Induction of ornithine decarboxylase activity is a necessary step for mitogen-activated protein kinase kinase-induced skin tumorigenesis

A transgenic mouse line overexpressing a constitutively active mutant of MEK1, a downstream effector of Ras, driven by the keratin 14 (K14) promoter, has been used to test the hypothesis that ornithine decarboxylase (ODC) induction during tumor promotion following a single initiating event [i.e., the activation of the Raf/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (Raf/MEK/ERK) pathway], is a necessary step in skin carcinogenesis. K14-MEK mice exhibit moderate hyperplasia, with spontaneous skin tumor development within 5 weeks of birth. Analysis of epidermis and dermis showed induction of MEK protein and ERK1/ERK2 phosphorylation, but no change in Akt-1, suggesting that the PI 3-kinase pathway, another pathway downstream of ras, is not activated. Examination of tumors revealed high levels of ODC protein and activity, indicating that activation of signaling cascades dependent on MEK activity is a sufficient stimulus for ODC induction. When K14-MEK mice were given alpha-difluoromethylornithine (DFMO), a suicide inactivator of ODC, in the drinking water from birth, there was a dramatic delay in the onset of tumor growth ( approximately 6 weeks), and only 25% of DFMO-treated mice developed tumors by 15 weeks of age. All untreated K14-MEK mice developed tumors by 6 weeks of age. Treatment of tumor-bearing mice with DFMO reduced both tumor size and tumor number within several weeks. Tumor regression was the result of both inhibition of proliferation and increased apoptosis in tumors. The results establish ODC activation as an important component of the Raf/MEK/ERK pathway, and identify K14-MEK mice as a valuable model with which to study the regulation of ODC in ras carcinogenesis.

The Bax N terminus is required for negative regulation by the mitogen-activated protein kinase kinase and Akt signaling pathways in T cells

The Bcl-2 family proapoptotic protein, Bax, redistributes to the mitochondrion in response to varied stimuli, triggering loss of mitochondrial integrity and apoptosis. Suppression of MAPK kinase (MEK1) by the reagent UO126 in activated T cells maintained in the cytokine IL-2 disrupts cytoplasmic localization of Bax and cell survival. UO126 triggers mitochondrial translocation of ectopically expressed Bax-GFP, and both UO126 and dominant negative MEK-1 (DN-MEK1) trigger increased apoptosis in Bax-GFP-expressing T cell lines. Because inhibition of PI3K or its target Akt also triggers mitochondrial translocation of Bax in T cells and apoptosis in Bax-transfected cell lines, we generated Bax deletion mutants to identify the region(s) that confers sensitivity to regulation by MEK1 and Akt. A deletion mutant (Bax(1-171)) without the C terminus mitochondrial targeting sequence or an Akt target site (Ser(184)) localizes to the cytoplasm and triggers low level apoptosis that is enhanced by DN-Akt or DN-MEK1. A construct that lacks the first 29 aa (Bax-delta29) largely localizes to mitochondria, is highly apoptogenic, and is not inhibited by Akt or MEK1. Furthermore, Bax-delta29 overcomes IL-2-dependent survival in a T cell line, whereas Bax triggers comparatively low levels of apoptosis in these cells. Cytoplasmic localization and regulation by MEK1 and Akt are restored in a mutant deleted of the first 13 aa (Bax-delta13). Taken together, our results identify a region in the Bax N terminus that determines cellular localization regulated by MEK- and Akt-dependent signaling in T cells.

Analysis of the ERK1,2 transcriptome in mammary epithelial cells

MAPK (mitogen-activated protein kinase) pathways constitute major regulators of cellular transcriptional programmes. We analysed the ERK1,2 (extracellular-signal-regulated kinase 1,2) transcriptome in a non-transformed MEC (mammary epithelial cell) line, MCF-12A, utilizing rAd MEK1EE, a recombinant adenovirus encoding constitutively active MEK1 (MAPK/ERK kinase 1). rAd MEK1EE infection induced morphological changes and DNA synthesis which were inhibited by the MEK1,2 inhibitor PD184352. Hierarchical clustering of data derived from seven time points over 24 h identified 430 and 305 co-ordinately up-regulated and down-regulated genes respectively. c-Myc binding sites were identified in the promoters of most of these up-regulated genes. A total of 46 candidate effectors of the Raf/MEK/ERK1,2 pathway in MECs were identified by comparing our dataset with previously reported Raf-1-regulated genes. These analyses led to the identification of a suite of growth factors co-ordinately induced by MEK1EE, including multiple ErbB ligands, vascular endothelial growth factor and PHRP (parathyroid hormone-related protein). PHRP is the primary mediator of humoral hypercalcaemia of malignancy, and has been implicated in metastasis to bone. We demonstrate that PHRP is secreted by MEK1EE-expressing cells. This secretion is inhibited by PD184352, but not by ErbB inhibitors. Our results suggest that, in addition to anti-proliferative properties, MEK1,2 inhibitors may be anti-angiogenic and possess therapeutic utility in the treatment of PHRP-positive tumours.

Insulin receptor, insulin receptor substrate-1, Raf-1, and Mek-1 during hormonal hepatocarcinogenesis by intrahepatic pancreatic islet transplantation in diabetic rats

Low-number transplantation of pancreatic islets into the livers of diabetic rats leads to transformation of the downstream liver acini into clear-cell foci of altered hepatocytes (FAHs). These FAHs correspond to the glycogen-storing (clear-cell) phenotype of hepatocellular preneoplasias and develop into hepatocellular adenomas (HCAs) and hepatocellular carcinomas (HCCs) within 6 to 24 months. In addition, they show metabolic alterations that resemble well-known insulin effects, most likely constituting the result of the local hyperinsulinemia. Thus, we investigated FAHs, HCAs, and HCCs for altered expression of insulin receptor, insulin receptor substrate-1 (IRS-1), Raf-1 and Mek-1. Light and electron microscopic immunohistochemistry revealed a translocation of insulin receptor from the plasma membrane (normal tissue) into the cytoplasm in clear-cell FAHs and an increase in insulin receptor expression in HCAs and HCCs. FAHs also showed an increase in IRS-1 gene expression, investigated by in situ hybridization and quantitative reverse transcription-PCR. IRS-1, Raf-1, and Mek-1 proteins were strongly overexpressed in FAHs and tumors, as compared with the unaltered liver tissue. These overexpressions were closely linked to the clear-cell phenotype of preneoplastic and neoplastic hepatocytes, because basophilic FAHs (later stages) and basophilic tumors showed no overexpressions. In this endocrine model of hepatocarcinogenesis, severe alterations of insulin signaling were induced by the pathological local action of islet hormones in the livers and may substantially contribute to the carcinogenic process.

Salicylate Regulates COX‐2 Expression Through ERK and Subsequent NF‐κB Activation in Osteoblasts

The expression of cyclooxygenase-2 (COX-2) is a characteristic response to inflammation and can be inhibited with sodium salicylate. TNF-alpha plus IFN-gamma can induce extracellular signal-regulated kinase (ERK), IKK, IkappaB degradation and NF-kappaB activation. The inhibition of the ERK pathway with selective inhibitor, PD098059, blocked cytokine-induced COX-2 expression and PGE2 release. Salicylate treatment inhibited COX-2 expression induced by TNF-alpha/IFN-gamma and regulated the activation of ERK, IKK and IkappaB degradation and subsequent NF-kappaB activation in MC3T3E1 osteoblasts. As well, antioxidant-catalase, N-acetyl-cysteine or reduced glutathione-attenuated COX-2 expression in combined cytokines-treated cells. These antioxidants also inhibited the activation of ERK, IKK and NF-kappaB in MC3T3E1 osteoblasts. In addition, TNF-alpha/IFN-gamma stimulated ROS release in the osteoblasts. However salicylate had no obvious effect on ROS release in DCFDA assay. The results showed that salicylate inhibited the activation of ERK and IKK, IkappaB degradation and NF-kappaB activation independent of ROS release and suggested that salicylate exerts its anti-inflammatory action in part through inhibition of the ERK, IKK, IkappaB, NF-kappaB and resultant COX-2 expression pathway.

Heat shock inhibition of lipopolysaccharide-mediated tumor necrosis factor expression is associated with nuclear induction of MKP-1 and inhibition of mitogen-activated protein kinase activation

OBJECTIVE

Application of heat shock before an inflammatory stimulus often results in an attenuated response to that stimulus. As a result, it has become increasingly appreciated that heat shock may induce cross-tolerance to a variety of stimuli based on in vitro and in vivo models. Circulating peripheral blood monocytes are key mediators of cytokine release following endotoxin challenge. The mitogen-activated protein kinases play a key role in the transcriptional regulation of this response including expression of tumor necrosis factor. As such, counterregulatory phosphatases that target mitogen-activated protein kinase may play a role in this heat shock-mediated effect. We hypothesized that prior heat shock to monocytes would induce a phosphatase, MKP-1, that regulated mitogen-activated protein kinase activity and subsequently conferred cross-tolerance to lipopolysaccharide stimulation.

DESIGN

Experimental.

SETTING

University research foundation laboratory.

SUBJECTS

THP-1 human monocyte cell line.

INTERVENTIONS

THP-1 cells were exposed to either heat shock (43 degrees C, 1 hr) or normothermia (37 degrees C, 1 hr) and allowed to recover before stimulation with endotoxin (lipopolysaccharide).

MEASUREMENTS AND MAIN RESULTS

Induction of a heat shock response was determined by heat shock protein-70 expression. Tumor necrosis factor and interleukin-10 were measured by enzyme-linked immunosorbent assay to assess heat shock inhibition of lipopolysaccharide-induced gene expression. The effect of heat shock on lipopolysaccharide-mediated activation of the p38 and ERK kinases was examined by measuring phospho-specific isoforms of p38 and ERK1/2 and correlated to in vitro kinase activity. Confirmatory data were generated from experiments employing either pharmacologic inhibition or genetic deletion of MKP-1. Heat shock induced the nuclear localized phosphatase, MKP-1, that attenuated p38 and ERK kinase activity resulting in significantly diminished tumor necrosis factor expression in response to lipopolysaccharide.

CONCLUSIONS

The effect of heat shock on decreasing the tumor necrosis factor response to lipopolysaccharide is conferred by induction of MKP-1, which negatively regulates p38 and ERK kinases. Modulation of phosphatase activity may be a potential strategy for attenuating acute inflammatory responses.

Transactivation of epidermal growth factor receptor mediates catecholamine-induced growth of vascular smooth muscle

Stimulation of alpha1-adrenoceptors induces proliferation of vascular smooth muscle cells (SMCs) and contributes to arterial remodeling. Although activation of NAD(P)H oxidase and generation of reactive oxygen species (ROS) are required, little is known about this pathway. In this study, we examined the hypothesis that epidermal growth factor receptor (EGFR) transactivation and extracellular regulated kinases (ERK) are involved in alpha1-adrenoceptor-mediated SMC growth. Phenylephrine increased protein synthesis in association with a rapid (< or =5 minutes) and sustained (> or =60 minutes) doubling of phosphorylation of EGFR and ERK1/2, but not p38 or JNK in the media of rat aorta maintained in organ culture. Antagonists of EGFR phosphotyrosine activity (AG-1478) and ERK phosphorylation (PD-98059, U-0126) abolished phenylephrine-induced protein synthesis, whereas antagonists of p38 or JNK phosphorylation had no specific effect. A competitive antagonist (P22) for heparin binding EGF-like growth factor (HB-EGF) blocked phenylephrine-induced protein synthesis, as did downregulation of pro-HB-EGF (CRM197). Phenylephrine-induced protein synthesis was inhibited by neutralizing antibody to HB-EGF and absent in HB-EGF-/- SMCs. Inhibitors of metalloproteinases (BiPS, KB-R7785) also blocked adrenergic growth. The neutralizing antibody against HB-EGF had no effect on the two-fold increase in ROS generation induced by phenylephrine (DCF fluorescence), suggesting that stimulation of NAD(P)H oxidase by alpha1-adrenoceptor occupation precedes HB-EGF release. Cell culture studies confirmed and extended these findings. These data suggest that alpha1-adrenoceptor-mediated SMC growth requires ROS-dependent shedding of HB-EGF, transactivation of EGFR, and activation of the MEK1/2-dependent MAP kinase pathway. This trophic pathway may link sympathetic activity to arterial wall growth in adaptive remodeling and hypertrophic disease.

Molecular Cross-talk between MEK1/2 and mTOR Signaling during Recovery of 293 Cells from Hypertonic Stress

To investigate the role for initiation factor phosphorylation in de novo translation, we have studied the recovery of human kidney cells from hypertonic stress. Previously, we have demonstrated that hypertonic shock causes a rapid inhibition of protein synthesis, the disaggregation of polysomes, the dephosphorylation of eukaryotic translation initiation factor (eIF)4E, 4E-BP1, and ribosomal protein S6, and increased association of 4E-BP1 with eIF4E. The return of cells to isotonic medium promotes a transient activation of Erk1/2 and the phosphorylation of initiation factors, promoting an increase in protein synthesis that is independent of a requirement for eIF4E phosphorylation. As de novo translation is associated with the phosphorylation of 4E-BP1, we have investigated the role of the signaling pathways required for this event by the use of cell-permeable inhibitors. Surprisingly, although rapamycin, RAD001, wortmannin, and LY294002 inhibited the phosphorylation of 4E-BP1 and its release from eIF4E, they did not prevent the recovery of translation rates. These data suggest that only a small proportion of the available eIF4F complex is required for maximal translation rates under these conditions. Similarly, prevention of Erk1/2 activity alone with low concentrations of PD184352 did not impinge upon de novo translation until later times of recovery from salt shock. However, U0126, which prevented the phosphorylation of Erk1/2, ribosomal protein S6, TSC2, and 4E-BP1, attenuated de novo protein synthesis in recovering cells. These results indicate that the phosphorylation of 4E-BP1 is mediated by both phosphatidylinositol 3-kinase-dependent rapamycin-sensitive and Erk1/2-dependent signaling pathways and that activation of either pathway in isolation is sufficient to promote de novo translation.

Basic fibroblast growth factor antagonizes transforming growth factor-beta1-induced smooth muscle gene expression through extracellular signal-regulated kinase 1/2 signaling pathway activation

OBJECTIVE

Transforming growth factor-beta1 (TGFbeta1) and fibroblast growth factor (FGF) families play a pivotal role during vascular development and in the pathogenesis of vascular disease. However, the interaction of intracellular signaling evoked by each of these growth factors is not well understood. The present study was undertaken to examine the molecular mechanisms that mediate the effects of TGFbeta1 and basic FGF (bFGF) on smooth muscle cell (SMC) gene expression.

METHODS AND RESULTS

TGFbeta1 induction of SMC gene expression, including smooth muscle protein 22-alpha (SM22alpha) and smooth muscle alpha-actin, was examined in the pluripotent 10T1/2 cells. Marked increase in these mRNA levels by TGFbeta1 was inhibited by c-Src-tyrosine kinase inhibitors and protein synthesis inhibitor cycloheximide. Functional studies with deletion and site-directed mutation analysis of the SM22alpha promoter demonstrated that TGFbeta1 activated the SM22alpha promoter through a CC(A/T-rich)6GG (CArG) box, which serves as a serum response factor (SRF)-binding site. TGFbeta1 increased SRF expression through an increase in transcription of the SRF gene. In the presence of bFGF, TGFbeta1 induction of SMC marker gene expression was significantly attenuated. Transient transfection assays showed that bFGF significantly suppressed induction of the SM22alpha promoter-driven luciferase activity by TGFbeta1, whereas bFGF had no effects on the TGFbeta1-mediated increase in SRF expression and SRF:DNA binding activity. Mitogen-activated protein kinase kinase-1 (MEK1) inhibitor PD98059 abrogated the bFGF-mediated suppression of TGFbeta1-induced SMC gene expression.

CONCLUSIONS

Our data suggest that bFGF-induced MEK/extracellular signal-regulated kinase signaling plays an antagonistic role in TGFbeta1-induced SMC gene expression through suppression of the SRF function. These data indicate that opposing effects of bFGF and TGFbeta1 on SMC gene expression control the phenotypic plasticity of SMCs.