We provide evidence of a cross-talk between nuclear receptor and Ser/Thr protein phosphatases and show that vitamin D receptor (VDR) interacts with the catalytic subunit of protein phosphatases, PP1c and PP2Ac, and induces their enzymatic activity in a ligand-dependent manner. PP1c specifically interacts with VDR but not retinoic acid receptor ␣ and retinoid X receptor ␣ in yeast. Although VDR-PP1c and VDR-PP2Ac interaction is ligand-independent in vivo, 1␣,25-dihydroxy-vitamin D 3 induces VDR-associated phosphatase activity. Further, VDR modulation of PP1c/PP2Ac activity results in a rapid and specific dephosphorylation and inactivation of their substrate, p70 S6 kinase (p70 S6k ). Finally, we demonstrate that the endogenous VDR, PP1c or PP2Ac, and p70S6k are present in a ternary complex in vivo, and the interaction of p70S6k with the VDR-PP complex is modulated by the phosphorylation state of the kinase.
Since p70S6k is essential for G 1 -S transition, our results provide a molecular basis of 1␣,25-dihydroxyvitamin D 3 -induced G 1 block in colon cancer cells.
Vitamin D receptor (VDR),1 a sequence-specific ligand-dependent transcription factor belonging to the family of nuclear receptors, mediates biological actions of 1␣,25-dihydroxy-vitamin D 3 (1,25(OH) 2 D 3 ). VDR heterodimerizes with retinoid X receptor (RXR), and at the molecular level VDR-RXR heterodimers induce gene expression via interaction with vitamin D response elements present in the promoter regions of responsive genes (1). This mode of action is known as the "genomic action" of VDR. However 1,25-(OH) 2 D 3 also induces gene expression by a mechanism distinct from its classical mode of action. For example, 1,25-(OH) 2 D 3 -induced expression of monocytic differentiation markers CD14 and CD11b in THP-1 cells requires phosphatidylinositol 3-kinase (PI 3-kinase) via liganddependent interaction of VDR with the regulatory (p85) subunit of PI 3-kinase (2). Similarly estrogen receptor interacts with the p85 regulatory subunit of the PI 3-kinase where estrogen receptor-PI 3-kinase interaction leads to the activation of protein kinase B/AKT and endothelial nitric-oxide synthase (3). It thus appears that cross-talk between nuclear receptors and other signal transduction pathways can lead to either induction of gene expression in a nuclear receptor-responsive element-independent manner or to extranuclear/non-genomic induction of enzymatic activities that are physiologically important, for example, in explaining the vasoprotective effects of estrogen (3). Further, nuclear receptor ligands (dexamethasone, triiodothyronine, and retinoic acid) also induce a rapid dephosphorylation of c-Jun N-terminal kinase independently of their transcriptional activation (4). Therefore, nuclear receptors appear to have a functional role both outside and inside the nucleus. Ser/Thr phosphatases are implicated in the regulation of a wide variety of cellular functions, namely metabolism, transcription, translation, development, cell growth, and differentiation (5). There are ...