1,25-dihydroxyvitamin D3 modulates phosphorylation of serine 205 in the human vitamin D receptor: site-directed mutagenesis of this residue promotes alternative phosphorylation

Mitogen-Activated Protein Kinase and Nuclear Hormone Receptor Crosstalk in Cancer Immunotherapy

The three major MAP-kinase (MAPK) pathways, ERK1/2, p38 and JNK/SAPK, are upstream regulators of the nuclear "hormone" receptor superfamily (NHRSF), with a prime example given by the estrogen receptor in breast cancer. These ligand-activated transcription factors exert non-genomic and genomic functions, where they are either post-translationally modified by phosphorylation or directly interact with components of the MAPK pathways, events that govern their transcriptional activity towards target genes involved in cell differentiation, proliferation, metabolism and host immunity. This molecular crosstalk takes place not only in normal epithelial or tumor cells, but also in a plethora of immune cells from the adaptive and innate immune system in the tumor-stroma tissue microenvironment. Thus, the drugability of both the MAPK and the NHRSF pathways suggests potential for intervention therapies, especially for cancer immunotherapy. This review summarizes the existing literature covering the expression and function of NHRSF subclasses in human tumors, both solid and leukemias, and their effects in combination with current clinically approved therapeutics against immune checkpoint molecules (e.g., PD1).

A Narrative Role of Vitamin D and Its Receptor: With Current Evidence on the Gastric Tissues

Vitamin D is a major steroid hormone that is gaining attention as a therapeutic molecule. Due to the general awareness of its importance for the overall well-being, vitamin D deficiency (VDD) is now recognized as a major health issue. The main reason for VDD is minimal exposure to sunlight. The vitamin D receptor (VDR) is a member of the steroid hormone receptors that induces a cascade of cell signaling to maintain healthy Ca²⁺ levels that serve to regulate several biological functions. However, the roles of vitamin D and its metabolism in maintaining gastric homeostasis have not yet been completely elucidated. Currently, there is a need to increase the vitamin D status in individuals worldwide as it has been shown to improve musculoskeletal health and reduce the risk of chronic illnesses, including some cancers, autoimmune and infectious diseases, type 2 diabetes mellitus, neurocognitive disorders, and general mortality. The role of vitamin D in gastric homeostasis is crucial and unexplored. This review attempts to elucidate the central role of vitamin D in preserving and maintaining the overall health and homeostasis of the stomach tissue.

Fine tuning of vitamin D receptor (VDR) activity by post-transcriptional and post-translational modifications

Vitamin D receptor (VDR) is a member of the nuclear receptor (NR) superfamily of ligand-activated transcription factors. Activated VDR is responsible for maintaining calcium and phosphate homeostasis, and is required for proper cellular growth, cell differentiation and apoptosis. The expression of both phases I and II drug-metabolizing enzymes is also regulated by VDR, therefore it is clinically important.

Post-translational modifications of NRs have been known as an important mechanism modulating the activity of NRs and their ability to drive the expression of target genes. The aim of this mini review is to summarize the current knowledge about post-transcriptional and post-translational modifications of VDR.

The impact of VDR expression and regulation in vivo

The vitamin D receptor (VDR) mediates the pleiotropic biological actions of 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃). These actions include orchestration of mineral homeostasis which is coordinated by the kidney, intestine, bone and parathyroid gland wherein the VDR transcriptionally regulates expression of the genes involved in this complex process. Mutations in human VDR (hVDR) cause hereditary vitamin D resistant rickets, a genetic syndrome characterized by hypocalcemia, hyperparathyroidism and rickets resulting from dysregulation of mineral homeostasis. Expression of the VDR is regulated by external stimuli in a tissue-specific manner. However, the mechanisms of this tissue-specificity remain unclear. Studies also suggest that phosphorylation of hVDR at serine 208 impacts the receptor's transcriptional activity. These experiments were conducted in vitro, however, and therefore limited in their conclusions. In this report, we summarize (1) our most recently updated ChIP-seq data from mouse tissues to identify regulatory regions responsible for the tissues-specific regulation of the VDR and (2) our studies to understand the mechanism of hormonal regulation of Vdr expression in bone and kidney in vivo using transgenic mouse strains generated by mouse mini-genes that contain comprehensive genetic information capable of recapitulating endogenous Vdr gene regulation and expression. We also defined the functional human VDR gene locus in vivo by using a human mini-gene comparable to that in the mouse to generate a humanized VDR mouse strain in which the receptor is expressed at normal levels (normal expressor). The present report also shows that a humanized mouse model in which the VDR is expressed at levels about 10-fold lower than the normal expressor mouse rescued the VDR-null phenotype despite its reduced transcriptional activity relative to wildtype expression. We also generated an additional humanized mouse model expressing hVDR bearing a mutation converting serine 208 to alanine (hVDR-S208A). In spite of the mutation, target gene expression induced by the ligand was unchanged relative to a mouse strain expressing comparable levels of wildtype hVDR. Further characterization also showed that serum calcium and parathyroid hormone levels were normal and alopecia was not observed in this hVDR-S208A mouse strain as well. Taken together, our in vivo studies using ChIP-seq analyses and the mini-gene transgenic mice improve our understanding of the tissue-specific regulatory mechanisms of controlling VDR expression and the mechanisms of action of the VDR.

Mouse and human BAC transgenes recapitulate tissue-specific expression of the vitamin D receptor in mice and rescue the VDR-null phenotype

The biological actions of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are mediated by the vitamin D receptor (VDR), which is expressed in numerous target tissues in a cell type-selective manner. Recent studies using genomic analyses and recombineered bacterial artificial chromosomes (BACs) have defined the specific features of mouse and human VDR gene loci in vitro. In the current study, we introduced recombineered mouse and human VDR BACs as transgenes into mice and explored their expression capabilities in vivo. Individual transgenic mouse strains selectively expressed BAC-derived mouse or human VDR proteins in appropriate vitamin D target tissues, thereby recapitulating the tissue-specific expression of endogenous mouse VDR. The mouse VDR transgene was also regulated by 1,25(OH)2D3 and dibutyryl-cAMP. When crossed into a VDR-null mouse background, both transgenes restored wild-type basal as well as 1,25(OH)2D3-inducible gene expression patterns in the appropriate tissues. This maneuver resulted in the complete rescue of the aberrant phenotype noted in the VDR-null mouse, including systemic features associated with altered calcium and phosphorus homeostasis and disrupted production of parathyroid hormone and fibroblast growth factor 23, and abnormalities associated with the skeleton, kidney, parathyroid gland, and the skin. This study suggests that both mouse and human VDR transgenes are capable of recapitulating basal and regulated expression of the VDR in the appropriate mouse tissues and restore 1,25(OH)2D3 function. These results provide a baseline for further dissection of mechanisms integral to mouse and human VDR gene expression and offer the potential to explore the consequence of selective mutations in VDR proteins in vivo.

The vitamin D receptor: new paradigms for the regulation of gene expression by 1,25-dihydroxyvitamin D3

This article represents a summary of what is known of the VDR protein and its molecular mechanism of action at target genes. New methodologies now used, such as ChIP-chip and ChIP-seq, as well as novel reporter studies using large BAC clones stably transfected into culture cells or introduced as transgenes in mice, are providing new insights into how 1,25(OH)2D3-activated VDR modulates the expression of genes at single gene loci and at the level of gene networks. Many of these insights are unexpected and suggest that gene regulation is even more complex than previously appreciated. These studies also highlight new technologies and their central role in establishing fundamental biologic principles.

The vitamin D receptor: new paradigms for the regulation of gene expression by 1,25-dihydroxyvitamin D(3)

The actions of the vitamin D hormone 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) are mediated by the vitamin D receptor (VDR), a ligand-activated transcription factor that functions to control gene expression. After ligand activation, the VDR binds directly to specific sequences located near promoters and recruits a variety of coregulatory complexes that perform the additional functions required to modify transcriptional output. Recent advances in transcriptional regulation, which permit the unbiased identification of the regulatory regions of genes, are providing new insight into how genes are regulated. Surprisingly, gene regulation requires the orchestrated efforts of multiple modular enhancers often located many kilobases upstream, downstream, or within the transcription units themselves. These studies are transforming our understanding of how 1,25(OH)(2)D(3) regulates gene transcription.

Evolution of our understanding of vitamin D

The development of our understanding of the function of vitamin D from its discovery in the second and third decades of the 20(th) century to its hormonal activation of its nuclear receptor and to its present position of an important factor in public health has been traced. The key discoveries of the conversion of vitamin D to its hormonal form, its regulation, and the evolving picture of its molecular mechanism of action are presented. The recognition of its role beyond mineralization of the skeleton to its role in skin, the immune system, and its protective role in some forms of malignancy represent more recent developments. The evolution of derivatives of 1alpha,25-dihydroxyvitamin D(3) as therapeutic agents suggests a richness of therapeutic potential. All of this nevertheless illustrates that much more remains to be discovered and applied to our armaments for preventing and treating disease.

Phosphorylation of human vitamin D receptor serine-182 by PKA suppresses 1,25(OH)2D3-dependent transactivation

The human vitamin D receptor (hVDR), which is a substrate for several protein kinases, mediates the actions of its 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) ligand to regulate gene expression. To determine the site, and functional impact, of cAMP-dependent protein kinase (PKA)-catalyzed phosphorylation of hVDR, we generated a series of C-terminally truncated and point mutant receptors. Incubation of mutant hVDRs with PKA and [gamma-32P]ATP, in vitro, or overexpressing them in COS-7 kidney cells labeled with [32P]orthophosphate, revealed that serine-182 is the predominant residue in hVDR phosphorylated by PKA. An aspartate substituted mutant (S182D), incorporating a negative charge to mimic phosphorylation, displayed only 50% of the transactivation capacity in response to 1,25(OH)2D3 of either wild-type or an S182A-altered hVDR. When the catalytic subunit of PKA was overexpressed, a similar reduction in wild-type but not S182D hVDR transactivity was observed. In a mammalian two-hybrid system, S182D bound less avidly than wild-type or S182A hVDR to the retinoid X receptor (RXR) heterodimeric partner that co-mediates vitamin D responsive element recognition and transactivation. These data suggest that hVDR serine-182 is a primary site for PKA phosphorylation, an event that leads to an attenuation of both RXR heterodimerization and resultant transactivation of 1,25(OH)2D3 target genes.

Rapid, membrane-initiated actions of 1,25 dihydroxyvitamin D: what are they and what do they mean?

Vitamin D is a conditionally required nutrient traditionally thought to influence physiology as the metabolite 1,25-dihydroxyvitamin D [1,25(OH)(2) D] by binding to the vitamin D receptor (VDR) and stimulating the transcription of genes through direct VDR-DNA interactions. However, over the past 15 y research has demonstrated that 1,25(OH)(2) D, as well as other steroid hormones, can rapidly stimulate ion fluxes and activate protein kinases by transcription-independent mechanisms. This review summarizes recent research on the rapid actions of 1,25(OH)(2) D and identifies questions that remain to be answered in this area.

Integration of hormone signaling in the regulation of human 25(OH)D3 24-hydroxylase transcription

The current study sought to define the molecular mechanisms involved in the cross talk between 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and activators of PKC in the regulation of 25(OH)D(3) 24-hydroxlyase [24(OH)ase]. Transfection of the h24(OH)ase promoter construct [-5,500/-22 luciferase; vitamin D response elements at -294/-274 and -174/-151; AP-1 site at -1,167/-1,160] in vitamin D receptor (VDR)-transfected COS-7 cells resulted in strong activation by 1,25(OH)(2)D(3). In these cells, cotreatment with the PKC activator TPA and 1,25(OH)(2)D(3) yielded a 27-fold increase in luciferase activity, which was 2- to 3-fold greater than activation obtained with 1,25(OH)(2)D(3) alone (P < 0.05). Similar results were observed using LLCPK-1 kidney cells, suggesting that the previously observed enhancement of 1,25(OH)(2)D(3)-induced renal 24(OH)ase mRNA and activity by PKC activation occurs at the level of transcription. The functional cooperation between PKC activation and VDR was not found to be mediated by the AP-1 site in the h24(OH)ase promoter or by enhanced binding of GRIP or DRIP205 to VDR and was also not due to PKC-mediated phosphorylation of VDR on Ser(51). Our study demonstrates that, in LLCPK-1 kidney cells, the PKC enhancement of 1,25(OH)(2)D(3)-stimulated transcription may be due, in part, to an increase in VDR concentration. In addition, inhibitors of the MAPK pathway were found to decrease the TPA enhancement (P < 0.05). Because activation of MAPK has been reported to result in the phosphorylation of SRC-1 and in functional cooperation between SRC-1 and CREB binding protein, we propose that the potentiation of VDR-mediated transcription may also be mediated through changes in the phosphorylation of specific VDR coregulators.

G2/M arrest by 1,25-dihydroxyvitamin D3 in ovarian cancer cells mediated through the induction of GADD45 via an exonic enhancer

1,25-Dihydroxyvitamin D3 suppresses the growth of multiple human cancer cell lines by inhibiting cell cycle progression and inducing cell death. The present study showed that 1,25-dihydroxyvitamin D3 causes cell cycle arrest at the G2/M transition through p53-independent induction of GADD45 in ovarian cancer cells. Detailed analyses have established GADD45 as a primary target gene for 1,25-dihydroxyvitamin D3. A DR3-type vitamin D response element was identified in the fourth exon of GADD45 that forms a complex with the vitamin D receptor.retinoid X receptor heterodimer in electrophoresis mobility shift assays and mediates the dose-dependent induction of luciferase activity by 1,25-dihydroxyvitamin D3 in reporter assays. Chromatin immunoprecipitation assays have shown that the vitamin D receptor is recruited in a ligand-dependent manner to the exonic enhancer but not to the GADD45 promoter regions. In ovarian cancer cells expressing GADD45 antisense cDNA or GADD45-null mouse embryo fibroblasts, 1,25-dihydroxyvitamin D3 failed to induce G2/M arrest. Taken together, these results identify GADD45 as an important mediator for the tumor-suppressing activity of 1,25-dihydroxyvitamin D3 in human ovarian cancer cells.

Effects of 1,25-dihydroxyvitamin D3 on the distribution of androgen and vitamin D receptors in human prostate neonatal epithelial cells

Although many studies have examined the mechanisms of 1,25-dihydroxyvitamin D(3) (calcitriol or 1,25 D) action in different prostate cancer cell lines, little is known regarding the influence of this steroid on the normal prostate. The presence of both VDR and AR in normal prostatic tissues raises the distinct possibility of an important role for this hormone in the normal gland. In order to ascertain the possible role of 1,25 D on both AR and VDR in the normal prostate, the effects of calcitriol and dihydrotestosterone (DHT) on the normal human neonatal prostatic epithelial cell line, 267B-1, were examined. These studies were approached by focusing on how 1,25 D in the presence or absence of DHT affects the distribution of AR and VDR in the cytoplasmic and nuclear compartments of the cells in terms of their protein levels and DNA binding activities. Immunoblot analyses show that 1,25 D increases the AR protein level in both the cytoplasmic and nuclear fractions but not the VDR protein level. On the other hand, the gel shift assays demonstrate that 1,25 D increases both the AR- and VDR-DNA binding activities in the nuclear fraction, whereas there is no increase in DNA binding activities in the cytoplasmic fraction. Addition of DHT along with 1,25 D does not affect the DNA binding activities of both AR and VDR. Overall, these studies suggest that 1,25 D actions on the normal prostate cells may be mediated independently through AR and VDR, respectively.

Induction of apoptosis by 1,25-dihydroxyvitamin D3 in MCF-7 Vitamin D3-resistant variant can be sensitized by TPA

Vitamin D(3) compounds offer an alternative approach to anti-hormonal therapies for human breast cancer. 1,25-Dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) acts through the nuclear Vitamin D(3) receptor (VDR), a phosphoprotein and ligand-dependent transcription factor. Our lab has shown that 1,25-(OH)(2)D(3) induces apoptosis in MCF-7 cells by disruption of mitochondrial function which is associated with Bax translocation to mitochondria, cytochrome c release, and production of reactive oxygen species (ROS). TPA, a protein kinase C (PKC) activator, does not induce cytochrome c release or Bax translocation, thus demonstrating that it has no effect on mitochondria and apoptosis on its own. However, when the MCF-7(D(3)Res) cells (a Vitamin D(3)-resistant variant) are treated with 1,25-(OH)(2)D(3) in the presence of TPA, the cells displayed apoptotic morphology and redistribution of both cytochrome c and Bax. TPA pretreatment greatly enhances 1,25-(OH)(2)D(3) stimulated 24-hydroxylase luciferase activity and VDR protein expression, although transactivation is lower in the MCF-7(D(3)Res) cells compared to the parental cell line. The observation that the phorbol ester TPA sensitizes the Vitamin D(3)-resistant variant to the effects of 1,25-(OH)(2)D(3) suggests an important role for phosphorylation in dictating sensitivity to Vitamin D(3)-mediated apoptosis. This study demonstrates that the effects of 1,25-(OH)(2)D(3) on mitochondrial disruption might be sensitized through activators of PKC.

Vector-averaged gravity-induced changes in cell signaling and vitamin D receptor activity in MG-63 cells are reversed by a 1,25-(OH)2D3 analog, EB1089

Skeletal unloading in an animal hindlimb suspension model and microgravity experienced by astronauts or as a result of prolonged bed rest causes site-specific losses in bone mineral density of 1%-2% per month. This is accompanied by reductions in circulating levels of 1,25-(OH)(2)D(3), the active metabolite of vitamin D. 1,25-(OH)(2)D(3), the ligand for the vitamin D receptor (VDR), is important for calcium absorption and plays a role in differentiation of osteoblasts and osteoclasts. To examine the responses of cells to activators of the VDR in a simulated microgravity environment, we used slow-turning lateral vessels (STLVs) in a rotating cell culture system. We found that, similar to cells grown in microgravity, MG-63 cells grown in the STLVs produce less osteocalcin, alkaline phosphatase, and collagen Ialpha1 mRNA and are less responsive to 1,25-(OH)(2)D(3). In addition, expression of VDR was reduced. Moreover, growth in the STLV caused activation of the stress-activated protein kinase pathway (SAPK), a kinase that inhibits VDR activity. In contrast, the 1,25-(OH)(2)D(3) analog, EB1089, was able to compensate for some of the STLV-associated responses by reducing SAPK activity, elevating VDR levels, and increasing expression of osteocalcin and alkaline phosphatase. These studies suggest that, not only does simulated microgravity reduce differentiation of MG-63 cells, but the activity of the VDR, an important regulator of bone metabolism, is reduced. Use of potent, less calcemic analogs of 1,25-(OH)(2)D(3) may aid in overcoming this defect.

Isolation of baculovirus-expressed human vitamin D receptor: DNA responsive element interactions and phosphorylation of the purified receptor

Two controversial aspects in the mechanism of human vitamin D receptor (hVDR) action are the possible significance of VDR homodimers and the functional role of receptor phosphorylation. To address these issues, milligram quantities of baculovirus-expressed hVDR were purified to 97% homogeneity, and then tested for binding to the rat osteocalcin vitamin D responsive element (VDRE) via electrophoretic mobility shift and half-site competition assays in the presence or absence of a CV-1 nuclear extract containing retinoid X receptor (RXR). Methylation interference analysis revealed that both the hVDR homodimer and the VDR-RXR heterodimer display similar patterns of VDRE G-base protection. However, in competition studies, the relative dissociation of the homodimeric hVDR complex from the VDRE was extremely rapid (t1/2 < 30 s) compared to the dissociation of the heteromeric complex (t1/2 > 5 min), thus illustrating the relative instability and low affinity of homodimeric VDR binding to DNA. These results indicate that VDR-RXR heterodimers are the preferred VDRE binding species. Further, two dimensional gel electrophoresis of hVDR demonstrated several isoelectric forms of the receptor, suggesting that it is subject to multiple phosphorylation events. In vitro kinase assays confirmed that purified hVDR is an efficient substrate for protein kinases A and Cbeta, as well as casein kinase II. In vivo studies of the expressed receptor in intact cells, namely baculovirus vector infected Sf9 insect cells and transfected mammalian COS-7 cells, demonstrated that hVDR was phosphorylated in a hormone-enhanced fashion. Functional consequences of hVDR phosphorylation were suggested by the observations that: (i) potato acid phosphatase (PAP)-treated hVDR no longer interacted with the VDRE as either a homodimer or a heteromeric complex with RXR, and (ii) treatment of transfected COS-7 cells with a phosphatase inhibitor (okadaic acid) along with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) resulted in a synergistic enhancement of both hVDR phosphorylation and transactivation of a VDRE-linked reporter gene, compared to the effect of treatment with either agent alone. These studies point to a significant role for phosphorylation of VDR in regulating high-affinity VDR-RXR interactions with VDREs, and also in modulating 1,25(OH)2D3-elicited transcriptional activation in target cells.

The p38 and JNK pathways cooperate to trans-activate vitamin D receptor via c-Jun/AP-1 and sensitize human breast cancer cells to vitamin D(3)-induced growth inhibition

The signaling connection between mitogen-activated protein kinases(MAPKs) and nuclear steroid receptors is complex and remains mostly unexplored. Here we report that stress-activated protein kinases p38 and JNK trans-activate nuclear steroid vitamin D receptor (VDR) gene and increase vitamin D(3)-dependent growth inhibition in human breast cancer cells. Activation of p38 and JNK by an active MAPK kinase 6 stimulates VDR promoter activity independently of the ligand vitamin D(3) and estrogen receptor expression. Moreover, stimulation of the endogenous stress pathways by adenovirus-mediated delivery of recombinant MAPK kinase 6 also activates VDR and sensitizes MCF-7 cells to vitamin D(3)-dependent growth inhibition. Both the p38 and JNK MAPK pathways and the downstream transcription factor c-Jun/AP-1 are required for the VDR stimulation, as revealed by application of their dominant negatives, the specific p38 inhibitor SB203580, and site-directed mutagenesis of the AP-1 element in the VDR promoter. The essential role of the p38 and JNK stress pathways in up-regulation of VDR expression is further confirmed by using the chemical stimulator arsenite. These results establish a signaling connection between the stress MAPK pathways and steroid hormone receptor VDR expression and thereby offer new insights into regulation of cell growth by the MAPK pathways through regulation of vitamin D(3)/VDR activity.

Enhancement of VDR-mediated transcription by phosphorylation: correlation with increased interaction between the VDR and DRIP205, a subunit of the VDR-interacting protein coactivator complex

When UMR-106 osteoblastic cells, LLCPK1 kidney cells, and VDR transfected COS-7 cells were transfected with the rat 24-hydroxylase [24(OH)ase] promoter (-1,367/+74) or the mouse osteopontin (OPN) promoter (-777/+79), we found that the response to 1,25dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] could be significantly enhanced 2- to 5-fold by the protein phosphatase inhibitor, okadaic acid (OA). Enhancement of 1,25-(OH)(2)D(3)-induced transcription by OA was also observed using a synthetic reporter gene containing either the proximal 24(OH)ase vitamin D response element (VDRE) or the OPN VDRE, suggesting that the VDRE is sufficient to mediate this effect. OA also enhanced the 1,25-(OH)(2)D(3)-induced levels of 24(OH)ase and OPN mRNA in UMR osteoblastic cells. The effect of OA was not due to an up-regulation of VDR or to an increase in VDR-RXR interaction with the VDRE. To determine whether phosphorylation regulates VDR-mediated transcription by modulating interactions with protein partners, we examined the effect of phosphorylation on the protein-protein interaction between VDR and DRIP205, a subunit of the vitamin D receptor-interacting protein (DRIP) coactivator complex, using glutathione-S-transferase pull-down assays. Similar to the functional studies, OA treatment was consistently found to enhance the interaction of VDR with DRIP205 3- to 4-fold above the interaction observed in the presence of 1,25-(OH)(2)D(3) alone. In addition, studies were done with the activation function-2 defective VDR mutant, L417S, which is unable to stimulate transcription in response to 1,25-(OH)(2)D(3) or to interact with DRIP205. However, in the presence of OA, the mutant VDR was able to activate 24(OH)ase and OPN transcription and to recruit DRIP205, suggesting that OA treatment may result in a conformational change in the activation function-2 defective mutant that creates an active interaction surface with DRIP205. Taken together, these findings suggest that increased interaction between VDR and coactivators such as DRIP205 may be a major mechanism that couples extracellular signals to vitamin D action.

Stress pathway activation induces phosphorylation of retinoid X receptor

Cellular stresses inhibit retinoid signaling, but the molecular basis for this phenomenon has not been revealed. Here, we present evidence that retinoid X receptor (RXR) is a substrate for both mitogen-activated protein kinase kinase-4 (MKK4/SEK1) and its downstream mediator c-Jun N-terminal kinase (JNK). MKK4/SEK1 and JNK recognized distinct features on RXR in the DE and AB regions, respectively. Phosphorylation by MKK4/SEK1 had profound effects on the biochemical properties of RXR, inhibiting the expression of genes activated by RXR-retinoic acid receptor complexes. Tyr-249 in the RXR DE region was required for the inhibitory effect of MKK4/SEK1. These effects were significantly reduced in MKK4/SEK1-null cells, indicating that MKK4/SEK1 is required for the suppression of retinoid signaling by stress. Findings presented here demonstrate that MKK4/SEK1 can directly modulate transcription by phosphorylating RXR, a novel MKK4/SEK1 substrate.

Differential effects of 20-epi vitamin D analogs on the vitamin D receptor homodimer

Vitamin D analogs have received increased attention because of their possible therapeutic benefits in treating osteoporosis and various proliferative disorders. Several analogs were examined for their effects on DNA binding of the vitamin D receptor (VDR) homodimer complex with the murine osteopontin vitamin D response element. All of the tested analogs increased complex binding by recombinant human VDR in the electrophoretic mobility shift assay and notable differences in mobility of these complexes were observed. A panel of C-terminal anti-VDR antisera were screened for their ability to interact with analog-bound VDR homodimer complexes or as a heterodimer complex with recombinant human retinoid X receptor alpha (rhRXR alpha). Like calcitriol, analog-bound heterodimer complexes were largely resistant to interaction with these antisera; however, striking differences were observed with the various antisera in an analogous homodimer binding experiment. KH1060 and CB1093, analogs with 20-epi conformations, produced homodimer complexes that were 3- to 6-fold more resistant to supershifting with Ab180 compared with the hormone or EB1089. Chymotrypsin digestion in combination with Western blotting using a C-terminal anti-VDR antiserum revealed similar digestion patterns for all ligands. However, KH1060- and CB1093-bound VDR complexes were more resistant to digestion than either calcitriol or EB1089. Finally, the ability of these compounds to yield stable homodimer complexes was assessed by challenging preformed homodimer with the exogenous addition of rhRXR alpha extracts. Although new heterodimer complexes appeared in a time-dependent fashion, the preformed homodimer complexes exhibited stable binding throughout the time course of the experiment. The results indicate that VDR homodimers are targets of vitamin D analogs with differential effects on C-terminal protein conformation that may partially explain the varied biological responses of these compounds.

Multisite autophosphorylation of p21-activated protein kinase gamma-PAK as a function of activation

p21-activated protein kinase (PAK) is a family of serine/threonine kinases whose activity is stimulated by binding to small G-proteins such as Cdc42 and subsequent autophosphorylation. Focusing on the ubiquitous gamma-isoform of PAK in this study, baculovirus-infected insect cells were used to obtain recombinant gamma-PAK, while native gamma-PAK was isolated from rabbit reticulocytes. Two-dimensional gel electrophoresis of gamma-PAK followed by immunoblot analysis revealed a similar profile for native and recombinant gamma-PAK, both consisting of multiple protein spots. Following Cdc42-stimulated autophosphorylation, the two-dimensional profiles of native and recombinant gamma-PAK were characterized by a similar acidic shift, suggesting a common response to Cdc42. To understand the effect of differential phosphorylation on its activation status, gamma-PAK autophosphorylation was conducted in the presence or absence of activators such as Cdc42 and histone II-AS, followed by tryptic digestion and comparative two-dimensional phosphopeptide mapping. The major phosphopeptides were subjected to a combination of manual and automated amino acid sequencing. Overall, eight autophosphorylation sites were identified in Cdc42-activated gamma-PAK, six of which are in common with those previously reported in alpha-PAK, while Ser-19 and Ser-165 appear to be uniquely phosphorylated in the gamma-form. Further, the phosphorylation of Ser-141, Ser-165, and Thr-402 was found to correlate with gamma-PAK activation.

Ecdysteroid receptor and ultraspiracle from Chironomus tentans (Insecta) are phosphoproteins and are regulated differently by molting hormone

Three different isotypes of the ecdysteroid receptor (cEcR) (66, 68 and 70 kDa) and several molecular variants of the dimerization partner "ultraspiracle" (cUSP) (58-77 kDa) can be separated electrophoretically in homogenates of the epithelial cell line from Chironomus tentans. After phosphatase treatment the bands with the lowest electrophoretic mobility disappear in both cases. Phosphorylation occurs exclusively at ser/thr in EcR and USP. Binding studies with 3H-ponasterone A using 0.4 M NaCl extracts revealed two classes of high-affinity binding (KD1 = 0.47 and KD2 = 7.2 nM) competable either with 20-OH-ecdysone or muristerone A. At least KD2 and Bmax2 are unchanged after dephosphorylation. In hormonally naive cells a considerable part of EcR and USP is already present in nuclei. The phosphorylation pattern of both transcription factors is the same in cytosol and nuclear fractions. Incubation with 20-OH-ecdysone (1 microM, up to 4 days) does not alter the extent and mode of phosphorylation of EcR, although EcR concentration increases. In contrast USP concentration remains constant but phosphorylation is enhanced.

Site-directed substitution of Ser1406 of hamster CAD with glutamic acid alters allosteric regulation of carbamyl phosphate synthetase II

Ser1406 of the allosteric region of the hamster CAD enzyme, carbamyl phosphate synthetase II (CPSase), is known to be phosphorylated in vitro by cAMP-dependent protein kinase (PKA). Metabolic labeling experiments described here demonstrate that CAD is phosphorylated in somatic cells in culture. Phosphorylation is stimulated by treating cells with 8-bromo-cAMP, a PKA activator. The stimulation is essentially prevented by pretreatment with H-89, a PKA specific inhibitor. Substitution of Ser1406 with alanine results in an enzyme with kinetics and allosteric regulation indistinguishable from unsubstituted CAD. However, substitution to glutamic acid increases CPSase activity by reducing the apparent Km (ATP). The UTP concentration required to give 50% inhibition is increased rendering this altered enzyme significantly less sensitive to feedback inhibition, but allosteric activation by PRPP is unaffected. While these data do not prove that Ser1406 is phosphorylated in vivo, they do indicate that a specific alteration at this residue can affect allosteric regulation.

Steroid hormone receptors and their regulation by phosphorylation

The steroid/thyroid hormone receptor superfamily of ligand-activated transcription factors encompasses not only the receptors for steroids, thyroid hormone, retinoids and vitamin D, but also a large number of proteins whose functions and/or ligands are unknown and which are thus termed orphan receptors. Recent studies have highlighted the importance of phosphorylation in receptor function. Although most of the phosphorylation sites are serine and threonine residues, a few of the family members are also phosphorylated on tyrosine. Those steroid receptor family members that are bound to heat-shock proteins in the absence of ligand typically are basally phosphorylated and exhibit increases in phosphorylation upon ligand binding. Most of these sites contain Ser-Pro motifs, and there is evidence that cyclin-dependent kinases and MAP kinases (mitogen-activated protein kinases) phosphorylate subsets of these sites. In contrast, phosphorylation sites identified thus far in members of the family that bind to DNA in the absence of hormone typically do not contain Ser-Pro motifs and are frequently casein kinase II or protein kinase A sites. Phosphorylation has been implicated in DNA binding, transcriptional activation and stability of the receptors. The finding that some of the steroid receptor family members can be activated in the absence of ligand by growth factors or neurotransmitters that modulate kinase and/or phosphatase pathways underscores the role of phosphorylation in receptor function. Hence this family of transcription factors integrates signals from ligands as well as from signal transduction pathways, resulting in alterations in mRNA and protein expression that are unique to the complex signals received.

Stoichiometry and site-specific phosphorylation of human progesterone receptor in native target cells and in the baculovirus expression system

Human progesterone receptor (PR) in T47D breast cancer cells is phosphorylated on nine different serine residues; three are hormone-inducible (Ser102, Ser294, and Ser345), while others are basal but hormone-stimulated. In the present study, we have compared the phosphorylation state of native and recombinant PR expressed in a baculovirus insect cell system. Stoichiometric measurements showed that unliganded native PR in T47D cells was approximately 50% phosphorylated ( approximately 4 phosphates/PR) and became essentially 100% phosphorylated ( approximately 9 phosphates/PR) when bound to hormone. Unliganded PR expressed in Sf9 insect cells was phosphorylated with a similar stoichiometry ( approximately 3 phosphates/PR), but the phosphate content did not change with hormone addition. Site-specific phosphorylation analyzed by tryptic phosphopeptide mapping and manual peptide sequencing revealed that expressed PR bound to hormone in the Sf9 insect cells was phosphorylated on all the same sites as hormone-treated PR in T47D cells. Only minor differences were detected in the relative proportion of three sites (two basal sites and Ser345) and phosphorylation did not occur on alternate sites. Interestingly, unliganded baculovirus-expressed PR was constitutively phosphorylated on hormone inducible sites and was phosphorylated on basal sites to the same extent as hormone treated PR. Thus, in the absence of hormone, the phosphorylation state of baculovirus-expressed PR resembled that of the hyperphosphorylated native PR. In contrast to native PR, the expressed receptor in cytosols of Sf9 cells did not form a large oligomeric complex suggesting that hyperphosphorylation may be due to dissociation of the complex in the absence of hormone. This study demonstrating phosphorylation on correct sites with a stoichiometry similar to that of native PR indicates that overexpressed PR in the baculovirus system is suitable for in vitro structure/function studies.

Role of phosphorylation on DNA binding and transcriptional functions of human progesterone receptors

To study the function of human progesterone receptor (hPR) phosphorylation, we have tested four sets of serine to alanine substitution mutants: 10 serine clusters, located in regions common to both hPR isoforms (the M-series mutants) were mutated in A-receptors and B-receptors; 6 serine clusters located in the B-upstream segment (BUS; the B-series mutants) were mutated individually and collectively and cloned into B-receptors and into BUS-DBD-NLS, a constitutive transactivator, in which the AF3 function of BUS is fused to the DNA binding domain (DBD) and nuclear localization signal (NLS) of hPR. Transcription by most of the M-series mutants resembles that of wild-type A- or B-receptors. Mutation of 3 sites, Ser190 at the N terminus of A-receptors, a cluster of serines just upstream of the DBD, or Ser676 in the hinge region, inhibits transcription by 20-50% depending on cell or promoter context. These sites lie outside the AF1 activation function. M-series mutants are substrates for a hormone-dependent phosphorylation step, and they all bind well to DNA. Progressive mutation of the B-series clusters leads to the gradual dephosphorylation of BUS, but only the 6-site mutant, involving 10 serine residues, is completely dephosphorylated. These data suggest that in BUS alternate serines are phosphorylated or dephosphorylated at any time. However, even when BUS is completely dephosphorylated, both BUS-DBD-NLS and full-length B-receptors remain strong transactivators. Mutant B-receptors also do not acquire the dominant negative properties of A-receptors, and they retain the ability to activate transcription in synergy with 8-Br-cAMP and antiprogestins. We conclude that phosphorylation has subtle effects on the complex transcriptional repertoire that distinguishes the two hPR isoforms and does not influence transactivation mediated by AF1 or AF3, but subserves other functions.

Genomic mechanisms involved in the pleiotropic actions of 1,25-dihydroxyvitamin D3

The biologically active metabolite of vitamin D (cholecalciferol), i.e. 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], is a secosteroid hormone whose mode of action involves stereospecific interaction with an intracellular receptor protein (vitamin D receptor; VDR). 1,25(OH)2D3 is known to be a principal regulator of calcium homeostasis, and it has numerous other physiological functions including inhibition of proliferation of cancer cells, effects on hormone secretion and suppression of T-cell proliferation and cytokine production. Although the exact mechanisms involved in mediating many of the different effects of 1,25(OH)2D3 are not completely defined, genomic actions involving the VDR are clearly of major importance. Similar to other steroid receptors, the VDR is phosphorylated; however, the exact functional role of the phosphorylation of the VDR remains to be determined. The VDR has been reported to be regulated by 1,25(OH)2D3 and also by activation of protein kinases A and C, suggesting co-operativity between signal transduction pathways and 1,25(OH)2D3 action. The VDR binds to vitamin D-responsive elements (VDREs) in the 5' flanking region of target genes. It has been suggested that VDR homodimerization can occur upon binding to certain VDREs but that the VDR/retinoid X receptor (RXR) heterodimer is the functional transactivating species. Other factors reported to be involved in VDR-mediated transcription include chicken ovalbumin upstream promoter (COUP) transcription factor, which is involved in active silencing of transcription, and transcription factor IIB, which has been suggested to play a major role following VDR/RXR heterodimerization. Newly identified vitamin D-dependent target genes include those for Ca2+/Mg(2+)-ATPase in the intestine and p21 in the myelomonocytic U937 cell line. Elucidation of the mechanisms involved in the multiple actions of 1,25(OH)2D3 will be an active area of future research.

Phosphorylation of Ser211 in the chicken progesterone receptor modulates its transcriptional activity

The chicken progesterone receptor has been shown to be phosphorylated in vivo at four major sites. Previous studies have shown that mutation of one of the hormone-dependent phosphorylation sites, Ser530, to alanine decreases the transcriptional activity of the receptor under conditions where ligand is limited. Here, we present evidence for the functional significance of another phosphorylation site, Ser211. Mutation of Ser211 to alanine results in a decrease in the transcriptional activity of the receptor and affects the phosphorylation-dependent decrease in mobility of the receptor in SDS-polyacrylamide gel electrophoresis. The degree of reduction in transcriptional activity is dependent on both the cell type and the reporters used in the studies but is independent of hormone concentration, suggesting that phosphorylation at Ser211 regulates the activity of the receptor through a mechanism distinct from Ser530 phosphorylation.

Human vitamin D receptor phosphorylation by casein kinase II at Ser-208 potentiates transcriptional activation

The potential functional significance of human 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] receptor (hVDR) phosphorylation at Ser-208 was evaluated by cotransfecting COS-7 kidney cells with hVDR constructs and the catalytic subunit of human casein kinase 11 (CK-11). Under these conditions, hVDR is intensely phosphorylated in a reaction that depends on both CK-II and the presence of Ser-208. The resulting hyperphosphorylated receptor is unaltered in its kinetics for binding the 1,25(OH)2D3 ligand, its partitioning into the nucleus, and its ability to associate with a vitamin D responsive element. Replacement of Ser-208 with glycine or alanine indicates that phosphorylation of hVDR at Ser-208 is not obligatory for 1,25(OH)2D3 action, but coexpression of wild-type hVDR and CK-11 elicits a dose-dependent enhancement of 1,25(OH)2D3-stimulated transcription of a vitamin D responsive element reporter construct. This enhancement by CK-II is abolished by mutating Ser-208 to glycine or alanine and does not occur with glucocorticoid receptor-mediated transcription. Therefore, phosphorylation of hVDR by CK-11 at Ser-208 specifically modulates its transcriptional capacity, suggesting that this covalent modification alters the conformation of VDR to potentiate its interaction with the machinery for DNA transcription.

Two types of anti-progestins have distinct effects on site-specific phosphorylation of human progesterone receptor

Human progesterone receptor (PR) is phosphorylated on multiple serine residues; three sites (Ser102, Ser294, and Ser345) are inducible by hormone agonist, while at least six others are basally phosphorylated and exhibit a general increase in response to hormone. In this study we have used high performance liquid chromatography phosphopeptide mapping and manual peptide sequencing to investigate how two different progestin antagonists, RU486 and ZK98299, affect site-specific phosphorylation of PR isolated from T47D breast cancer cells. As compared to the progestin agonist R5020, RU486 stimulated a similar increase in overall incorporation of [32P]phosphate per PR molecule (2.5-2.6-fold for PR-A and 2.1-fold for PR-B), and at the site-specific level, RU486 stimulated both the basal and inducible sites to the same extent as R5020. In contrast, ZK98299 produced only a minimal increase in overall phosphorylation (1.2-fold for PR-A and 1.1-fold for PR-B) which was due to a reduced stimulation of the basal sites and failure to induce any of the three hormone-dependent sites. No inappropriate phosphorylation sites were detected in response to either RU486 or ZK98299. In cotreatment studies, ZK98299 blocked the increase in overall phosphorylation of PR induced by R5020, demonstrating that the failure of this antagonist to stimulate specific phosphorylation sites is not due to an inefficient interaction with PR in the intact cell. These results indicate that the biological effects of RU486 are not mediated by an alternation in the phosphorylation state of PR, whereas failure to promote phosphorylation of certain sites may contribute to the antagonist action of ZK98299. Additionally these results support the concept of two mechanistic classes of anti-progestins that affect PR differently in vivo.

Role of acidic and phosphorylated residues in gene activation by the glucocorticoid receptor

To investigate the role of acidic and phosphorylated amino acids in the function of the major transactivation domain (tau 1) of the glucocorticoid receptor, we have performed a mutagenesis study. Aspartic and glutamic acid residues were neutralized in clusters of 2 to 4 amino acids throughout the tau 1 domain. The activity of the mutant proteins was determined using transactivation assays in yeast and mammalian cells. Some acidic residues in the core region of tau 1 appear to play a minor role in tau 1 activity, but, generally, individual acidic residues are not critical for activity. Mutagenesis of five serine residues that are phosphorylated in the mouse glucocorticoid receptor and which are conserved in the human receptor did not affect the transactivation activity of the tau 1 domain in yeast. As in mouse cells, these serine residues are the predominant sites of phosphorylation for ectopically expressed receptor in yeast, since the mutant protein lacking all five sites had a severely reduced phosphorylation level. Mutant proteins in which larger numbers of acidic residues are neutralized show a progressive decrease in activity indicating that acidity in general is important for tau 1 function. However, our results are not consistent with the "acid blob" theory of transactivator function that has been suggested for some other activator proteins. Other putative roles for the acidity of tau 1 are discussed.

Receptor mediated genomic action of the 1,25(OH)2D3 hormone: expression of the human vitamin D receptor in E. coli

The nuclear vitamin D receptor (VDR) binds the 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) hormone with high affinity and elicits its actions to stimulate gene expression in target cells by binding to the vitamin D-responsive element (VDRE). VDREs in such positively controlled genes as osteocalcin, osteopontin, beta 3 integrin and vitamin D-24-OHase are direct hexanucleotide repeats with a spacer of three nucleotides. The present studies of VDR/VDRE interaction utilized full-length human vitamin D receptor (hVDR) that was overexpressed in E. coli, purified to near homogeneity (> 95%), and its authenticity confirmed by demonstrating high affinity hormone binding and reactivity to monoclonal antibody 9A7 gamma. The expressed hVDR displays strict dependence on the family of retinoid X receptors (RXRs) for binding to the vitamin D-responsive element (VDRE) in the rat osteocalcin gene. Similar overexpression in E. coli of the DNA binding domain (delta 134), containing only residues 4-133 of hVDR, generated a receptor species that possesses intrinsic DNA binding activity. Both full-length and delta 134 hVDRs retain similar DNA binding specificities when tested with several natural hormone responsive elements, indicating that the N-terminal zinc finger region determines hVDR-DNA sequence selectivity. The C-terminal region of the molecule is required for hormone binding and confers the receptor with the property of very high affinity DNA binding, via heterodimerization between hVDR and RXR. A natural ligand for the RXR co-receptor, 9-cis retinoic acid, suppresses both VDR-RXR binding to the VDRE and 1,25(OH)2D3 stimulated transcription, indicating that 9-cis retinoic acid recruits RXR away from VDR to instead form RXR homodimers.

Phosphorylation sites in ligand-induced and ligand-independent activation of the progesterone receptor

Steroid hormone receptors are phosphoproteins that undergo hyperphosphorylation upon binding of hormone. The mechanism and the role of this reaction remain poorly understood. Two-dimensional analysis of ligand-free progesterone receptor (PR) tryptic digests showed the existence of seven main phosphopeptides. Incubation of the cells with the progestin R5020 led to a global increase in the levels of PR phosphorylation. However, the same phosphopeptides were seen, and their levels of labeling relative to each other were unchanged. A similar result was observed after incubation of cells with the antiprogestin RU486. The antiprogestin ZK98299 demonstrated only half of the activity of RU486 in terms of receptor hyperphosphorylation, but the same phosphopeptides, proportionally labeled to the same extent, were observed by chromatography electrophoresis. Ligand-induced DNA binding did not play a role in receptor hyperphosphorylation since the mutant delta 547-592, which is devoid of the first zinc finger region, exhibited the same phosphopeptides, labeled to the same extent, as did wild-type receptor after incubation of cells with hormone. These results suggest that the same kinase(s) act in vivo on ligand-free and on agonist or antagonist-bound progesterone receptor. Binding of different ligands produces different conformational changes in the ligand binding domain of the receptor which enhance, to varying extents, affinity of the receptor for the kinase(s). The DNA binding region also plays a role in the interaction with the kinase(s), although binding to DNA per se is not necessary for the hyperphosphorylation of the receptor to take place.(ABSTRACT TRUNCATED AT 250 WORDS)