Nutrition, Growth, and Cell Cycle Control in Drosophila

Bruce A. Edgar, Leslie J. Saucedo, Jessica Britton, Ling Li, Tania Reis, Huaqi Jiang, Dayna Hall, Savraj Grewal, and Aida Flor A. de la Cruz. Division of Basic Sciences, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Seattle, WA 98109, USA

Cell growth in the differentiated, polyploid tissues of Drosophila larvae is tightly coupled to the availability of dietary protein (1). Previously, we reported that activating Inr/PI3K signaling in starved animals cell-autonomously bypasses the nutritional requirement for cell growth and showed, using a pleckstrin homology (PH)-green fluorescent protein (GFP), that starvation suppresses PI3K activity in vivo (2). These and recent observations from other labs (3, 4) indicate that an essential function of insulin/PI3K signaling is to coordinate cellular metabolism with the availability of dietary protein. In a genetic screen for genes capable of driving growth we identified Rheb, a member of the Ras superfamily of GTP-binding proteins that functions in the insulin signaling pathway downstream of tuberous sclerosis complex (TSC1/2) and upstream of the target of rapamycin (TOR) gene (5). Like the insulin receptor (InR) or PI3K, Rheb is required for cell growth and, when overexpressed, it can drive cell growth and DNA replication in starved larvae. TSC2 acts as a Rheb-specific GTPase activating protein (GAP)(6, 7), suggesting that the TSC1/2 complex suppresses growth by inhibiting growth promotion by Rheb. We have suggested that Rheb may stimulate cell growth and activate TOR by stimulating nutrient import, and will present data pertaining to this issue.

A model for growth-dependent endocycle progression will also be presented, along with supporting data. In this model endocycles run using an oscillator that employs both positive and negative feedback between the transcription factor, E2F1/Dp, and the cell cycle regulatory kinase Cyclin E/Cdk2, which triggers S-phase initiation. This oscillator produces alternating pulses of E2F activity and cycE transcription, and the resulting pulses of CycE protein expression time the alternation of G and S phases. In this model the frequency of oscillations is determined by rates of CycE protein synthesis and degradation. One or both of these parameters is controlled, in turn, by rates of cell growth (8). The model predicts that cell cycle progression will cease whenever cell growth rates lower the net rate of CycE protein accumulation below what is required for S-phase initiation. This is what we imagine happens when starvation attenuates insulin signaling, arresting cell growth. In situations of enhanced growth, the endocycle oscillator can run faster, yielding increased ploidy without marked increases in levels of cell cycle gene transcription. This is what we imagine happens when the insulin/PI3K/Tsc/Rheb/Tor pathway is activated in response to nutrition or experimental manipulation. Data will also be presented that relates these findings to growth-dependent cell cycle regulation in mitotically proliferating cells in the Drosophila wing and eye imaginal discs.

1. Britton, J.S. and Edgar, B.A.  (1998) Environmental control of the cell cycle in Drosophila: nutrition activates mitotic and endoreplicative cells by distinct mechanisms.  Development 125, 2149-2158.
2. Britton, J.S., Lockwood, W.B., Cohen, S.M., and Edgar, B.A.  (2002) Drosophila’s Insulin/PI3-Kinase Pathway Coordinates Cellular Metabolism with Nutritional Conditions. Developmental Cell 2, 239–249.
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5. Saucedo, L.J., Gao, X., Chiarelli, D.C., Li, L., Pan, D., and Edgar, B.A. (2003) The small GTPase, Rheb, promotes cell growth and is a component of the insulin/TOR signaling network. Nature Cell Biology 5, 566-571.
6. Zhang, Y., Gao, X., Ru, B., Edgar, B.A., and Pan, D. (2003) Small GTPase Rheb is a direct target of the tuberous sclerosis tumor suppressor proteins. Nature Cell Biology 5, 578-581.
7. Garami A, Zwartkruis FJ, Nobukuni T, Joaquin M, Roccio M, Stocker H, Kozma SC, Hafen E, Bos JL, Thomas G. (2003). Insulin activation of Rheb, a mediator of mTOR/S6K/4E-BP signaling, is inhibited by TSC1 and 2. Mol Cell. 11:1457-66.
8. Prober, D.A. and Edgar, B.A. (2000) Ras promotes cellular growth in the Drosophila wing. Cell 100, 435-446.