PPARγ-active triterpenoid CDDO enhances ATRA-induced differentiation in APL

Acute promyelocytic leukemia (APL) is associated with oncogenic PML-RARα that acts as a dominant negative transcriptional repressor of retinoic acid (RA) receptor target genes by recruiting histone deacetylase (HDAC). The peroxisome proliferator-activated receptor-γ (PPARγ) is a member of the nuclear receptor family that forms heterodimers with retinoid X receptor (RXR). In addition to RAR targets, PML-RARα silence a wide range of nuclear receptor target genes including PPARγ targets. All-trans-retinoic acid (ATRA), a ligand for the RA receptor (RAR), restores normal retinoid signaling and induces terminal differentiation of APL cells; however, APL cells can develop resistance to ATRA. Using ATRA sensitive NB4 and ATRA-resistant derivative MR2 cell lines, we demonstrate that PPARγ ligand 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid (CDDO) enhances pro-apoptotic and differentiating effects of ATRA in ATRA-sensitive NB4 cells and partially reverses ATRA resistance in MR2 cells. The CDDO/ATRA combination synergistically induces RAR β2 expression both in ATRA-sensitive and -resistant APL cells. RAR α2 mRNA induction by CDDO/ATRA was mediated in part by enhanced H3-Lys9 acetylation in the RAR β2 promoter which in turn increased the affinity of RAR β for βRARE. PPARγ specific inhibitor T007 and silencing of PPARγ by siRNA diminished CDDO-induced maturation and RAR β2 mRNA along with PPARγ induction indicating that PPARγ activation is at least partially responsible for the RAR β2 transcription and maturation induction. In an in vivo mouse model of APL, CDDO derivative CDDO-methyl ester markedly enhanced ATRAinduced maturation and extended the survival of mice. In summary, these results provide rationale for the combined targeting of RAR and PPARγ nuclear receptors in the therapy of APL.