Insulin delays the progression of Drosophila cells through G2/M by activating the dTOR/dRaptor complex

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Abstract
In Drosophila and mammals, insulin signalling can increase growth, progression through G1/S, cell size and tissue size. Here, we analyse the way insulin affects cell size and cell‐cycle progression in two haemocyte‐derived Drosophila cell lines. Surprisingly, we find that although insulin increases cell size, it slows the rate at which these cells increase in number. By using BrdU pulse‐chase to label S‐phase cells and follow their progression through the cell cycle, we show that insulin delays progression through G2/M, thereby slowing cell division. The ability of insulin to slow progression through G2/M is independent of its ability to stimulate progression through G1/S, so is not a consequence of feedback by the cell‐cycle machinery to maintain cell‐cycle length. Insulin's effects on progression through G2/M are mediated by dTOR/dRaptor signalling. Partially inhibiting dTOR/dRaptor signalling by dsRNAi or mild rapamycin treatment can increase cell number in cultured haemocytes and the Drosophila wing, respectively. Thus, insulin signalling can influence cell number depending on a balance between its ability to accelerate progression through G1/S and delay progression through G2/M.