NO and TNF-α released from activated macrophages stabilize HIF-1α in resting tubular LLC-PK₁ cells

Hypoxic/ischemic conditions provoke activation of the transcription factor hypoxia-inducible factor-1 (HIF-1). HIF-1 is composed of HIF-1alpha (subjected to protein stability regulation) and constitutively expressed HIF-1beta. Besides hypoxia, diverse agonists are identified that stabilize HIF-1alpha during normoxia. Here we used a coculture system of RAW 264.7 macrophage cells and tubular LLC-PK(1) cells to establish that lipopolysaccharide- and interferon-gamma-stimulated but not resting macrophages elicited HIF-1alpha accumulation in LLC-PK(1) cells. Via pharmacological interventions such as blockade of nitric oxide (NO) production in macrophages, scavenging of NO with the use of 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, or application of tumor necrosis factor-alpha (TNF-alpha)-neutralizing antibodies, we identified NO and TNF-alpha as signaling molecules. Working in concert, NO and TNF-alpha have a stronger response when allowed direct cell-to-cell contact instead of contact with only the cell supernatant of activated macrophages. We show that signal transmission by NO with TNF-alpha in LLC-PK(1) cells is mediated via the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway, because it is blocked by wortmannin or dominant-negative forms of PI3-K as well as protein kinase B. We conclude that NO and TNF-alpha, derived from activated macrophages, provoke HIF-1alpha stabilization in LLC-PK(1) cells under normoxic conditions, which underscores HIF-1alpha stabilization due to intercellular regulation.