Although cisplatin has been used for decades to treat human cancer, some toxic side effects and resistance are observed. It has been suggested that gold(III) complexes, containing metal centers isoelectronic and isostructural to cisplatin, are promising anticancer drugs. Gold(III) dithiocarbamate complexes were shown to exhibit in vitro cytotoxicity, comparable with and even greater than cisplatin; however, the involved mechanism of action remained unknown. Because we previously reported that copper(II) dithiocarbamates are potent proteasome inhibitors, we hypothesized that gold(III) dithiocarbamate complexes could suppress tumor growth via direct inhibition of the proteasome activity. Here, for the first time, we report that a synthetic gold(III) dithiocarbamate (compound 2) potently inhibits the activity of a purified rabbit 20S proteasome and 26S proteasome in intact highly metastatic MDA-MB-231 breast cancer cells, resulting in the accumulation of ubiquitinated proteins and the proteasome target protein p27 and induction of apoptosis. The compound 2-mediated proteasome inhibition and apoptosis induction were completely blocked by addition of a reducing agent DTT or N-acetyl-L-cysteine, showing that process of oxidation is required for proteasome inhibition by compound 2. Treatment of MDA-MB-231 breast tumor-bearing nude mice with compound 2 resulted in significant inhibition of tumor growth, associated with proteasome inhibition and massive apoptosis induction in vivo. Our findings reveal the proteasome as a primary target for gold(III) dithiocarbamates and support the idea for their potential use as anticancer therapeutics. (Cancer Res 2006; 66(21): 10478-86)

Curcumin (diferuloylmethane) is the major active ingredient of turmeric (Curcuma longa) used in South Asian cuisine for centuries. Curcumin has been shown to inhibit the growth of transformed cells and to have a number of potential molecular targets. However, the essential molecular targets of curcumin under physiologic conditions have not been completely defined. Herein, we report that the tumor cellular proteasome is most likely an important target of curcumin. Nucleophilic susceptibility and in silico docking studies show that both carbonyl carbons of the curcumin molecule are highly susceptible to a nucleophilic attack by the hydroxyl group of the NH 2 -terminal threonine of the proteasomal chymotrypsin-like (CT-like) subunit. Consistently, curcumin potently inhibits the CT-like activity of a purified rabbit 20S proteasome (IC 50 = 1.85 Mmol/L) and cellular 26S proteasome. Furthermore, inhibition of proteasome activity by curcumin in human colon cancer HCT-116 and SW480 cell lines leads to accumulation of ubiquitinated proteins and several proteasome target proteins, and subsequent induction of apoptosis. Furthermore, treatment of HCT-116 colon tumor-bearing ICR SCID mice with curcumin resulted in decreased tumor growth, associated with proteasome inhibition, proliferation suppression, and apoptosis induction in tumor tissues. Our study shows that proteasome inhibition could be one of the mechanisms for the chemopreventive and/or therapeutic roles of curcumin in human colon cancer. Based on its ability to inhibit the proteasome and induce apoptosis in both HCT-116 and metastatic SW480 colon cancer cell lines, our study suggests that curcumin could potentially be used for treatment of both early-stage and late-stage/refractory colon cancer. [Cancer Res 2008;68(18):7283-92]

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