Tyrosine kinase and MAPK inhibition of TNF-alpha- and EGF-stimulated IEC-6 cell growth

Dual-specificity phosphatase 6 deletion protects the colonic epithelium against inflammation and promotes both proliferation and tumorigenesis

The Ras/mitogen‐activated protein kinase (MAPK) pathway controls fundamental cellular processes such as proliferation, differentiation, and apoptosis. The dual‐specificity phosphatase 6 (DUSP6) regulates cytoplasmic MAPK signaling by dephosphorylating and inactivating extracellular signal‐regulated kinase (ERK1/2) MAPK. To determine the role of DUSP6 in the maintenance of intestinal homeostasis, we characterized the intestinal epithelial phenotype of Dusp6 knockout (KO) mice under normal, oncogenic, and proinflammatory conditions. Our results show that loss of Dusp6 increased crypt depth and epithelial cell proliferation without altering colonic architecture. Crypt regeneration capacity was also enhanced, as revealed by ex vivo Dusp6 KO organoid cultures. Additionally, loss of Dusp6 induced goblet cell expansion without affecting enteroendocrine and absorptive cell differentiation. Our data also demonstrate that Dusp6 KO mice were protected from acute dextran sulfate sodium‐induced colitis, as opposed to wild‐type mice. In addition, Dusp6 gene deletion markedly enhanced tumor load in Apc^Min/+ mice. Decreased DUSP6 expression by RNA interference in HT29 colorectal cancer cells enhanced ERK1/2 activation levels and promoted both anchorage‐independent growth in soft agar as well as invasion through Matrigel. Finally, DUSP6 mRNA expression in human colorectal tumors was decreased in advanced stage tumors compared with paired normal tissues. These results demonstrate that DUSP6 phosphatase, by controlling ERK1/2 activation, regulates colonic inflammatory responses, and protects the intestinal epithelium against oncogenic stress.

Transmural pressure loading enhances gastric mucosal cell proliferation

AIM

Although increased intraluminal pressure in the stomach due to gastric outlet obstruction or functional gastric motor dysfunction, including gastroparesis, may affect gastric mucosal integrity, the direct effect of mechanical pressure on gastric mucosal cells has not yet been fully investigated. The aims of this study were to determine whether exposure to transmural pressure would affect the proliferation of gastric mucosal cells and to elucidate the intracellular signaling pathways involved.

METHODS

Cellular proliferation and DNA synthesis were evaluated in rat gastric epithelial cells exposed to high transmural pressures. The levels of activation of 3 MAP kinases, ERK, JNK, and p38, were assessed, and the induction of immediate early gene expression was examined. The activation of nuclear factor activator protein-1 (AP-1) was evaluated by an electrophoretic mobility shift assay.

RESULTS

Exposure to high transmural pressure significantly increased DNA synthesis within 24 h, with the most marked increase observed after exposure to a pressure of 80 mmHg, and this increase was inhibited by the MEK1 inhibitor PD98059. Early activation of ERK kinase, but not of JNK or p38 kinase, was detected after pressure loading. Early induction of the c-fos and c-myc genes and activation of the AP-1 transcription factor were also demonstrated within 3 h of exposure to 80 mmHg of pressure.

CONCLUSION

Gastric mucosal cell proliferation induced by exposure to high transmural pressure may be related to early activation of ERK, the induction of c-fos and c-myc, and the activation of AP-1.

The EGF receptor and HER2 participate in TNF-α-dependent MAPK activation and IL-8 secretion in intestinal epithelial cells

TNF-α activates multiple mitogen-activated protein kinase (MAPK) cascades in intestinal epithelial cells (IECs) leading to the secretion of interleukin 8 (IL-8), a neutrophil chemoattractant and an angiogenic factor with tumor promoting properties. As the epidermal growth factor receptor (EGFR) is a known transducer of proliferative signals and a potent activator of MAPKs, we hypothesized that the EGFR participates in TNF-dependent MAPK activation and IL-8 secretion by intestinal epithelial cells (IECs). We show that the EGFR is tyrosine-phosphorylated following treatment of IECs (HT-29 and IEC-6) with TNF-α. This requires EGFR autophosphorylation as it was blocked by the EGFR kinase inhibitor AG1478. Autophosphorylation was also inhibited by both a Src-kinase inhibitor and the metalloproteinase inhibitor batimastat. TNF treatment of IECs resulted in the accumulation of soluble TGF-α; treatment of IECs with batimastat suppressed TGF-α release and immunoneutralization of TGF-α resulted in decreased EGFR and ERK phosphorylations. TNF-α treatment of IECs resulted in an association between EGFR and HER2 and inhibition of HER2 using a specific inhibitor AG879 in combination with AG1478-suppressed TNF-α-dependent ERK phosphorylation and IL-8 release. Downregulation of HER2 via siRNA resulted in a significant decrease in ERK phosphorylation and a 50% reduction in IL-8 secretion.

In vitro studies on the inhibition of colon cancer by butyrate and carnitine

OBJECTIVE

Epidemiologic studies support an association between diet and the incidence of colorectal cancer. Butyrate, a short-chain fatty acid present in dietary fiber and dairy products, is a potential anticarcinogenic compound. We previously showed that carnitine can enhance the bioavailability of butyrate in vivo. In the present study, we evaluated the effects of butyrate alone and in combination with carnitine on colon cancer cells in vitro, examining proliferation and apoptosis and the molecular mechanisms by which these nutrients may inhibit colon cancer.

METHODS

Caco-2 cells, a well-established cell model, were incubated with butyrate (2.5-20mM) with or without carnitine (10mM) for various incubation periods. Proliferation was measured by incorporation of (3)H-thymidine, and apoptosis was detected using flow cytometry, and then confirmed by analyzing the presence of single-strand DNA breaks typical of apoptotic cells. Prostaglandin E(2) production was assayed and Bcl-2 and cyclo-oxygenase-2 expressions were examined by western blotting.

RESULTS

Butyrate and carnitine inhibited Caco-2 cell proliferation (P<0.05) and induced apoptosis (P<0.05). Prostaglandin E(2) production was decreased in treated Caco-2 cells. At the molecular level, the expression of proapoptotic Bax and Bak proteins were increased in cells incubated with butyrate and carnitine, whereas expression of antiapoptotic Bcl-x(L) was decreased. Cyclo-oxygenase-2 expression was decreased in cells incubated with butyrate and carnitine.

CONCLUSIONS

Butyrate and carnitine inhibit human colon carcinoma cell proliferation and induce apoptosis in human colon carcinoma cells. This is accompanied by an appreciable alteration of the Bax-to-Bcl-x(L) and Bak-to-Bcl-x(L) ratios in favor of apoptosis. This study provides a scientific rationale to study the effects of carnitine and butyrate in colon cancer in vivo.

Inhibitory effect of TNF-α on the intestinal absorption of galactose

Sepsis is a systemic response to infection in which toxins, such as bacterial lipopolysaccharide (LPS), stimulate the production of inflammatory mediators like the cytokine tumor necrosis factor alpha (TNF-alpha). Previous studies from our laboratory have revealed that LPS inhibits the intestinal absorption of L-leucine and D-fructose in rabbit when it was intravenously administered, and that TNF-alpha seems to mediate this effect on amino acid absorption. To extend this work, the present study was designed to evaluate the possible effect of TNF-alpha on D-galactose intestinal absorption, identify the intracellular mechanisms involved and establish whether this cytokine mediates possible LPS effects. Our findings indicate that TNF-alpha decreases D-galactose absorption both in rabbit intestinal tissue preparations and brush-border membrane vesicles. Western blot analysis revealed reduced amounts of the Na+/glucose cotransporter (SGLT1) protein in the plasma membrane attributable to the cytokine. On the contrary, TNF-alpha increased SGLT1 mRNA levels. Specific inhibitors of the secondary messengers PKC, PKA, the MAP kinases p38 MAP, JNK, MEK1/2 as well as the proteasome, diminished the TNF-alpha-evoked inhibitory effect. LPS inhibition of the uptake of the sugar was blocked by a TNF-alpha antagonist. In conclusion, TNF-alpha inhibits D-galactose intestinal absorption by decreasing the number of SGLT1 molecules at the enterocyte plasma membrane through a mechanism in which several protein-like kinases are involved.

Adenosine is a negative regulator of NF-κB and MAPK signaling in human intestinal epithelial cells

Previous studies suggest that adenosine possesses anti-inflammatory properties, however, the mechanisms by which adenosine affects immune function remain unclear, particularly in the intestine. In this study, we hypothesized that adenosine directly affects pro-inflammatory gene expression in intestinal epithelial cells through modulation of NF-kappaB signaling. HT-29 cells were treated with adenosine prior to incubation with various stimuli and pro-inflammatory gene expression and signal transduction analyzed. Adenosine pretreatment resulted in a reduction in IL-8 expression and secretion in response to TNF-alpha, IL-1, LPS, and PMA. This effect was paralleled by inhibition of kappaB-driven luciferase expression and a reduction in recruitment of NF-kappaB to the IL-8 promoter. Pretreatment of HT-29 cells also resulted in reduced ERK, p38, and JNK MAPK phosphorylation, following TNF-alpha treatment. The observed effects in this study occurred independently of known surface adenosine receptors. This study identifies adenosine as a potent negative regulator of pro-inflammatory signaling in intestinal epithelial cells.

Interleukin-17 is a potent immuno-modulator and regulator of normal human intestinal epithelial cell growth

Upregulation of the T-cell derived cytokine interleukin (IL-17) was reported in the inflamed intestinal mucosa of patients with inflammatory bowel disorders. In this study, we analyzed the effect of IL-17 on human intestinal epithelial cell (HIEC) turnover and functions. Proliferation and apoptosis in response to IL-17 was monitored in HIEC (cell counts, [(3)H]thymidine incorporation method, and annexinV-PI-apoptosis assay). Signalling pathways were analyzed by Western blots, electromobility shift assay, and immunofluorescence studies. IL-17 proved to be a potent inhibitor of HIEC proliferation without any pro-apoptotic/necrotic effect. The growth inhibitory effect of IL-17 was mediated via the p38 stress kinase. Consequently, the p38-SAPkinase-inhibitor SB203580 abrogated this anti-mitotic effect. In parallel, IL-17 provoked the degradation of IkappaBalpha, allowing nuclear translocation of the p65 NF-kappaB subunit and induction of the NF-kappaB-controlled genes IL-6 and -8. IL-17 potently blocks epithelial cell turnover while at the same time amplifying an inflammatory response in a positive feedback manner.

Long term endocrine regulation of nucleoside transporters in rat intestinal epithelial cells

We studied the regulation of nucleoside transporters in intestinal epithelial cells upon exposure to either differentiating or proliferative agents. Rat intestinal epithelial cells (line IEC-6) were incubated in the presence of differentiating (glucocorticoids) or proliferative (EGF and TGF-α) agents. Nucleoside uptake rates and nucleoside transporter protein and mRNA levels were assessed. The signal transduction pathways used by the proliferative stimuli were analyzed. We found that glucocorticoids induce an increase in sodium-dependent, concentrative nucleoside transport rates and in protein and mRNA levels of both rCNT2 and rCNT1, with negligible effects on the equilibrative transporters. EGF and TGF-α induce an increase in the equilibrative transport rate, mostly accounted for by an increase in rENT1 activity and mRNA levels, rENT2 mRNA levels remaining unaltered. This effect is mimicked by another proliferative stimulus that functions as an in vitro model of epithelial wounding. Here, rENT1 activity and mRNA levels are also increased, although the signal transduction pathways used by the two stimuli are different. We concluded that differentiation of rat intestinal epithelial cells is accompanied by increased mature enterocyte features, such as concentrative nucleoside transport (located at the brush border membrane of the enterocyte), thus preparing the cell for its ultimate absorptive function. A proliferative stimulus induces the equilibrative nucleoside activities (mostly through ENT1) known to be located at the basolateral membrane, allowing the uptake of nucleosides from the bloodstream for the increased demands of the proliferating cell.

Prevention of TNF-alpha-induced apoptosis in polyamine-depleted IEC-6 cells is mediated through the activation of ERK1/2

It has been documented that polyamines play a critical role in the regulation of apoptosis in intestinal epithelial cells. We have recently reported that protection from TNF-alpha/cycloheximide (CHX)-induced apoptosis in epithelial cells depleted of polyamines is mediated through the inactivation of a proapoptotic mediator, JNK. In this study, we addressed the involvement of the MAPK pathway in the regulation of apoptosis after polyamine depletion of IEC-6 cells. Polyamine depletion by alpha-difluromethylornithine (DFMO) resulted in the sustained activation of ERK in response to TNF-alpha/CHX treatment. Pretreatment of polyamine-depleted IEC-6 cells with a cell membrane-permeable MEK1/2 inhibitor, U-0126, significantly inhibited TNF-alpha/CHX-induced ERK phosphorylation and significantly increased DNA fragmentation, JNK activity, and caspase-3 activity in response to TNF-alpha/CHX. Moreover, the dose dependency of U-0126-mediated inhibition of TNF-alpha/ CHX-induced ERK phosphorylation correlated with the reversal of the antiapoptotic effect of DFMO. IEC-6 cells expressing constitutively active MEK1 had decreased TNF-alpha/CHX-induced JNK phosphorylation and were significantly protected from apoptosis. Conversely, a dominant-negative MEK1 resulted in high basal activation of JNK, cytochrome c release, and spontaneous apoptosis. Polyamine depletion of the dominant-negative MEK1 cells did not prevent JNK activation or cytochrome c release and failed to confer protection from both TNF-alpha/CHX and camptothecin-induced apoptosis. Finally, expression of a dominant-negative mutant of JNK significantly protected IEC-6 cells from TNF-alpha/CHX-induced apoptosis. These data indicate that polyamine depletion results in the activation of ERK, which inhibits JNK activation and protects cells from apoptosis.

Polyamines are required for activation of c-Jun NH2-terminal kinase and apoptosis in response to TNF-alpha in IEC-6 cells

Intracellular polyamine homeostasis is important for the regulation of cell proliferation and apoptosis and is necessary for the balanced growth of cells and tissues. Polyamines have been shown to play a role in the regulation of apoptosis in many cell types, including IEC-6 cells, but the mechanism is not clear. In this study, we analyzed the mechanism by which polyamines regulate the process of apoptosis in response to tumor necrosis factor-alpha (TNF-alpha). TNF-alpha or cycloheximide (CHX) alone did not induce apoptosis in IEC-6 cells. Significant apoptosis was observed when CHX was given along with TNF-alpha, as indicated by a significant increase in the detachment of cells, caspase-3 activity, and DNA fragmentation. Polyamine depletion by treatment with alpha-difluoromethylornithine significantly reduced the level of apoptosis, as judged by DNA fragmentation and the caspase-3 activity of attached cells. Apoptosis in IEC-6 cells was accompanied by the activation of upstream caspases-6, -8, and -9 and NH2-terminal c-Jun kinase (JNK). Inhibition of JNK activation prevented caspase-9 activation. Polyamine depletion prevented the activation of JNK and of caspases-6, -8, -9, and -3. SP-600125, a specific inhibitor of JNK activation, prevented cytochrome c release from mitochondria, JNK activation, DNA fragmentation, and caspase-9 activation in response to TNF-alpha/CHX. In conclusion, we have shown that polyamine depletion delays and decreases TNF-alpha-induced apoptosis in IEC-6 cells and that apoptosis is accompanied by the release of cytochrome c, the activation of JNK, and of upstream caspases as well as caspase-3. Polyamine depletion prevented JNK activation, which may confer protection against apoptosis by modulation of upstream caspase-9 activation.

Intraepithelial lymphocyte-derived interferon-gamma evokes enterocyte apoptosis with parenteral nutrition in mice

Total parenteral nutrition (TPN) results in an increase in intraepithelial lymphocyte (IEL)-derived interferon-gamma (IFN-gamma) expression as well as an increase in epithelial cell (EC) apoptosis. This study examined the role that IEL-derived IFN-gamma has in the increase in EC apoptosis. Mice received either TPN or oral feedings for 7 days. Small bowel EC apoptosis significantly rose in mice receiving TPN. The administration of TPN also significantly increased IEL-derived IFN-gamma and Fas ligand (FasL) expression. EC apoptosis in IFN-gamma knockout (IFNKO) mice that received TPN was significantly lower than in wild-type TPN mice. Sensitivity of EC to Fas-mediated apoptosis in IFNKO mice was significantly lower than in wild-type TPN mice. Apoptosis in Fas-deficient and FasL-deficient mice that received TPN was significantly lower than in wild-type mice that received TPN. The TPN-induced increase in IFN-gamma expression appears to result in an increase in Fas-L expression and EC sensitivity to Fas, with a resultant increase in EC apoptosis. This may well be one of the mediators of increased EC apoptosis observed with TPN administration.

Induction of marked apoptosis in mammalian cancer cell lines by antisense DNA treatment to abolish expression of DENN (differentially expressed in normal and neoplastic cells)

We previously reported the isolation of the novel human DENN gene, which is differentially expressed in normal and neoplastic cells. DENN is identical to MADD (mitogen-activated protein kinase-activating death domain), which interacts with tumor necrosis factor receptor 1 through their death domains. DENN is also homologous to Rab3 GEP, a rat Rab3 GDP/GTP exchange protein. Real-time reverse transcription-polymerase chain reaction analysis showed that DENN expression in cancer cell lines was 26-50 times that in normal cells. The Jurkat human leukemia, PLC/PRF/5 human hepatoma, and NS-1 mouse myeloma cell lines as well as the MRC-5 human fetal lung and Vero monkey kidney cell lines were treated successfully with four separate DENN-targeted antisense oligodeoxynucleotides (ODNs) to abrogate DENN expression. Quantitative assessment of cell viability and apoptosis by flow cytometry via fluorescein diacetate and propidium iodide membrane-integrity tests, terminal deoxynucleotidyl transferase-mediated deoxyuridine 5-triphosphate-biotin nick end-labeling, and annexin V assays showed that antisense silencing of DENN resulted in markedly more pronounced cell death in cancer cells compared with nonmalignant cells. Antisense-treated cell lines exhibited extensive loss of DNA content, forming distinct sub-G(1) peaks, while cell proliferation diminished significantly. Ultrastructural features of programmed cell death in cells subjected to antisense ODNs were authenticated by electron microscopy. In contrast, transfection of cell lines with a plasmid construct to achieve DENN overexpression augmented cellular proliferation and could reverse the apoptotic effect of antisense and staurosporine treatment. Our findings suggest that DENN is intimately involved in anti-apoptotic and cell-survival processes.

MAP kinases contribute to IL-8 secretion by intestinal epithelial cells via a posttranscriptional mechanism

The intracellular pathways that regulate intestinal epithelial gene expression are poorly understood. In this study we examined the roles of extracellular signal-regulated kinase (ERK) and p38 in the expression of interleukin-8 (IL-8) and intercellular adhesion molecule-1 (ICAM-1) using the human intestinal cell line HT-29. HT-29 cells were treated with tumor necrosis factor-alpha (TNF-alpha) in the presence or absence of ERK and p38 pathway inhibitors. TNF-alpha treatment resulted in increased IL-8 and ICAM-1 protein and mRNA synthesis, increased ERK and p38 activity, and activation of the transcription factors activator protein-1 (AP-1) and nuclear factor-kappaB (NF-kappaB). Inhibition of the ERK and p38 pathways attenuated IL-8 secretion but did not alter ICAM-1 expression. Furthermore, AP-1 and NF-kappaB DNA binding was not affected by ERK and p38 inhibition. In contrast, ERK and p38 inhibition resulted in the accelerated degradation of the IL-8 mRNA, suggesting that in HT-29 cells, p38 and ERK contribute to TNF-alpha-stimulated IL-8 secretion by intestinal epithelial cells via a posttranscriptional mechanism that involves stabilization of the IL-8 transcript.

ANG II and LPA induce Pyk2 tyrosine phosphorylation in intestinal epithelial cells: role of Ca2+, PKC, and Rho kinase

The G protein-coupled receptor agonists angiotensin II (ANG II) and lysophosphatidic acid (LPA) rapidly induce tyrosine phosphorylation of the cytosolic proline-rich tyrosine kinase 2 (Pyk2) in IEC-18 intestinal epithelial cells. The combined Pyk2 tyrosine phosphorylation induced by phorbol 12,13-dibutyrate, a direct agonist of protein kinase C (PKC), and ionomycin, a Ca2+ ionophore, was equal to that induced by ANG II. Inhibition of either PKC or Ca2+ signaling attenuated the effect of ANG II and LPA, although simultaneous inhibition of both pathways failed to completely abolish Pyk2 tyrosine phosphorylation. Cytochalasin D, which disrupts stress fibers, strongly inhibited the response of Pyk2 to ANG II or LPA. The distinct Rho-associated kinase (ROK) inhibitors HA-1077 and Y-27632, as well as the Rho inhibitor Clostridium botulinum C3 exoenzyme, also significantly attenuated ANG II- and LPA-stimulated Pyk2 tyrosine phosphorylation. Simultaneous inhibition of PKC, Ca2+, and either actin assembly or ROK completely abolished the Pyk2 response. Together, these results show that ANG II and LPA rapidly induce Pyk2 tyrosine phosphorylation in intestinal epithelial cells via separate Ca2+-, PKC-, and Rho-mediated pathways.

Fatty acids enhance GRO/CINC-1 and interleukin-6 production in rat intestinal epithelial cells

Intestinal mucosal immunity is modulated by cytokine release from intestinal cells, but little is known about the relation between nutrient absorption and cytokine release. In this study, we examined how exposure to fatty acids affects the production of growth-regulated oncogene/cytokine-induced neutrophil chemoattractant-1 (GRO/CINC-1) and interleukin (IL)-6 in rat intestinal epithelial cells (IEC). The long-chain fatty acids, oleic, linoleic and arachidonic acids, and the middle-chain fatty acid octanoic acid were administered to subconfluent cultures of IEC-6 cells alone, or in combination with IL-1beta and transforming growth factor (TGF)-beta. The GRO/CINC-1 and IL-6 concentrations in culture media were determined by sandwich enzyme immunoassay. In epithelial cells, GRO/CINC-1 and IL-6 mRNA expression were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and mitogen-activated protein kinase (MAPK) activities determined by immunoblotting. Administration of long-chain fatty acids significantly increased the GRO/CINC-1 and IL-6 secretion into culture media, and this secretion was markedly increased (P < 0.05) in the presence of IL-1beta or TGF-beta. Octanoic acid had no effect on GRO/CINC-1 or IL-6 production. Furthermore, treatment with long-chain fatty acids significantly enhanced the GRO/CINC-1 and IL-6 expression that was induced by IL-1beta or TGF-beta. MAPK activity was significantly enhanced by treatment with long-chain fatty acids. Inhibitors of phospholipase C, protein kinase C or MAPK significantly reduced the fatty acid-induced increase in GRO/CINC-1 secretion, whereas a calcium/calmodulin inhibitor did not attenuate the secretion. These results suggest that long-chain fatty acids enhance cytokine release under conditions of inflammatory stimulation in the intestinal mucosa.

Transcription factor NF-kappaB participates in regulation of epithelial cell turnover in the colon

The transcription factor nuclear factor (NF)-kappaB regulates the expression of genes that can influence cell proliferation and death. Here we analyze the contribution of NF-kappaB to the regulation of epithelial cell turnover in the colon. Immunohistochemical, immunoblot, and DNA binding analyses indicate that NF-kappaB complexes change as colonocytes mature: p65-p50 complexes predominate in proliferating epithelial cells of the colon, whereas the p50-p50 dimer is prevalent in mature epithelial cells. NF-kappaB1 (p50) knockout mice were used to study the role of NF-kappaB in regulating epithelial cell turnover. Knockout animals lacked detectable NF-kappaB DNA binding activity in isolated epithelial cells and had significantly longer crypts with a more extensive proliferative zone than their wild-type counterparts (as determined by proliferating cell nuclear antigen staining and in vivo bromodeoxyuridine labeling). Gene expression profiling reveals that the NF-kappaB1 knockout mice express the potentially growth-enhancing tumor necrosis factor (TNF)-alpha and nerve growth factor-alpha genes at elevated levels, with in situ hybridization localizing some of the TNF-alpha expression to epithelial cells. TNF-alpha is NF-kappaB regulated, and its upregulation in NF-kappaB1 knockouts may result from an alleviation of p50-p50 repression. NF-kappaB complexes may therefore influence cell proliferation in the colon through their ability to selectively activate and/or repress gene expression.

Glutamine metabolism stimulates intestinal cell MAPKs by a cAMP-inhibitable, Raf-independent mechanism

BACKGROUND & AIMS

Infectious diarrhea caused by viruses plus enterotoxigenic bacteria is often more severe than diarrhea induced by either pathogen alone. We postulated that the increased cell adenosine 3',5'-cyclic monophosphate (cAMP) concentration observed during infection by enterotoxigenic organisms retards the intestinal repair process by blocking activation of mitogen-activated protein kinases (MAPKs) in proliferating intestinal cells.

METHODS

We evaluated the effects of glutamine on MAPK activity, thymidine incorporation, and cell number in glutamine-starved and -sufficient rat intestinal crypt cells (IEC-6).

RESULTS

In glutamine-starved cells, 10 mmol/L glutamine in the absence of serum stimulated [(3)H]thymidine incorporation 8-fold. This effect was inhibited by 60% with 8-(4-chlorophenylthio) (8-CPT)-cAMP (100 micromol/L) + isobutyl methylxanthine (100 micromol/L). In cells not starved of glutamine, glutamine stimulated thymidine incorporation by 3-fold, and 8-CPT-cAMP completely blocked the mitogenic effect. Inhibition of proliferation by cAMP persisted for at least 68 hours after cAMP removal. In vitro kinase assays showed that glutamine signaling requires an intact ERK (extracellular signal-related kinase) pathway in unstarved cells. In starved cells, at least one other pathway (JNK) was activated by glutamine, and the mitogenic inhibition by 8-CPT-cAMP was incomplete. Other intestinal fuels (glucose and acetate) were not mitogenic.

CONCLUSIONS

Increased levels of intracellular cAMP inhibit ERKs but only partially reduce glutamine-stimulated proliferation in enterocytes adapted to low glutamine.

Involvement of JAK2 and MAPK on type II nitric oxide synthase expression in skin-derived dendritic cells

In this report, we demonstrate that a fetal mouse skin-derived dendritic cell line produces nitric oxide (NO) in response to the endotoxin [lipopolysaccharide (LPS)] and to cytokines [tumor necrosis factor-alpha (TNF-alpha) and granulocyte-macrophage colony-stimulating factor (GM-CSF)]. Expression of the inducible isoform of NO synthase (iNOS) was confirmed by immunofluorescence with an antibody against iNOS. The tyrosine kinase inhibitor genistein decreased LPS- and GM-CSF-induced nitrite (NO(-2)) production. The effect of LPS and cytokines on NO(-2) production was inhibited by the Janus kinase 2 (JAK2) inhibitor tyrphostin B42. The p38 mitogen-activated protein kinase (p38 MAPK) inhibitor SB-203580 also reduced the NO(-2) production evoked by LPS, TNF-alpha, or GM-CSF, but it was not as effective as tyrphostin B42. Inhibition of MAPK kinase with PD-098059 also slightly reduced the effect of TNF-alpha or GM-CSF on NO(-2) production. Immunocytochemistry studies revealed that the transcription factor nuclear factor-kappaB was translocated from the cytoplasm into the nuclei of fetal skin-derived dendritic cells (FSDC) stimulated with LPS, and this translocation was inhibited by tyrphostin B42. Our results show that JAK2 plays a major role in the induction of iNOS in FSDC.

Susceptibility to FAS-induced apoptosis in human nontumoral enterocytes: role of costimulatory factors

FAS-FAS ligand interaction has been implicated in increased enterocyte apoptosis seen in immune-mediated bowel injury. However, scant information exists on the role of FAS in physiological enterocyte turnover. In the present study, the regulation of enterocyte FAS and FAS ligand expression by cytokines and its functional role in human intestinal epithelial cell apoptosis and proliferation were analyzed with two different models: a nontransformed human intestinal epithelial cell line (HIEC) and normal colonic explant cultures. HIEC constitutively expressed FAS, as analyzed by flow cytometry. However, stimulation with agonistic anti-FAS antibody (1-500 ng/ml) did not induce HIEC apoptosis. In contrast, in the presence of tumor necrosis factor alpha (TNFalpha) and/or interferon gamma (IFNgamma), HIEC became highly susceptible to FAS-induced apoptosis. The sensitizing effect to FAS-induced apoptosis was mediated via TNFalpha- and IFNgamma-induced upregulation of FAS expression (maximally 348%). Receptor studies showed that the effect of TNFalpha on FAS was mediated via the p55 TNF receptor. In colonic organ cultures, IFNgamma and TNFalpha also enhanced colonocyte FAS expression, resulting in a markedly increased apoptotic response to stimulation of this receptor, as shown by in situ terminal deosyuridine triphosphate nick-end staining. Neither FAS ligand expression nor its induction by cytokines was observed in HIEC or colonic explants. Proliferation studies showed that FAS is not implicated in regulating HIEC growth. These findings suggest that, despite the fact that normal human enterocytes express FAS, costimulatory factors, such as TNFalpha or IFNgamma, abundantly secreted under inflammatory conditions, are necessary to sensitize intestinal epithelial cells to FAS-induced apoptosis by upregulating this receptor.

TNFalpha-induced IEC-6 cell apoptosis requires activation of ICE caspases whereas complete inhibition of the caspase cascade leads to necrotic cell death

Tumor necrosis factor (TNF)alpha is considered to play a key pathogenetic role in inflammatory bowel diseases. In this study we analyzed the mechanisms by which TNFalpha induces intestinal epithelial cell apoptosis. TNFalpha alone, and more potently in combination with IFNgamma, induced a high degree of IEC-6 cell apoptosis. This effect was more than 100-fold stronger if both of the TNF-R were stimulated, compared to stimulation of the p55-TNF-R alone, indicating an important apoptosis enhancing effect of the p75-TNF-R. TNFalpha-induced apoptosis required activation of ICE caspases and was completely abolished by its inhibitor, zVAD-fmk. Specific inhibition of caspase-3 with zDEVD-fmk did not alter the effect of TNFalpha. Western blot analyses confirmed that caspase-3 was not activated in response to TNFalpha. In the presence of complete inhibition of the caspase cascade with zVAD-fmk (>/=50 microM), TNFalpha induced cell necrosis rather than apoptosis. Our data reveal that TNFalpha can trigger enterocyte cell death via apoptosis or necrosis, depending upon the activation or blockade of specific caspases.

Conversion of TNF alpha from antiproliferative to proliferative ligand in mouse intestinal epithelial cells by regulating mitogen-activated protein kinase

The mechanisms regulating the balance between intestinal epithelial cell proliferation and differentiation are essential to maintaining an intact mucosal barrier. Mitogen-activated protein (MAP) kinases appear to be key transducers of extracellular signals in these pathways. The goal of this study was to investigate the regulation of MAP kinase by tumor necrosis factor alpha (TNFalpha) and epidermal growth factor (EGF) in intestinal epithelial cells. The young adult mouse colon cell line was studied for TNFalpha and/or EGF regulation of MAP kinase in the presence or absence of the MAP kinase kinase (MEK1) inhibitor PD 98059. Proliferation was determined by hemocytometry, and activated MAP kinase was identified by Western blot analysis, in vitro kinase assay, and confocal laser immunofluorescent microscopy. TNFalpha stimulated sustained nuclear MAP kinase activity, while EGF stimulated transient cytoplasmic MAP kinase activity. Changing TNFalpha's sustained MAP kinase activation to transient converted TNFalpha from an anti-proliferative to a proliferative ligand. These findings demonstrate that both TNFalpha and EGF activate MAP kinase in intestinal epithelial cells. The kinetics and subcellular distribution of this enzyme activity may be pivotal in the transduction of divergent cellular responses in the intestinal epithelium with implications for altered proliferative signals in inflammatory bowel disease.

Implication of TNF Receptor-I-Mediated Extracellular Signal-Regulated Kinases 1 and 2 (ERK1/2) Activation in Growth of AIDS-Associated Kaposi’s Sarcoma Cells: A Possible Role of a Novel Death Domain Protein MADD in TNF-α-Induced ERK1/2 Activation in Kaposi’s Sarcoma Cells

TNF-α is a key pathogenic mediator of infectious and inflammatory diseases. HIV infection stimulates and dysregulates the immune system, leading to abnormal production of TNF-α. Despite its cytotoxic effect on some tumor cell lines, TNF-α functions as a growth stimulator for Kaposi’s sarcoma (KS), a common malignancy in HIV-infected patients. However, signaling pathways linked to TNF-α-induced mitogenic responses are not well understood. We found that extracellular signal-regulated kinases 1 and 2 (ERK1/2) in KS cells were significantly activated by TNF-α through tyrosine/threonine phosphorylation. Using neutralizing anti-TNFR-I and TNFR-II mAbs, we have now obtained evidence that TNF-α-induced KS cell growth and ERK1/2 activation are mediated exclusively by TNFR-I, not by TNFR-II. A selective inhibitor for ERK1/2 activator kinases, PD98059, profoundly inhibited not only the activation of ERK1/2, but also the TNF-α-induced KS cell proliferation. We therefore propose that the TNFR-I-ERK1/2 pathway plays a pivotal role in transmitting to KS cells the mitogenic signals of TNF-α. TNFR-I possesses no intrinsic kinase activity, suggesting that TNFR-I-associated proteins may provide a link between TNFR-I and ERK1/2 activation. We found that actinomycin D treatment of KS cells selectively abolished expression of mitogen-activated protein kinase-activating death domain protein (MADD), a novel TNFR-I-associated death domain protein. TNF-α failed to induce ERK1/2 activation in the actinomycin D-treated cells. MADD may couple TNFR-I with the ERK1/2 signaling pathway required for KS cell proliferation.