Proteasomal inhibition reduces parkin mRNA in PC12 and SH-SY5Y cells

Mutations in the gene encoding the E3 ubiquitin-protein ligase parkin have been shown to be a common genetic cause of familial early-onset Parkinson's disease (PD). In addition to its function in the ubiquitin-proteasome system (UPS), parkin has been ascribed general neuroprotective properties. Stress and mutation induced decreases in parkin solubility leading to compromised cytoprotection have recently been reported. We systematically investigated whether PD-related stresses including MG132 and epoxomicin (proteasomal impairment), tunicamycin (unfolded protein stress), and rotenone (mitochondrial dysfunction) resulted in expressional changes of parkin and other E3 ubiquitin ligases (dorfin, SIAH-1). Rotenone and tunicamycin did not change parkin mRNA levels, whereas proteasomal inhibition resulted in a reduction of parkin mRNA in PC12 cells as well as in SH-SY5Y cells. Therefore, surprisingly, cells did not react with a compensatory parkin upregulation under proteasomal inhibition, although, in parallel, parkin protein shifted to the insoluble fraction, reducing soluble parkin levels in the cytosol. Since the mRNA of the parkin-coregulated gene PACRG paralleled the parkin mRNA at least partly, we suspect a promoter-driven mechanism. Our study, therefore, shows a link between proteasomal impairment and parkin expression levels in cell culture, which is intriguing in the context of the described and debated proteasomal dysfunction in the substantia nigra of PD patients.