A hyperosmotic stimulus elevates intracellular calcium and inhibits proliferation of a human keratinocyte cell line

Occlusion has previously been used to treat psoriatic plaques and was shown to improve the condition. We investigated the consequences of applying a mechanical stress, in vitro, on the HaCaT keratinocyte cell line. A mechanical load applied to cells can be mimicked by a hyperosmotic stimulus. Exposure of HaCaT keratinocytes to different hyperosmotic solutions (final osmolarity in the range 350-600 mOsm, produced by sucrose addition) resulted in an inhibition of cell proliferation after 96 h of treatment. As keratinocyte maturation is regulated by calcium levels, we measured hyperosmotic-stimulus-induced changes of intracellular calcium ([Ca²⁺](i)) by single-cell image analysis employing FURA-2/AM. The hyperosmotic stimulus produced a rapid transient 2.6-fold elevation of [Ca²⁺](i) followed by a gradual decay to the basal level. The transients originated from extracellular as well as from intracellular calcium pools and did not respond to voltage-sensitive calcium channel blockers. The hyperosmotic stimulus was shown to increase the cellular expression of involucrin, a differentiation marker, following 72h of incubation, as measured by flow cytometry. Treatment of cells with the [Ca²⁺](i) chelator BAPTA/AM almost completely blocked the [Ca²⁺](i) elevation, but did not alter cellular growth or the induction of differentiation observed after hyperosmotic stimulus. It is suggested that treatment of keratinocytes with hyperosmotic stimulus can induce short-time effects (calcium transients) as well as long-term cellular maturation.