Oxygen homeostasis represents an essential organizing principle of metazoan evolution and biology. Hypoxia-inducible factor 1 (HIF-1) regulates transcription in response to changes in O2 concentration. HIF-1 is a heterodimeric transcription factor that consists of HIF-1alpha and HIF-1beta subunits. O2 -dependent degradation of the HIF-1alpha subunit is mediated by prolyl hydroxylase (PHD), the von Hippel-Lindau (VHL)/Elongin-C/Elongin-B E3 ubiquitin ligase, and the proteasome. Inhibitors of heat shock protein 90 (HSP90) dissociate HSP90 from HIF-1alpha and induce O2/PHD/VHL-independent degradation of HIF-1alpha. Recently, we reported the identification of receptor of activated protein C kinase (RACK1) as a novel HIF-1alpha interacting protein. RACK1 promotes the O2/PHD/VHL-independent and proteasome-dependent degradation of HIF-1alpha. RACK1 competes with HSP90 for binding to the PAS-A domain of HIF-1alpha. RACK1 activity is required for the mechanism of action for the HSP90 inhibitor 17-allylaminogeldanamycin to induce HIF-1alpha degradation. RACK1 binds to Elongin-C and recruits Elongin-B and other components of E3 ubiquitin ligase to HIF-1alpha. The ubiquitination and degradation of HIF-1alpha are promoted by RACK1. RACK1 is an essential component of an O2/PHD/VHL-independent system for regulating HIF-1alpha stability through competition with HSP90 and recruitment of the Elongin-C/B ubiquitin ligase complex. Here we discuss how this system may be regulated.
Oxygen homeostasis represents an important organizing principle for human development and physiology. The essential requirement for oxidative phosphorylation to generate ATP is balanced by the risk of oxidative damage to cellular lipids, nucleic acids, and proteins. As a result, cellular and systemic O2 concentrations are tightly regulated via shortand long-acting response pathways that affect the activity and expression of a multitude of cellular proteins (for review, see Semenza 1999a). This delicate balance is disrupted in heart disease, cancer, cerebrovascular disease, and chronic obstructive pulmonary disease, which represent the most common causes of mortality and account for two-thirds of all deaths in the U.S. (Greenlee 2000). Appreciation of the fundamental importance of oxygen homeostasis for development, physiology, and disease pathophysiology is growing but still incomplete. Knowledge acquisition is presently exponential when one includes areas, such as the role of angiogenesis in ischemic or neoplastic disease, in which investigators are studying oxygen homeostasis even though they may not interpret their studies within this broad physiological context. Vascular endothelial growth factor (VEGF) plays an essential role in angiogenesis (for review, see Ferrara and Davis-Smyth 1997; Ferrara 1999). The regulation of VEGF expression illustrates how reduced O2 availability (hypoxia) can elicit physiological responses via multiple molecular mechanisms. VEGF expression is induced when most cell types are subjected to hypoxia, thus providing a mechanism by which tissue perfusion can be optimized to demand. Steady state levels of VEGF mRNA increase in hypoxic cells as a result of increased production (transcriptional activation) and decreased destruction (mRNA stabilization). Whereas overall protein synthesis is inhibited in response to hypoxia, VEGF mRNA is efficiently translated into protein by use of an internal ribosome entry site (Stein et al. 1998). Finally, expression of the VEGF receptor FLT-1 is also induced when endothelial cells are exposed to hypoxia (Gerber et al. 1997). The essential first step in this process, transcriptional activation, is mediated by the binding of hypoxia-inducible factor 1 (HIF-1) to a cis-acting hypoxia-response element located 1 kb 5 to the transcriptional start site of the human VEGF gene (Forsythe et al. 1996). HIF-1 is a basic helix–loop–helix PAS protein consisting of HIF-1 and HIF-1 subunits (Wang and Semenza 1995; Wang et al. 1995). HIF-1 expression and HIF-1 transcriptional activity are precisely regulated by cellular O2 concentration (for review, see Semenza 1999b, 2000a; Wenger 2000). The molecular mechanisms of sensing and signal transduction by which changes in O2 concentration result in changes in HIF-1 activity are poorly understood, but recent data suggest that the O2 signal is converted to a redox signal (Chandel et al. 2000; Haddad et al. 2000) that may trigger a kinase cascade and/or regulate HIF-1 directly (for review, see Semenza 1999a,b; Chandel and Schumacker 2000). The regulation of HIF-1 activity occurs at multiple levels. Whereas HIF-1 mRNA is constitutively expressed in tissue culture cells, it is markedly induced by hypoxia or ischemia in vivo (Yu et al. 1998; Bergeron et al. 1999). HIF-1 protein expression is negatively regulated in nonhypoxic cells by ubiquitination and proteasomal degradation (Salceda and Caro 1997; Huang et al. 1998; Kallio et al. 1999). Under hypoxic conditions, HIF-1 protein levels increase dramatically and the fraction that is ubiquitinated decreases (Sutter et al. 2000). Nuclear localization of HIF-1 may also be induced by hypoxia (Kallio et al. 1998). The carboxy-terminal half of HIF-1 contains two transactivation domains that are also negatively regulated under nonhypoxic conditions (Jiang et al. 1997b; Pugh et al. 1997). The interaction of these domains with the coactivators CBP, p300, SRC-1, and TIF2 is regulated by the cellular O2 concentration and redox state (Kallio et al. 1998; Ema et al. 1999; Carrero et al. 2000). Finally, species–specific alternative splicing of human and mouse HIF-1 RNA has also been reported (Wenger et al. 1997; Iyer et al. 1998b; Gothie et al. 2000). Hypoxia results in the rapid accumulation of HIF-1 in the nucleus (Wang et al. 1995) where it dimerizes with HIF-1 and binds to the core DNA sequence 5 -RCGTG3 (Semenza 2000a), leading to the transcriptional activation of VEGF and several dozen other known target genes (Table 1). HIF-1 and HIF-1 expression are required for embryonic survival in mice (Kozak et al. 1997; Maltepe et al. 1997; Iyer et al. 1998a; Ryan et al. 1998; 1E-MAIL gsemenza@jhmi.edu; FAX (410) 955-0484.
Journal Article Hinf I polymorphism 3′ to the human β-globin gene detected by the polymerase chain reaction (PCR) Get access Gregg L. Semenza, Gregg L. Semenza Genetics Unit, Department of Pediatries, The Johns Hopkins University School of MedicineBaltimore, MD 21205, USA Search for other works by this author on: Oxford Academic PubMed Google Scholar Carol E. Dowling, Carol E. Dowling Genetics Unit, Department of Pediatries, The Johns Hopkins University School of MedicineBaltimore, MD 21205, USA Search for other works by this author on: Oxford Academic PubMed Google Scholar Haig H. Kazazian, Jr. Haig H. Kazazian, Jr. Genetics Unit, Department of Pediatries, The Johns Hopkins University School of MedicineBaltimore, MD 21205, USA Search for other works by this author on: Oxford Academic PubMed Google Scholar Nucleic Acids Research, Volume 17, Issue 6, 25 March 1989, Page 2376, https://doi.org/10.1093/nar/17.6.2376 Published: 25 March 1989
Tumors contain well-oxygenated (aerobic) and poorly oxygenated (hypoxic) regions, which were thought to utilize glucose for oxidative and glycolytic metabolism, respectively. In this issue of the JCI, Sonveaux et al. show that human cancer cells cultured under hypoxic conditions convert glucose to lactate and extrude it, whereas aerobic cancer cells take up lactate via monocarboxylate transporter 1 (MCT1) and utilize it for oxidative phosphorylation (see the related article beginning on page 3930). When MCT1 is inhibited, aerobic cancer cells take up glucose rather than lactate, and hypoxic cancer cells die due to glucose deprivation. Treatment of tumor-bearing mice with an inhibitor of MCT1 retarded tumor growth. MCT1 expression was detected exclusively in nonhypoxic regions of human cancer biopsy samples, and in combination, these data suggest that MCT1 inhibition holds potential as a novel cancer therapy.
Cancer cells feature altered glucose metabolism that allows their rapid growth. They consume large amounts of glucose to produce lactate, even in the presence of ample oxygen, which is known as the Warburg effect. Pyruvate kinase M2 (PKM2) contributes to the Warburg effect by previously unknown mechanisms. Hypoxia-inducible factor 1 (HIF-1) mediates PKM2 gene transcription and glucose reprogramming in cancer cells. The recent discovery of novel physical and functional interactions between PKM2 and HIF-1 in cancer cells has provided insight into molecular mechanisms underlying the Warburg effect.
