HIF-1 alpha protein is an essential factor for protection of myeloid cells against LPS-induced depletion of ATP and apoptosis that supports Toll-like receptor 4-mediated production of IL-6

Sepsis is the leading cause of death in intensive Care units, which reflects detrimental host response to infection where lipopolysaccharide (LPS) shared by Gram-negative bacteria acts as a potent activator of immune cells via Toll-like receptor 4 (TLR4). Recently it was found that TLR4 downstream signalling leads to the accumulation of hypoxia-inducible factor 1 alpha (HIF-1 alpha), which is important for TLR4-dependent expression of pro-inflammatory cytokines, however, basic biochemical mechanisms of involvement of this protein in TLR4 downstream signalling remains unclear. Here we found that knockdown of the expression of HIF-1 alpha a protein by siRNA led to the depletion of ATP, which corresponded to the constant increase in the activity of apoptosis signal-regulating kinase 1 (ASK1) and therefore apoptosis as estimated based on the increase in the activity of caspase 3. On the other hand, LPS-dependent production of IL-6 was attenuated. Treatment of HIF-1 alpha knockdown cells with extracellular ATP in combination with LPS preserved the IL-6 expression but not the activity of ASK1 on the level observed in LPS-stimulated control cells. We therefore suggested that HIF-1 alpha protein supports LPS-dependent expression of IL-6 by preventing depletion of ATP. On the other hand HIF-1a protein is selectively required for down-regulation of ASK1 activated during LPS-induced TLR4 downstream signalling.