Gastrin stimulates expression of protooncogene c-myc through a process involving polyamines in IEC-6 cells

Inhibition of Smurf2 translation by miR-322/503 modulates TGF-β/Smad2 signaling and intestinal epithelial homeostasis

Smurf2 is an E3 ubiquitin ligase that regulates TGF-β/Smad signaling and is implicated in a wide variety of cellular responses. miR-322 and miR-503 repress Smurf2 translation and thus modulate TGF-β/Smad2 signaling and intestinal epithelial homeostasis.

Smad ubiquitin regulatory factor 2 (Smurf2) is an E3 ubiquitin ligase that regulates transforming growth factor β (TGF-β)/Smad signaling and is implicated in a wide variety of cellular responses, but the exact mechanisms that control Smurf2 abundance are largely unknown. Here we identify microRNA-322 (miR-322) and miR-503 as novel factors that regulate Smurf2 expression posttranscriptionally. Both miR-322 and miR-503 interact with Smurf2 mRNA via its 3′-untranslated region (UTR) and repress Smurf2 translation but do not affect total Smurf2 mRNA levels. Studies using heterologous reporter constructs reveal a greater repressive effect of miR-322/503 through a single binding site in the Smurf2 3′-UTR, whereas point mutation of this site prevents miR-322/503–induced repression of Smurf2 translation. Increased levels of endogenous Smurf2 via antagonism of miR-322/503 inhibits TGF-β–induced Smad2 activation by increasing degradation of phosphorylated Smad2. Furthermore, the increase in Smurf2 in intestinal epithelial cells (IECs) expressing lower levels of miR-322/503 is associated with increased resistance to apoptosis, which is abolished by Smurf2 silencing. These findings indicate that miR-322/503 represses Smurf2 translation, in turn affecting intestinal epithelial homeostasis by altering TGF-β/Smad2 signaling and IEC apoptosis.

αV integrin: A new gastrin target in human pancreatic cancer cells

AIM: To analyse α_V integrin expression induced by gastrin in pancreatic cancer models.

METHODS: α_V integrin mRNA expression in human pancreatic cancer cells was analysed using a “cancer genes” array and confirmed by real-time reverse transcription-polymerase chain reaction (PCR). Western blotting and semi-quantitative immunohistochemistry were used to examine protein levels in human pancreatic cancer cell lines and pancreatic tissues, respectively. The role of α_V integrin on gastrin-induced cell adhesion was examined using blocking anti-α_V integrin monoclonal antibodies. Adherent cells were quantified by staining with crystal violet.

RESULTS: Using a “cancer genes” array we identified α_V integrin as a new gastrin target gene in human pancreatic cancer cells. A quantitative real-time PCR approach was used to confirm α_V integrin gene expression. We also demonstrate that Src family kinases and the PI 3-kinase, two signalling pathways specifically activated by the CCK-2 receptor (CCK2R), are involved in gastrin-mediated α_V integrin expression. In contrast, inhibition of the ERK pathway was without any effect on α_V integrin expression induced by gastrin. Our results also show that gastrin modulates cell adhesion viaα_V integrins. Indeed, in vitro adhesion assays performed on fibronectin show that gastrin significantly increases adhesion of pancreatic cancer cells. The use of blocking anti-α_V integrin monoclonal antibodies completely reversed the increase in cell-substrate adhesion induced by gastrin. In addition, we showed in vivo that the targeted CCK2R expression in the pancreas of Elas-CCK2 mice, leads to the overexpression of α_V integrin. This process may contribute to pancreatic tumour development observed in these transgenic animals.

CONCLUSION: α_V integrin is a new gastrin target in pancreatic cancer models and contributes to gastrin effects on cell adhesion.

Polyamines and the intestinal tract

Owing to their high turnover, the intestinal mucosal cells have a particularly high requirement for polyamines. Therefore, they are an excellent charcol for the study of polyamine function in rapid physiological growth and differentiation. After a cursory introduction to the major aspects of polyamine metabolism, regulation, and mode of action, we discuss the contribution of the polyamines to the maintenance of normal gut function, the maturation of the intestinal mucosa, and its repair after injuries. Repletion of cellular polyamine pools with (D,L)-2-(difluoromethyl)ornithine has considerably improved our understanding of how the polyamines are involved in the regulation of normal and neoplastic growth. Unfortunately, the attempts to exploit polyamine metabolism as a cancer therapeutic target have not yet been successful. However, the selective inactivation of ornithine decarboxylase appears to be a promising chemopreventive method in familial adenomatous polyposis. Presumably, it relies on the fact that ornithine decarboxylase is a critical regulator of the proliferative response of the protooncogene c-myc, but not of its apoptotic response.

Polyamine-modulated c-Myc expression in normal intestinal epithelial cells regulates p21Cip1 transcription through a proximal promoter region

Maintenance of intestinal mucosal epithelial integrity requires cellular polyamines that regulate expression of various genes involved in cell proliferation, growth arrest and apoptosis. Our previous studies have shown that polyamines are essential for expression of the c-myc gene and that polyamine-induced c-Myc plays a critical role in stimulation of normal IEC (intestinal epithelial cell) proliferation, but the exact downstream targets of induced c-Myc are still unclear. The p21Cip1 protein is a major player in cell cycle control, which is primarily regulated at the transcriptional level. The current study was designed to determine whether induced c-Myc stimulates normal IEC proliferation by repressing p21Cip1 transcription following up-regulation of polyamines. Overexpression of the ODC (ornithine decarboxylase) gene increased levels of cellular polyamines, induced c-Myc expression and inhibited p21Cip1 transcription, as indicated by repression of p21Cip1 promoter activity and a decrease in p21Cip1 protein levels. In contrast, depletion of cellular polyamines by inhibiting ODC enzyme activity with alpha-difluoromethylornithine decreased c-Myc, but increased p21Cip1 transcription. Ectopic expression of wild-type c-myc not only inhibited basal levels of p21Cip1 transcription in control cells, but also prevented increased p21Cip1 in polyamine-deficient cells. Experiments using different p21Cip1 promoter mutants showed that transcriptional repression of p21Cip1 by c-Myc was mediated through Miz-1- and Sp1-binding sites within the proximal region of the p21Cip1 promoter in normal IECs. These findings confirm that p21Cip1 is one of the direct mediators of induced c-Myc following increased polyamines and that p21Cip1 repression by c-Myc is implicated in stimulation of normal IEC proliferation.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

Cholecystokinin-2 receptor modulates cell adhesion through beta 1-integrin in human pancreatic cancer cells

Several lines of evidence suggest that gastrin and the CCK-2 receptor (CCK2R) could contribute to pancreatic carcinogenesis by modulating processes such as proliferation, cell adhesion or migration. In the current study, we used a 'cancer gene array' and identified beta1-integrin subunit as a new gastrin-regulated gene in human pancreatic cancer cells. We also demonstrated that Src family kinases and the phosphatidylinositol-3-kinase (PI-3-kinase) pathway play a crucial role in the expression of beta1-integrin induced by gastrin. Our results also showed that gastrin modulates cell-substrate adhesion via beta1-integrin. Indeed, using blocking anti-beta1-integrin monoclonal antibodies, we completely reversed the increase in cell-substrate adhesion induced by gastrin. In addition, we observed that in response to gastrin, beta1-integrin is tyrosine phosphorylated by Src family kinases and associates with paxillin, a scaffold protein involved in focal adhesion and integrin signalling. This mechanism might be involved in gastrin-induced cell adhesion. Moreover, we showed in vivo that targeted CCK2R expression in the pancreas of Elas-CCK2 mice leads to the overexpression of beta1-integrin. This process may contribute to pancreatic tumour development observed in these transgenic animals.

Gastrin stabilises beta-catenin protein in mouse colorectal cancer cells

As gastrin may play a role in the pathophysiology of gastrointestinal (GI) malignancies, the elucidation of the mechanisms governing gastrin-induced proliferation has recently gained considerable interest. Several studies have reported that a large percentage of colorectal tumours overexpress or stabilise the β-catenin oncoprotein. We thus sought to determine whether gastrin might regulate β-catenin expression in colorectal tumour cells. Amidated gastrin-17 (G-17), one of the major circulating forms of gastrin, not only enhanced β-catenin protein expression, but also one of its target genes, cyclin D1. Furthermore, activation of β-catenin-dependent transcription by gastrin was confirmed by an increase in LEF-1 reporter activity, as well as enhanced cyclin D1 promoter activity. Finally, G-17 prolonged the τ_1/2 of β-catenin protein, demonstrating that gastrin appears to exert its mitogenic effects on colorectal tumour cells, at least in part, by stabilising β-catenin.

Polyamine-modulated expression of c-myc plays a critical role in stimulation of normal intestinal epithelial cell proliferation

The nuclear protein c-Myc is a transcription factor involved in the control of cell cycle. Our previous studies indicated that cellular polyamines are absolutely required for cell proliferation in crypts of small intestinal mucosa and that polyamines have the ability to stimulate expression of the c-myc gene. The current study went further to determine whether induced nuclear c-Myc plays a role in stimulation of cell proliferation by polyamines in intestinal crypt cells (IEC-6 line). Exposure of normal quiescent cells after 24-h serum deprivation to 5% dialyzed fetal bovine serum (dFBS) increased both cellular polyamines and expression of the c-myc gene. Increased c-Myc protein formed heterodimers with its binding partner, Max, and specifically bound to the Myc/Max binding site, which was associated with an increase in DNA synthesis. Depletion of cellular polyamines by pretreatment with alpha-difluoromethylornithine (DFMO) prevented increases in c-myc expression and DNA synthesis induced by 5% dFBS. c-Myc gene transcription and cell proliferation decreased in polyamine-deficient cells, whereas the natural polyamine spermidine given together with DFMO maintained c-myc gene expression and cell growth at normal levels. Disruption of c-myc expression using specific c-myc antisense oligomers not only inhibited normal cell growth (without DFMO) but also prevented the restoration of cell proliferation by spermidine in polyamine-deficient cells. Ectopic expression of wild-type c-myc by recombinant adenoviral vector containing c-myc cDNA increased cell growth. These results indicate that polyamine-induced nuclear c-Myc interacts with Max, binds to the specific DNA sequence, and plays an important role in stimulation of normal intestinal epithelial cell proliferation.

Progastrin1-80 stimulates growth of intestinal epithelial cells in vitro via high-affinity binding sites

Proliferation and carcinogenesis of the large intestinal epithelial cells (IEC) cells is significantly increased in transgenic mice that overexpress the precursor progastrin (PG) peptide. It is not known if the in vivo growth effects of PG on IEC cells are mediated directly or indirectly. Full-length recombinant human PG (rhPG(1-80)) was generated to examine possible direct effects of PG on IEC cells. Surprisingly, rhPG (0.1-1.0 nM) was more effective than the completely processed gastrin 17 (G17) peptide as a growth factor. Even though IEC cells did not express CCK(1) and CCK(2) receptors (-R), fluorescently labeled G17 and Gly-extended G17 (G-Gly) were specifically bound to the cells, suggesting the presence of binding proteins other than CCK(1)-R and CCK(2)-R on IEC cells. High-affinity (K(d) = 0.5-1.0 nM) binding sites for (125)I-rhPG were discovered on IEC cells that demonstrated relative binding affinity for gastrin-like peptides in the order PG >or= COOH-terminally extended G17 >or= G-Gly > G17 > *CCK-8 (* significant difference; P < 0.05). In conclusion, our studies demonstrate for the first time direct growth effects of the full-length precursor peptide on IEC cells in vitro that are apparently mediated by the high-affinity PG binding sites that were discovered on these cells.

Effects of dietary 1,4-diaminobutane (putrescine) on eggshell quality and laying performance of hens laying thin-shelled eggs

Experiments were conducted to evaluate the potential for dietary 1,4-diaminobutane (putrescine) to influence eggshell quality and overall laying performance in hens. Forty-eight, 60-wk-old White Leghorn hens laying thin-shelled eggs were fed a corn and soybean meal-based diet supplemented with 0.00 (control), 0.05, 0.10, or 0.15% putrescine for 4 wk. Twelve hens that laid thick-shelled eggs were also fed the control diet. The feeding of supplemental putrescine decreased feed consumption; however, egg weight decreased only at higher levels of supplementation. Increasing dietary levels of putrescine responded quadratically in eggshell deformation, eggshell weight, and eggshell weight as a percentage of egg weight (P < 0.05). There were no significant differences in shell deformation, shell thickness, or shell weight when comparing hens laying thick-shelled eggs and those laying thin-shelled eggs that were fed 0.05% supplemental putrescine. Calcium intake, calcium retention, and calcium balance decreased linearly (P < 0.05) with increasing levels of dietary putrescine. Pancreatic putrescine concentrations were significantly higher (P < 0.05) in hens laying thick-shelled eggs compared with hens laying thin-shelled eggs. It appeared that pancreatic cells synthesized more polyamines in hens laying thick-shelled eggs. This increase in polyamines might have caused improved eggshell quality by increasing calcium transport. It was concluded that 0.05% supplemental putrescine improved eggshell quality; however, higher levels proved to be toxic.

Cloning and characterization of the mouse polyamine-modulated factor-1 (mPMF-1) gene: an alternatively spliced homologue of the human transcription factor

The natural polyamines and their analogues have been implicated in transcriptional regulation of specific genes. Human polyamine-modulated factor-1 (hPMF-1) was the first polyamine-responsive transcription factor identified. Here the mouse homologue of the hPMF-1 gene is described. Interestingly, the mouse gene (mPMF-1) codes for two alternatively spliced mRNAs. Both of the mouse splice variants, mPMF-1S and mPMF-1L, possess C-terminal coiled-coil domains nearly identical to that found in hPMF-1 and are highly homologous with the human protein. The C-terminal coiled-coil structure is necessary for transcriptional activation. However, the shorter protein, mPMF-1S, does not contain an N-terminal coiled-coil region as do both hPMF-1 and the longer mPMF-1L. mPMF-1L mRNA codes for a protein of 202 amino acids, 37 amino acids longer than the human protein. By contrast, mPMF-1S codes for only 133 amino acids, as a result of two exons being omitted compared with mPMF-1L. Both mouse transcription factors can interact with Nrf-2 (nuclear factor-E2-related factor 2), the normal partner of hPMF-1, substantiating the importance of the C-terminal coiled-coil region responsible for this interaction. Finally, the expression of mPMF-1 is induced when mouse M1 myeloid leukaemia cells are exposed to polyamine analogues, suggesting control similar to that observed for the hPMF-1.

Effects of dietary 1, 4-diaminobutane (putrescine) on eggshell quality and laying performance of older hens

We conducted an experiment to evaluate the potential for dietary 1,4-diaminobutane (putrescine) to promote eggshell quality and overall laying hen performance. A total of 128, 60-wk-old Barred Rock hens were fed a corn and soybean meal-based layer diet supplemented with 0.0 (control), 0.2, 0.4, and 0.6% free base 1,4-diaminobutane for 4 wk. The feeding of supplemental putrescine decreased feed consumption (P < or = 0.05) and egg mass (P < or = 0.05) and tended to decrease egg production (P < 0.08). Albumen quality was not significantly affected (P < 0.09) by the end of the experiment, as determined by Haugh units. Eggshell thickness was not significantly improved with lower levels of dietary putrescine (P < 0.08). Although dietary putrescine did not have any effect on the relative weights of duodenum, jejunum + ileum, or pancreas, there was a linear increase in putrescine concentrations in tissues (P < or = 0.05). Supplementation of dietary putrescine also resulted in increased putrescine and spermidine concentrations in egg (P < or = 0.05). Egg weight and eggshell deformation increased over time; however, eggshell weight, eggshell weight as percentage of egg weight, and eggshell thickness decreased (P < or = 0.05). It appeared that eggshell quality declined regardless of diet over the 4-wk experimental period. It was concluded that the lack of effect of dietary putrescine on egg parameters, with the exception of albumen quality and eggshell thickness, was due to putrescine toxicity. Hens transferred excess dietary putrescine and metabolites to eggs.

NF-kappaB activation and susceptibility to apoptosis after polyamine depletion in intestinal epithelial cells

The maintenance of intestinal mucosal integrity depends on a balance between cell renewal and cell death, including apoptosis. The natural polyamines, putrescine, spermidine, and spermine, are essential for mucosal growth, and decreasing polyamine levels cause G(1) phase growth arrest in intestinal epithelial (IEC-6) cells. The present study was done to determine changes in susceptibility of IEC-6 cells to apoptosis after depletion of cellular polyamines and to further elucidate the role of nuclear factor-kappaB (NF-kappaB) in this process. Although depletion of polyamines by alpha-difluoromethylornithine (DFMO) did not directly induce apoptosis, the susceptibility of polyamine-deficient cells to staurosporine (STS)-induced apoptosis increased significantly as measured by changes in morphological features and internucleosomal DNA fragmentation. In contrast, polyamine depletion by DFMO promoted resistance to apoptotic cell death induced by the combination of tumor necrosis factor-alpha (TNF-alpha) and cycloheximide. Depletion of cellular polyamines also increased the basal level of NF-kappaB proteins, induced NF-kappaB nuclear translocation, and activated the sequence-specific DNA binding activity. Inhibition of NF-kappaB binding activity by sulfasalazine or MG-132 not only prevented the increased susceptibility to STS-induced apoptosis but also blocked the resistance to cell death induced by TNF-alpha in combination with cycloheximide in polyamine-deficient cells. These results indicate that 1) polyamine depletion sensitizes intestinal epithelial cells to STS-induced apoptosis but promotes the resistance to TNF-alpha-induced cell death, 2) polyamine depletion induces NF-kappaB activation, and 3) disruption of NF-kappaB function is associated with altered susceptibility to apoptosis induced by STS or TNF-alpha. These findings suggest that increased NF-kappaB activity after polyamine depletion has a proapoptotic or antiapoptotic effect on intestinal epithelial cells determined by the nature of the death stimulus.

Characterization of the interaction between the transcription factors human polyamine modulated factor (PMF-1) and NF-E2-related factor 2 (Nrf-2) in the transcriptional regulation of the spermidine/spermine N1-acetyltransferase (SSAT) gene

Polyamines and polyamine analogues have been demonstrated to modulate the transcription of various genes. Spermidine/spermine N(1)-acetyltransferase (SSAT) is transcriptionally regulated through the interaction of at least two trans-acting transcription factors, NF-E2-related factor 2 (Nrf-2) and PMF-1 (polyamine modulated factor-1). Nrf-2 has previously been shown to regulate transcription of other genes through interactions between its C-terminal leucine zipper and the leucine-zipper region of other members of the small Maf protein family (the term "Maf" is derived from MusculoAponeurotic-Fibrosarcoma virus). Here it is demonstrated that the interaction between Nrf-2 and PMF-1 is mediated through the binding of the leucine-zipper region of Nrf-2 and a C-terminal coiled-coil region of PMF-1 that does not contain a leucine zipper. Mutations that interrupt either the leucine zipper of Nrf-2 or the coiled-coil region of PMF-1 are demonstrated to alter the ability of these factors to interact, thus their ability to regulate the transcription of the SSAT gene is lost.

Role of autocrine and endocrine gastrin-like peptides in colonic carcinogenesis

Colon carcinogenesis is a multistep process that involves deletions, mutations, and changes in expression of genes that regulate growth, differentiation, and apoptosis. Hyperproliferation can initiate dysplastic growth, resulting in accumulation of genetic defects and progression of colon cancer. Although genetic instability, because of inheritance of specific genetic defects, plays a dominant role in familial cancers, in the majority of sporadic cancers hyperproliferation is likely to play a permissive role in initiation and progression of the disease. Thus factors that regulate growth, differentiation, and apoptosis are likely to play an important role in colon carcinogenesis. Autocrine gastrins, insulin-like growth factor-II, transforming growth factor-alpha, and endocrine gastrins have been implicated in the tumorigenic potential of colon cancer cells. In this article we focus on the role of endocrine and autocrine gastrins in colon cancer and review recent advances that suggest a role of processing intermediates of gastrin in colon carcinogenesis.

Regulatory effect and mechanism of gastrin and its antagonists on colorectal carcinoma

AIM: To explore the effect and mechanism of gastrin and its an tagonists proglumide and somatostatin on colorectal carcinoma and their clinical significance.

METHODS: A model of transplanted human colonic carcinoma was established from SW480 cell line in gymnomouse body. The volume and weight of transplanted carcinoma was observed under the effect of pentagatrin (PG), proglumide (PGL) and octapeptide somotostatin (SMS201-995, SMS). The cAMP content of carcinoma cell was determined by radioimmunoassay and the DNA, protein content and cell cycle were determined by flow-cytometry. The amount of viable cells was determined by MTT colorimetric analysis, IP₃ content was determined by radioimmuno assay, Ca²⁺ concentration in cell by fluorometry and PKC activity by isotopic enzymolysis. The expression of gastrin, c-myc, c-fos and rasP21 in 48 case s of colorectal carcinoma tissue was detected by the immuno-cytochemistry SP method. Argyrophilia nucleolar organizer regions was determined with argyrophilia stain.

RESULTS: The volume, weight, cAMP, DNA and protein content in carcinoma cell, cell amount and proliferation index of S and G₂M phase in PG group were all significantly higher than those of control group. When PG was at the concentration of 25 mg/L, the amount of viable cells, IP₃ content and Ca²⁺ concentration in cell and membrane PKC activity in PG group were significantly higher than those in control group; when PGL was at a concentration of 32 mg/L, they dropped to the lowest level in PG (25 mg/L) + PGL group, but without significant difference from the control group. The positive expression rate of gastrin, c-myc, c-fos and rasP21 in carcinoma tissue was 39.6%, 54.2%, 47.9% and 54.2% respectively and significantly higher than that in mucosa 3 cm and 6 cm adjacent to carcinoma tissue and normal colorectal mucosa. The positive expression rate of gastrin of highly-differentiated adenocarcinoma group was significantly higher than that of poorly-differentiated and mucinous adenoc arcinoma groups. The AgNORs count of carcinoma tissue was significantly higher than that in mucosa 3 cm and 6 cm adjacent to carcinoma tissue and norm al colorectal mucosa; and the positive expression of c-myc and c-fos and the A gNORs count in gastrin-positive group was significantly higher than those in gastrin-negative group.

CONCLUSION: Pentagastrin has a promoting effect on the growth of transplanted human colonic carcinoma from SW480 cell line. PGL has no obvious effect on the growth of human colonic carcinoma SW480 cell line, but could inhibit the growth promoting effect of PG on transplanted carcinoma. Somatostatin can not only inhibit the growth of transplanted human colonic carcinoma from SW480 cell line directly but also depress the growth-promoting effect of gastrin on the transplanted carcinoma. Some colorectal carcinoma cells can produce and secrete gastrin through autocrine, highly-differentiated adenocarcinoma express the highest level gastrin. Endogenous gastrin can stimulate the cell division and proliferation of carcinoma cell and promote the growth of colorectal carcinoma regulating the expression of oncogene c-myc, c-fos. Our study has provided experimental basis for the adjuvant treatment using gastrin antagonist such as PGL, so matostatin of patients with colorectal carcinoma.

Inhibition of polyamine synthesis induces p53 gene expression but not apoptosis

The nuclear phosphoprotein p53 acts as a transcription factor and is involved in growth inhibition and apoptosis. The present study was designed to examine the effect of decreasing cellular polyamines on p53 gene expression and apoptosis in small intestinal epithelial (IEC-6) cells. Cells were grown in DMEM containing 5% dialyzed fetal bovine serum in the presence or absence of alpha-difluoromethylornithine (DFMO), a specific inhibitor of polyamine biosynthesis, for 4, 6, and 12 days. The cellular polyamines putrescine, spermidine, and spermine in DFMO-treated cells decreased dramatically at 4 days and remained depleted thereafter. Polyamine depletion by DFMO was accompanied by a significant increase in expression of the p53 gene. The p53 mRNA levels increased 4 days after exposure to DFMO, and the maximum increases occurred at 6 and 12 days after exposure. Increased levels of p53 mRNA in DFMO-treated cells were paralleled by increases in p53 protein. Polyamines given together with DFMO completely prevented increased expression of the p53 gene. Increased expression of the p53 gene in DFMO-treated cells was associated with a significant increase in G1 phase growth arrest. In contrast, no features of programmmed cell death were identified after polyamine depletion: no internucleosomal DNA fragmentation was observed, and no morphological features of apoptosis were evident in cells exposed to DFMO for 4, 6, and 12 days. These results indicate that 1) decreasing cellular polyamines increases expression of the p53 gene and 2) activation of p53 gene expression after polyamine depletion does not induce apoptosis in intestinal crypt cells. These findings suggest that increased expression of the p53 gene may play an important role in growth inhibition caused by polyamine depletion.

Epidermal growth factor-stimulated rabbit oesophageal mucosal growth: Role of polyamines

Factors operative in oesophageal mucosal growth are poorly characterized. Epidermal growth factor (EGF) has been shown to stimulate mucosal growth throughout the gastrointestinal tract. The present study was performed to test the hypothesis that polyamine biosynthesis is required for stimulation of epithelial growth by EGF in oesophageal mucosa. Using an in vitro explant model, oesophageal mucosal growth was quantified in the presence or absence of EGF and α-difluoromethylornithine (DFMO), a specific inhibitor of polyamine synthesis. Administration of 50 nmol/L EGF significantly increased the rate of epithelial growth in oesophageal explants. Treatment with DFMO for 3 and 7 days not only depleted the tissue polyamines putrescine, spermidine and spermine, but also significantly impaired mucosal growth. The inhibitory effect of DFMO on mucosal growth was partially but significantly prevented when exogenous putrescine was given. Additionally, stimulation of epithelial growth by EGF was also blocked by depletion of cellular polyamines in DFMO-treated oesophageal explants. These results indicate that EGF stimulates epithelial growth of oesophageal mucosa at least partly through a process involving polyamine biosynthesis.

Effects of polyamine levels on the degradation of short-lived and long-lived proteins in cultured L-132 human lung cells

Biogenic polyamines have important regulatory functions in various biological processes and it has also been suggested that they could modulate intracellular protein degradation. For an overall assessment of the role of polyamines in this process, we have investigated the effect that the decrease in intracellular polyamine levels caused by inhibitors of polyamine biosynthesis brings about on the degradation of the pools of short- and long-lived proteins in cultured L-132 human lung cells. Treatment of cells with 100 microM (2R,5R)-delta-methyl acetylenic putrescine (MAP), a potent enzyme-activated irreversible inhibitor of ornithine decarboxylase, or with 100 microM MAP plus 50 microM N-butyl 1,3-diaminopropane, a specific inhibitor of spermine synthase, caused a similar decrease (65-70% of control) in the total intracellular levels of polyamines, although they affected the concentrations of spermidine and spermine differently. The effect of the two treatments on protein degradation was essentially the same. In polyamine-depleted cells we observed an inhibition of degradation in long-lived proteins of 16% (P<0.05), with a significant increase in the half-life (t12) of this pool from 100.5 to 120.1 h. This was concomitant with an increase of 26% (P<0. 05) in degradation in short-lived proteins, with a significant decrease in the t12 of this pool from 0.85 to 0.67 h. Recovery of polyamine levels by the addition of 50 microM spermidine to polyamine-depleted cells resulted in a restoration of the degradation rates in both pools of proteins. The way(s) by which polyamines could modulate proteolysis are discussed.

Expression of the transforming growth factor-beta gene during growth inhibition following polyamine depletion

Polyamine depletion and cytokine transforming growth factor-beta (TGF-beta) inhibit cell proliferation. The current study tests the hypothesis that polyamine depletion results in growth inhibition by altering expression of the TGF-beta gene in intestinal epithelial cells. Studies were conducted in the IEC-6 cell line derived from rat small intestinal crypt cells. Cells were grown in DMEM in the presence or absence of alpha-difluoromethylornithine (DFMO), a specific inhibitor of polyamine biosynthesis, for 6 and 12 days. Administration of DFMO not only depleted intracellular polyamines but also significantly increased the mRNA levels of TGF-beta. Increased TGF-beta mRNA in DFMO-treated cells was paralleled by an increase in TGF-beta content. Depletion of intracellular polyamines by DFMO had no effect on the rate of TGF-beta gene transcription, as measured by nuclear run-on assay. The half-life of mRNA for TGF-beta in normal cells was approximately 65 min and increased to >16 h in cells treated with DFMO for 6 or 12 days. Exogenous polyamine, when given together with DFMO, prevented the increased half-life of TGF-beta mRNA in IEC-6 cells. TGF-beta added to the culture medium significantly decreased the rate of DNA synthesis and final cell number in normal and polyamine-deficient cells. Furthermore, growth inhibition caused by polyamine depletion was partially but significantly blocked by addition of immunoneutralizing anti-TGF-beta antibody. These results indicate that 1) depletion of intracellular polyamines induces the activation of the TGF-beta gene through posttranscriptional regulation and 2) increased expression of the TGF-beta gene plays an important role in the process of growth inhibition following polyamine depletion.

Pharmacological evidence for the contribution of polyamines as mediators of the transcriptional component involved in smooth muscle relaxation elicited by forskolin

To study whether cAMP-dependent transcriptional effect and polyamines might play a modulatory role on smooth muscle, the effect of forskolin on KCl (60 mM)-induced contractions in isolated rat uterus and its modification by inhibitors of cAMP-dependent protein kinase (PKA) (Rp-cAMPS and TPCK), transcription (actinomycin D), protein synthesis (cycloheximide) and ornithine decarboxylase (alpha-difluoromethyl-ornithine, DFMO), and a polyamine (spermine) have been assayed. Forskolin (0.1 to 6 microM) induced concentration-dependent relaxation on KCl-induced tonic contractions in rat uterus (IC50: 0.55 +/- 0.12 microM) which was antagonized (p<0.05) by Rp-cAMPS (30 microM), TPCK (3 microM), cycloheximide (300 microM), actinomycin D (4 and 12 microM) and TPCK (3 microM) plus actinomycin D (12 microM). The IC50 values of forskolin in the presence of these drugs were 3.75 +/- 1.53 microM, 12.08 +/- 8.18 microM, 6.88 +/- 5.02 microM, 3.80 +/- 2.35 and 5.31 +/- 2.80 microM, and 4.26 +/- 3.65 microM respectively. Furthermore, DFMO (10 mM) also shifted the relaxation curve to forskolin to the right (IC50: 3.06 +/- 2.66 microM, p<0.05) but DFMO (10 mM) plus actinomycin D (12 microM) (IC50: 1.78 +/- 1.33 microM) did not. However, DFMO (10 mM) and actinomycin D (12 microM) did not antagonize the spermine (1-30 mM)-elicited relaxation (IC50s: 7.8 +/- 0.7 mM vs 7.28 +/- 1.4 mM and 4.67 +/- 0.44 mM in the presence of DFMO and actinomycin D, respectively). Moreover, spermine (1 mM) did not decrease the forskolin induced relaxation and counteracted the antagonism produced by actinomycin D and DFMO. Our results suggest that, in rat uterus, forskolin: a) produced cAMP-dependent relaxation, as this is antagonized by Rp-cAMP and TPCK, and b) increased the activity of ornithine decarboxylase, as this is inhibited by DFMO. Therefore, polyamines could be the mediator of the cAMP-dependent transcriptional component involved in forskolin relaxation, since, as mentioned, DFMO antagonized this relaxation and spermine counteracted the displacement produced by DFMO and actinomycin D. Thus, a plasma membrane-nucleus interaction might, at least partially, explain the mechanisms involved in forskolin induced relaxation in smooth muscle of rat uterus under the present experimental conditions.

Specific requirement of putrescine for the mitogenic action of juvenile hormone on adult insect neuroblasts

Persistent neurogenesis in an adult insect brain was recently shown to be stimulated by juvenile hormone (JH). This morphogenetic hormone was also shown to act on polyamine biosynthesis. To analyze the possible involvement of polyamines in the neurogenic action of JH, two series of experiments were carried out with adult female crickets, Acheta domesticus: (i) inhibition of the first key enzyme in polyamine biosynthesis, ornithine decarboxylase, with alpha-difluoromethylornithine (alpha-DFMO), and examination of the effects of this treatment on the neuroblast proliferation response to JH; and (ii) examination of the effects of putrescine supplementation on the mitotic index of JH-deprived and alpha-DFMO-treated females. In control females, alpha-DFMO treatment, as well as JH deprivation, greatly reduced neuroblast proliferation. Putrescine supplementation in alpha-DFMO-treated insects overcame the effects of alpha-DFMO, and allowed for detection of putrescine in the neural tissue and stimulation of brain neurogenesis. In JH-deprived females, alpha-DFMO treatment completely prevented the stimulatory action of JH on neuroblast proliferation and on brain putrescine levels. By contrast, putrescine feeding of JH-deprived animals was able to mimic the stimulatory effect of JH: brain putrescine levels increased and neuroblast proliferation was restored. To our knowledge, this report demonstrates for the first time that in vivo administration of putrescine can mimic the effects of a morphogenetic hormone on adult neuroblast proliferation, and shows the importance of polyamines, especially putrescine, in the transduction of JH message in neural tissue.