Regulation of proteasome activity the role of insulin/IGF-1 signaling

The ubiquitin-proteasome system (UPS) is the major pathway for controlled protein degradation in the cell. Substrates destined for degradation are polyubiquitinated by ubiquitin ligating enzymes and degraded by the proteasome, which is a complex multisubunit enzyme of over 2.5 MDa. Due to its essential role in maintenance of the cellular protein homeostasis, dysfunctions in proteasome activity have been linked to many severe disorders, such as Alzheimer s, Parkinson s and Huntington s disease and some cancers. Although the proteasome takes part in almost all cellular processes, it is still unclear which factors contribute to regulation of proteasome function. The aim of this thesis was to elucidate mechanisms of how proteasome activity is regulated in a multicellular organism. To enable monitoring and quantification of proteasome activity in vivo, a photoconvertible reporter system was developed to live image proteasome activity in C. elegans. By using this system, it was uncovered that proteasome activity varies between different tissues, as GABAergic and dopaminergic neurons showed faster reporter degradation than muscle cells, and that aging affects proteasome activity in a tissue-dependent manner. Subsequently, it was investigated whether signaling pathways regulating aging also affect proteasome activity. Insulin/IGF-1 signaling (IIS) is an evolutionarily conserved signaling pathway regulating lifespan in worms, flies, rodents and possibly humans. By using C. elegans as a model organism, this thesis shows that IIS regulates proteasome activity through the FOXO/DAF-16 and Nrf/SKN-1 transcription factors. Moreover, it was demonstrated that DAF-16, which is activated by reduced IIS, inhibits the expression of the proteasome-associated deubiquitinating enzyme UBH-4, which was shown to function as a tissue-specific proteasome inhibitor. The role of UBH-4 appears to be well-conserved, as downregulation of its mammalian ortholog, UCHL-5, increases proteasome activity and degradation of proteotoxic proteins in mammalian cells. Taken together, this thesis provides tools to study proteasome activity in vivo, and establishes a molecular mechanism linking IIS to efficiency of proteasomal degradation. These results can be utilized when designing new therapies for proteasome-associated diseases, especially for disorders that may be alleviated with tissue-specific modulation of proteasome activity.