Upregulation of vitamin D receptor levels by 1,25(OH)₂ vitamin D₃ in cultured human keratinocytes

Abstract 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/μg protein) than in differentiated cultures (90 pg/μg 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.