A Vitamin D Receptor-Ser/Thr Phosphatase-p70 S6 Kinase Complex and Modulation of Its Enzymatic Activities by the Ligand

Abstract

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 ...

Functional Diversity of Protein Phosphatase-1, a Cellular Economizer and Reset Button

“…It has recently been demonstrated that both PP1 and the p70 S6 kinase interact with the vitamin D receptor (37). However, the p70 S6 kinase was only recruited in its phosphorylated form and in the absence of ligand.…”

“…IV). The p70 S6 kinase only binds to the vitamin D receptor in its phosphorylated form and is kept dephosphorylated by receptor-associated PP1 as long as vitamin D is bound (37). Whatever the mechanism of the mutually exclusive binding of PP1 and p70 S6 kinase to the Neurabin complex, the dissociation of PP1 from the complex may represent a key event in stress-fiber formation.…”

Promise of vitamin D analogues in the treatment of hyperproliferative conditions

“…In breast cancer cells, 1a,25(OH) 2 D 3 enhances the expression of HOXA10, a homeobox protein that causes G 1 arrest (43). In a VDR-serine/threonine protein phosphatase PP1c/PP2A-p70S6 kinase complex, 1a,25(OH) 2 D 3 seems to induce the VDR-associated phosphatase activity that in turn inactivates their substrate p70S6 kinase (44). Because p70S6 kinase is essential for G 1 -S-phase transition, these results provided further evidence of a 1a,25(OH) 2 D 3 -mediated G 1 block in colon cancer cells.…”

Signaling pathway in development of Camellia oleifera nurse seedling grafting union

“…With the deceasing PPI-1 activity, MARRS activity was increasing (Bettoun et al 2002;Farach-Carson and Nemere 2003;Zhimin and Hunter 2014). In addition, selenium proteins, as MARRS receptor (Schütze et al 1998), can promote the cell proliferation (Zeng 2009;Zhang et al 2013), and, on the other side, regulate the reduction of CENP that mediates mitotic progress (Cardoso et al 2015), with the heat shock proteins (HSP) acting upon transcription (Kochupillai 2008).…”