genes are induced constitutively in cells transformed by the v-src oncoprotein. This induction is generally dependent on the activation of transcription factors binding to src-responsive elements of the promoter. In previous studies, we showed that the induction of the CEF-4/9E3 cytokine gene by pp60v-src is dependent on the PRDII/kappa B domain of the promoter (Dehbi et al., 1992). In this investigation, we describe the activation of the HIV-1 LTR by pp60v-src and show that a region of 30 bp containing the two NF-kappa B binding sites is critical for activation of the promoter. The induction was dependent on transformation since non-transforming forms of pp60v-src had little or no effect on the promoter. The expression of proviral DNA and the release of p24 antigen were also increased by v-src indicating that viral replication was stimulated in src-transformed cells. The effect of v-src on HIV-1 gene expression occurred in the presence or in the absence of the tat viral trans-activator, in fibroblasts and in Jurkat T lymphocytes. These results indicate that several promoters controlled by PRDII/kappa B may be activated constitutively in v-src transformed cells and suggest that oncogenic tyrosine kinases may play a role in the induction of viruses with a PRDII/kappa B-controlled promoter.

Quiescence-dependent activation of the p20K promoter in growth-arrested chicken embryo fibroblasts

We described the synthesis of a quiescence-specific p20K protein in quiescent chicken heart mesenchymal cells and contact-inhibited chicken embryo fibroblasts (Bédard, P.-A., Balk, S.D., Gunther, H.S., Morisi, A., and Erikson, R.L. (1987a) Mol. Cell. Biol. 7, 1450-1459). We now report that the expression of p20K is enhanced in cells rendered quiescent by other conditions of growth arrest such as serum starvation or treatment with hydroxyurea. Chicken embryo fibroblasts transformed by the Rous sarcoma virus also expressed p20K upon serum/medium depletion. In all conditions investigated, the synthesis of p20K was correlated with decreased DNA synthesis, indicating that growth arrest regulates the expression of p20K in fibroblasts. The abundance of p20K mRNAs was elevated in quiescent cells, and the p20K gene was more active in nuclear run-on transcription assays in conditions of growth arrest. The p20K gene was isolated, and the promoter region was analyzed in transient expression assays. Serum starvation increased the activity of the promoter, indicating that the expression of p20K is controlled at least in part at the transcriptional level. Deletion analysis revealed that a region of less than 217 base pairs (bp) is sufficient for quiescence-dependent activity of the promoter. Within this region, a segment of 48 bp was essential to basal and quiescence-induced activity of the promoter in dividing and nondividing cells, respectively. The 48-bp region enhanced the activity of a minimal heterologous promoter in quiescent cells but had no effect in conditions of proliferation, indicating that it functions as a quiescence-responsive unit (QRU). Therefore, the transcription of the p20K gene in quiescent fibroblasts is controlled by the one or several growth-regulated elements located in the 48-bp QRU of the promoter.

Cytokine-induced expression of mRNAs for chemotactic factors in human synovial cells and fibroblasts

In response to interleukin 1 or tumor necrosis factor, human synovial cells and fibroblasts expressed several genes encoding known chemotactic factors or related proteins. Transcripts for interleukin 8 (IL-8), gro/MGSA, pAT 464, IP-10, pAT 744 and Monocyte Chemotactic and Activating Factor (MCAF) accumulated rapidly in IL-1 and TNF-treated cells. The inhibition of protein synthesis led to the superinduction of IL-8 and gro/MGSA mRNAs in IL-1, but not in TNF-treated cells. Thus, IL-1 and TNF are likely to regulate the expression of these mRNAs by different mechanisms. Important cell-specific differences in mRNA accumulation characterized the expression of chemotactic factor genes. Moreover, only a subset of the same genes was activated in quiescent cells stimulated by serum. Therefore, genes encoding closely related proteins each had a distinct pattern of expression. continuous stimulation of fibroblasts and synovial cells with IL-1 resulted in high and prolonged expression of IL-8 and gro/MGSA mRNAs. These results extend the list of chemotactic factor genes expressed by mesenchymal cells in vitro and suggest a pivotal role for these cells in processes such as chronic inflammation.

Regulation of gene expression in oncogenically transformed cells

Several genes expressed in response to growth factors are also regulated aberrantly in oncogenically transformed cells. The constitutive expression of genes encoding extracellular proteases, transcription factors, and cytokines is often correlated with cell transformation. In several instances, the uncontrolled expression of these genes is the result of transcriptional activation. Therefore, much attention has been devoted to the study of promoter function in transformed cells. We now review the results of recent investigations on transformation-dependent gene expression. The activation of several transcription factors in oncogenically-transformed cells is described. Results regarding the regulation of promoters through PRD II/kappa B are presented for cells transformed by a variety of oncogenes. Finally, we discuss the significance of transcription factor activation in the process of cell transformation.

Transcriptional activation of the CEF-4/9E3 cytokine gene by pp60v-src

The CEF-4/9E3 gene is expressed constitutively in Rous sarcoma virus (RSV)-transformed cells. This expression is largely determined by an increase in transcription of the gene. In this report, we characterize the regulatory elements responsible for the transformation-dependent activation of CEF-4/9E3. Three sequences corresponding to AP-1, PRD II/kappa B, and TAACGCAATT are involved in the process and therefore define the src-responsive unit (SRU) of the CEF-4 promoter. In constructs containing a deletion of the SRU, multiple copies of AP-1 or PRD II/kappa B, but not TAACGCAATT, led to activation of the promoter. Thus, factors interacting with these elements are constitutively activated in RSV-transformed chicken embryo fibroblasts. In agreement with the results of transient expression assays, protein binding to AP-1, PRD II/kappa B, and TAACGCAATT were more abundant in the nuclei of transformed cells. The expression of the CEF-4 promoter was investigated in cells infected by a temperature-sensitive mutant of RSV. No significant increase in CEF-4 promoter activity was detected early after activation of pp60v-src. In contrast, a substantial activation of the CEF-4 promoter was detected late after a temperature shift. Factors interacting with the TAACGCAATT, PRD II/kappa B, and AP-1 elements accumulated gradually over a period of several hours. Therefore, transcriptional activation plays an important role in the late, constitutive expression of the CEF-4 gene in stably transformed cells.

Complex expression pattern of the CEF-4 cytokine in transformed and mitogenically stimulated cells

The transformation of chicken embryo fibroblasts (CEF) by the Rous sarcoma virus leads to the constitutive expression of the cellular gene designated CEF-4. With specific antisera, we confirmed that transformed cells actively synthesize and secrete a 6 kDa polypeptide corresponding to the CEF-4 gene product. The expression of CEF-4 was investigated by Northern and immunoprecipitation analyses. Upon activation of pp60v-src in cells infected by a ts mutant of RSV, the expression of CEF-4 was biphasic with an early transient and a late constitutive period of expression. CEF-4 was expressed in cells transformed by a variety of oncogenes, but the level of constitutive expression differed quantitatively among transformed cells. Cells transformed by v-myc alone did not express CEF-4. Unlike other members of the interleukin 8 gene family, CEF-4 was induced in response to a broad spectrum of growth factors and inflammatory agents. Dexamethasone repressed the induction of CEF-4 by lipopolysaccharides but had little or no effect on the response to serum or pp60v-src. These data emphasize the complexity of CEF-4 expression and suggest the existence of multiple levels or pathways of CEF-4 regulation in normal and transformed cells.

Tandem duplication and divergence of a sea urchin protein belonging to the troponin C superfamily

The Spec1 and Spec2 proteins of the sea urchin Strongylocentrotus purpuratus are related to calmodulin, troponin C, and myosin light chains by sequence similarity in their four calcium binding domains. These domains, the EF-hands, are distinct helix-loop-helix structures of about 40 amino acids. The Spec1 and Spec2 genes are expressed specifically in aboral ectoderm cells of the developing embryo; however, the function of the Spec proteins in these cells is unknown. To find conserved regions of the proteins that might be important for structure and function, Spec homologues from Lytechinus pictus, a distantly related sea urchin, were sought. L. pictus embryos do not synthesize detectable amounts of the 14,000-17,000-Da Spec proteins as determined by two-dimensional gel electro-phoresis, but do synthesize three 34,000-Da proteins that cross-react with Spec1 antibodies and display a similar ontogenetic pattern of expression. cDNA clones were isolated by hybridization to a synthetic oligonucleotide corresponding to the EF-hand. One clone, LpS1, encodes an mRNA with developmental properties like those of the S. purpuratus Spec mRNAs. However, LpS1 contains an open reading frame for a protein of 34,000 Da rather than 17,000 Da, and antibodies raised against part of the LpS1 reading frame demonstrate that LpS1 encodes a 34,000-Da protein in L. pictus embryos. The sequence of LpS1 reveals the presence of eight EF-hand domains, which share structural homology with the Spec1 or Spec2 EF-hands; however, little else in the protein sequence is conserved. The results support the hypothesis that the LpS1 gene arose from a duplication of an ancestral Spec gene and that the overall structural features of the Spec family of proteins are more conserved than the amino acid sequences.

Repression of quiescence-specific polypeptides in chicken heart mesenchymal cells transformed by Rous sarcoma virus

Chicken heart mesenchymal cells do not proliferate in medium of physiological composition containing plasma (S. Balk, Proc. Natl. Acad. Sci. USA 77:6606-6610, 1980). To understand the molecular events involved in cell quiescence and in the initiation of cell division under physiological conditions, we examined the differences in the patterns of protein synthesis of quiescent, hormone-stimulated, and Rous sarcoma virus-transformed chicken heart mesenchymal cells. We describe the expression of a 20,000-kilodalton (kDa) polypeptide actively synthesized by quiescent cells but not by their transformed counterparts. Normal chicken heart mesenchymal cells stimulated with epidermal growth factor and insulin also repressed the synthesis of the 20,000-kDa polypeptide while actively growing but synthesized increasing amounts of the protein at high cell density (confluence). The synthesis of the 20,000-kDa protein is not restricted to chicken heart mesenchymal cells, since confluent, density-arrested chicken embryo fibroblasts also expressed high levels of the protein. Transformed chicken heart mesenchymal cells and embryo fibroblasts did not synthesize the protein even at high cell density. The 20,000-kDa polypeptide accumulated in the culture medium.

Translational activation of maternal mRNA encoding the heat-shock protein hsp90 during sea urchin embryogenesis

Changes in protein synthesis induced by heat shock of Strongylocentrotus purpuratus gastrulae were analyzed bt two-dimensional electrophoresis. Hyperthermia induces the synthesis of polypeptides having molecular masses of 90, 70, 50, 40, and 38 kDa. One of these, hsp90, appears as a pair of polypeptides which comigrates with proteins synthesized at normal temperature in eggs and embryos; these comigrating spots produce indistinguishable patterns upon electrophoretic analysis of partial V8 protease digests, indicating that hsp90 is synthesized throughout embryogenesis. The relative rate of incorporation of methionine into hsp90 is low in eggs and zygotes, but increases abruptly in morulae, constituting a rare and striking change in protein synthesis during early development. Cell-free translation analyses indicate that most of the mRNA encoding hsp90 resides in the pool of free ribonucleoprotein particles in eggs and early embryos, but shifts to polysomes by the 64-cell stage while remaining constant in mass. Thus the increase in synthesis of hsp90 appears to be via the selective activation of translation of a stored maternal mRNA. The shift of hsp90 mRNA to polysomes is accompanied by polyadenylation. Heat shock of eggs or zygotes did not result in translational activation of hsp90 mRNA. The sea urchin hsp90 doublet of spots comigrates with hsp90 induced by heat shock of chicken embryo fibroblasts, a conserved protein abundant in many cells of a variety of species.

Patterns of protein synthesis and metabolism during sea urchin embryogenesis

We have analyzed the patterns of protein synthesis in developing embryos of the sea urchin Strongylocentrotus purpuratus by two-dimensional gel electrophoresis. There was an increase in the number of proteins detectably synthesized during development, as well as significant changes in relative rates of synthesis involving approximately 20% of the nearly 900 newly synthesized polypeptides. The majority of these changes were increases rather than decreases in synthesis; about half were of at least 10-fold, while a few were of more than 100-fold. Very few changes were detected upon fertilization and during the first several hours of development, while about 60% of the changes detected occurred between the hatching and the beginning of invagination. An analysis of proteins detected by silver staining indicated that most remained nearly constant in mass during embryonic development, but several increased or declined substantially. Many proteins present in eggs were not detectably synthesized in either eggs or embryos.