Curcumin homing to the nucleolus: mechanism for initiation of an apoptotic program.

Curcumin is a plant-derived polyphenol that displays antitumor properties. Incubation of cultured SF-767 glioma cells with curcumin gave rise to intense intranuclear foci of curcumin fluorescence. In vitro studies revealed that nuclear homing by curcumin is not a result of DNA/chromatin binding. On the other hand, curcumin fluorescence colocalized with nucleophosmin, a nucleolus marker protein. To determine the temporal relationship between curcumin-induced apoptosis and nucleolar homing, confocal live cell imaging was performed. The data show that curcumin localization to the nucleolus occurs prior to cell surface exposure of phosphatidylserine. In studies of the mechanism of curcumin-induced apoptosis in SF-767 cells, its effect on the subcellular location of p14(ARF) was determined. Whereas p14(ARF) was confined to the nucleolus in untreated cells, 2 h following incubation with curcumin, it displayed a diffuse nuclear distribution. Given the role of nuclear p14(ARF) in binding the E3 ubiquitin ligase, mouse double minute 2 homolog (MDM2), the effect of curcumin treatment on cellular levels of the tumor suppressor protein, p53, was examined. Between 2 and 4 h following curcumin treatment, p53 levels increased with maximum levels reached by 8 h. Thus, curcumin homing to the nucleolus induces redistribution of p14(ARF) to the nucleoplasm where interaction with MDM2 leads to stabilization of p53, with subsequent initiation of apoptosis.