Protonophore activity of short-chain fatty acids induces their intracellular accumulation and acidification.

Short-chain fatty acids (SCFAs), produced by dietary fiber fermentation in the colon, play essential roles in cellular metabolism, with butyrate notably modulating immune responses and epigenetic regulation. Their production contributes to an acidic colonic environment where protonated SCFAs permeate membranes, leading to intracellular acidification and SCFA accumulation. Using our method to measure intracellular pH, we investigated how extracellular pH influences butyrate-induced acidification and immunomodulatory effects in human macrophages. Our data show that butyrate accumulates and acidifies cells at acidic extracellular pH due to the permeability of its protonated form. While inflammatory cytokine production was mildly influenced by extracellular pH, butyrate-induced histone acetylation exhibited a pH dependence, underscoring the importance of considering extracellular pH when assessing the SCFA's functions.