1,25-DIHYDROXYVITAMIN D-3 POTENTIATES THE KERATINOCYTE RESPONSE TO CALCIUM

Extracellular calcium (Ca-0) stimulates the differentiation of keratin ocytes; 1,25 dihydroxyvitamin D-3 (1,25(OH)(2)D-3) does likewise. Sinc e 1,25(OH)(2)D-3 regulates calcium flux in other cells, we hypothesize d that 1,25(OH)(2)D-3 acted through and promoted the effects of calciu m on keratinocyte differentiation. To test this hypothesis, we evaluat ed the effects of calcium and 1,25(OH)(2)D-3 alone and in combination on the mRNA and protein levels of involucrin and transglutaminase in n eonatal human keratinocytes. Ca-0 alone increased these mRNA levels in a dose-dependent fashion (0.03 to 1.2 mM) over a 24-h period. This in crease in mRNA levels was associated with a stimulation by calcium of involucrin and transglutaminase gene transcription. However, by 72 h, the mRNA levels of involucrin and transglutaminase decreased. At 0.03 mM Ca-0, 1,25(OH)(2)D-3 showed a dose-dependent stimulation of involuc rin and transglutaminase mRNA for up to 48 h and potentiated the initi al (4-h) stimulation by Ca-0 of involucrin and transglutaminase mRNA A s for calcium alone, this increase in mRNA was associated with an incr ease in transcription of the involucrin and transglutaminase genes. Ho wever, by 24 h of exposure to both calcium and 1,25(OH)(2)D-3, a dose- dependent fall in mRNA levels was seen. The mRNA levels of involucrin and transglutaminase were stable for 24 h when neonatal human keratino cytes were grown in serum-free keratinocyte growth medium containing 0 .03 or 1.2 mM Ca-0 alone. However, the mRNAs of both genes underwent r apid degradation when neonatal human keratinocytes were treated with 1 ,25(OH)(2)D-3, especially in high Ca-0. 1,25(OH)(2)D-3 and Ca-0 increa sed the protein levels of involucrin and transglutaminase activity in a synergistic fashion throughout the 48-h time course. These data supp ort the hypothesis that 1,25(OH)(2)D-3 promotes calcium-induced differ entiation at the level of both gene expression and mRNA stability.