Valtrate as a novel therapeutic agent exhibits potent anti-pancreatic cancer activity by inhibiting Stat3 signaling

Abstract

Background: Valtrate is a novel epoxy iridoid ester isolated from Chinese herbal medicine Valeriana jatamansi Jones with anti-proliferative activity against various human cancer cell lines. However, its efficacy and molecular mechanisms against pancreatic cancer (PC) cells are largely unclear.

Purpose: To investigate the anti-cancer effects of valtrate on PC cell lines and its underlying mechanisms.

Methods: MTT assay was first performed to detect the effect of valtrate on cell viability in human PC cell lines and normal pancreatic epithelial cells HPDE. Cell apoptosis and cycle phase assay were detected by flow cytometry. The relative mRNA expressions of Bax, Bcl-2, c-Myc, and CyclinB1 were tested by quantitative PCR (qPCR) assay. The expression of relative proteins was detected by Western blotting (WB). A PANC-1^luc cells xenograft mouse model in nu/nu female mice was used to elucidate the effect of valtrate on tumor growth in vivo.

Results: Valtrate significantly inhibited the growth of PC cells without affecting the growth of normal pancreatic epithelial cells HPDE, induced significant apoptosis and cell cycle arrest in G2/M phase. Moreover, valtrate inhibited the tumor growth of PC cell PANC-1 in xenograft mice by 61%. Further mechanism study demonstrated that valtrate could increase the expression level of Bax, suppress Bcl-2 as well as c-Myc and Cyclin B1, inhibit the transcriptional activity of Stat3, while valtrate decreased the expression level of Stat3 and phosphated-Stat3 (Tyr705) and induced the high molecular aggregation of Stat3. Molecular docking analysis predicted that valtrate might interact with Cys712 of Stat3 protein. Valtrate could also induce a transient depleted intracellular glutathione (GSH) level and increased reactive oxygen species (ROS). NAC (N-acetylcysteine), a reducer reversed valtrate-induced the depletion of Stat3, p-Stat3, c-Myc, and Cyclin B1.

Conclusion: Valtrate exerts anti-cancer activity against PC cells by directly targeting Stat3 through a covalent linkage to inhibit Stat3 activity, which causes apoptosis and cell cycle arrest.

Keywords: Epoxy iridoid esters; Pancreatic cancer; Reactive oxygen species; Stat3; Valtrate.