Starvation and oxidative stress resistance in Drosophila are mediated through the eIF4E-binding protein, d4E-BP

Abstract

eIF4E, the mRNA 5 cap-binding protein, is regulated by its binding protein (4E-BP), a downstream target of phosphatidylinositol-3-OH kinase [PI(3)K] signaling. We show thatDrosophila 4E-BP (d4E-BP) activity becomes critical for survival under dietary restriction and oxidative stress, and is linked to life span. The Drosophila forkhead transcription factor (dFOXO) activates d4E-BP transcription. We show that ectopic expression of d4E-BP in dFOXO-null flies restores oxidative stress resistance to control levels. Thus, d4E-BP is an important downstream effector of a dFOXO phenotype, and regulation of translation by eIF4E is vital during environmental stress. A rapid response is a crucial early line of defense in preventing cellular death in situations of stress . Translational regulation allows an organism to generate quick responses to environmental cues by controlling the expression of protein from existing cellular mRNAs (Johnstone and Lasko 2001). Translation initiation of most eukaryotic mRNAs requires binding of eIF4F, a protein complex made up of eIF4A, eIF4G, and eIF4E, to the 5Ј cap structure (Gingras et al. 1999). eIF4E activity is highly regulated both by Mnk1/Mnk2-dependent phosphorylation (Waskiewicz et al. 1997) and by repressor proteins termed eIF4E-binding proteins (4E-BPs), which compete with eIF4G for the same binding site on eIF4E (Haghighat et al. 1995;Mader and Sonenberg 1995). 4E-BPs themselves are negatively regulated by phosphorylation, and are downstream effectors of the PI3K/TOR pathway (Miron et al. 2001). Under nutritionally favorable conditions, the evolutionarily conserved TOR pathway is active and results in 4E-BP phosphorylation. This prevents 4E-BP binding to eIF4E, thus upregulating translation (Schmelzle and Hall 2000;Hay and Sonenberg 2004). Conversely, poor nutrition causes inhibition of the TOR pathway, such that unphosphorylated 4E-BP represses translation through eIF4E binding.Activation of the PI3K pathway stimulates Akt activity. Akt directly phosphorylates the forkhead transcription factor dFOXO, which is a transcriptional activator of d4E-BP in flies, resulting in reduced d4E-BP transcription (Jünger et al. 2003;Puig et al. 2003). Akt also activates TOR through tuberous sclerosis complex 2 (TSC2), which functions as a GTPase-activating protein (GAP) for the GTPase protein Rheb that activates TOR (Hay and Sonenberg 2004). Thus, activation of the PI3K pathway in Drosophila represses both the expression of the d4E-BP gene and the activity of d4E-BP protein.Here we investigated whether d4E-BP is essential under starvation and oxidative stress conditions, because dFOXO activates the transcription of d4E-BP and d4E-BP mRNA levels increase upon starvation (Zinke et al. 2002). We also provide evidence that d4E-BP activity is linked to life span, as overexpression of dFOXO is linked to increased longevity (Giannakou et al. 2004;Hwangbo et al. 2004). We present data that support both connections and indicate that d4E-BP is the critical effector of the dFOXO-induced stress-sensitive...

Nutritional sensitivity of fifth instar prothoracic glands in the tobacco hornworm, Manduca sexta

“…It was expected that transcript levels of both insulin receptor and 4EBP would increase due to starvation as is seen in flies (Tettweiler et al, 2005;Puig and Tjian, 2006). Figure 10A reveals that there was only a slight increase in insulin receptor transcript between larvae that have been starved for 30 hours and their age-matched fed counterparts (V2) (one-way ANOVA, F=1.757, p=0.2063).…”

“…Insulin receptor transcript levels have been shown to increase in response to starvation (Chen et al, 1996;Tettweiler et al, 2005;Puig and Tjian, 2006). 4EBP is a negative regulator of growth and as such has been shown to be concomitantly upregulated in response to starvation (Tettweiler et al, 2005;Puig and Tjian, 2006).…”

“…Interestingly, the size of the larvae is not affected in mutants lacking 4EBP activity, but this mutation does make them more sensitive to starvation (Teleman et al, 2005;Tettweiler et al, 2005).…”

Fasting vs dietary restriction in cellular protection and cancer treatment: from model organisms to patients

“…In flies, fasting-dependent protection against oxidative stress is mediated by d4E-BP, which acts downstream of the PI3K/Akt/dFOXO3 pathway (Tettweiler et al, 2005). d4E-BP binds to eIF4E and represses translation.…”

Stress and mTORture signaling

“…Studies in yeast and higher eukaryotes have shown that lowered TOR (and/or decreased IIS in higher eukaryotes) signaling pathway activity increases lifespan (Kenyon et al, 1993;Guarente and Kenyon, 2000;Vellai et al, 2003;Hwangbo et al, 2004;Kapahi et al, 2004;Kaeberlein et al, 2005;Powers et al, 2006). In addition, decreased IIS/TOR signaling activity is associated with increased resistance to some types of stress, suggesting that this pathway plays also an important role in the adaption to different stress conditions (Scott et al, 2002;Holzenberger et al, 2003;Broughton et al, 2005;Teleman et al, 2005;Tettweiler et al, 2005;Powers et al, 2006). In the following section, we will review extra-and intracellular cues that modulate TOR signaling.…”