Credit: NPG

E2F transcription factors (E2F–DP (differentiation-related polypeptide) heterodimers) have a crucial role in the induction of apoptosis following DNA damage in Drosophila melanogaster by directly regulating the transcription of key apoptotic genes. However, Dp-mutant D. melanogaster cells, in which E2f1 is functionally inactivated, fail to undergo cell death in response to irradiation despite apoptotic gene induction. This study shows that E2f1 also regulates mitochondrial function that is required for cell death, in addition to its role in pro-apoptotic gene expression.

in D. melanogaster and mammalian cells, mitochondria-associated genes are directly regulated by E2F1–DP

Gene expression analysis of larval eye discs from wild-type and Dp-mutant D. melanogaster showed that the Dp-mutant cells induce the same set of apoptotic genes as wild-type cells in response to irradiation, and the level of expression of apoptotic genes was the same or higher in irradiated Dp-mutant cells. So, the failure of Dp-mutant cells to undergo apoptosis is not the result of a failure to induce pro-apoptotic gene transcription.

Of the genes that were downregulated in Dp-mutant but not wild-type eye discs after irradiation, there was a marked enrichment of mitochondrial genes associated with metabolic and oxidative phosphorylation pathways. Dp and E2f1 were shown to bind directly to the promoters of many of these D. melanogaster genes, and eight human homologues of these genes also bound E2F1 in a DP-dependent manner. So, in D. melanogaster and mammalian cells, mitochondria-associated genes are directly regulated by E2F1–DP.

The authors next showed that DP mutation leads to mitochondrial defects in both D. melanogaster and human cells. Mitochondrial membrane potential was decreased in Dp-mutant compared with wild-type D. melanogaster eye discs. Also, the mitochondria of Dp-mutant D. melanogaster cells, and of human cells in which E2F1 was inactivated, were more globular and swollen. The downregulation of mitochondria-associated direct gene targets of E2f1–Dp was sufficient to partially phenocopy these mitochondrial defects and to mediate protection from irradiation-induced apoptosis, with the severity of the defects correlating well with the degree of protection.

The results show that E2F transcription factors regulate mitochondrial function in D. melanogaster and human cells. The mechanism by which mitochondrial dysfunction provides protection against irradiation-induced apoptosis remains to be determined.