Retinoic acid stimulation of human dermal fibroblast proliferation is dependent on suboptimal extracellular Ca²⁺ concentration

Human dermal fibrobtasts failed to proliferate when cultured in medium containing 0.15 mmol/ l (millimolar) Ca²⁺ (keratinocyte growth medium [KGM]) but did when the external Ca²⁺ concentration was raised to 1.4 mmol/l. All-trans retinoic acid (retinoic acid) stimulated proliferation in KGM but did not further stimulate growth in Ca²⁺-supplemented KGM. The ability of retinoic acid to stimulate proliferation was inhibited in KGM pre-pared without Ca²⁺ or prepared with 0.03 mmol/l Ca²⁺ and in KGM treated with 1 mmol/l ethylene-glycol-bis-(β-aminoethyl ether)N,N′-tetra acetic acid. Using ⁴⁵Ca²⁺ to measure Ca²⁺ influx and efflux, it was found that retinoic acid minimally increased Ca²⁺ uptake into flbroblasts. In contrast, retinoic acid treatment of fibroblasts that had been pre-equilibrated for 1 day with ⁴⁵Ca²⁺ inhibited release of intracellular Ca²⁺ into the extracellular fluid. Retinoic acid also stimulated ³⁵S-methionine incorporation into trichloroacetic acid-precipitable material but in contrast to its effect on proliferation, stimulation of ³⁵S-methionine incorporation occurred in both high-Ca²⁺ and low-Ca²⁺ medium. These data indicate that retinoic acid stimulation of proliferation, but not protein synthesis, is dependent on the concentration of Ca²⁺ in the extracellular environment.