Expression of PPARgamma is reduced by medium supplementation with L-glutamine in human colorectal Caco-2 cells.

Peroxisome proliferator-activated receptor-gamma (PPARgamma) belongs to the nuclear hormone receptor family. This receptor is implicated in colon cell differentiation and in colon cancer. Receptor activation by specific agonists has been shown to protect against colon cancer progression. PPARgamma protein content within cells is modulated by several mechanisms, including proteasome degradation, activation of Wnt signalling pathways and presence of fermentation products such as butyrate. Herein, we investigated the impact of L-glutamine on PPARgamma expression during the differentiation of Caco-2 cells grown in medium containing dialyzed fetal calf serum supplemented or not with L-glutamine. Using RT-PCR and Western blotting, we demonstrated that PPARgamma expression was decreased when L-glutamine was added to the medium. Using immunohistochemistry, we demonstrated that PPARgamma immunostaining was mainly found in cytoplasm when cells were cultured with L-glutamine while it was found in nuclei and cytoplasm when cells were grown without the addition of L-glutamine. Supershift retardation assays demonstrated a decrease of PPARgamma binding onto consensus peroxisome proliferator response element. We concluded that L-glutamine modulated PPARgamma expression in Caco-2 cells.

Different contribution of apoptosis to the antiproliferative effects of diosgenin and other plant steroids, hecogenin and tigogenin, on human 1547 osteosarcoma cells.

Regulation of growth arrest and apoptosis are, in part, controlled by the tumor suppressor p53 after its phosphorylation which causes a determinant role in its functional activation. Moreover, PPAR regulate many functions such as proliferation and apoptosis. We compared the biological activity of diosgenin with hecogenin and tigogenin, plant steroids structurally close to diosgenin, on proliferation rate, cell cycle distribution and apoptosis in human 1547 osteosarcoma cells. We found that all three molecules have an antiproliferative effect but gel shift analysis demonstrated that none of the plant steroids transactivated PPAR in human 1547 osteosarcoma cells whereas these molecules induced NF-kappaB binding to DNA. Although these plant steroids have a very close structure, only diosgenin caused a cell cycle arrest associated with strong apoptosis. This biological action seems correlated with a large increase of p53 protein expression. This fact was showed by immunofluorescence analysis which confirmed that diosgenin strongly enhanced the activation of p53 in contrast to hecogenin and tigogenin actions.