ABSTRACT
Members of the Myc family of proto-oncogenes have long been implicated in regulating proliferation, apoptosis and oncogenesis. Recently, transcriptional and biological studies have suggested a direct role for Myc in regulating growth. We have used dm(4), a new null allele of the Drosophila diminutive (dm) gene, which encodes dMyc on the X chromosome, to investigate a role for dMyc in larval endoreplicating tissues, where cellular growth and DNA replication occur in the absence of cell division. Hemizygous dm(4)/Y mutants arrest as second instar larvae, and fat body nuclei of dm(4)/Y mutants fail to attain normal size and normal levels of DNA, resulting from a reduced frequency of S-phase. Thus, dMyc is required for endoreplication and larval growth. In support of this, dMyc, as well as its antagonist dMnt, are expressed in larval tissues in a pattern consistent with their involvement in regulating endoreplication. Overexpression of dMyc in endoreplicating cells results in dramatic increases in nuclear DNA content and cell and nucleolar size, whereas dMnt overexpression has the opposite effect. BrdU incorporation and Cyclin E protein levels continue to oscillate in dMyc-overexpressing cells, indicating that the normal cell cycle control mechanisms are not disrupted. dMyc driven growth and endoreplication are strongly attenuated when the endocycle is blocked with Cyclin E or the cdk inhibitor p21. By contrast, the ability of dMyc to promote growth and endoreplication is only partly reduced when PI3K activity is blocked, suggesting that they influence distinct growth pathways. Our results indicate that larval growth and endoreplication are coupled processes that, although linked to cell cycle control mechanisms, are regulated by dMyc and dMnt.
Subject(s)
DNA-Binding Proteins/genetics , Drosophila Proteins/genetics , Drosophila/embryology , Repressor Proteins/genetics , Transcription Factors/genetics , Animals , Base Sequence , Cell Division , Chromosome Mapping , DNA Primers , DNA Replication/genetics , DNA-Binding Proteins/metabolism , Drosophila/growth & development , Drosophila Proteins/metabolism , Gene Expression Regulation, Developmental/genetics , Larva/growth & development , Repressor Proteins/metabolism , Reverse Transcriptase Polymerase Chain Reaction , Transcription Factors/metabolism , X ChromosomeABSTRACT
Studies in Drosophila have characterized insulin receptor/phosphoinositide 3-kinase (Inr/PI3K) signaling as a potent regulator of cell growth, but its function during development has remained uncertain. Here we show that inhibiting Inr/PI3K signaling phenocopies the cellular and organismal effects of starvation, whereas activating this pathway bypasses the nutritional requirement for cell growth, causing starvation sensitivity at the organismal level. Consistent with these findings, studies using a pleckstrin homology domain-green fluorescent protein (PH-GFP) fusion as an indicator for PI3K activity show that PI3K is regulated by the availability of dietary protein in vivo. Hence we surmise that an essential function of insulin/PI3K signaling in Drosophila is to coordinate cellular metabolism with nutritional conditions.
