Decreased expression of the melatonin receptor 1 in human colorectal adenocarcinomas.

Melatonin exerts anti-proliferative and pro-apoptotic effects in various cancer cell lines. Furthermore, there is evidence for impaired melatonin secretion in human breast and colorectal cancer. Additionally, several studies revealed a modulated expression of the melatonin receptor 1 (MT1), in human breast cancer specimens. Since melatonin binding sites were already identified in the human intestine, our aim is to identify the expression and to characterize the localization of the MT1 receptor in the human colon and in particular to compare MT1 expression levels between non-malignant and malignant colonic tissue. We assessed MT1 transcript levels with real time RT-PCR in colon adenocarcinomas and the adjacent normal colonic mucosa of 39 patients and observed a significant decrease of MT1 mRNA expression in colorectal cancer compared with the healthy adjacent mucosa tissue (0.67 mean difference, P < 0.0001). The results were confirmed at the protein level by Western blot analysis and by immunohistochemistry. MT1 was localized mainly supranuclear in colonic epithelial cells lining the crypts. We also evaluated mRNA expression of different clock genes in the colon samples and found a significant correlation between MT1 and Cryptochrome 1 (Cry1) expression (P < 0.01 for normal and P < 0.05 for tumour tissue). In conclusion, the decreased expression of MT1 in human colorectal cancer could point to a role of melatonin in this disease.

CYP24A1 splice variants--implications for the antitumorigenic actions of 1,25-(OH)2D3 in colorectal cancer.

25-hydroxyvitamin D3 24-hydroxylase (CYP24A1), the catabolizing enzyme of the active vitamin D3, is often overexpressed in solid tumors. The unbalanced high levels of CYP24A1 seem to be a determinant of vitamin D resistance in tumors. Splice variants of CYP450 enzymes are common. Existence of CYP24A1 isoforms has been reported recently. We have investigated the presence of CYP24A1 splicing variants (SV) in human colon cancer cell lines and tissue samples. Using a set of primer combination we have screened the entire coding sequence of CYP24A1 and identified three splice variants in colon cancer cell lines. The presence of these SVs in human colon tissue samples showed a correlation with histological type of the tissue and gender of patients. The sequencing of the alternatively spliced fragments showed that two have lost the mitochondrial target domain, while the third lacks the heme-binding domain. All SVs retained their sterol binding domain. Translation of these variants would lead to a dysfunctional enzyme without catalytic activity that still binds its substrates therefore they might compete for substrate with the synthesizing and catabolizing enzymes of vitamin D.

The role of Foxp3+ T cells in long-term efficacy of prophylactic and therapeutic mucosal tolerance induction in mice.

BACKGROUND: Mucosal tolerance induction is suggested as treatment strategy for allergic diseases. Using a murine model of birch pollen (BP) allergy we investigated the long-term efficacy and the underlying mechanisms of mucosal tolerance induction with two structurally different molecules in a prophylactic and in a therapeutic set-up. METHODS: The three-dimensional major BP allergen Bet v 1 or a nonconformational hypoallergenic fragment thereof was intranasally applied before (prophylaxis) or after sensitization (therapy). RESULTS: In the prophylactic application both the Bet v 1 allergen and the fragment prevented allergic sensitization, and this effect lasted for 1 year. In the therapeutic approach established allergic immune responses were also suppressed after treatment with either of the molecules. However, a long-lasting curative effect (6 months) was only achieved with the Bet v 1 allergen but not with the Bet v 1 fragment. Real-time reverse transcriptase polymerase chain reaction (RT-PCR) analysis of splenocytes revealed that tolerance induction with the Bet v 1 allergen was associated with enhanced expression of transforming growth factor (TGF)-beta, interleukin (IL)-10, and Foxp3 mRNA in CD4+ T cells, whereas treatment with the fragment led to the induction of either Foxp3 (prophylaxis) or IL-10 (therapy) alone. CONCLUSION: From these data we concluded (i) that the mechanisms underlying peripheral tolerance are linked to the conformation of the antigen, (ii) that mucosal tolerance is mediated by separate regulatory cell subsets, and (iii) that the long-term efficacy of immunosuppression is associated with the presence of Foxp3+ T cells.

Vitamin D receptor activity and prevention of colonic hyperproliferation and oxidative stress.

Unimpaired vitamin D action has been implicated in human cancer prevention. We have previously demonstrated the effectiveness of 1 alpha-dihydroxyvitamin D3 (1,25-D3) to reduce proliferation and increase differentiation in human colon cancer cells. The aim of this study was to investigate, on the one hand, expression of the vitamin D receptor (VDR) and of 25-hydroxyvitamin D(3)-1 alpha-hydroxylase (1 alpha-hydroxylase) in human normal and malignant colonic tissue and, on the other hand, to determine consequences of reduced or lacking VDR action in a VDR knockout mouse model. In low-grade malignancies of the human colon we found increased VDR and 1 alpha-hydroxylase mRNA expression. However, in late-stage high-grade tumors the vitamin D system is severely compromised. In the mouse colon we found an inverse relationship between VDR levels and proliferation in colon descendens, a tissue known to be specifically affected by nutrients during carcinogenesis. Expression of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a marker of oxidative DNA damage, was significantly augmented with complete loss of VDR. These data suggest that genomic 1,25-D(3) action is necessary to protect against nutrition-linked hyperproliferation and oxidative DNA damage.

Characterization of a vitamin D receptor knockout mouse as a model of colorectal hyperproliferation and DNA damage.

The vitamin D receptor knockout (VDR-KO) mouse presents with a skeletal phenotype typical for complete lack of genomic 1,25-dihydroxycholecalciferol effects. Our previous data from human colorectal tissue suggest that the steroid hormone and its receptor may have protective function against tumour progression. In order to investigate the relevance of the vitamin D system for pre-malignant site-directed changes in the colon, we characterized the amount and site-specific distribution of the VDR along the large intestine in wild-type (WT), heterozygote (HT) and KO mice. We also evaluated expression of proliferating cell nuclear antigen (PCNA), of cyclin D1 and the levels of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a marker of oxidative stress. In colon ascendens, proliferative cells were dispersed all along the crypt and expression levels of all three markers were high in WT mice. A decrease of VDR expression did not affect expression significantly. In colon descendens, however, fewer proliferative cells were solely located in the lower third of the crypt, and an inverse relationship between VDR reduction, PCNA positivity and cyclin D1 expression was found in HT and KO mice. In parallel to enhanced proliferation a highly significant increase of 8-OHdG positivity occurred. Therefore, the sigmoid colon of VDR-KO mice, fed on an appropriate lactose/calcium-enriched diet to alleviate impaired calcium homeostasis-related phenotypic changes, is an excellent model for investigating induction and prevention of pre-malignant changes in one of the hotspots for human colorectal cancer incidence.

Dietary calcium and growth modulation of human colon cancer cells: role of the extracellular calcium-sensing receptor.

Using the human colon adenocarcinoma-derived cell line Caco-2, we investigated the possible role of the Ca2+-sensing receptor (CaR) in mediating effects of extracellular Ca2+ on cellular proliferation. Caco-2 cells respond to low ambient [Ca2+]o by activation of the protein kinase C-signaling pathway, leading to upregulation of c-myc mRNA expression and thereby, finally, to alleviation from the G1/S phase control of the cell cycle. This proliferative response can be reverted by activation of the CaR either through raising [Ca2+]o or, respectively, by using the CaR agonist Gd3+ as a substitute for Ca2+. The inhibitory effect of [Ca2+]o on cell replication exhibits saturation kinetics (IC50 = 0.045 mM), indicating the existence of a highly sensitive CaR operating at low ambient [Ca2+]o. Specific immunostaining revealed the presence of CaR-positive cells in the crypt epithelium of normal human colonic mucosa as well as in glandular (i.e., differentiated structures) of carcinomatous lesions. This could provide a rationale for use of calcium supplements for intervention in early phases of colon tumorigenesis.

Immunocytochemical localization of the extracellular calcium-sensing receptor in normal and malignant human large intestinal mucosa.

We identified the parathyroid type Ca(2+)-sensing receptor (CaR) in normal human colon mucosa and in cancerous lesions at the mRNA and protein level. Polymerase chain reaction produced an amplification product from reverse-transcribed large intestinal RNA which corresponded in size and length to a 537-bp sequence from exon 7 of the CaR gene. With a specific antiserum against its extracellular domain, the CaR could be detected by immunostaining in normal human colon mucosa in cells preferentially located at the crypt base. The CaR protein was also expressed in tumors of the large bowel in all 20 patients examined. However, the great majority of CaR-positive cells in the adenocarcinomas inspected were confined to more differentiated areas exhibiting glandular-tubular structures. Poorly or undifferentiated regions were either devoid of specific immunoreactivity or contained only isolated CaR-positive cells. In the normal mucosa and in glandular-tubular structures of cancerous lesions, the CaR was exclusively expressed in chromogranin A-positive enteroendocrine cells and in only a small fraction of PCNA-positive cells.

Growth regulation of human colon cancer cells by epidermal growth factor and 1,25-dihydroxyvitamin D3 is mediated by mutual modulation of receptor expression.

The human colon adenocarcinoma-derived cell line Caco-2 was used as a model system to study the interaction of epidermal growth factors (EGF) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in control of colorectal cancer cell growth. The mitogenic stimulus of EGF was rapidly transduced via apical and basal membrane receptors alike into elevation of c-myc expression, causing a shift of Caco-2 cells from the G0/G1 into the S phase of the cell cycle. The stimulatory effect of EGF on cell division was effectively counteracted by 1,25(OH)2D3: the presence of the steroid hormone prevents the negative effect of EGF on vitamin D receptor abundance and concurrently minimises ligand-occupied EGF receptor numbers on both sides of Caco-2 cell monolayers. Our data suggest that EGF and 1,25-(OH)2D3 actions on mutual receptor levels represent a specific feature of the potent antimitogenic effect of the steroid hormone on colon cancer cells.

Calcium-dependent c-myc proto-oncogene expression and proliferation of Caco-2 cells: a role for a luminal extracellular calcium-sensing receptor.

The human colonic cell line Caco-2 responds to a reduction of ambient [Ca++]o to levels at and below 0.25 mM by a twofold increase in [3H]thymidine labelling of their DNA. [Ca++]o signals sensed preferentially across the luminal aspect of Caco-2 cells, are rapidly (4 h) transduced via PKC activation into up to sixfold increases in c-myc expression. This suggests the presence of a [Ca++]o-sensing membrane receptor (CaR) similar to that described by Brown et al. (1) in parathyroid and kidney cells. By RT-PCR we were able to amplify a 426 bp fragment from Caco-2 mRNA with 98% nucleotide identity to a part of the coding region for the extracellular domain of the parathyroid CaR. Immunohistochemical staining with a monoclonal antiparathyroid CaR antibody demonstrates CaR protein at the plasma membrane of confluent Caco-2 cells. Our results imply that the intestinal CaR is a potential mediator for the transduction of low luminal [Ca++]o into tumor promoting signals in human colonocytes.

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Vitamin D receptor and cytokeratin expression may be progression indicators in human colon cancer.

Epidemiological data suggest the protective role of vitamin D against the development of colorectal carcinoma in man. This could be due to the anti-mitogenic effect of the steroid hormone on human colon carcinoma cells which is mediated by a specific nuclear vitamin D receptor (VDR). Western blot analysis showed that VDR expression increases during the transition from normal mucosa to polyps and later to pT3 tumors. In later stages, however, VDR is dramatically reduced. Cytokeratin 20, which was monitored as a differentiation marker, decreases in parallel with advancing proliferation and disappears from "normal" mucosa adjacent to later stage carcinoma. Interestingly, VDR density was conspicuously higher in all tumors tested when compared to adjacent "normal" tissue. This suggest that, up to a certain degree of dedifferentiation, malignant colonocytes can upregulate the VDR, probably as a counteractive measure in response to tumor cell growth, but that this ability is finally lost in highly undifferentiated carcinoma cells.

Growth control of human colon-adenocarcinoma-derived Caco-2 cells by vitamin-D compounds and extracellular calcium in vitro: relation to c-myc-oncogene and vitamin-D-receptor expression.

The human colon-cancer cell line Caco-2, though of malignant origin, is still able to express the c-myc proto-oncogene in a regulable fashion. Transition from the logarithmic growth phase into the quiescent, i.e., confluent state, is accompanied by a significant increase in the number of cells in the G0/G1 phase of the cell cycle and a concomitant reduction of c-myc mRNA and of nuclear association of c-myc protein. Conversely, growth stimulation by lowering extracellular [Ca++]0 to 0.25 mM results in up-regulation of c-myc expression levels and consequently inhibition of re-entry of Caco-2 cells into the G0/G1 phase. In contrast, regulation of c-myc in Caco-2 cells is completely resistant to vitamin-D sterols, since the anti-mitogenic action of I alpha, 25-dihdroxyvitamin D3 (I alpha, 25(OH)2D3) and of 2 synthetic analogs, I alpha, 25(OH)2-16-ene-23-yne-D3 and I alpha, 25(OH)2-26, 27-F6-16-ene-23-yne-D3, occurred independently of any change in c-myc mRNA and nuclear protein levels. Although the antiproliferative effect of the vitamin-D sterols requires high-affinity binding to the cytoplasmic vitamin-D receptor (VDR), vitamin-D sterols have no effect on VDR mRNA levels in Caco-2 cells. However, VDR mRNA expression changed in an antiparallel fashion to c-myc regulation upon transition between different growth states. This suggests that VDR mRNA abundance could nevertheless be important for vitamin-D-related c-myc-independent growth control in Caco-2 cells.

Retinoic acid induced death of ovarian carcinoma cells correlates with c-myc stimulation.

In the ovarian adenocarcinoma subline N.I, all-trans retinoic acid (ATRA) induced substantial cell death. This response was elicited only at decreased serum levels. Exposure of N.I cells to increasing concentrations of ATRA was accompanied by a considerable up-regulation of c-myc transcript levels that correlated with the rate of cell killing, which itself was an active process as judged by sustained transcriptional expression. ATRA-triggered rounding and detaching of single cells from the substratum was accompanied by degradation of genomic DNA. We show that the N.I model cell line, which is otherwise highly ATRA-resistant, can undergo an ATRA-triggered suicide program when serum is limited. The accompanying c-myc up-regulation seems to be mediated by retinoic-acid-receptor-independent pathways involving membrane-associated phospholipases instead, because manoalide partly suppressed c-myc induction by ATRA but left constitutive c-myc expression unaffected.