JNK inhibition blocks piperlongumine-induced cell death and transcriptional activation of heme oxygenase-1 in pancreatic cancer cells

Piperlongumine (PL) is an alkaloid that inhibits glutathione S-transferase pi 1 (GSTP1) activity, resulting in elevated reactive oxygen species (ROS) levels and cancer-selective cell death. We aimed to identify stress-associated molecular responses to PL treatment in pancreatic ductal adenocarcinoma (PDAC) cells. GSTP1 directly interacts with JNK, which is activated by oxidative stress and can lead to decreased cancer cell proliferation and cell death. Therefore, we hypothesized that JNK pathways are activated in response to PL treatment. Our results show PL causes dissociation of GSTP1 from JNK; robust JNK, c-Jun, and early ERK activation followed by suppression; increased expression of cleaved caspase-3 and cleaved PARP; and nuclear translocation of Nrf2 and c-Myc in PDAC cells. Gene expression analysis revealed PL caused a > 20-fold induction of heme oxygenase-1 (HO-1), which we hypothesized was a survival mechanism for PDAC cells under enhanced oxidative stress. HO-1 knockout resulted in enhanced PL-induced PDAC cell death under hypoxic conditions. Similarly, high concentrations of the HO-1 inhibitor, ZnPP (10 mu M), sensitized PDAC cells to PL; however, lower concentrations ZnPP (10 nM) and high or low concentrations of SnPP both protected PDAC cells from PL-induced cell death. Interestingly, the JNK inhibitor significantly blocked PL-induced PDAC cell death, Nrf-2 nuclear translocation, and HMOX-1 mRNA expression. Collectively, the results demonstrate JNK signaling contributes to PL-induced PDAC cell death, and at the same time, activates Nrf-2 transcription of HMOX-1 as a compensatory survival mechanism. These results suggest that elevating oxidative stress (using PL) while at the same time impairing antioxidant capacity (inhibiting HO-1) may be an effective therapeutic approach for PDAC.