Contribution of Growth and Cell Cycle Checkpoints to Radiation Survival in Drosophila

Abstract

Cell cycle checkpoints contribute to survival after exposure to ionizing radiation (IR) by arresting the cell cycle and permitting repair. As such, yeast and mammalian cells lacking checkpoints are more sensitive to killing by IR. We reported previously that Drosophila larvae mutant for grp (encoding a homolog of Chk1) survive IR as well as wild type despite being deficient in cell cycle checkpoints. This discrepancy could be due to differences either among species or between unicellular and multicellular systems. Here, we provide evidence that Grapes is needed for survival of Drosophila S2 cells after exposure to similar doses of IR, suggesting that multicellular organisms may utilize checkpoint-independent mechanisms to survive irradiation. The dispensability of checkpoints in multicellular organisms could be due to replacement of damaged cells by regeneration through increased nutritional uptake and compensatory proliferation. In support of this idea, we find that inhibition of nutritional uptake (by starvation or onset of pupariation) or inhibition of growth factor signaling and downstream targets (by mutations in cdk4, chico, or dmyc) reduced the radiation survival of larvae. Further, some of these treatments are more detrimental for grp mutants, suggesting that the need for compensatory proliferation is greater for checkpoint mutants. The difference in survival of grp and wild-type larvae allowed us to screen for small molecules that act as genotype-specific radiation sensitizers in a multicellular context. A pilot screen of a small molecule library from the National Cancer Institute yielded known and approved radio-sensitizing anticancer drugs. Since radiation is a common treatment option for human cancers, we propose that Drosophila may be used as an in vivo screening tool for genotype-specific drugs that enhance the effect of radiation therapy.

Induction of apoptosis by Drosophila Myc

“…A similar sensitivity to X-irradiation has recently been reported for growth impaired flies in general, i.e. animals that were either nutritionally starved or mutant for dmyc, chico, or cdk4 (Jaklevic et al, 2006). After irradiation with 40 Gy of X-rays such animals show similar levels of apoptosis as wild type larvae (consistent with our observations for dmyc mutant larvae irradiated with 50 Gy).…”

“…However, wild type animals can compensate for the dead cells by a process called compensatory proliferation, whereby dying cells emit growth factors (such as Wg and Dpp) that stimulate proliferation of the surrounding intact cells (reviewed in Gallant, 2005). These proliferation signals are likely to be present in the irradiated growth impaired larvae as well, but the recipients of these signals cannot proliferate fast enough to compensate for the lost cells, resulting in increased organismal lethality (Jaklevic et al, 2006). A similar defect in compensatory proliferation might also be responsible for the lethality we observe in our experiments.…”

HorkaD, a Chromosome Instability-Causing Mutation in Drosophila, Is a Dominant-Negative Allele of lodestar

“…For example, only $20% of embryos hatch from eggs of females that are both homozygous for the strong mnk mutant alleles and lack Chk2 function (Xu et al 2001;Masrouha et al 2003;Takada et al 2003;Xu and Du 2003;Brodsky et al 2004). Similarly, the grp homozygous females, which lack checkpoint kinase 1, are sterile; their embryos suffer from abnormal cortical nuclear divisions and do not cellularize (Yu et al 2000;Jaklevic et al 2006;Takada et al 2007). Females homozygous for the Ataxia telangiectasia-related mei-41 strong mutant alleles are, in effect, sterile (Laurencon et al 2003;Larocque et al 2007).…”

Astrocyte elevated gene-1 in relation to colorectal cancer development and radiotherapy response

“…However, despite the advantages of the model, there are several disadvantages: nude mice husbandry is very expensive and requires specialised facilities, the process from implantation to endpoint measurement can take several months, and it is very difficult to image single cells (275). To overcome those shortcomings various attempts have been made to develop non-mammalian animal models (276)(277)(278). A promising model is the Zebrafish (Danio rerio).…”