Nucleo-cytoplasmic Cycling of the Vitamin D Receptor in the Enterocyte-Like Cell Line, Caco-2

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. © 2006 Wiley-Liss, Inc.