Hydration Status Regulates Sodium Flux and Inflammatory Pathways through Epithelial Sodium Channel (ENaC) in the Skin

Abstract

Although it is known that the inflammatory response that results from disruption of epithelial barrier function after injury results in excessive scarring, the upstream signals remain unknown. It has also been observed that epithelial disruption results in reduced hydration status and that the use of occlusive dressings that prevent water loss from wounds decreases scar formation. We hypothesized that hydration status changes sodium homeostasis and induces sodium flux in keratinocytes, which result in activation of pathways responsible for keratinocyte-fibroblast signaling and ultimately lead to activation of fibroblasts. Here, we demonstrate that perturbations in epithelial barrier function lead to increased sodium flux in keratinocytes. We identified that sodium flux in keratinocytes is mediated by epithelial sodium channels (ENaCs) and causes increased secretion of proinflammatory cytokines, which activate fibroblast via the cyclooxygenase 2 (COX-2)/prostaglandin E2 (PGE2) pathway. Similar changes in signal transduction and sodium flux occur by increased sodium concentration, which simulates reduced hydration, in the media in epithelial cultures or human ex vivo skin cultures. Blockade of ENaC, prostaglandin synthesis, or PGE2 receptors all reduce markers of fibroblast activation and collagen synthesis. In addition, employing a validated in vivo excessive scar model in the rabbit ear, we demonstrate that utilization of either an ENaC blocker or a COX-2 inhibitor results in a marked reduction in scarring. Other experiments demonstrate that the activation of COX-2 in response to increased sodium flux is mediated through the PIK3/Akt pathway. Our results indicate that ENaC responds to small changes in sodium concentration with inflammatory mediators and suggest that the ENaC pathway is a potential target for a strategy to prevent fibrosis.

Capsaicin Used on Skin Influences Ion Transport Pathways: An in vitro Study

“…It seems that the adverse effect may be related to changes primarily in sodium ion transport, by the epidermis and dermis, especially by immunological cells and nerve fibers. However, as AMI blocks ENaC channels, which are present on keratinocytes but not on nerve fibers [38,47,48], the observed electrophysiological effect of the interaction of capsaicin in the presence of a sodium channel blocker depended on keratinocytes only.…”

“…One of the mechanisms of this interaction is the modification of sodium ion transport, resulting in their influx into the cells [25,38,45,49]. It is proven that increased sodium ion transport in the nerve fibers ending under the influence of capsaicin causes increased pain [25,50].…”

“…ENaC are localized on keratinocytes [38,48] and engage in the ion transport changes in association with the TRPV receptor [51]. In keratinocytes, sodium influx into the cells is related to the secretion of proinflammatory factors, e.g., prostaglandin E 2 [38]. Additionally, AMI-sensitive sodium channels were found on mast cells, demonstrating their participation with secretion of proinflammatory agents and inflammatory process reaction [52].…”

“…Sodium channels are also located in other skin cells, such as keratinocytes; it is therefore reasonable to assume that capsaicin can modify their action. ENaC are localized on keratinocytes [38,48] and engage in the ion transport changes in association with the TRPV receptor [51]. In keratinocytes, sodium influx into the cells is related to the secretion of proinflammatory factors, e.g., prostaglandin E 2 [38].…”

“…It is known that the skin's principal epithelial sodium channel (ENaC) plays a crucial role in physiological processes involved in the development of skin barrier functions. These involve synthesis of differentiation-specific proteins [36], lipid formation [37], and the regulation of sodium flux in the epidermis through inflammatory mediators [38]. …”