A2a and a2b adenosine receptors affect HIF-1α signaling in activated primary microglial cells

Microglia are central nervous system (CNS)-resident immune cells, that play a crucial role in neuroinflammation. Hypoxia-inducible factor-1 (HIF-1 alpha), the main transcription factor of hypoxia-inducible genes, is also involved in the immune response, being regulated in normoxia by inflammatory mediators. Adenosine is an ubiquitous nucleoside that has an influence on many immune properties of microglia through interaction with four receptor subtypes. The aim of this study was to investigate whether adenosine may affect microglia functions by acting on HIF-1 alpha modulation. Primary murine microglia were activated with lipopolysaccharide (LPS) with or without adenosine, adenosine receptor agonists and antagonists and HIF-1 alpha accumulation and downstream genes regulation were determined. Adenosine increased LPS-induced HIF-1 alpha accumulation leading to an increase in HIF-1 alpha target genes involved in cell metabolism [glucose transporter-1 (GLUT-1)] and pathogens killing [inducible nitric-oxide synthase (iNOS)] but did not induce HIF-1 alpha dependent genes related to angiogenesis [vascular endothelial growth factor (VEGF)] and inflammation [tumor necrosis factor-alpha(TNF-alpha)]. The stimulatory effect of adenosine on HIF-1 alpha and its target genes was essentially exerted by activation of A(2A) through p44/42 and A(2B) subtypes via p38 mitogen-activated protein kinases (MAPKs) and Akt phosphorylation. Furthermore the nucleoside raised VEGF and decreased TNF-alpha levels, by activating A(2B) subtypes. In conclusion adenosine increases GLUT-1 and iNOS gene expression in a HIF-1 alpha-dependent way, through A(2A) and A(2B) receptors, suggesting their role in the regulation of microglial cells function following injury. However, inhibition of TNF-alpha adds an important anti-inflammatory effect only for the A(2B) subtype.