Structure-dependent activity of glycyrrhetinic acid derivatives as peroxisome proliferator-activated receptor γ agonists in colon cancer cells

Glycyrrhizin, a pentacyclic triterpene glycoside, is the major phytochemical in licorice. This compound and its hydrolysis product glycyrrhetinic acid have been associated with the multiple therapeutic properties of licorice extracts. We have investigated the effects of 2-cyano substituted analogues of glycyrrhetinic acid on their cytotoxicities and activity as selective peroxisome proliferator-activated receptor γ (PPARγ) agonists. Methyl 2-cyano-3,11-dioxo-18β-olean-1,12-dien-30- oate (β-CDODA-Me) and methyl 2-cyano-3,11-dioxo-18α-olean-1,12-dien- 30-oate (α-CDODA-Me) were more cytotoxic to colon cancer cells than their descyano analogues and introduction of the 2-cyano group into the pentacyclic ring system was necessary for the PPARγ agonist activity of α-CDODA-Me and β-CDODA-Me isomers. However, in mammalian two-hybrid assays, both compounds differentially induced interactions of PPARγ with coactivators, suggesting that these isomers, which differ only in the stereochemistry at C18 which affects conformation of the E-ring, are selective receptor modulators. This selectivity in colon cancer cells was shown for the induction of two proapoptotic proteins, namely caveolin-1 and the tumor-suppressor gene Krüppel-like factor-4 (KLF-4). β-CDODA-Me but not α-CDODA-Me induced caveolin-1 in SW480 colon cancer cells, whereas caveolin-1 was induced by both compounds in HT-29 and HCT-15 colon cancer cells. The CDODA-Me isomers induced KLF-4 mRNA levels in HT-29 and SW480 cells but had minimal effects on KLF-4 expression in HCT-15 cells. These induced responses were inhibited by cotreatment with a PPARγ antagonist. This shows for the first time that PPARγ agonists derived from glycyrrhetinic acid induced cell-dependent caveolin-1 and KLF-4 expression through receptor-dependent pathways.