Abstract

Schweinfurthins are potent inhibitors of cancer cell growth especially against human-derived central nervous system (CNS) tumor lines, such as the SF-295 cells. However, the mechanism(s) by which these compounds impede cell growth is (are) not fully understood. In an effort to understand the basis for schweinfurthin effects, we present a fluorescent schweinfurthin 3-deoxyschweinfurthin B-like p-nitro bis-stilbene (3dSB-PNBS), which displays similar biological activity to 3-deoxyschweinfurthin B (3dSB). These two schweinfurthins retain the unique differential activity of the natural schweinfurthins as evidenced by the spindle-like morphologic changes induced in the SF-295 cells and unaltered appearance of the human-derived lung carcinoma A549 cells. Here we demonstrate that incubation with 3dSB or 3dSB-PNBS results in poly ADP ribose polymerase (PARP) and caspase-9 cleavage, both markers of apoptosis. Co-incubation of 3dSB or 3dSB-PNBS with the caspase-9 inhibitor Z-LEHD-FMK prevents PARP cleavage. Therapeutics that induce apoptosis often activate cellular stress pathways. A common marker for multiple stress pathways is the phosphorylation of eukaryotic initiation factor (eIF2α), which is phosphorylated in response to 3dSB and 3dSB-PNBS. Glucose-regulated protein 78 (GRP78) and protein disulfide isomerase (PDI), both ER chaperones, are upregulated with schweinfurthin exposure. Utilizing the fluorescent properties of 3dSB-PNBS and DMP-PNBS, a control compound; we show that the intracellular levels of 3dSB-PNBS are higher than rhodamine 123 or DMP-PNBS in the SF-295 and A549 cells.