During metazoan life, protein homeostasis (proteostasis) declines with age due to impaired proteasome activity. The signalling pathways activated by growth factors that regulate proteostasis and their importance to ageing are just emerging. In this issue, Liu et al report a crucial role of epidermal growth factor (EGF) in regulating Caenorhabditis elegans lifespan through the RAF/MAPK pathway. EGF signalling activates the ubiquitin–proteosome system (UPS) and represses the chaperone machinery by modulating the expression of several ageing‐related genes (gerontogenes). This strategic switch in controlling global protein turnover provides novel insights into the molecular mechanisms driving ageing in multi‐cellular organisms.
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Normal cellular functions and viability are controlled by regulated turnover of non‐functional proteins by the proteasome machinery (Varshavsky, 2005; Finley, 2009). During ageing, quality control of proteostasis declines resulting in the accumulation of misfolded and oxidized proteins that lead to cytotoxicity and neurodegenerative disorders like Alzheimer's and Parkinson's diseases. Proteins doomed for degradation are decorated by ubiquitin chains, which are recognized by ubiquitin receptors at the proteasomes (Dikic et al, 2009). Proteins can be conjugated with mono, multi‐mono or polyubiquitin chains of various kinds that in turn determine the localization, functionality and stability of the proteins (Ravid and Hochstrasser, 2008; Grabbe et al, 2011). Accumulation of polyubiquitinated or oxidized proteins in ageing organisms is contributed by several factors including deceased subunit expression of proteasome subunits (Vernace et al, 2007). Much of the studies unveiling the signalling pathways regulating ageing processes were performed in short‐lived organisms including yeast, flies and worms. Yet, they provide profound insights into mechanisms behind healthy lifespan in organisms that live for decades.
The genetic programmes controlling ageing has been extensively studied in invertebrates, which revealed a crucial role for insulin/IGF‐1 pathway, dietary restriction, TOR signalling, AMP kinase and Sirtuins (Kenyon, 2010). In worms, loss of daf‐2, the hormone receptor of IGF activates Forkhead box O (FOXO) transcription factor DAF‐16, which in turn regulates expression of genes contributing to increased lifespan. DAF‐16 can also be activated directly by SIRT1‐dependent deacetylation during oxidative stress (Kenyon, 2010). IGF‐1 signalling in regulating ageing is evolutionarily conserved as revealed by several studies in higher vertebrates. It is well known that small dogs with reduced IGF‐1 levels live longer and that the mutations impairing insulin receptor function are highly represented in cohorts of centernarians (Kenyon, 2010). Despite the crucial role of EGF in larva growth and development, their possible roles in regulating ageing are not well understood. EGF regulates normal growth and differentiation by activation of various signalosomes in distinct subcellular compartments (Hoeller et al, 2005). Recent studies revealed, a major unexpected role of EGF in promoting healthy ageing as revealed by gain‐of‐function mutants at the organismal level (Yu and Driscoll, 2011). The interaction of EGF ligand in C. elegans (LIN‐3) with its receptor is regulated by ligand binding proteins HPA‐1 and HPA‐2 leading to controlled IP3R activation, calcium release and healthy ageing (Iwasa et al, 2010; Yu and Driscoll, 2011). Apart from PLC‐γ activation, Let23/EGFR activation also triggers the highly conserved RAS (LET‐60)/RAF (LIN‐45)/MAPK pathway, which has a crucial role in the normal development of multi‐cellular organisms. The study by Chris Rongo and colleagues revealed an important role for this pathway in controlling the expression of gerontogenes, which regulate the proteasome and chaperone machinery. Gene expression profile studies revealed that EGF signalling activates several anti‐ageing genes through the EOR‐1 and EOR‐2 PLZF transcription factors (Figure 1). Surprisingly, members of the ubiquitin–proteasome machinery were upregulated and HSP16 family were found to be downregulated by EGF/EOR‐1 signalling (Liu et al, 2011). The proper folding and conformation of the proteins are regulated by molecular chaperones and ageing cells often accumulate misfolded proteins. EGF signalling alters the timing of protein aggregation as mutants with decreased EGF signalling accumulate Poly(Q)40∷YFP aggregates at a slower rate consistent with the reduction in HSP levels. In the same vein, EGF signalling augments protein turnover by enhancing the expression of skr‐5, an adaptor linking several F‐box proteins to Cullin‐E3 ubiquitin ligases. The ubiquitination mechanisms regulating the ageing phenotypes are not well deciphered and recent studies revealed a role for E3 ubiquitin ligase WWP‐1 and E2 enzyme UBC‐18 as a positive regulator of lifespan in C. elegans in response to dietary restriction (Carrano et al, 2009). In another study, a role for the chaperone‐like AAA ATPase CDC‐48/p97 and a deubiquitinase ATX‐3 in coupling longevity and protein homeostasis was demonstrated. Combined loss of these two factors surprisingly led to a lifespan extension in worms and the effect seems to depend on the DUB activity of ATX‐3 (Kuhlbrodt et al, 2011). These results reveal that controlled ubiquitin editing regulates the selection of polyubiquitinated substrates for the 26S proteasomes. Loss of ATX‐3 and CDC‐48 stabilizes the components of IGF‐1 signalling pathway, leading to transcriptional activation of DAF‐16 targets thus promoting lifespan in worms (Kuhlbrodt et al, 2011). Targeted degradation of polyubiquitin proteins during ageing is also clearly dependent on the UFD complex as mutation therein also inhibit global protein turn over. The study by Hoppe and colleagues suggest that ATX‐3 might preferentially edit K48‐linked chains extended by the UFD complex to regulate protein degradation (Kuhlbrodt et al, 2011). Interestingly, Liu et al reveal that conjugation of both K48‐ and K29‐linked ubiquitin chains are required for global protein turnover in ageing adult worms. Whether these or other ubiquitin chains are conjugated to selected substrates of the IGF pathway at endogenous levels during ageing needs to be further elucidated.
Altogether, these studies reveal the highly sophisticated molecular machinery modulating the UPS to regulate proteostasis during ageing (Figure 1). The antagonizing role(s) of IGF and EGF in regulating ageing is intriguing as dietary restrictions have been normally shown to prolong lifespan in various organisms. Knocking out PI3K causes worms to prolong their lifespan dramatically and inhibition of mTOR and S6 kinase extend the lifespan in invertebrates by triggering autophagy (Kenyon, 2010) (Figure 1). It is also interesting to note that serum EGF levels decline in ageing human adults and loss of EGFR ligands exhibit hair loss and weight loss with ageing in mice (Yu and Driscoll, 2011). Much of our knowledge on EGF signalling components in regulating ageing is primarily limited by the developmental defects observed in knockout mice. Tissue‐specific deletion of these components could uncover the role of these proteins in healthy lifespan. As anti‐ageing therapeutics are being developed, the crucial role of EGF signalling in augmenting healthy ageing opens up new avenues; though care should be taken as augmented EGFR signalling is also associated with epithelial cancers.
The authors declare that they have no conflict of interest.
Research in the ID laboratory is supported by the Deutsche Forschungsgemeinschaft, the Cluster of Excellence ‘Macromolecular Complexes’ of the Goethe University Frankfurt (EXC115) and a European Research Council Advanced Grant and in the KR laboratory through an ENP grant (RA1739/1‐1) from the DFG.
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