Nucleo‐cytoplasmic cycling of the vitamin D receptor in the enterocyte‐like cell line, Caco‐2

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Abstract
We examined the effects of 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) on the distribution and mobility of the vitamin D receptor (VDR) in the enterocyte‐like Caco‐2 cell. Confocal microscopy showed that a green fluorescent protein‐vitamin D receptor (GFP‐VDR) fusion protein is predominantly nuclear (58%) and it does not associate with the apical or basolateral membrane of proliferating or polarized, differentiated cells. In contrast to the previously studied cell types, neither endogenous VDR nor GFP‐VDR levels accumulate in the nucleus following 1,25(OH)2D3 treatment (100 nM, 30 min). However, in nuclear photobleaching experiments nuclear GFP‐VDR import was significantly increased by 1,25(OH)2D3 during both an early (0–5 min) and later (30–35 min) period (20% per 5 min). Compared to the natural ligand, nuclear import of GFP‐VDR was 60% lower in cells treated with the 1,25(OH)2D3 analog, 1‐alpha‐fluoro‐16‐ene‐20‐epi‐23‐ene‐26,27‐bishomo‐25‐hydroxyvitamin D3 (Ro‐26‐9228, 5 min, 100 nM). Downstream events like ligand‐induced association of VDR with chromatin at 1 h and the accumulation of CYP24 mRNA were significantly lower in Ro‐26‐9228 treated cells compared to 1,25(OH)2D3 (60 and 95% lower, respectively). Collectively our data are consistent with a role for ligand‐induced nuclear VDR import in receptor activation. In addition, ligand‐dependent VDR nuclear import appears to be balanced by export, thus accounting for the lack of nuclear VDR accumulation even when VDR import is significantly elevated. J. Cell. Biochem. 100: 617–628, 2007.

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