The first study in this line of evidence described the longevity of worms with reduced levels of TOR signaling

The first study in this line of evidence described the longevity of worms with reduced levels of TOR signaling. exacerbated by inhibition of TOR via rapamycin feeding. Under conditions of starvation or low TOR signaling levels, knockdown of FKH attenuates the size reduction associated with these conditions. Subcellular localization of endogenous FKH protein is shifted from predominantly cytoplasmic on a high-protein Gpc4 diet to a pronounced nuclear accumulation in animals with reduced levels of TOR or fed with rapamycin. Two putative FKH target genes,CG6770andcabut, are transcriptionally induced by rapamycin or FKH expression, and silenced by FKH knockdown. Induction of both target genes in heterozygousTORmutant animals is suppressed by mutations infkh. Furthermore, TOR signaling levels and FKH impact on transcription of the dFOXO target gened4E-BP, implying a point of crosstalk with the insulin pathway. In summary, our observations show that an alteration of FKH levels has an effect on cellular and organismal size, and that FKH function is required for the growth inhibition and target gene induction caused by low TOR signaling levels. == Introduction == Transcription factors belonging to the winged helix/forkheadbox (Fox) family are implicated in a variety of biological processes ranging from embryonic development to the regulation of metabolism, growth, cell death and organismal lifespan[1]. Members of this transcription factor family share a conserved 110-residue DNA binding domain first discovered in theDrosophilaFork Head protein, which therefore and because of structural reasons is referred to as the forkhead or winged helix domain. The family is divided into subclasses labeled with the letters A to S, and this categorization is based on amino acid sequence similarity in the forkhead domain[2]. For most Fox proteins, knowledge is scarce about how they are interfaced with upstream signaling pathways. A well characterized group is the FoxO subfamily which, among input from other pathways, is regulated by the insulin signaling LY-900009 module in a way that is conserved between the nematodeCaenorhabditis elegansand humans. This involves direct phosphorylation by insulin-induced kinases, binding to 14-3-3 proteins and nucleocytoplasmic LY-900009 shuttling[3]. The insulin-forkhead connection was first described inC. elegans, where mutations in the insulin receptor genedaf-2are completely suppressed by mutations in the FoxO transcription factor genedaf-16[4],[5]. Members of the FoxA subfamily are also important players in metabolism and regulated by insulin, but whether LY-900009 the exact mechanism also involves nuclear exclusion is still a matter of debate and less clear than for FoxO proteins[6]. Once again, pioneering research inC. elegansstarted to uncover a link between a growth control pathway and a forkhead transcription factor, this time between the worm Target of rapamycin (TOR) homolog LET-363 and the FoxA protein PHA-4. The first study in this line of evidence described the longevity of worms with reduced levels of TOR signaling. In contrast to thedaf-16-dependent lifespan increase of insulin receptor mutant worms, long life conferred by low levels of TOR is not affected bydaf-16mutations[7]. This suggested that insulin and TOR signaling regulate lifespan through distinct downstream transcriptional regulators. Another condition which, similar to reduced TOR signaling, can prolong life in nematodes in a DAF-16-independent fashion is dietary restriction. LET-363 is involved in the dietary restriction response induced by mutations in eithereat-2which lead to reduced food intake by impaired pharyngeal pumping[8], or inpep-2mutants which display compromised intestinal uptake of dietary peptides[9]. PHA-4 was recently identified as the forkhead transcription factor which is necessary to increase lifespan under multiple conditions of dietary restriction, such as lowering the concentration of bacteria fed to the worms in culture oreat-2mutations, but not under conditions of lowered insulin signaling[10], making it a candidate for a transcriptional effector downstream of TOR signaling. This working hypothesis was confirmed by the finding that PHA4 is required for the lifespan extension elicited by reduced LET-363/TOR or RSKS-1/S6 kinase levels, both of which are independent LY-900009 of DAF-16 function[11]. These observations prompted us to investigate a possible link between TOR signaling and the transcription.