Caspase-mediated cleavage of APC results in an amino-terminal fragment with an intact armadillo repeat domain

Hydrogen sulfide-releasing aspirin inhibits the growth of leukemic Jurkat cells and modulates β-catenin expression

Hydrogen sulfide-releasing aspirin (HS-ASA) is a novel compound with potential against cancer. It inhibited the growth of Jurkat T-leukemia cells with an IC₅₀ of 1.9 ± 0.2 μM whereas that of ASA was >5000 μM. It dose-dependently inhibited proliferation and induced apoptosis in these cells, causing a G₀/G₁ cell cycle arrest. HS-ASA down-regulated β-catenin protein levels and reduced mRNA and protein expression of β-catenin/TCF downstream target genes cyclinD1 and c-myc. Aspirin up to 5 mM had no effect on β-catenin expression. HS-ASA also increased caspase-3 protein levels and dose-dependently increased its activity. These effects were substantially blocked by z-VAD-fmk, a pan-caspase inhibitor.

C-terminal binding proteins are essential pro-survival factors that undergo caspase-dependent downregulation during neuronal apoptosis

C-terminal binding proteins (CtBPs) are transcriptional co-repressors that are subject to proteasome-dependent downregulation during apoptosis. Alternative mechanisms that regulate CtBP expression are currently under investigation and the role of CtBPs in neuronal survival is largely unexplored. Here, we show that CtBPs are downregulated in cerebellar granule neurons (CGNs) induced to undergo apoptosis by a variety of stressors. Moreover, antisense-mediated downregulation of CtBP1 is sufficient to cause CGN apoptosis. Similarly, the CtBP inhibitor, 4-methylthio-2-oxobutyric acid, induces expression of the CtBP target Noxa and causes actinomycin-sensitive CGN apoptosis. Unexpectedly, we found that the mechanism of CtBP downregulation in CGNs undergoing apoptosis varies in a stimulus-specific manner involving either the proteasome or caspases. In the case of CGNs deprived of depolarizing potassium (5K apoptotic condition), caspases appear to play a dominant role in CtBP downregulation. However, incubation in 5K does not enhance the kinetics of CtBP1 degradation and recombinant CtBP1 is not cleaved in vitro by caspase-3. In addition, 5K has no significant effect on CtBP transcript expression. Finally, mouse embryonic stem cells display caspase-dependent downregulation of CtBP1 following exposure to staurosporine, an effect that is not observed in DGCR8 knockout cells which are deficient in miRNA processing. These data identify caspase-dependent downregulation of CtBPs as an alternative mechanism to the proteasome for regulation of these transcriptional co-repressors in neurons undergoing apoptosis. Moreover, caspases appear to regulate CtBP expression indirectly, at a post-transcriptional level, and via a mechanism that is dependent upon miRNA processing. We conclude that CtBPs are essential pro-survival proteins in neurons and their downregulation contributes significantly to neuronal apoptosis via the de-repression of pro-apoptotic genes.

Mechanisms Regulating Microtubule Binding, DNA Replication, and Apoptosis are Controlled by the Intestinal Tumor Suppressor APC

Colorectal cancer (CRC) results from the progressive accumulation of both genetic and epigenetic alterations that lead to the transformation of normal colorectal epithelium to benign (adenoma) and invasive (carcinoma) disease. Since its discovery in mutated form as the causative gene for familial adenomatous polyposis coli (FAP), as well as in many sporadic CRCs, the APC tumor suppressor has been shown to possess numerous functions within the cell including regulation of WNT signaling pathways and its transcriptional effects, cell migration, and chromosome separation. In recent years, other novel roles for APC have been investigated and suggest that APC can also repress DNA replication and enhance apoptosis. Further insights into the mechanisms by which APC contributes to tumor suppression will accelerate the diagnosis and treatment of CRC.

Inhibition of proprotein convertase SKI-1 blocks transcription of key extracellular matrix genes regulating osteoblastic mineralization

Mineralization, a characteristic phenotypic property of osteoblastic lineage cells, was blocked by 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride (AEBSF) and decanoyl-Arg-Arg-Leu-Leu-chloromethyl ketone (dec-RRLL-cmk), inhibitors of SKI-1 (site 1; subtilisin kexin like-1) protease. Because SKI-1 is required for activation of SREBP and CREB (cAMP-response element-binding protein)/ATF family transcription factors, we tested the effect of these inhibitors on gene expression. AEBSF decreased expression of 140 genes by 1.5-3.0-fold including Phex, Dmp1, COL1A1, COL11A1, and fibronectin. Direct comparison of AEBSF and dec-RRLL-cmk, a more specific SKI-1 inhibitor, demonstrated that expression of Phex, Dmp1, COL11A1, and fibronectin was reduced by both, whereas COL1A2 and HMGCS1 were reduced only by AEBSF. AEBSF and dec-RRLL-cmk decreased the nuclear content of SKI-1-activated forms of transcription factors SREBP-1, SREBP-2, and OASIS. In contrast to AEBSF, the actions of dec-RRLL-cmk represent the sum of its direct actions on SKI-1 and indirect actions on caspase-3. Specifically, dec-RRLL-cmk reduced intracellular caspase-3 activity by blocking the formation of activated 19-kDa caspase-3. Conversely, overexpression of SKI-1-activated SREBP-1a and CREB-H in UMR106-01 osteoblastic cells increased the number of mineralized foci and altered their morphology to yield mineralization nodules, respectively. In summary, SKI-1 regulates the activation of transmembrane transcription factor precursors required for expression of key genes required for mineralization of osteoblastic cultures in vitro and bone formation in vivo. Our results indicate that the differentiated phenotype of osteoblastic cells and possibly osteocytes depends upon the non-apoptotic actions of SKI-1.

"Where, O death, is thy sting?" A brief review of apoptosis biology

Apoptosis was a term introduced in 1972 to distinguish a mode of cell death with characteristic morphology and apparently regulated, endogenously driven mechanisms. The effector processes responsible for apoptosis are now mostly well known, involving activation of caspases and Bcl2 family members in response to a wide variety of physiological and injury-induced signals. The factors that lead of the decision to activate apoptosis as opposed to adaptive responses to such signals (e.g. autophagy, cycle arrest, protein synthesis shutoff) are less well understood, but the intranuclear Promyelocytic Leukaemia Body (PML body) may create a local microenvironment in which the audit of DNA damage may occur, informed by the extent of the damage, the adequacy of its repair and other aspects of cell status.

The mitochondrial protein hTID-1 partners with the caspase-cleaved adenomatous polyposis cell tumor suppressor to facilitate apoptosis

BACKGROUND & AIMS

The adenomatous polyposis cell (APC) tumor suppressor is a multifunctional protein involved in cell migration, proliferation, differentiation, and apoptosis. Cleavage of APC and the subsequent release of an amino-terminal segment are necessary for a transcription-independent mechanism of APC-mediated apoptosis. The aim of the current study is to elucidate the mechanism by which the amino-terminus of APC contributes to the enhancement of apoptosis.

METHODS

Previous yeast 2-hybrid screens, using the armadillo repeat domain of APC as bait, identified hTID-1 as a potential binding partner. Coimmunoprecipitations, coimmunofluorescence, and binding assays confirm a direct interaction between caspase-cleaved APC and hTID-1 in vivo at the mitochondria. Overexpression and small interfering RNA (siRNA) knockdown studies were designed to determine the significance of this interaction.

RESULTS

These experiments have identified hTID-1 as a directly interacting protein partner of caspase-cleaved APC. hTID-1 is an apoptosis modulator: 2 of its known mitochondrial protein isoforms, 43-kilodaltons and 40-kilodaltons, have opposing effects in apoptosis. We demonstrate that the amino-terminal segment of APC interacts with both hTID-1 isoforms directly, although there is a stronger association with the apoptotic suppressor 40-kilodalton isoform in vitro. This interaction localizes to amino acids 202-512 of APC, a region including 2 of the 7 armadillo repeats. Overexpression of the 40-kilodalton hTID-1 isoform partially rescues cells from apoptosis mediated by APC 1-777, whereas siRNA knockdown of this hTID-1 isoform enhances apoptosis.

CONCLUSIONS

These data suggest that the amino-terminal segment of APC promotes cell sensitivity to apoptosis modulated through its binding to 40- and 43-kilodalton hTID-1 isoforms.

Caspase cleavage of the APC tumor suppressor and release of an amino-terminal domain is required for the transcription-independent function of APC in apoptosis

The adenomatous polyposis coli (APC) tumor suppressor is inactivated by mutation in most colorectal tumors. APC is a component of the Wnt signaling pathway and is best known for its ability to downregulate beta-catenin and consequent effects on transcriptional regulation. Previous work demonstrated that APC accelerates apoptosis-associated caspase activity independently of transcription, and suggested novel tumor suppressor functions of APC. In this work, we have mapped the APC apoptosis-accelerating region to amino acids (aa) 1-760 by testing a series of non-overlapping APC segments. Interestingly, this segment corresponds to a stable group II caspase cleavage product of APC released during apoptosis that includes the amino-terminal aa1-777. Mutation of the APC aspartic acid residue at position 777 to an alanine completely abolished in vitro cleavage of APC by a recombinant group II caspase and rendered the full-length protein unable to accelerate apoptosis in vitro. A truncated APC protein associated with familial and sporadic colorectal cancer, also unable to accelerate apoptosis in vitro and in vivo, is resistant to group II caspase cleavage. These results demonstrate that cleavage of APC and the subsequent release of an amino-terminal segment are necessary for the transcription-independent mechanism of APC-mediated apoptosis.

Artesunate attenuates the growth of human colorectal carcinoma and inhibits hyperactive Wnt/β-catenin pathway

Artesunate (ART), a remarkable antimalarial agent, also inhibited the growth of human colorectal carcinoma. As determined by MTT assay, flow cytometry analysis on apoptosis and indirect immunofluorescence analysis on the proliferation-associated marker Ki67, ART suppressed the proliferation and promoted the apoptosis of colorectal cancer cells in a dose-dependent manner. Furthermore, immunofluorescence analysis on beta-catenin and RT-PCR analysis on Wnt/beta-catenin target genes demonstrated ART translocated beta-catenin from nucleus to adherent junctions of membrane and reduced transcription mediated by beta-catenin. These results suggested the anticancer activity of ART correlated with the inhibition of hyperactive Wnt/beta-catenin signaling pathway. In vivo, ART significantly slowed the growth of colorectal tumor xenografts. Bioluminescent imaging also revealed that ART decreased the physiological activity of tumor xenografts and delayed spontaneous liver metastasis. These antitumor effects were related to the membranous translocation of beta-catenin and the inhibition of the unrestricted activation of Wnt/beta-catenin pathway, which was confirmed by the immunohistochemical staining of tumor tissues. These results and the known low toxicity are clues that ART might be a promising candidate drug for the treatment of colorectal carcinoma.

Epithelial proliferation induces novel changes in APC expression

The role of wild-type adenomatous polyposis coli (APC) protein in native epithelia is poorly understood. The present study examined the relationships between wild-type APC and beta-catenin expression in an established model of hyperproliferation, transmissible murine colonic hyperplasia (TMCH). Distal colonic crypts isolated from normal or TMCH mice were: (i) fractionated into cytosolic and nuclear components for Western blotting and immunoprecipitation (IP), (ii) extracted for total RNA isolation for Northern blotting and, (iii) analysed immunohistochemically by confocal microscopy. Western blots performed sequentially through day 12 TMCH with N-terminal APC antibodies revealed increased abundance of approximately 312 kDa (p312) protein by day 6 (4.0 +/- 0.75-fold, n = 6) that peaked by day 9, before declining by day 12. A approximately 130 kDa (p130) band appeared at day 9 and increased by day 12 (1.5 +/- 0.11-fold, n = 6). A C-terminal antibody detected only p312. APC mRNA level did not change during TMCH and appearance of p130 was not due to alternative splicing. Co-IP with N-terminal anti-APC antibodies, revealed APC's association with beta-catenin both at day 6 and day 12. p130, but not p312, associated predominantly with beta-catenin at day 12 during co-IP with anti-beta-catenin. p130 also selectively accumulated in the nucleus, bound to nuclear beta-catenin at day 12. Immunocytochemistry with N-terminal antibodies revealed an increasing crypt base : surface gradient of APC within the apical pole/apical-lateral membranes at day 6. At day 12, intense apical/cytoplasmic and occasional nuclear staining along the longitudinal crypt axis was observed. Full-length APC increases during epithelial hyperproliferation and may represent a homoeostatic response. The dramatic increase in cytoplasmic and sporadic nuclear APC staining at day 12 with N-terminal antibodies may represent p130. The nuclear accumulation of p130 may be a novel mechanism regulating nuclear beta-catenin function during TMCH.

Drosophila caspase transduces Shaggy/GSK-3beta kinase activity in neural precursor development

Caspases are well known for their role in the execution of apoptotic programs, in which they cleave specific target proteins, leading to the elimination of cells, and for their role in cytokine maturation. In this study, we identified a novel substrate, which, through cleavage by caspases, can regulate Drosophila neural precursor development. Shaggy (Sgg)46 protein, an isoform encoded by the sgg gene and essential for the negative regulation of Wingless signaling, is cleaved by the Dark-dependent caspase. This cleavage converts it to an active kinase, which contributes to the formation of neural precursor (sensory organ precursor (SOP)) cells. Our evidence suggests that caspase regulation of the wingless pathway is not associated with apoptotic cell death. These results imply a novel role for caspases in modulating cell signaling pathways through substrate cleavage in neural precursor development.

The APC tumor suppressor promotes transcription-independent apoptosis in vitro

The APC tumor suppressor is found in nonproliferating epithelial cells of the colonic crypts and is mutated in most colorectal tumors. To understand the function of APC in normal epithelium and how its loss leads to tumor formation, we tested whether APC is a mediator of apoptosis using an in vitro assay that monitors caspase-3-mediated cleavage of lamin B protein or a colorimetric substrate in a cell-free Xenopus egg extract. Recombinant APC protein accelerates apoptosis-associated caspase activity independently of ongoing transcription and protein synthesis. Conversely, the addition of mutant APC and immunodepletion of Xenopus APC decelerates apoptosis-associated caspase activity. Acceleration of apoptosis by APC is abolished by the caspase-8 inhibitor Z-IETD-FMK, demonstrating that caspase-8 is an essential component of APC-mediated apoptosis. These results suggest that the induction of apoptosis may be one role of APC in tumor suppression and that this mechanism is independent of beta-catenin-mediated effects on transcription.

Zinc stabilizes adenomatous polyposis coli (APC) protein levels and induces cell cycle arrest in colon cancer cells

In the present study, we investigated the mechanisms by which zinc causes growth arrest in colon cancer cells. The results suggest that zinc treatment stabilizes the levels of the wild-type adenomatous polyposis coli (APC) protein at the post-translational level since the APC mRNA levels and the promoter activity of the APC gene were decreased in HCT-116 cells (which express the wild-type APC gene) after treatment with ZnCl2. Increased levels of wild-type but not truncated APC proteins were required for the ZnCl2-mediated G2/M phase arrest in different colon cancer cell lines. We further tested whether serum-stimulation, which induces cell cycle arrest in the S phase, can relieve ZnCl2-induced G2/M phase arrest of HCT-116 cells. Results showed that in the HCT-116 cells pretreated with ZnCl2, the serum-stimulation neither changed the distribution of G2/M phase arrested cells nor the increased levels of APC protein. The G2/M phase arrest correlated with retarded growth of HCT-116 cells. To further establish that wild-type APC protein plays a role in ZnCl2-induced G2/M arrest, we treated SW480 colon cancer cells that express truncated APC protein. We found that ZnCl2 treatment did not induce G2/M phase arrest in SW480 cells; however, the cell growth was retarded due to the loss of E-cadherin and alpha-tubulin levels. These results suggest that ZnCl2 inhibits the proliferation of colon cancer cells (which carry the wild-type APC gene) through stabilization of the APC protein and cell cycle arrest in the G2/M phase. On the other hand, ZnCl2 inhibits the proliferation of colon cancer cells (which carry the mutant APC gene) by disrupting cellular attachment and microtubule stability.

Anti-cancer effects of butyrate: use of micro-array technology to investigate mechanisms

Epidemiological evidence suggests that a high intake of resistant starch and NSP protects against colo-rectal cancer. The mechanisms underlying this protection are thought to be mediated by the short-chain fatty acid butyrate, which is present in the colonic lumen in millimolar concentrations as a result of bacterial fermentation of carbohydrates that have resisted digestion in the small intestine. In vitro studies have shown that butyrate displays a host of chemo-preventative properties including increased apoptosis, reduced proliferation, down regulation of angiogenesis, enhanced immunosurveillance and anti-inflammatory effects in colo-rectal cancer cell lines. However, the molecular mechanisms underlying the apparent chemo-preventative actions of butyrate are largely unknown. The evidence supporting the role of butyrate as an anti-cancer agent is reviewed, with particular emphasis on those studies that have attempted to elucidate the mechanism of action of butyrate. Our understanding of the mechanistic action of butyrate and its role in cancer prevention is likely to advance considerably in this post-genomic era with the application of genomic and proteomic technologies. Studies are described that have used gene array and proteomic techniques to investigate the response of colo-rectal cancer cells to butyrate. These pioneering studies illustrate the potential of these technologies to help characterise the molecular responses of the cancer cell to butyrate, and to define the role of butyrate (and other nutrients) in the prevention of colo-rectal cancer.

Many cuts to ruin: a comprehensive update of caspase substrates

Apoptotic cell death is executed by the caspase-mediated cleavage of various vital proteins. Elucidating the consequences of this endoproteolytic cleavage is crucial for our understanding of cell death and other biological processes. Many caspase substrates are just cleaved as bystanders, because they happen to contain a caspase cleavage site in their sequence. Several targets, however, have a discrete function in propagation of the cell death process. Many structural and regulatory proteins are inactivated by caspases, while other substrates can be activated. In most cases, the consequences of this gain-of-function are poorly understood. Caspase substrates can regulate the key morphological changes in apoptosis. Several caspase substrates also act as transducers and amplifiers that determine the apoptotic threshold and cell fate. This review summarizes the known caspase substrates comprising a bewildering list of more than 280 different proteins. We highlight some recent aspects inferred by the cleavage of certain proteins in apoptosis. We also discuss emerging themes of caspase cleavage in other forms of cell death and, in particular, in apparently unrelated processes, such as cell cycle regulation and cellular differentiation.

Germline APC mutation on the beta-catenin binding site is associated with a decreased apoptotic level in colorectal adenomas

Germline mutations in APC tumor suppressor gene are responsible for familial adenomatous polyposis (FAP). A major role of these genetic changes is the constitutive activation of beta-catenin-Tcf-4 mediated transcription of nuclear target genes, but other cellular functions can be misregulated. To assess how different APC mutations can drive the early steps of colonic tumorigenesis, we studied the effect of 10 different germline-truncating alterations on the phenotype of the corresponding adenomas. A significant reduction of apoptosis, uncoupled with an increased c-myc and cyclin-D1 expression, was seen with a frameshift mutation on codon 1383, in the 20-aa repeats of the beta-catenin degradation domain, independent of a somatic alteration on the wild-type allele. The decreased apoptotic level was associated with a higher incidence of cancerization. No other APC mutation was linked with a similar effect, even in presence of a somatic allelic loss. These findings suggest that mutations in critical sites of the beta-catenin degradation domain of APC gene can convey a selective advantage to the colonic neoplastic clones by altering the apoptotic surveillance rather than enhancing the beta-catenin-Tcf-4 transcription of growth-promoting genes.

Different molecular events account for butyrate-induced apoptosis in two human colon cancer cell lines

We studied the molecular events underlying butyrate-induced apoptosis in two different colon cancer cell lines: Caco-2, a well defined cancer cell and RSB, a cell line obtained from a colonic tumor of an ulcerative colitis patient. Caco-2 and RSB cells were exposed to 2, 5 and 10 mmol/L butyrate for 48 h. Caspase-1 was cleaved in Caco-2-cells at all butyrate concentrations, whereas in RSB-cells caspase-1 expression was undetectable. In RSB cells, butyrate dose-dependently induced caspase-3 cleavage, whereas in Caco-2-cells, butyrate up-regulated expression of the caspase-3 active subunit. Caspase-3-specific activity, cytoplasmic nucleosome concentration and growth were directly correlated with butyrate doses in both cell lines; however, the response was more pronounced in Caco-2 than in RSB cells. Expression of the cleaved poly(ADP-ribose) polymerase (PARP) product was elevated in both cell lines at the highest butyrate concentration. Bak expression gradually increased as a function of butyrate concentrations in both cell lines. At 10 mmol/L butyrate, expression increased by fivefold and sevenfold in Caco-2 and RSB cells, respectively. The highest expression of Bcl-2 was observed in control Caco-2 cells, and expression decreased with increasing butyrate concentration. This effect was not observed in RSB cells. Inactivation of caspase-1 with Z-YVAD-FMK abrogated butyrate-induced apoptosis in Caco-2 but not in RSB cells. Inactivation of caspase-3 with Z-DVED-FMK completely inhibited butyrate-induced apoptosis in RSB cells whereas this effect was less pronounced in Caco-2 cells. Our data demonstrate that butyrate-induced apoptosis is activated via different apoptotic pathways in diversely stratified colon cancers.

Attenuated APC alleles produce functional protein from internal translation initiation

Some truncating mutations of the APC tumor suppressor gene are associated with an attenuated phenotype of familial adenomatous polyposis coli (AAPC). This work demonstrates that APC alleles with 5' mutations produce APC protein that down-regulates beta-catenin, inhibits beta-catenin/T cell factor-mediated transactivation, and induces cell-cycle arrest. Transfection studies demonstrate that cap-independent translation is initiated internally at an AUG at codon 184 of APC. Furthermore, APC coding sequence between AAPC mutations and AUG 184 permits internal ribosome entry in a bicistronic vector. These data suggest that AAPC alleles in vivo may produce functional APC by internal initiation and establish a functional correlation between 5' APC mutations and their associated clinical phenotype.

Cleavage and shedding of E-cadherin after induction of apoptosis

Apoptotic cell death induces dramatic molecular changes in cells, becoming apparent on the structural level as membrane blebbing, condensation of the cytoplasm and nucleus, and loss of cell-cell contacts. The activation of caspases is one of the fundamental steps during programmed cell death. Here we report a detailed analysis of the fate of the Ca(2+)-dependent cell adhesion molecule E-cadherin in apoptotic epithelial cells and show that during apoptosis fragments of E-cadherin with apparent molecular masses of 24, 29, and 84 kDa are generated by two distinct proteolytic activities. In addition to a caspase-3-mediated cleavage releasing the cytoplasmic domain of E-cadherin, a metalloproteinase sheds the extracellular domain from the cell surface during apoptosis. Immunofluorescence analysis confirmed that concomitant with the disappearance of E-cadherin staining at the cell surface, the E-cadherin cytoplasmic domain accumulates in the cytosol. In the presence of inhibitors of caspase-3 and/or metalloproteinases, cleavage of E-cadherin was almost completely blocked. The simultaneous cleavage of the intracellular and extracellular domains of E-cadherin may provide a highly efficient mechanism to disrupt cadherin-mediated cell-cell contacts in apoptotic cells, a prerequisite for cell rounding and exit from the epithelium.

The nonsteroidal anti-inflammatory drugs aspirin and indomethacin attenuate beta-catenin/TCF-4 signaling

Increasing epidemiological and experimental evidence implicates non-steroidal anti-inflammatory drugs (NSAIDs) as anti-tumorigenic agents. The precise mechanisms whereby NSAIDs exert their anti-neoplastic effects remain poorly understood. Studies from hereditary and sporadic colorectal cancer (CRC) patients suggest that NSAIDs may interfere with initiating steps of carcinogenesis, i.e. disturbances within the beta-catenin signaling pathway. We therefore investigated beta-catenin/TCF signaling in response to aspirin or indomethacin, respectively, in four CRC cell lines (SW948, SW480, HCT116, LoVo). Both, aspirin and indomethacin inhibited transcription of a beta-catenin/TCF-responsive reporter gene in a dose dependent manner. In addition, the beta-catenin/TCF transcriptional target cyclin D1 was downregulated by both drugs. Endogenous beta-catenin levels remained unaffected by either drug. Moreover, indirect immunofluorescence studies revealed no significant changes of subcellular beta-catenin localization in either cell line after NSAID treatment. Likewise, binding of the beta-catenin/TCF complex to its specific DNA-binding sites was not altered, as demonstrated by electrophoretic mobility shift assay (EMSA) of nuclear extracts derived from NSAID treated cells. These results strongly suggest that aspirin and indomethacin attenuate the transcription of beta-catenin/TCF-responsive genes, by modulating TCF activity without disrupting beta-catenin/TCF complex formation.

Kill and cure: dietary augmentation of immune defences against colon cancer

At its most fundamental, cancer is a genetic disease resulting from inherited or acquired mutations in tumour suppressor genes and proto-oncogenes. Environmental factors, including ingested food components, interact with genetic inheritance to determine individual cancer risk. There is growing evidence that the immune system exerts selective pressure during neoplastic development. Tumour cells that evade this immunosurveillance because they are non-antigenic or because they defend themselves successfully against immune attack have a survival advantage. Effective chemopreventative agents will include dietary components that enhance the immune system's ability to identify transformed cells and to target them for apoptosis.