α-synuclein stimulates differentiation of osteosarcoma cells -: Relevance to down-regulation of proteasome activity

Because a limited study previously showed that alpha-synuclein (alpha-syn), the major pathogenic protein for Parkinson disease, was expressed in differentiating brain tumors as well as various peripheral cancers, the main objective of the present study was to determine whether a-syn might be involved in the regulation of tumor differentiation. For this purpose, a-syn and its non-amyloidogenic homologue beta-syn were stably transfected to human osteosarcoma MG63 cell line. Compared with beta-syn-overexpressing and vector-transfected cells, alpha-syn-overexpressing cells exhibited distinct features of differentiated osteoblastic phenotype, as shown by up-regulation of alkaline phosphatase and osteocalcin as well as inductive matrix mineralization. Further studies revealed that proteasome activity was significantly decreased in a-syn-overexpressing cells compared with other cell types, consistent with the fact that proteasome inhibitors stimulate differentiation of various osteoblastic cells. In a-syn-overexpressing cells, protein kinase C (PKC) activity was significantly decreased, and reactivation of PKC by phorbol ester significantly restored the proteasome activity and abrogated cellular differentiation. Moreover, activity of lysosome was up-regulated in a-syn-overexpressing cells, and treatment of these cells with autophagy-lysosomal inhibitors resulted in a decrease of proteasome activity associated with up-regulation of a-syn expression, leading to enhance cellular differentiation. Taken together, these results suggest that the stimulatory effect of a-syn on tumor differentiation may be attributed to down-regulation of proteasome, which is further modulated by alterations of various factors, such as protein kinase C signaling pathway and a autophagy-lysosomal degradation system. Thus, the mechanism of a-syn regulation of tumor differentiation and neuropathological effects of a-syn may considerably overlap with each other.