Hypoxia induces major effects on cell cycle kinetics and protein expression in Drosophila melanogaster embryos

Hypoxia induces a stereotypic response in Drosophila melanogaster embryos: depending on the time of hypoxia, embryos arrest cell cycle activity either at metaphase or just before S phase. To understand the mechanisms underlying hypoxia-induced arrest, two kinds of experiments were conducted. First, embryos carrying a kinesin-green fluorescent protein construct, which permits in vivo confocal microscopic visualization of the cell cycle, showed a dose-response relation between O-2 level and cell cycle length. For example, mild hypoxia (PO2 similar to55 Torr) had no apparent effect on cell cycle length, whereas severe hypoxia (PO2 similar to25-35 Torr) or anoxia (PO2 = 0 Torr) arrested the cell cycle. Second, we utilized Drosophila embryos carrying a heat shock promoter driving the string (cdc25) gene (HS-STG3), which permits synchronization of embryos before the start of mitosis. Under conditions of anoxia, we induced a stabilization or an increase in the expression of several G1/S (e.g., dE2F1, RBF2) and G2/M (e.g., cyclin A, cyclin B, dWee1) proteins. This study suggests that, in fruit fly embryos, 1) there is a dose-dependent relationship between cell cycle length and O-2 levels in fruit fly embryos, and 2) stabilized cyclin A and E2F1 are likely to be the mediators of hypoxia-induced arrest at metaphase and pre-S phase.