UP-REGULATION OF VITAMIN-D-RECEPTOR LEVELS BY 1,25(OH)(2) VITAMIN-D-3IN CULTURED HUMAN KERATINOCYTES

The natural biologically active form of vitamin D-3, 1,25-dihydroxyvit amin D-3 (1,25(OH)(2)D-3), possesses antiproliferative, prodifferentia ting and immunomodulatory properties. The actions of 1,25(OH),D, are m ediated through the intracellular vitamin D receptor (VDR), and the le vel of VDR is believed to determine the cellular responsiveness to vit amin D-3. In the present study we examined the effects of 1,25(OH)(2)D -3 on the expression of VDR and its message in cultured human keratino cytes. Western analysis showed the mean VDR content to be higher in un differentiated cultures (175 pg/mu g protein) than in differentiated c ultures (90 pg/mu g protein). Incubation with 1,25(OH),D, induced an i ncrease in the VDR in both undifferentiated and differentiated keratin ocytes. The VDR increase was detectable after 2 h and maximal (approxi mately twofold stimulation) after 8 h. The 1,25(OH)(2)D-3-induced stim ulation of VDR levels was dose dependent with a maximum at 10(-7) M. T he VDR mRNA levels as determined by the ribonuclease protection assay showed a peak (50% stimulation) after approximately 2 h. Although this increase in VDR mRNA was not statistically significant, the results i ndicate that the ligand-induced upregulation of VDR involves increased transcription. The upregulation of VDR levels may increase the respon siveness to 1,25(OH)(2)D-3 and may, therefore, be an important mechani sm for regulating the effects of 1,25(OH)(2)D-3 on keratinocyte prolif eration and differentiation.