The natural biologically active form of vitamin D3, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), possesses antiproliferative, prodifferentiating and immunomodulatory properties. The actions of 1,25(OH)2D3 are mediated through the intracellular vitamin D receptor (VDR), and the level of VDR is believed to determine the cellular responsiveness to vitamin D3. In the present study we examined the effects of 1,25(OH)2D3 on the expression of VDR and its message in cultured human keratinocytes. Western analysis showed the mean VDR content to be higher in undifferentiated cultures (175 pg/microgram protein) than in differentiated cultures (90 pg/microgram protein). Incubation with 1,25(OH)2D3 induced an increase in the VDR in both undifferentiated and differentiated keratinocytes. The VDR increase was detectable after 2 h and maximal (approximately two-fold stimulation) after 8 h. The 1,25(OH)2D3-induced stimulation of VDR levels was dose dependent with a maximum at 10(-7) M. The 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 indicate that the ligand-induced upregulation of VDR involves increased transcription. The upregulation of VDR levels may increase the responsiveness to 1,25(OH)2D3 and may, therefore, be an important mechanism for regulating the effects of 1,25(OH)2D3 on keratinocyte proliferation and differentiation.