Peroxisome proliferator-activated receptor (PPAR ), a fatty acid-activated nuclear receptor, is implicated in adipocyte differentiation and insulin sensitisation. In view of the association of dietary fat intake and bowel disease, the expression of PPAR in rodent and human intestine was studied. Expression of PPAR mRNA was examined by Northern blot hybridisation, RNase protection, and/or competitive RT-PCR assays, whereas PPAR protein levels were evaluated by immunoblotting and immunohistochemistry. PPAR mRNA and protein were abundantly expressed in colon relative to the small intestine both in rodents and in man. Interestingly, expression of PPAR was primarily localised in the more differentiated epithelial cells in the colon. The level of expression of PPAR in colon was similar to the levels seen in adipose tissue. Expression of PPAR increased from proximal to distal segments of the colon in man. In Caco-2 and HT-29 human adenocarcinoma cells, PPAR expression increased upon differentiation, consistent with PPAR being associated with a differentiated epithelial phenotype. High-level expression of PPAR was observed in the colon, but not in the small intestine, suggesting a potential role of this nuclear receptor in the colon.

Fibroblasts are known to be present in variable amounts in human breast adenocarcinoma tissue. In order to investigate if they influence in some way the proliferation rate of the carcinoma cells, we developed a coculture model in which cells of well characterized breast epithelial cell lines were seeded and grown in microchamber slides along with fibroblasts derived from breast tumor biopsies. As representatives of hormone dependent and independent tumor cells, we used MCF-7 and BT-20 cell lines. A third line, NPM-21T, derived from non proliferating mastopathy cells immortalized by SV-40 T DNA transfection, was representative of non tumor epithelial cells. The proliferation rate of the adenocarcinoma and epithelial cells was assessed by measurement of the BrdU labeling index, the cells being identified by specific beta-actin immunostaining. It was found that the proliferation of the adenocarcinoma cells was significantly increased in the presence of fibroblasts, while that of immortalized cells was not. Moreover, 1,25(OH)2D3, which was known to be a negative regulator of carcinoma cell growth, was found to be able also to blunt the overgrowth in the presence of fibroblasts. The absence of response of NPM-21T cells to the presence of fibroblasts suggests that the tumor cells could be the origin of their own overgrowth, through an indirect mechanism mediated by the fibroblasts. The factors which are involved and the 1,25(OH)2D3 mechanism of action are not yet identified.

The biological role of 1,25(OH)2D3 in controlling Ca++ homeostasis in the body has been identified and widely investigated for a long time. More recently its effect in regulating cell proliferation or differentiated activity was described in a variety of normal and malignant cells. The present study was carried out to investigate the different aspects and biological mechanisms of this activity and to determine if the use of 1,25(OH)2D3 in the treatment of breast cancer patients could be considered. It is found that 1,25(OH)2D3 reduces the proliferation of MCF-7 and BT-20 cells lines regardless of their sex steroid receptor status. This effect is related to the concentration, from 10(-12) M to 10(-8) M. Its amplitude is less in other cell lines, but it opposes the EGF-induced increase of proliferation. It is observed that the proliferation rate of MCF-7 and BT-20 cells is increased when these tumor cells are cocultured with fibroblasts derived from breast tumor biopsies and that 1,25(OH)2D3 reverses this process. Moreover, experiments on DMBA induced mammary tumors in Sprague Dawley rats found that 1,25(OH)2D3 given at non toxic doses reduces significantly the tumor proliferation. These data showed that 1,25(OH)2D3 at low doses is effective on the proliferation of BT-20 and MCF-7 cells and on the paracrine growth stimulatory effect observed in the presence of fibroblasts. They suggest that 1,25(OH)2D3 or related synthetic molecules which are less active on Ca++ metabolism could be useful in the treatment of breast cancer patients.

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