Hypoxia is a major regulator of angiogenesis and plays a central role in the pathophysiology of common clinical conditions such as ischemic heart disease, cancer, and stroke. Under conditions of oxygen deprivation, animal cells have the capacity to alter their gene expression pattern and restore homeostasis through the so-called transcriptional response to hypoxia. In mammals, this transcriptional response is mediated by the hypoxia-inducible factor (HIF), an α/β heterodimer composed of two basic helix-loop-helix-PAS (bHLH-PAS) proteins. The β-subunit is constitutive and the α-subunit is oxygen-regulated by several mechanisms that include conditional recruitment of transcriptional co-activators, control of subcellular localization, and regulation of protein stability. Our previous work led to the definition of a hypoxia-responsive system in Drosophila that is homologous to mammalian HIF. We identified the bHLH-PAS proteins Sima and Tango as, respectively, the Drosophila HIF–α- and –β-subunits and found that Sima protein stability is regulated by an oxygen-dependent prolyl-4-hydroxylase enzyme, which we named Fatiga.
One major goal in our laboratory is to define novel cellular components of the machinery that mediates the transcriptional response to hypoxia. To this end, we have conducted a genome-wide RNAi-based screen in Drosophila cells in culture; we found 35 genes that participate in the hypoxic response. Nine of these genes are previously known elements of the hypoxia responsive pathway, whereas 31 genes are novel HIF regulators. Importantly, these novel regulators belong to just a few protein complexes or signal transduction pathways, including elements of the RISC complex, translational regulators from the eIF3 complex, and components of chromatin remodelling complexes. By combining genetic studies in vivo with biochemical and molecular approaches in cell culture, we intend to define the molecular function of these novel genes in the transcriptional response to hypoxia.
Another line of research in the laboratory focuses on the role of Fatiga and Sima in body growth regulation and cell size control. In this context, we have shown that Sima-dependent transcription is controlled by the insulin/PI3K and TOR pathways through a mechanism that involves upregulation of Sima protein levels and increased nuclear localization. Given that the PI3K and TOR pathways are well known regulators of cell growth, we are currently analyzing in detail the molecular participation of Sima in growth control, as well as the cross-talk of the insulin/TOR pathways with the hypoxia-responsive machinery.We are also characterizing novel genes that mediate oxygen-dependent regulation of cell growth.
Last updated September 2010
