2005)

2005). activity of purified 20S proteasomes and human breast cancer cellular 26S proteasomes. Direct binding of tin atoms to cellular proteasomes is responsible for the observed irreversible inhibition. Inhibition of cellular proteasomes by TPT in several human cell lines results in the accumulation of ubiquitinated proteins and natural proteasome target proteins, accompanied by induction of cell death. == Conclusions == The proteasome is one of the molecular targets of environmental toxic organotins in human cells, and proteasome inhibition by organotins contributes to their cellular toxicity. Keywords:cell death, molecular target, organotins, proteasome, proteasome inhibitors, TPT In eukaryotes, more than 80% of intra-cellular proteins are degraded through the ubiquitin/proteasome-dependent pathway (Ciechanover 1998;Dou and Li 1999;Goldberg 1995). The ubiquitin/proteasome- dependent pathway plays an essential role in antigen Olmutinib (HM71224) presentation, cellular aging, apoptosis, and other major cellular processes. The cellular Olmutinib (HM71224) proteasome, commonly called 26S proteasome, is composed of two 19S regulatory particles and a 20S core particle. The latter is usually a multicatalytic threonine protease with at least three distinct catalytic activities: chymotrypsin (CT)-like (cleavage after hydrophobic residues mediated by the 5 subunit), trypsin-like (cleavage after basic residues by the 2 2 subunit), and peptidyl-glutamyl peptide-hydrolyzing (PGPH)-like (cleavage after acidic residues by the 1 subunit) (Goldberg 1995;Groll et al. 1997). Inhibition of proteasome CT-like activity by various compounds is associated with cell apoptosis (An et al. 1998;Lopes et al. 1997). Since the 1960s, organotins, especially triphenyltin (TPT) Rabbit polyclonal to FAT tumor suppressor homolog 4 and tributyltin (TBT), have been extensively used as antifouling boat paints, polyvinyl chloride stabilizers, agricultural pesticides, and industrial catalysts. Consequently, organotin contamination is found in various environmental media (Fent 1996). Because of their lipophilic property, organotins can be accumulated through the food chain and reach higher concentrations in top predators. For example, levels of organotins in stranded whales reached 1.01.1 mg/kg (Harino et al. 2007) and in the liver of harbor porpoises reached 684,605 mg/kg (Strand et al. 2005). Organotin chemicals have also been found in the tissues of humans contaminated by organotin insecticides or food (Kannan et al. 1999). Exposure to TPT or TBT can also affect sex differentiation, resulting in masculinization of females or infertility in males (McAllister and Kime 2003;Smith 1996). TBT at low concentrations is usually toxic to cortical neurons by triggering glutamate excitotoxicity (Nakatsu et al. 2006). TBT also induces the differentiation of adipo-cytesin vitroand increases adipose massin vivo, perhaps through activation of the retinoid X receptor and the peroxisome-proliferator activated receptor (Grun et al. 2006). Both TPT and TBT can interfere with the cytotoxic function of natural killer (NK) cells (Snoeij et al. 1987), associated with increased cancer incidence. However, the toxicologic mechanism for organotin compounds is not completely comprehended, and the essential cellular target of Olmutinib (HM71224) organotins has not been identified. The authentic proteasome inhibitor clasto-lactacystin lactone contains an electrophilic-ester bond carbon that is responsible for its biological inhibition of the proteasome (Fenteany et al. 1995). We hypothesize that this electrophilic tin (Sn) atom in organotins could also be attacked by the Oatom of the N-terminal threonine (Thr-1) of the proteasome 5 subunit, causing irreversible inhibition. This hypothesis is usually supported by the present results fromin silicodocking, byin vitroproteasome activity assay using purified 20S proteasomes and human breast cancer MDA-MB-231 cells and human peripheral blood Jurkat T cells treated with TPT and other organotins, by analysis of CT-like activity, 5 proteasome subunit expression, and Sn levels in isolated proteasome complexes from the treated cells. == Materials and Methods == == Materials == We obtained organotin compounds monophenyltin trichloride (MPT), diphenyltin dichloride (DPT), triphenyltin chloride (TPT), tetraphenyltin (TePT), monobutyltin trichloride (MBT), dibutyltin dichloride (DBT), tributyltin chloride (TBT), and tetrabutyltin (TeBT) from Acros Organics (Morris Plains, NJ, USA). A 50-mmol/L stock solution of each organotin [dissolved.