Strontium induces murine keratinocyte differentiation in vitro in the presence of serum and calcium

Primary mouse keratinocytes in culture are induced to terminally differentiate by increasing extracellular Ca 2+ concentrations (Ca O ) from 0.05 mM to ≥ 0.1 mM. The addition of Sr 2+ (≥ 2.5 mM) to medium containing 0.05 mM Ca 2+ induces focal stratification and terminal differentiation, which are similar to that found after increasing the Ca O to 0.12 mM. Sr 2+ in 0.05 mM Ca 2+ medium induces the expression of the differentiation‐specific keratins, keratin 1 (K1), keratin 10 (K10), and the granular cell marker, filaggrin, as determined by both immunoblotting and immunofluorescence. Sr 2+ induces the expression of those differentiation markers in a dose dependent manner, with an optimal concentration of 5 mM. In the absence of Ca 2+ in the medium, the Sr 2+ effects are reduced, and Sr 2+ is ineffective when both Ca 2+ and serum are deleted from the medium. Sr 2+ treatment increases the ratio of fluorescence intensity of the intracellular Ca 2+ sensitive probe, fura‐2, indicating an associated rise in the level of intracellular free Ca 2+ and/or Sr 2+ . At doses sufficient to induce differentiation, Sr 2+ also increases the level of inositol phosphates in primary keratinocytes within 30 min. The uptake curves of 85 Sr 2+ by primary keratinocytes are similar to those of 45 Ca 2+ . At low concentrations, the initial uptake of both 45 Ca 2+ and 85 Sr 2+ reaches a plateau within 1 hr; at higher concentrations, the uptake of both 45 Ca 2+ and 85 Sr 2+ increases continuously for 12 hr. In keratinocytes pre‐equilibrated with 45 Ca 2+ in 0.05 mM Ca 2+ medium, Sr 2+ causes an increase of 45 Ca 2+ uptake, which is dependent on the presence of serum. These results suggest that Sr 2+ utilizes the same signalling pathway as Ca 2+ to induce keratinocyte terminal differentiation and that Ca 2+ may be required to exert these effects. © 1993 Wiley‐Liss, Inc.