Sumac Induces Mitochondria‐Dependent Cellular Toxicity in Human Pancreatic Cancer Cell Lines

Abstract

Pancreatic cancer is a devastating disease that kills about 57,000 Americans and 250,000 people worldwide each year. Chemoresistance, lack of symptoms, and ineffective therapy have resulted in gaps in our understanding of the molecular events underlying its etiology. We aim to identify natural small molecule compounds that are synthetic lethal in cellular models of mutant KRAS‐dependent pancreatic adenocarcinoma. Our preliminary experiments demonstrate that aqueous extracts of the Middle Eastern spice‐plant sumac induce cytotoxicity in two cultured human pancreatic cancer cell lines with KRAS gene mutations; MIA PaCa‐2 (G12C) and PANC‐1 (G12V and G12D) but not in normal fibroblasts. Pancreatic cancer patients, 90% of whom have mutations in the KRAS oncogene, do not respond well to traditional chemotherapy. We hypothesized that the phytochemical sumac added as an adjunct to the therapeutic regimen would skew the otherwise chemoresistant pancreatic cancer cells into an apoptotic phenotype. We hypothesized that sumac’s cytotoxicity was a consequence of mitochondrial targeting, specifically a derangement of mitochondrial membrane potential, leading to mitochondrial compromise, and eventually apoptosis. To test our hypothesis, we treated MIA PaCa‐2 and PANC‐1 cells in vitro with increasing concentrations of aqueous extracts of sumac (0, 0.03, 0.3, and 3.0 mg/mL) for varying time points and measured 1) mitochondrial membrane potential using TMRE assays, 2) ROS production using DHT assays, 3) ATP levels using fluorimetry, and 4) activated caspases 3 and 7. Our data show that sumac treatment 1) ameliorated ROS production, 2) abrogated mitochondrial membrane potential, 3) activated caspases 3 and 7, and 4) reduced cellular ATP levels in a dose‐dependent manner. These changes were accompanied by a dose‐dependent change in the protein expression of the energy‐sensing mitochondrial PGC‐1α, which increased in both cell lines, the anti‐apoptotic protein Bcl‐2, which decreased in the PANC‐1 cell line but not in the MIA PaCa‐2 cell line, the pro‐apoptotic Bax, and mitofusin‐1, which is responsible for the fusion of the mitochondrial outer membranes. These data suggest that sumac mediates “synthetic lethality” in pancreatic cancer cells based on the type of KRAS gene mutation. We also hypothesized that the observed ATP reduction would alter levels of SIRT3, which is major mitochondrial physiology regulating deacetylase enzyme. Immunoblot data show a paradoxical increase in SIRT3 in a dose‐dependent manner, suggesting other roles for SIRT‐3 in this pathway. Furthermore, using HPLC‐based NMR spectroscopy, we identified the active compound in the aqueous sumac extract to be [(2R, 3R, 4S, 5R)‐3, 4, 5, 6‐tetrakis [(2‐deuterio‐3, 4, 5‐trihydroxybenzoyl)oxy]oxan‐2‐yl] methyl‐2‐deuterio‐3, 4, 5‐trihydroxybenzoate. In conclusion, sumac targets hitherto unknown mitochondrial signal transduction pathways and is synthetic lethal for the KRAS mutant G12V and G12D.