E2F Transcription Factors are Critical Regulators of the Cell Cycle, Apoptosis and Mitochondrial Function

The retinoblastoma (pRB) family of tumor suppressor proteins and the downstream E2F transcription factors that they regulate, play an instrumental role in both cell proliferation, and apoptosis. The importance of the Rb-E2F pathway is underscored by the obligatory inactivation of pRB control in most cancers. While the large number of E2F proteins presents an obstacle to genetic analysis in mammals, the highly conserved Rb-E2F families in Drosophila consist of only two E2F members, an activator, dE2f1, and a repressor, dE2f2. Previous studies have shown that proliferation of de2f1 mutant cells is severely reduced due to the unchecked activity of dE2f2. To better understand the mechanistic basis of this cell proliferation, our lab undertook a screen to identify suppressor genes, which when mutated, rescue the reduced cell proliferation of de2f1 mutants. In addition to identifying suppressors that compromise dE2f2-mediated repression, the screen also uncovered suppressors that leave this repression intact. One of the suppressors I investigated in greater detail was the gene belle which encodes an RNA helicase. Mutation of belle leads to a down-regulation of the p21/p27 Cyclin-Dependent Kinase Inhibitor homolog, Dacapo, and a downregulation in Hedgehog pathway activity, which is important for rescuing proliferation of de2f1 mutant cells. In addition to investigating the role of E2F in cell proliferation, my work revealed that E2f1 directly regulates mitochondrial function by regulating the expression of mitochondria associated genes, a novel class of E2F targets. In vivo, the genetic ablation of E2F leads to severe mitochondrial defects, which are phenocopied by targeted downregulation of several of these E2F target genes. These defects are due to the loss of the normal dE2f/dDP-dependent regulation of mitochondria associated targets. Furthermore, I identified a context, the normal induction of apoptosis in response to irradiation, in which this previously unappreciated role of E2F in the regulation of mitochondrial function is critical. Importantly, the function of E2f in regulating mitochondria associated gene expression, as well as the functional significance in respect to DNA damage-induced apoptosis, is conserved in mammalian cells.