Showing papers by "Michael K. Lee published in 2008"

Lysine 63-linked ubiquitination promotes the formation and autophagic clearance of protein inclusions associated with neurodegenerative diseases

Abstract: Although ubiquitin-enriched protein inclusions represent an almost invariant feature of neurodegenerative diseases, the mechanism underlying their biogenesis remains unclear. In particular, whether the topology of ubiquitin linkages influences the dynamics of inclusions is not well explored. Here, we report that lysine 48 (K48)- and lysine 63 (K63)-linked polyubiquitination, as well as monoubiquitin modification contribute to the biogenesis of inclusions. K63-linked polyubiquitin is the most consistent enhancer of inclusions formation. Under basal conditions, ectopic expression of K63 mutant ubiquitin in cultured cells promotes the accumulation of proteins and the formation of intracellular inclusions in the apparent absence of proteasome impairment. When co-expressed with disease-associated tau and SOD1 mutants, K63 ubiquitin mutant facilitates the formation of tau- and SOD-1-positive inclusions. Moreover, K63-linked ubiquitination was found to selectively facilitate the clearance of inclusions via autophagy. These data indicate that K63-linked ubiquitin chains may represent a common denominator underlying inclusions biogenesis, as well as a general cellular strategy for defining cargo destined for the autophagic system. Collectively, our results provide a novel mechanistic route that underlies the life cycle of an inclusion body. Harnessing this pathway may offer innovative approaches in the treatment of neurodegenerative disorders.

Amyloid pathology is associated with progressive monoaminergic neurodegeneration in a transgenic mouse model of Alzheimer's disease.

Abstract: beta-Amyloid (Abeta) pathology is an essential pathogenic component in Alzheimer's disease (AD). However, the significance of Abeta pathology, including Abeta deposits/oligomers and glial reactions, to neurodegeneration is unclear. In particular, despite the Abeta neurotoxicity indicated by in vitro studies, mouse models with significant Abeta deposition lack robust and progressive loss of forebrain neurons. Such results have fueled the view that Abeta pathology is insufficient for neurodegeneration in vivo. In this study, because monoaminergic (MAergic) neurons show degenerative changes at early stages of AD, we examined whether the APPswe/PS1DeltaE9 mouse model recapitulates progressive MAergic neurodegeneration occurring in AD cases. We show that the progression forebrain Abeta deposition in the APPswe/PS1DeltaE9 model is associated with progressive losses of the forebrain MAergic afferents. Significantly, axonal degeneration is associated with significant atrophy of cell bodies and eventually leads to robust loss (approximately 50%) of subcortical MAergic neurons. Degeneration of these neurons occurs without obvious local Abeta or tau pathology at the subcortical sites and precedes the onset of anxiety-associated behavior in the mice. Our results show that a transgenic mouse model of Abeta pathology develops progressive MAergic neurodegeneration occurring in AD cases.

Ubiquitin proteasome system stress underlies synergistic killing of ovarian cancer cells by bortezomib and a novel HDAC6 inhibitor.

Abstract: Purpose: Elevated metabolic activity of ovarian cancer cells causes increased ubiquitin-proteasome-system (UPS) stress, resulting in their greater sensitivity to the toxic effects of proteasomal inhibition. The proteasomes and a potentially compensatory histone deacetylase 6 (HDAC6)-dependent lysosomal pathway mediate eukaryotic protein turnover. We hypothesized that up-regulation of the HDAC6-dependent lysosomal pathway occurs in response to UPS stress and proteasomal inhibition, and thus, ovarian cancer cell death can be triggered most effectively by coinhibition of both the proteasome- and HDAC6-dependent protein degradation pathways. Experimental Design: To address this hypothesis, we examined HDAC6 expression patterns in normal and cancerous ovarian tissues and used a novel HDAC6-specific inhibitor, NK84, to address HDAC6 function in ovarian cancer. Results: Abnormally high levels of HDAC6 are expressed by ovarian cancer cells in situ and in culture relative to benign epithelium and immortalized ovarian surface epithelium, respectively. Specific HDAC6 inhibition acts in synergy with the proteasome inhibitor Bortezomib (PS-341) to cause selective apoptotic cell death of ovarian cancer cells at doses that do not cause significant toxicity when used individually. Levels of UPS stress regulate the sensitivity of ovarian cancer cells to proteasome/HDAC6 inhibition. Pharmacologic inhibition of HDAC6 also reduces ovarian cancer cell spreading and migration consistent with its known function in regulating microtubule polymerization via deacetylation of α-tubulin. Conclusion: Our results suggest the elevation of both the proteasomal and alternate HDAC6-dependent proteolytic pathways in ovarian cancer and the potential of combined inhibition of proteasome and HDAC6 as a therapy for ovarian cancer.