DAF-16/FoxO Directly Regulates an Atypical AMP-Activated Protein Kinase Gamma Isoform to Mediate the Effects of Insulin/IGF-1 Signaling on Aging in Caenorhabditis elegans

Abstract

The DAF-16/FoxO transcription factor controls growth, metabolism and aging in Caenorhabditis elegans. The large number of genes that it regulates has been an obstacle to understanding its function. However, recent analysis of transcript and chromatin profiling implies that DAF-16 regulates relatively few genes directly, and that many of these encode other regulatory proteins. We have investigated the regulation by DAF-16 of genes encoding the AMP-activated protein kinase (AMPK), which has α, β and γ subunits. C. elegans has 5 genes encoding putative AMP-binding regulatory γ subunits, aakg-1-5. aakg-4 and aakg-5 are closely related, atypical isoforms, with orthologs throughout the Chromadorea class of nematodes. We report that ∼75% of total γ subunit mRNA encodes these 2 divergent isoforms, which lack consensus AMP-binding residues, suggesting AMP-independent kinase activity. DAF-16 directly activates expression of aakg-4, reduction of which suppresses longevity in daf-2 insulin/IGF-1 receptor mutants. This implies that an increase in the activity of AMPK containing the AAKG-4 γ subunit caused by direct activation by DAF-16 slows aging in daf-2 mutants. Knock down of aakg-4 expression caused a transient decrease in activation of expression in multiple DAF-16 target genes. This, taken together with previous evidence that AMPK promotes DAF-16 activity, implies the action of these two metabolic regulators in a positive feedback loop that accelerates the induction of DAF-16 target gene expression. The AMPK β subunit, aakb-1, also proved to be up-regulated by DAF-16, but had no effect on lifespan. These findings reveal key features of the architecture of the gene-regulatory network centered on DAF-16, and raise the possibility that activation of AMP-independent AMPK in nutritionally replete daf-2 mutant adults slows aging in C. elegans. Evidence of activation of AMPK subunits in mammals suggests that such FoxO-AMPK interactions may be evolutionarily conserved.

The SNF1 Kinase Ubiquitin-associated Domain Restrains Its Activation, Activity, and the Yeast Life Span

“…We have determined that this is due to loss of Fkh1/2-dependent SNF1 expression. SNF4 expression was not affected despite the analogous Daf16/ FOXO transcription factor in Caenorhabditis elegans contributing to the expression of the ␥ subunit of an atypical AMPKrelated enzyme (60). In yeast, Fkh1 alone binds to the SNF1 promoter, consistent with its greater impact on returning SNF1 expression toward normal as compared with Fkh2 (Fig.…”

Feedback regulation via AMPK and HIF-1 mediates ROS-dependent longevity in Caenorhabditis elegans

“…We also showed that hif-1 mutations up-regulate AMPK through an unknown mechanism. Recently, HIF-1 was shown to inhibit DAF-16/FOXO activity (61), and DAF-16 elevates AMPK activity by increasing transcription of the genes that encode AMPK subunits (62). Thus, hif-1 mutations may increase AMPK activity by modulating DAF-16.…”

Post-translational modifications of FOXO family proteins

“…Notably, DAF-16 can induce the expression of genes encoded by AMPK, which slows aging in Caenorhabiditis elegans. This suggests that DAF-16 and AMPK form a positive feedback loop (64). In mammals, however, the research about this feedback loop has not detected any changes in the γ substrate of AMPK depending on FOXO protein activation (65).…”