15-Deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2), an endogenous ligand for peroxisome proliferator-activated receptor-γ, has been shown to induce apoptosis in various types of cells. However, this cyclopentenone prostaglandin, characterized by the presenc ...
15-Deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2), an endogenous ligand for peroxisome proliferator-activated receptor-γ, has been shown to induce apoptosis in various types of cells. However, this cyclopentenone prostaglandin, characterized by the presence of a highly reactive α,β-unsaturated carbonyl group, also exerts cytoprotective effects. In the present work, we found that treatment of human breast cancer (MCF-7) cells with 15d-PGJ2 led to concentration- and time-dependent increases in the expression and activity of heme oxygenase-1 (HO-1), a ubiquitous stress-responsive enzyme. Elevated expression or activity of HO-1 has been reported to stimulate proliferation and to accelerate angiogenesis in several tumor cells. 15d-PGJ2 at 30 μM induced phosphorylation of Akt and ERK1/2 in 6 h and 12 h, respectively. Up-regulation of HO-1 by 15d-PGJ2 was abolished by pretreatment with the phosphatidyl inositol-3 kinase (PI3K) inhibitor LY294002, but not with the MEK inhibitor U0126. To confirm whether the HO-1 expression is mediated by Akt that is downstream of PI3K, we transfected MCF-7 cells with full length Akt or Kinase-Dead Akt (KD-Akt) vector. KD-Akt transfection abolished HO-1 expression as well as HO-1 activity. These results suggest that induction of HO-1 expression can be regulated via the PI3K-Akt pathway. Nrf2, a basic-leucine zipper transcription factor, has been reported to positively regulate the antioxidant response element (ARE)-mediated expression of various phase-2 detoxifying or antioxidant enzymes including HO-1. 15d-PGJ2 induced increased nuclear translocation as well as expression of Nrf2, leading to elevated Nrf2-ARE binding. The promoter activity assessed by using the luciferase reporter construct harbouring ARE consensus sequences was increased by 15d-PGJ2 in transiently transfected MCF-7 cells. Mutation of the GC box in the ARE core sequence obliterated the 15d-PGJ2-induced transcriptional activity of Nrf2. Treatment of MCF-7 cells with LY294002 or transient transfection with KD-Akt abrogated 15d-PGJ2-induced Nrf2 activation and ARE binding activity. MCF-7 cells transfected with dominant negative Nrf2 were less responsive to 15d-PGJ2-induced expression and activity of HO-1. 9,10-Dihydro-15d-PGJ2, a 15d-PGJ2 analog lacking the reactive α,β-unsaturated carbonyl group, failed to induce HO-1 expression as well as Nrf2 activation and Akt phosphorylation.