MEK/ERK regulates adherens junctions and migration through Rac1

Polyamine depletion with the ornithine decarboxylase inhibitor alpha-difluoromethyl ornithine (DFMO), prevents Rac1 activation causing the formation of a thick actin cortex at the cell periphery and inhibits migration of intestinal epithelial cells. In the present study, we demonstrate that MEK activation by EGF increased Rac1 activation, dissociation of intercellular contacts, and migration in both control and polyamine-depleted cells, while U0126, a specific inhibitor of MEK1, prevented disruption of junctions as well as EGF-induced Rac1 activation. Constitutively active MEK1 (CA-MEK) expression altered cell-cell contacts in control and polyamine depleted cells. The expression of constitutively active Rac1 (CA-Rac1) restored beta-catenin to the cell periphery and prevented the formation of actin cortex and caused the appearance of F-actin stress fibers in polyamine-depleted cells. Inhibition of Rac activation by NSC23766, a specific inhibitor of Tiam1, an upstream guanidine nucleotide exchange factor for Rac1, reproduced the beta-catenin localization and actin structure of polyamine-depleted cells. Tiam1 localized more extensively with beta-catenin at the cell periphery in CA-Rac1 cells compared to vector cells. Polyamine depletion decreased the expression of E-cadherin to a greater extent compared to beta-catenin. Subcellular fractionation further confirmed our immuno-localization and western blotting observations. These data suggest that EGF acting through MEK1/ERK to activate Rac1 regulates cell-cell contacts. Thus, decreased migration in polyamine depleted cells may be due to the inhibition of Tiam1 activation of Rac1 and the subsequent decreased expression of beta-catenin and E-cadherin leading to reduced cell-cell contacts.

Role of myosin regulatory light chain and Rac1 in the migration of polyamine-depleted intestinal epithelial cells

We have previously shown that polyamine depletion decreased migration, Rac activation, and protein serine threonine phosphatase 2A activity. We have also shown that polyamine depletion increased cortical F-actin and decreased lamellipodia and stress fibers. In this study, we used staurosporine (STS), a potent, cell-permeable, and broad-spectrum serine/threonine kinase inhibitor, and studied migration. STS concentrations above 100 nM induced apoptosis. However, in polyamine-depleted cells, a lower concentration of STS (5 nM) increased attachment, spreading, Rac1 activation, and, subsequently, migration without causing apoptosis. STS-induced migration was completely prevented by a Rac1 inhibitor (NSC-23766) and dominant negative Rac1. These results imply that STS restores migration in polyamine-depleted cells through Rac1. The most important finding in this study was that polyamine depletion increased the association of phosphorylated myosin regulatory light chain (pThr(18)/Ser(19)-MRLC) at the cell periphery, which colocalized with thick cortical F-actin. Localization of pThr(18)- and pSer(19)-MRLC was found with stress fibers and nuclei, respectively. STS decreased the phosphorylation of cellular and peripheral pThr(18)-MRLC without any effect on nuclear pSer(19)-MRLC, dissolved thick cortical F-actin, and increased lamellipodia and stress fiber formation in polyamine-depleted cells. In control and polyamine-depleted cells, focal adhesion kinase (FAK) colocalized with stress fibers and the actin cortex, respectively. STS reorganized FAK, paxillin, and the cytoskeleton. These results suggest that polyamine depletion prevents the dephosphorylation of MRLC and thereby prevents the dynamic reorganization of the actin cytoskeleton and decreases lamellipodia formation resulting in the inhibition of migration.

Akt-mediated GSK-3beta inhibition prevents migration of polyamine-depleted intestinal epithelial cells via Rac1

The rapid migration of intestinal epithelial cells (IEC) is important for the healing of mucosal wounds. We have previously shown that polyamine depletion inhibits migration of IEC-6 cells. Akt activation and its downstream target GSK-3beta have been implicated in the regulation of migration. Here we investigated the significance of elevated phosphatidylinositol 3-kinase (PI3K)/Akt signaling on migration of polyamine-depleted cells. Polyamine-depleted cells had high Akt (Ser473) and GSK-3beta (Ser9) phosphorylation. Pretreatment with 20 microM LY294002 (PI3K inhibitor) for 30 min inhibited phosphorylation of Akt, increased migration by activating Rac1 in polyamine-depleted IEC-6 cells, and restored the actin structure similar to that in cells grown in control medium. Treatment of cells with a GSK-3beta inhibitor (AR-A014418) altered the actin cytoskeleton and inhibited migration, mimicking the effects of polyamine depletion. Thus, our results indicate that sustained activation of Akt in response to polyamine depletion inhibits migration through GSK-3beta and Rac1.

Rac1 mediates intestinal epithelial cell apoptosis via JNK

Apoptosis plays a key role in the maintenance of a constant cell number and a low incidence of cancer in the mucosa of the intestine. Although the small GTPase Rac1 has been established as an important regulator of migration of intestinal epithelial cells, whether Rac1 is also involved in apoptosis is unclear. The present study tested the hypothesis that Rac1 mediates TNF-alpha-induced apoptosis in IEC-6 cells. Rac1 is activated during TNF-alpha-induced apoptosis as judged by the level of GTP-Rac1, the level of microsomal membrane-associated Rac1, and lamellipodia formation. Although expression of constitutively active Rac1 does not increase apoptosis in the basal condition, inhibition of Rac1 either by NSC-23766 (Rac1 inhibitor) or expression of dominant negative Rac1 protects cells from TNF-alpha-induced apoptosis by inhibiting caspase-3, -8, and -9 activities. Inhibition of Rac1 before the administration of apoptotic stimuli significantly prevents TNF-alpha-induced activation of JNK1/2, the key proapoptotic regulator in IEC-6 cells. Inhibition of Rac1 does not modulate TNF-alpha-induced ERK1/2 and Akt activation. Inhibition of ERK1/2 and Akt activity by U-0126 and LY-294002, respectively, increased TNF-alpha-induced apoptosis. However, inhibition of Rac1 significantly decreased apoptosis in the presence of ERK1/2 and Akt inhibitors, similar to the effect observed with NSC-23766 alone in response to TNF-alpha. Thus, Rac1 inhibition protects cells independently of ERK1/2 and Akt activation during TNF-alpha-induced apoptosis. Although p38 MAPK is activated in response to TNF-alpha, inhibition of p38 MAPK did not decrease apoptosis. Rac1 inhibition did not alter p38 MAPK activity. Thus, these results indicate that Rac1 mediates apoptosis via JNK and plays a key role in proapoptotic pathways in intestinal epithelial cells.

Individual and joint actions of selenate and methylmercury on the development and survival of insect detritivore Megaselia scalaris (Diptera: Phoridae)

Despite the important roles played by insects in most ecosystems, surprisingly little is known about how anthropogenic pollutants or their mixtures interact to affect insect populations. The independent and joint actions of selenate and methylmercury on a ubiquitous insect detritivore, Megaselia scalaris (Loew), were determined in this study. Ovipositing females did not distinguish between untreated food sources and those contaminated with toxic concentrations of selenate, methylmercury, or both chemicals in combination. Even at the highest concentrations of pollutants, no negative effects were observed for the egg stage. However, larval survival was significantly decreased and development significantly prolonged by selenate and methylmercury individually at low or intermediate ecologically relevant treatment levels. Potentiation was strongly evident because mixtures containing concentrations as little as only 1% of the respective individual median lethal tolerances (LC(50)s) caused significantly more mortality and delayed larval development than would be expected from the responses selenate and methylmercury elicit individually. However, survival and pupal development was not affected at any rate tested. Female fecundity was significantly decreased by methylmercury but not by selenate or mixture treatments. The relative toxicity to M. scalaris of each of the individual and joint treatments was selenate (LC(50) = 260 microg/g) < methylmercury (LC(50) = 22 microg/g) < the mixture at approximately 5% of the LC(50) concentration of each of the components (12 microg/g selenate plus 1.0 microg/g methylmercury). The increased mortality and delayed larval development within sites contaminated by selenate, methylmercury, or combination of the two have substantial implications for the ecology, population dynamics, and sustainability of M. scalaris populations. If these results can be extrapolated to other arthropod detritivores, ecosystem food-web function may be substantially affected.

Myosin light chain kinase regulates synaptic plasticity and fear learning in the lateral amygdala

Learning and memory depend on signaling molecules that affect synaptic efficacy. The cytoskeleton has been implicated in regulating synaptic transmission but its role in learning and memory is poorly understood. Fear learning depends on plasticity in the lateral nucleus of the amygdala. We therefore examined whether the cytoskeletal-regulatory protein, myosin light chain kinase, might contribute to fear learning in the rat lateral amygdala. Microinjection of ML-7, a specific inhibitor of myosin light chain kinase, into the lateral nucleus of the amygdala before fear conditioning, but not immediately afterward, enhanced both short-term memory and long-term memory, suggesting that myosin light chain kinase is involved specifically in memory acquisition rather than in posttraining consolidation of memory. Myosin light chain kinase inhibitor had no effect on memory retrieval. Furthermore, ML-7 had no effect on behavior when the training stimuli were presented in a non-associative manner. Anatomical studies showed that myosin light chain kinase is present in cells throughout lateral nucleus of the amygdala and is localized to dendritic shafts and spines that are postsynaptic to the projections from the auditory thalamus to lateral nucleus of the amygdala, a pathway specifically implicated in fear learning. Inhibition of myosin light chain kinase enhanced long-term potentiation, a physiological model of learning, in the auditory thalamic pathway to the lateral nucleus of the amygdala. When ML-7 was applied without associative tetanic stimulation it had no effect on synaptic responses in lateral nucleus of the amygdala. Thus, myosin light chain kinase activity in lateral nucleus of the amygdala appears to normally suppress synaptic plasticity in the circuits underlying fear learning, suggesting that myosin light chain kinase may help prevent the acquisition of irrelevant fears. Impairment of this mechanism could contribute to pathological fear learning.

Localization of glucocorticoid receptors at postsynaptic membranes in the lateral amygdala

Glucocorticoids, released in high concentrations from the adrenal cortex during stressful experiences, bind to glucocorticoid receptors in nuclear and peri-nuclear sites in neuronal somata. Their classically known mode of action is to induce gene promoter receptors to alter gene transcription. Nuclear glucocorticoid receptors are particularly dense in brain regions crucial for memory, including memory of stressful experiences, such as the hippocampus and amygdala. While it has been proposed that glucocorticoids may also act via membrane bound receptors, the existence of the latter remains controversial. Using electron microscopy, we found glucocorticoid receptors localized to non-genomic sites in rat lateral amygdala, glia processes, presynaptic terminals, neuronal dendrites, and dendritic spines including spine organelles and postsynaptic membrane densities. The lateral nucleus of the amygdala is a region specifically implicated in the formation of memories for stressful experiences. These newly observed glucocorticoid receptor immunoreactive sites were in addition to glucocorticoid receptor immunoreactive signals observed using electron and confocal microscopy in lateral amygdala principal neuron and GABA neuron soma and nuclei, cellular domains traditionally associated with glucocorticoid immunoreactivity. In lateral amygdala, glucocorticoid receptors are thus also localized to non-nuclear-membrane translocation sites, particularly dendritic spines, where they show an affinity for postsynaptic membrane densities, and may have a specialized role in modulating synaptic transmission plasticity related to fear and emotional memory.

Polyamine depletion inhibits irradiation-induced apoptosis in intestinal epithelia

Our group has previously shown that polyamine depletion delays apoptosis in rat intestinal epithelial (IEC-6) cells (Ray RM, Viar MJ, Yuan Q, and Johnson LR, Am J Physiol Cell Physiol 278: C480-C489, 2000). Here, we demonstrate that polyamine depletion inhibits gamma-irradiation-induced apoptosis in vitro and in vivo. Pretreatment of IEC-6 cells with 5 mM alpha-difluoromethylornithine (DFMO) for 4 days significantly reduced radiation-induced caspase-3 activity and DNA fragmentation. This protective effect was prevented by the addition of 10 muM exogenous putrescine. Radiation exposure to mice resulted in a high frequency of apoptosis over cells positioned fourth to seventh in crypt-villus units. Pretreatment of mice with 2% DFMO in drinking water significantly reduced apoptotic cells from approximately 2.75 to 1.61 per crypt-villus unit, accompanied by significant decreases in caspase-3 levels. Further examination showed that DFMO pretreatment inhibited the radiation-induced increase in the proapoptotic protein Bax. Moreover, DFMO pretreatment significantly enhanced the intestinal crypt survival rate by 2.1-fold subsequent to radiation and ameliorated mucosal structural damage. We conclude that polyamine depletion by DFMO inhibits gamma-irradiation-induced apoptosis of intestinal epithelial cells both in vitro and in vivo through inhibition of Bax and caspase-3 activity, which leads to attenuation of radiation-inflicted intestinal injury. These data indicate that DFMO may be therapeutically useful to counteract the gastrointestinal toxicity caused by chemoradiotherapy. This is the first demonstration that polyamines are required for apoptosis in vivo.

Protein Phosphatase 2A Regulates Apoptosis in Intestinal Epithelial Cells

Polyamine depletion prevents apoptosis by increasing serine/threonine phosphorylation leading to either inactivation or activation of pro- and anti-apoptotic proteins, respectively. Despite evidence that protein kinases are regulators of apoptosis, a specific role for protein phosphatases in regulating cell survival has not been established. In this study, we show that polyamine depletion inhibits serine/threonine phosphatase 2A (PP2A). Inhibition of PP2A in cells depleted of polyamines correlated well with increased phosphorylation of Bad at Ser112. Bad Ser112 phosphorylation in response to tumor necrosis factor (TNF)-alpha treatment decreased with time in cells grown in control as well as those grown in the presence of alpha-difluoromethylornithine plus putrescine. However, a sustained increase in the levels of Bad Ser112 phosphorylation was maintained in response to TNF-alpha treatment in cells grown in the presence of alpha-difluoromethylornithine. Inhibition of PP2A by okadaic acid and fostriecin or PP2A small interfering RNA transfection significantly decreased TNF-alpha-induced apoptosis in control and polyamine-depleted cells. Inhibition of PP2A by okadaic acid: 1) increased Bad and Bcl-2 phosphorylation at Ser112 and Ser70, respectively; 2) increased ERK activity; 3) prevented JNK activation; 4) prevented cytochrome c release, and activation of caspases-9 and -3 in response to TNF-alpha. Inhibition of MEK1 by U0126 prevented phosphorylation of Bad at Ser112. These results indicate that polyamines regulate PP2A activity, and inhibition of PP2A in response to polyamine depletion increases steady state levels of Bad and Bcl-2 proteins and their phosphorylation and thereby prevents cytochrome c release, caspase-9, and caspase-3 activation.

Immunocytochemical localization of polyamines during attachment and spreading of retinal pigment epithelial and intestinal epithelial cells

In order to form and maintain a protective barrier for photoreceptors, the retinal pigment epithelium relies on integrin signaling and related pathways to form adhesion complexes, undergo cell spreading, and establish a confluent cellular monolayer. Polyamines are multifunctional polycations that are essential for cell attachment and spreading, although their exact mechanisms of action are as yet unclear. We report new immunocytochemical evidence suggesting that in the cells of retinal pigment epithelium and also the intestinal epithelium, polyamines are present in a population of intracellular vesicles that appear transiently during initial stages of cell spreading. In newly attached cells with minimal spreading, the vesicles are seen near the nucleus, whereas in more highly spread cells, the vesicles are localized to the plasma membrane, near, but not precisely co-localized with an enzyme marker for adhesion complexes, focal adhesion kinase. We also observe pronounced nuclear staining in newly attached cells that have not spread, whereas this staining is decreased in cells that have spread. Nuclear staining has been previously reported in other cell types and has been attributed to DNA binding of polyamines, which is known to stabilize chromatin structure. We hypothesize that the appearance of polyamine vesicles near focal adhesions of cells undergoing attachment and spreading may reflect the mechanism by which polyamine pools are targeted to appropriate interaction sites necessary for the assembly of adhesion complexes. Alternatively, the vesicles could represent the mechanism by which polyamines are removed from the nucleus and possibly released from the cell.

MEK1 restores migration of polyamine-depleted cells by retention and activation of Rac1 in the cytoplasm

We previously showed that polyamines are required for proliferation and migration both in vivo and in a cultured intestinal epithelial cell (IEC-6) model. Wounding of the IEC-6 monolayer induced transient ERK activation, which was further enhanced by EGF. EGF stimulated migration in control and polyamine-depleted cells, but the degree of stimulation was significantly less in polyamine-depleted cells. Inhibition of MEK1 inhibited basal as well as EGF-induced ERK activation and migration. Expression of constitutively active (CA)-MEK and dominant-negative (DN)-MEK had significant effects on F-actin structure. CA-MEK increased stress fiber and lamellipodia formation, while DN-MEK showed loss of stress fibers and abnormal actin cytoskeletal structure. Unlike EGF, CA-MEK significantly increased migration of both control and polyamine-depleted cells. The most important and significant finding in this study was that polyamine depletion caused localization of Rac1 and RhoA to the nuclear as well as perinuclear regions. Interestingly, CA-MEK completely reversed the subcellular distribution of Rac1 and RhoA proteins in polyamine-depleted cells. Polyamine depletion increased Rac1 in the nuclear fraction and decreased it in the cytoplasmic and membrane fractions of vector-transfected cells. CA-MEK prevented accumulation of Rac1 in the nucleus. Polyamine depletion significantly decreased Rac1 activity during 6-h migration in vector-transfected cells. Cells transfected with CA-MEK had almost identical levels of activated Rac1 in all three groups. These results suggest that polyamine depletion prevents activation of Rac1 and RhoA by sequestering them to the nucleus and that expression of constitutively active MEK reverses this effect, creating the cellular localization required for activation.

Sprouty regulates cell migration by inhibiting the activation of Rac1 GTPase

Sprouty (SPRY) protein negatively modulates fibroblast growth factor and epidermal growth factor actions. We showed that human SPRY2 inhibits cell growth and migration in response to serum and several growth factors. Using rat intestinal epithelial (IEC-6) cells, we investigated the involvement of the Rho family of GTPases, RhoA, Rac1, and cdc42 in SPRY2-mediated inhibition of cell migration and proliferation. The ability of TAT-tagged SPRY2 to inhibit proliferation and migration of IEC-6 cells transfected with constitutively active mutants of RhoA(G14V), Rac1(G12V), and cdc42 (F28L) was determined. Constitutively active RhoA(G14V), Rac1(G12V), or cdc42(F28L) did not protect cells from the anti-proliferative actions of TAT-SPRY2. The ability of TAT-hSPRY2 to inhibit migration was not altered by of RhoA(G14V) and cdc42(F28L). However, Rac1(G12V) obliterated the ability of SPRY2 to inhibit cell autonomous or serum-induced migration. Also, the activation of endogenous Rac1 was attenuated by TAT-SPRY2. Thus, SPRY2 mediates its anti-migratory actions by inhibiting Rac1 activation.

Lesions in the bed nucleus of the stria terminalis disrupt corticosterone and freezing responses elicited by a contextual but not by a specific cue-conditioned fear stimulus

The bed nucleus of the stria terminalis (BNST) is believed to be a critical relay between the central nucleus of the amygdala (CE) and the paraventricular nucleus of the hypothalamus in the control of hypothalamic-pituitary-adrenal (HPA) responses elicited by conditioned fear stimuli. If correct, lesions of CE or BNST should block expression of HPA responses elicited by either a specific conditioned fear cue or a conditioned context. To test this, rats were subjected to cued (tone) or contextual classical fear conditioning. Two days later, electrolytic or sham lesions were placed in CE or BNST. After 5 days, the rats were tested for both behavioral (freezing) and neuroendocrine (corticosterone) responses to tone or contextual cues. CE lesions attenuated conditioned freezing and corticosterone responses to both tone and context. In contrast, BNST lesions attenuated these responses to contextual but not tone stimuli. These results suggest CE is indeed an essential output of the amygdala for the expression of conditioned fear responses, including HPA responses, regardless of the nature of the conditioned stimulus. However, because lesions of BNST only affected behavioral and endocrine responses to contextual stimuli, the results do not support the notion that BNST is critical for HPA responses elicited by conditioned fear stimuli in general. Instead, the BNST may be essential specifically for contextual conditioned fear responses, including both behavioral and HPA responses, by virtue of its connections with the hippocampus, a structure essential to contextual conditioning. The results are also not consistent with the hypothesis that BNST is only involved in unconditioned aspects of fear and anxiety.

Optimal lysophosphatidic acid-induced DNA synthesis and cell migration but not survival require intact autophosphorylation sites of the epidermal growth factor receptor

Lysophosphatidic acid (LPA)-elicited transphosphorylation of receptor tyrosine kinases has been implicated in mediating extracellular signal-regulated kinase (ERK) 1/2 activation, which is necessary for LPA-induced cell proliferation, migration, and survival. B82L cells lack epidermal growth factor receptor (EGFR) but express LPA(1-3), platelet-derived growth factor (PDGF), ErbB2, and insulin-like growth factor receptor transcripts, yet LPA caused no detectable transphosphorylation of these receptor tyrosine kinases. LPA equally protected B82L cells, or transfectants expressing EGFR, the kinase dead EGFR(K721A), EGFR(Y5F) receptor mutant, which lacks five autophosphorylation sites, or EGFR(Y845F), which lacks the Src phosphorylation site from tumor necrosis factor-alpha-induced apoptosis. In contrast, LPA-elicited DNA synthesis and migration were augmented in cells expressing EGFR, EGFR(K721A), or EGFR(Y845F), but not EGFR(Y5F), although the PDGF responses were indistinguishable. LPA-induced transphosphorylation of the EGFR, ErbB2, or PDGF receptor was not required for its antiapoptotic effect. EGFR with or without intrinsic kinase activity or without the Src-phosphorylation site augmented, but was not required for, LPA-elicited cell proliferation or migration. In B82L cells, augmentation of these two LPA responses required intact autophosphorylation sites because among the four EGFR mutants, only cells expressing the EGFR(Y5F) mutant showed no enhancement. In EGFR(Y5F)-expressing cells, LPA failed to elicit tyrosine phosphorylation of Src homologous and collagen protein (SHC) and caused only a modest increase in ERK1/2 phosphorylation similar to that in wild-type B82L cells. The present data pinpoint the lack of importance of the intrinsic kinase activity in contrast to the importance of autophosphorylation sites of the EGFR for SHC phosphorylation in the enhancement of select ERK1/2-dependent LPA responses.

Lysophosphatidic acid induces neointima formation through PPARgamma activation

Neointimal lesions are characterized by accumulation of cells within the arterial wall and are a prelude to atherosclerotic disease. Here we report that a brief exposure to either alkyl ether analogs of the growth factor–like phospholipid lysophosphatidic acid (LPA), products generated during the oxidative modification of low density lipoprotein, or to unsaturated acyl forms of LPA induce progressive formation of neointima in vivo in a rat carotid artery model. This effect is completely inhibited by the peroxisome proliferator-activated receptor (PPAR)γ antagonist GW9662 and mimicked by PPARγ agonists Rosiglitazone and 1-O-hexadecyl-2-azeleoyl-phosphatidylcholine. In contrast, stearoyl-oxovaleryl phosphatidylcholine, a PPARα agonist and polypeptide epidermal growth factor, platelet-derived growth factor, and vascular endothelial growth factor failed to elicit neointima. The structure-activity relationship for neointima induction by LPA analogs in vivo is identical to that of PPARγ activation in vitro and disparate from that of LPA G protein–coupled receptor activation. Neointima-inducing LPA analogs up-regulated the CD36 scavenger receptor in vitro and in vivo and elicited dedifferentiation of cultured vascular smooth muscle cells that was prevented by GW9662. These results suggest that selected LPA analogs are important novel endogenous PPARγ ligands capable of mediating vascular remodeling and that activation of the nuclear transcription factor PPARγ is both necessary and sufficient for neointima formation by components of oxidized low density lipoprotein.

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.

RhoA stimulates IEC-6 cell proliferation by increasing polyamine-dependent Cdk2 activity

Although RhoA plays an important role in cell proliferation and in Ras transformation in fibroblasts and mammary epithelial cells, its role in intestinal epithelial cells (IEC) is unknown. In a previous study (Ray RM, Zimmerman BJ, McCormack SA, Patel TB, and Johnson LR. Am J Physiol Cell Physiol 276: C684-C691, 1999), we showed that polyamine depletion [dl-alpha-difluoromethylornithine (DFMO) treatment] strongly inhibits the proliferation of IEC. In this report, we examined the effect of RhoA on IEC-6 cell proliferation and whether polyamine depletion inhibits cell proliferation in the presence of constitutively active RhoA. Constitutively active RhoA and vector-transfected IEC-6 cell lines were grown in the presence or absence of DFMO, which causes polyamine depletion by inhibiting ornithine decarboxylase, the first rate-limiting step in polyamine synthesis. Constitutively active RhoA significantly increased the rate of cell proliferation. These cells also lost contact inhibition and formed conspicuous foci when they were fully confluent. Decreased p21Waf1/Cip1 expression and increased cyclin-dependent kinase (Cdk2) mRNA levels and activity accompanied the increased proliferation. The inhibition of p21Waf1/Cip1 was independent of p53. There was no activation of the Ras-Raf-MEK-ERK pathway in the RhoA-transfected cell line. Polyamine depletion totally prevented the effect of activated RhoA on IEC-6 cell proliferation, focus formation, and Cdk2 expression. The stability of mRNA and protein for Cdk2 and p21Waf1/Cip1 in V14-RhoA cells was not significantly different from that of vector-transfected cells. In conclusion, RhoA activation decreased p21Waf1/Cip1 expression and increased basal and serum-induced ornithine decarboxylase activity, Cdk2 expression, Cdk2 protein, and Cdk2 activity, leading to the stimulation of IEC proliferation and transformation. Polyamine depletion totally prevented RhoA's effect on proliferation by decreasing Cdk2 expression and activity.

Polyamines in cultured rabbit corneal cells

PURPOSE

To determine whether polyamines are present in corneal cells, whether corneal cell polyamines can be depleted by blocking the first rate-limiting enzyme in the polyamine synthesis pathway, ornithine decarboxylase (ODC), and whether polyamines are required for proliferation in all three corneal cell types.

METHODS

Cultured corneal epithelial cells, keratocytes, and endothelial cells were exposed to the specific ODC blocker difluoromethylornithine (DFMO), and ODC activity, intracellular polyamine concentrations, and cell proliferation were measured.

RESULTS

DFMO blocked ODC activity in a dose- and time-dependent manner in all three cell types. DFMO treatment completely depleted putrescine and spermidine by 2 days and also significantly depleted spermine. DFMO treatment also inhibited cell growth in all three cell types and this inhibition could be completely reversed by adding exogenous putrescine to the culture medium.

CONCLUSIONS

Polyamines are present in all cell types of the cornea, their formation is catalyzed at least in part by ODC, and they are an important component of corneal cell proliferation.

Inter-species extrapolation of skin heating resulting from millimeter wave irradiation: modeling and experimental results

This study reports measurements of the skin surface temperature elevations during localized irradiation (94 GHz) of three species: rat (irradiated on lower abdomen), rhesus monkey (posterior forelimb), and human (posterior forearm). Two exposure conditions were examined: prolonged, low power density microwaves (LPM) and short-term, high power density microwaves (HPM). Temperature histories were compared with calculations from a bio-heat transfer model. The mean peak surface temperature increase was approximately 7.0 degrees C for the short-term HPM exposures for all three species/locations, and 8.5 degrees C (monkey, human) to 10.5 degrees C (rat) for the longer-duration LPM exposures. The HPM temperature histories are in close agreement with a one-dimensional conduction heat transfer model with negligible blood flow. The LPM temperature histories were compared with calculations from the bio-heat model, evaluated for various (constant) blood flow rates. Results suggest a variable blood flow model, reflecting a dynamic thermoregulatory response, may be more suited to describing skin surface temperature response under long-duration MMW irradiation.

LPA protects intestinal epithelial cells from apoptosis by inhibiting the mitochondrial pathway

We previously showed (Gastroenterology 123: 206-216, 2002) that lysophosphatidic acid (LPA) protects and rescues rat intestinal epithelial cells (IEC-6) from apoptosis. Here, we provide evidence for the LPA-elicited inhibition of the mitochondrial apoptotic pathway leading to attenuation of caspase-3 activation. Pretreatment of IEC-6 cells with LPA inhibited campothecin-induced caspase-9 and caspase-3 activation and DNA fragmentation. A caspase-9 inhibitor peptide mimicked the LPA-elicited antiapoptotic activity. LPA elicited ERK1/ERK2 and PKB/Akt phosphorylation. The LPA-elicited antiapoptotic activity and inhibition of caspase-9 activity were abrogated by pertussis toxin, PD 98059, wortmannin, and LY 294002. LPA reduced cytochrome c release from mitochondria and prevented activation of caspase-9. LPA prevented translocation of Bax from cytosol to mitochondria and increased the expression of the antiapoptotic Bcl-2 mRNA and protein. LPA had no effect on Bcl-xl, Bad, and Bak mRNA or protein expression. These data indicate that LPA protects IEC-6 cells from camptothecin-induced apoptosis through G(i)-coupled inhibition of caspase-3 activation mediated by the attenuation of caspase-9 activation due to diminished cytochrome c release, involving upregulation of Bcl-2 protein expression and prevention of Bax translocation.

The requirement for polyamines for intestinal epithelial cell migration is mediated through Rac1

The rapid migration of intestinal epithelial cells is important to the healing of mucosal ulcers and wounds. This cell migration requires the presence of polyamines and the activation of RhoA. RhoA activity, however, is not sufficient for migration because polyamine depletion inhibited the migration of IEC-6 cells expressing constitutively active RhoA. The current study examines the role of Rac1 and Cdc42 in cell migration and whether their activities are polyamine-dependent. Polyamine depletion with alpha-difluoromethylornithine inhibited the activities of RhoA, Rac1, and Cdc42. This inhibition was prevented by supplying exogenous putrescine in the presence of alpha-difluoromethylornithine. IEC-6 cells transfected with constitutively active Rac1 and Cdc42 migrated more rapidly than vector-transfected cells, whereas cells expressing dominant negative Rac1 and Cdc42 migrated more slowly. Polyamine depletion had no effect on the migration of cells expressing Rac1 and only partially inhibited the migration of those expressing Cdc42. Although polyamine depletion caused the disappearance of actin stress fibers in cells transfected with empty vector, it had no effect on cells expressing Rac1. Constitutively active Rac1 increased RhoA and Cdc42 activity in both normal and polyamine-depleted cells. These results demonstrate that Rac1, RhoA, and Cdc42 are required for optimal epithelial cell migration and that Rac1 activity is sufficient for cell migration in the absence of polyamines due to its ability to activate RhoA and Cdc42 as well as its own effects on the process of cell migration. These data imply that the involvement of polyamines in cell migration occurs either at Rac1 itself or upstream from Rac1.