A nuclear localization-deficient VDR loses its growth-inhibitory activity in breast cancer cells. (a) VDR binds PTPH1 in breast cancer cells. T47D cells stably expressed with VDR or its mutant were analyzed for protein complex formation by V5 IP and WB analyses. (b) Regulatory effects of vitamin D3 on VDR and its mutant nuclear translocation. Cells were cultured in the absence and presence of 10 nM vitamin D3 for 24 h and collected for cell fractionation analyses. (c, d) Indicated cells were assessed for proliferation (c) or colony formation (d) (* P < 0.05 vs. vector or VDR expressed cells) as described in Figures 4c and and3,3, respectively. (e) PTPH1 stimulates breast cancer growth through increasing cytoplasmic VDR expression. Nuclear VDR is known to be growth inhibitory, whereas our results presented here suggest that cytoplasmic VDR loses this inhibitory function. Since PTPH1 promotes breast cancer growth by a mechanism that couples with its activity to bind and stabilize VDR protein and to increase cytoplasmic VDR expression, our results of a VDR-dependent growth-stimulation by PTPH1 together with a decreased PTPH1 protein expression / stability in VDR depleted cells suggest that PTPH1 may increase breast cancer growth through cooperation with cytoplasmic VDR via mutual stabilization. This model suggests that regulation of PTPH1 expression and/or VDR localization may be a new approach to control breast cancer growth and progression.