Oxygen sensing, homeostasis, and disease

Ubiquitin-specific protease 19 (USP19) regulates hypoxia-inducible factor 1α (HIF-1α) during hypoxia

A proper cellular adaptation to low oxygen levels is essential for processes such as development, growth, metabolism, and angiogenesis. The response to decrease in oxygen supply, referred to as hypoxia, is also involved in numerous human diseases including cancer, inflammatory conditions, and vascular disease. The hypoxia-inducible factor 1-α (HIF-1α), a key player in the hypoxic response, is kept under stringent regulation. At normoxia, the levels are kept low as a consequence of the efficient degradation by the ubiquitin-proteasome system, and in response to hypoxia, the degradation is blocked and the accumulating HIF-1α promotes a transcriptional response essential for proper adaptation and survival. Here we show that the ubiquitin-specific protease-19 (USP19) interacts with components of the hypoxia pathway including HIF-1α and rescues it from degradation independent of its catalytic activity. In the absence of USP19, cells fail to mount an appropriate response to hypoxia, indicating an important role for this enzyme in normal or pathological conditions.

Hypoxic stress simultaneously stimulates vascular endothelial growth factor via hypoxia-inducible factor-1α and inhibits stromal cell-derived factor-1 in human endometrial stromal cells

BACKGROUND

Hypoxia of the human endometrium is a physiologic event occurring during the perimenstrual period and the local stimulus for angiogenesis. The aim of this study was to investigate the effects of hypoxic stress on the regulation of vascular endothelial growth factor (VEGF) and stromal cell-derived factor-1 (SDF-1/CXCL12), and the potential role of hypoxia-inducible factor-1α (HIF-1α) in the endometrium.

METHODS

Human endometrial stromal cells (ESCs, n= 22 samples) were studied in vitro. ESCs were cultured under hypoxic and normoxic conditions and treated with cobalt chloride (CoCl₂; a hypoxia-mimicking agent) and/or echinomycin, a small-molecule inhibitor of HIF-1α activity. The mRNA levels and production of VEGF and SDF-1 were assessed by real-time PCR and ELISA, respectively. The HIF-1α protein levels were measured using western blot analysis.

RESULTS

Hypoxia simultaneously induced the expression of mRNA and production of VEGF and attenuated the expression and production of SDF-1 from ESCs in a time-dependent manner. Similar changes were observed in the ESCs after stimulation with CoCl₂ in a dose-dependent manner. CoCl₂ significantly induced the expression of HIF-1α protein, and its highest expression was observed at 6 h. Echinomycin inhibited hypoxia-induced VEGF production without affecting the HIF-1α protein level and cell toxicity and had no effect on SDF-1 secretion (P < 0.01).

CONCLUSIONS

Hypoxia simultaneously acts to increase VEGF via HIF-1α and to decrease SDF-1 in a HIF-1α-independent manner in ESCs. These results indicate a potential mechanism for the action of hypoxic conditions that could influence angiogenesis in the human endometrium.

A myeloid hypoxia-inducible factor 1α-Krüppel-like factor 2 pathway regulates gram-positive endotoxin-mediated sepsis

Although gram-positive infections account for the majority of cases of sepsis, the molecular mechanisms underlying their effects remains poorly understood. We investigated how cell wall components of gram-positive bacteria contribute to the development of sepsis. Experimental observations derived from cultured primary macrophages and the cell line indicate that gram-positive bacterial endotoxins induce hypoxia-inducible factor 1α (HIF-1α) mRNA and protein expression. Inoculation of live or heat-inactivated gram-positive bacteria with macrophages induced HIF-1 transcriptional activity in macrophages. Concordant with these results, myeloid deficiency of HIF-1α attenuated gram-positive bacterial endotoxin-induced cellular motility and proinflammatory gene expression in macrophages. Conversely, gram-positive bacteria and their endotoxins reduced expression of the myeloid anti-inflammatory transcription factor Krüppel-like transcription factor 2 (KLF2). Sustained expression of KLF2 reduced and deficiency of KLF2 enhanced gram-positive endotoxins induced HIF-1α mRNA and protein expression in macrophages. More importantly, KLF2 attenuated gram-positive endotoxins induced cellular motility and proinflammatory gene expression in myeloid cells. Consistent with these results, mice deficient in myeloid HIF-1α were protected from gram-positive endotoxin-induced sepsis mortality and clinical symptomatology. By contrast, myeloid KLF2-deficient mice were susceptible to gram-positive sepsis induced mortality and clinical symptoms. Collectively, these observations identify HIF-1α and KLF2 as critical regulators of gram-positive endotoxin-mediated sepsis.

Structural adaptation of normal and tumour vascular networks

Vascular networks are dynamic structures, adapting to changing conditions by structural remodelling of vessel diameters and by growth of new vessels and regression of existing vessels. The vast number of blood vessels in the circulatory system, more than 10⁹, implies that vessels' arrangement and structure are not under individual genetic control but emerge as a result of generic responses of each segment to the various stimuli that it experiences. To obtain insight into the types of response that are needed, a network-oriented approach has been used, in which theoretical models are used to simulate structural adaptation in vascular networks, and the results are compared with experimental observations. With regard to the structural control of vessel diameters, this approach shows that responses to both haemodynamic and metabolic stimuli are needed for the formation of functionally adequate and efficient network structures. Furthermore, information transfer in both upstream and downstream directions is essential for balancing flows between long and short flow pathways. Otherwise, functional shunting occurs, that is, short pathways become enlarged and flow bypasses longer pathways. Information transfer in the upstream direction is achieved by conducted responses communicated along vessel walls. Simulations of structural adaptation in tumour microvascular networks indicate that impaired vascular communication, resulting in functional shunting, may be an important factor causing the dysfunctional microcirculation and local hypoxia typically observed in tumours. Anti-angiogenic treatment of tumours may restore vascular communication and thereby improve or normalize flow distribution in tumour vasculature.

Sterols for Oxygen: The Metabolic Burden of Microbial SREBP

In this issue of Molecular Cell, Lee et al. (2011) report a novel mechanism for oxygen-sensing in S. pombe, whereby the 2-OG-Fe(II) dioxygenase Ofd protein regulates both the DNA-binding activity and the degradation of the hypoxia regulated transcription factor, Sre1p.

Priming of hypoxia-inducible factor by neuronal nitric oxide synthase is essential for adaptive responses to severe anemia

Cells sense and respond to changes in oxygen concentration through gene regulatory processes that are fundamental to survival. Surprisingly, little is known about how anemia affects hypoxia signaling. Because nitric oxide synthases (NOSs) figure prominently in the cellular responses to acute hypoxia, we defined the effects of NOS deficiency in acute anemia. In contrast to endothelial NOS or inducible NOS deficiency, neuronal NOS (nNOS)(-/-) mice demonstrated increased mortality during anemia. Unlike wild-type (WT) animals, anemia did not increase cardiac output (CO) or reduce systemic vascular resistance (SVR) in nNOS(-/-) mice. At the cellular level, anemia increased expression of HIF-1α protein and HIF-responsive mRNA levels (EPO, VEGF, GLUT1, PDK1) in the brain of WT, but not nNOS(-/-) mice, despite comparable reductions in tissue PO(2). Paradoxically, nNOS(-/-) mice survived longer during hypoxia, retained the ability to regulate CO and SVR, and increased brain HIF-α protein levels and HIF-responsive mRNA transcripts. Real-time imaging of transgenic animals expressing a reporter HIF-α(ODD)-luciferase chimeric protein confirmed that nNOS was essential for anemia-mediated increases in HIF-α protein stability in vivo. S-nitrosylation effects the functional interaction between HIF and pVHL. We found that anemia led to nNOS-dependent S-nitrosylation of pVHL in vivo and, of interest, led to decreased expression of GSNO reductase. These findings identify nNOS effects on the HIF/pVHL signaling pathway as critically important in the physiological responses to anemia in vivo and provide essential mechanistic insight into the differences between anemia and hypoxia.

Perinatal intermittent hypoxia alters γ-aminobutyric acid: a receptor levels in rat cerebellum

Perinatal hypoxia commonly causes brain injury in infants, but the time course and mechanisms underlying the preferential male injury are unclear. Intermittent hypoxia disturbs cerebellar γ-aminobutyric (GABA)-A receptor profiles during the perinatal period, possibly responding to transient excitatory processes associated with GABA(A) receptors. We examined whether hypoxic insults were particularly damaging to the male rodent cerebellum during a specific developmental time window. We evaluated cerebellar injury and GABA(A) receptor profiles following 5-h intermittent hypoxia (IH: 20.8% and 10.3% ambient oxygen, switched every 240s) or room-air control in groups of male and female rat pups on postnatal d 1-2, wk 1, or wk 3. The cerebella were harvested and compared between groups. The mRNA levels of GABA(A) receptors α6, normalized to a house-keeping gene GAPDH, and assessed using real-time reverse-transcriptase PCR assays were up-regulated by IH at wk 1, more extensively in male rats, with sex influencing the regulatory time-course. In contrast, GABA(A) α6 receptor protein expression levels, assessed using Western blot assays, reached a nadir at wk 1 in both male and female rats, possibly indicating involvement of a post-transcriptional mechanism. The extent of cerebellar damage and level of apoptosis, assessed by DNA fragmentation, were greatest in the wk 3 IH-exposed group. The findings suggest partial protection for female rats against early hypoxic insult in the cerebellum, and that down-regulation of GABA(A) receptors, rather than direct neural injury assessed by DNA fragmentation may modify cerebellar function, with potential later motor and other deficits.