Mild acidosis delays neutrophil apoptosis via multiple signaling pathways and acts in concert with inflammatory mediators

Abstract

Accumulating evidence indicates development of local extracellular acidosis in inflamed tissues in response to infection and tissue injury. Activation of infiltrating neutrophils contributes to a transient decrease in pH, which, in turn, triggers innate immunity. In this study, we investigated the impact of extracellular acidosis on neutrophil apoptosis, a critical determinant of the outcome of the inflammatory response and analyzed the underlying signaling pathways. Culture of human isolated neutrophils in mildly acidotic conditions (pH 6.5-7.0) resulted in activation of NF-κB; intracellular accumulation of cAMP; and phosphorylation of Akt, ERK, and p38 MAPK; and preservation of Mcl-1 expression. Consequently, extracellular acidosis prevented disruption of mitochondrial transmembrane potential and translocation of cytochrome and apoptosis-inducing factor from the mitochondria to cytoplasm and nuclei, respectively and inhibited caspase-3 activity. Pharmacological inhibition of ERK, PI3K, NF-κB, or PKA partially reversed survival cues by extracellular acidosis and redirected neutrophils to apoptosis. Conversely, dibutyryl cAMP (100-500 μM) delayed apoptosis of neutrophils cultured at pH 7.4. Extracellular acidosis-generated survival cues were additive to the potent prosurvival signals from bacterial DNA, LPS, modified C-reactive protein, and serum amyloid A. Acidosis increased CpG DNA uptake by neutrophils and augmented phosphorylation of ERK and Akt, leading to preservation of Mcl-1 expression. Our results identified extracellular acidosis as a survival signal for neutrophils by suppressing the constitutive apoptotic machinery and suggest that transient decreases in local pH can enhance neutrophil responses to inflammatory stimuli, thereby contributing to amplification or prolongation of the inflammatory response.

Possible influence of free fatty acid receptors on pH regulation in the ruminal epithelium of sheep

“…An influence of pH i is described for cAMP production by the G‐protein‐independent sAC, but this AC's activity is not affected by forskolin (Bitterman et al, ), so its involvement is rather unlikely in our experimental set‐up. To date, no impact of H + on the G‐protein coupled tAC has been recorded (El Kebir, Oliveira Lima Dos Santos, Mansouri, Sekheri, & Filep, ). Thus, the butyrate‐induced decrease in cAMP levels at low mucosal pH is probably due to an elevated transepithelial butyrate transfer and not caused by cytosolic acidification as a result of butyrate dissociation.…”

Neutrophil heterogeneity and fate in inflamed tissues: implications for the resolution of inflammation

“…Activated neutrophils secrete protons that contribute to interstitial acidification, yielding pH between 5.5 and 7.0, in inflammatory loci (64,88,195). Extracellular acidosis is a danger signal (231), which through activation of multiple signaling pathways, including ERK 1/2, phosphoinositide 3-kinase (PI3K)/Akt, NF-B, and cAMP/ PKA leads to preventing degradation of Mcl-1, a key regulator of neutrophil survival (62), and subsequently suppresses PMN apoptosis and enhances the apoptosis-delaying actions of inflammatory mediators (65). Extracellular acidosis affects the outcome of inflammation in a context-dependent manner.…”

Serum Amyloid A1 (SAA1) Revisited: Restricted Leukocyte-Activating Properties of Homogeneous SAA1

“…A number of studies have demonstrated the capacity of rSAA1 to promote the survival of leukocytes namely neutrophils via the activation of FPR2 (44,45). More interestingly, also plasmaderived SAA, which lacks the inflammatory capacity displayed by its recombinant counterpart, was shown to promote neutrophil survival (46).…”