Overload or dysfunction of ubiquitin-proteasome system (UPS) is implicated in mechanisms of neurodegeneration associated with neurodegenerative diseases, e.g. Parkinson and Alzheimer disease, and ischemia-reperfusion injury. The aim of this study was to investigate the possible association between viability of neuroblastoma SH-SY5Y and glioblastoma T98G cells treated with bortezomib, inhibitor of 26S proteasome, and accumulation of ubiquitin-conjugated proteins with respect to direct cytotoxicity of aggregates of ubiquitin-conjugated proteins. Bortezomib-induced death of SH-SY5Y cells was documented after 24 h of treatment while death of T98G cells was delayed up to 48 h. Already after 4 h of treatment of both SH-SY5Y and T98G cells with bortezomib, increased levels of both ubiquitin-conjugated proteins with molecular mass more than 150 kDa and Hsp70 were observed whereas Hsp90 was elevated in T98G cells and decreased in SH-SY5Y cells. With respect to the cell death mechanism, we have documented bortezomib-induced activation of caspase 3 in SH-SY5Y cells that was probably a result of increased expression of pro-apoptotic proteins, PUMA and Noxa. In T98G cells, bortezomib-induced expression of caspase 4, documented after 24 h of treatment, with further activation of caspase 3, observed after 48 h of treatment. The delay in activation of caspase 3 correlated well with the delay of death of T98G cells. Our results do not support the possibility about direct cytotoxicity of aggregates of ubiquitin-conjugated proteins. They are more consistent with a view that proteasome inhibition is associated with both transcription-dependent and -independent changes in expression of pro-apoptotic proteins and consequent cell death initiation associated with caspase 3 activation.
Keywords: Apoptosis; Bcl-2 family proteins; Caspases; Cell death; Heat shock proteins; Ubiquitin proteasome system.