Mammalian cells respond to low 02 levels (hypoxia; P02=3-30 mm Hg) by activating adaptive responses that help to restore 02 supply and prevent hypoxic injury. Controversy exists regarding the identity of the cellular 02 sensor triggering these responses. Our studies have revealed that mitochondria act as 02 sensors by increasing their release of reactive oxygen species (ROS) during hypoxia. These ROS function as early signals in the pathway linking the 02 sensor to the downstream responses, which include the activation of the Hypoxia-Inducible Factor-1 (HIF-1). HIF-1 activation triggers the increased expression of glycolytic enzymes, glucose transporters, and other genes during hypoxia. This application proposes to test the hypothesis that mitochondria act as the 02 sensor responsible for activation of HIF-1 during hypoxia, by releasing ROS. Studies using pharmacological tools in the previous funding period implicated mitochondrial Complex Ill as the site of increased ROS production. To test this more definitively, in Aim 1 we will generate a targeted knockout of the Rieske iron-sulfur protein (RISP), a nuclear-encoded gene that is required for the generation of ROS at Complex III. In murine embryonic stem cells lacking this gene, we predict that hypoxic stabilization of HIF-1 will be lost, while responses to anoxia, cobalt and exogenous H202 will be retained. Recent studies indicate that the small GTPase racl is required for HIF-1 alpha stabilization during hypoxia. Rac1 may act by amplifying the mitochondrial ROS signal generated during hypoxia by engaging additional oxidase systems, by amplifying mitochondrial ROS generation, or by triggering the relocation of mitochondria toward the nucleus. Aim 2 will determine whether mitochondrial ROS signals activate rac- 1 during hypoxia, and whether rac 1 then promotes the stabilization of HIF-1 alpha by amplifying the ROS signal. We hypothesize that the activity of prolyl hydroxylase, the enzyme that targets the HIF-l alpha subunit for proteasomal degradation during normoxia, is not intrinsically an 02 sensor but rather is regulated by signals initiated by the mitochondria. Aim 3 will clarify the role of increased mitochondrial ROS signaling in the stabilization of HIF-1 alpha during hypoxia by studying the regulation of prolyl hydroxylase activity by mitochondrial ROS signals. This will be tested in cultured cells and in an in vitro system where hydroxylation of a recombinant protein is assessed. Collectively, these studies will provide a more conclusive test of the hypothesis that mitochondrial ROS are the site of 02 sensing responsible for HIF-1 activation and gene expression during hypoxia.