Colonic polyamine content and ornithine decarboxylase activity as markers for adenomas

Urinary Excretion of Free and Acetylated Polyamines in Hepatocellular Carcinoma

Polyamines (putrescine, spermidine and spermine) are essential for the proliferation of normal and neoplastic cells, and have been repeatedly recommended as tumor markers, with contrasting and elusive results.

In the present study the urinary excretion of free and acetylated polyamines was measured in patients with hepatocellular carcinoma (HCC), cirrhotics and control subjects.

Separation and quantification of dansyl-derivatives of free, acetylated and total polyamines was performed by reverse-phase high-performance liquid chromatography.

The results show that the urinary excretion of total, free, and acetylated polyamines is significantly higher in HCC patients than in cirrhotics and controls (p<0.001). The N1/N8 acetyl-spermidine molar ratio was found to be higher in HCC patients than in cirrhotics and controls (p<0.001). No correlation was found between urinary excretion of polyamines and serum alpha-fetoprotein, tumor size and severity of liver cirrhosis. The results show that increased urinary excretion of free and acetylated polyamines, as well as an altered N1/N8-acetyl-spermidine molar ratio, is a sensible but not specific feature of HCC patients; polyamines may play a role in human carcinogenesis, but their determination does not seem reliable for the early detection of liver cancer.

Effects of probiotic bacteria on mucosal polyamines levels in dogs with IBD and colonic polyps: a preliminary study

Spermine (SPM) and its precursor putrescine (PUT), regulated by ornithine decarboxylase (ODC) and diamino-oxidase (DAO), are polyamines required for cell growth and proliferation. Only a few studies have investigated the anti-inflammatory and tumour inhibitory properties of probiotics on mucosal polyamine levels. We investigated the effects of a high concentration multistrain probiotic for human use on colonic polyamine biosynthesis in dogs. Histological sections (inflammatory bowel disease, n=10; polyposis, n=5) were assessed after receiving 112 to 225×10⁹ lyophilised bacteria daily for 60 days at baseline (T0) and 30 days after treatment end (T90). Histology scores, expression of PUT, SPM, ODC and DAO, and a clinical activity index (CIBDAI) were compared at T0 and T90. In polyps, cellular proliferation (Ki-67 expression), and apoptosis (caspase-3 protein expression) were also evaluated. After treatment, in inflammatory bowel disease significant decreases were observed for CIBDAI (P=0.006) and histology scores (P<0.001); PUT, SPM and ODC expression increased (P<0.01). In polyps, a significant decrease in polyamine levels, ODC activity, and Ki-67, and a significant increase in caspase-3 positivity and DAO expression (P=0.005) was noted. Our results suggest potential anti-proliferative and anti-inflammatory effects of the probiotic mixture in polyps and inflammation, associated with reduced mucosal infiltration and up-regulation of PUT, SPM, and ODC levels.

Potentiating NK cell activity by combination of Rosuvastatin and Difluoromethylornithine for effective chemopreventive efficacy against Colon Cancer

Colorectal cancer (CRC) is the second highest cause of cancer-related deaths. A successful strategy to improve chemopreventive efficacies is by down-regulating tumor polyamines and enhancing NK cell activities. Colonic carcinogenesis was induced by azoxymethane (AOM) in male F344 rats. Eight weeks after AOM treatment, animals were fed diets containing Rosuvastatin and difluromethylornithine (DFMO) individually and in combination for 40 weeks. Both agents showed significant suppression of adenocarcinoma multiplicity and incidence with no toxicity compared to untreated rats. Low-dose Rosuvastatin plus DFMO suppressed colon adenocarcinoma multiplicity by 76% compared to low-dose Rosuvastatin (29%) and DFMO (46%), suggesting additive efficacy. Furthermore, low-dose combination caused a delay in colonic adenocarcinoma progression. DFMO, Rosuvastatin and/or combinations significantly decreased polyamine content and increased intra-tumoral NK cells expressing perforin plus IFN-γ compared to untreated colon tumors. Further ex-vivo analysis of splenic NK cells exposed to DFMO, Rosuvastatin or combination resulted in an increase of NKs with perforin expression. This is the first report on Rosuvastatin alone or combination strategy using clinically relevant statin plus DFMO doses which shows a significant suppression of colon adenocarcinomas, and their potential in increasing functional NK cells. This strategy has potential for further testing in high risk individuals for colon cancer.

Inhibition of the Polyamine Synthesis Pathway Is Synthetically Lethal with Loss of Argininosuccinate Synthase 1

Argininosuccinate synthase 1 (ASS1) is the rate-limiting enzyme for arginine biosynthesis. ASS1 expression is lost in a range of tumor types, including 50% of malignant pleural mesotheliomas. Starving ASS1-deficient cells of arginine with arginine blockers such as ADI-PEG20 can induce selective lethality and has shown great promise in the clinical setting. We have generated a model of ADI-PEG20 resistance in mesothelioma cells. This resistance is mediated through re-expression of ASS1 via demethylation of the ASS1 promoter. Through coordinated transcriptomic and metabolomic profiling, we have shown that ASS1-deficient cells have decreased levels of acetylated polyamine metabolites, together with a compensatory increase in the expression of polyamine biosynthetic enzymes. Upon arginine deprivation, polyamine metabolites are decreased in the ASS1-deficient cells and in plasma isolated from ASS1-deficient mesothelioma patients. We identify a synthetic lethal dependence between ASS1 deficiency and polyamine metabolism, which could potentially be exploited for the treatment of ASS1-negative cancers.

Inhibition of polyamine oxidase prevented cyclin-dependent kinase inhibitor-induced apoptosis in HCT 116 colon carcinoma cells

Roscovitine and purvalanol are novel cyclin-dependent kinase (CDK) inhibitors that prevent cell proliferation and induce apoptotic cell death in various cancer cell lines. Although a number of studies have demonstrated the potential apoptotic role of roscovitine, there is limited data about the therapeutic efficiency of purvalanol on cancer cells. The natural polyamines (PAs) putrescine, spermidine, and spermine have essential roles in the regulation of cell differentiation, growth, and proliferation, and increased levels of these compounds have been associated with cancer progression. Recently, depletion of intracellular PA levels because of modulation of PA catabolic enzymes was shown to be an indicator of the efficacy of chemotherapeutic agents. In this study, our aim was to investigate the potential role of PA catabolic enzymes in CDK inhibitor-induced apoptosis in HCT 116 colon carcinoma cells. Exposure of cells to roscovitine or purvalanol decreased cell viability in a dose- and time-dependent manner. The selected concentrations of roscovitine and purvalanol inhibited cell viability by 50 % compared with control cells and induced apoptosis by activating the mitochondria-mediated pathway in a caspase-dependent manner. However, the apoptotic effect of purvalanol was stronger than that of roscovitine in HCT 116 cells. In addition, we found that CDK inhibitors decreased PA levels and significantly upregulated expression of key PA catabolic enzymes such as polyamine oxidase (PAO) and spermine oxidase (SMO). MDL-72,527, a specific inhibitor of PAO and SMO, decreased apoptotic potential of CDK inhibitors on HCT 116 cells. Moreover, transient silencing of PAO was also reduced prevented CDK inhibitor-induced apoptosis in HCT 116 cells. We conclude that the PA catabolic pathway, especially PAO, is a critical target for understanding the molecular mechanism of CDK inhibitor-induced apoptosis.

Advances in biophotonics detection of field carcinogenesis for colon cancer risk stratification

The process of neoplastic transformation of the colon involves a progression through hyperproliferative epithelium through the aberrant crypt foci→small adenoma→large adenoma→invasive cancer→metastatic disease. These are orchestrated by sequential genetic and epigenetic events which provide the underpinnings of cellular alterations such as early induction in proliferation/suppression of apoptosis, along with the late stage increase in invasiveness. Colorectal cancer (CRC) averages 49-111 mutations per tumor encompassing 10-15 critical signaling pathways[1]. Accumulating such a high number of mutations requires a fertile mutational field, which is the hallmark of colon carcinogenesis.

While genetic susceptibility to colorectal cancer is well-known, at least half of the risk is believed to be due to exogeneous factors (e.g., obesity, diet, exercise). Understanding these risk factors represents a promising mode of tailoring screening modality and intensity. However, previous attempts using these factors (i.e., NCI risk calculator) have only been modestly successful with an area under receiver operating characteristics (ROC) curve (AUC) of just 0.61. One of the most important concepts is that risk is the interaction between these genetic and environmental components and is driven by the variety of polymorphisms. Thus, predicting risk is difficult given the complexity. On the other hand, the colonic mucosa represents the end product of the complex interplay between these multiple factors. The power of field carcinogenesis is that it reflects this interplay between genetics and environment.

Role of dietary polyamines in a phase III clinical trial of difluoromethylornithine (DFMO) and sulindac for prevention of sporadic colorectal adenomas

Background:

The polyamine-inhibitory regimen difluoromethylornithine (DFMO)+sulindac has marked efficacy in preventing metachronous colorectal adenomas. Polyamines are synthesised endogenously and obtained from dietary sources. Here we investigate dietary polyamine intake and outcomes in the DFMO+sulindac colorectal adenoma prevention trial.

Methods:

Dietary polyamine data were available for 188 of 267 patients completing the study. Total dietary polyamine content was derived by the sum of dietary putrescine, spermine and spermidine values and categorised into two groups: highest (>75–100%) vs the lower three quartiles (0–25, 25–50 and 50–75%). Baseline tissue polyamine concentration and ODC1 genotype were determined. Logistic regression models were used for risk estimation.

Results:

A significant interaction was detected between dietary polyamine group and treatment with regard to adenoma recurrence (P=0.012). Significant metachronous adenoma risk reduction was observed after DFMO+sulindac treatment in dietary polyamine quartiles 1–3 (risk ratio (RR) 0.19; 95% confidence interval (CI) 0.08–0.42; P<0.0001) but not in quartile 4 (RR 1.51; 95% CI 0.53–4.29; P=0.44). However, a lower number of events in the placebo group within dietary quartile 4 confound the aforementioned risk estimates.

Conclusion:

These preliminary findings reveal complex relationships between diet and therapeutic prevention, and they support further clinical trial-based investigations where the dietary intervention itself is controlled.

Polyamines and the intestinal tract

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

Peripheral blood leucocytes ornithine decarboxylase activity in chronic myeloid leukemia patients: prognostic and therapeutic implications

Leukocytes ornithine decarboxylase (ODC) activity was measured in normal individuals and in patients with chronic myeloid leukemia (CML) in chronic phase (CML-CP) as well as in accelerated phase (CML-AP), with an aim to examine the role of ODC activity in prognostic evaluation of CML patients. Our results showed that ODC activity was significantly higher in CML-CP (41.02+/-25.57nmol/h per 10(7) cells, P<0.005) and CML-AP (67.71+/-44.42nmol/h per 10(7) cells, P<0.001) patients than in normal subjects (3.12+/-1.34nmol/h per 10(7) cells). Furthermore, patients with CML-AP showed higher ODC activity than CML-CP patients (P<0.005). Patients with CML-CP who converted to accelerated phase within 24 months had higher ODC activity (84.58+/-12.81nmol/h per 10(7) cells) than patients who did not convert to accelerated phase (31.13+/-18.24nmol/h per 10(7) cells). The high value of ODC activity was also associated with less clinico-hematological response. We suggest that ODC activity reflects the neoplastic proliferative activity in CML patients and may serve as an additional prognostic marker.

Dietary soy protein is associated with reduced intestinal mucosal polyamine concentration in male Wistar rats

Quantitation of polyamine levels has been correlated with biomarkers of proliferation in the colon mucosa where dysregulated epithelial hyperproliferation is associated with colorectal cancer risk. This study was performed to assess the response of polyamine measurements to dietary factors in an animal model. Male Wistar rats were fed purified diet or diets substituted by 20% lard fat, 20% beet fiber and 20% soy protein. After 2 wk, mucosal polyamines were measured along intestinal tracts by HPLC. In rats fed the control diet (n = 10), mucosal polyamines were found at high levels in the duodenum, jejunum and ileum but at low levels in the cecum, colon and rectum. Compared with rats fed the control diet, those fed the 20% lard diet showed greater polyamine levels in the large intestine (P < 0.05, n = 10), but those fed the 20% fiber diet exhibited lower polyamine levels in the small intestine (P < 0.05, n = 9). However, rats fed the 20% soy protein diet had lower polyamine levels in both small and large intestines (P < 0.05, n = 15). Significant linear correlations were observed between rectal polyamine levels and the dietary energy intakes in these four diet groups (r = 0.972-0.991, P < 0.001). Supplementation of 0.1% soy isoflavones to the basal diet or 0.3% DL-methionine to the 20% soy protein diet for 4 wk did not affect polyamine levels. The results indicate that soy protein reduced mucosal polyamine levels, at least in part, through reduction of energy intakes. Further studies are warranted to verify that polyamine levels in intestinal mucosa are useful as an intermediate endpoint of the dietary risk factors.

Type of dietary fiber, not fat, alters phospholipase D and ornithine decarboxylase activities in the rat large intestine

We investigated the effect of dietary fatty acid composition (n-6 vs. n-3) and fiber (highly fermentable vs. less fermentable) on the activities of phospholipase D (PLD) and ornithine decarboxylase (ODC) in the rat large intestine (cecum and proximal and distal colon). Twenty-four Sprague-Dawley rats (215-270 g) ate synthetic diets with 2% safflower oil plus 21.5% safflower or fish oil and 10% cellulose or guar gum for four weeks. Cecal bile acids and free fatty acids were higher in rats fed guar gum than in rats fed cellulose. Rats fed fish oil had more proximal colonic mucosal and cecal bile acids than those fed safflower oil. PLD activity was 23% lower in the proximal colon of rats fed guar gum than in those fed cellulose, but the mucosal weight was not different. ODC activity was lower but cecal mucosal wet weight was higher in the cecum of the rats fed guar gum than in the cecum of the rats fed cellulose. The activities of PLD and ODC are affected by dietary fiber and may not be accurate markers for tissue growth in the colonic mucosa.

Mucosal polyamine measurements and colorectal cancer risk

Polyamines are short-chain aliphatic amines required for normal cellular growth that are ubiquitously found in all living tissues. Polyamine content has been shown to correlate with cellular proliferation. Quantitation of polyamines may thus provide a biochemical measure of proliferation in the colorectal mucosa where dysregulated epithelial proliferation is associated with colorectal cancer risk. A case-control study was conducted to validate the hypothesized association between mucosal polyamine measurements and colorectal cancer risk. Polyamines were measured in 4-6 multiple rectal mucosal biopsies from 11 normal control subjects and seven case patients with colon cancer. Compared with the controls, mean polyamine measurements, after adjustment for age and sex, were significantly increased for spermidine (P < 0.003) and spermine (P < 0.017). Subsequent analysis indicated that in controls 1-4 biopsies appeared adequate to characterize an individual. However, mucosal polyamines in the cases exhibited more sampling variability, requiring 4-8 biopsies to achieve an acceptable level of reliability. After adjustment for age and sex, the odds ratios for spermidine and spermine levels, compared to the controls, were 4.8 (95% confidence interval: 1.6-33.7) and 2.3 (1.2-6.3), respectively. The results of this study indicate that increases of mucosal polyamine measurements, after taking the sampling and methodological variability into account, are significantly associated with colorectal cancer risk, and suggest that polyamine measurements in rectal mucosa may play an important role as biomarkers for identifying high-risk individuals and/or for using as intermediate endpoints in prevention trials.

Ornithine decarboxylase levels in patients with normal colonic mucosa

We describe a systematic examination of ornithine decarboxylase activity in 120 colonic mucosal samples which were obtained from 20 subjects without colonic disease to establish the normal mean and standard deviation from proximal to distal colon. Ornithine decarboxylase activity was determined by releasing CO2 from DL-[1-14C]ornithine. The mean ornithine decarboxylase levels (CO2 liberated) ranged from 0.26 +/- 0.08 nmol/h.mg protein in the caecum to 0.44 +/- 0.16 nmol/h.mg protein in the rectum. There was no difference between sex and age. Ornithine decarboxylase was not stimulated by guanosine 5'-triphosphate. alpha-Difluoromethylornithine showed an ornithine decarboxylase inhibition of 97.1%. Ornithine decarboxylase activity can be measured with reliable precision and reproducibility. The knowledge of the normal range of ornithine decarboxylase activity in normal human colonic mucosa is necessary for the determination of ornithine decarboxylase activity in pathological findings, especially in malignant transformation.

Comodulation of cellular polyamines and proliferation: biomarker application to colorectal mucosa

Polyamines are low molecular weight aliphatic amines required for normal cellular growth which are ubiquitously found in all living tissues. Polyamine biosynthesis is known to increase with mitogenesis, and elevated polyamine concentrations are found in hyperproliferative tissues. Quantitation of tissue polyamine content may thus provide a biochemical measure of proliferation, with potential biomarker application to the colonic mucosa where dysregulated epithelial proliferation is associated with cancer risk. This study was performed to validate polyamine analyses as a measure of cellular proliferation, and to preliminarily assess polyamine characteristics when applied to clinical samples. Using FHC, a human colonic epithelial cell line, for in vitro experimentation, deoxycholic acid or retinol was added to freshly passaged cultures to either stimulate or inhibit proliferation, respectively. Parallel cultures were then assayed for (1) proliferation by sulforhodamine B staining; and (2) polyamine content by a high-performance liquid chromatographic method. Deoxycholic acid stimulated, and retinol inhibited proliferation in dose-dependent fashion. Polyamine content, specifically the spermidine content and the spermidine/spermine ratio, also increased or decreased in response to culture with deoxycholic acid or retinol, respectively. Significant linear correlations between proliferation and spermidine (r = 0.858, P < 0.001), and with the spermidine/spermine ratio (r = 0.574, P < 0.05) were observed. When quantitative polyamine analyses were applied to human colonic specimens, replicate mucosal sampling revealed a high degree of intra-individual variability, indicating a heterogeneous distribution of polyamines within anatomically confined colonic segments. The results support a role for quantitative polyamine analyses as a correlative measure of colonic epithelial proliferation; however, intraindividual variability may limit the utility of colorectal biomarker measurements.

Protein kinase C activity as a potential marker for colorectal neoplasia

Protein kinase C (PKC) is a mediator of transmembrane signal transduction, important in cell growth and differentiation. Cell activation by extracellular signals is associated with a translocation of PKC from the cytosol to the membrane. We measured and compared PKC activity in cytosol and membrane fractions of normal and neoplastic colorectal tissue. Total and membrane-associated PKC activity in normal colorectal tissue was greater in patients (N = 16) with colorectal cancer compared to that from patients with a normal colonoscopy (N = 16), P < 0.01. A similar trend was noted in PKC activity of normal colorectal tissue from patients with adenomas compared to patients with a normal colonoscopy. PKC activity (total, membrane-associated, percent membrane) was not different in neoplastic colorectal tissue compared to that of adjacent normal tissue. However, there was a considerable range of PKC activity noted in all groups, which would limit the utility of PKC activity as a marker for colorectal neoplasia.

Effects of gastrin and difluoromethylornithine on growth of human colon cancer

The effect of difluoromethylornithine (DFMO), a specific inhibitor of ornithine decarboxylase activity, was evaluated in vivo and in vitro on the growth of a gastrin-sensitive human colon carcinoma (WiDr). In vivo, mice bearing the tumor treated with pentagastrin had larger tumors with higher ornithine decarboxylase activity and polyamine content (P < 0.05) than mice not treated with pentagastrin. Difluoromethylornithine treatment significantly decreased ornithine decarboxylase in both the pentagastrin-treated and the untreated animals; however, DFMO had no effect on tumor volume, weight, protein, or DNA content. In cell culture, gastrin treatment increased WiDr cell number and [3H]thymidine incorporation in the presence or absence of serum. In serum-free conditions, however, gastrin stimulated cell growth without concomitantly increasing ODC activity. DFMO, on the other hand, decreased both ODC activity and growth. These studies suggest that the trophic effect of gastrin on WiDr human colon cancer is independent of ODC activity. Since gastrin treatment increased ODC activity in vivo, gastrin may interact in vitro with other factors present in serum that can alter ODC activity.

Precursors of colorectal carcinoma. Biopsy and biologic markers

The term biologic marker (biomarker) of colorectal cancer refers in this article to an early preclinical phenotypic characteristic that relates to the risk for developing this cancer. Putative biologic markers in the normal colorectal mucosa of patients at risk include abnormal cell proliferation as determined by kinetic studies, ornithine decarboxylase activity, and polyamine synthesis. Alterations of mucin synthesis have been studied using both histochemical stains and lectin-binding techniques. Blood group and related carbohydrate antigens also have been evaluated as potential biomarkers in the normal mucosa. Biopsy small (less than 5 mm) polyps encountered at endoscopy has become a standard practice. Although a small polyp found to be an adenoma has a low likelihood of harboring high-grade dysplasia or invasive carcinoma, it represents an indicator of risk for colorectal neoplasia. Hyperplastic polyps, however, even though they have certain epidemiologic associations with colorectal neoplasia, are controversial as putative biomarkers of clinical relevance. Current research supports a concept of a field defect of the colorectal mucosa at risk for neoplasia, which may be identified by phenotypic abnormalities of the normal mucosa and the development of small adenomas.

Correlation between bromodeoxyuridine labelling and ornithine decarboxylase levels in normal rectal mucosa of patients with colorectal adenoma

We studied rectal cell proliferation by means of bromodeoxyuridine labelling and ornithine decarboxylase activity assay in 16 patients with colorectal adenoma. In each patient, three rectal biopsy specimens taken from normal-appearing mucosa were incubated with bromodeoxyuridine (BrdU), fixed in ethanol and stained with avidin-biotin peroxidase complex using a monoclonal antibody against BrdU. In addition, two biopsies were homogenized and incubated with [1-14C]-ornithine for ornithine decarboxylase (ODC) assay. A direct, significant correlation was found between BrdU-labelling index and ODC levels in the mucosa (r = 0.6511, P less than 0.01). We conclude that BrdU labelling and ODC activity assay give comparable results in the analysis of cell proliferation rate of rectal mucosa. These methods are useful to investigate rectal cell proliferation pattern of patients with increased risk of colorectal cancer.

Protein kinase C activity as marker for colorectal cancer

Protein kinase C (PKC) activity was measured in normal-appearing colonic mucosa from patients undergoing colonic resection. Comparisons were made between cases with and without colorectal cancers. PKC activity in cytosolic and particulate fractions was significantly reduced in patients with colorectal cancer: 40 +/- 9 and 114 +/- 29 (pmol/min/mg) respectively, compared with patients without cancer; 129 +/- 11 and 250 +/- 38 (pmol/min/mg) (p less than 0.05). Normal-appearing colonic mucosa in patients with cancer showed significantly decreased total PKC activities in the cytosolic and particulate fractions compared with those in patients without cancer (10 +/- 1 and 20 +/- 3 vs. 30 +/- 2 and 33 +/- 6) (pmol/min/g tissue) (p less than 0.05). The cellular distribution (% particulate fraction) of PKC activity in normal-appearing mucosa in patients with cancer (64%) was higher than in patients without cancer (49%) (p less than 0.05). OUr data suggest that PKC activity may be used as a biological marker of risk of developing colorectal cancer or risk of bearing an asymptomatic tumor.

Colorectal ornithine decarboxylase activity in human mucosa and tumors: elevation of enzymatic activity in distal mucosa

To examine the value of ornithine decarboxylase (ODC) assay as a biological marker of potential malignancy in large bowel, we harvested 43 colorectal carcinoma, 7 adenoma, 6 polyps, and 77 normal-appearing mucosa at surgery from patients with colorectal carcinoma. In addition, 13 normal rectal mucosa were obtained at biopsy from patients with benign diseases or diseases unrelated to colorectal carcinoma as normal control. ODC activity was significantly higher in polyps and adenocarcinomas than in normal-appearing mucosa from patients with colorectal carcinoma. ODC activity in normal-appearing mucosa varied throughout the large intestine, with significantly higher activities in the distal segment of the large bowel. The higher ODC activity detected in the sigmoid colon and rectum correlates with the larger incidence of tumor development in this region of the large bowel. This observation needs to be taken into consideration when ODC activities of the colorectal mucosa are measured as biological markers of potential malignancies.

Calcium and the prevention of colon cancer

Diet is a major determinant of colon cancer risk. Calcium may protect against colon cancer, presumably by binding cytotoxic bile acids and fatty acids. Numerous studies support this proposition. In subjects at risk for colon cancer oral calcium supplementation has been shown to reduce rectal epithelial proliferation rate, thereby supposedly decreasing cancer risk. In contrast to the original hypothesis that phosphate counteracts the effect of calcium, evidence has now been provided that phosphate is crucial for the intraluminal binding of bile acids in complexes of calcium, phosphate, and bile acids. Supplemental calcium has been shown to reduce the cytotoxic potential of fecal water, which is probably attributable to the profound effect of calcium on bile acid and fatty acid metabolism. However, some reservation with regard to the protective ability of calcium seems to be warranted as we found that oral calcium supplementation caused an increase in epithelial proliferation rate in the sigmoid of patients with adenomatous polyps. Further controlled studies evaluating the effects of calcium on the epithelium of different parts of the colon should now be performed.