Dysfunction of the UPS (ubiquitin—proteasome system) has been implicated in dopaminergic neuronal death in PD (Parkinson's disease). Recent studies suggest that unregulated cell cycle events play a key role in neuronal death. In this study, the effects of UPS dysfunction on cell cycle events in neuronal differentiated PC12 cells were analysed using a specific inhibitor of proteasome, lactacystin. Lactacystin induced apoptosis, G2/M cell cycle arrest and sustained the phosphorylation of the pRB (retinoblastoma protein), the key molecular process of G1/S transition, in neuronal PC12 cells. Furthermore, inhibition of cell cycle progression protected against lactacystin-induced cell apoptosis. Finally, we determined that lactacystin activated the ERK signalling pathway. Inhibition of ERK1/2 activation by MEK-1 inhibitor PD98059 decreased cell cycle aberrant and prevented apoptosis induced by lactacystin. These results indicate that aberrant cell cycle events contribute to apoptotic death induced by UPS dysfunction.