Hypoxia-inducible factors (HIFs) are key elements for controlling immune cell metabolism and functions. While HIFs are known to be involved in T cells and macrophages activation, their functions in B lymphocytes are poorly defined. Here, we show that hypoxia-inducible factor-1α (HIF-1α) contributes to IL-10 production by B cells. HIF-1α regulates IL-10 expression, and HIF-1α-dependent glycolysis facilitates CD1dhiCD5+ B cells expansion. Mice with B cell-specific deletion of Hif1a have reduced number of IL-10-producing B cells, which result in exacerbated collagen-induced arthritis and experimental autoimmune encephalomyelitis. Wild-type CD1dhiCD5+ B cells, but not Hif1a-deficient CD1dhiCD5+ B cells, protect recipient mice from autoimmune disease, while the protective function of Hif1a-deficient CD1dhiCD5+ B cells is restored when their defective IL-10 expression is genetically corrected. Taken together, this study demonstrates the key function of the hypoxia-associated transcription factor HIF-1α in driving IL-10 expression in CD1dhiCD5+ B cells, and in controlling their protective activity in autoimmune disease.

Hypoxia-inducible protein 2 (HIG2) has been implicated in canonical Wnt signaling, both as target and activator. The potential link between hypoxia and an oncogenic signaling pathway might play a pivotal role in renal clear-cell carcinoma characterized by constitutive activation of hypoxia-inducible factors (HIFs), and hence prompted us to analyze HIG2 regulation and function in detail. HIG2 was up-regulated by hypoxia and HIF inducers in all cell types and mouse organs investigated and abundantly expressed in renal clear-cell carcinomas. Promoter analyses, gel shifts, and siRNA studies revealed that HIG2 is a direct and specific target of HIF-1, but not responsive to HIF-2. Surprisingly, HIG2 was not secreted, and HIG2 overexpression neither stimulated proliferation nor activated Wnt signaling. Instead, we show that HIG2 decorates the hemimembrane of lipid droplets, whose number and size increase on hypoxic inhibition of fatty acid β-oxidation, and colocalizes with the lipid droplet proteins adipophilin and TIP47. Normoxic overexpression of HIG2 was sufficient to increase neutral lipid deposition in HeLa cells and stimulated cytokine expression. HIG2 could be detected in atherosclerotic arteries and fatty liver disease, suggesting that this ubiquitously inducible HIF-1 target gene may play an important functional role in diseases associated with pathological lipid accumulation.

A key adaptation to environmental hypoxia is an increase in erythropoiesis, driven by the hormone erythropoietin (EPO) through what is traditionally thought to be primarily a renal response. However, both neurons and astrocytes (the largest subpopulation of glial cells in the CNS) also express EPO following ischemic injury, and this response is known to ameliorate damage to the brain. To investigate the role of glial cells as a component of the systemic response to hypoxia, we created astrocyte-specific deletions of the murine genes encoding the hypoxia-inducible transcription factors HIF-1α and HIF-2α and their negative regulator von Hippel-Lindau (VHL) as well as astrocyte-specific deletion of the HIF target gene Vegf. We found that loss of the hypoxic response in astrocytes does not cause anemia in mice but is necessary for approximately 50% of the acute erythropoietic response to hypoxic stress. In accord with this, erythroid progenitor cells and reticulocytes were substantially reduced in number in mice lacking HIF function in astrocytes following hypoxic stress. Thus, we have demonstrated that the glial component of the CNS is an essential component of hypoxia-induced erythropoiesis. IntroductionThe erythropoietin (EPO) hormone is the chief regulator of red blood cell production in mammals, and its rate of synthesis is highly responsive to changes in physiologic oxygenation (1). Studies of this relationship between oxygen and red blood cell production date back to Viault's work at the end of the nineteenth century (2). Dissection of the mechanisms for induction of the Epo gene by low-oxygen conditions led to the discovery of a hypoxia responsive element (HRE) in the 3′ enhancer region of the gene; a binding site within the Epo gene was found for a family of transcription factors, which were subsequently termed hypoxia-inducible factors (HIFs) (3).HIF is a heterodimeric DNA-binding complex composed of 2 basic helix-loop-helix proteins of the PAS family: the constitutive non-oxygen-responsive subunit HIF-1β (also termed ARNT) and 1 of either hypoxia-inducible α-subunits, HIF-1α or HIF-2α (reviewed in refs. 4, 5). HIF-α subunits are rapidly degraded in normoxia but highly inducible by hypoxia. The interface between oxygen and the HIF-α subunits in normoxia is in part the hydroxylation of 2 prolyl residues in the oxygen-dependent degradation domain of the α subunits. HIF-α hydroxylation under normoxia regulates the interaction with the von Hippel-Lindau tumor suppressor protein (pVHL), which targets HIF-α for proteolysis by the ubiquitin-proteasome pathway (6).EPO synthesis is only detectable in a small number of tissues, including the kidney, liver, and the CNS (reviewed in refs. 7, 8). The kidney produces a large fraction of the circulating EPO in adult mammals (9), and it is clear that nonrenal EPO does not compensate for the loss of renal EPO production in patients with chronic

Mitochondrial stress induces PARL-mediated cleavage and cytosolic release of the mitochondrial phosphatase Pgam5. In the cytosol, Pgam5 interacts with the Wnt pathway component axin and dephosphorylates axin-bound β-catenin, thereby cell-intrinsically activating Wnt/β-catenin signaling to induce mitochondrial biogenesis.