Deoxycholic acid induces intracellular signaling through membrane perturbations

Comprehensive cytotoxic evaluation of morin, a bioflavonoid against verapamil on rat gastrointestinal epithelium for novel pharmaceutical application involving P-glycoprotein inhibition

OBJECTIVE

In this study, a comprehensive and comparative cytotoxic evaluation of morin against verapamil on rat intestinal epithelium as P-gp inhibitors through in-vitro gastrointestinal short-term toxicity assays involving permeability studies for safety evaluation was investigated.

METHODS

In this study, the effect of morin (1 mM or 10 mM) or verapamil (1 mM or 10 mM) or sodium deoxycholate (10 mM) was investigated on intestinal epithelium and isolated brush border membrane using biomarker assays. Cytotoxicity was determined using 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT) assay. The nutrients transport was assessed using everted sacs studies. Paracellular permeability was measured using Lucifer yellow, followed by morphometric analysis of intestinal sacs.

KEY FINDINGS

Our results indicated that morin was effective in maintaining cell viability with no significant changes (P > 0.05) in the activity of intestinal brush border markers, membrane integrity and morphometric analysis as compared with control. On the contrary, dramatic (P < 0.01) changes were noticed in the release of membrane markers, cell viability and surface characteristics of intestinal segments when treated with verapamil or sodium deoxycholate as compared with control or morin.

CONCLUSIONS

Our findings confirm that morin is non-toxic to rat intestinal epithelium against verapamil demonstrating the potential use of bioflavonoid as safe and novel pharmaceutical adjuvant as P-gp inhibitor.

Bile acid signaling through farnesoid X and TGR5 receptors in hepatobiliary and intestinal diseases

BACKGROUND

The well-known functions of bile acids (BAs) are the emulsification and absorption of lipophilic xenobiotics. However, the emerging evidences in the past decade showed that BAs act as signaling molecules that not only autoregulate their own metabolism and enterohepatic recirculation, but also as important regulators of integrative metabolism by activating nuclear and membrane-bound G protein-coupled receptors. The present review was to get insight into the role of maintenance of BA homeostasis and BA signaling pathways in development and management of hepatobiliary and intestinal diseases.

DATA SOURCES

Detailed and comprehensive search of PubMed and Scopus databases was carried out for original and review articles.

RESULTS

Disturbances in BA homeostasis contribute to the development of several hepatobiliary and intestinal disorders, such as non-alcoholic fatty liver disease, liver cirrhosis, cholesterol gallstone disease, intestinal diseases and both hepatocellular and colorectal carcinoma.

CONCLUSION

Further efforts made in order to advance the understanding of sophisticated BA signaling network may be promising in developing novel therapeutic strategies related not only to hepatobiliary and gastrointestinal but also systemic diseases.

Interactions of flavan-3-ols and procyanidins with membranes: mechanisms and the physiological relevance

Flavonoids are a type of phenolic compound widely present in edible plants. A great number of health benefits have been ascribed to flavonoid consumption in the human population. However, the molecular mechanisms involved in such effects remain to be identified. The flavan-3-ols (-)-epicatechin and (+)-catechin, and their related oligomers (procyanidins) have been thoroughly studied because of their capacity to interact with cell membranes. Starting with these interactions, procyanidins could modulate multiple biochemical processes, such as enzyme activities, receptor-ligand binding, membrane-initiated cell signaling, and molecule transport across membranes. This review focuses on molecular aspects of procyanidin interactions with membrane lipid components, and the resulting protection of the membranes against mechanical and/or oxidative damage, resulting in the maintenance of cell functions.

Differential regulation of EGFR-MAPK signaling by deoxycholic acid (DCA) and ursodeoxycholic acid (UDCA) in colon cancer

A high-fat diet coincides with increased levels of bile acids. This increase in bile acids, particularly deoxycholic acid (DCA), has been strongly associated with the development of colon cancer. Conversely, ursodeoxycholic acid (UDCA) may have chemopreventive properties. Although structurally similar, DCA and UDCA present different biological and pathological effects in colon cancer progression. The differential regulation of cancer by these two bile acids is not yet fully understood. However, one possible explanation for their diverging effects is their ability to differentially regulate signaling pathways involved in the multistep progression of colon cancer, such as the epidermal growth factor receptor (EGFR)-mitogen-activated protein kinase (MAPK) pathway. This review will examine the biological effects of DCA and UDCA on colon cancer development, as well as the diverging effects of these bile acids on the oncogenic signaling pathways that play a role in colon cancer development, with a particular emphasis on bile acid regulation of the EGFR-MAPK pathway.

Deoxycholic acid induced changes in electrophysiological parameters and macromolecular permeability in murine small intestine with and without functional enteric nervous system plexuses

BACKGROUND

We have previously shown in mice that the fecal proportion and concentration of the hydrophobic bile acid deoxycholic acid (DCA) is elevated with high-fat feeding and that these changes are able to disrupt the intestinal barrier function. The aim of this study was to investigate whether these changes are mediated by the enteric nervous system (ENS).

METHODS

The function of the ENS in the small intestinal tissues of mice was compromised by two different methods: by removing the seromuscular layer and by incubating the intact tissues with tetrodotoxin (TTX), a neural conduction blocker, before DCA treatment. Tissues with or without functional plexuses were mounted into a Ussing chamber system and treated with 3 mM DCA for 20 min. After DCA treatment, the intestinal permeability to fluorescein was assessed. Short-circuit current (Isc ) and transepithelial resistance (TER) were recorded throughout the experiment.

KEY RESULTS

DCA increased intestinal fluorescein permeability only in tissues where the seromuscular layer was removed. In tissues with intact seromuscular layer, DCA induced a significant increase in TER, which was attenuated by blocking of the neural function by TTX.

CONCLUSIONS & INFERENCES

The results of this study suggest that the DCA-induced increase observed in fluorescein permeability is not mediated through neural pathways, but more due to a direct effect on the epithelium. However, as TTX was able to attenuate the DCA-induced increase in TER, it can be speculated that DCA is also able to elicit responses through neural pathways.

Safety evaluation of naringenin upon experimental exposure on rat gastrointestinal epithelium for novel optimal drug delivery

OBJECTIVE

To assess the effect of naringenin on the intestinal biochemical composition, function and histology for gastrointestinal toxicity since it has not yet been adequately exploited for safety through standard assays.

METHODS

Here, we describe naringenin (1 mM, 10 mM and 100 mM, respectively) or sodium deoxycholate (10 mM) effects on isolated brush border membrane from intestinal segments with single pass intestinal perfusion using lactate dehydrogenase, alkaline phosphatase and protein assays. MTT assay was used for cytotoxicity studies. Everted gut sac studies were used for evaluating the transport of nutrients across the intestinal segments. Lucifer yellow was used for paracellular permeability, followed by histological changes and surface characteristic studies of intestinal sacs.

RESULTS

The results indicated no significant alterations with naringenin, although significant (p < 0.01) changes were noticed with sodium deoxycholate in the activity of the rat intestinal brush border associated enzymes such as LDH, followed by intact cell viability with marked decrease in the villi height of the intestinal segments.

CONCLUSIONS

These observations indicate that naringenin was harmless upon exposure to rat gastrointestinal epithelium, clearly demonstrating the potential use of naturally occurring bioflavonoid as safe and novel pharmaceutical adjuvant in oral dosage forms as P-gp inhibitor.

Secondary bile acids: an underrecognized cause of colon cancer

Bile acids were first proposed as carcinogens in 1939. Since then, accumulated evidence has linked exposure of cells of the gastrointestinal tract to repeated high physiologic levels of bile acids as an important risk factor for gastrointestinal cancers. High exposure to bile acids may occur in a number of settings, but most importantly, is prevalent among individuals who have a high dietary fat intake. A rapid effect on cells of high bile acid exposure is the generation of reactive oxygen species and reactive nitrogen species, disruption of the cell membrane and mitochondria, induction of DNA damage, mutation and apoptosis, and development of reduced apoptosis capability upon chronic exposure. Here, we review the substantial evidence of the mechanism of secondary bile acids and their role in colon cancer.

Linkages between mitochondrial lipids and life history in temperate and tropical birds

Temperate birds tend to have a fast pace of life and short life spans with high reproductive output, whereas tropical birds tend to have a slower pace of life, invest fewer resources in reproduction, and have higher adult survival rates. How these differences in life history at the organismal level are rooted in differences at the cellular level is a major focus of current research. Here, we cultured fibroblasts from phylogenetically paired tropical and temperate species, isolated mitochondria from each, and compared their mitochondrial membrane lipids. We also correlated the amounts of these lipids with an important life history parameter, clutch size. We found that tropical birds tended to have less mitochondrial lipid per cell, especially less cardiolipin per cell, suggesting that cells from tropical birds have fewer mitochondria or less inner mitochondrial membrane per cell. We also found that the mitochondria of tropical birds and the species with the smallest clutch sizes had higher amounts of plasmalogens, a lipid that could serve as an antioxidant. Overall, our findings are consistent with the idea that there are underlying molecular and cellular physiological traits that could account for the differences in whole-animal physiology between animals with different life histories.

Bile acids modulate signaling by functional perturbation of plasma membrane domains

Eukaryotic cell membranes are organized into functional lipid and protein domains, the most widely studied being membrane rafts. Although rafts have been associated with numerous plasma membrane functions, the mechanisms by which these domains themselves are regulated remain undefined. Bile acids (BAs), whose primary function is the solubilization of dietary lipids for digestion and absorption, can affect cells by interacting directly with membranes. To investigate whether these interactions affected domain organization in biological membranes, we assayed the effects of BAs on biomimetic synthetic liposomes, isolated plasma membranes, and live cells. At cytotoxic concentrations, BAs dissolved synthetic and cell-derived membranes and disrupted live cell plasma membranes, implicating plasma membrane damage as the mechanism for BA cellular toxicity. At subtoxic concentrations, BAs dramatically stabilized domain separation in Giant Plasma Membrane Vesicles without affecting protein partitioning between coexisting domains. Domain stabilization was the result of BA binding to and disordering the nonraft domain, thus promoting separation by enhancing domain immiscibility. Consistent with the physical changes observed in synthetic and isolated biological membranes, BAs reorganized intact cell membranes, as evaluated by the spatial distribution of membrane-anchored Ras isoforms. Nanoclustering of K-Ras, related to nonraft membrane domains, was enhanced in intact plasma membranes, whereas the organization of H-Ras was unaffected. BA-induced changes in Ras lateral segregation potentiated EGF-induced signaling through MAPK, confirming the ability of BAs to influence cell signal transduction by altering the physical properties of the plasma membrane. These observations suggest general, membrane-mediated mechanisms by which biological amphiphiles can produce their cellular effects.

Changes in cultured dermal fibroblasts during early passages across five wild bird species

With the advent of the usage of primary fibroblasts in comparative and evolutionary biology, it is important for researchers to know the extent to which cells might be altered during the culturing process and how much species might differ in response to cell culture. We compared early changes in cell size and lipid composition of primary dermal fibroblasts grown at physiologically relevant oxygen concentrations (5% O2) from wild-caught species of birds. Fibroblasts from American Robins (Turdus migratorius L., 1766) and Bobwhite Quails (Colinus virginianus (L., 1758)) increased in size early in the culture process and cells from all five species of wild-caught birds exhibited changes in lipid-class composition. The two most common phospholipids, phosphatidylcholine and phosphatidylethanolamine, increased in concentration in all species between early passages and later passages of fibroblasts. Some less abundant lipid species, such as cardiolipin and sphingomyelin, exhibited similar concentrations in all three passages that we measured. Other lipid classes, such as cholesterol, increased in some species in later passages and decreased in others. Although results may vary with cell-culture conditions, this study points to a need for researchers comparing multiple species to take precautions when using cell culture, such as experimenting on the earliest possible passage of cells.

Resistance to cisplatin-induced cell death conferred by the activity of organic anion transporting polypeptides (OATP) in human melanoma cells

Expression of organic anion transporting polypeptides (OATP) transporters can be modified with potential incidence in cancers, yet they have not been considered in melanoma. Here, we demonstrate transcriptional and protein expression of OATP members in human melanoma cell lines with sodium-independent organic anion uptake activity. Importantly, uptake of different organic anions over 24 h led to a common resistance signal to apoptotic cell death, induced further by cisplatin in 24 h. The mechanism is not dependent on the transport of cisplatin by the OATP, as it is not an OATP substrate. The resistance signal was modulated by PKC, disclosing it as signal mediator. This study suggests that OATP, which can be constantly activated by endobiotics, may contribute to melanoma chemotherapeutic resistance, thereby justifying the development of OATP targeting strategies.

Anticancer steroids: linking natural and semi-synthetic compounds

Steroids, a widespread class of natural organic compounds occurring in animals, plants and fungi, have shown great therapeutic value for a broad array of pathologies. The present overview is focused on the anticancer activity of steroids, which is very representative of a rich structural molecular diversity and ability to interact with various biological targets and pathways. This review encompasses the most relevant discoveries on steroid anticancer drugs and leads through the last decade and comprises 668 references.

Bile acids in the colon, from healthy to cytotoxic molecules

Bile acids are natural detergents mainly involved in facilitating the absorption of dietary fat in the intestine. In addition to this absorptive function, bile acids are also essential in the maintenance of the intestinal epithelium homeostasis. To accomplish this regulatory function, bile acids may induce programmed cell death fostering the renewal of the epithelium. Here we first discuss on the different molecular pathways of cell death focusing on apoptosis in colon epithelial cells. Bile acids may induce apoptosis in colonocytes through different mechanisms. In contrast to hepatocytes, the extrinsic apoptotic pathway seems to have a low relevance regarding bile acid cytotoxicity in the colon. On the contrary, these molecules mainly trigger apoptosis through direct or indirect mitochondrial perturbations, where oxidative stress plays a key role. In addition, bile acids may also act as regulatory molecules involved in different cell signaling pathways in colon cells. On the other hand, there is increasing evidence that the continuous exposure to certain hydrophobic bile acids, due to a fat-rich diet or pathological conditions, may induce oxidative DNA damage that, in turn, may lead to colorectal carcinogenesis as a consequence of the appearance of cell populations resistant to bile acid-induced apoptosis. Finally, some bile acids, such as UDCA, or low concentrations of hydrophobic bile acids, can protect colon cells against apoptosis induced by high concentrations of cytotoxic bile acids, suggesting a dual behavior of these agents as pro-death or pro-survival molecules.

Bile Acid Receptors and Liver Cancer

Liver cancer, particularly hepatocellular carcinoma (HCC), is the third leading cause of cancer death in the world. Bile acids (BAs) are liver-produced amphipathic molecules that are required to facilitate the absorption of cholesterol, fat-soluble vitamins, and lipids in the intestine. However, BAs are also known to act as potential carcinogens and deregulation of BA homeostasis has been linked to HCC formation. Two key BA receptors, farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (TGR5), were recently identified, which provides great insights into BAs' normal physiological functions as well as their carcinogenic effects. In this review, we focus on the potential links among BAs, two BA receptors, and HCC. FXR and TGR5 not only play key roles in regulating BA homeostasis but also are essential in suppressing BAs' carcinogenic effects on liver cancer.

Nanosecond pulsed electric fields act as a novel cellular stress that induces translational suppression accompanied by eIF2α phosphorylation and 4E-BP1 dephosphorylation

Recent advances in electrical engineering enable the generation of ultrashort electric fields, namely nanosecond pulsed electric fields (nsPEFs). Contrary to conventional electric fields used for DNA electroporation, nsPEFs can directly reach intracellular components without membrane destruction. Although nsPEFs are now recognized as a unique tool in life sciences, the molecular mechanism of nsPEF action remains largely unclear. Here, we present evidence that nsPEFs act as a novel cellular stress. Exposure of HeLa S3 cells to nsPEFs quickly induced phosphorylation of eIF2α, activation of its upstream stress-responsive kinases, PERK and GCN2, and translational suppression. Experiments using PERK- and GCN2-knockout cells demonstrated dual contribution of PERK and GCN2 to nsPEF-induced eIF2α phosphorylation. Moreover, nsPEF exposure yielded the elevated GADD34 expression, which is known to downregulate the phosphorylated eIF2α. In addition, nsPEF exposure caused a rapid decrease in 4E-BP1 phosphorylation irrespective of the PERK/GCN2 status, suggesting participation of both eIF2α and 4E-BP1 in nsPEF-induced translational suppression. RT-PCR analysis of stress-inducible genes demonstrated that cellular responses to nsPEFs are distinct from those induced by previously known forms of cellular stress. These results provide new mechanistic insights into nsPEF action and implicate the therapeutic potential of nsPEFs for stress response-associated diseases.

Protective effects of nonionic triblock copolymers on bile acid-mediated epithelial barrier disruption

Translocation of bacteria and other luminal factors from the intestine following surgical injury can be a major driver of critical illness. Bile acids have been shown to play a key role in the loss of intestinal epithelial barrier function during states of host stress. Experiments to study the ability of nonionic block copolymers to abrogate barrier failure in response to bile acid exposure are described. In vitro experiments were performed with the bile salt sodium deoxycholate on Caco-2 enterocyte monolayers using transepithelial electrical resistance to assay barrier function. A bisphenol A coupled triblock polyethylene glycol (PEG), PEG 15-20, was shown to prevent sodium deoxycholate-induced barrier failure. Enzyme-linked immunosorbent assay, lactate dehydrogenase, and caspase 3-based cell death detection assays demonstrated that bile acid-induced apoptosis and necrosis were prevented with PEG 15-20. Immunofluorescence microscopic visualization of the tight junctional protein zonula occludens 1 (ZO-1) demonstrated that PEG 15-20 prevented significant changes in tight junction organization induced by bile acid exposure. Preliminary transepithelial electrical resistance-based studies examining structure-function correlates of polymer protection against bile acid damage were performed with a small library of PEG-based copolymers. Polymer properties associated with optimal protection against bile acid-induced barrier disruption were PEG-based compounds with a molecular weight greater than 10 kd and amphiphilicity. The data demonstrate that PEG-based copolymer architecture is an important determinant that confers protection against bile acid injury of intestinal epithelia.

Sodium deoxycholate inhibits chick duodenal calcium absorption through oxidative stress and apoptosis

High concentrations of sodium deoxycholate (NaDOC) produce toxic effects. This study explores the effect of a single high concentration of NaDOC on the intestinal Ca(2+) absorption and the underlying mechanisms. Chicks were divided into two groups: 1) controls and 2) treated with different concentrations of NaDOC in the duodenal loop for variable times. Intestinal Ca(2+) absorption was measured as well as the gene and protein expressions of molecules involved in the Ca(2+) transcellular pathway. NaDOC inhibited the intestinal Ca(2+) absorption, which was concentration dependent. Ca(2+)-ATPase mRNA decreased by the bile salt and the same occurred with the protein expression of Ca(2+)-ATPase, calbindin D(28k) and Na(+)/Ca(2+) exchanger. NaDOC produced oxidative stress as judged by ROS generation, mitochondrial swelling and glutathione depletion. Furthermore, the antioxidant quercetin blocked the inhibitory effect of NaDOC on the intestinal Ca(2+) absorption. Apoptosis was also triggered by the bile salt, as indicated by the TUNEL staining and the cytochrome c release from the mitochondria. As a compensatory mechanism, enzyme activities of the antioxidant system were all increased. In conclusion, a single high concentration of NaDOC inhibits intestinal Ca(2+) absorption through downregulation of proteins involved in the transcellular pathway, as a consequence of oxidative stress and mitochondria mediated apoptosis.

Bile acids down-regulate caveolin-1 in esophageal epithelial cells through sterol responsive element-binding protein

Bile acids are synthesized from cholesterol and are major risk factors for Barrett adenocarcinoma (BAC) of the esophagus. Caveolin-1 (Cav1), a scaffold protein of membrane caveolae, is transcriptionally regulated by cholesterol via sterol-responsive element-binding protein-1 (SREBP1). Cav1 protects squamous epithelia by controlling cell growth and stabilizing cell junctions and matrix adhesion. Cav1 is frequently down-regulated in human cancers; however, the molecular mechanisms that lead to this event are unknown. We show that the basal layer of the nonneoplastic human esophageal squamous epithelium expressed Cav1 mainly at intercellular junctions. In contrast, Cav1 was lost in 95% of tissue specimens from BAC patients (n = 100). A strong cytoplasmic expression of Cav1 correlated with poor survival in a small subgroup (n = 5) of BAC patients, and stable expression of an oncogenic Cav1 variant (Cav1-P132L) in the human BAC cell line OE19 promoted proliferation. Cav1 was also detectable in immortalized human squamous epithelial, Barrett esophagus (CPC), and squamous cell carcinoma cells (OE21), but was low in BAC cell lines (OE19, OE33). Mechanistically, bile acids down-regulated Cav1 expression by inhibition of the proteolytic cleavage of 125-kDa pre-SREBP1 from the endoplasmic reticulum/Golgi apparatus and nuclear translocation of active 68-kDa SREBP1. This block in SREBP1's posttranslational processing impaired transcriptional activation of SREBP1 response elements in the proximal human Cav1 promoter. Cav1 was also down-regulated in esophagi from C57BL/6 mice on a diet enriched with 1% (wt/wt) chenodeoxycholic acid. Mice deficient for Cav1 or the nuclear bile acid receptor farnesoid X receptor showed hyperplasia and hyperkeratosis of the basal cell layer of esophageal epithelia, respectively. These data indicate that bile acid-mediated down-regulation of Cav1 marks early changes in the squamous epithelium, which may contribute to onset of Barrett esophagus metaplasia and progression to BAC.

Electroporation enhances the ability of lactobacilli to remove cholesterol

The objective of the present study was to evaluate the effect of electroporation on the membrane properties of lactobacilli and their ability to remove cholesterol in vitro. The growth of lactobacilli cells treated at 7.5 kV/cm for 4 ms was increased by 0.89 to 1.96 log(10) cfu/mL upon fermentation at 37 °C for 20 h, the increase being attributed to the reversible and transient formation of pores and defragmentation of clumped cells. In addition, an increase of cholesterol assimilation as high as 127.2% was observed for most cells electroporated at a field strength of 7.5 kV/cm for 3.5 ms compared with a lower field strength of 2.5 kV/cm. Electroporation also increased the incorporation of cholesterol into the cellular membrane, as shown by an increased cholesterol:phospholipids ratio (50.0-59.6%) upon treatment at 7.5 kV/cm compared with treatment at 2.5 kV/cm. Saturation of cholesterol was observed in different regions of the membrane bilayer such as upper phospholipids, apolar tail, and polar heads, as indicated by fluorescence anisotropy using 3 fluorescent probes. Electroporation could be a useful technique to increase the ability of lactobacilli to remove cholesterol for possible use as cholesterol-lowering adjuncts in the future.

Deoxycholic and chenodeoxycholic bile acids induce apoptosis via oxidative stress in human colon adenocarcinoma cells

The continuous exposure of the colonic epithelium to high concentrations of bile acids may exert cytotoxic effects and has been related to pathogenesis of colon cancer. A better knowledge of the mechanisms by which bile acids induce toxicity is still required and may be useful for the development of new therapeutic strategies. We have studied the effect of deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA) treatments in BCS-TC2 human colon adenocarcinoma cells. Both bile acids promote cell death, being this effect higher for CDCA. Apoptosis is detected after 30 min-2 h of treatment, as observed by cell detachment, loss of membrane asymmetry, internucleosomal DNA degradation, appearance of mitochondrial transition permeability (MPT), and caspase and Bax activation. At longer treatment times, apoptosis is followed in vitro by secondary necrosis due to impaired mitochondrial activity and ATP depletion. Bile acid-induced apoptosis is a result of oxidative stress with increased ROS generation mainly by activation of plasma membrane enzymes, such as NAD(P)H oxidases and, to a lower extent, PLA2. These effects lead to a loss of mitochondrial potential and release of pro-apoptotic factors to the cytosol, which is confirmed by activation of caspase-9 and -3, but not caspase-8. This initial apoptotic steps promote cleavage of Bcl-2, allowing Bax activation and formation of additional pores in the mitochondrial membrane that amplify the apoptotic signal.

Large procyanidins prevent bile-acid-induced oxidant production and membrane-initiated ERK1/2, p38, and Akt activation in Caco-2 cells

Procyanidins are oligomers of flavanol subunits present in large amounts in fruits and vegetables. Their consumption is associated with health benefits against colonic inflammation and colorectal cancer (CRC). Large procyanidins (with more than three subunits) are not absorbed by intestinal epithelial cells but could exert biological actions through their interactions with the cell membrane. This study investigated the capacity of hexameric procyanidins (Hex) to prevent oncogenic events initiated by deoxycholic acid (DCA), a secondary bile acid linked to the promotion of CRC. Hex interacted with Caco-2 cell membranes preferentially at the water-lipid interface. Hex (2.5-20 μM) inhibited DCA-triggered increase in cellular calcium, NADPH oxidase activation, and oxidant production. DCA promoted the activation of protein kinase B (Akt), of the mitogen-activated protein kinases ERK1/2 and p38, and of the downstream transcription factor AP-1. This activation was not triggered by calcium or oxidant increases. Hex caused a dose-dependent inhibition of DCA-mediated activation of all these signals. DCA also triggered alterations in the cell monolayer morphology and apoptotic cell death, events that were delayed by Hex. The capacity of large procyanidins to interact with the cell membrane and prevent those cell membrane-associated events can in part explain the beneficial effects of procyanidins on CRC.

Chemoprevention of colorectal cancer: a role for ursodeoxycholic acid, folate and hormone replacement treatment?

Chemoprevention of colorectal cancer has been an intense focus of research for many years. Among the possible candidate agents, ursodeoxycholic acid, folate, and hormone replacement therapy have been recently investigated with conflicting data. Experimental evidence shows that UDCA, folate and HRT target critical molecular events important for colon carcinogenesis. In animal models of sporadic, familial and inflammatory-associated cancers, they have shown to reduce colonic neoplasms. Observational studies have shown compelling evidence of possible protective effects of all three agents. However, randomised-controlled studies have yielded disappointing results, raising the issues of possible harm rather than protective effect for some of them. In this review experimental and clinical data on UDCA, folate and HRT as potential chemopreventive agents are discussed.

Bile acids and colon cancer: Solving the puzzle with nuclear receptors

Colorectal cancer is the third most common malignancy worldwide and is often linked to obesity, a sedentary lifestyle, carbohydrate- and fat-rich diets and elevated fecal excretion of secondary bile acids. Accumulation of toxic bile acids triggers oxidative damage, mitochondrial dysfunction and tumor progression. Nuclear receptors are transcription factors crucially involved in the regulation of bile acid metabolism and detoxification, and their activation may confer protection from bile acid tumor-promoting activity. In this review, we explore the tangled relationships among bile acids, nuclear receptors and the intestinal epithelium, with particular emphasis on the role of the farnesoid X receptor in colorectal cancer prevention and on novel nuclear receptor-based approaches to expand the portfolio of chemotherapeutic agents.

Plasma membrane sterol complexation, generated by filipin, triggers signaling responses in tobacco cells

The effects of changes in plasma membrane (PM) sterol lateral organization and availability on the control of signaling pathways have been reported in various animal systems, but rarely assessed in plant cells. In the present study, the pentaene macrolide antibiotic filipin III, commonly used in animal systems as a sterol sequestrating agent, was applied to tobacco cells. We show that filipin can be used at a non-lethal concentration that still allows an homogeneous labeling of the plasma membrane and the formation of filipin-sterol complexes at the ultrastructural level. This filipin concentration triggers a rapid and transient NADPH oxidase-dependent production of reactive oxygen species, together with an increase in both medium alkalinization and conductivity. Pharmacological inhibition studies suggest that these signaling events may be regulated by phosphorylations and free calcium. By conducting FRAP experiments using the di-4-ANEPPDHQ probe and spectrofluorimetry using the Laurdan probe, we provide evidence for a filipin-induced increase in PM viscosity that is also regulated by phosphorylations. We conclude that filipin triggers ligand-independent signaling responses in plant cells. The present findings strongly suggest that changes in PM sterol availability could act as a sensor of the modifications of cell environment in plants leading to adaptive cell responses through regulated signaling processes.

Regulation of Nur77 expression by β-catenin and its mitogenic effect in colon cancer cells

The orphan nuclear receptor Nur77 is an immediate-early response gene whose expression is rapidly induced by various extracellular stimuli. The aims of this study were to study the role of Nur77 expression in the growth and survival of colon cancer cells and the mechanism by which Nur77 expression was regulated. We showed that levels of Nur77 were elevated in a majority of human colon tumors (9/12) compared to their nontumorous tissues and that Nur77 expression could be strongly induced by different colonic carcinogens including deoxycholic acid (DCA). DCA-induced Nur77 expression resulted in up-regulation of antiapoptotic BRE and angiogenic VEGF, and it enhanced the growth, colony formation, and migration of colon cancer cells. In studying the mechanism by which Nur77 was regulated in colon cancer cells, we found that β-catenin was involved in induction of Nur77 expression through its activation of the transcriptional activity of AP-1 (c-Fos/c-Jun) that bound to and transactivated the Nur77 promoter. Together, our results demonstrate that Nur77 acts to promote the growth and survival of colon cancer cells and serves as an important mediator of the Wnt/β-catenin and AP-1 signaling pathways.

Bile acids initiate lineage-addicted gastroesophageal tumorigenesis by suppressing the EGF receptor-AKT axis

While bile acids are a risk factor for tumorigenesis induced by reflux disease, the mechanisms by which they contribute to neoplasia remain undefined. Here, we reveal that in gastroesophageal junction (GEJ) cells bile acids activate a tissue-specific developmental program defining the intestinal epithelial cell phenotype characterizing GEJ metaplasia. Deoxycholic acid (DCA) inhibited phosphorylation of EGF receptors (EGFRs) suppressing the proto-oncogene AKT. Suppression of EGFRs and AKT by DCA actuated an intestine-specific cascade in which NF-kappaB transactivated the tissue-specific transcription factor CDX2. In turn, CDX2 orchestrated a lineage-specific differentiation program encompassing genes characterizing intestinal epithelial cells. Conversely, progression from metaplasia to invasive carcinoma in patients, universally associated with autonomous activation of EGFRs and/or AKT, was coupled with loss of this intestinal program. Thus, bile acids induce intestinal metaplasia at the GEJ by activating the lineage-specific differentiation program involving suppression of EGFR and AKT, activating the NF-kappaB-CDX2 axis. Induction of this axis provides the context for lineage-addicted tumorigenesis, in which autonomous activation of AKT corrupts adaptive intestinal NF-kappaB signaling, amplifying tumorigenic programs.

Identifying apoptotic cells with the 3-hydroxyflavone derivative F2N12S, a ratiometric fluorescent small molecule probe selective for plasma membranes: a possible general mechanism for selective uptake into apoptotic cells

The mechanism of selective targeting of the plasma membrane of apoptotic cells by F2N12S, a recently reported ratiometric, fluorescent small molecule probe, was analyzed using decision-rule QSAR models. Selectivity was determined by a combination of the probe's weak amphiphilicity and slow flip-flop with the increased plasma membrane fluidity of apoptotic cells. The probable chemical features required for such probes may be defined in terms of numerical structural parameters as: 3.5 < AI < ∼ 5.5; log P < 5.0; HGS > 400 (where AI, log P and HGS parameters model amphiphilicity, lipophilicity and headgroup size, respectively). When HGS is <400, compounds are initially membrane selective, but subsequently are internalized.

The molecular pathogenesis of Barrett's esophagus: common signaling pathways in embryogenesis metaplasia and neoplasia

Although Barrett's esophagus has been recognized for over 50 years, the cellular and molecular mechanisms leading to the replacement of squamous esophageal epithelium with a columnar type are largely unknown. Barrett's is known to be an acquired process secondary to chronic gastroesophageal reflux disease and occurs in the presence of severe disruption of the gastroesophageal barrier and reflux of a mixture of gastric and duodenal content. Current hypothesis suggest that epithelial change occurs due to stimulation of esophageal stem cells present in the basal layers of the epithelium or submucosal glands, toward a columnar epithelial differentiation pathway. The transcription factor CDX2 seems to play a key role in promoting the cellular biology necessary for columnar differentiation, and can be induced by bile salt and acid stimulation. Several cellular signaling pathways responsible for modulation of intestinal differentiation have also been identified and include WNT, Notch, BMP, Sonic HH and TGFB. These also have been shown to respond to stimulation by bile acids, acid or both and may influence CDX2 expression. Their relative activity within the stem cell population is almost certainly responsible for the development of the esophageal columnar epithelial phenotype we know as Barrett's esophagus.

Protective effects of glycoursodeoxycholic acid in Barrett's esophagus cells

Barrett's esophagus (BE) is a premalignant condition associated with the development of esophageal adenocarcinoma (EAC). Previous studies have implicated hydrophobic bile acids and gastric acid in BE and EAC pathogenesis. In this study, we tested the hypothesis that DNA damage, cytotoxicity and oxidative stress induced by bile acids and gastric acid can be attenuated by the cytoprotective, hydrophilic bile acid glycoursodeoxycholic acid (GUDCA). Non-dysplastic BE cells were exposed for 10 min to pH 4 and/or bile acid cocktail or to pH 4 and a modified cocktail consisting of a mixture of bile acids and GUDCA. DNA damage was evaluated by the comet assay; cell viability and proliferation were measured by trypan blue staining and the MTS assay; reactive oxygen species (ROS) were measured using hydroethidium staining; oxidative DNA/RNA damage was detected by immunostaining with antibody against 8-OH-dG; thiol levels were measured by 5-chloromethylfluorescein diacetate (CMFDA) staining; and the expression of antioxidant proteins was evaluated by western blotting. DNA damage and oxidative stress were significantly increased, while thiol levels were decreased in BE cells treated with pH 4 and bile acid cocktail compared with cells treated with pH 4 alone or untreated cells. Bile acids and low pH also significantly decreased cell proliferation. Expression of the antioxidant enzymes, MnSOD and CuZnSOD, was elevated in the cells treated with bile acids and low pH. When GUDCA was included in the medium, all these effects of pH 4 and bile acids were markedly reduced. In conclusion, treatment of BE cells with acidified medium and a bile acid cocktail at physiologically relevant concentrations induces DNA damage, cytotoxicity, and ROS. The cytoprotective bile acid, GUDCA, inhibits these deleterious effects by inhibiting oxidative stress.

Bile acids in regulation of intestinal physiology

In addition to their roles in facilitating lipid digestion and absorption, bile acids are recognized as important regulators of intestinal function. Exposure to bile acids can dramatically influence intestinal transport and barrier properties; in recent years, they have also become appreciated as important factors in regulating cell growth and survival. Indeed, few cells reside within the intestinal mucosa that are not altered to some degree by exposure to bile acids. The past decade saw great advances in the knowledge of how bile acids exert their actions at the cellular and molecular levels. In this review, we summarize the current understanding of the role of bile acids in regulation of intestinal physiology.

Ca2+-dependent K+ efflux regulates deoxycholate-induced apoptosis of BHK-21 and Caco-2 cells

BACKGROUND & AIMS

Deoxycholate (DC) has proapoptotic and tumorigenic effects in different cell types of the gastrointestinal tract. Exposure of BHK-21 (stromal) cells to DC induces Ca(2+) entry at the plasma membrane, which affects intracellular Ca(2+) signaling. We assessed whether DC-induced increases in [Ca(2+)] can impinge on plasma membrane properties (eg, ionic conductances) involved in cell apoptosis.

METHODS

Single- and double-barreled microelectrodes were used to measure membrane potential (V(m)) and extracellular [K(+)] in BHK-21 fibroblasts and Caco-2 colon carcinoma cells. Apoptosis was assessed by Hoechst labeling, propidium iodide staining, and caspase-3 and caspase-7 assays.

RESULTS

DC-induced cell membrane hyperpolarization was directly measured with intracellular microelectrodes in both cell lines. Diverse Ca(2+) mobilizing agents, such as membrane receptor agonists, an inhibitor of the sarco/endoplasmic reticulum Ca(2+) adenosine triphosphatase and a Ca(2+) ionophore, also induced increases in V(m). Removal of extracellular Ca(2+) reduced the agonist- and DC-induced membrane hyperpolarization by approximately 15% and 60%, respectively. These findings indicate a prominent role for Ca(2+) entry at the plasma membrane in the action of this bile salt. Blockade of Ca(2+)-activated K(+) conductances by charybdotoxin and apamin reduced DC-induced hyperpolarization by 75% and 64% in BHK-21 and Caco-2 cells, respectively. These inhibitors also reduced the DC-induced increase in extracellular [K(+)] by 75% and cell apoptosis by approximately 50% in both cell lines.

CONCLUSIONS

Ca(2+)-dependent K(+) conductance is an important regulator of DC-induced apoptosis in stromal and colon cancer cells.

Growth suppression by ursodeoxycholic acid involves caveolin-1 enhanced degradation of EGFR

Ursodeoxycholic acid (UDCA) has been shown to prevent colon tumorigenesis in animal models and in humans. In vitro work indicates that this bile acid can suppress cell growth and mitogenic signaling suggesting that UDCA may be an anti-proliferative agent. However, the mechanism by which UDCA functions is unclear. Previously we showed that bile acids may alter cellular signaling by acting at the plasma membrane. Here we utilized EGFR as a model membrane receptor and examined the effects that UDCA has on its functioning. We found that UDCA promoted an interaction between EGFR and caveolin-1 and this interaction enhanced UDCA-mediated suppression of MAP kinase activity and cell growth. Importantly, UDCA treatment led to recruitment of the ubiquitin ligase, c-Cbl, to the membrane, ubiquitination of EGFR, and increased receptor degradation. Moreover, suppression of c-Cbl activity abrogated UDCA's growth suppression activities suggesting that receptor ubiquitination plays an important role in UDCA's biological activities. Taken together these results suggest that UDCA may act to suppress cell growth by inhibiting the mitogenic activity of receptor tyrosine kinases such as EGFR through increased receptor degradation.

Deoxycholate, an endogenous cytotoxin/genotoxin, induces the autophagic stress-survival pathway: implications for colon carcinogenesis

We report that deoxycholate (DOC), a hydrophobic bile acid associated with a high-fat diet, activates the autophagic pathway in non-cancer colon epithelial cells (NCM-460), and that this activation contributes to cell survival. The DOC-induced increase in autophagy was documented by an increase in autophagic vacuoles (detected using transmission electron microscopy, increased levels of LC3-I and LC3-II (western blotting), an increase in acidic vesicles (fluorescence spectroscopy of monodansycadaverine and lysotracker red probes), and increased expression of the autophagic protein, beclin-1 (immunohistochemistry/western blotting). The DOC-induced increase in beclin-1 expression was ROS-dependent. Rapamycin (activator of autophagy) pre-treatment of NCM-460 cells significantly (P < .05) decreased, and 3-MA (inhibitor of autophagy) significantly (P < .05) increased the cell loss caused by DOC treatment, alone. Rapamycin pre-treatment of the apoptosis-resistant colon cancer cell line, HCT-116RC (developed in our laboratory), resulted in a significant decrease in DOC-induced cell death. Bafilomycin A(1) and hydroxychloroquine (inhibitors of the autophagic process) increased the DOC-induced percentage of apoptotic cells in HCT-116RC cells. It was concluded that the activation of autophagy by DOC has important implications for colon carcinogenesis and for the treatment of colon cancer in conjunction with commonly used chemotherapeutic agents.

Bile-inducible efflux transporter from Bifidobacterium longum NCC2705, conferring bile resistance

Bifidobacteria are normal inhabitants of the human gut. Some strains of this genus are considered health promoting or probiotic, being included in numerous food products. In order to exert their health benefits, these bacteria must overcome biological barriers, including bile salts, to colonize and survive in specific parts of the intestinal tract. The role of multidrug resistance (MDR) transporters in bile resistance of probiotic bacteria and the effect of bile on probiotic gene expression are not fully understood. In the present study, the effect of subinhibitory concentrations of bile on the expression levels of predicted MDR genes from three different bifidobacterial strains, belonging to Bifidobacterium longum subsp. longum, Bifidobacterium breve, and Bifidobacterium animalis subsp. lactis, was tested. In this way, two putative MDR genes whose expression was induced by bile, BL0920 from B. longum and its homolog, Bbr0838, from B. breve, were identified. The expression of the BL0920 gene in Escherichia coli was shown to confer resistance to bile, likely to be mediated by active efflux from the cells. To the best of our knowledge, this represents the first identified bifidobacterial bile efflux pump whose expression is induced by bile.

Bile acid alone, or in combination with acid, induces CDX2 expression through activation of the epidermal growth factor receptor (EGFR)

OBJECTIVES

Bile acids and acid are implicated in the development of Barrett's esophagus. Evidence suggests that Barrett's esophagus intestinal metaplasia may occur via induction of caudal homeobox gene 2 (CDX2). We hypothesized that induction of CDX2 by bile acids may be due to ligand-dependent transactivation of epidermal growth factor receptor (EGFR).

METHODS

Human mucosal epithelial cells (SEG-1) were treated for 0 to 24 h with up to 300 microM deoxycholic acid (DCA) at pH 7 or 5 with or without (w/wo) antibodies against EGFR ligand-binding site (Mab528, 3-5 mug/ml). Treatment with 100 ng/ml EGF served as control. CDX2 mRNA expression was determined by real-time polymerase chain reaction. EGFR activation was analyzed by Westerns of phosphorylated EGFR tyrosines.

RESULTS

Acid (pH 5) increased the induction of CDX2 mRNA expression caused by DCA. CDX2 mRNA induction was markedly reduced by EGFR blockade with Mab528. Each treatment (pH 5, DCA or pH 5 plus DCA) activated the EGFR on all tyrosines tested but in different time courses. Phosphorylation by DCA was inhibited by Mab528. Activation of EGFR by DCA at pH 5 resulted in EGFR degradation, while that by DCA alone did not.

CONCLUSION

Thus, CDX2 induction by DCA w/wo acid occurs through ligand-dependent transactivation of the EGFR. The variations in EGFR degradation pattern with DCA or DCA at pH 5 indicate differential transactivation pathways. The molecular pathogenesis of Barrett's esophagus may occur via bile-stimulated cell signaling through the EGFR.

Overexpression of OATP1B3 confers apoptotic resistance in colon cancer

Organic anion transporting polypeptide 1B3 (OATP1B3, SLCO1B3) is normally expressed in hepatocytes. In this study, we showed frequent overexpression of OATP1B3 in colorectal adenocarcinomas. Quantitative reverse transcription-PCR analysis of 17 colon tumors indicated tumoral overexpression of OATP1B3 by approximately 100-fold, compared with 20 normal colon samples (P < 0.0001). Using immunohistochemistry on a tissue microarray containing 93 evaluable colon tumor specimens, we detected immunostaining of OATP1B3 in 75 colon adenocarcinomas (81%) and no immunostaining in normal samples. To determine the functional effects of OATP1B3 expression on drug-induced apoptosis, we used camptothecin and oxaliplatin on a panel of colorectal cancer cell lines stably overexpressing OATP1B3. The results indicated that OATP1B3 overexpression enhanced cell survival in RKO, HCT-8, and HCT116(p53+/+) cells that harbor wild-type p53 but not in Caco-2 and HCT116(p53-/-) cells that lack p53, compared with the respective empty vector controls (P < 0.01). The terminal deoxynucleotidyl transferase-mediated nick-end labeling assay confirmed that HCT116(p53+/+) cells overexpressing OATP1B3 had significantly lower apoptotic levels compared with empty vector control (P < 0.001). The overexpression of OATP1B3 reduced the transcriptional activity of p53, with subsequent reductions in transcript and protein levels of its downstream transcription targets (P21WAF1 and PUMA). Overexpression of a point mutation (G583E) variant of OATP1B3 lacking transport activity did not confer an antiapoptotic effect or affect p53 transcriptional activity, suggesting that the antiapoptotic effect of OATP1B3 may be associated with its transport activity. Taken together, our results suggest that OATP1B3 overexpression in colorectal cancer cells may provide a survival advantage by altering p53-dependent pathways.

The role of membrane cholesterol in determining bile acid cytotoxicity and cytoprotection of ursodeoxycholic acid

In cholestatic liver diseases, the ability of hydrophobic bile acids to damage membranes of hepatocytes/ductal cells contributes to their cytotoxicity. However, ursodeoxycholic acid (UDC), a hydrophilic bile acid, is used to treat cholestasis because it protects membranes. It has been well established that bile acids associate with and solubilize free cholesterol (CHOL) contained within the lumen of the gallbladder because of their structural similarities. However, there is a lack of understanding of how membrane CHOL, which is a well-established membrane stabilizing agent, is involved in cytotoxicity of hydrophobic bile acids and the cytoprotective effect of UDC. We utilized phospholipid liposomes to examine the ability of membrane CHOL to influence toxicity of individual bile acids, such as UDC and the highly toxic sodium deoxycholate (SDC), as well as the cytoprotective mechanism of UDC against SDC-induced cytotoxicity by measuring membrane permeation and intramembrane dipole potential. The kinetics of bile acid solubilization of phosphatidylcholine liposomes containing various levels of CHOL was also characterized. It was found that the presence of CHOL in membranes significantly reduced the ability of bile acids to damage synthetic membranes. UDC effectively prevented damaging effects of SDC on synthetic membranes only in the presence of membrane CHOL, while UDC enhances the damaging effects of SDC in the absence of CHOL. This further demonstrates that the cytoprotective effects of UDC depend upon the level of CHOL in the lipid membrane. Thus, changes in cell membrane composition, such as CHOL content, potentially influence the efficacy of UDC as the primary drug used to treat cholestasis.

Hydrophobic bile acids, genomic instability, Darwinian selection, and colon carcinogenesis

Sporadic colon cancer is caused predominantly by dietary factors. We have selected bile acids as a focus of this review since high levels of hydrophobic bile acids accompany a Western-style diet, and play a key role in colon carcinogenesis. We describe how bile acid-induced stresses cause cell death in susceptible cells, contribute to genomic instability in surviving cells, impose Darwinian selection on survivors and enhance initiation and progression to colon cancer. The most likely major mechanisms by which hydrophobic bile acids induce stresses on cells (DNA damage, endoplasmic reticulum stress, mitochondrial damage) are described. Persistent exposure of colon epithelial cells to hydrophobic bile acids can result in the activation of pro-survival stress-response pathways, and the modulation of numerous genes/proteins associated with chromosome maintenance and mitosis. The multiple mechanisms by which hydrophobic bile acids contribute to genomic instability are discussed, and include oxidative DNA damage, p53 and other mutations, micronuclei formation and aneuploidy. Since bile acids and oxidative stress decrease DNA repair proteins, an increase in DNA damage and increased genomic instability through this mechanism is also described. This review provides a mechanistic explanation for the important link between a Western-style diet and associated increased levels of colon cancer.

Overexpression of OATP1B3 confers apoptotic resistance in colon cancer

Organic anion transporting polypeptide 1B3 (OATP1B3, SLCO1B3) is normally expressed in hepatocytes. In this study, we showed frequent overexpression of OATP1B3 in colorectal adenocarcinomas. Quantitative reverse transcription-PCR analysis of 17 colon tumors indicated tumoral overexpression of OATP1B3 by approximately 100-fold, compared with 20 normal colon samples (P < 0.0001). Using immunohistochemistry on a tissue microarray containing 93 evaluable colon tumor specimens, we detected immunostaining of OATP1B3 in 75 colon adenocarcinomas (81%) and no immunostaining in normal samples. To determine the functional effects of OATP1B3 expression on drug-induced apoptosis, we used camptothecin and oxaliplatin on a panel of colorectal cancer cell lines stably overexpressing OATP1B3. The results indicated that OATP1B3 overexpression enhanced cell survival in RKO, HCT-8, and HCT116(p53+/+) cells that harbor wild-type p53 but not in Caco-2 and HCT116(p53-/-) cells that lack p53, compared with the respective empty vector controls (P < 0.01). The terminal deoxynucleotidyl transferase-mediated nick-end labeling assay confirmed that HCT116(p53+/+) cells overexpressing OATP1B3 had significantly lower apoptotic levels compared with empty vector control (P < 0.001). The overexpression of OATP1B3 reduced the transcriptional activity of p53, with subsequent reductions in transcript and protein levels of its downstream transcription targets (P21WAF1 and PUMA). Overexpression of a point mutation (G583E) variant of OATP1B3 lacking transport activity did not confer an antiapoptotic effect or affect p53 transcriptional activity, suggesting that the antiapoptotic effect of OATP1B3 may be associated with its transport activity. Taken together, our results suggest that OATP1B3 overexpression in colorectal cancer cells may provide a survival advantage by altering p53-dependent pathways.

Characterization of enantiomeric bile acid-induced apoptosis in colon cancer cell lines

Bile acids are steroid detergents that are toxic to mammalian cells at high concentrations; increased exposure to these steroids is pertinent in the pathogenesis of cholestatic disease and colon cancer. Understanding the mechanisms of bile acid toxicity and apoptosis, which could include nonspecific detergent effects and/or specific receptor activation, has potential therapeutic significance. In this report we investigate the ability of synthetic enantiomers of lithocholic acid (ent-LCA), chenodeoxycholic acid (ent-CDCA), and deoxycholic acid (ent-DCA) to induce toxicity and apoptosis in HT-29 and HCT-116 cells. Natural bile acids were found to induce more apoptotic nuclear morphology, cause increased cellular detachment, and lead to greater capase-3 and -9 cleavage compared with enantiomeric bile acids in both cell lines. In contrast, natural and enantiomeric bile acids showed similar effects on cellular proliferation. These data show that bile acid-induced apoptosis in HT-29 and HCT-116 cells is enantiospecific, hence correlated with the absolute configuration of the bile steroid rather than its detergent properties. The mechanism of LCA- and ent-LCA-induced apoptosis was also investigated in HT-29 and HCT-116 cells. These bile acids differentially activate initiator caspases-2 and -8 and induce cleavage of full-length Bid. LCA and ent-LCA mediated apoptosis was inhibited by both pan-caspase and selective caspase-8 inhibitors, whereas a selective caspase-2 inhibitor provided no protection. LCA also induced increased CD95 localization to the plasma membrane and generated increased reactive oxygen species compared with ent-LCA. This suggests that LCA/ent-LCA induce apoptosis enantioselectively through CD95 activation, likely because of increased reactive oxygen species generation, with resulting procaspase-8 cleavage.

Methyl jasmonate decreases membrane fluidity and induces apoptosis through tumor necrosis factor receptor 1 in breast cancer cells

In recent years, studies with plant compounds have shown both chemotherapeutic and chemopreventive properties. This study with plant stress hormones (jasmonates) showed growth inhibitory effects in breast cancer cells. cis-Jasmone and methyl jasmonate (MJ) inhibited the long-term proliferation of MDA-MB-435 and MCF-7 cells. Cell cycle analysis showed G0/G1 and S-phase arrest with increasing apoptotic population. Cellular signaling studies with MJ showed decreased membrane fluidity and activation of extrinsic and intrinsic apoptotic pathways. Specifically in extrinsic apoptotic pathway increased expression of TNF receptor 1, activation of mitogen-activated protein kinase and caspase-8 was observed. MJ also decreased the mitochondrial membrane potential and activated caspase-3 in breast cancer cells. In conclusion our results revealed novel-signaling mechanism of MJ in breast cancer cells, indicating that MJ could have potential applications for chemotherapeutic purposes.

Activation of focal adhesion kinase and JNK contributes to the extracellular matrix and cAMP-GEF mediated survival from bile acid induced apoptosis in rat hepatocytes

BACKGROUND/AIMS

Adherence to an extracellular matrix (ECM) rescues hepatocytes from apoptosis, but how hepatocytes adhered to different ECM and respond to apoptotic and cytoprotective stimuli is unknown.

METHODS

Rat hepatocytes were plated on type 1 collagen (CI), laminin (LM) or polylysine (PL) and the amount of apoptosis induced by glycochenodeoxycholate (GCDC), deoxycholate (DCA), Fas ligand or serum withdrawal was determined by Hoechst staining. The response to cytoprotection by cAMP-guanine exchange factor (cAMP-GEF) activation was determined. Kinase activation was determined by immunoblotting with phosphospecific antibodies.

RESULTS

Hepatocytes on LM and PL had more apoptosis in response to all apoptotic stimuli. GCDC increased c-jun-N-terminal kinase (JNK) phosphorylation 2-fold in hepatocytes on CI, but 15- and 30-fold in hepatocytes on PL or LM. SP-600125, a JNK inhibitor, prevented LM and PL potentiation of bile acid apoptosis. GCDC induced dephosphorylation of focal adhesion kinase (FAK) was prevented by cAMP-GEF activation. Cytochalasin B which decreased FAK phosphorylation prevented cAMP-GEF cytoprotection.

CONCLUSIONS

JNK activation augments apoptosis in hepatocytes plated on PL and LM. Decreased FAK phosphorylation as seen in cells treated with bile acids or attached to PL and LM promotes hepatocyte apoptosis.

Ceramide signaling in cancer and stem cells

Most of the previous work on the sphingolipid ceramide has been devoted to its function as an apoptosis inducer. Recent studies, however, have shown that in stem cells, ceramide has additional nonapoptotic functions. In this article, ceramide signaling will be reviewed in light of 'systems interface biology': as an interconnection of sphingolipid metabolism, membrane biophysics and cell signaling. The focus will be on the metabolic interconversion of ceramide and sphingomyelin or sphingosine-1-phosphate. Lipid rafts and sphingolipid-induced protein scaffolds will be discussed as a membrane interface for lipid-controlled cell signaling. Ceramide/sphingomyelin and ceramide/sphingosine-1-phosphate-interdependent cell-signaling pathways are significant for the regulation of cell polarity, apoptosis and/or proliferation, and as novel pharmacologic targets in cancer and stem cells.

Bile acids modulate tight junction structure and barrier function of Caco-2 monolayers via EGFR activation

Intestinal and systemic illnesses have been linked to increased gut permeability. Bile acids, whose luminal profile can be altered in human disease, modulate intestinal paracellular permeability. We investigated the mechanism by which selected bile acids increase gut permeability using a validated in vitro model. Human intestinal Caco-2 cells were grown in monolayers and challenged with a panel of bile acids. Transepithelial electrical resistance and luminal-to-basolateral fluxes of 10-kDa Cascade blue-conjugated dextran were used to monitor paracellular permeability. Immunoprecipitation and immunoblot analyses were employed to investigate the intracellular pathway. Redistribution of tight junction proteins was studied by confocal laser microscopy. Micromolar concentrations of cholic acid, deoxycholic acid (DCA), and chenodeoxycholic acid (CDCA) but not ursodeoxycholic acid decreased transepithelial electrical resistance and increased dextran flux in a reversible fashion. Coincubation of 50 muM CDCA or DCA with EGF, anti-EGF monoclonal antibody, or specific src inhibitor 4-Amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP-2) abolished the effect. A concentration of 50 muM of either CDCA or DCA also induced EGF receptor phosphorylation, occludin dephosphorylation, and occludin redistribution at the tight junction level in the same time frame and in a reversible fashion. We conclude that selected bile acids modulate intestinal permeability via EGF receptor autophosphorylation, occludin dephosphorylation, and rearrangement at the tight junction level. The effect is mediated by the src family kinases and is abolished by EGF treatment. These data also support the role of bile acids in the genesis of necrotizing enterocolitis and the protective effect of EGF treatment.

Deoxycholyltaurine rescues human colon cancer cells from apoptosis by activating EGFR-dependent PI3K/Akt signaling

Recent studies indicate that secondary bile acids promote colon cancer cell proliferation but their role in maintaining cell survival has not been explored. We found that deoxycholyltaurine (DCT) markedly attenuated both unstimulated and TNF-alpha-stimulated programmed cell death in colon cancer cells by a phosphatidylinositol 3-kinase (PI3K)-dependent mechanism. To examine the role of bile acids and PI3K signaling in maintaining colon cancer cell survival, we explored the role of signaling downstream of bile acid-induced activation of the epidermal growth factor receptor (EGFR) in regulating both apoptosis and proliferation of HT-29 and H508 human colon cancer cells. DCT caused dose- and time-dependent Akt (Ser(473)) phosphorylation, a commonly used marker of activated PI3K/Akt signaling. Both EGFR kinase and PI3K inhibitors attenuated DCT-induced Akt phosphorylation and Akt activation, as demonstrated by reduced phosphorylation of a GSK-3-paramyosin substrate. Transfection of HT-29 cells with kinase-dead EGFR (K721M) reduced DCT-induced Akt phosphorylation. In HT-29 cells, EGFR and PI3K inhibitors as well as transfection with dominant negative AKT attenuated DCT-induced cell proliferation. DCT-induced PI3K/Akt activation resulted in downstream phosphorylation of GSK-3 (Ser(21/9)) and BAD (Ser(136)), and nuclear translocation (activation) of NF-kappaB, thereby confirming that DCT-induced activation of PI3K/Akt signaling regulates both proproliferative and prosurvival signals. Collectively, these results indicate that DCT-induced activation of post-EGFR PI3K/Akt signaling stimulates both colon cancer cell survival and proliferation.