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

Acute promyelocytic leukemia (APL) is associated with oncogenic PML-RAR alpha 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-gamma (PPAR gamma) is a member of the nuclear receptor family that forms heterodimers with retinoid X receptor (RXR). In addition to RAR targets, PML-RAR alpha silence a wide range of nuclear receptor target genes including PPAR gamma 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 PPAR gamma 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 beta 2 expression both in ATRA-sensitive and -resistant APL cells. RAR beta 2 mRNA induction by CDDO/ATRA was mediated in part by enhanced H3-Lys9 acetylation in the RAR beta 2 promoter which in turn increased the affinity of RAR beta for beta RARE. PPAR gamma specific inhibitor T007 and silencing of PPAR gamma by siRNA diminished CDDO-induced maturation and RAR beta 2 mRNA along with PPAR gamma induction indicating that PPAR gamma activation is at least partially responsible for the RAR beta 2 transcription and maturation induction. In an in vivo mouse model of APL, CDDO derivative CDDO-methyl ester markedly enhanced ATRA-induced maturation and extended the survival of mice. In summary, these results provide rationale for the combined targeting of RAR and PPAR gamma nuclear receptors in the therapy of APL.