Inhibitory Effect Of Ophiobolin O On Cell Proliferation Of Human Breast Cancer MCF-7Cells And Its Mechanisms

Breast cancer remains one of the most important health problems in all ages ofwomen worldwide. However, most women eventually experience short-term efficacy, andthe therapies are still far from optimal. Other than traditional systemic treatment, newmolecular and gene-mediated therapies have played an essential role in the treatment oflife-threatening diseases in recent years. Ophiobolins are a family of naturally occurringsesquiterpenes that are characterised by a tricyclic（5-7-5）ring system. As secondarymetabolites that are produced by pathogenic fungi that attack rice, maize and sorghum,ophiobolins show a broad spectrum of inhibitory activity against nematodes, fungi andbacteria as well as cytotoxic activity against cancer cells.Ophiobolin A (cochliobolin) hasbeen isolated from Helminthosporium oryzae and has an absolute structure that has beendetermined by X-ray crystallography of the bromomethoxy derivative. In addition, severalmembers of the ophiobolin family have been isolated from fermentation broths.Ophiobolin K and6-epiophiobolin K were originally discovered as potent nematocidalagents. Ophiobolin M and6-epiophiobolin M isolated from Cochliobolus heterostrophushave been also reported to have potent nematocidal activity. Ophiobolin O, which belongsto the family of ophiobolin, is a natural compound that has been isolated from Aspergillusustus094102. However, the anticancer effect and the mechanism of ophiobolin O remainunknown. In the current study, we describe the isolation and chemical structure ofophiobolin O and report the potent anticancer activity of this compound for the first time.In recent years, many reports have demonstrated that the MAPK family membershave either positive, negative, or no influence on cell cycle arrest and apoptosis underdifferent experimental conditions. The ERK pathway plays an essential role in apoptosis,differentiation and cell cycle progression by post-translational phosphorylation ofapoptotic regulatory molecules such as Bad, Bim, Mcl-1, caspase9and Bcl-2. TheRaf/MEK/ERK cascade activates the ERK pathway, resulting in the phosphorylation of theantiapoptotic Mcl-1protein and pro-apoptotic Bim protein. The phosphorylation of Bimsubsequently leads to ubiquitination and proteosomal degradation, causing disassociationof Bim from proteins such as Bcl-2, Bcl-XL and Mcl-1. In addition, Bim becomesubiquitinated and targeted to the proteosome. Bcl-2(with enhanced anti-apoptotic activity),Bcl-XL and Mcl-1bind Bax to block Bax activation and the formation of Bax homodimers,inhibiting apoptosis. Previous studies have shown that Bim may be phosphorylated by PI3K/Akt pathways at S87, which attenuates the apoptotic effect of Bim and promotes thebinding of Bim to14-3-3proteins. Controversially, the phosphorylation of Bim at S65byJNK can cause apoptosis and promotes the formation of Bax homodimers. JNK may alsophosphorylate14-3-3family members, leading to Bax translocation from the cytosol to themitochondria membrane, which is the site that promotes apoptosis. JNK and p38MAPKexert tumour suppressive and oncogenic functions in a cell type-specific orcontext-specific manner. Some studies have shown that JNK and p38kinase maycooperatively regulate the cancer cell cycle progression. Previous studies havedemonstrated that anti-cancer agents activate MSK1, MNK1, and MK2, which mediate thetumour-suppressive function of p38.This, is the first study to demonstrate theanti-proliferative effect of ophiobolin O in human breast cancer MCF-7cells. The resultsof present study show that ophiobolin O induced cycle G0/G1phase arrest in MCF-7cellsusing a cell cycle analysis. In addition, we demonstrated that ophiobolin O reduced theviability of human breast cancer MCF-7cells in a time-and dose-dependent manner andefficiently induced apoptosis in MCF-7cells using the Annexin V/PI binding assay.Ophiobolin O also caused the activation of JNK (c-Jun NH2-terminal kinase), p38MAPK(mitogen activated protein kinase) and ERK (extracellular signal-regulated kinase) as wellas the degradation of Bcl-2phosphorylation (Ser70). Bax protein expression was notchanged in ophiobolin O-treated cells.In summary, the present study demonstrated the following:1) breast cancer MCF-7cells were highly sensitive to ophiobolin O-induced growth inhibition;2) ophiobolin Oinhibited the cell cycle progression at the G0/G1phase;3) ophiobolin O induced apoptosisin MCF-7cells;4) ophiobolin O-induced cell growth inhibition was regulated viaJNK-mediated Bcl-2phosphorylation in the breast cancer MCF-7cells;5) Whetherophiobolin O may be used as a potential drug target for anti-cancer (breast cancer) therapyrequires further experimental clinical research.