Generation of free fatty acids from phospholipids regulates stratum corneum acidification and integrity

There is evidence that the "acid mantle" of the stratum corneum is importan t for both permeability barrier formation and cutaneous antimicrobial defen se. The origin of the acidic pH of the stratum corneum remains conjectural, however. Both passive (e.g., eccrine/sebaceous secretions, proteolytic) an d active (e.g., proton pumps) mechanisms have been proposed. We assessed he re whether the free fatty acid pool, which is derived from phospholipase-me diated hydrolysis of phospholipids during cornification, contributes to str atum corneum acidification and function. Topical applications of two chemic ally unrelated secretory phospholipase sPLA(2) inhibitors, bromphenacylbrom ide and 1-hexadecyl-3 -trifluoroethylglycero-sn-2-phosphomethanol, for 3 d produced an increase in the pH of murine skin surface that was paralleled n ot only by a permeability barrier abnormality but also altered stratum corn eum integrity (number of strippings required to break the barrier) and decr eased stratum corneum cohesion (protein weight removed per stripping). Not only stratum corneum pH but also all of the functional abnormalities normal ized when either palmitic, stearic, or linoleic acids were coapplied with t he inhibitors. Moreover, exposure of intact murine stratum corneum to a neu tral pH for as little as 3 h produced comparable abnormalities in stratum. corneum integrity and cohesion, and further amplified the inhibitor-induced functional alterations. Furthermore, short-term applications of an acidic pH buffer to inhibitor-treated skin also reversed the abnormalities in stra tum corneum integrity and cohesion, despite the ongoing decrease in free fa tty acid levels, Finally, the secretory-phospholipase-inhibitor-induced alt erations in integrity/cohesion were in accordance with premature dissolutio n of desmosomes, demonstrated both by electron microscopy and by reduced de smoglein 1 levels in the stratum corneum (shown by immunofluorescence stain ing and vizualized by confocal microscopy), Together, these results demonst rate: (i) the importance of phospholipid-to-free-fatty-acid processing for normal stratum corneum acidification; and (ii) the potentially important ro le of this pathway not only for barrier homeostasis but also for the dual f unctions of stratum corneum integrity and cohesion.