Fetal bovine serum requirement for pyrrolidine dithiocarbamate-induced apoptotic cell death of MCF-7 breast tumor cells.

Pyrrolidine dithiocarbamate (PDTC) can form a complex with metal ions and then act as a proteasome inhibitor, which leads to tumor cell apoptosis, and could therefore be developed as an anticancer agent. In our efforts to find factors that induce PDTC-mediated apoptosis of tumor cells, the effect of serum concentration on the apoptotic activity of PDTC was investigated. PDTC could not induce MCF-7 breast tumor cell death in serum-free media but significantly induced cell death in a dose-dependent manner at concentrations of ≥25 μM in media containing 10% fetal bovine serum. PDTC-mediated cell death was also dependent on serum concentration. PDTC-mediated cell death occurred through apoptosis. Similar to that in normal FBS, PDTC-mediated apoptotic cell death was also induced in media containing dialyzed FBS, indicating that PDTC-mediated apoptosis does not require metal ions or salts, but rather proteins in fetal bovine serum. In addition, differential apoptotic effects of PDTC were not observed with inhibitors of NF-κB activation such as N-acetylcysteine (NAC), Fenofibrate and carbobenzoxyl-l-leucyl-l-leucyl-l-leucinal (MG132) or with the metal-binding agent, 5-chloro-7-iodo-8-hydroxyquinoline (Clioquinol). These results indicate that serum is required for PDTC-mediated apoptosis and that zinc-binding compounds such as PDTC, N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) and Clioquinol may each have their own mechanisms by which they induce tumor cell death, even though they are all classified as zinc-binding compounds.