HIF-1–dependent repression of equilibrative nucleoside transporter (ENT) in hypoxia

Abstract

Extracellular adenosine (Ado) has been implicated as central signaling molecule during conditions of limited oxygen availability (hypoxia), regulating physiologic outcomes as diverse as vascular leak, leukocyte activation, and accumulation. Presently, the molecular mechanisms that elevate extracellular Ado during hypoxia are unclear. In the present study, we pursued the hypothesis that diminished uptake of Ado effectively enhances extracellular Ado signaling. Initial studies indicated that the half-life of Ado was increased by as much as fivefold after exposure of endothelia to hypoxia. Examination of expressional levels of the equilibrative nucleoside transporter (ENT)1 and ENT2 revealed a transcriptionally dependent decrease in mRNA, protein, and function in endothelia and epithelia. Examination of the ENT1 promoter identified a hypoxia inducible factor 1 (HIF-1)–dependent repression of ENT1 during hypoxia. Using in vitro and in vivo models of Ado signaling, we revealed that decreased Ado uptake promotes vascular barrier and dampens neutrophil tissue accumulation during hypoxia. Moreover, epithelial Hif1 α mutant animals displayed increased epithelial ENT1 expression. Together, these results identify transcriptional repression of ENT as an innate mechanism to elevate extracellular Ado during hypoxia.

Direct or indirect stimulation of adenosine A 2A receptors enhances bone regeneration as well as bone morphogenetic protein‐2

“…Thus, it has been reported that chronic hypoxia in cardiomyocytes down-regulates ent1 activity, thus modulating release and/or uptake of adenosine (40). Moreover, hypoxia increases the half-life of extracellular adenosine in part by HIF-1-regulated ent1 repression in vascular endothelial and mucosal epithelial cells (41), whereas HIF1a-dependent repression of ent2 increases mucosal adenosine signaling and attenuates hypoxia-associated inflammation of the intestine (42). Solid tumors express low levels of nucleoside transporters, which lead to an increase in adenosine signaling in the cancer environment (43), and decreased nucleoside transporter expression in tumor tissue may contribute to reduced drug uptake and the development of resistance (43).…”

Adenosine Generation and Signaling during Acute Kidney Injury

“…58,23 Extracellular adenosine has a very short half-life in the extracellular space. This is mainly due to its rapid uptake into the intracellular space by nucleoside transporters, 24,34,29 where it is rapidly metabolized into inosine by adenosine deaminase or into AMP by adenosine kinase. 61,28 …”

“…23 Several studies implicate extracellular adenosine generation and signaling in adaptation to hypoxia. 24,23,25 As such, extracellular adenosine production is enhanced during limited oxygen availability 23,26 -29 and is critical to maintaining cellular functions during hypoxia, 30,23,31 or to dampen hypoxia-driven inflammation. [32][33][34]28,17 On the basis of such findings, recent studies in the kidneys implicate extracellular adenosine generation and signaling as pharmacologic targets for protection from ischemia.…”

Neutrophils as Sources of Extracellular Nucleotides: Functional Consequences at the Vascular Interface

“…These proteins include enzymes involved in anaerobic metabolism, the angiogenic cytokine vascular endothelial growth factor (VEGF), and inducible nitric oxide synthase (Semenza et al 2000). Important for this review, and as depicted in Figure 1, HIF coordinates extracellular nucleotide metabolism (CD73) (Synnestvedt et al 2002), adenosine signaling (A2BR) (Kong et al 2006) and the extracellular nucleoside transporter (ENT) (Eltzschig et al 2005).…”