Expression of peroxisome proliferator-activated receptors alpha and gamma in differentiating human colon carcinoma Caco-2 cells

The expression of peroxisome proliferator-activated receptors α (PPARα) and γ (PPARγ) was studied in the human adenocarcinoma Caco-2 cells induced to differentiate by long term culture (15 days). The differentiation of Caco-2 cells was attested by increases in the activities of sucrase-isomaltase and alkaline phosphatase (two brush border enzymes), fatty acyl-CoA oxidase (AOX) and catalase (two peroxisomal enzymes), by an elevation in the protein levels of villin (a brush border molecular marker), AOX, peroxisomal bifunctional enzyme (PBE), catalase and peroxisomal membrane protein of 70 kDa (PMP70), and by the appearance of peroxisomes. The expression of PPARα and PPARγ was investigated by Western blotting, immunocytochemistry, Northern blotting and S1 nuclease protection assay during the differentiation of Caco-2 cells. The protein levels of PPARα, PPARγ, and PPARγ 2 increased gradually during the time-course of Caco-2 cell differentiation. Immunocytochemistry revealed that PPARα and γ were localized in cell nuclei. The PPARγ 1 protein was encoded by PPARγ 3 mRNA because no signal was obtained for PPARγ 1 mRNA using a specific probe in S1 nuclease protection assay. The amount of PPARγ 3 mRNA increased concomitantly to the resulting PPARγ 1 protein. On the other hand, the mRNA of PPARα and PPARγ 2 were not significantly changed, suggesting that the increase in their respective protein was due to an elevation of the translational rate. The role played by the PPAR subtypes in Caco-2 cell differentiation is discussed.