The Accumulation of Neurotoxic Proteins, Induced by Proteasome Inhibition, is Reverted by Trehalose, an Enhancer of Autophagy, in Human Neuroblastoma Cells

Neurodegenerative diseases like Parkinson's disease, Alzheimer's disease, Huntington's disease and others are due to accumulation of abnormal proteins which fold improperly and impair neuronal function. Accumulation of these proteins could be achieved by several mechanisms including mutation, overproduction or impairment of its degradation. Inhibition of the normal protein degradation is produced by blockade of the ubiquitin proteasome system. We have shown that epoxomicin, a proteasome inhibitor, increases the levels of proteins involved in neurodegenerative disorders such as α-synuclein and hyper phosphorylated tau in NB69 human neuroblastoma cells and that such increase correlates with an enhanced rate of cell death. We then investigated whether the stimulation of autophagy, an alternative mechanism for elimination of abnormal proteins, by treatment with trehalose, counteracts the effects of proteasomal blockade. Trehalose, a disaccharide present in many non-mammalian species, known to enhance autophagy, protects cells against various environmental stresses. Treatment with trehalose produced a dose and time-dependent increase in the number of autophagosomes and markers of autophagy in NB69 cells. Trehalose did not change the number of total neither the number of dividing cells in the culture but it completely prevented the necrosis of NB69 induced by epoxomicin. In addition, the treatment with trehalose reverted the accumulation, induced by epoxomicin, of polyubiquitinated proteins, total and phosphorylated tau, p-GSK-3, and α-synuclein, as well as the α-synuclein intracellular aggregates. The effects of trehalose were not mediated through activation of free radical scavenging compounds, like GSH, or mitochondrial proteins, like DJ1, but trehalose reduced the activation of ERK and chaperone HSP-70 induced by epoxomicin. Inhibition of ERK phosphorylation prevented the epoxomicin-induced cell death. Inhibition of autophagy reverted the neuroprotective effects of trehalose in epoxomicin-induced cell death. These results suggest that trehalose is a powerful modifier of abnormal protein accumulation in neurodegenerative diseases.