Abstract
The active metabolite of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), suppresses the proliferation while promoting the differentiation of keratinocytes through the vitamin D receptor (VDR). β-Catenin, on the other hand, promotes proliferation and blocks epidermal differentiation, although it stimulates hair follicle differentiation. In intestinal epithelia VDR binds β-catenin and blocks its proliferative effects. In this study we investigated the role of 1,25(OH)2D3/VDR on β-catenin regulated gene transcription during keratinocyte proliferation and differentiation. 1,25(OH)2D3 suppressed promoter reporter activity driven by synthetic and natural TCF/β-catenin response elements. Over-expression of VDR further suppressed these TCF/β-catenin promoter activities. 1,25(OH)2D3 also suppressed the mRNA expression of the β-catenin regulated gene Gli1 through VDR. These data were consistent with our previous observations that VDR silencing resulted in keratinocyte hyperproliferation with increased expression of Gli1 in vitro, whereas VDR null skin showed hyperproliferation in vivo. In contrast, 1,25(OH)2D3 induced expression of another β-catenin regulated gene, PADI1, important for both epidermal and hair follicle differentiation. Deletion of VDR resulted in defects in hair differentiation in vivo, with decreased expression of β-catenin regulated hair differentiation genes such as PADI1, hair keratin KRT31 and calcium binding protein S100a3. These genes possess vitamin D response elements (VDRE) adjacent to TCF/β-catenin response elements and are regulated by both VDR and β-catenin signaling. Therefore, we propose that VDR and β-catenin interact reciprocally to promote VDR stimulation of genes involved with differentiation that contain both VDR and β-catenin response elements while inhibiting β-catenin stimulation of genes involved with proliferation. Thus the major finding of this study is that while 1,25(OH)2D3/VDR inhibits the actions of β-catenin to promote keratinocyte proliferation, 1,25(OH)2D3/VDR promotes the ability of β-catenin to stimulate hair follicle differentiation. This article is part of a Special Issue entitled '16th Vitamin D Workshop'.