TI - dFOXO controls the reduction in cell number in body-size mutants . AB - Genetic analysis of the control of body size in Drosophila has revealed two classes of mutations . Flies carrying mutations in chico or viable allelic combinations of DInr , Dp110 and dPKB are reduced in body size by up to 50% owing to a reduction in both cell size and cell number . Conversely , flies mutant for dS6K exhibit a more moderate reduction in body size , caused almost exclusively by a reduction in cell size [36] . This suggests that the pathways controlling cell number and cell size bifurcate at or below dPKB . Although dFOXO single mutants have no obvious size phenotype , loss of dFOXO substantially suppresses the cell -number reduction observed in insulin-signaling mutants . It appears that dFOXO mediates the repression of proliferation in flies mutant for DInr , chico , Dp110 and dPKB without being required for the reduction in cell size . Chico-dFOXO double mutant flies even have slightly smaller cells than chico mutants , suggesting that removal of dFOXO permits cell -cycle acceleration under conditions of impaired insulin signaling . The pathway controlling body size in response to insulin therefore bifurcates at the level of dPKB : dPKB controls cell number by inhibiting dFOXO function and dPKB controls cell size , atleast under some conditions , by regulating S6K activity by PHOSphorylation of dTSC2 [29] . The signaling systems controlling cell size and cell number are tightly interconnected . Genetic and biochemical analyses have revealed five different links between the dTSC-dTOR-dS6K pathway and the DInr-dPKB-dFOXO pathway . First , under conditions of unnaturally high insulin-signaling activity ( that is , following the oncogenic activation of dPKB ) dPKB PHOSphorylates and inactivates dTSC2 , resulting in increased activation of dS6K [29] . Under normal culture conditions this regulation does not seem critical , however , loss of dPKB function does not lower dS6K activity in larval extracts [54] . Second , under physiological conditions , dPDK1 regulates dPKB as well as dS6K [63] . Third , dS6K itself downregulates dPKB activity in a negative feedback loop [31] . Fourth , under severe starvation conditions , nuclear dFOXO presumably activates target genes that reduce cell proliferation . One of these target genes is d4E-BP , which encodes an inhibitor of translation initiation . When conditions improve , the insulin and TOR signaling pathways can stimulate translation by disrupting the 4E-BP/eIF4E complex via PHOSphorylation of 4E-BP , and in parallel by repressing FOXO -dependent 4E-BP expression . Fifth , under even more severe starvation or stress conditions , full activation of dFOXO upregulates expression of the insulin receptor itself , thus rendering the cell hypersensitive to low insulin levels (see [52]) . These multiple positive and negative interactions ensure a continuous fine adjustment of the growth rate to changing environmental conditions .