Sestrin as a Feedback Inhibitor of TOR That Prevents Age-Related Pathologies

Abstract

Sestrins are conserved proteins that accumulate in cells exposed to stress and potentiate adenosine monophosphate-activated protein kinase (AMPK) and inhibit activation of target of rapamycin (TOR). We show that abundance of Drosophila Sestrin (dSesn) is increased upon chronic TOR activation through accumulation of reactive oxygen species (ROS) that cause activation of c-Jun N-terminal kinase (JNK) and transcription factor FoxO (Forkhead box O). Loss of dSesn resulted in age-associated pathologies including triglyceride accumulation, mitochondrial dysfunction, muscle degeneration and cardiac malfunction, which were prevented by pharmacological activation of AMPK or inhibition of TOR. Hence, dSesn appears to be a negative feedback regulator of TOR that integrates metabolic and stress inputs and prevents pathologies caused by chronic TOR activation, that may result from diminished autophagic clearance of damaged mitochondria, protein aggregates, or lipids.TOR (target of rapamycin) is a key protein kinase that regulates cell growth and metabolism to maintain cellular and organismal homeostasis (1-3). Insulin (Ins) and insulin-like growth factors (IGF) are major TOR activators that operate through phosphoinositide 3-kinase (PI3K) and the protein kinase AKT (2). Conversely, adenosine monophosphate activated protein kinase (AMPK), which is activated upon energy depletion, caloric restriction (CR), or genotoxic damage, is a stress-responsive inhibitor of TOR activation (2,4). TOR stimulates cell growth and anabolism by increasing protein and lipid synthesis through p70 S6 kinase (S6K), eukaryotic translation initiation factor 4E-binding protein (4E-BP), and sterol response element binding protein (SREBP) (1-3,5) and by decreasing autophagic † To whom correspondence should be addressed. karinoffice@ucsd.edu. Prolonged TOR signaling induces dSesnPersistent TOR activation in wing discs by a constitutively active form of insulin receptor (InR CA ) resulted in prominent dSesn protein accumulation, not seen in a dSesn-null larvae (Fig. 1, A to C). InR CA also induced accumulation of dSesn RNA (Fig. 1, D to F), indicating that dSesn accumulation is due to increased transcription or mRNA stabilization. As dSesn accumulation was restricted to cells in which TOR was activated, the response is likely to be cell autonomous. dSesn was also induced when TOR was chronically activated by overexpression of the small guanine triphosphatase Rheb (Fig. 1G), or clonal loss of PTEN (phosphatase and tensin homolog) or TSC1 (tuberous sclerosis complex 1) (Fig. 1, H TOR signaling generates ROS to induce dSesnIn mammals, transcription of Sesn genes is increased in cells exposed to oxidative stress (9,11) and we observed ROS accumulation, detected by oxidation of dihydroethidium (DHE), in the same region of the imaginal discs in which InR CA or Rheb were expressed (Fig. 2, A (Fig. 2F).FoxO and p53 are ROS-activated transcription factors that control mammalian Sesn genes (9-12,14). The dSesn locus contains 8 perfect FoxO-response elemen...

Role of the MAPK/cJun NH 2 -terminal kinase signaling pathway in starvation-induced autophagy

“…Indeed, previous studies have demonstrated that MAPK8/9 can be activated in response to TORC1 signaling [35] and that TORC1 activation can require MAPK8/9 signaling [28]. However, we found no requirement for MAPK8/9 upstream (Figure 1) or downstream (Figure 2) of TORC1 during starvation-induced autophagy.…”

“…The requirement of MAPK8 for starvation-induced autophagy [5] may therefore reflect a role for MAPK8 upstream or down-stream of MTOR. Indeed, previous studies have demonstrated that MAPK8/9 can be activated in response to TORC1 signaling [35] and that TORC1 activation can require MAPK8/9 signaling [28]. Alternatively, MAPK8/9 and TORC1 may function in parallel pathways that control autophagy.…”

Long live FOXO: unraveling the role of FOXO proteins in aging and longevity

“…In fact, it has been shown that FOXO factors participate in the regulation of genes responsible for two main mechanisms of intracellular clearance: autophagy and the ubiquitin‐proteasome system (Webb & Brunet, 2014). Defects in autophagy, the process of degradation and recycling of cytoplasmic proteins and organelles in response to starvation have been associated with premature aging and age‐related disorders (Hara et al ., 2006; Komatsu et al ., 2006; Jung et al ., 2008; Pickford et al ., 2008; Masiero et al ., 2009; Lee et al ., 2010a, 2010b). FOXOs affect the expression of genes involved in autophagy and mitophagy (muscle‐specific autophagy) in muscle cells from flies (dFOXO) to mammals (FOXO3), allowing adaptation of the tissues to starvation (Zhao et al ., 2007; Sengupta et al ., 2009; Demontis & Perrimon, 2010).…”

“…The mechanism(s) by which Foxo factors contribute to lifespan remain elusive. As previously mentioned, decreased autophagy has been related to premature aging and age‐related associated disorders (Hara et al ., 2006; Komatsu et al ., 2006; Jung et al ., 2008; Pickford et al ., 2008; Masiero et al ., 2009; Lee et al ., 2010a, 2010b). Foxo factors have been shown to regulate autophagy in mouse muscle, particularly Foxo3, which induces the expression of several autophagy genes and increases autophagosome formation (Mammucari et al ., 2007; Zhao et al ., 2007; Webb & Brunet, 2014).…”

Autophagy, Apoptosis, Mitoptosis and Necrosis: Interdependence Between Those Pathways and Effects on Cancer

“…Sestrins are highly conserved small proteins that accumulate in the cells exposed to stress and play key roles in regulation of aging and cell metabolism (Lee et al 2010). p53 interacts with target genes SESTRIN1 and SESTRIN2 which subsequently inhibit mammalian target of rapamycin (mTOR) through an indirect mechanisms involving stimulation of adenosine monophosphate-activated protein kinase (AMPK) (Budanov and Karin 2008;Sanli et al 2012).…”