Ambiguine I Isonitrile from Fischerella ambigua Induces Caspase-Independent Cell Death in MCF-7 Hormone Dependent Breast Cancer Cells

Ambiguine I isonitrile (AmbI) obtained from the cultured cyanobacterium Fischerella ambigua was identified as a potent NF-κB inhibitor (IC50=30 nM). The cytotoxic effect was evaluated in both HT-29 colon cancer cell line (EC₅₀=4.35 μM) and MCF-7 breast cancer cell line (EC₅₀=1.7 μM) using the SRB assay. In the cells treated with AmbI, an increased population of cells was detected in sub G₁-phase. The apoptotic effect was associated with block in G₁-phase of the cell cycle in treated cells; however, cell death was induced independently of caspase-7. The NF-κB expression of p50 and p65 units were also examined in treated cells and compared with the positive control, rocaglamide (IC₅₀=75 nM). Moreover, the expression of mediators of the NF-κB pathway such as kinase IKKκ was studied at increasing concentrations of AmbI. The down stream effect of NF-κB inhibition and the effect on the expression of TNF-α induced ICAM-1 was evaluated. Thus, the dose-dependent and time-dependent effect of AmbI on MCF-7 cells was examined in an attempt to investigate its potential mechanism of action on inducing apoptosis.