Differential HIF and NOS responses to acute anemia: defining organ-specific hemoglobin thresholds for tissue hypoxia.
Tissue hypoxia likely contributes to anemia-induced organ injury and mortality. Severe anemia activates hypoxia-inducible factor (HIF) signaling by hypoxic- and neuronal nitric oxide (NO) synthase- (nNOS) dependent mechanisms. However, organ-specific hemoglobin (Hb) thresholds for increased HIF expression have not been defined. To assess organ-specific Hb thresholds for tissue hypoxia, HIF-α (oxygen-dependent degradation domain, ODD) luciferase mice were hemodiluted to mild, moderate, or severe anemia corresponding to Hb levels of 90, 70, and 50 g/l, respectively. HIF luciferase reporter activity, HIF protein, and HIF-dependent RNA levels were assessed. In the brain, HIF-1α was paradoxically decreased at mild anemia, returned to baseline at moderate anemia, and then increased at severe anemia. Brain HIF-2α remained unchanged at all Hb levels. Both kidney HIF-1α and HIF-2α increased earlier (Hb ∼70-90 g/l) in response to anemia. Liver also exhibited an early HIF-α response. Carotid blood flow was increased early (Hb ∼70, g/l), but renal blood flow remained relatively constant, only increased at Hb of 50 g/l. Anemia increased nNOS (brain and kidney) and endothelia NOS (eNOS) (kidney) levels. Whereas anemia-induced increases in brain HIFα were nNOS-dependent, our current data demonstrate that increased renal HIFα was nNOS independent. HIF-dependent RNA levels increased linearly (∼10-fold) in the brain. However, renal HIF-RNA responses (MCT4, EPO) increased exponentially (∼100-fold). Plasma EPO levels increased near Hb threshold of 90 g/l, suggesting that the EPO response is sensitive. Collectively, these observations suggest that each organ expresses a different threshold for cellular HIF/NOS hypoxia responses. This knowledge may help define the mechanism(s) by which the brain and kidney maintain oxygen homeostasis during anemia.
Department of Anesthesia, St. Michael's Hospital, University of Toronto, Ontario, Canada; Keenan Research Centre for Biomedical Science and Li Ka Shing Knowledge Institute of St. Michael's Hospital, Toronto, Ontario, Canada;
Albert K Y
AK
Tsui
Keenan Research Centre for Biomedical Science and Li Ka Shing Knowledge Institute of St. Michael's Hospital, Toronto, Ontario, Canada; Department of Medicine, Division of Nephrology, St. Michael's Hospital, University of Toronto, Toronto, Ontario, Canada;
Philip A
PA
Marsden
Department of Anesthesia, St. Michael's Hospital, University of Toronto, Ontario, Canada; Keenan Research Centre for Biomedical Science and Li Ka Shing Knowledge Institute of St. Michael's Hospital, Toronto, Ontario, Canada; Department of Physiology, University of Toronto, Toronto, Ontario, Canada;
C David
CD
Mazer
Department of Medical Biophysics, University of Toronto, Mouse Imaging Centre, Hospital for Sick Children, Toronto, Ontario, Canada.
John G
JG
Sled
Department of Anesthesia, St. Michael's Hospital, University of Toronto, Ontario, Canada;
Keith M
KM
Lee
Department of Medical Biophysics, University of Toronto, Mouse Imaging Centre, Hospital for Sick Children, Toronto, Ontario, Canada.
R Mark
RM
Henkelman
Department of Medical Biophysics, University of Toronto, Mouse Imaging Centre, Hospital for Sick Children, Toronto, Ontario, Canada.
Lindsay S
LS
Cahill
Department of Medical Biophysics, University of Toronto, Mouse Imaging Centre, Hospital for Sick Children, Toronto, Ontario, Canada.
Yu-Qing
YQ
Zhou
Department of Anesthesia, St. Michael's Hospital, University of Toronto, Ontario, Canada;
Neville
N
Chan
Department of Anesthesia, St. Michael's Hospital, University of Toronto, Ontario, Canada; Keenan Research Centre for Biomedical Science and Li Ka Shing Knowledge Institute of St. Michael's Hospital, Toronto, Ontario, Canada;
Elaine
E
Liu
Department of Anesthesia, St. Michael's Hospital, University of Toronto, Ontario, Canada; Keenan Research Centre for Biomedical Science and Li Ka Shing Knowledge Institute of St. Michael's Hospital, Toronto, Ontario, Canada; Department of Physiology, University of Toronto, Toronto, Ontario, Canada; hareg@smh.ca.
Gregory M T
GM
Hare
D000208:Acute Disease; D000740:Anemia / Q000097:blood / Q000201:enzymology* / Q000209:etiology / Q000235:genetics / Q000503:physiopathology; D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins / Q000378:metabolism*; D015415:Biomarkers / Q000097:blood; D001921:Brain / Q000098:blood supply / Q000201:enzymology; D002560:Cerebrovascular Circulation; D004195:Disease Models, Animal; D004921:Erythropoietin / Q000378:metabolism; D006438:Hemodilution; D006439:Hemodynamics; D006454:Hemoglobins / Q000378:metabolism*; D006801:Humans; D000860:Hypoxia / Q000097:blood / Q000201:enzymology* / Q000209:etiology / Q000235:genetics / Q000503:physiopathology; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000235:genetics / Q000378:metabolism*; D007668:Kidney / Q000098:blood supply / Q000201:enzymology; D008099:Liver / Q000098:blood supply / Q000201:enzymology; D049409:Luciferases, Firefly / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D027501:Monocarboxylic Acid Transporters / Q000378:metabolism; D009124:Muscle Proteins / Q000378:metabolism; D052248:Nitric Oxide Synthase Type I / Q000378:metabolism*; D052250:Nitric Oxide Synthase Type III / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012079:Renal Circulation; D012720:Severity of Illness Index; D000099162:Endothelial PAS Domain-Containing Protein 1
D051792:Basic Helix-Loop-Helix Proteins; D015415:Biomarkers; D004921:Erythropoietin; D006454:Hemoglobins; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D049409:Luciferases, Firefly; D027501:Monocarboxylic Acid Transporters; D009124:Muscle Proteins; D052248:Nitric Oxide Synthase Type I; D052250:Nitric Oxide Synthase Type III; D011993:Recombinant Fusion Proteins; D000099162:Endothelial PAS Domain-Containing Protein 1; C497442:HIF1A protein, human; C498648:Slc16a4 protein, mouse; C496321:Nos1 protein, mouse; C496315:Nos3 protein, mouse
anemia; hemoglobin threshold; hypoxia inducible factor; nitric oxide synthase