Activation of phospholipase C-?1 in human keratinocytes by hyperosmolar shock without enzyme phosphorylation

Protective effects of glycerol and xylitol in keratinocytes exposed to hyperosmotic stress

Purpose: Our goal was to study whether glycerol and xylitol provide protection against osmotic stress in keratinocytes. Methods: The experiments were performed on HaCaT keratinocytes. Hyperosmotic stress was induced by the addition of sorbitol (450, 500 and 600 mOsm). Both polyols were applied at two different concentrations (glycerol: 0.027% and 0.27%, xylitol: 0.045% and 0.45%). Cellular viability and cytotoxicity were assessed, intracellular Ca²⁺ concentration was measured, and the RNA expression of inflammatory cytokines was determined by means of PCR. Differences among groups were analyzed with one-way ANOVA and Holm-Sidak post-hoc test. When the normality test failed, Kruskal-Wallis one-way analysis of variance on ranks, followed by Dunn's method for pairwise multiple comparison was performed. Results: The higher concentrations of the polyols were effective. Glycerol ameliorated the cellular viability while xylitol prevented the rapid Ca²⁺ signal. Both polyols suppressed the expression of IL-1α but only glycerol decreased the expression of IL-1β and NFAT5. Conclusions: Glycerol and xylitol protect keratinocytes against osmotic stress. Despite their similar chemical structure, the effect of these polyols displayed differences. Hence, joint application of glycerol and xylitol may be a useful therapeutic approach for different skin disorders.

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.