Massive programmed cell death in intestinal epithelial cells induced by three-dimensional growth conditions: suppression by mutant c-H-ras oncogene expression

Nuclear lipid droplets in Caco2 cells originate from nascent precursors and in situ at the nuclear envelope

Intestinal epithelial cells convert excess fatty acids into triglyceride (TAG) for storage in cytoplasmic lipid droplets and secretion in chylomicrons. Nuclear lipid droplets (nLDs) are present in intestinal cells but their origin and relationship to cytoplasmic TAG synthesis and secretion is unknown. nLDs and related lipid-associated promyelocytic leukemia structures (LAPS) were abundant in oleate-treated Caco2 but less frequent in other human colorectal cancer cell lines and mouse intestinal organoids. nLDs and LAPS in undifferentiated oleate-treated Caco2 cells harbored the phosphatidate phosphatase Lipin1, its product diacylglycerol, and CTP:phosphocholine cytidylyltransferase (CCT)α. CCTα knockout Caco2 cells had fewer but larger nLDs, indicating a reliance on de novo PC synthesis for assembly. Differentiation of Caco2 cells caused large nLDs and LAPS to form regardless of oleate treatment or CCTα expression. nLDs and LAPS in Caco2 cells did not associate with apoCIII and apoAI and formed dependently of microsomal triglyceride transfer protein expression and activity, indicating they are not derived from endoplasmic reticulum luminal LDs precursors. Instead, undifferentiated Caco2 cells harbored a constitutive pool of nLDs and LAPS in proximity to the nuclear envelope that expanded in size and number with oleate treatment. Inhibition of TAG synthesis did affect the number of nascent nLDs and LAPS but prevented their association with promyelocytic leukemia protein, Lipin1α, and diacylglycerol, which instead accumulated on the nuclear membranes. Thus, nLD and LAPS biogenesis in Caco2 cells is not linked to lipoprotein secretion but involves biogenesis and/or expansion of nascent nLDs by de novo lipid synthesis.

Integrins Cooperate With the EGFR/Ras Pathway to Preserve Epithelia Survival and Architecture in Development and Oncogenesis

Adhesion to the extracellular matrix (ECM) is required for normal epithelial cell survival. Disruption of this interaction leads to a specific type of apoptosis known as anoikis. Yet, there are physiological and pathological situations in which cells not connected to the ECM are protected from anoikis, such as during cell migration or metastasis. The main receptors transmitting signals from the ECM are members of the integrin family. However, although integrin-mediated cell-ECM anchorage has been long recognized as crucial for epithelial cell survival, the in vivo significance of this interaction remains to be weighed. In this work, we have used the Drosophila wing imaginal disc epithelium to analyze the importance of integrins as survival factors during epithelia morphogenesis. We show that reducing integrin expression in the wing disc induces caspase-dependent cell death and basal extrusion of the dead cells. In this case, anoikis is mediated by the activation of the JNK pathway, which in turn triggers expression of the proapoptotic protein Hid. In addition, our results strongly suggest that, during wing disc morphogenesis, the EGFR pathway protects cells undergoing cell shape changes upon ECM detachment from anoikis. Furthermore, we show that oncogenic activation of the EGFR/Ras pathway in integrin mutant cells rescues them from apoptosis while promoting their extrusion from the epithelium. Altogether, our results support the idea that integrins promote cell survival during normal tissue morphogenesis and prevent the extrusion of transformed cells.

Oncogenic RAS drives the CRAF-dependent extracellular vesicle uptake mechanism coupled with metastasis

Oncogenic RAS impacts communication between cancer cells and their microenvironment, but it is unclear how this process influences cellular interactions with extracellular vesicles (EVs). This is important as intercellular EV trafficking plays a key role in cancer invasion and metastasis. Here we report that overexpression of mutant RAS drives the EV internalization switch from endocytosis (in non-transformed cells) to macropinocytosis (in cancer cells) resulting in enhanced EV uptake. This process depends on the surface proteoglycan, fibronectin and EV engulfment mechanism regulated by CRAF. Both mutant RAS and activated CRAF expression is associated with formation of membrane ruffles to which they colocalize along with actin, sodium-hydrogen exchangers (NHEs) and phosphorylated myosin phosphatase (pMYPT). RAS-transformed cells internalize EVs in the vicinity of ruffled structures followed by apparent trafficking to lysosome and degradation. NHE inhibitor (EIPA) suppresses RAS-driven EV uptake, along with adhesion-independent clonal growth and experimental metastasis in mice. Thus, EV uptake may represent a targetable step in progression of RAS-driven cancers.

Oncogenic RAS-induced downregulation of ATG12 is required for survival of malignant intestinal epithelial cells

Activating mutations of RAS GTPase contribute to the progression of many cancers, including colorectal carcinoma. So far, attempts to develop treatments of mutant RAS-carrying cancers have been unsuccessful due to insufficient understanding of the salient mechanisms of RAS signaling. We found that RAS downregulates the protein ATG12 in colon cancer cells. ATG12 is a mediator of autophagy, a process of degradation and reutilization of cellular components. In addition, ATG12 can kill cells via autophagy-independent mechanisms. We established that RAS reduces ATG12 levels in cancer cells by accelerating its proteasomal degradation. We further observed that RAS-dependent ATG12 loss in these cells is mediated by protein kinases MAP2K/MEK and MAPK1/ERK2-MAPK3/ERK1, known effectors of RAS. We also demonstrated that the reversal of the effect of RAS on ATG12 achieved by the expression of exogenous ATG12 in cancer cells triggers both apoptotic and nonapoptotic signals and efficiently kills the cells. ATG12 is known to promote autophagy by forming covalent complexes with other autophagy mediators, such as ATG5. We found that the ability of ATG12 to kill oncogenic RAS-carrying malignant cells does not require covalent binding of ATG12 to other proteins. In summary, we have identified a novel mechanism by which oncogenic RAS promotes survival of malignant intestinal epithelial cells. This mechanism is driven by RAS-dependent loss of ATG12 in these cells.

ErbB2-dependent downregulation of a pro-apoptotic protein Perp is required for oncogenic transformation of breast epithelial cells

The ability of breast cancer cells to resist anoikis, apoptosis caused by detachment of the non-malignant epithelial cells from the extracellular matrix (ECM), is thought to be critical for breast tumor growth, invasion and metastasis. ErbB2, an oncoprotein that is often overproduced in breast tumors, can block breast cancer cell anoikis via mechanisms that are understood only in part. In an effort to understand them better we found that detachment of the non-malignant human breast epithelial cells from the ECM upregulates a protein Perp in these cells. Perp is a component of the desmosomes, multiprotein complexes involved in cell-to-cell adhesion. Perp can cause apoptosis via unknown mechanisms. We demonstrated that Perp upregulation by cell detachment is driven by detachment-induced loss of epidermal growth factor receptor (EGFR). We also found that Perp knockdown by RNA interference (RNAi) rescues detached cells from death which indicates that Perp contributes to their anoikis. We observed that ErbB2, when overexpressed in detached breast epithelial cells, causes Perp downregulation. Furthermore, ErbB2-directed RNAi or treatment with lapatinib, an ErbB2/EGFR small-molecule inhibitor used for breast cancer therapy, upregulated Perp in ErbB2-positive human breast and ovarian carcinoma cells. We established that ErbB2 downregulates Perp by activating an ErbB2 effector protein kinase Mek that blocks detachment-induced EGFR loss in a manner that requires the presence of a signaling protein Sprouty-2. Finally, we observed that restoration of the wild-type Perp levels in ErbB2-overproducing breast epithelial cells increases their anoikis susceptibility and blocks their clonogenicity in the absence of adhesion to the ECM. In summary, we have identified a novel mechanism of ErbB2-mediated mechanism of anoikis resistance of ErbB2-overproducing breast epithelial cells. This mechanism allows such cells to grow without adhesion to the ECM and is driven by ErbB2-induced activation of Mek, subsequent EGFR upregulation and further EGFR-dependent Perp loss.

Nuclear-localized CTP:phosphocholine cytidylyltransferase α regulates phosphatidylcholine synthesis required for lipid droplet biogenesis

Phosphatidylcholine (PC) synthesis by the CDP-choline pathway in mammalian cells regulates lipid droplet size and triacylglycerol storage. Mechanistically this involves increased CCTα expression and translocation to the nuclear envelope. CCTα regulates the PC synthesis required for lipid droplet biogenesis without associating with the surface of the particle.

The reversible association of CTP:phosphocholine cytidylyltransferase α (CCTα) with membranes regulates the synthesis of phosphatidylcholine (PC) by the CDP-choline (Kennedy) pathway. Based on results with insect CCT homologues, translocation of nuclear CCTα onto cytoplasmic lipid droplets (LDs) is proposed to stimulate the synthesis of PC that is required for LD biogenesis and triacylglycerol (TAG) storage. We examined whether this regulatory mechanism applied to LD biogenesis in mammalian cells. During 3T3-L1 and human preadipocyte differentiation, CCTα expression and PC synthesis was induced. In 3T3-L1 cells, CCTα translocated from the nucleoplasm to the nuclear envelope and cytosol but did not associate with LDs. The enzyme also remained in the nucleus during human adipocyte differentiation. RNAi silencing in 3T3-L1 cells showed that CCTα regulated LD size but did not affect TAG storage or adipogenesis. LD biogenesis in nonadipocyte cell lines treated with oleate also promoted CCTα translocation to the nuclear envelope and/or cytoplasm but not LDs. In rat intestinal epithelial cells, CCTα silencing increased LD size, but LD number and TAG deposition were decreased due to oleate-induced cytotoxicity. We conclude that CCTα increases PC synthesis for LD biogenesis by translocation to the nuclear envelope and not cytoplasmic LDs.

Upregulation of ATG3 contributes to autophagy induced by the detachment of intestinal epithelial cells from the extracellular matrix, but promotes autophagy-independent apoptosis of the attached cells

Detachment of nonmalignant intestinal epithelial cells from the extracellular matrix (ECM) triggers their growth arrest and, ultimately, apoptosis. In contrast, colorectal cancer cells can grow without attachment to the ECM. This ability is critical for their malignant potential. We found previously that detachment-induced growth arrest of nonmalignant intestinal epithelial cells is driven by their detachment-triggered autophagy, and that RAS, a major oncogene, promotes growth of detached cells by blocking such autophagy. In an effort to identify the mechanisms of detachment-induced autophagy and growth arrest of nonmalignant cells we found here that detachment of these cells causes upregulation of ATG3 and that ATG3 upregulation contributes to autophagy and growth arrest of detached cells. We also observed that when ATG3 expression is artificially increased in the attached cells, ATG3 promotes neither autophagy nor growth arrest but triggers their apoptosis. ATG3 upregulation likely promotes autophagy of the detached but not that of the attached cells because detachment-dependent autophagy requires other detachment-induced events, such as the upregulation of ATG7. We further observed that those few adherent cells that do not die by apoptosis induced by ATG3 become resistant to apoptosis caused by cell detachment, a property that is critical for the ability of normal epithelial cells to become malignant. We conclude that cell-ECM adhesion can switch ATG3 functions: when upregulated in detached cells in the context of other autophagy-promoting events, ATG3 contributes to autophagy. However, when overexpressed in the adherent cells, in the circumstances not favoring autophagy, ATG3 triggers apoptosis.

Anoikis of colon carcinoma cells triggered by β-catenin loss can be enhanced by tumor necrosis factor receptor 1 antagonists

Detachment of non-malignant epithelial cells from the extracellular matrix causes their apoptosis, a phenomenon called anoikis. By contrast, carcinoma cells are anoikis-resistant, and this resistance is thought to be critical for tumor progression. Many oncogenes trigger not only anti- but also pr-apoptotic signals. The proapoptotic events represent an aspect of a phenomenon called oncogenic stress, which acts as a safeguard mechanism blocking tumor initiation. In cells that become malignant, oncogene-induced antiapoptotic signals outbalance the proapoptotic ones. It is now thought that treatments blocking the antiapoptotic events but preserving the proapoptotic signals can be particularly effective in killing tumor cells. Whether or not oncogenes induce any proanoikis signals that can be used for enhancing the efficiency of approaches aimed at triggering anoikis of cancer cells has never been explored. β-Catenin is a major oncoprotein that is often activated in colorectal cancer and promotes tumor progression via mechanisms that are understood only in part. We found here that β-catenin triggers both anti- and proanoikis signals in colon cancer cells. We observed that the antianoikis signals prevail and the cells become anoikis-resistant. We further established that one proanoikis signal in these cells is triggered by β-catenin-induced downregulation of an apoptosis inhibitor tumor necrosis factor receptor 1 (TNFR1) and subsequent reduction of the activity of a transcription factor NF-κB (nuclear factor-κB), a mediator of TNFR1 signaling. We also found that the effect of β-catenin on TNFR1 requires the presence of transcription factor TCF1, a β-catenin effector. We demonstrated that ablation of β-catenin in colon cancer cells triggers their anoikis and that this anoikis is enhanced even further if low TNFR1 or NF-κB activity is artificially preserved in the β-catenin-deprived cells. Thus, inhibition of TNFR1 or NF-κB activity can be expected to enhance the efficiency of approaches aimed at blocking β-catenin-driven anoikis resistance of colon carcinoma cells.

3D tissue-engineered model of Ewing's sarcoma

Despite longstanding reliance upon monolayer culture for studying cancer cells, and numerous advantages from both a practical and experimental standpoint, a growing body of evidence suggests that more complex three-dimensional (3D) models are necessary to properly mimic many of the critical hallmarks associated with the oncogenesis, maintenance and spread of Ewing's sarcoma (ES), the second most common pediatric bone tumor. And as clinicians increasingly turn to biologically-targeted therapies that exert their effects not only on the tumor cells themselves, but also on the surrounding extracellular matrix, it is especially important that preclinical models evolve in parallel to reliably measure antineoplastic effects and possible mechanisms of de novo and acquired drug resistance. Herein, we highlight a number of innovative methods used to fabricate biomimetic ES tumors, encompassing both the surrounding cellular milieu and the extracellular matrix (ECM), and suggest potential applications to advance our understanding of ES biology, preclinical drug testing, and personalized medicine.

Oncogenic ras-driven cancer cell vesiculation leads to emission of double-stranded DNA capable of interacting with target cells

Cell free DNA is often regarded as a source of genetic cancer biomarkers, but the related mechanisms of DNA release, composition and biological activity remain unclear. Here we show that rat epithelial cell transformation by the human H-ras oncogene leads to an increase in production of small, exosomal-like extracellular vesicles by viable cancer cells. These EVs contain chromatin-associated double-stranded DNA fragments covering the entire host genome, including full-length H-ras. Oncogenic N-ras and SV40LT sequences were also found in EVs emitted from spontaneous mouse brain tumor cells. Disruption of acidic sphingomyelinase and the p53/Rb pathway did not block emission of EV-related oncogenic DNA. Exposure of non-transformed RAT-1 cells to EVs containing mutant H-ras DNA led to the uptake and retention of this material for an extended (30days) but transient period of time, and stimulated cell proliferation. Thus, our study suggests that H-ras-mediated transformation stimulates vesicular emission of this histone-bound oncogene, which may interact with non-transformed cells.

Oxysterol-binding protein (OSBP)-related protein 4 (ORP4) is essential for cell proliferation and survival

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) comprise a large gene family with sterol/lipid transport and regulatory activities. ORP4 (OSBP2) is a closely related paralogue of OSBP, but its function is unknown. Here we show that ORP4 binds similar sterol and lipid ligands as OSBP and other ORPs but is uniquely required for the proliferation and survival of cultured cells. Recombinant ORP4L and a variant without a pleckstrin homology (PH) domain (ORP4S) bind 25-hydroxycholesterol and extract and transfer cholesterol between liposomes. Two conserved histidine residues in the OSBP homology domain ORP4 are essential for binding phosphatidylinositol 4-phosphate but not sterols. The PH domain of ORP4L also binds phosphatidylinositol 4-phosphate in the Golgi apparatus. However, in the context of ORP4L, the PH domain is required for normal organization of the vimentin network. Unlike OSBP, RNAi silencing of all ORP4 variants (including a partial PH domain truncation termed ORP4M) in HEK293 and HeLa cells resulted in growth arrest but not cell death. ORP4 silencing in non-transformed intestinal epithelial cells (IEC)-18 caused apoptosis characterized by caspase 3 and poly(ADP-ribose) polymerase processing, DNA cleavage, and JNK phosphorylation. IEC-18 transformed with oncogenic H-Ras have increased expression of ORP4L and ORP4S proteins and are resistant to the growth-inhibitory effects of ORP4 silencing. Results suggest that ORP4 promotes the survival of rapidly proliferating cells.

Fructooligosacharides reduce Pseudomonas aeruginosa PAO1 pathogenicity through distinct mechanisms

Pseudomonas aeruginosa is ubiquitously present in the environment and acts as an opportunistic pathogen on humans, animals and plants. We report here the effects of the prebiotic polysaccharide inulin and its hydrolysed form FOS on this bacterium. FOS was found to inhibit bacterial growth of strain PAO1, while inulin did not affect growth rate or yield in a significant manner. Inulin stimulated biofilm formation, whereas a dramatic reduction of the biofilm formation was observed in the presence of FOS. Similar opposing effects were observed for bacterial motility, where FOS inhibited the swarming and twitching behaviour whereas inulin caused its stimulation. In co-cultures with eukaryotic cells (macrophages) FOS and, to a lesser extent, inulin reduced the secretion of the inflammatory cytokines IL-6, IL-10 and TNF-α. Western blot experiments indicated that the effects mediated by FOS in macrophages are associated with a decreased activation of the NF-κB pathway. Since FOS and inulin stimulate pathway activation in the absence of bacteria, the FOS mediated effect is likely to be of indirect nature, such as via a reduction of bacterial virulence. Further, this modulatory effect is observed also with the highly virulent ptxS mutated strain. Co-culture experiments of P. aeruginosa with IEC18 eukaryotic cells showed that FOS reduces the concentration of the major virulence factor, exotoxin A, suggesting that this is a possible mechanism for the reduction of pathogenicity. The potential of these compounds as components of antibacterial and anti-inflammatory cocktails is discussed.

Ras and Ras mutations in cancer

Ras genes are pre-eminent genes that are frequently linked with cancer biology. The functional loss of ras protein caused by various point mutations within the gene, is established as a prognostic factor for the genesis of a constitutively active Ras-MAPK pathway leading to cancer. Ras signaling circuit follows a complex pathway, which connects many signaling molecules and cells. Several strategies have come up for targeting mutant ras proteins for cancer therapy, however, the clinical benefits remain insignificant. Targeting the Ras-MAPK pathway is extremely complicated due its intricate networks involving several upstream and downstream regulators. Blocking oncogenic Ras is still in latent stage and requires alternative approaches to screen the genes involved in Ras transformation. Understanding the mechanism of Ras induced tumorigenesis in diverse cancers and signaling networks will open a path for drug development and other therapeutic approaches.

Impact of host ageing on the metastatic phenotype

Ageing impacts multiple host mechanisms involved in cancer progression. Here we show that poorly metastatic Lewis lung carcinoma (LLC) cells form less bulky metastatic deposits in aged mice (>52 weeks) relative to their young (4-6 weeks) counterparts. Serial selection of LLC cells for increased metastatic capability in either young or old mice led in both cases to exaggerated growth of pulmonary nodules after only 5 cycles of in vivo passage. The respective metastatic cellular variants established in young (Y-series) or old (O-series) mice differed in cell morphology and constitutive activity of growth factor receptors, especially phospho-PDGFRa and phospho-EPHA7. These cell lines also exhibited marked differences in their time dependent profiles of cellular impedance (CI), which reflects their physical properties, such as cell shape, adhesion and interactions with substrata. In confluent monolayer culture Y-series cell lines generated high and increasing CI values, while these values remained low and constant in the O-series of cell lines. These observations suggest that the selective pressure of the metastatic microenvironment in young versus old hosts is sufficiently different to results in the enrichment of distinct, age-related metastatic phenotypes of cancer cells. Thus, age could inform therapeutic approaches to metastatic cancers.

ras-Induced up-regulation of CTP:phosphocholine cytidylyltransferase α contributes to malignant transformation of intestinal epithelial cells

Cancer cells have enhanced lipogenic capacity characterized by increased synthesis of fatty acids and complex lipids, including phosphatidylcholine (PC). As the rate-limiting enzyme in the CDP-choline pathway for PC synthesis, CTP:phosphocholine cytidylyltransferase α (CCTα) is implicated in the provision of membranes and bioactive lipids necessary of cell proliferation. In this study, we assessed the role of CCTα in malignant intestinal epithelial cells transformed with activated H-ras (IEC-ras). Three IEC-ras clones had significant up-regulation CCTα expression, but PC synthesis and in vitro activity of CCTα were similar to control IEC. RNA interference of CCTα in adherent IEC-ras did not affect PC synthesis, confirming that the enzyme was relatively inactive. However, CCTα silencing in ras-transformed IEC reduced anchorage-independent growth, a criterion for malignant transformation, as well as tumorigenicity in mice. Relative to their adherent counterparts, detached IEC-ras had increased PC synthesis that was attenuated by inducible CCTα silencing. Detachment of IEC-ras was accompanied by increased CCTα phosphorylation and cytosolic enzyme activity. We conclude that the expanded pool of CCTα in IEC-ras is activated by detachment. This provides the increased PC biosynthetic capacity that contributes to malignant transformation of intestinal epithelial cells when detached from the extracellular matrix.

Integrins, adhesion and apoptosis

Integrin-mediated adhesion to extracellular matrix proteins is required for survival of many cell types. This phenomenon appears to be a mechanism of tumour suppression and to participate in embryogenesis. Here, our current understanding of how integrin-dependent signals prevent apoptosis and implications of anchorage-dependent survival for development, physiology and pathology are discussed.

New insights into the kinetic resistance to anticancer agents

Kinetic resistance plays a major role in the failure of chemotherapy towards many solid tumors. Kinetic resistance to cytotoxic drugs can be reproduced in vitro by growing the cells as multicellular spheroids (Multicellular Resistance) or as hyperconfluent cultures (Confluence-Dependent Resistance). Recent findings on the cell cycle regulation have permitted a better understanding why cancer cells which arrest in long quiescent phases are poorly sensitive to cell-cycle specific anticancer drugs. Two cyclin-dependent kinase inhibitors (CDKI) seem particularly involved in the cell cycle arrest at the G1 to S transition checkpoint: the p53-dependent p21(cip1) protein which is activated by DNA damage and the p27(kip1) which is a mediator of the contact inhibition signal. Cell quiescence could alter drug-induced apoptosis which is partly dependent on an active progression in the cell cycle and which is facilitated by overexpression of oncogenes such as c-Myc or cyclins. Investigations are yet necessary to determine the influence of the cell cycle on the balance between antagonizing (bcl-2, bcl-X(L)...) or stimulating (Bax, Bcl-X(S), Fas...) factors in chemotherapy-induced apoptosis. Quiescent cells could also be protected from toxic agents by an enhanced expression of stress proteins, such as HSP27 which is induced by confluence. New strategies are required to circumvent kinetic resistance of solid tumors: adequate choice of anticancer agents whose activity is not altered by quiescence (radiation, cisplatin), recruitment from G1 to S/G2 phases by cell pretreatment with alkylating drugs or attenuation of CDKI activity by specific inhibitors.

Oncogenic ras-induced down-regulation of pro-apoptotic protease caspase-2 is required for malignant transformation of intestinal epithelial cells

Resistance of carcinoma cells to anoikis, apoptosis that is normally induced by loss of cell-to-extracellular matrix adhesion, is thought to be essential for the ability of these cells to form primary tumors, invade adjacent tissues, and metastasize to distant organs. Current knowledge about the mechanisms by which cancer cells evade anoikis is far from complete. In an effort to understand these mechanisms, we found that ras, a major oncogene, down-regulates protease caspase-2 (which initiates certain steps of the cellular apoptotic program) in malignant human and rat intestinal epithelial cells. This down-regulation could be reversed by inhibition of a protein kinase Mek, a mediator of Ras signaling. We also found that enforced down-regulation of caspase-2 in nonmalignant intestinal epithelial cells by RNA interference protected them from anoikis. Furthermore, the reversal of the effect of Ras on caspase-2 achieved by the expression of exogenous caspase-2 in detached ras-transformed intestinal epithelial cells promoted well established apoptotic events, such as the release of the pro-apoptotic mitochondrial factors cytochrome c and HtrA2/Omi into the cytoplasm of these cells, significantly enhanced their anoikis susceptibility, and blocked their long term growth in the absence of adhesion to the extracellular matrix. Finally, the blockade of the effect of Ras on caspase-2 substantially suppressed growth of tumors formed by the ras-transformed cells in mice. We conclude that ras-induced down-regulation of caspase-2 represents a novel mechanism by which oncogenic Ras protects malignant intestinal epithelial cells from anoikis, promotes their anchorage-independent growth, and allows them to form tumors in vivo.

Oncogenic ras-induced down-regulation of autophagy mediator Beclin-1 is required for malignant transformation of intestinal epithelial cells

Detachment of non-malignant epithelial cells from the extracellular matrix causes their growth arrest and, ultimately, death. By contrast, cells composing carcinomas, cancers of epithelial origin, can survive and proliferate without being attached to the extracellular matrix. These properties of tumor cells represent hallmarks of malignant transformation and are critical for cancer progression. Previously we identified several mechanisms by which ras, a major oncogene, blocks detachment-induced apoptosis of intestinal epithelial cells, but mechanisms by which Ras promotes proliferation of those cells that remain viable following detachment are unknown. We show here that detachment of non-malignant intestinal epithelial cells promotes formation of autophagosomes, vacuole-like structures that mediate autophagy (a process of cellular self-cannibalization), and that oncogenic ras prevents this autophagosome formation. We also found that ras activates a GTPase RhoA, that RhoA promotes activation of a protease calpain, and that calpain triggers degradation of Beclin-1, a critical mediator of autophagy, in these cells. The reversal of the effect of ras on Beclin-1 (achieved by expression of exogenous Beclin-1) promoted autophagosome formation following cell detachment, significantly reduced the fraction of detached cells in the S phase of the cell cycle and their rate of proliferation without affecting their viability. Furthermore, RNA interference-induced Beclin-1 down-regulation in non-malignant intestinal epithelial cells prevented detachment-dependent reduction of the fraction of these cells in the S phase of the cell cycle. Thus, ras oncogene promotes proliferation of those malignant intestinal epithelial cells that remain viable following detachment via a distinct novel mechanism that involves Ras-induced down-regulation of Beclin-1.

Constitutive CXCL12 expression induces anoikis in colorectal carcinoma cells

BACKGROUND & AIMS

CXCL12 and CXCR4 signaling plays critical roles in development, homeostasis, and tumor metastasis. Previously, we have shown that epigenetic silencing of CXCL12 in colorectal and mammary carcinomas promotes metastasis. Anoikis is an essential process of colonic epithelial turnover and limits the metastatic progression of carcinoma. We sought to determine the role for anoikis in limiting tumor metastasis following reexpression of CXCL12 in human colorectal carcinoma cells.

METHODS

Tumor formation and metastasis of colonic carcinoma cells was monitored using in vivo bioluminescence imaging. Anoikis was defined by using caspase-3/7, focal adhesion kinase (FAK) and p130Cas cleavage, DNA fragmentation, and cell survival assays. Phosphorylation of extracellular-regulated kinase-1/2 (ERK1/2) was monitored by immunoblot and immunohistochemistry, and activity was inhibited by using U0126.

RESULTS

Constitutive expression of CXCL12 in human colorectal carcinoma cells reduced orthotopic tumor formation and inhibited metastasis in severe combined immunodeficient mice. Further, CXCL12 expression induced apoptosis specifically in nonadherent colorectal carcinoma cells. Apoptotic cell death was preceded by hypophosphorylation and cleavage of FAK and p130Cas, leading to increased cellular detachment in culture, and depended on alterations in the extracellular matrix. Similar to in vivo colonic epithelium, CXCL12-induced anoikis of carcinoma cells depended on basal ERK1/2 activation.

CONCLUSIONS

These data significantly expand the current paradigm of chemokine signaling in carcinogenesis by showing that endogenous CXCL12, in marked contrast to exogenous ligand, inhibits tumor metastasis through increased anoikis. Altered ERK1/2 signaling provides a mechanism for the dichotomy between the physiologic and pathophysiologic roles of CXCL12-CXCR4 signaling in the intestinal epithelium.

Oncogene addiction: setting the stage for molecularly targeted cancer therapy

In pugilistic parlance, the one-two punch is a devastating combination of blows, with the first punch setting the stage and the second delivering the knock-out. This analogy can be extended to molecularly targeted cancer therapies, with oncogene addiction serving to set the stage for tumor cell killing by a targeted therapeutic agent. While in vitro and in vivo examples abound documenting the existence of this phenomenon, the mechanistic underpinnings that govern oncogene addiction are just beginning to emerge. Our current inability to fully exploit this weakness of cancer cells stems from an incomplete understanding of oncogene addiction, which nonetheless represents one of the rare chinks in the formidable armor of cancer cells.

DR5 receptor mediates anoikis in human colorectal carcinoma cell lines

As human colorectal cancer (CRC) cells metastasize to distant sites, they are susceptible to detachment-induced cell death or anoikis - a form of apoptosis that occurs when anchorage-dependent CRC cells go into suspension. Our goal was to identify whether tumor necrosis factor receptor apoptosis-inducing ligand (TRAIL) receptors mediate anoikis in human CRC cells. First, we assessed whether caspases of the extrinsic (caspase-8) or intrinsic (caspase-9) death pathways were involved. Caspase-8 was cleaved during exposure to suspension culture in four CRC lines, and cell death was inhibited by caspase-3 and caspase-8 inhibitors but not by a caspase-9 inhibitor. Gene transcripts in macrophage inflammatory protein-101 (MIP-110), a weakly metastatic human CRC, were increased at least 2-fold for TRAIL-R2 (DR5) and TRAIL after 24 h of suspension culture compared with cells in monolayer culture. The increased expression of DR5 was confirmed at the protein level at 24 h, and exposure of MIP-101 cells to an antagonistic antibody to DR5 decreased caspase-8 activation. The antagonistic antibody to DR5 inhibited anoikis in four human CRC lines. Treatment with an antagonistic DR4 antibody or a neutralizing antibody to TRAIL ligand did not reduce anoikis consistently. Knockdown of DR5 or TRAIL also inhibited anoikis, whereas exogenous TRAIL or FasL did not consistently increase anoikis. In summary, DR5 receptor mediates death signals for anoikis in human CRC cells through the extrinsic apoptotic pathway.

E-cadherin cell-cell adhesion in ewing tumor cells mediates suppression of anoikis through activation of the ErbB4 tyrosine kinase

Ability to grow under anchorage-independent conditions is one of the major hallmarks of transformed cells. Key to this is the capacity of cells to suppress anoikis, or programmed cell death induced by detachment from the extracellular matrix. To model this phenomenon in vitro, we plated Ewing tumor cells under anchorage-independent conditions by transferring them to dishes coated with agar to prevent attachment to underlying plastic. This resulted in marked up-regulation of E-cadherin and rapid formation of multicellular spheroids in suspension. Addition of calcium chelators, antibodies to E-cadherin (but not to other cadherins or beta(1)-integrin), or expression of dominant negative E-cadherin led to massive apoptosis of spheroid cultures whereas adherent cultures were unaffected. This correlated with reduced activation of the phosphatidylinositol 3-kinase-Akt pathway but not the Ras-extracellular signal-regulated kinase 1/2 cascade. Furthermore, spheroid cultures showed profound chemoresistance to multiple cytotoxic agents compared with adherent cultures, which could be reversed by alpha-E-cadherin antibodies or dominant negative E-cadherin. In a screen for potential downstream effectors of spheroid cell survival, we detected E-cadherin-dependent activation of the ErbB4 receptor tyrosine kinase but not of other ErbB family members. Reduction of ErbB4 levels by RNA interference blocked Akt activation and spheroid cell survival and restored chemosensitivity to Ewing sarcoma spheroids. Our results indicate that anchorage-independent Ewing sarcoma cells suppress anoikis through a pathway involving E-cadherin cell-cell adhesion, which leads to ErbB4 activation of the phosphatidylinositol 3-kinase-Akt pathway, and that this is associated with increased resistance of cells to cytotoxic agents.

Detachment-induced upregulation of XIAP and cIAP2 delays anoikis of intestinal epithelial cells

Detachment of normal epithelial cells from the extracellular matrix triggers apoptosis, a phenomenon called anoikis. Conversely, carcinoma cells tend to be relatively more anoikis-resistant than their normal counterparts, and this increased resistance represents a critical feature of the malignant phenotype. Mechanisms that control susceptibility and resistance to anoikis are not fully understood. It is now known that detachment of non-malignant epithelial cells triggers both pro- and antiapoptotic signals, and it is the balance between these signals and the duration of detachment that determine further fate of the cells. Detachment-induced antiapoptotic events delay anoikis and if cells reattach relatively soon after detachment they survive. Direct regulators of apoptosis responsible for this delay of anoikis are unknown. We found that detachment of non-malignant intestinal epithelial cells triggers upregulation of inhibitors of apoptosis protein (IAP) family, such as X-chromosome-linked inhibitor of apoptosis protein and cellular inhibitor of apoptosis-2 (cIAP2). We demonstrated that this upregulation requires detachment-dependent activation of the transcription factor nuclear factor-kappaB. We further observed that various IAP antagonists accelerate anoikis, indicating that upregulation of the IAPs delays detachment-triggered apoptosis. We conclude that the IAPs are important regulators of the balance between detachment-triggered life and death signals. Perhaps, not by coincidence, these proteins are often upregulated in carcinomas, tumors composed of cells that tend to be anoikis-resistant.

Oncogenic Ras inhibits anoikis of intestinal epithelial cells by preventing the release of a mitochondrial pro-apoptotic protein Omi/HtrA2 into the cytoplasm

Resistance of cancer cells to anoikis, apoptosis induced by cell detachment from the extracellular matrix, is thought to represent a critical feature of the malignant phenotype. Mechanisms that control anoikis of normal and cancer cells are understood only in part. Previously we found that anoikis of non-malignant intestinal epithelial cells is driven by detachment-induced down-regulation of Bcl-X(L), a protein that blocks apoptosis through preventing the release of death-promoting factors from the mitochondria. Mitochondrial proteins the release of which causes anoikis are presently unknown. Similar to what was previously observed by others for keratinocytes and fibroblasts, we show here that anoikis of intestinal epithelial cells does not involve caspase-9, a target of a mitochondrial protein cytochrome c. Furthermore, Smac/Diablo, another mitochondrial pro-apoptotic factor, does not appear to play a role in detachment-dependent apoptosis of these cells either. Instead, anoikis of intestinal epithelial cells is triggered by the release of a mitochondrial protein Omi/HtrA2, an event driven by detachment-induced down-regulation of Bcl-X(L). Moreover, we established that oncogenic ras inhibits anoikis by preventing the release of Omi/HtrA2. This effect of ras required ras-induced down-regulation of a pro-apoptotic protein Bak and could be blocked by an inhibitor of phosphoinositide 3-kinase, a target of Ras that was previously implicated by us in the down-regulation of Bak and blockade of anoikis. We conclude that Omi/HtrA2 is an inducer of anoikis and an important regulator of ras-induced transformation.

Down-regulation of DNA mismatch repair proteins in human and murine tumor spheroids: implications for multicellular resistance to alkylating agents

Similar to other anticancer agents, intrinsic or acquired resistance to DNA-damaging chemotherapeutics is a major obstacle for cancer therapy. Current strategies aimed at overcoming this problem are mostly based on the premise that tumor cells acquire heritable genetic mutations that contribute to drug resistance. Here, we present evidence for an epigenetic, tumor cell adhesion-mediated, and reversible form of drug resistance that is associated with a reduction of DNA mismatch repair proteins PMS2 and/or MLH1 as well as other members of this DNA repair process. Growth of human breast cancer, human melanoma, and murine EMT-6 breast cancer cell lines as multicellular spheroids in vitro, which is associated with increased resistance to many chemotherapeutic drugs, including alkylating agents, is shown to lead to a reproducible down-regulation of PMS2, MLH1, or, in some cases, both as well as MHS6, MSH3, and MSH2. The observed down-regulation is in part reversible by treatment of tumor spheroids with the DNA-demethylating agent, 5-azacytidine. Thus, treatment of EMT-6 mouse mammary carcinoma spheroids with 5-azacytidine resulted in reduced and/or disrupted cell-cell adhesion, which in turn sensitized tumor spheroids to cisplatin-mediated killing in vitro. Our results suggest that antiadhesive agents might sensitize tumor spheroids to alkylating agents in part by reversing or preventing reduced DNA mismatch repair activity and that the chemosensitization properties of 5-azacytidine may conceivably reflect its role as a potential antiadhesive agent as well as reversal agent for MLH1 gene silencing in human tumors.

Intestinal epithelial stem cells and progenitors

The adult intestinal epithelium contains a relatively simple, highly organized, and readily accessible stem cell system. Excellent methods exist for the isolation of intestinal epithelium from adults, and as a result collecting large quantities of intestinal stem and progenitor cells for study or culture and subsequent clinical applications should be routine. It is not, however, for two reasons: (1) adult intestinal epithelial cells rapidly initiate apoptosis on detachment from the basement membrane, and (2) in vitro conditions necessary for survival, proliferation, and differentiation are poorly understood. Thus to date the study of intestinal stem and progenitor cells has been largely dependent on in vivo approaches. We discuss existing in vivo assays for stem and progenitor cell behavior as well as current methods for isolating and culturing the intestinal epithelium.

Three-dimensional tissue culture models in cancer biology

Three-dimensional (3D) tissue culture models have an invaluable role in tumour biology today providing some very important insights into cancer biology. As well as increasing our understanding of homeostasis, cellular differentiation and tissue organization they provide a well defined environment for cancer research in contrast to the complex host environment of an in vivo model. Due to their enormous potential 3D tumour cultures are currently being exploited by many branches of biomedical science with therapeutically orientated studies becoming the major focus of research. Recent advances in 3D culture and tissue engineering techniques have enabled the development of more complex heterologous 3D tumour models.

A More Aggressive Breast Cancer Spheroid Model Coupled to an Electronic Capillary Sensor System for a High-Content Screening of Cytotoxic Agents in Cancer Therapy: 3-Dimensional In Vitro Tumor Spheroids as a Screening Model

One major problem in cancer therapy is the immortality of tumor cells showing an active telomerase, which is responsible for the elongation of the telomeres after each cellular division and the knocking down of apoptotic suppressors. A further phenomenon occurring during cancer therapies is the problem of multicellular resistance. To develop therapeutic anticancer approaches inducing cellular apoptosis, gene-modified biological in vitro systems were established and evaluated for drug screening in a capillary system for a real-time, impedimertic monitoring. Multicellular spheroids of the human breast cancer cell line T-47D clone 11 were transfected with 1) antisense caspase-3 cDNA expression vectors for knocking down the main cell death molecule and 2) sense Bcl-xl cDNA expression vectors for overexpressing the apoptotic suppressor, resulting in more aggressive tumor models. These gene-modified tumor spheroids less sensitive for apoptosis were developed for screening drugs such as methotrexate in tumor spheroid–based biosensor systems via impedance spectroscopy. In this report, it is demonstrated that this could successfully exhibit that this real-time monitoring system with tumor spheroids positioned in a capillary system with a 4-electrode configuration is the most efficient high-content screening module for impedimetric measurements of physiological alterations during gene modification and drug application.

ras Oncogene triggers up-regulation of cIAP2 and XIAP in intestinal epithelial cells: epidermal growth factor receptor-dependent and -independent mechanisms of ras-induced transformation

Detachment of normal epithelial cells from the extracellular matrix (ECM) triggers apoptosis, a phenomenon called anoikis. Conversely, carcinomas (cancers of epithelial origin) represent three-dimensional disorganized multicellular masses in which cells are deprived of adhesion to the ECM but remain viable. Resistance of cancer cells to anoikis is thought to be critical for tumor progression. However, the knowledge about molecular mechanisms of this type of resistance remains limited. Herein we report that ras oncogene, an established inhibitor of anoikis, triggers a significant upregulation of anti-apoptotic proteins cIAP2 and XIAP in intestinal epithelial cells. We also observed that the effect of ras on cIAP2 requires ras-induced autocrine production of transforming growth factor alpha (TGF-alpha), a ligand for epidermal growth factor receptor, whereas ras-triggered up-regulation of XIAP is TGF-alpha-independent. Moreover, overexpression of either cIAP2 or XIAP in nonmalignant intestinal epithelial cell was found to block anoikis. In addition, an established IAP antagonist Smac or Smac-derived cell-permeable peptide suppressed ras-induced anoikis resistance and subsequent anchorage-independent growth of ras-transformed cells. We conclude that ras-induced overexpression of cIAP2 and XIAP significantly contributes to the ability of ras-transformed intestinal epithelial cells to survive in the absence of adhesion to the ECM and grow in a three-dimensional manner.

A transient increase in the activity of Src-family kinases induced by cell detachment delays anoikis of intestinal epithelial cells

Detachment of epithelial cells from the basement membrane (BM) induces apoptosis, a phenomenon now widely known as anoikis. Studies in mammary and intestinal epithelial cells have shown that the loss of attachment to the BM rapidly triggers reversible proapoptotic events from which the cells can recover if they reattach within a certain period. Thus, cells seem to be transiently protected from the initial detachment-induced proapoptotic events. The molecular mechanisms underlying such transient protection against anoikis are unknown. In this paper, we present evidence indicating that detachment of intestinal epithelial cells triggers a transient, yet significant increase in the activity of the tyrosine kinases c-Src and c-Fyn, and that this activation of Src-family kinases (SFK) contributes to the transient protection against anoikis in these cells. The protective signals from SFK are mediated by the PI3K pathway, and caveolin-1. In addition, we show that the MEK1-ERK1/2 pathway acts in a synergistic manner with SFK to protect intestinal epithelial cells from anoikis.

Increased plasma vascular endothelial growth factor (VEGF) as a surrogate marker for optimal therapeutic dosing of VEGF receptor-2 monoclonal antibodies

A major obstacle compromising the successful application of many of the new targeted anticancer drugs, including angiogenesis inhibitors, is the empiricism associated with determining an effective biological/therapeutic dose because many of these drugs express optimum therapeutic activity below the maximum tolerated dose, if such a dose can be defined. Hence, surrogate markers are needed to help determine optimal dosing. Here we describe such a molecular marker, increased plasma levels of vascular endothelial growth factor (VEGF), in normal or tumor-bearing mice that received injections of an anti-VEGF receptor (VEGFR)-2 monoclonal antibody, such as DC101. Rapid increases of mouse VEGF (e.g., within 24 hours) up to 1 order of magnitude were observed after single injections of DC101 in non-tumor-bearing severe combined immunodeficient or nude mice; similar increases in human plasma VEGF were detected in human tumor-bearing mice. RAFL-1, another anti-VEGFR-2 antibody, also caused a significant increase in plasma VEGF. In contrast, increases in mouse VEGF levels were not seen when small molecule VEGFR-2 inhibitors were tested in normal mice. Most importantly, the increases in plasma VEGF were induced in a dose-dependent manner, with the maximum values peaking when doses previously determined to be optimally therapeutic were used. Plasma VEGF should be considered as a possible surrogate pharmacodynamic marker for determining the optimal biological dose of antibody drugs that block VEGFR-2 (KDR) activity in a clinical setting.

Three-dimensional in vitro tissue culture models of breast cancer-- a review

Three-dimensional (3D) in vitro breast tumour models have an invaluable role in tumour biology today providing some very important insights into breast cancer. As well as increasing our understanding of homeostasis, cellular differentiation and tissue organization they provide a well defined environment for cancer research in contrast to the complex host environment of an in vivo model. With the recent availability of relevant stromal elements together with the vast array of extracellular matrix constituents available, in vivo like microenvironments can be recreated. These tissue like structures more realistically model the structural architecture and differentiated function of breast cancer than a cellular monolayer providing in vivo like responses to therapeutic agents. Three dimensional in vitro models allow the study of cell-cell and cell-extracellular matrix interactions, in addition to the influence of the microenvironment on cellular differentiation, proliferation, apoptosis and gene expression. Due to their enormous potential 3D cultures are currently being exploited by many other branches of biomedical science with therapeutically orientated studies becoming the major focus of research. In return great progress in 3D culture techniques have been made, largely due to this greater interaction. At present they are being used in studies ranging from investigating the role of adhesion molecules (e.g., E-cadherin) in invasion/metastasis; VEGF and angiogenesis, to tissue modelling and remodelling. Progress in the development of complex 3D culture systems is more productive than ever, however further research is vital.

The dark side of Ras: regulation of apoptosis

Mutational activation of Ras promotes oncogenesis by disrupting a multitude of normal cellular processes. Perhaps, best characterized and understood are the mechanisms by which oncogenic Ras promotes deregulated cell cycle progression and uncontrolled cellular proliferation. However, it is now clear that oncogenic Ras can also deregulate processes that control apoptosis. In light of the diversity of downstream effector targets known to facilitate Ras function, it is perhaps not surprising that Ras regulation of cell survival is complex, involving the balance and interplay of multiple signaling networks. While our understanding of these events is still far from complete, and is complicated by cell type and signaling context differences, several important mechanisms have begun to emerge. We review the role and mechanism of specific effectors in regulating the antiapoptotic (Raf, phosphatidylinositol 3-kinase and Tiam1) and apoptotic (Nore1 and RASSF1) actions of oncogenic Ras, and discuss the possibility that the effector actions of p120RasGAP make a significant contribution to Ras regulation of apoptotic events.

Angiogenesis in transgenic models of multistep angiogenesis

The histopathology and the epidemiology of human cancers, as well as studies of animal models of tumorigenesis, have led to a widely accepted notion that multiple genetic and epigenetic changes have to accumulate for progression to malignancy. Formation of new blood vessels (tumor angiogenesis) has been recognized, in addition to proliferative capabilities and ability to down-modulate cell death (apoptosis), as essential for the progressive growth and expansion of solid tumors. Mice overexpressing activated forms of oncogenes or carrying targeted mutations in tumor suppressor genes have proven extremely useful for linking the function of these genes with specific tumor features such as continuous proliferation, escape from apoptosis, invasion and neo-angiogenesis. The interbreeding of these mice allows for studying the extent of cooperativity between different genetic lesions in disease progression, leading to a greater understanding of multi-stage nature of tumorigenesis.

Bread wheat gliadin cytotoxicity: a new three-dimensional cell model

BACKGROUND

In an attempt to clarify the role of gliadin toxicity in the pathogenesis of gluten intolerance (celiac disease), previous in vitro studies have been based on two-dimensional human cell cultures. However, the specific morphological and biochemical properties of in vivo tissue are better maintained in three-dimensional cell cultures (multicellular spheroids, MCS). The aim of this study was to develop a three-dimensional in vitro model to investigate the effects of gliadin on epithelial cells and broaden our understanding of the early tissue damage occurring in celiac disease.

METHODS

The three-dimensionally growing Lovo cell line was exposed to increasing concentrations of peptic-tryptic-digested bread wheat gliadin (from 125 to 1000 microg/mL) for 7 days in order to evaluate cell viability (colony-forming assay), and at the standard concentration of 500 microg/mL for 7 days in order to evaluate MCS diameters, volumes and cell morphology using light and electron microscopy.

RESULTS

In comparison with the controls, the cell viability of the gliadin-treated MCS was significantly reduced (20-80%), but there was no difference in size. Various degrees of cell damage (autophagic vacuoles and intra-cytoplasmic lipid-like droplets) were detected by both light and electron microscopy.

CONCLUSION

This is the first study investigating the effects of gliadin on MCS. Lovo MCS seem to be responsive to gliadin exposure, thus confirming previous results obtained using two-dimensional cell cultures. The data suggest that three-dimensional cell cultures may be useful in broadening our understanding of some of the early effects of gliadin peptides on epithelial cells.

Release of Vascular Endothelial Growth Factor from a Human Melanoma Cell Line, WM35, Is Induced by Hypoxia but Not Ultraviolet Radiation and Is Potentiated by Activated Ras Mutation

Angiogenesis, the formation of blood vessels, is a major factor influencing tumor growth and metastatic capacity, and VEGF is the prototype angiogenic factor. VEGF expression is also found in the dermis and tumor stroma during the course of melanoma progression. Various oncogenes such as c-Src, v-Raf, and Ras, and multiple environmental stimuli, including hypoxia and ultraviolet radiation (UVR), can regulate VEGF expression under certain conditions. We have constructed several cell lines from a radial growth phase, primary human melanoma cell line, WM35. We have stably transfected WM35 cells with mutant activated H-ras, N-ras, dominant negative p53, or empty vector. In this report, we determined how VEGF expression and release from these melanoma cell lines were affected by the following important factors associated with melanoma initiation and progression: hypoxia, UVR, activated Ras, dominant negative p53, and culture conditions mimicking radial growth phase melanoma (monolayer culture) and vertical growth phase melanoma (spheroid culture). We found that hypoxia, but not UVR, up-regulates VEGF mRNA expression and protein release in these melanoma cells. In addition, activated Ras and dominant negative p53 enhances the hypoxia-induced VEGF protein release. We propose that hypoxia-induced VEGF release promotes tumor progression, especially in melanomas with Ras or p53 mutations.

Cyclin D1 is necessary but not sufficient for anchorage-independent growth of rat mammary tumor cells and is associated with resistance of the Copenhagen rat to mammary carcinogenesis

To identify genes associated with the resistance of Copenhagen (Cop) rats to mammary carcinogenesis, we infused a retrovirus harboring v-Ha-ras directly into the main mammary ducts of resistant F1 rats from a cross between Cop and susceptible Wistar Furth (WF) rats. Adenocarcinomas formed in approximately 50% of infused glands. Cell lines derived from these tumors were clonal, but did not share a common viral integration site, suggesting that a high level of v-Ha-ras expression was able to overcome resistance in the F1 rats. Some of the cell lines were able to grow in soft agar, but a significant number did not display anchorage-independent growth. These growth characteristics were independent of v-Ha-ras expression levels. The ability to grow in soft agar was associated with the size of tumors induced by injecting the cells into nude mice, and showed a striking positive association with the expression of cyclin D1. Furthermore, while resistance to anchorage-independent growth was fully overcome by transfection of cyclin D1 in some clones, in the others the effect was partial. A similar pattern of cyclin D1 upregulation and growth in soft agar was also observed when the cells were transfected with an active form of beta-catenin. Hybrid cells from the somatic fusion of an anchorage-dependent to an anchorage-independent clone did not grow in soft agar. These results suggest that while a high expression level of cyclin D1 is necessary for anchorage-independent growth in all clones, it is not sufficient for full growth capacity in soft agar, raising the possibility that the loss of a tumor suppressor gene in the cell lines is required to fully confer anchorage-independent growth. Our anchorage-dependent and -independent rat mammary tumor-derived cell lines may recapitulate the resistance and susceptibility of Cop and WF rats, respectively, to mammary carcinogenesis that could facilitate the identification of breast cancer susceptibility genes.

Differential expression of CD44(S) and variant isoforms v3, v10 in three-dimensional cultures of mouse melanoma cell lines

Multi-cellular spheroids (MCS) generated from tumor cells serve as excellent in vitro models for understanding the mechanisms of tumor progression and micro-metastasis. We have compared the expression of molecular markers with reference to their growth as conventional adherent monolayers (2-D) and anchorage independent cultures (3-D) using two mouse melanoma cell lines, B16F10 and Clone M3. The two cell lines differed in their ability to form spheroids with respect to their aggregation potential, with B16F10 forming large clusters compared to Clone M3. A panel of molecular markers comprising cell adhesion molecules, cyclin dependent kinase inhibitors and members of the cadherin-catenin complex were analyzed by flow cytometry in 2-D and 3-D cultures. There was a distinct difference in the patterns of expression of CD44(S) and variant isoforms v3, v10 in spheroids compared to cells grown as monolayers in both cell lines. Also, there was an increase in cells positive for CDK inhibitor p27 in 3-D cultures from the B16F10 cell line. The expression of alpha and gamma catenin was down regulated in spheroids. As these molecules are implicated in the regulation of cell proliferation, alterations in the expression of these molecules in 3-D cultures compared to their 2-D counterparts suggests the importance of spheroids as experimental model for tumorigenesis.

Increased Bcl-xL expression mediates v-Src-induced resistance to anoikis in intestinal epithelial cells

Acquisition of resistance to anoikis (detachment-induced apoptosis) is considered to be a requirement for transformed intestinal epithelial cells to invade surrounding tissues and metastasize to distant organs. Increased Src kinase activity, which is a feature of a large proportion of colorectal cancers, has been identified as one of the factors that can contribute to anoikis resistance. However, the molecular mechanism by which high levels of Src activity contribute to anoikis resistance in intestinal epithelial cells is unknown. Here we show that high Src activity confers resistance to anoikis in intestinal epithelial cells, at least in part, by inducing Bcl-xL overexpression, and that this induction is mediated by the MEK/MAPK pathway. Based on the findings reported here, and on our previous study showing that Bcl-xL plays a critical role in ras-induced resistance to anoikis, we propose that the increased Bcl-xL levels found in colorectal cancers play a significant role in the induction of resistance to anoikis during the progression of this disease.

Human duodenal spheroids for noninvasive intracellular pH measurement and quantification of regulation mechanisms under physiological conditions

Three-dimensional cell cultures (spheroids) of biopsies of human duodenum were used to develop a new noninvasive method for studying intercellular and intracellular mechanisms. Through examinations of intracellular pH regulation, high functional similarity to native tissue could be shown, as already evidenced morphologically. A special microperfusion chamber was developed to fix individual spheroids physically to a nylon net, via laminar perfusion flow through the chamber. A significant improvement over current fixation methods was shown by the increase of cell viability almost up to 100%. Viability of the spheroids was confirmed by trypan blue exclusion, by a LIVE/DEAD viability/cytotoxicity kit, and by BCECF distribution. Intracellular pH was measured by use of the pH-sensitive fluorescence dye BCECF. To investigate the intracellular pH regulation, spheroid-like vesicles were acidified by NH4Cl prepulse technique. The subsequent active intracellular pH recovery was blocked with Na+-free Krebs Henseleit (KH) solution, with amiloride KH (inhibitor of the Na+-H+-exchanger), or with H2DIDS KH (inhibitor of the HCO3(-)-Cl(-)-exchanger and Na+-HCO3(-)-cotransporter). The intracellular pH of the spheroids was 7.31 +/- 0.05. pH-backregulation after acidification was prevented by sodium-free buffer, amiloride, and H2DIDS. These experiments indicated the presence of a Na+-H+-exchanger and a Na+-HCO3(-)-cotransporter. In conclusion, the human duodenal spheroid is an excellent physiological system for in vitro studies of the human duodenum.

Transforming growth factor-alpha prevents detachment-induced inhibition of c-Src kinase activity, Bcl-XL down-regulation, and apoptosis of intestinal epithelial cells

Detachment of epithelial cells from the extracellular matrix (ECM) results in apoptosis, a phenomenon often referred to as anoikis. Acquisition of anoikis resistance is now thought to be a prerequisite for the progression of carcinomas. Colorectal cancer cells frequently secrete epidermal growth factor receptor (EGFR) ligands, which are known to have anti-apoptotic activity. However, whether these ligands have the ability to inhibit anoikis of intestinal epithelial cells is unclear, since at least in some cell types efficient EGFR signaling requires cell-ECM adhesion. Here we report that transforming growth factor-alpha (TGF-alpha), an EGFR ligand that is frequently secreted by colorectal cancer cells, strongly inhibits anoikis of the non-malignant rat intestinal epithelial cell lines, IEC-18 and RIE-1. TGF-alpha exerts its anti-anoikis effect by preventing detachment-induced inhibition of c-Src kinase activity. We also show that Fas activation, a molecular event known to play a critical role in anoikis, is not suppressed by TGF-alpha. On the other hand, this growth factor strongly inhibits the detachment-induced down-regulation of Bcl-X(L), another change that is involved in the induction of anoikis. We further demonstrate that this inhibition occurs in a c-Src-dependent manner. We conclude that TGF-alpha has the ability to suppress anoikis of intestinal epithelial cells, at least in part, by reverting the loss of c-Src activity and Bcl-X(L) expression induced by detachment from the ECM.

Endogenous IL-1 and type II IL-1 receptor expression modulate anoikis in intestinal epithelial cells

We previously reported that IL-1beta and the decoy receptor for IL-1 (IL-1RII) are expressed by intestinal epithelial cells (IEC) during detachment-induced cell death, or "anoikis." We now investigated whether IL-1 regulates anoikis. Skewing the balance in favor of IL-1, by blocking IL-1RII or by adding IL-1beta to detached rat IEC-18 cells, reduced cell death. The protective effect of anti-IL-1RII was reversed by blocking IL-1beta, confirming the anti-apoptotic effect was due to endogenous IL-1beta. Added IL-1beta also rescued cells from anoikis and was associated with considerable aggregation of the detached cells. Aggregate formation and the anti-apoptotic effect of added IL-1beta were prevented by blocking E-cadherin, indicating that IL-1 promoted aggregation and indirectly, survival. On the other hand, treating detached cells with IL-1beta and an anti-beta(1) integrin antibody abolished the protective effect of IL-1beta but not the aggregates. We conclude that the anti-apoptotic effect of IL-1 is mediated through a beta(1) integrin-dependent event secondary to cell-cell adhesion. This illustrates a previously uncharacterized role for IL-1 in the intestine wherein this cytokine may facilitate the preservation of the epithelial monolayer integrity.

Ras regulation of vascular endothelial growth factor and angiogenesis

Given the multifaceted role of Ras in tumor angiogenesis, pharmacologic targeting of such proteins may bring about at least three important consequences: (1) partial obliteration of the angiogenic competence of tumor cells, (2) an increase in vascular dependence and sensitization to apoptosis, and (3) a direct inhibition of endothelial cell responses to proangiogenic stimuli. Exploration of some of these possibilities, using various pharmacological compounds and antibodies, has already begun. An intriguing possibility is that Ras antagonists and signal transduction inhibitors may synergize with a number of other antiangiogenic modalities such as direct acting antiangiogenic agents (e.g., endostatin) or antivascular regimens involving low-dose continuous chemotherapy as a vasculature-targeting strategy.

Tomorrow's anticancer agents: inhibitors of Ras farnesylation

The purpose of this chapter is to concentrate on what can be considered as definite milestones on the way from examples of inhibitors of farnesyl-protein transferase (FPTase) to candidate drugs actually being considered for or already being evaluated in clinical trials. Emphasis will be placed on results obtained using experimental tumour models in vivo, with a detailed discussion of these results and of the questions which remain to be studied or are still unanswered. The data discussed here are almost exclusively based on published reports, with only brief reference, in the chapter "use of the FPTase inhibitors in the clinic", to some of the newer compounds reported on during recent meetings, details of which have not yet appeared in the peer-reviewed literature. For those requiring a more extensive review of the catalogue of FPTase inhibitors now discovered, some excellent reviews have been committed to this purpose [1-3].

Apoptosis: A Current Molecular Analysis

Apoptosis is a cell suicide program characterized by distinct morphological (cell shrinkage, membrane blebbing, pyknosis, chromatin margination, denser cytoplasmic images) and biochemical (e.g., DNA fragmentation into distinct ladders; degradation of apoptotic markers such as PARP and nuclear lamins) features. It is involved in multiple physiological processes examplified by involution of mammary tissues, embryonic development, homeostatic maintenance of tissues and organs, and maturation of the immune system, as well as in many pathological conditions represented by neurologic degeneration (Alzeimer's disease), autoimmune and inflammatory diseases, etiology of atherosclerosis, AIDS, and oncogenesis and tumor progression. Numerous molecular entities have been shown to regulate the apoptotic process. This review provides a concise summary of the recent data on the role of oncogenes/tumor suppressor genes, cytokines and growth factors/growth factor receptors, intracellular signal transducers, cell cycle regulators, reactive oxygen species or other free radicals, extracellular matrix regulators/cell adhesion molecules, and specific endonucleases and cytoplasmic proteases (the ICE family proteins) in regulating cell survival and apoptosis. Elucidation of the molecular mechanisms regulating apoptosis bears tremendous impact on enhancing our understanding of many diseases inflicting the human beings and undoubtedly brings us hope for the cure of these diseases.

Farnesyl transferase inhibitors: a novel targeted tnerapy for cancer

Activating oncogenic mutations of the RAS gene are common in cancer, occurring in 30% of solid tumours in adults. Inhibitors of the enzyme farnesyl protein transferase prevent a key step in the post-translational processing of the RAS protein, and were developed initially as a therapeutic strategy to inhibit cell signalling in RAS-transformed cells. As more has been learnt about the biological effects of farnesyl transferase inhibitors on cancer cells, it has become increasingly clear that tumours without oncogenic RAS mutations may also be targets for farnesyl transferase inhibitor therapy. Encouraging results from phase I and II clinical trials have emerged, creating both enthusiasm and new challenges for the optimum clinical development of this important new class of anticancer drug.

Anti-tumorigenic effect of a K-ras ribozyme against human lung cancer cell line heterotransplants in nude mice

Approximately 15-30% of human non-small cell lung cancers (NSCLC) carry K-ras mutations, among which point mutations at codon 12 are the most common. This study characterizes the anti-tumor effect of an anti-K-ras ribozyme adenoviral vector (KRbz-ADV; replication-deficient, E1-deleted Ad5 backbone) against NSCLC lines that express the relevant mutation (K-ras codon 12 GGT --> GTT; H441 and H1725). KRbz-ADV significantly inhibited tumor cell growth (38-94% reduction by 3H-thymidine uptake) in a time- and dose-dependent manner, but produced minimal growth inhibition on normal epithelial cells, or NSCLC H1650 cells that lack the relevant mutation. The in vivo anti-tumorigenic effect of KRbz-ADV treatment was characterized with cell line xenografts in nu/nu mice. Pre-treatment with KRbz-ADV (10 or 20 p.f.u. per cell) completely abrogated subcutaneous engraftment of H441 (n = 13) or H1725 cells (n = 8), as compared with a 100% tumor take and progressive tumor growth in animals that received untreated tumor cells, or control vector (luciferase-adenovirus/Luc-ADV)-treated tumor cells. Pre-treatment with a mutant anti-K-ras ribozyme adenoviral vector (mutKRbz-ADV), which has the same specificity as KRbz but lacks ribozyme catalytic activity, did not produce an anti-tumorigenic effect. The in vivo effect of KRbz-ADV treatment was further examined by initiating injections (2 x 10(9) p.f.u.) at 7 days after tumor induction. Pre-existing tumor growth was reduced by 39% by a single intratumoral injection. Repeat injections (three or five KRbz-ADV-intratumoral injections at 2 x 10(9) p.f.u. every other day) resulted in complete tumor regression in five of seven mice. In contrast, single or multiple injections of control vector Luc-ADV did not significantly alter tumor xenograft outcome. Ribozyme expression was confirmed in H441 cells that demonstrated reduced growth after KRbz-ADV treatment. Reduced growth corresponded to significantly lowered levels of K-ras mRNA, as defined by RT-PCR (51% of untreated level, n = 3) and RNase protection assay (56% of untreated level, n = 4) analyses. Further, 37.5% of KRbz-ADV-treated cells underwent apoptosis, as compared with 11.7%, and 19.0% in untreated and Luc-ADV-treated cultures, respectively. A significantly higher proportion of KRbz-ADV-treated H441 cells (58.2%) underwent apoptosis when maintained under anchor-independent conditions that simulate in vivo tumorigenesis ('anoikis'). This is the first report that demonstrates that KRbz-ADV can effectively inhibit in vivo tumorigenesis, and produces regression of pre-existing human lung tumor xenografts having the relevant K-ras mutation.