Polyamines are necessary for normal expression of the transforming growth factor-beta gene during cell migration

Putrescine mitigates intestinal atrophy through suppressing inflammatory response in weanling piglets

Background

Polyamines are essential for cell growth and beneficial for intestinal maturation. To evaluate the effects of putrescine on alleviating intestinal atrophy and underlying molecular mechanisms, both in vivo feeding trial and in vitro cell culture were conducted. Weanling pigs were fed a diet supplemented with 0, 0.1%, 0.2% or 0.3% putrescine dihydrochloride, whereas porcine intestinal epithelial cells (IPEC-J2) were challenged with lipopolysaccharide (LPS) in the presence of 200 μmol/L putrescine.

Results

Dietary supplementation with 0.2% putrescine dihydrochloride decreased the incidence of diarrhea with an improvement in intestinal integrity. Inhibition of ornithine decarboxylase activity decreased the proliferation and migration of IPEC-J2 cells, and this effect was alleviated by the supplementation with putrescine. The phosphorylation of extracellular signal regulated kinase and focal adhesion kinase was enhanced by putrescine. LPS increased the expression of inflammatory cytokines [tumor necrosis factor α (TNF-α), interleukin 6 (IL-6) and IL-8], and inhibited cell proliferation and migration in IPEC-J2 cells. Adding exogenous putrescine suppressed the expression of TNF-α, IL-6 and IL-8, and recovered cell migration and proliferation in LPS-treated IPEC-J2 cells. Dietary putrescine supplementation also reduced the mRNA levels of TNF-α, IL-6 and IL-8 and their upstream regulator nuclear receptor kappa B p65 subunit in the jejunal mucosa of piglets.

Conclusions

Dietary supplementation with putrescine mitigated mucosal atrophy in weanling piglets through improving anti-inflammatory function and suppressing inflammatory response. Our results have important implications for nutritional management of intestinal integrity and health in weanling piglets and other neonates.

Electronic supplementary material

The online version of this article (10.1186/s40104-019-0379-9) contains supplementary material, which is available to authorized users.

Role of Polyamines in Asthma Pathophysiology

Asthma is a complex disease of airways, where the interactions of immune and structural cells result in disease outcomes with airway remodeling and airway hyper-responsiveness. Polyamines, which are small-sized, natural super-cations, interact with negatively charged intracellular macromolecules, and altered levels of polyamines and their interactions have been associated with different pathological conditions including asthma. Elevated levels of polyamines have been reported in the circulation of asthmatic patients as well as in the lungs of a murine model of asthma. In various studies, polyamines were found to potentiate the pathogenic potential of inflammatory cells, such as mast cells and granulocytes (eosinophils and neutrophils), by either inducing the release of their pro-inflammatory mediators or prolonging their life span. Additionally, polyamines were crucial in the differentiation and alternative activation of macrophages, which play an important role in asthma pathology. Importantly, polyamines cause airway smooth muscle contraction and thus airway hyper-responsiveness, which is the key feature in asthma pathophysiology. High levels of polyamines in asthma and their active cellular and macromolecular interactions indicate the importance of the polyamine pathway in asthma pathogenesis; therefore, modulation of polyamine levels could be a suitable approach in acute and severe asthma management. This review summarizes the possible roles of polyamines in different pathophysiological features of asthma.

L-arginine uptake by cationic amino acid transporter 2 is essential for colonic epithelial cell restitution

Restoration of the colonic epithelial barrier is an important response during colitis. L-arginine (L-Arg) is a semiessential amino acid that reduces murine colitis induced by Citrobacter rodentium. Cationic amino acid transporter (CAT) proteins increase L-Arg uptake into cells. L-Arg is utilized to produce nitric oxide (NO), by inducible NO synthase (iNOS), or L-ornithine (L-Orn) by arginase (Arg) enzymes. The latter is followed by generation of polyamines by ornithine decarboxylase (ODC) and L-proline (L-Pro) by ornithine aminotransferase (OAT). We show that L-Arg enhanced epithelial restitution in conditionally immortalized young adult mouse colon (YAMC) cells in a wound repair model, and in isolated mouse colonic epithelial cells (CECs), using a cell migration assay. Restitution was impaired by C. rodentium. Wounding induced CAT2, and inhibition of L-Arg uptake by the competitive inhibitor L-lysine (L-Lys) or by CAT2 shRNA, but not CAT1 shRNA, decreased restitution. Migration was impaired in CECs treated with L-Lys or from CAT2(-/-) mice. Wounding increased Arg1 expression, and inhibition of arginase with S-(2-boronoethyl)-L-cysteine (BEC) or Arg1 shRNA inhibited restitution in YAMC cells; cell migration in CECs was also impaired by BEC. Inhibition of ODC or iNOS did not alter restitution. L-Orn or L-Pro restored restitution in cells treated with BEC or Arg1 shRNA, whereas the polyamine putrescine had no benefit. Wounding increased OAT levels, OAT shRNA inhibited restitution, and L-Pro restored restitution in cells with OAT knockdown. Uptake of L-Arg, and its metabolism by Arg1 to L-Orn and conversion to L-Pro by OAT is essential for colonic epithelial wound repair.

L-arginine supplementation improves responses to injury and inflammation in dextran sulfate sodium colitis

Inflammatory bowel disease (IBD), consisting of Crohn's disease and ulcerative colitis (UC), results in substantial morbidity and is difficult to treat. New strategies for adjunct therapies are needed. One candidate is the semi-essential amino acid, L-arginine (L-Arg), a complementary medicine purported to be an enhancer of immunity and vitality in the lay media. Using dextran sulfate sodium (DSS) as a murine colonic injury and repair model with similarities to human UC, we assessed the effect of L-Arg, as DSS induced increases in colonic expression of the y(+) cationic amino acid transporter 2 (CAT2) and L-Arg uptake. L-Arg supplementation improved the clinical parameters of survival, body weight loss, and colon weight, and reduced colonic permeability and the number of myeloperoxidase-positive neutrophils in DSS colitis. Luminex-based multi-analyte profiling demonstrated that there was a marked reduction in proinflammatory cytokine and chemokine expression with L-Arg treatment. Genomic analysis by microarray demonstrated that DSS-treated mice supplemented with L-Arg clustered more closely with mice not exposed to DSS than to those receiving DSS alone, and revealed that multiple genes that were upregulated or downregulated with DSS alone exhibited normalization of expression with L-Arg supplementation. Additionally, L-Arg treatment of mice with DSS colitis resulted in increased ex vivo migration of colonic epithelial cells, suggestive of increased capacity for wound repair. Because CAT2 induction was sustained during L-Arg treatment and inducible nitric oxide (NO) synthase (iNOS) requires uptake of L-Arg for generation of NO, we tested the effect of L-Arg in iNOS(-/-) mice and found that its benefits in DSS colitis were eliminated. These preclinical studies indicate that L-Arg supplementation could be a potential therapy for IBD, and that one mechanism of action may be functional enhancement of iNOS activity.

MMP7 expression regulated by endocrine therapy in ERbeta-positive colon cancer cells

Background

Many studies have shown that colon cancer is an estrogen-dependent carcinoma. This study explored the efficacy of endocrine therapy in colon cancer cells with high metastatic potential (HT29). We investigated the proliferation of HT29 cells after exposure to endocrine therapy (tamoxifen) and 5-FU.

Methods

Apoptosis was evaluated using flow cytometry. The expression of matrix metalloproteinases 7 (MMP-7) and estrogen receptor beta (ERβ) was measured by reverse transcription-polymerase chain reaction (RT-PCR) and western blot. The migration capability of treated cells was determined with wound scratch assay.

Results

Tamoxifen alone, 5-FU alone, and the combination of the two drugs can significantly inhibit HT29 cell proliferation and migration, block the cells in G₂/M phase and induce cell apoptosis. These drugs also can down-regulate MMP7 and ERβ expression.

Conclusion

Our findings suggest that endocrine therapy is an efficient therapy for inhibiting ERβ-positive colon cancer cell proliferation and migration via down-regulation of MMP7.

Bombesin enhances TGF-beta growth inhibitory effect through apoptosis induction in intestinal epithelial cells

Mammalian intestinal epithelium undergoes continuous cell turn over, with cell proliferation in the crypts and apoptosis in the villus. Both transforming growth factor (TGF)-beta and gastrin-releasing peptide (GRP) are involved in the regulation of intestinal epithelial cells for division, differentiation, adhesion, migration and death. Previously, we have shown that TGF-beta and bombesin (BBS) synergistically induce cyclooxygenase-2 (COX-2) expression and subsequent prostaglandin E(2) (PGE2) production through p38(MAPK) in rat intestinal epithelial cell line stably transfected with GRP receptor (RIE/GRPR), suggesting the interaction between TGF-beta signaling pathway and GRPR. The current study examined the biological responses of RIE/GRPR cells to TGF-beta and BBS. Treatment with TGF-beta1 (40 pM) and BBS (100 nM) together synergistically inhibited RIE/GRPR growth and induced apoptosis. Pretreatment with SB203580 (10 microM), a specific inhibitor of p38(MAPK), partially blocked the synergistic effect of TGF-beta and BBS on apoptosis. In conclusion, BBS enhanced TGF-beta growth inhibitory effect through apoptosis induction, which is at least partially mediated by p38(MAPK).

Polyamines regulate the stability of activating transcription factor-2 mRNA through RNA-binding protein HuR in intestinal epithelial cells

Maintenance of intestinal mucosal epithelial integrity requires polyamines that modulate the expression of various genes involved in cell proliferation and apoptosis. Recently, polyamines were shown to regulate the subcellular localization of the RNA-binding protein HuR, which stabilizes its target transcripts such as nucleophosmin and p53 mRNAs. The activating transcription factor-2 (ATF-2) mRNA encodes a member of the ATF/CRE-binding protein family of transcription factors and was computationally predicted to be a target of HuR. Here, we show that polyamines negatively regulate ATF-2 expression posttranscriptionally and that polyamine depletion stabilizes ATF-2 mRNA by enhancing the interaction of the 3'-untranslated region (UTR) of ATF-2 with cytoplasmic HuR. Decreasing cellular polyamines by inhibiting ornithine decarboxylase (ODC) with alpha-difluoromethylornithine increased the levels of ATF-2 mRNA and protein, whereas increasing polyamines by ectopic ODC overexpression repressed ATF-2 expression. Polyamine depletion did not alter transcription via the ATF-2 gene promoter but increased the stability of ATF-2 mRNA. Increased cytoplasmic HuR in polyamine-deficient cells formed ribonucleoprotein complexes with the endogenous ATF-2 mRNA and specifically bound to 3'-UTR of ATF-2 mRNA on multiple nonoverlapping 3'-UTR segments. Adenovirus-mediated HuR overexpression elevated ATF-2 mRNA and protein levels, whereas HuR silencing rendered the ATF-2 mRNA unstable and prevented increases in ATF-2 mRNA and protein. Furthermore, inhibition of ATF-2 expression prevented the increased resistance of polyamine-deficient cells to apoptosis induced by treatment with tumor necrosis factor-alpha and cycloheximide. These results indicate that polyamines modulate the stability of ATF-2 mRNA by altering cytoplasmic HuR levels and that polyamine-modulated ATF-2 expression plays a critical role in regulating epithelial apoptosis.

Identification of apoptotic genes mediating TGF-beta/Smad3-induced cell death in intestinal epithelial cells using a genomic approach

Transforming growth factor (TGF)-beta-dependent apoptosis is important in the elimination of damaged or abnormal cells from normal tissues in vivo. Previously, we have shown that TGF-beta inhibits the growth of rat intestinal epithelial (RIE)-1 cells. However, RIE-1 cells are relatively resistant to TGF-beta-induced apoptosis due to a low endogenous Smad3-to-Akt ratio. Overexpression of Smad3 sensitizes RIE-1 cells (RIE-1/Smad3) to TGF-beta-induced apoptosis by altering the Smad3-to-Akt ratio in favor of apoptosis. In this study, we utilized a genomic approach to identify potential downstream target genes that are regulated by TGF-beta/Smad3. Total RNA samples were analyzed using Affymetrix oligonucleotide microarrays. We found that TGF-beta regulated 518 probe sets corresponding to its target genes. Interestingly, among the known apoptotic genes included in the microarray analyses, only caspase-3 was induced, which was confirmed by real-time RT-PCR. Furthermore, TGF-beta activated caspase-3 through protein cleavage. Upstream of caspase-3, TGF-beta induced mitochondrial depolarization, cytochrome c release, and cleavage of caspase-9, which suggests that the intrinsic apoptotic pathway mediates TGF-beta-induced apoptosis in RIE-1/Smad3 cells.

Synthesis, characterization and cytocompatibility of polyurethaneurea elastomers with designed elastase sensitivity

In designing a synthetic scaffold for engineering soft, mechanically active tissues, desirable properties include elasticity, support of cell adhesion and growth, ease of processability, and responsiveness to in vivo remodeling. To achieve these properties, we have developed a family of thermoplastic elastomers, polyurethaneureas (PUs), that possess enzymatic remodeling capabilities in addition to simple hydrolytic lability. PUs were synthesized using either polycaprolactone or triblock copolymer polycaprolactone-b-poly(ethylene glycol)-b-polycaprolactone as the soft segment, 1,4-butanediisocyanate as the hard segment, and the peptide Ala-Ala-Lys as a chain extender. The synthesized PUs had high molecular weights, low glass transition temperatures (< -54 degrees C), and were flexible with breaking strains of 670-890% and tensile strengths of 15-28 MPa. Incubation in buffered saline without elastase for 8 weeks resulted in mass loss from 12% to 18% depending on soft segment composition. The degradation significantly increased (p < 0.05) in the presence of elastase, ranging from 19% to 34% with degradation products showing no cytotoxicity. To encourage cell adhesion, PUs were surface-modified with radio frequency glow discharge followed by coupling of Arg-Gly-Asp-Ser (RGDS). Endothelial cell adhesion was >140% of tissue culture polystyrene on PU surfaces and >200% on RGDS-modified surfaces. The synthesized PUs thus combine mechanical, chemical, and bioresponsive properties that might be employed in soft-tissue engineering applications.

Preparation and characterization of highly porous, biodegradable polyurethane scaffolds for soft tissue applications

In the engineering of soft tissues, scaffolds with high elastance and strength coupled with controllable biodegradable properties are necessary. To fulfill such design criteria we have previously synthesized two kinds of biodegradable polyurethaneureas, namely poly(ester urethane)urea (PEUU) and poly(ether ester urethane)urea (PEEUU) from polycaprolactone, polycaprolactone-b-polyethylene glycol-b-polycaprolactone, 1,4-diisocyanatobutane and putrescine. PEUU and PEEUU were further fabricated into scaffolds by thermally induced phase separation using dimethyl sulfoxide (DMSO) as a solvent. The effect of polymer solution concentration, quenching temperature and polymer type on pore morphology and porosity was investigated. Scaffolds were obtained with open and interconnected pores having sizes ranging from several mum to more than 150 microm and porosities of 80-97%. By changing the polymer solution concentration or quenching temperature, scaffolds with random or oriented tubular pores could be obtained. The PEUU scaffolds were flexible with breaking strains of 214% and higher, and tensile strengths of approximately 1.0 MPa, whereas the PEEUU scaffolds generally had lower strengths and breaking strains. Scaffold degradation in aqueous buffer was related to the porosity and polymer hydrophilicity. Smooth muscle cells were filtration seeded in the scaffolds and it was shown that both scaffolds supported cell adhesion and growth, with smooth muscle cells growing more extensively in the PEEUU scaffold. These biodegradable and flexible scaffolds demonstrate potential for future application as cell scaffolds in cardiovascular tissue engineering or other soft tissue applications.

Regulation of adherens junctions and epithelial paracellular permeability: a novel function for polyamines

Maintenance of intestinal mucosal epithelial integrity requires polyamines that are involved in the multiple signaling pathways controlling gene expression and different epithelial cell functions. Integrity of the intestinal epithelial barrier depends on a complex of proteins composing different intercellular junctions, including tight junctions, adherens junctions, and desmosomes. E-cadherin is primarily found at the adherens junctions and plays a critical role in cell-cell adhesions that are fundamental to formation of the intestinal epithelial barrier. The current study determined whether polyamines regulate intestinal epithelial barrier function by altering E-cadherin expression. Depletion of cellular polyamines by alpha-difluoromethylornithine (DFMO) reduced intracellular free Ca2+ concentration ([Ca2+]cyt), decreased E-cadherin expression, and increased paracellular permeability in normal intestinal epithelial cells (IEC-6 line). Polyamine depletion did not alter expression of tight junction proteins such as zona occludens (ZO)-1, ZO-2, and junctional adhesion molecule (JAM)-1. Addition of exogenous polyamine spermidine reversed the effects of DFMO on [Ca2+]cyt and E-cadherin expression and restored paracellular permeability to near normal. Elevation of [Ca2+]cyt by the Ca2+ ionophore ionomycin increased E-cadherin expression in polyamine-deficient cells. In contrast, reduction of [Ca2+]cyt by polyamine depletion or removal of extracellular Ca2+ not only inhibited expression of E-cadherin mRNA but also decreased the half-life of E-cadherin protein. These results indicate that polyamines regulate intestinal epithelial paracellular barrier function by altering E-cadherin expression and that polyamines are essential for E-cadherin expression at least partially through [Ca2+]cyt.

Polyamines regulate Rho-kinase and myosin phosphorylation during intestinal epithelial restitution

Polyamines are required for the early phase of mucosal restitution that occurs as a consequence of epithelial cell migration. Our previous studies have shown that polyamines increase RhoA activity by elevating cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)) through controlling voltage-gated K(+) channel expression and membrane potential (E(m)) during intestinal epithelial restitution. The current study went further to determine whether increased RhoA following elevated [Ca(2+)](cyt) activates Rho-kinase (ROK/ROCK) resulting in myosin light chain (MLC) phosphorylation. Studies were conducted in stable Cdx2-transfected intestinal epithelial cells (IEC-Cdx2L1), which were associated with a highly differentiated phenotype. Reduced [Ca(2+)](cyt), by either polyamine depletion or exposure to the Ca(2+)-free medium, decreased RhoA protein expression, which was paralleled by significant decreases in GTP-bound RhoA, ROCK-1, and ROKalpha proteins, Rho-kinase activity, and MLC phosphorylation. The reduction of [Ca(2+)](cyt) also inhibited cell migration after wounding. Elevation of [Ca(2+)](cyt) induced by the Ca(2+) ionophore ionomycin increased GTP-bound RhoA, ROCK-1, and ROKalpha proteins, Rho-kinase activity, and MLC phosphorylation. Inhibition of RhoA function by a dominant negative mutant RhoA decreased the Rho-kinase activity and resulted in cytoskeletal reorganization. Inhibition of ROK/ROCK activity by the specific inhibitor Y-27632 not only decreased MLC phosphorylation but also suppressed cell migration. These results indicate that increase in GTP-bound RhoA by polyamines via [Ca(2+)](cyt) can interact with and activate Rho-kinase during intestinal epithelial restitution. Activation of Rho-kinase results in increased MLC phosphorylation, leading to the stimulation of myosin stress fiber formation and cell migration.

Polyamines regulate beta-catenin tyrosine phosphorylation via Ca(2+) during intestinal epithelial cell migration

Polyamines are essential for early mucosal restitution that occurs by epithelial cell migration to reseal superficial wounds after injury. Normal intestinal epithelial cells are tightly bound in sheets, but they need to be rapidly disassembled during restitution. beta-Catenin is involved in cell-cell adhesion, and its tyrosine phosphorylation causes disassembly of adhesion junctions, enhancing the spreading of cells. The current study determined whether polyamines are required for the stimulation of epithelial cell migration by altering beta-catenin tyrosine phosphorylation. Migration of intestinal epithelial cells (IEC-6 line) after wounding was associated with an increase in beta-catenin tyrosine phosphorylation, which decreased the binding activity of beta-catenin to alpha-catenin. Polyamine depletion by alpha-difluoromethylornithine reduced cytoplasmic free Ca(2+) concentration ([Ca(2+)](cyt)), prevented induction of beta-catenin phosphorylation, and decreased cell migration. Elevation of [Ca(2+)](cyt) induced by the Ca(2+) ionophore ionomycin restored beta-catenin phosphorylation and promoted migration in polyamine-deficient cells. Decreased beta-catenin phosphorylation through the tyrosine kinase inhibitor herbimycin-A or genistein blocked cell migration, which was accompanied by reorganization of cytoskeletal proteins. These results indicate that beta-catenin tyrosine phosphorylation plays a critical role in polyamine-dependent cell migration and that polyamines induce beta-catenin tyrosine phosphorylation at least partially through [Ca(2+)](cyt).

Activation of K(+) channels and increased migration of differentiated intestinal epithelial cells after wounding

Early mucosal restitution occurs by epithelial cell migration to reseal superficial wounds after injury. Differentiated intestinal epithelial cells induced by forced expression of the Cdx2 gene migrate over the wounded edge much faster than undifferentiated parental cells in an in vitro model. This study determined whether these differentiated intestinal epithelial cells exhibit increased migration by altering voltage-gated K(+) (Kv) channel expression and cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)). Stable Cdx2-transfected IEC-6 cells (IEC-Cdx2L1) with highly differentiated phenotype expressed higher basal levels of Kv1.1 and Kv1.5 mRNAs and proteins than parental IEC-6 cells. Neither IEC-Cdx2L1 cells nor parental IEC-6 cells expressed voltage-dependent Ca(2+) channels. The increased expression of Kv channels in differentiated IEC-Cdx2L1 cells was associated with an increase in whole cell K(+) currents, membrane hyperpolarization, and a rise in [Ca(2+)](cyt). The migration rates in differentiated IEC-Cdx2L1 cells were about four times those of parental IEC-6 cells. Inhibition of Kv channel expression by polyamine depletion decreased [Ca(2+)](cyt), reduced myosin stress fibers, and inhibited cell migration. Elevation of [Ca(2+)](cyt) by ionomycin promoted myosin II stress fiber formation and increased cell migration. These results suggest that increased migration of differentiated intestinal epithelial cells is mediated, at least partially, by increasing Kv channel activity and Ca(2+) influx during restitution.

Ca2+-RhoA signaling pathway required for polyamine-dependent intestinal epithelial cell migration

Expression of voltage-gated K(+) (Kv) channel genes is regulated by polyamines in intestinal epithelial cells (IEC-6 line), and Kv channel activity is involved in the regulation of cell migration during early restitution by controlling membrane potential (E(m)) and cytosolic free Ca2+ concentration ([Ca2+](cyt)). This study tests the hypothesis that RhoA of small GTPases is a downstream target of elevated ([Ca2+](cyt)) following activation of K(+) channels by increased polyamines in IEC-6 cells. Depletion of cellular polyamines by alpha-difluoromethylornithine (DFMO) reduced whole cell K+ currents [I(K(v))] through Kv channels and caused membrane depolarization, which was associated with decreases in ([Ca2+](cyt)), RhoA protein, and cell migration. Exogenous polyamine spermidine reversed the effects of DFMO on I(K(v)), E(m), ([Ca2+](cyt)), and RhoA protein and restored cell migration to normal. Elevation of ([Ca2+](cyt)) induced by the Ca2+ ionophore ionomycin increased RhoA protein synthesis and stimulated cell migration, while removal of extracellular Ca2+ decreased RhoA protein synthesis, reduced protein stability, and inhibited cell motility. Decreased RhoA activity due to Clostridium botulinum exoenzyme C(3) transferase inhibited formation of myosin II stress fibers and prevented restoration of cell migration by exogenous spermidine in polyamine-deficient cells. These findings suggest that polyamine-dependent cell migration is partially initiated by the formation of myosin II stress fibers as a result of Ca2+-activated RhoA activity.

Expression of the TGF-beta receptor gene and sensitivity to growth inhibition following polyamine depletion

Our previous studies have shown that inhibition of polyamine biosynthesis increases the sensitivity of intestinal epithelial cells to growth inhibition induced by exogenous transforming growth factor-beta (TGF-beta). This study went further to determine whether expression of the TGF-beta receptor genes is involved in this process. Studies were conducted in the IEC-6 cell line, derived from rat small intestinal crypt cells. Administration of alpha-difluoromethylornithine (DFMO), a specific inhibitor of ornithine decarboxylase (the rate-limiting enzyme for polyamine synthesis), for 4 and 6 days depleted cellular polyamines putrescine, spermidine, and spermine in IEC-6 cells. Polyamine depletion by DFMO increased levels of the TGF-beta type I receptor (TGF-betaRI) mRNA and protein but had no effect on the TGF-beta type II receptor expression. The induced TGF-betaRI expression after polyamine depletion was associated with an increased sensitivity to growth inhibition induced by exogenous TGF-beta but not by somatostatin. Extracellular matrix laminin inhibited IEC-6 cell growth without affecting the TGF-beta receptor expression. Laminin consistently failed to induce the sensitivity of TGF-beta-mediated growth inhibition. In addition, decreasing TGF-betaRI expression by treatment with retinoic acid not only decreased TGF-beta-mediated growth inhibition in normal cells but also prevented the increased sensitivity to exogenous TGF-beta in polyamine-deficient cells. These results indicate that 1) depletion of cellular polyamines by DFMO increases expression of the TGF-betaRI gene and 2) increased TGF-betaRI expression plays an important role in the process through which polyamine depletion sensitizes intestinal epithelial cells to growth inhibition induced by TGF-beta.

Role of K(+) channel expression in polyamine-dependent intestinal epithelial cell migration

Polyamines are essential for cell migration during early mucosal restitution after wounding in the gastrointestinal tract. Activity of voltage-gated K(+) channels (Kv) controls membrane potential (E(m)) that regulates cytoplasmic free Ca(2+) concentration ([Ca(2+)](cyt)) by governing the driving force for Ca(2+) influx. This study determined whether polyamines are required for the stimulation of cell migration by altering K(+) channel gene expression, E(m), and [Ca(2+)](cyt) in intestinal epithelial cells (IEC-6). The specific inhibitor of polyamine synthesis, alpha-difluoromethylornithine (DFMO, 5 mM), depleted cellular polyamines (putrescine, spermidine, and spermine), selectively inhibited Kv1.1 channel (a delayed-rectifier Kv channel) expression, and resulted in membrane depolarization. Because IEC-6 cells did not express voltage-gated Ca(2+) channels, the depolarized E(m) in DFMO-treated cells decreased [Ca(2+)](cyt) as a result of reduced driving force for Ca(2+) influx through capacitative Ca(2+) entry. Migration was reduced by 80% in the polyamine-deficient cells. Exogenous spermidine not only reversed the effects of DFMO on Kv1.1 channel expression, E(m), and [Ca(2+)](cyt) but also restored cell migration to normal. Removal of extracellular Ca(2+) or blockade of Kv channels (by 4-aminopyridine, 1-5 mM) significantly inhibited normal cell migration and prevented the restoration of cell migration by exogenous spermidine in polyamine-deficient cells. These results suggest that polyamine-dependent intestinal epithelial cell migration may be due partially to an increase of Kv1.1 channel expression. The subsequent membrane hyperpolarization raises [Ca(2+)](cyt) by increasing the driving force (the electrochemical gradient) for Ca(2+) influx and thus stimulates cell migration.

Putrescine does not support the migration and growth of IEC-6 cells

The migration of IEC-6 cells is inhibited when the cells are depleted of polyamines by inhibiting ornithine decarboxylase with alpha-difluoromethylornithine (DFMO). Exogenous putrescine, spermidine, and spermine completely restore cell migration inhibited by DFMO. Because polyamines are interconverted during their synthesis and catabolism, the specific role of individual polyamines in intestinal cell migration, as well as growth, remains unclear. In this study, we used an inhibitor of S-adenosylmethionine decarboxylase, diethylglyoxal bis(guanylhydrazone)(DEGBG), to block the synthesis of spermidine and spermine from putrescine. We found that exogenous putrescine does not restore migration and growth of IEC-6 cells treated with DFMO plus DEGBG, whereas exogenous spermine does. In addition, the normal distribution of actin filaments required for migration, which is disrupted in polyamine-deficient cells, could be achieved by adding spermine but not putrescine along with DFMO and DEGBG. These results indicate that putrescine, by itself, is not essential for migration and growth, but that it is effective because it is converted into spermidine and/or spermine.

Healing of Gastrointestinal Mucosa: Involvement of Polyamines

Polyamines are involved in the processes of cell migration and proliferation that result in the repair of mucosal lesions. Depletion of polyamines dramatically alters the arrangement of the cytoskeleton, EGF receptor function, the activities of signal transduction proteins, the levels of several protooncogenes, and the expression and cellular content of at least one growth factor involved in these processes.

Two distinct classes of rat intestinal mucosal enzymes incorporating putrescine into protein

Tissue-transglutaminase (t-TGase) is a family of calcium-dependent enzymes. A Ca2+-independent soluble enzyme, in addition to t-TGase, capable of incorporating polyamines into proteins was demonstrated in rat intestinal mucosa. The Ca2+-independent enzyme was stimulated 2- to 5-fold by Fe2+ and Co2+ ions but inhibited by Cu2+ and Zn2+ ions. The Ca2+-stimulated t-TGase activity was inhibited by divalent ions in the following order: Zn2+, Fe2+ >Co2+ > Cu2+. The opposite effects of EGTA, Fe2+ and Co2+ on these two enzyme activities indicate that they are two distinct classes of enzymes. Competition studies demonstrated differential preferences of the two enzymes for substrates. The Ca2+-dependent enzyme preferred putrescine, monodansylcadaverine > cadaverine, spermidine, spermine > 1,10-diaminodecane > triethylbutylamine. On the other hand, the Ca2+-independent enzyme preferred putrescine > cadaverine > spermine, I,10-diaminodecane > spermidine > monodansylcadaverine > triethylbutylamine. Further studies with divalent ions excluded the possible association of this novel Ca2+-independent enzyme with diamine oxidase. Finally, the Ca2+-independent enzyme had a higher affinity for putrescine (Km = 0.02 mM) than did Ca2+-dependent t-TGase (0.2 mM). As judged by gel filtration on HiPrep Sephacryl 200 column, the Ca2+-independent enzyme had a molecular weight of approximately 48 kDa, the intestinal Ca2+-dependent t-TGase was about 188 kDa while that of testicular t-TGase was about 96 kDa. In conclusion, the Ca2+-independent enzyme is stimulated by cobalt or ferric ions, and selectively incorporates aliphatic diamines or polyamines with symmetric amino groups. The observed Ca2+-independent enzyme activity is not related to diamine oxidase or its products. With a 10 times greater affinity for putrescine, the calcium-independent, 48-kDa intestinal enzyme may mediate polyamine function better than calcium dependent, 188-kDa intestinal tissue transglutaminase in the intestinal mucosa.