Bardoxolone methyl and a related triterpenoid downregulate cMyc expression in leukemia cells

Structurally related pentacyclic triterpenoids methyl 2-cyano-3,- 12-dioxoolean-1,9-dien-28-oate [bardoxolone-methyl (Bar-Me)] and methyl 2-trifluoromethyl-3,11-dioxoolean-1,12-dien-30-oate (CF₃DODA-Me) contain 2-cyano-1-en-3-one and 2-trifluoromethyl- 1-en-3-one moieties, respectively, in their A-rings and differ in the position of their en-one structures in ring C. Only Bar-Me forms a Michael addition adduct with glutathione (GSH) and inhibits IKKb phosphorylation. These differences may be due to steric hindrance by the 11-keto group in CF₃DODA-Me, which prevents Michael addition by the conjugated en-one in the A-ring. In contrast, both Bar-Me and CF₃DODA-Me induce reactive oxygen species in HL-60 and Jurkat leukemia cells, inhibit cell growth, induce apoptosis and differentiation, and decrease expression of specificity proteins (Sp) 1, 3, and 4, and cMyc, and these effects are significantly attenuated after cotreatment with the antioxidant GSH. In contrast to solid tumor-derived cells, cMyc and Sp transcriptions are regulated independently and cMyc plays a more predominant role than Sp transcription factors in regulating HL-60 or Jurkat cell proliferation and differentiation compared with that observed in cells derived from solid tumors.