No abstract

The principal pathological features of Alzheimer's disease (AD) are extracellular amyloid plaques and intracellular neurofibrillary tangles, the latter composed of the microtubule-binding protein tau assembled into paired helical and straight filaments. Recent studies suggest that these pathological entities may be functionally linked, although the mechanisms by which amyloid deposition promotes pathological tau filament assembly are poorly understood. Here, we report that tau is proteolyzed by multiple caspases at a highly conserved aspartate residue (Asp 421 ) in its C terminus in vitro and in neurons treated with amyloid-␤ (A␤) (1-42) peptide. Tau is rapidly cleaved at Asp 421 in A␤-treated neurons (within 2 h), and its proteolysis appears to precede the nuclear events of apoptosis. We also demonstrate that caspase cleavage of tau generates a truncated protein that lacks its C-terminal 20 amino acids and assembles more rapidly and more extensively into tau filaments in vitro than wild-type tau. Using a monoclonal antibody that specifically recognizes tau truncated at Asp 421 , we show that tau is proteolytically cleaved at this site in the fibrillar pathologies of AD brain. Taken together, our results suggest a novel mechanism linking amyloid deposition and neurofibrillary tangles in AD: A␤ peptides promote pathological tau filament assembly in neurons by triggering caspase cleavage of tau and generating a proteolytic product with enhanced polymerization kinetics. A lzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by accelerated neuronal cell death leading to dementia (1). Its hallmark pathologic features are extracellular amyloid plaques and intraneuronal fibrillar structures, the latter including neurofibrillary tangles (NFTs), neuropil threads, and dystrophic neurites invading amyloid plaques (2). Amyloid plaques are formed by the extracellular deposition of proteolytic fragments of the amyloid precursor protein (APP) termed amyloid-␤ (A␤) (1, 3), whereas the fibrillar pathologies are composed of the microtubule-associated protein tau assembled into polymeric filaments (paired helical and straight filaments) (2). The pathogenic role of amyloid deposition in AD is underscored by the evidence that each of the disease-causing mutations in familial AD results in enhanced production of amyloidogenic A␤ peptides; these peptides are sufficient to induce apoptosis in cultured neurons (1, 3). Furthermore, the recent observation that tau mutations cause hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), a class of diseases characterized by NFT-like deposition of polymeric tau and dementia without amyloid plaques, emphasizes the critical role that tau plays in neurodegenerative events (4-6). Although amyloid plaques and NFTs have been largely regarded as independent neuropathologic entities, recent work suggests they may be functionally linked: mutation of APP that results in amyloid deposition or direct intracranial injection of A␤ peptide increase...

A novel cell-targeting, pH-sensitive polymeric carrier was employed in this study for delivery of the anticancer drug bortezomib (BTZ) to cancer cells. Our strategy is based on facile conjugation of BTZ to catechol-containing polymeric carriers that are designed to be taken up selectively by cancer cells through cell surface receptor-mediated mechanisms. The polymer used as a building block in this study was poly(ethylene glycol), which was chosen for its ability to reduce nonspecific interactions with proteins and cells. The catechol moiety was exploited for its ability to bind and release borate-containing therapeutics such as BTZ in a pH-dependent manner. In acidic environments, such as in cancer tissue or the subcellular endosome, BTZ dissociates from the polymer-bound catechol groups to liberate the free drug, which inhibits proteasome function. A cancer-cell-targeting ligand, biotin, was presented on the polymer carriers to facilitate targeted entry of drug-loaded polymer carriers into cancer cells. Our study demonstrated that the cancer-targeting drug–polymer conjugates dramatically enhanced cellular uptake, proteasome inhibition, and cytotoxicity toward breast carcinoma cells in comparison with nontargeting drug–polymer conjugates. The pH-sensitive catechol–boronate binding mechanism provides a chemoselective approach for controlling the release of BTZ in targeted cancer cells, establishing a concept that may be applied in the future toward other boronic acid-containing therapeutics to treat a broad range of diseases.

Caspases are universal effectors of apoptosis. The mitochondrial and death receptor pathways activate distinct apical caspases (caspase-9 and -8, respectively) that converge on the proteolytic activation of the downstream executioner caspase-3. Caspase-9 and -8 cleave procaspase-3 to produce a p24 processing intermediate (composed of its prodomain and large subunit), which then undergoes autoproteolytic cleavage to remove the prodomain from the active protease. Recently, several heat shock proteins have been shown to selectively inhibit the mitochondrial apoptotic pathway by disrupting the activation of caspase-9 downstream of cytochrome c release. We report here that the small heat shock protein ␣B-crystallin inhibits both the mitochondrial and death receptor pathways. In S-100 cytosolic extracts treated with cytochrome c/dATP or caspase-8, ␣B-crystallin inhibits the autoproteolytic maturation of the p24 partially processed caspase-3 intermediate. In contrast, neither the closely related small heat shock protein family member Hsp27 nor Hsp70 inhibited the maturation of the p24 intermediate. We also demonstrate that ␣B-crystallin co-immunoprecipitates with the p24 partially processed caspase-3 in vivo. Taken together, our results demonstrate that ␣B-crystallin is a novel negative regulator of apoptosis that acts distally in the conserved cell death machinery by inhibiting the autocatalytic maturation of caspase-3.

Recent gene profiling studies have identified a new breast cancer subtype, the basal-like group, which expresses genes characteristic of basal epithelial cells and is associated with poor clinical outcomes. However, the genes responsible for the aggressive behavior observed in this group are largely unknown. Here we report that the small heat shock protein α-basic-crystallin (αB-crystallin) was commonly expressed in basal-like tumors and predicted poor survival in breast cancer patients independently of other prognostic markers. We also demonstrate that overexpression of αB-crystallin transformed immortalized human mammary epithelial cells (MECs). In 3D basement membrane culture, αB-crystallin overexpression induced luminal filling and other neoplastic-like changes in mammary acini, while silencing αB-crystallin by RNA interference inhibited these abnormalities. αB-Crystallin overexpression also induced EGF-and anchorage-independent growth, increased cell migration and invasion, and constitutively activated the MAPK kinase/ERK (MEK/ERK) pathway. Moreover, the transformed phenotype conferred by αB-crystallin was suppressed by MEK inhibitors. In addition, immortalized human MECs overexpressing αB-crystallin formed invasive mammary carcinomas in nude mice that recapitulated aspects of human basal-like breast tumors. Collectively, our results indicate that αB-crystallin is a novel oncoprotein expressed in basal-like breast carcinomas that independently predicts shorter survival. Our data also implicate the MEK/ERK pathway as a potential therapeutic target for these tumors.