Targeting STAT3 signaling with essential oils: a potential strategy for adjuvant cancer therapy

STAT3 is signaling is aberrantly active in most of the solid and hematological cancers. Constitutive activation of STAT3 plays a critical role in regulating the hallmarks of cancer, therefore STAT3 is considered as a promising target for cancer therapy. There is a growing interest in exploring natural compounds as potential anticancer agents due to the toxic effects of synthetic anticancer drugs on healthy cells and the development of chemoresistance. Essential oils (EOs) are phyto-complexes that exhibit diverse anticancer effects. In this work, from a panel of EOs, we aimed to identify EOs with potential anti-STAT3 activity and anticancer effects in DU145 human prostate cancer cells that exhibit constitutive STAT3 activation. EOs of Pinus mugo, Lavandula angustifolia, Pinus sylvestris, and Cupressus sempervirens were selected as the most potent EOs in inhibiting constitutive STAT3 phosphorylation and inducing cytotoxicity. Notably, Pinus mugo EO (PMEO) showed low cytotoxicity in non-transformed human fibroblasts, suggesting the specificity to efficiently target cancer cells. The molecular mechanism of anti-STAT3 activity was further evaluated through spectrophotometric and fluorometric analyses, and the biological effect of STAT3 inhibition was analyzed by western blotting, qRT-PCR and flow cytometry and wound healing assay. PMEO treatment induced a rapid decline in glutathione (GSH) levels and an increase in reactive oxygen species (ROS) levels, leading to oxidative stress. Pre-treatment of cells with N-acetyl-cysteine (NAC), a cell permeable ROS scavenger, reversed the inhibitory action of PMEO on STAT3 phosphorylation, suggesting that the inhibition of STAT3 activation by PMEO is mediated by ROS. The suppression of the STAT3 signaling cascade reduced the expression of pro-proliferative and anti-apoptotic genes at mRNA and protein levels, leading to the inhibition of cell migration and apoptotic cell death. Additionally, a combination treatment revealed that PMEO acts synergistically with cisplatin in inducing cytotoxicity in cancer cells. Furthermore, a nanoformulation was developed by loading PMEO into PLGA nanoparticles (PMEO-NPs) to improve the efficiency of the EO. PMEO-NPs displayed sustained release of PMEO and effective uptake by MDA-MB-231 cells in vitro and exhibited more potent anticancer activities, including enhanced cytotoxicity through ROS generation, and increased apoptotic morphology in MDAMB- 231 cells. Moreover, both PMEO and PMEO-NPs effectively suppressed breast cancer stem cell markers, indicating their potential to combat cancer stemness and aggressiveness. Additionally, the results suggest the potential use of these EOs as adjuvant or complementary therapies to enhance the effectiveness of conventional chemotherapy in the treatment of aggressive cancers.