Inhibition of myogenesis by okadaic acid, an inhibitor of protein phosphatases, 1 and 2A, correlates with the induction of AP1

Role of the Transcription Factor FOSL1 in Organ Development and Tumorigenesis

The transcription factor FOSL1 plays an important role in cell differentiation and tumorigenesis. Primarily, FOSL1 is crucial for the differentiation of several cell lineages, namely adipocytes, chondrocytes, and osteoblasts. In solid tumors, FOSL1 controls the progression of tumor cells through the epithelial-mesenchymal transformation. In this review, we summarize the available data on FOSL1 expression, stabilization, and degradation in the cell. We discuss how FOSL1 is integrated into the intracellular signaling mechanisms and provide a comprehensive analysis of FOSL1 influence on gene expression. We also analyze the pathological changes caused by altered Fosl1 expression in genetically modified mice. In addition, we dedicated a separate section of the review to the role of FOSL1 in human cancer. Primarily, we focus on the FOSL1 expression pattern in solid tumors, FOSL1 importance as a prognostic factor, and FOSL1 perspectives as a molecular target for anticancer therapy.

Wnt signaling in regulation of biological functions of the nurse cell harboring Trichinella spp

Background

The nurse cell (NC) constitutes in mammalian skeletal muscles a confined intracellular niche to support the metabolic needs of muscle larvae of Trichinella spp. encapsulating species. The main biological functions of NC were identified as hypermitogenic growth arrest and pro-inflammatory phenotype, both inferred to depend on AP-1 (activator protein 1) transcription factor. Since those functions, as well as AP-1 activity, are known to be regulated among other pathways, also by Wnt (Wingless-Type of Mouse Mammary Tumor Virus Integration Site) signaling, transcription profiling of molecules participating in Wnt signaling cascades in NC, was performed.

Methods

Wnt signaling-involved gene expression level was measured by quantitative RT-PCR approach with the use of Qiagen RT² Profiler PCR Arrays and complemented by that obtained by searching microarray data sets characterizing NC transcriptome.

Results

The genes involved in inhibition of canonical Wnt/β-catenin signaling cascade as well as leading to β-catenin degradation were found expressed in NC at high level, indicating inhibition of this cascade activity. High expression in NC of genes transmitting the signal of Wnt non-canonical signaling cascades leading to activation of AP-1 transcription factor, points to predominant role of non-canonical Wnt signaling in a long term maintenance of NC biological functions.

Conclusions

Canonical Wnt/β-catenin signaling cascade is postulated to play a role at the early stages of NC formation when muscle regeneration process is triggered. Following mis-differentiation of infected myofiber and setting of NC functional specificity, are inferred to be controlled among other pathways, by Wnt non-canonical signaling cascades.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1770-4) contains supplementary material, which is available to authorized users.

Revised rat multi-organ carcinogenesis bioassay for whole-body detection of chemopreventive agents: modifying potential of S-methylcysteine

The DMBDD rat multi-organ carcinogenesis model based on two-stage carcinogenesis theory was revised to make more suitable assay system for detecting chemopreventive effects of chemical substances by increasing the doses of two carcinogens, 1,2-dimethylhydrazine dihydrochloride (DMH) and N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN). The revised bioassay resulted in increasing preneoplastic or neoplastic lesions in the colon, urinary bladder and liver. S-Methylcysteine (SMC), a water-soluble organosulfur compound, was used as a test chemical in the new initiation regimen. Though SMC did not express clear-cut inhibitory effects in tumor levels, it showed modifying effects on the development of lung hyperplastic and colon preneoplastic lesions. In conclusion, the present model featuring high yields of preneoplastic and neoplastic lesions with low mortality in a short period (30 weeks), might be suitable for testing the efficacy of possible chemopreventive chemicals at the whole-body level.

Opposing functions of ATF2 and Fos-like transcription factors in c-Jun-mediated myogenin expression and terminal differentiation of avian myoblasts

With the aim to identify the oncoprotein partners implicated in the c-Jun myogenic influence, we carried out stable transfection experiments of c-Jun and/or ATF2, Fra2, c-Fos overexpression in avian myoblasts. Before induction of differentiation, c-Jun repressed myoblast withdrawal from the cell cycle, as did a TPA treatment. However, after serum removal, unlike TPA, c-Jun significantly stimulated myoblast differentiation. In search for specific partners involved in this dual influence, we found that a reduction in the amounts of c-Fos and Fra2 and an increase in c-Jun proteins occurred at cell confluence, a situation likely to favor cooperation between c-Jun and ATF2 during terminal differentiation. Whereas c-Fos and Fra2 cooperated with c-Jun to abrogate myoblast withdrawal from the cell cycle and terminal differentiation, ATF2 co-expression potentiated the positive myogenic c-Jun influence. In addition, myogenin expression was a positive target of this cooperation and this regulation occurred through a stimulation of myogenin promoter activity: (1) whereas c-Fos or Fra2 co-expression abrogated c-Jun stimulatory activity on this promoter, ATF2 co-expression potentiated this influence; (2) using a dominant negative ATF2 mutant, we established that c-Jun transcriptional activity required functionality of endogenous ATF2. These data suggest that through this dual myogenic influence due to cooperations with different partners, c-Jun is involved in the control of duration of myoblast proliferation and thereafter of fusion efficiency.

Critical activities of Rac1 and Cdc42Hs in skeletal myogenesis: antagonistic effects of JNK and p38 pathways

The Rho family of GTP-binding proteins plays a critical role in a variety of cellular processes, including cytoskeletal reorganization and activation of kinases such as p38 and C-jun N-terminal kinase (JNK) MAPKs. We report here that dominant negative forms of Rac1 and Cdc42Hs inhibit the expression of the muscle-specific genes myogenin, troponin T, and myosin heavy chain in L6 and C2 myoblasts. Such inhibition correlates with decreased p38 activity. Active RhoA, RhoG, Rac1, and Cdc42Hs also prevent myoblast-to-myotube transition but affect distinct stages: RhoG, Rac1, and Cdc42Hs inhibit the expression of all muscle-specific genes analyzed, whereas active RhoA potentiates their expression but prevents the myoblast fusion process. We further show by two different approaches that the inhibitory effects of active Rac1 and Cdc42Hs are independent of their morphogenic activities. Rather, myogenesis inhibition is mediated by the JNK pathway, which also leads to a cytoplasmic redistribution of Myf5. We propose that although Rho proteins are required for the commitment of myogenesis, they differentially influence this process, positively for RhoA and Rac1/Cdc42Hs through the activation of the SRF and p38 pathways, respectively, and negatively for Rac1/Cdc42Hs through the activation of the JNK pathway.

Regulation of HMGIC expression: an architectural transcription factor involved in growth control and development

The HMGIC gene has been implicated in the control of cell proliferation and development. We show here that HMGIC has multiple mRNA isoforms that arise by transcription initiation from alternative tandem promoters. These transcripts are not only differentially expressed between cell lines, but they can also differ within an individual cell line, in response to particular stimuli. Whereas quiescent 3T3-L1 preadipocytes express low levels of HMGIC mRNA, stimulation by serum results in a dramatic upregulation with the characteristics of a delayed-early response gene. Characterization of involved signal transduction pathways showed that both FGF-1 and PDGF-BB are strong inducers of HMGIC expression mediated via both the PI-3 kinase and MAP kinase pathways. In order to characterize the regulatory elements, sequences upstream of the translation initiation site of HMGIC were assayed for promoter activity. The HMGIC 5' flanking sequences had constitutive promoter activity in all cell lines tested, suggesting that HMGIC is regulated by negative regulatory elements that were not present in the 5'-flanking regions analysed here.

BTG1: a triiodothyronine target involved in the myogenic influence of the hormone

The product of the B-cell translocation gene 1 (BTG1), a member of an antiproliferative protein family including Tis-21/PC3 and Tob, is thought to play an important role in the regulation of cell cycle progression. We have shown in a previous work that triiodothyronine (T3) stimulates quail myoblast differentiation, partly through a cAMP-dependent mechanism involved in the stimulation of cell cycle withdrawal. Furthermore, we found that T3 or 8-Br-cAMP increases BTG1 nuclear accumulation in confluent myoblast cultures. In this study, we report that BTG1 is essentially expressed at cell confluence and in differentiated myotubes. Whereas neither T3 nor cAMP exerted a direct transcriptional control upon BTG1 expression, we found that AP-1 activity, a crucial target involved in the triiodothyronine myogenic influence, repressed BTG1 expression, thus probably explaining the low BTG1 expression level in proliferating myoblasts. In transient transfection studies, we demonstrated that an AP-1-like sequence located in the BTG1 promoter was involved in this negative regulation. Our present data also bring evidence that the stimulation of BTG1 nuclear accumulation by T3 or 8-Br-cAMP probably results from an increased nuclear import or retention in the nucleus. Lastly, BTG1 overexpression in quail myoblasts mimicked the T3 or 8-Br-cAMP myogenic influence: (i) inhibition of myoblast proliferation due to an increased rate of myoblast withdrawal from the cell cycle; and (ii) stimulation of terminal differentiation. These data suggest that BTG1 is probably involved in T3 and cAMP myogenic influences. In conclusion, BTG1 is a T3 target involved in the regulation of myoblast differentiation.

Cytokine induction of NO synthase II in human DLD-1 cells: roles of the JAK-STAT, AP-1 and NF-kappaB-signaling pathways

1. In human epithelial-like DLD-I cells, nitric oxide synthase (NOS) II expression was induced by interferon-gamma (100 u ml(-1)) alone and, to a larger extent, by a cytokine mixture (CM) consisting of interferon-gamma, interleukin-1beta (50 u ml(-1)) and tumor necrosis factor-alpha (10 ng ml(-1)). 2. CM-induced NOS II expression was inhibited by tyrphostin B42 (mRNA down to 1%; nitrite production down to 0.5% at 300 microM) and tyrphostin A25 (mRNA down to 24%, nitrite production down to 1% at 200 microM), suggesting the involvement of janus kinase 2 (JAK-2). Tyrphostin B42 also blocked the CM-induced JAK-2 phosphorylation (kinase assay) and reduced the CM-stimulated STAT1alpha binding activity (gel shift analysis). 3. CM reduced the nuclear binding activity of transcription factor AP-1. A heterogenous group of compounds, that stimulated the expression of c-fos/c-jun, enhanced the nuclear binding activity of AP-1. This group includes the protein phosphatase inhibitors calyculin A, okadaic acid, and phenylarsine oxide, as well as the inhibitor of translation anisomycin. All of these compounds reduced CM-induced NOS II mRNA expression (to 9% at 50 nM calyculin A; to 28% at 500 nM okadaic acid; to 18% at 10 microM phenylarsine oxide; and to 19% at 100 ng ml(-1) anisomycin) without changing NOS II mRNA stability. In cotransfection experiments, overexpression of c-Jun and c-Fos reduced promoter activity of a 7 kb DNA fragment of the 5'-flanking sequence of the human NOS II gene to 63%. 4. Nuclear extracts from resting DLD-1 cells showed significant binding activity for transcription factor NF-kappaB, which was only slightly enhanced by CM. The NF-kappaB inhibitors dexamethasone (1 microM), 3,4-dichloroisocoumarin (50 microM), panepoxydone (5 microg ml(-1)) and pyrrolidine dithiocarbamate (100 microM) produced no inhibition of CM-induced NOS II induction. 5. We conclude that in human DLD-1 cells, the interferon-gamma-JAK-2-STAT1alpha pathway is important for NOS II induction. AP-1 (that is downregulated by CM) seems to be a negative regulator of NOS II expression. NF-kappaB, which is probably important for basal activity of the human NOS II promoter, is unlikely to function as a major effector of CM in DLD-1 cells.

Protein phosphatase 1 isoforms in differentiating C2C12 myocytes

In muscle protein phosphatase 1 (PP1) is involved in growth factor signal transduction and metabolic regulations. Three isoforms of the catalytic subunit are found in mammalian cells (PP1alpha, PP1gamma1 and PP1delta), with potentially different functions. We investigated the changes in the PP1 isoforms in differentiating C2C12 myoblasts. Few hours after differentiation induction the soluble PP1 activity was reversibly increased, displaying a peak at 6h. This was due to activation mainly of PP1alpha, with no change in the immunodetected protein. A further indication of PP1alpha involvement came from the observation that electroporation of inactive PP1alpha into myoblasts induced a differentiation delay of at least 24h. Subsequently, starting from 9-12 h, the activities and protein levels of all the three soluble PP1 isoforms decreased, reaching a minimum around 48 h. By this time the cells had undergone morphological changes and myosin became immunodetectable. We conclude that PP1 may be involved in myoblast differentiation, based on: 1) its higher activity in myoblasts than in myocytes, 2) the reversible activation of soluble PP1alpha during the first 6h from differentiation induction, 3) the delay in differentiation onset following electroporation of inactive PP1alpha into myoblasts.

Expression of the tumor necrosis factor alpha gene and early response genes by nodularin, a liver tumor promoter, in primary cultured rat hepatocytes

Nodularin is a new liver carcinogen possessing a potent tumor-promoting activity in rat liver, mediated through inhibition of protein phosphatases 1 and 2A, and a weak initiating activity. Since we previously reported evidence that nodularin up-regulated expression of the tumor necrosis factor alpha gene (TNF alpha) and early-response genes in rat liver after its i.p. administration, and since TNF alpha had tumor-promoting activity in vitro, it is possible that TNF alpha itself is involved in liver tumor promotion. We investigated whether hepatocytes themselves induce expression of the TNF alpha gene and early-response genes in primary cultured rat hepatocytes treated with nodularin. Like nodularin, microcystin-LR, which is another liver tumor promoter belonging to the okadaic acid class, strongly induced TNF alpha gene expression in rat hepatocytes, as well as TNF alpha release from those cells into the medium. On the other hand, 12-O-tetradecanoylphorbol-13-acetate, which has been reported to induce no tumor promotion in rat liver, induced no apparent expression of the TNF alpha gene in primary cultured rat hepatocytes. As for the expression of early-response genes, 1 microM nodularin or microcystin-LR induced expression of the c-jun, jun B, jun D, c-fos, fos B and fra-1 genes in the hepatocytes, and the expression of these genes was prolonged up to 24 h, suggesting mRNA stabilization induced by inhibition of protein phosphatases 1 and 2A. This paper presents new evidence that the TNF alpha gene and early-response genes were expressed in hepatocytes treated with a liver tumor promoter.

The phosphatase inhibitor okadaic acid stimulates the TSH-induced G1-S phase transition in thyroid cells

Protein phosphorylation plays an essential role in regulating many cellular processes in eukaryotes. Signal transduction mechanisms that are reversibly controlled by protein phosphorylation require also protein phosphatases (PPs). Okadaic acid (OA), which is a potent inhibitor of protein phosphatase 2A (PP2A) and protein phosphatase 1, elicits phosphorylation of many proteins in unstimulated cells and induces different cellular responses, including transcriptional activation, shape changes, and pseudomitotic state. In this study, the effects of OA on rat thyroid cells (FRTL-5 strain) were analyzed to evaluate the role of serine/threonine phosphatases in hormone-induced thyroid cell proliferation. OA at a concentration range between 0.1 and 1 nM stimulated thyroid cell growth. Furthermore, 0.25 nM OA increased about 3.5-fold the thyrotropin (TSH)-induced DNA synthesis in quiescent cells. OA treatment also stimulated cell proliferation induced by drugs that mimic TSH effect, such as 8Br-cAMP and cholera toxin, suggesting that PP2A activity was relevant in the cAMP pathway activated by the hormone. Flow cytometry experiments showed that OA significantly increased the fraction of TSH-stimulated quiescent cells entering the S phase. In order to define the mechanisms underlying the observed stimulatory effect of OA on thyroid cell growth, expression of genes relevant in the G1-S phase transition was evaluated. A 2-fold increase in the level of cyclin D1 mRNA expression was found by Northern blot analysis in OA-treated cells. Although cdk2 gene expression was not modulated by the same OA treatment, an increase in Cdk2 protein was revealed by immunoprecipitation experiments. Moreover, OA modifies the phosphorylation pattern of the tumor suppressor retinoblastoma protein, a key event in the G1-S phase transition. Therefore, these experiments reveal that PP2A phosphatases play an important role in thyroid cell growth and can act at multiple sites in the TSH pathways driving cells to S phase.

Epidermal growth factor and okadaic acid stimulate Sp1 proteolysis

Sp1 nuclear levels have been shown to directly correlate with the proliferative state of the cell. We therefore studied changes in the abundance of Sp1 in a rat pituitary cell line GH4 whose growth rate is regulated by epidermal growth factor (EGF). Nuclear extracts from GH4 cells treated with 10 nM EGF for at least 16 h showed a 50% decrease in Sp1 binding to a GC-rich element present in the gastrin promoter. The decrease in binding correlated with a decrease in cell proliferation, a loss of nuclear Sp1 protein and a 50-60% decrease in Sp1-mediated transactivation through an Sp1 enhancer element in transfection assays. Okadaic acid, a phosphatase inhibitor, was synergistic with the effect of EGF on Sp1 protein levels suggesting that the loss of Sp1 was mediated by phosphorylation events. This result was confirmed by showing a 2-fold increase in orthophosphate-labeled Sp1 with EGF and okadaic acid. Cycloheximide prevented the expected loss of Sp1 mediated by EGF and okadaic acid suggesting that the synthesis of a protease may mediate these events. This hypothesis was tested directly by showing that the cysteine protease inhibitor leupeptin prevented Sp1 degradation. Using the PEST-FIND computer program, the computed PEST score for human and rat Sp1 is 10.4 and 13.7, respectively, indicating that Sp1 has a domain with a high concentration of proline, glutamic acid, serine, and threonine residues as reported for a number of proteins with inducible rates of degradation. Collectively, these results indicate that sustained stimulation of GH4 cells by EGF initiates a cascade of phosphorylation events that promotes Sp1 proteolysis, decreased Sp1 nuclear levels and decreased cellular proliferation.

S-methylcysteine and cysteine are inhibitors of induction of glutathione S-transferase placental form-positive foci during initiation and promotion phases of rat hepatocarcinogenesis

S-Methylcysteine (SMC) occurs in a variety of plants, including Allium sativum, Phaseolus vulgaris, and Cruciferae. In this study, we synthesized five organosulfur compounds (OSCs), SMC and four analogs, and examined their modifying effects on diethylnitrosamine-induced neoplasia of the liver in male F344 rats, using the medium-term bioassay system of Ito (Ito test) based on the two-step model of hepatocarcinogenesis. In addition, we investigated the modifying effects of SMC and cysteine on the initiation stage of rat hepatocarcinogenesis. Carcinogenic potential was scored by comparing the numbers and areas of induced glutathione S-transferase placental form (GST-P)-positive hepatocellular focl. All OSCs examined had a tendency to decrease the number of GST-P-positive foci when given in the promotion stage of the Ito test, and in particular SMC and cysteine exerted significant inhibitory effects. When given during the initiation stage, these two OSCs also significantly inhibited focus formation. Regarding the mechanism underlying the inhibitory effects of SMC and cysteine, measurement of ornithine decarboxylase in SMC- and cysteine-treated liver tissues after partial hepatectomy (PH) revealed a significantly reduced activity, and the proportion of hepatocytes positive for proliferating cell nuclear antigen was significantly decreased by SMC or cysteine administration. Moreover, examination of the expression of the early response proto-oncogenes, c-fos, c-jun, and c-myc, after PH demonstrated down-regulated induction of c-jun mRNA transcripts by SMC, sustained for an eight-hour period. Our results support the view that SMC and cysteine are chemopreventive agents for rat hepatocarcinogenesis and that their intake may be importance for cancer prevention.

Induction of c-Erb A-AP-1 interactions and c-Erb A transcriptional activity in myoblasts by RXR. Consequences for muscle differentiation

We have previously shown that c-Erb A and v-Erb A display a cell-specific activity in avian myoblasts. In this work, we have compared the molecular basis of thyroid hormone action in HeLa cells and in QM7 myoblasts. The transcriptional activity of c-Erb A alpha 1 through a palindromic thyroid hormone response element (TRE) was similar in both cell types. However, c-Erb A did not activate gene transcription through a direct repeat sequence (DR) 4 TRE in myoblasts in contrast to results obtained in HeLa cells. Moreover, whereas retinoic acid receptor-AP-1 interactions were functional in both cell types, thyroid hormone receptor (T3R)-AP-1 interactions were only functional in HeLa cells. Using electrophoretic mobility shift assays, functional tests, and Northern blot experiments, we observed that RXR isoforms are not expressed in proliferating myoblasts. Expression of RXR gamma in these cells did not influence T3R transcriptional activity through a palindromic TRE but induced such an activity through a DR4 TRE. Moreover, it restored c-Erb A-AP-1 functionality in QM7 myoblasts and enhanced the myogenic influence of T3. We also observed that c-Jun overexpression in proliferating QM7 cells restored T3R transcriptional activity through a DR4 TRE. Therefore, alternative mechanisms are involved in the induction of T3R transcriptional activity according to the cell status (proliferation: c-Jun; differentiation: RXR). In addition we provide the first evidence that RXR is required to allow inhibition of AP-1 activity by ligand-activated T3R. Lastly, we demonstrate the importance of RXR in the regulation of myoblast differentiation by T3.

Okadaic acid-induced transcriptional downregulation of type I collagen gene expression is mediated by protein phosphatase 2A

Expression of type I collagen genes is highly regulated and becomes abnormal in various pathological conditions, from excessive collagen production in fibrotic diseases to their downregulation in transformed cells. Some inflammatory cytokines and other ligands, capable of eliciting intracellular phosphorylation, can profoundly alter collagen gene expression. We investigated the role of serine/threonine protein phosphatases (PP) in the regulation of collagen gene expression. Biosynthesis of the endogenous type I procollagen, and expression of Pro alpha 1(I) promoter-luciferase (Luc) constructs transfected in NIH3T3 fibroblasts, were evaluated in response to PP2A and PP1 inhibitor okadaic acid (OA) and exogenously expressed PP catalytic subunits. OA suppressed type I collagen gene expression as judged by reduced rates of protein synthesis, steady state levels of Pro alpha 1(I) collagen mRNA and expression of Luc driven by Pro alpha 1 (I) collagen promoter in OA-treated cells. Co-transfection of Pro alpha 1(I)-Luc with expression vectors containing PP2A, but not PP1, stimulated collagen promoter activity. These results strongly suggest that OA acts via PP2A-mediated dephosphorylation of an unidentified transcription factor(s) or cofactor(s) needed to activate Pro alpha 1(I) collagen promoter.

Production of an endogenous inhibitor of protein kinase C by embryonic myoblasts undergoing differentiation

The current studies were undertaken to determine whether embryonic myoblasts or myogenic satellite cells undergoing differentiation and fusion contained endogenous modulators of protein kinase C (PKC). Clonal-derived turkey embryonic myoblast and satellite cell cultures were harvested at confluency and at approximately 40% fusion (embryonic myoblasts) or 75% fusion (satellite cells). PKC activity in cystosolic preparations of the cells and myotubes was undetectable. Cytosolic extracts (0.065 mg protein) of confluent and fused satellite cell cultures and confluent embryonic myoblasts had no effect on control PKC activity (control: 14.9 pmol/min, control + cytosols: 15.2, 13.9 and 13.5 pmol/min, respectively). Cytosolic preparations (0.065 mg protein) of embryonic myoblast-derived myotubes inhibited control PKC activity (4.0 pmol/min). In a time-course study, PKC-inhibitory activity was present in embryonic myoblasts at the earliest time point examined (30% fusion). Additionally, protein phosphatase activity correlated with PKC inhibitory activity. Thus, PKC-inhibitory activity appears as embryonic myoblasts begin to undergo fusion to form myotubes, but is not present in differentiating satellite cells.

Fos-related antigen (Fra-1), junB, and junD activate human involucrin promoter transcription by binding to proximal and distal AP1 sites to mediate phorbol ester effects on promoter activity

Human involucrin (hINV) is a cornified envelope precursor that is specifically expressed in the suprabasal epidermal layers. We previously demonstrated that 2500 base pairs of the hINV gene upstream regulatory region confers differentiation appropriate regulation in transgenic mice. An analysis of the hINV gene sequence upstream of the transcription start site reveals five potential AP1 binding sites (AP1-1 to 5). Using reporter gene constructs in human keratinocytes, we show that the most distal (AP1-5) and most proximal (AP1-1) AP1 sites are essential for high level transcriptional activity. Simultaneous mutation of these sites reduces transcription by 80%. Gel supershift experiments indicate the interaction of these sites with Fra-1, junB, and junD. Involucrin mRNA levels increase 10-fold and promoter activity 5-11-fold when differentiation is induced by phorbol ester. Functional studies implicate AP1-1 and AP1-5 in mediating the phorbol ester-dependent increase in promoter activity. No involucrin promoter activity or involucrin mRNA was detected in 3T3 fibroblasts. We conclude that (i) two AP1 sites in the hINV promoter are important elements required for keratinocyte-specific expression, (ii) these AP1-1 sites mediate the phorbol ester-dependent increase in promoter activity, and (iii) Fra-1, junB, and junD may be important regulators of hINV expression in epidermis.

Elevation of transforming growth factor-alpha mRNA and protein expression by diverse tumor promoters in SENCAR mouse epidermis

The study presented here was designed to further investigate the role of transforming growth factor-alpha (TGF alpha) in skin tumor promotion by examining the ability of 12-O-tetradecanoylphorbol-13-acetate (TPA) and several non-phorbol ester promoters to alter TGF alpha mRNA and protein levels in mouse epidermis. Total RNA was isolated from SENCAR mouse epidermis at various times after single topical treatments with TPA (3.4 nmol), chrysarobin (220 nmol), okadaic acid (2.5 nmol), and thapsigargin (8.5 nmol). Northern analyses of these isolated RNA samples revealed that all four tumor promoters transiently elevated TGF alpha mRNA levels. Whereas TPA, okadaic acid, and thapsigarin elevated TGF alpha mRNA levels over similar time courses (peak at 4-8 h), chrysarobin elevated TGF alpha mRNA levels with a markedly delayed time course (peak at 24-48 h). More detailed studies with TPA also revealed that multiple treatments (four over a 2-wk period) transiently elevated TGF alpha mRNA in both the epidermis and the dermis. The time courses for changes in TGF alpha mRNA after multiple TPA treatments were similar for both tissues. To facilitate studies of altered TGF alpha mRNA expression in mouse epidermis and possibly other mouse tissues, a semiquantitative reverse transcriptase-polymerase chain reaction method was developed. This method faithfully revealed changes in TGF alpha mRNA levels with all four tumor-promoting agents similar to those determined by northern blot analyses. Immunofluorescence analysis of frozen sections from promoter-treated skin revealed elevated TGF alpha protein levels in both epidermis and dermis, although staining was most intense in the epidermal layer. Immunofluorescence analysis of epidermal hyperplasia adjacent to a full-thickness wound also demonstrated significant epidermal TGF alpha staining. Collectively, these results indicate that mechanistically diverse tumor promoter stimuli elevate TGF alpha mRNA and protein in SENCAR mouse epidermis. Elevated levels of TGF alpha may play an essential role in mitogenic stimulation during tumor promotion by diverse promoting stimuli.

Effects of okadaic acid on expression of phosphoenolpyruvate carboxykinase in cultured rat hepatocytes

In normal rat hepatocytes in primary culture the level of mRNA encoding the key gluconeogenic enzyme phospho enol pyruvate carboxykinase (PEPCK) is increased by the cyclic AMP analogue, chlorophenylthio cyclic AMP (cpt cAMP), and this response is reversed by insulin. The protein-phosphatase inhibitor okadaic acid diminished the stimulatory effects of cpt cAMP on PEPCK mRNA. Protein kinase A remained fully active in the presence of okadaic acid, therefore, the insulin-mimetic actions of okadaic acid were localised to a site subsequent to initial protein kinase A activation. Insulin produced a decrease in PEPCK mRNA expression which was similar to that of okadaic acid both in extent and mechanism (i.e., lack of change in protein kinase A activation). The effects of okadaic acid on PEPCK mRNA amount were not additive with those of insulin and the effects of insulin were not abolished by okadaic acid. These data suggest that okadaic acid and insulin may interact with the cAMP regulation of the PEPCK gene expression at a common site. The mechanisms by which this may be attained are discussed in relation to what is known about the control of specific protein kinases and protein phosphatases by insulin and okadaic acid and of the importance of protein phosphorylation state to regulation of gene-transcriptional processes.

Effect of okadaic acid on apo B and apo A-I secretion by CaCo-2 cells

The effect of protein phosphorylation on the synthesis and secretion of apo B and apo A-I by CaCo-2 cells was investigated. Okadaic acid, a potent inhibitor of protein serine/threonine phosphatases 1 and 2A, caused a significant increase in total cellular protein phosphorylation. Apo B-48 was phosphorylated in control cells and this was increased significantly in the presence of okadaic acid. Under the experimental conditions, the phosphorylation of apo B-100 or apo A-I was not observed. No evidence of tyrosine phosphorylation of apo B-100, B-48, or apo A-I was found. Okadaic acid did not change the amount of apo B mass within cells but apo B mass secreted into the basolateral medium was decreased by 40%. Apo A-I mass within cells or in the basolateral medium was unaffected by okadaic acid. Despite causing an 18% decrease in total protein synthesis, okadaic acid did not alter the rate of synthesis of apo B-100, apo B-48, or apo A-I. Cellular turnover of labeled apo B-100 in cells incubated with okadaic acid was similar to controls, whereas apo B-48 and apo A-I turnover were slowed by okadaic acid. Compared to controls, however, 1 microM okadaic acid caused a 75% and 50% decrease in the secretion of newly synthesized apo B-100 and apo B-48, respectively, while decreasing labeled apo A-I secretion by 35%. In contrast to apo A-I mRNA levels, which were not altered by okadaic acid, apo B mRNA levels were significantly decreased by the polyether fatty acid. Despite differences observed in the phosphorylation state of apo B-100 and apo B-48, okadaic acid decreased the secretion of both forms of apo B without altering their synthesis. Okadaic acid, by increasing cellular protein phosphorylation, significantly disrupts the secretory processing of apo B by CaCo-2 cells.