Proteasomal inhibition causes the formation of protein aggregates containing a wide range of proteins, including nitrated proteins

Mutations in Cu,Zn‐superoxide dismutase (SOD‐1) are associated with some familial cases of amyotrophic lateral sclerosis (ALS), but it is not known how they result in cell death. We examined effects of overexpression of wild‐type SOD‐1 or the G37R or G85R mutations on the accumulation of ubiquitinated and nitrated proteins, and on loss of cell viability induced by the proteasome inhibitor, lactacystin. Wild‐type SOD‐1 had no effect on proteasomal activity, but the mutants decreased it somewhat. Treatment with lactacystin (1 µm) caused only limited cell viability loss, even though it induced a marked inhibition of proteasomal activities. However, viability loss due to apoptosis was substantial in response to lactacystin when cells were overexpressing a mutant SOD‐1. The frequency of cells showing immunoreactivity against ubiquitinated‐ or nitrated‐proteins was enhanced when wild‐type and mutant SOD‐1 s were overexpressed. Ubiquitinated or nitrated α‐tubulin, SOD‐1, α‐synuclein and 68K neurofilaments were observed in the aggregates. Similar aggregates were observed in cells overexpressing mutant parkin (Del3–5, T240R and Q311′X). The nitric oxide synthase inhibitor, l‐NAME, decreased viability loss and aggregation, suggesting that nitration of proteins may play an important role in aggregation and in the cell death accompanying it.