Nutrient stress alters the glycosylation status of LGR5 resulting in reduced protein stability and membrane localisation in colorectal tumour cells: implications for targeting cancer stem cells

BACKGROUND

LGR5 is an important marker of intestinal stem cells and performs its vital functions at the cell membrane. Despite the importance of LGR5 to both normal and cancer stem cell biology, it is not known how microenvironmental stress affects the expression and subcellular distribution of the protein.

METHODS

Nutrient stress was induced through glucose starvation. Glycosylation status was assessed using endoglycosidase or tunicamycin treatment. Flow cytometry and confocal microscopy were used to assess subcellular distribution of LGR5.

RESULTS

Glucose deprivation altered the glycosylation status of LGR5 resulting in reduced protein stability and cell surface expression. Furthermore, inhibiting LGR5 glycosylation resulted in depleted surface expression and reduced localisation in the cis-Golgi network.

CONCLUSIONS

Nutrient stress within a tumour microenvironment has the capacity to alter LGR5 protein stability and membrane localisation through modulation of LGR5 glycosylation status. As LGR5 surface localisation is required for enhanced Wnt signalling, this is the first report to show a mechanism by which the microenvironment could affect LGR5 function.

BAG-1 suppresses expression of the key regulatory cytokine transforming growth factor β (TGF-β1) in colorectal tumour cells

As colorectal cancer remains the second highest cause of cancer-related deaths in much of the industrialised world, identifying novel strategies to prevent colorectal tumour development remains an important challenge. BAG-1 is a multi-functional protein, the expression of which is up-regulated at relatively early stages in colorectal tumorigenesis. Importantly, BAG-1 is thought to enhance colorectal tumour progression through promoting tumour cell survival. Here we report for the first time a novel role for BAG-1, establishing it as a suppressor of transforming growth factor beta [TGF-β1] expression in colorectal tumour cells. Microarray analysis first highlighted the possibility that BAG-1 may regulate TGF-β1 expression, a key cytokine in normal colonic tissue homeostasis. Q-RT-PCR and ELISA demonstrated TGFB1 mRNA and protein expression to be significantly increased when BAG1 levels were reduced by siRNA; additionally, induction of BAG-1L caused suppression of TGFB1 mRNA in colorectal tumour cells. Using reporter and ChIP assays, a direct association of BAG-1 with the TGFB1 gene regulatory region was identified. Immunohistochemistry and Weiser fraction data indicated levels of BAG-1 and TGF-β1 are inversely correlated in the normal colonic epithelium in vivo, consistent with a role for BAG-1-mediated repression of TGF-β1 production. In vitro studies showed that the change in TGF-β1 production following manipulation of BAG-1 is functionally relevant; through induction of anchorage-independent growth in TGF-β1 dependent NRK fibroblasts and regulation of SMAD2 phosphorylation in TGF-β1 sensitive adenoma cells. Taken together, this study identifies the anti-apoptotic protein BAG-1 as a suppressor of the inhibitory growth factor TGF-β1, suggesting that high expression of BAG-1 can impact on a number of the hallmarks of cancer, of potential importance in promoting the early stages of colorectal tumorigenesis. Establishing BAG-1 as a repressor of TGF-β1 has important biological implications, and highlights a new role for BAG-1 in colorectal tumorigenesis.

The retinoblastoma protein (Rb) as an anti-apoptotic factor: expression of Rb is required for the anti-apoptotic function of BAG-1 protein in colorectal tumour cells

Although the retinoblastoma-susceptibility gene RB1 is inactivated in a wide range of human tumours, in colorectal cancer, the retinoblastoma protein (Rb) function is often preserved and the RB locus even amplified. Importantly, we have previously shown that Rb interacts with the anti-apoptotic Bcl-2 associated athanogene 1 (BAG-1) protein, which is highly expressed in colorectal carcinogenesis. Here we show for the first time that Rb expression is critical for BAG-1 anti-apoptotic activity in colorectal tumour cells. We demonstrate that Rb expression not only increases the nuclear localisation of the anti-apoptotic BAG-1 protein, but that expression of Rb is required for inhibition of apoptosis by BAG-1 both in a γ-irradiated Saos-2 osteosarcoma cell line and colorectal adenoma and carcinoma cell lines. Further, consistent with the fact that nuclear BAG-1 has previously been shown to promote cell survival through increasing nuclear factor (NF)-κB activity, we demonstrate that the ability of BAG-1 to promote NF-κB activity is significantly inhibited by repression of Rb expression. Taken together, data presented suggest a novel function for Rb, promoting cell survival through regulating the function of BAG-1. As BAG-1 is highly expressed in the majority of colorectal tumours, targeting the Rb–BAG-1 complex to promote apoptosis has exciting potential for future therapeutic development.

Clonal evolution and tumor progression in 2 human colorectal adenoma-derived cell-lines invitro - the involvement of chromosome-1 abnormalities

Two human colorectal adenoma cell lines, S/RG and S/AN, have been continuously passaged in vitro to determine whether they would immortalize and if specific cytogenetic changes were involved in immortalization and tumor progression. At passage 7, S/RG was highly aneuploid, but had no abnormalities of chromosome 1 (Paraskeva et al, Cancer Res 49: 1282-1286, 1989). With continued passage under two independent sets of growth conditions an isochromosome Iq and derivatives of this isochromosome occurred as specific abnormalities. S/AN was near-diploid at passage 10, with a deletion in lp and monosomy 18. The karyotype at passage 44 showed no change. The cell lines are stable in that they have remained anchorage-dependent and non-tumorigenic after several years in culture and S/AN has retained a near diploid karyotype. These cell lines are therefore highly valuable for further studies of tumor progression in human colorectal carcinogenesis.

β-catenin negatively regulates expression of the prostaglandin transporter PGT in the normal intestinal epithelium and colorectal tumour cells: a role in the chemopreventive efficacy of aspirin?

BACKGROUND

Levels of the pro-tumorigenic prostaglandin PGE(2) are increased in colorectal cancer, previously attributed to increased synthesis through COX-2 upregulation and, more recently, to decreased catabolism. The functionally linked genes 15-prostaglandin dehydrogenase (15-PGDH) and the prostaglandin transporter PGT co-operate in prostaglandin degradation and are downregulated in colorectal cancer. We previously reported repression of 15-PGDH expression by the Wnt/β-catenin pathway, commonly deregulated during early colorectal neoplasia. Here we asked whether β-catenin also regulates PGT expression.

METHODS

The effect of β-catenin deletion in vivo was addressed by PGT immunostaining of β-catenin(-/lox)-villin-cre-ERT2 mouse tissue. The effect of siRNA-mediated β-catenin knockdown and dnTCF4 induction in vitro was addressed by semi-quantitative and quantitative real-time RT-PCR and immunoblotting.

RESULTS

This study shows for the first time that deletion of β-catenin in murine intestinal epithelium in vivo upregulates PGT protein, especially in the crypt epithelium. Furthermore, β-catenin knockdown in vitro increases PGT expression in both colorectal adenoma- and carcinoma-derived cell lines, as does dnTCF4 induction in LS174T cells.

CONCLUSIONS

These data suggest that β-catenin employs a two-pronged approach to inhibiting prostaglandin turnover during colorectal neoplasia by repressing PGT expression in addition to 15-PGDH. Furthermore, our data highlight a potential mechanism that may contribute to the non-selective NSAID aspirin's chemopreventive efficacy.

Long-range epigenetic silencing of chromosome 5q31 protocadherins is involved in early and late stages of colorectal tumorigenesis through modulation of oncogenic pathways

Loss of tumour suppressor gene function can occur as a result of epigenetic silencing of large chromosomal regions, referred to as long-range epigenetic silencing (LRES), and genome-wide analyses have revealed that LRES is present in many cancer types. Here we utilize Illumina Beadchip methylation array analysis to identify LRES across 800 kb of chromosome 5q31 in colorectal adenomas and carcinomas (n=34) relative to normal colonic epithelial DNA (n=6). This region encompasses 53 individual protocadherin (PCDH) genes divided among three gene clusters. Hypermethylation within these gene clusters is asynchronous; while most PCDH hypermethylation occurs early, and is apparent in adenomas, PCDHGC3 promoter methylation occurs later in the adenoma-carcinoma transition. PCDHGC3 was hypermethylated in 17/28 carcinomas (60.7%) according to methylation array analysis. Quantitative real-time reverse transcription-polymerase chain reaction showed that PCDHGC3 is the highest expressed PCDH in normal colonic epithelium, and that there was a strong reciprocal relationship between PCDHGC3 methylation and expression in carcinomas (R=-0.84). PCDH LRES patterns are reflected in colorectal tumour cell lines; adenoma cell lines are not methylated at PCDHGC3 and show abundant expression at the mRNA and protein level, while the expression is suppressed in hypermethylated carcinoma cell lines (R=-0.73). Short-interfering RNA-mediated reduction of PCDHGC3 led to a decrease of apoptosis in RG/C2 adenoma cells, and overexpression of PCDHGC3 in HCT116 cells resulted in the reduction of colony formation, consistent with tumour suppressor capabilities for PCDHGC3. Further functional analysis showed that PCDHGC3 can suppress Wnt and mammalian target of rapamycin signalling in colorectal cancer cell lines. Taken together, our data suggest that the PCDH LRES is an important tumour suppressor locus in colorectal cancer, and that PCDHGC3 may be a strong marker and driver for the adenoma-carcinoma transition.

The proapoptotic BH3-only protein Bim is downregulated in a subset of colorectal cancers and is repressed by antiapoptotic COX-2/PGE(2) signalling in colorectal adenoma cells

Overexpression of cyclooxygenase-2 (COX-2) and elevated levels of its enzymatic product prostaglandin E2 (PGE₂) occur in the majority of colorectal cancers and play important roles in colorectal tumorigenesis. However, despite the established prosurvival role of PGE₂ in cancer, the underlying mechanisms are not fully understood. Here, we have shown that PGE₂ suppresses apoptosis via repression of the proapoptotic BH3-only protein Bim in human colorectal adenoma cells. Repression of Bim expression was dependent upon PGE₂-mediated activation of the Raf-MEK-ERK1/2 pathway which promoted Bim phosphorylation and proteasomal degradation. Reduction of Bim expression using RNA interference reduced spontaneous apoptosis in adenoma cells and abrogated PGE₂-dependent apoptosis suppression. Treatment of COX-2-expressing colorectal carcinoma cells with COX-2-selective NSAIDs induced Bim expression, suggesting that Bim repression via PGE₂ signalling may be opposed by COX-2 inhibition. Examination of Bim expression in two established in vitro models of the adenoma-carcinoma sequence revealed that downregulation of Bim expression was associated with tumour progression towards an anchorage-independent phenotype. Finally, immunohistochemical analyses revealed that Bim expression is markedly reduced in approximately 40% of human colorectal carcinomas in vivo. These observations highlight the COX-2/PGE₂ pathway as an important negative regulator of Bim expression in colorectal tumours and suggest that Bim repression may be an important step during colorectal cancer tumorigenesis.

The actin-bundling protein fascin is overexpressed in colorectal adenomas and promotes motility in adenoma cells in vitro

Background:

Fascin is overexpressed in many cancers, including colorectal, but its role in the malignant transformation of benign colorectal adenomas is unclear.

Methods:

Immunohistochemical analysis of fascin expression was carried out in resected human colorectal adenoma specimens. The effects of forced overexpression of fascin on adenoma cell motility were also analysed.

Results:

We show fascin overexpression in adenomas increasing with tumour size, histological type, and degree of dysplasia and increased cell motility in adenoma cell lines following fascin transfection.

Conclusion:

These data suggest an important role for fascin in the malignant progression of colorectal tumours.

Inhibition of COX-2 with NS-398 decreases colon cancer cell motility through blocking epidermal growth factor receptor transactivation: possibilities for combination therapy

The use of non-steroidal anti-inflammatory drugs has proved of great interest in the prevention and treatment of colorectal cancer, although their precise mechanisms of action remain unclear. Overexpression of cyclooxygenase-2 (COX-2) and subsequent prostaglandin production promote metastasis and have been shown to increase cell motility in vitro.

OBJECTIVE

We have aimed to elucidate whether specific inhibition of COX-2 with NS-398 (NS-398 is a selective inhibitor of COX-2) would be able to inhibit motility of colorectal cancer cells and whether this was modulated through epidermal growth factor receptor (EGFR) transactivation.

MATERIALS AND METHODS

A transwell filter assay was used to study cell motility. Expression of COX-2, EGFR phosphorylation and prostaglandin E(2) (PGE(2)) receptors were assessed by Western blot analysis and reverse transcriptase-polymerase chain reaction. PGE(2) concentrations after NS-398 treatment were estimated by enzyme immunoassay.

RESULTS

Treatment with NS-398 significantly reduced PGE(2) levels and reduced cell migration in the HT29 and HCA7 colorectal carcinoma cell lines and this effect was rescued by addition of PGE(2). Furthermore, specific inhibition of COX-2 with NS-398 reduced EGFR phosphorylation in colorectal cancer cells. Direct inhibition of EGFR activity with AG1478 reduced PGE(2)-stimulated motility, clearly demonstrating that PGE(2 )acts via the EGFR-signalling pathway. The novel combination of NS-398 and AG1478 dramatically reduced migration of colorectal cancer cells.

CONCLUSION

The data presented indicate that the use of NS-398 in chemoprevention and adjuvant therapy for colorectal cancer may work in part, through the inhibition of cell motility. Furthermore, our data suggest that the combined use of non-steroidal anti-inflammatory drugs with EGFR antagonists could be explored further for future use in the clinic.

Insulin-like growth factor binding protein 3 (IGFBP-3) potentiates TRAIL-induced apoptosis of human colorectal carcinoma cells through inhibition of NF-kappaB

There is growing evidence that the insulin-like growth factor-binding protein 3 (IGFBP-3) can have IGF-independent effects on cell growth. However, despite the fact that IGFBP-3 has been reported to be both antiproliferative and proapoptotic, the molecular mechanisms underlying the action of IGFBP-3 have not been elucidated. We report that although addition of IGFBP-3 (either synthetic or secreted protein) had no effect on cell survival, IGFBP-3 (100 ng/ml) significantly enhanced TNF-related apoptosis-inducing ligand (TRAIL)-induced cell death in colonic carcinoma-derived cell lines (20-30% depending on cell line), whereas it had no effect on the survival of the TRAIL-resistant adenoma-derived cells. Both addition of IGFBP-3 protein to cell cultures or enforced expression of IGFBP-3 in the HT29 carcinoma cell line inhibited nuclear factor kappa B (NF-kappaB) activation in response to the induction of apoptosis by TRAIL. We propose that IGFBP-3 is a non-toxic NF-kappaB inhibitor, which could be used as an adjuvant in the treatment of colon cancer.

Prospects in NSAID-derived chemoprevention of colorectal cancer

There is strong evidence for an important role for increased COX (cyclo-oxygenase)-2 expression and PG (prostaglandin) E2 production in colorectal tumorigenesis. PGE(2) acts through four E-prostanoid receptors (EP1-4). COX-2 has therefore become a target for the potential chemoprevention and therapy of colorectal cancer. However, any therapeutic/preventive strategy has the potential to have an impact on physiological processes and hence result in side effects. General COX (COX-1 and -2) inhibition by traditional NSAIDs (non-steroidal anti-inflammatory drugs), such as aspirin, although chemopreventive, has some side effects, as do some conventional COX-2-selective NSAIDs. As PGE2 is thought to be the major PG species responsible for promoting colorectal tumorigenesis, research is being directed to a number of protein targets downstream of COX-2 that might allow the selective inhibition of the tumour-promoting activities of PGE2, while minimizing the associated adverse events. The PGE synthases and E-prostanoid receptors (EP1-4) have therefore recently attracted considerable interest as potential novel targets for the prevention/therapy of colorectal cancer. Selective (and possibly combinatorial) inhibition of the synthesis and signalling of those PGs most highly associated with colorectal tumorigenesis may have some advantages over COX-2-selective inhibitors.

DMBT1 expression and glycosylation during the adenoma-carcinoma sequence in colorectal cancer

The gene DMBT1 (deleted in malignant brain tumour-1) has been proposed to play a role in brain and epithelial cancer, but shows unusual features for a classical tumour-suppressor gene. On the one hand, DMBT1 has been linked to mucosal protection, whereas, on the other, it potentially plays a role in epithelial differentiation. Thus its function in a particular tissue is of mechanistic importance for its role in cancer. Because the former function requires secretion to the lumen and the latter function may depend on its presence in the extracellular matrix, we decided to investigate DMBT1 expression, location and its mode of secretion during malignant transformation in colorectal cancer. Using human colorectal PC/AA cell lines and tissue sections from individual patients, we have examined the expression of DMBT1 and its glycosylation in the adenoma-carcinoma sequence leading to the adenocarcinoma phenotype.

The endogenous cannabinoid, anandamide, induces cell death in colorectal carcinoma cells: a possible role for cyclooxygenase 2

BACKGROUND AND AIMS

Cyclooxygenase 2 (COX-2) is upregulated in most colorectal cancers and is responsible for metabolism of the endogenous cannabinoid, anandamide, into prostaglandin-ethanolamides (PG-EAs). The aims of this study were to determine whether anandamide and PG-EAs induce cell death in colorectal carcinoma (CRC) cells, and whether high levels of COX-2 in CRC cells could be utilised for their specific targeting for cell death by anandamide.

METHODS

We determined the effect of anandamide on human CRC cell growth by measuring cell growth and cell death, whether this was dependent on COX-2 protein expression or enzyme activity, and the potential involvement of PG-EAs in induction of cell death.

RESULTS

Anandamide inhibited the growth of CRC cell lines HT29 and HCA7/C29 (moderate and high COX-2 expressors, respectively) but had little effect on the very low COX-2 expressing CRC cell line, SW480. Induction of cell death in HT29 and HCA7/C29 cell lines was partially rescued by the COX-2 selective inhibitor NS398. Cell death induced by anandamide was neither apoptosis nor necrosis. Furthermore, inhibition of fatty acid amide hydrolase potentiated the non-apoptotic cell death, indicating that anandamide induced cell death was mediated via metabolism of anandamide by COX-2, rather than its degradation into arachidonic acid and ethanolamine. Interestingly, both PGE2-EA and PGD2-EA induced classical apoptosis.

CONCLUSIONS

These findings suggest anandamide may be a useful chemopreventive/therapeutic agent for colorectal cancer as it targets cells that are high expressors of COX-2, and may also be used in the eradication of tumour cells that have become resistant to apoptosis.

Nuclear BAG-1 expression inhibits apoptosis in colorectal adenoma-derived epithelial cells

BAG-1 is an anti-apoptotic protein that is frequently deregulated in a variety of malignancies including colorectal cancer. There are three isoforms: BAG-1L is located in the nucleus, BAG-1M and BAG-1S are located both in the nucleus and the cytoplasm. In colon cancer, the expression of nuclear BAG-1 is associated with poorer prognosis and is potentially a useful predictive factor for distant metastasis. However, the function of BAG-1 in colonic epithelial cells has not been studied. Having previously shown a predominant nuclear localisation of BAG-1 in adenoma-derived cell lines, we wanted to determine the function of nuclear BAG-1 in these non-tumourigenic cells, to identify whether nuclear BAG-1 was implicated in tumour progression in the colon. In the current report we established that nuclear BAG-1 inhibits apoptosis in a colorectal adenoma-derived cell line. We demonstrate that apoptosis induced by gamma-radiation or the vitamin D analogue EB1089 in the non-tumourigenic human colorectal adenoma-derived S/RG/C2 cell line, was preceded by a decrease in nuclear and an increase in cytoplasmic BAG-1 expression. This change in subcellular localisation of BAG-1 was due to the redistribution of the BAG-1M isoform. In addition, we have shown that the maintenance of high nuclear BAG-1 through enforced expression of the nuclear localised BAG-1L isoform enhanced cellular survival after gamma-radiation or exposure to EB1089. Furthermore the expression of cytoplasmic BAG-1S isoform fused with a nuclear localisation signal protected against gamma-radiation induced apoptosis. This demonstrates that nuclear localisation of the BAG-1 protein confers a survival advantage in colorectal adenoma-derived cells and that nuclear BAG-1 could potentially be an important survival factor in colorectal carcinogenesis.

Colorectal cancer prognosis: is it all mutation, mutation, mutation?

For the 500,000 new cases of colorectal cancer in the world each year, identification of patients with a worse prognosis and those who are more likely to respond to treatment is a challenge. There is an increasing body of evidence correlating genetic mutations with outcome in tumours derived from human colorectal cancer cohorts. K-ras, but not p53 or APC, mutations appear to be associated with poorer overall survival in colorectal cancer patients.

Action of a library of O-glycosylation inhibitors on the growth of human colorectal cancer cells in culture

O-glycosylation is thought to play a significant role in the regulation of cell growth. However, only limited information is available, and few specific and selective inhibitors have been found. We have synthesized a library of O-glycosylation inhibitors based on benzyl-O-N-acetyl-D-galactosamine. These inhibitors were tested with an established series of human colorectal cancer cell lines, which model the adenoma-carcinoma sequence. Cancer cells were incubated with the inhibitors, and examined for cell growth patterns, and cellular and subcellular glycosylation using a range of lectins with confocal microscopy. The specificity of O-glycan inhibition was confirmed for the library, relative to other forms of glycosylation. All inhibitors tested resulted in smaller cell yields. However, a differential effect on O-glycosylation was detected using the lectins showing variation of localization at a subcellular level in the various cell lines. Further differential action of the inhibitor library was observed for apoptosis and on the cell cycle with the cell lines tested. This work demonstrates that O-glycosylation is closely involved in the regulation of cell growth in colorectal cancer cells and that the generation of a library of low-molecular-mass inhibitors offers a valuable means of examining this regulation at the molecular level.

Cannabinoids and cancer: potential for colorectal cancer therapy

Despite extensive research into the biology of CRC (colorectal cancer), and recent advances in surgical techniques and chemotherapy, CRC continues to be a major cause of death throughout the world. Therefore it is important to develop novel chemopreventive/chemotherapeutic agents for CRC. Cannabinoids are a class of compounds that are currently used in the treatment of chemotherapy-induced nausea and vomiting, and in the stimulation of appetite. However, there is accumulating evidence that they could also be useful for the inhibition of tumour cell growth by modulating key survival signalling pathways. The chemotherapeutic potential for plant-derived and endogenous cannabinoids in CRC therapy is reviewed.

The role of the retinoblastoma protein (Rb) in the nuclear localization of BAG-1: implications for colorectal tumour cell survival

Although the retinoblastoma susceptibility gene RB1 is inactivated in a wide variety of human cancers, the retinoblastoma protein (Rb) has been shown to be overexpressed in colon cancers, which is linked to the anti-apoptotic function of the protein. However, the mechanisms by which Rb regulates apoptosis are yet to be fully elucidated. We have established that Rb interacts with the anti-apoptotic BAG-1 (Bcl-2 associated athanogene-1) protein, and that a decrease in nuclear localization of BAG-1 is detectable when the interaction between Rb and BAG-1 is disrupted by expression of the E7 viral oncoprotein. Interestingly, although reported as deregulated in colorectal cancers, we have found that BAG-1 expression is also altered in small adenomas, where its localization was found to be predominantly nuclear. In addition, we have established that maintenance of high nuclear BAG-1 in vitro increases the resistance of adenoma-derived cells to γ-radiation-induced apoptosis. Our work suggests a novel function for Rb, involving modulation of the subcellular localization of BAG-1. We have found predominant nuclear BAG-1 localization in small adenomas, and suggest that BAG-1 may promote colorectal tumour cell survival by making colonic epithelial cells less sensitive to DNA damage.

Increased sensitivity to TRAIL-induced apoptosis occurs during the adenoma to carcinoma transition of colorectal carcinogenesis

The death ligand TRAIL (Apo2L) has potential for cancer therapy, since tumour cells are thought to be more sensitive than normal cells. We investigated whether sensitivity to TRAIL increases during the adenoma to carcinoma transition of colorectal carcinogenesis. Under the same culture conditions, we compared the extent of TRAIL-induced apoptosis in four premalignant adenoma and three carcinoma cell lines. Although TRAIL induced some apoptosis in adenoma cultures, the carcinoma cell lines were significantly more sensitive (P<0.001). This finding was recapitulated in an in vitro model of tumour progression in which conversion of the adenoma cell line AA/C1 to a tumorigenic phenotype was associated with increased TRAIL sensitivity (P<0.001). Increased TRAIL sensitivity during colorectal carcinogenesis has been previously attributed to changes in the balance between TRAIL receptors TRAIL-R1 and -R2 and ‘decoy’ receptors TRAIL-R3 and -R4 during malignant progression. To address this, cell surface receptor expression was measured by flow cytometry. In summary, during colorectal carcinogenesis, there is a marked increase in sensitivity to TRAIL-induced apoptosis associated with progression from benign to malignant tumour that could be exploited for colon cancer therapy, but alterations in cell surface TRAIL receptor expression may not be the primary reason for this change.

Induced apoptosis in the prevention of colorectal cancer by non-steroidal anti-inflammatory drugs

Epidemiological, clinical and animal studies indicate non-steroidal anti-inflammatory drugs (NSAIDs) to be chemopreventive for colorectal cancer. The best established target for NSAIDs are the two isoforms of cyclooxygenase (COX), a key enzyme in the biosynthesis of prostaglandins. Recent investigations using human colorectal tumor cell lines have focused on the cellular and molecular mechanisms potentially underlying the chemopreventive effect of NSAIDs. These studies have used 'traditional' NSAIDs and their metabolites which either do not inhibit COX, are non-selective for the COX isoforms or selectively inhibit COX-1 over COX-2, and recently developed NSAIDs that are highly selective for COX-2. In vitro, apoptosis is the dominant anti-proliferative effect of each of these classes of NSAID and sensitivity to NSAID-induced apoptosis increases with the malignant potential of the tumor cells. Limited in vivo evidence backs up these findings. Cell cycle arrest also contributes to the in vitro growth inhibitory effect of traditional NSAIDs. The induction of apoptosis by NSAIDs may result from the inhibition of the COX isoforms but other as yet undefined paths to NSAID-induced apoptosis clearly exist. A member of each class of NSAID is under trial as a chemopreventive agent for colorectal cancer.

Increased NF-kappaB DNA binding but not transcriptional activity during apoptosis induced by the COX-2-selective inhibitor NS-398 in colorectal carcinoma cells

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit colorectal neoplasia, an effect that is associated with their ability to induce apoptosis. Although NSAIDs have been reported to inhibit NF-κB, more recent studies show activation of NF-κB by NSAIDs. NF-κB commonly shows antiapoptotic activity and is implicated in the therapeutic resistance of cancer cells. The effects of highly COX-2-selective NSAIDs such as NS-398 on NF-κB in colorectal tumour cells have not been reported. Therefore, we addressed whether NF-κB has a role in NS-398-induced apoptosis of colorectal cancer cells. Treatment of HT-29 colorectal carcinoma cells with doses of NS-398 (50–75 μM) known to induce apoptosis had no effect on NF-κB for up to 48 h. However after 72 and 96 h NF-κB DNA-binding activity was increased by NS-398, in parallel with apoptosis induction. NS-398-treated HT-29 cells showed increased p50 homodimer binding and an induction of p50/p65 heterodimers, as demonstrated by supershift assay. However, although NS-398 increased NF-κB DNA binding it did not increase NF-κB-dependent reporter activity and inhibition of NF-κB DNA binding did not enhance NS-398-induced apoptosis. This indicates that NF-κB activated by NS-398 is transcriptionally inactive and is an encouraging result for the use of COX-2-selective NSAIDs not only in chemoprevention but also as novel therapies for colon cancer.

Butyrate downregulates alpha2beta1 integrin: a possible role in the induction of apoptosis in colorectal cancer cell lines

BACKGROUND

Integrins mediate cell matrix adhesion and regulate cell growth and survival. In colonic epithelial cells, alpha(2)beta(1) integrin controls glandular differentiation and proliferation. Butyrate stimulates differentiation and induces apoptosis in vitro.

AIMS

We investigated whether butyrate induction of apoptosis was associated with perturbation of integrin mediated cell matrix adhesion.

METHODS

Three colonic cancer cell lines (SW1222, SW620, LS174T) were studied. Adhesion to extracellular matrix proteins, expression of alpha(2)beta(1) integrin, and apoptosis were studied in adherent cells after treatment with 4 mM butyrate.

RESULTS

Butyrate decreased the attachment to type I collagen in SW620 cells and type I and IV collagen in LS174T cells. The decreased cell attachment was associated with downregulation of alpha(2)beta(1) integrin and increased apoptosis in adherent cells. No changes in alpha(2)beta(1) expression or matrix adhesion were seen in SW1222 cells, which were also found to be less sensitive to butyrate induction of apoptosis. Downregulation of alpha(2)beta(1) integrin preceded the detection of apoptosis.

CONCLUSION

Apoptosis induced by butyrate is associated with downregulation of expression and functional activity of alpha(2)beta(1) integrin. Perturbation of cell matrix adhesion may be a novel mechanism by which butyrate induces apoptosis in colorectal cancer cells.

Transcriptional upregulation of the insulin-like growth factor binding protein IGFBP-3 by sodium butyrate increases IGF-independent apoptosis in human colonic adenoma-derived epithelial cells

Sodium butyrate (NaBt) and the pro-apoptotic IGFBP-3 protein, expressed at the top of the normal colonic crypt, have both been implicated in the regulation of apoptosis in colonic epithelial cells. Recent studies in human breast and hepatic cell lines have shown that NaBt can transcriptionally upregulate IGFBP-3 expression. However, the role of butyrate in the regulation of IGFBP-3 expression in the colon is less clear, with reports of both up- and downregulation of the IGFBP-3 protein in colorectal cancer cell lines. In this study we have shown that the level of IGFBP-3 protein expression in colonic epithelial cells correlates with the p53 status of the cells; wildtype p53 cells secrete higher levels of IGFBP3 protein than mutant p53 cell lines. Data presented shows that, when treated with a dose of NaBt that induced significant apoptosis (4 mM for 48 h), there was an upregulation of IGFBP-3 protein in both wildtype and mutant p53 expressing cell lines. The NaBt-induced increase in secreted IGFBP-3 protein was associated with transcriptional upregulation of the IGFBP-3 gene. Using a transfected derivative of the S/RG/C2 adenoma-derived cell line, which stably expressed exogenous IGFBP-3 protein at levels equivalent to that secreted by the 4 mM NaBt-treated parental line (1-3 ng/10(6) cells), we have shown a >2-fold increase in the sensitivity of the cells to NaBt-induced apoptosis when compared with the vector control and parental cell lines. Furthermore, inhibition of the secreted IGFBP-3 protein, by addition of neutralizing antibodies, resulted in a significant decrease in NaBt-induced apoptosis. These data suggest that IGFBP-3 may act as a positive regulator of NaBt-induced apoptosis in colonic epithelial cells, and represents a potentially important mechanism whereby the sensitivity of colonic epithelial cells to NaBt-induced apoptosis can be increased.

Expression of CDX2 in normal and neoplastic human colon tissue and during differentiation of an in vitro model system

BACKGROUND

The Cdx genes are expressed in the colorectal epithelium and are frequently downregulated during tumorigenesis. Overexpression of Cdx genes has been shown previously to result in cellular differentiation.

AIM

To study expression of CDX2 in normal and neoplastic human colon using a newly isolated monoclonal antibody. To define expression of CDX1 and CDX2 in an in vitro model system of colorectal tumour progression and to ascertain whether these are subject to regulation during differentiation.

METHODS

Normal and neoplastic human colon was immunostained for CDX2. CDX1 and CDX2 expression was assayed in cell lines derived from premalignant colonic adenomas by western blotting. Differentiation was induced by sodium butyrate treatment or post confluent growth, and changes in CDX expression compared with carcinoma cell lines with low levels of CDX expression.

RESULTS

CDX2 protein displayed no gradient of expression within the colonic crypt. Cell lines derived from adenomas, with high levels of CDX1 and CDX2, showed no regulation of these proteins when induced to differentiate by butyrate or confluency. CDX expression in these cell lines was independent of their APC or Ras status. CDX1 and CDX2 were expressed at very low levels in some carcinoma cell lines and were modestly upregulated on differentiation but were not restored to levels seen in adenoma cells.

CONCLUSION

The lack of significant regulation on cellular differentiation and the absence of a detectable gradient in the crypt implies that CDX2 may confer tissue specificity but may not play the previously suggested role in crypt patterning.

Abrogation of the radiation-induced G2 checkpoint by the staurosporine derivative UCN-01 is associated with radiosensitisation in a subset of colorectal tumour cell lines

Ionising radiation is commonly used in the treatment of colorectal cancer. Tumour cells with mutant p53 undergo cell cycle arrest at G2/M after ionising radiation and evidence suggests that abrogation of this G2 arrest can lead to a premature, aberrant mitosis, thus enhancing ionising radiation-induced cell killing. The G2 checkpoint inhibitor UCN-01 was thus investigated to determine whether it would abrogate the G2 checkpoint induced by 5 Gy ionising radiation in a range of colorectal tumour cell lines. Data presented show that, at doses that are alone non-toxic to the cells, UCN-01 inhibits the ionising radiation-induced G2 checkpoint in five colorectal tumour cell lines with mutant p53. The ability of UCN-01 to sensitise cells to ionising radiation-induced growth inhibition and apoptosis was also investigated and UCN-01 was found to radiosensitise two out of five cell lines. These results were confirmed by long-term colony forming efficiency studies. These results demonstrate that abrogation of the ionising radiation-induced G2 checkpoint is not necessarily associated with sensitisation to ionising radiation, however, some colorectal tumour cell lines can be radiosensitised by UCN-01. Although the mechanism of radiosensitisation is not clear, this may still be an important treatment strategy.

The protrusive phase and full development of integrin-dependent adhesions in colon epithelial cells require FAK- and ERK-mediated actin spike formation: deregulation in cancer cells

Integrins play an important role in tumour progression by influencing cellular responses and matrix-dependent adhesion. However, the regulation of matrix-dependent adhesion assembly in epithelial cells is poorly understood. We have investigated the integrin and signalling requirements of cell-matrix adhesion assembly in colon carcinoma cells after plating on fibronectin. Adhesion assembly in these, and in the adenoma cells from which they were derived, was largely dependent on alpha v beta 6 integrin and required phosphorylation of FAK on tyrosine-397. The rate of fibronectin-induced adhesion assembly and the expression of both alpha v beta 6 integrin and FAK were increased during the adenoma-to-carcinoma transition. The matrix-dependent adhesion assembly process, particularly the final stages of complex protrusion that is required for optimal cell spreading, required the activity of extracellular signal-regulated kinase (ERK). Furthermore, phosphorylated ERK was targeted to newly forming cell--matrix adhesions in the carcinoma cells but not the adenoma cells, and inhibition of FAK--tyrosine-397 phosphorylation or MEK suppressed the appearance of phosphorylated ERK at peripheral sites. In addition, inhibition of MEK--ERK activation blocked the formation of peripheral actin microspikes that were necessary for the protrusive phase of cell-matrix adhesion assembly. Thus, MEK--ERK--dependent peripheral actin re-organization is required for the full development of integrin-induced adhesions and this pathway is stimulated in an in vitro model of colon cancer progression.

Transcriptional down-regulation of the retinoblastoma protein is associated with differentiation and apoptosis in human colorectal epithelial cells

The aim of this study was to investigate the regulation of Rb protein expression in relation to increased differentiation and induction of apoptosis in colonic epithelial cells. In vivo, Rb protein expression was found to be down-regulated towards the top of the normal colonic crypt, coincident with the region of differentiation and apoptosis, but highly expressed in colonic carcinoma tissue. Using in vitro models to study the regulation of Rb expression in pre-malignant colonic epithelial cells, we have been able to show for the first time that Rb protein expression is transcriptionally down-regulated in differentiated pre-malignant cells (in post-confluent cultures) but not in malignant colorectal epithelial cells. Furthermore, suppression of rb protein function by the HPV-E7 viral oncoprotein increased both spontaneous and DNA damage-induced apoptosis. These results suggest that Rb is able to act as a survival factor in colonic epithelial cells by suppressing apoptosis, and that over-expression of pRb in colorectal tumour cells can cause a loss of sensitivity to apoptotic signalling, resulting in aberrant cell survival and resistance to therapy.

Apoptosis induction and cyclooxygenase-2 regulation in human colorectal adenoma and carcinoma cell lines by the cyclooxygenase-2-selective non-steroidal anti-inflammatory drug NS-398

We determined the effect of the highly selective cyclooxygenase-2 (COX-2) inhibitor NS-398 on proliferation, apoptosis and COX-2 regulation in 3 pre-malignant human colorectal adenoma cell lines (RG/C2, AA/C1, RR/C1) and compared its effect on 3 colorectal carcinoma cell lines (HT29, KS, JW2). COX-2 protein was expressed in each cell line derived from an adenoma, thus providing evidence that COX-2 is expressed in the tumour cells themselves at an early stage in human colorectal adenoma formation. NS-398 (20 to 100 microM for 96 h) induced apoptosis and inhibited the proliferation of the adenoma cell lines. Of the 3 carcinoma lines, only HT29 expressed COX-2 protein, yet each line was similarly sensitive to NS-398. There was a positive correlation between overall sensitivity of the cell lines (determined by the attached cell yield) and sensitivity to NS-398-induced apoptosis, suggesting that apoptosis is the dominant anti-proliferative effect of NS-398. Two of the 3 adenoma cell lines (RG/C2, AA/C1) were less sensitive than the carcinoma cell lines. NS-398 up-regulated COX-2 protein expression in the HT29 and adenoma cell lines. This was studied further in HT29 cultures, where treatment with NS-398 inhibited COX-2 activity, reducing prostaglandin E(2) secretion. Here, neither the increase in COX-2 protein expression nor the anti-proliferative and apoptosis-inducing effect of NS-398 was prevented by addition of exogenous prostaglandin E(2). Apoptosis appears to be the dominant anti-proliferative effect of NS-398 and, in COX-2 expressing cells, may be mechanistically linked to the observed induction of COX-2 protein expression upon treatment with NS-398.

Apoptosis induction and cyclooxygenase-2 regulation in human colorectal adenoma and carcinoma cell lines by the cyclooxygenase-2-selective non-steroidal anti-inflammatory drug NS-398

We determined the effect of the highly selective cyclooxygenase-2 (COX-2) inhibitor NS-398 on proliferation, apoptosis and COX-2 regulation in 3 pre-malignant human colorectal adenoma cell lines (RG/C2, AA/C1, RR/C1) and compared its effect on 3 colorectal carcinoma cell lines (HT29, KS, JW2). COX-2 protein was expressed in each cell line derived from an adenoma, thus providing evidence that COX-2 is expressed in the tumour cells themselves at an early stage in human colorectal adenoma formation. NS-398 (20 to 100 microM for 96 h) induced apoptosis and inhibited the proliferation of the adenoma cell lines. Of the 3 carcinoma lines, only HT29 expressed COX-2 protein, yet each line was similarly sensitive to NS-398. There was a positive correlation between overall sensitivity of the cell lines (determined by the attached cell yield) and sensitivity to NS-398-induced apoptosis, suggesting that apoptosis is the dominant anti-proliferative effect of NS-398. Two of the 3 adenoma cell lines (RG/C2, AA/C1) were less sensitive than the carcinoma cell lines. NS-398 up-regulated COX-2 protein expression in the HT29 and adenoma cell lines. This was studied further in HT29 cultures, where treatment with NS-398 inhibited COX-2 activity, reducing prostaglandin E(2) secretion. Here, neither the increase in COX-2 protein expression nor the anti-proliferative and apoptosis-inducing effect of NS-398 was prevented by addition of exogenous prostaglandin E(2). Apoptosis appears to be the dominant anti-proliferative effect of NS-398 and, in COX-2 expressing cells, may be mechanistically linked to the observed induction of COX-2 protein expression upon treatment with NS-398.

Apoptosis is induced by the active metabolite of vitamin D3 and its analogue EB1089 in colorectal adenoma and carcinoma cells: possible implications for prevention and therapy

Vitamin D3 is believed to reduce the risk of colon cancer, and serum levels inversely correlate with colorectal cancer incidence. The active metabolite, 1alpha,25-dihydroxyvitamin D3, has previously been shown to inhibit growth and promote differentiation of colon cancer cells. The vitamin D analogue, EB1089, is currently under clinical trial in a variety of cancers because of its growth-inhibitory effects in vitro and reduced hypercalcemic effects in vivo. The mechanism of growth inhibition by EB1089, however, remained to be determined. In this study we examined the effects of alpha,25-dihydroxyvitamin D3 and EB1089 on five colorectal tumor cell lines (two adenoma and three carcinoma) to determine the mechanism of growth inhibition and to ascertain whether premalignant adenoma cells were responsive to these agents. 1alpha,25-Dihydroxyvitamin D3 and EB1089 induced p53-independent apoptosis in adenoma and carcinoma cell lines in a dose-dependent manner between 10(-10) and 10(-6) M. EB1089, as well as inducing apoptosis, increased the proportion of cells in the G1 phase, particularly in the adenoma cell lines. In two of the three carcinoma cell lines (SW620 and PC/JW), levels of apoptosis induced by EB1089 were similar or greater than those induced by 1alpha,25-dihydroxyvitamin D3. Although the carcinoma cell line HT29 was relatively resistant to apoptosis induced by EB1089 compared with 1alpha,25-dihydroxyvitamin D3, EB1089 markedly inhibited cell yields. These observations offer promise for the clinical use of EB1089. To determine whether the induction of apoptosis by 1alpha,25-dihydroxyvitamin D3 and EB1089 was potentially via a differentiation pathway, alkaline phosphatase activity was measured as a marker of differentiation. Induction of alkaline phosphatase was observed in the floating apoptotic cells as well as in the adherent population. A link between the induction of differentiation and apoptosis by 1alpha,25-dihydroxyvitamin D3 and EB1089 is suggested by the occurrence of apoptosis subsequent to the induction of differentiation. To investigate the molecular pathway to apoptosis induction, members of the Bcl-2 family of proteins were examined (Bcl-2, Bcl-x, Bax, and Bak). Decreased Bcl-2 was observed in some cell lines, particularly in response to EB1089, but was not essential for apoptosis. Levels of the proapoptotic protein Bak, however, were consistently increased in all of the five cell lines in association with apoptosis induced by either agent. The results implicate Bak protein in the induction of apoptosis by 1alpha,25-dihydroxyvitamin D3 or its analogue EB1089. The ability of EB1089 to induce apoptosis in colorectal carcinoma cells suggests that this or other vitamin D analogues may prove clinically effective for the treatment of colorectal cancer. Furthermore, the fact that it induces cell cycle arrest and apoptosis in the premalignant adenoma cells may suggest an application in colorectal cancer chemoprevention.

Increased p53-dependent apoptosis by the insulin-like growth factor binding protein IGFBP-3 in human colonic adenoma-derived cells

We investigated the expression of insulin-like growth factor binding protein 3 (IGFBP-3) in normal human colonic epithelium and whether IGFBP-3 is involved in the induction of apoptosis in colonic epithelial cells. A gradient of IGFBP-3 protein expression was observed within the normal colonic crypt, and increased IGFBP-3 expression was coincident with the region of increased differentiation and apoptosis. Treatment of human colonic tumor cell lines with IGFBP-3 alone was shown to have no effect on growth. However, an increase in p53-dependent apoptosis was observed in the presence of 100 ng/ml IGFBP-3 24 h after the induction of DNA damage by gamma-irradiation. These results suggest that IGFBP-3 enhances the p53-dependent apoptotic response of colorectal cells to DNA damage.

The catalytic activity of the Src family kinases is required to disrupt cadherin-dependent cell-cell contacts

Despite the importance of epithelial cell contacts in determining cell behavior, we still lack a detailed understanding of the assembly and disassembly of intercellular contacts. Here we examined the role of the catalytic activity of the Src family kinases at epithelial cell contacts in vitro. Like E- and P-cadherin, Ca(2+) treatment of normal and tumor-derived human keratinocytes resulted in c-Yes (and c-Src and Fyn), as well as their putative substrate p120(CTN), being recruited to cell-cell contacts. A tyrosine kinase inhibitor with selectivity against the Src family kinases, PD162531, and a dominant-inhibitory c-Src protein that interferes with the catalytic function of the endogenous Src kinases induced cell-cell contact and E-cadherin redistribution, even in low Ca(2+), which does not normally support stable cell-cell adhesion. Time-lapse microscopy demonstrated that Src kinase inhibition induced stabilization of transiently formed intercellular contacts in low Ca(2+). Furthermore, a combination of E- and P-cadherin-specific antibodies suppressed cell-cell contact, indicating cadherin involvement. As a consequence of contact stabilization, normal cells were unable to dissociate from an epithelial sheet formed at high density and repair a wound in vitro, although individual cells were still motile. Thus, cadherin-dependent contacts can be stabilized both by high Ca(2+) and by inhibiting Src activity in low (0.03 mM) Ca(2+) in vitro.

A cyclooxygenase-2 (COX-2) selective non-steroidal anti-inflammatory drug enhances the growth inhibitory effect of butyrate in colorectal carcinoma cells expressing COX-2 protein: regulation of COX-2 by butyrate

Epidemiological, clinical, animal and laboratory studies have all provided evidence for the protective effects of non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin, against colorectal cancer. The main established target for NSAID action is cyclooxygenase (COX) and the inducible isoform, COX-2, is up-regulated in colorectal cancer. Rat intestinal epithelial cells transfected with a COX-2 expression vector have previously been found to be resistant to butyrate-induced apoptosis. Butyrate, a by-product of dietary fibre fermentation, is known to induce differentiation and apoptosis in colorectal tumour cells in vitro. In recent years there has been considerable interest in the possible role of dietary fibre/resistant starch in the prevention of colorectal cancer. In this study we investigated whether inhibition of COX-2 with a highly selective COX-2 inhibitor (NS-398) would sensitize human colorectal carcinoma cells to the growth inhibitory effect of butyrate. HT29 and S/KS colorectal carcinoma cell lines were treated for 72 h with 2 mM butyrate and/or 10 microM NS-398. Addition of 10 microM NS-398 alone (to inhibit COX-2 activity) did not result in detectable growth inhibition in either of the cell lines. NS-398 enhanced sensitivity to the growth inhibitory effect of butyrate in HT29 cells expressing COX-2 protein. In contrast, NS-398 did not sensitize S/KS cells lacking detectable COX-2 protein and function (as determined by prostaglandin E(2) production) to the growth inhibitory effect of butyrate. In addition, we report that butyrate treatment of carcinoma (HT29) and adenoma (PC/AA/C1) cells leads to up-regulation of COX-2 protein. Thus NS-398 only appears to sensitize human colorectal carcinoma cells expressing COX-2 protein to the growth inhibitory effect of butyrate. As COX-2 is up-regulated in colorectal carcinogenesis, this could have important implications for the selective inhibition of cells expressing COX-2 protein over those lacking COX-2 protein expression and for dietary modification to be considered alongside NSAIDs in the prevention, and possibly treatment, of colorectal cancer.

Increased dosage and amplification of the focal adhesion kinase gene in human cancer cells

Focal adhesion kinase (pp125FAK) is present at sites of cell/extracellular matrix adhesion and has been implicated in the control of cell behaviour. In particular, as a key component of integrin-stimulated signal transduction pathways, pp125FAK is involved in cellular processes such as spreading, motility, growth and survival. In addition, a number of reports have indicated that pp125FAK may be up-regulated in human tumour cells of diverse origin, and consequently, a role has been proposed for pp125FAK in the development of invasive cancers. However, to date the mechanisms that lead to elevated pp125FAK expression in tumour cells have not been determined. Here we used in situ hybridization to confirm chromosome 8q as the genomic location of the human fak gene and report that elevation of pp125FAK protein in cell lines derived from invasive squamous cell carcinomas is accompanied by gains in copy number of the fak gene in all cases examined. In addition, we observed increased fak copy number in frozen sections of squamous cell carcinomas. Furthermore, increased dosage of the fak gene was also observed in many cell lines derived from human tumours of lung, breast and colon, including two cell lines Calu3 and HT29, in which fak was amplified. In addition, in an in vitro model for human colon cancer progression there was a copy number gain of the fak gene during conversion from adenoma to carcinoma, which was associated with increased pp125FAK protein expression. Thus, we show for the first time that many cell lines derived from invasive epithelial tumours have increased dosage of the fak gene, which may contribute to the elevated protein expression commonly observed. Although other genes near the fak locus are co-amplified or increased in copy number, including the proto-oncogene c-myc, the biological properties of pp125FAK in controlling the growth, survival and invasiveness of tumour cells, suggest that it may contribute to the selection pressure for maintaining increased dosage of the region of chromosome 8q that encodes these genes.

Coordinated modulation of the fibroblast growth factor dual receptor mechanism during transformation from human colon adenoma to carcinoma

Basic fibroblast growth factor (bFGF) is dependent on heparan sulphate for its ability to activate the cell surface signal transducing receptor. We have investigated the FGF dual receptor mechanism in a novel model of the transformation from human colon adenoma to carcinoma in vitro. Reverse transcription-polymerase chain reaction showed that mRNA for FGF receptors 1 and 2 were expressed in both the adenoma and carcinoma cells whereas immunocytochemistry showed that the expression of the FGF R1 was reduced significantly in the carcinoma cells. We have reported previously that the composition and sequence of human colon adenoma and carcinoma heparan sulphate (HS) differ in a defined and specific manner. The functional significance of these changes was assessed by affinity co-electrophoresis, which showed that the affinity of adenoma HS for bFGF was 10-fold greater than that of the carcinoma HS (Kd 220 nM vs. 2493 nM, respectively). In addition, Northern studies of the expression of syndecan 1 and 4 mRNA showed that proteoglycan core protein expression was reduced significantly in the carcinoma cells. These findings were associated with a reduced biological response to bFGF in the carcinoma cells that could be partially reversed by the addition of exogenous heparin, suggesting that both the proteoglycan and signal transducing receptor control the cells' response to bFGF.

Butyrate- but not TGFβ1-induced apoptosis of colorectal adenoma cells is associated with increased expression of the differentiation markers E-cadherin and alkaline phosphatase

Sodium butyrate and transforming growth factor beta (TGFbeta1) are growth inhibitory to colonic adenoma cell lines. Butyrate induces apoptosis, whereas in some adenoma cell lines, TGFbeta1 can be growth inhibitory without apoptosis. In this report, we show that the adenoma cell line PC/BH/C1 undergoes apoptosis in response to TGFbeta1. Butyrate induced cell death is preceded by the induction of two markers of colonic differentiation--alkaline phosphatase (ALP) activity and E-cadherin protein expression. However, TGFbeta1-induced apoptosis was not accompanied by induction of these differentiation markers. It is possible that the apoptosis induced by TGFbeta1 in the adenoma cell line PC/BH/C1 is due to conflicting signals, as downregulation of c-myc protein in response to TGFbeta1 occurs only slowly in this cell line. Development of resistance to TGFbeta1 in colonic tumours may involve two separate stages--resistance to growth inhibition and resistance to TGFbeta1-induced apoptosis. Our results indicate that sodium butyrate induces apoptosis via differentiation, but TGFbeta1 induces apoptosis by a differentiation-independent mechanism. As for butyrate, the induction of E-cadherin expression is a potentially important chemopreventative action, since increased E-cadherin expression has been correlated with decreased metastatic potential. This is the first report of induction of E-cadherin by a naturally occurring factor in the diet. Butyrate may reduce tumour growth and invasion, not only as a result of the induction of apoptosis, but also through increased expression of E-cadherin.

A study of the intracellular and secreted forms of the MUC2 mucin from the PC/AA intestinal cell line

In this study we present data on the entire population of MUC2 molecules secreted from and within the cell layer of an intestinal cell line. The molecular size distribution of the extracted molecules and their reactivity with two different MUC2 polypeptide antibodies indicated the presence of precursor and mature forms of the mucin. Oligomerized forms of the mucin were found in both the cell layer and medium; however, precursor forms were confined to the cell layer. Isopycnic density gradient centrifugation gave good resolution of mature and precursor forms of MUC2 as assessed by agarose gel electrophoresis. Three different populations of MUC2 were identified: one at low density (>1.3 g/ml) containing the N-glycosylated, non-O-glycosylated polypeptide; a second at intermediate density (1.3-1.35 g/ml) which may represent partially O-glycosylated intermediates; and a third at high density (1.36-1.48 g/ml) containing the mature MUC2 mucins. Rate-zonal centrifugation and agarose electrophoretic analysis of the low-density fraction indicated that the N-glycosylated MUC2 polypeptide was present as putative monomer and dimer/oligomer species. The combination of isopycnic density gradient centrifugation with agarose electrophoresis provides a new and simple approach that allows us to follow the MUC2 gene product from polypeptide through to the mature glycosylated mucin.

Reduction of sialic acid O-acetylation in human colonic mucins in the adenoma-carcinoma sequence

The oligo-O-acetylation of sialic acids found in normal colonic mucins is greatly reduced in colorectal cancer. Mucins prepared from cancer tissue in adenocarcinoma showed this reduction, while normal O-acetylation was detected in resection margin and control cases and total mucin sialic acid content was significantly decreased in cancer vs. control samples. A reduction of the O-acetyl transferase activity catalysing the O-acetylation reaction was also found. A series of cultured human colorectal cell lines derived from the same premalignant adenomatous line, and representative of the adenoma-carcinoma sequence were examined and revealed a depletion of oligo-O-acetylation in the original diploid premalignant line, re-expression in a further premalignant line and reduction in malignant mucinous and adenocarcinoma cell lines. Reduction of sialic acid O-acetylation appears as an early event in the process of malignant transformation in human colorectal cancer.

An acidic environment leads to p53 dependent induction of apoptosis in human adenoma and carcinoma cell lines: implications for clonal selection during colorectal carcinogenesis

As tumours are known to acidify their microenvironment and fluctuations in lumenal pH have been reported in a number of colonic disease conditions, we investigated whether loss of p53 function, commonly associated with the adenoma to carcinoma transition in human colorectal epithelium, was implicated in the cellular response to changes in extracellular pH. Human colonic adenoma and carcinoma derived cell lines were incubated at an initial pH range of 5.5-8.0 and the attached cell yield and apoptotic cell yield determined after 4 days. Exposure of all cell lines to an acidic growth environment was associated with a G1 arrest, down regulation of the retinoblastoma protein (pRb) protein and switch to the hypophosphorylated form of the protein, and increased expression of the p21 protein. However, induction of apoptosis, associated with increased p53 protein expression but not with changes in Bcl-2 expression, was only detected in the adenoma derived BH/C1 and AA/C1 cell lines which express wild type p53 activity. Furthermore, this induction of apoptosis was inhibited in the transfected cell line AA/273p53/B, in which the wild type p53 function has been abrogated. These results suggest that acidification of the microenvironment would provide a selective growth advantage for cells that have lost wild type p53 function, leading to clonal expansion of aberrant cell populations.

Decreased levels of p26-Bcl-2, but not p30 phosphorylated Bcl-2, precede TGFbeta1-induced apoptosis in colorectal adenoma cells

Bcl-2 expression is confined to the base of the colonic crypt, whereas transforming growth factor beta (TGFbeta) is expressed in the upper crypt, as are the apoptotic death promoters, Bak and Bax. In colonic adenoma cells, TGFbeta induces a growth arrest. In some adenoma cell lines, this is accompanied by apoptosis and in others it is not. In this study, we used two human colonic adenoma cell lines: RG/C2, in which TGFbeta induces a G1 arrest without apoptosis, and BH/C1, in which TGFbeta induces both a G1 arrest and apoptosis. TGFbeta does not induce apoptosis in RG/C2 cells even if hydrocortisone and insulin are removed from the culture medium. In BH/C1 cells, TGFbeta induces apoptosis in the presence of insulin and hydrocortisone. Apoptosis induced by TGFbeta is preceded by a reduction in p26-Bcl-2 protein levels. There was no change in the levels of the p30 phosphorylated form of Bcl-2 or in levels of the proapoptotic proteins Bax or Bak. RG/C2 cells did not show decreased Bcl-2 levels in response to TGFbeta-induced growth inhibition. Therefore, TGFbeta regulates Bcl-2 expression in colonic adenoma cells which undergo apoptosis in response to TGFbeta, but not in those which are growth inhibited, but resistant to TGFbeta-induced apoptosis. TGFbeta may play an important role in the colonic epithelium, not only in the inhibition of cell proliferation, but also in the regulation of apoptosis.

The adenomatous polyposis coli protein and retinoblastoma protein are cleaved early in apoptosis and are potential substrates for caspases

Apoptosis in human monocytic THP.1 tumour cells, induced by diverse stimuli, was accompanied by proteolytic cleavage of the adenomatous polyposis coli gene product (APC) and by sequential cleavage of the retinoblastoma susceptibility gene product (Rb). Cleavage of poly(ADP-ribose) polymerase (PARP), APC and the initial cleavage of Rb at the carboxy terminal region all occurred at a similar time, early in the apoptotic process. Subsequently, Rb underwent a secondary cleavage to 43 kDa and 30 kDa protein fragments. Two caspase inhibitors, benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethyl ketone (Z-VAD.FMK) and acetyl-Tyr-Val-Ala-Asp chloromethyl ketone (YVAD.CMK), had markedly different effects on the induction of apoptosis. Z-VAD.FMK inhibited the primary and secondary cleavage of Rb, cleavage of APC and PARP, and apoptosis assessed by flow cytometry. In marked contrast, YVAD.CMK inhibited cleavage of APC and the secondary cleavage of Rb to the 43 kDa and 30 kDa protein fragments but did not inhibit the primary carboxy terminal cleavage of Rb, PARP proteolysis or apoptosis assessed by flow cytometry. These results suggest that different caspases are responsible for the cleavage of different substrates at different stages during the apoptotic process and that a caspase may either cleave APC directly or may be involved in the pathway leading to APC proteolysis. This is the first report suggesting that a cytoplasmic tumour suppressor gene (APC) may be cleaved by a caspase during apoptosis.

Heparan sulfate undergoes specific structural changes during the progression from human colon adenoma to carcinoma in vitro

We report a detailed analysis of heparan sulfate (HS) structure using a model of human colon carcinogenesis. Metabolically radiolabeled HS was isolated from adenoma and carcinoma cells. The chain length of HS was the same in both cell populations (Mr 20,000; 45-50 disaccharides), and the chains contained on average of two sulfated domains (S domains), identified by heparinase I scission. This enzyme produced fragments of approximate size 7 kDa, suggesting that the S domains were evenly spaced in the intact HS chain. The degree of polymer sulfation and the patterns of sulfation were strikingly different between the two HS species. When compared with adenoma HS, the iduronic acid 2-O-sulfate content of the carcinoma-derived material was reduced by 33%, and the overall level of N-sulfation was reduced by 20%. However, the level of 6-O-sulfation was increased by 24%, and this was almost entirely attributable to an enhanced level of N-sulfated glucosamine 6-O-sulfate, a species whose data implied was mainly located in the mixed sequences of alternating N-sulfated and N-acetylated disaccharides. The results indicate that in the transition to malignancy in human colon adenoma cells, the overall molecular organization of HS is preserved, but there are distinct modifications in both the S domains and their flanking mixed domains that may contribute to the aberrant behavior of the cancer cell.