Identification of a phosphorylation site in the hinge region of the human progesterone receptor and additional amino-terminal phosphorylation sites

c-Jun N-terminal kinase contributes to aberrant retinoid signaling in lung cancer cells by phosphorylating and inducing proteasomal degradation of retinoic acid receptor alpha

Retinoic acid (RA) is the ligand for nuclear RA receptors (RARs and RXRs) and is crucial for normal epithelial cell growth and differentiation. During malignant transformation, human bronchial epithelial cells acquire a block in retinoid signaling caused in part by a transcriptional defect in RARs. Here, we show that activation of c-Jun N-terminal kinase (JNK) contributes to RAR dysfunction by phosphorylating RARalpha and inducing degradation through the ubiquitin-proteasomal pathway. Analysis of RARalpha mutants and phosphopeptide mapping revealed that RARalpha residues Thr181, Ser445, and Ser461 are phosphorylated by JNK. Mutation of these residues to alanines prevented efficient ubiquitination of RARalpha and increased the stability of the protein. We investigated the importance of RARalpha phosphorylation by JNK as a mediator of retinoid resistance in lung cancer. Mice that develop lung cancer from activation of a latent K-ras oncogene had high intratumoral JNK activity and low RARalpha levels and were resistant to treatment with an RAR ligand. JNK inhibition in a human lung cancer cell line enhanced RARalpha levels, ligand-induced activity of RXR-RAR dimers, and growth inhibition by RA. These findings point to JNK as a key mediator of aberrant retinoid signaling in lung cancer cells.

Systematic development of an enzymatic phosphorylation assay compatible with mass spectrometric detection

The enzymatic peptide phosphorylation by cAMP-dependent protein kinase A (PKA) was optimized and monitored by means of electrospray ionization mass spectrometry (ESI-MS). The direct detection of phosphorylated peptides by MS renders labeling unnecessary, reduces time and labor, due to less initial sample pretreatment. In this study the phosphorylation of the peptide malantide by PKA was performed in batch and reaction compounds were detected by ESI-MS after the incubation time. The subsequent product quantitation was accomplished by using one-point normalization. Applying this set-up, optimum solvent conditions (such as salt and modifier content), concentrations of essential reaction compounds (such as cAMP, Mg2+ and ATP), and the influence of reaction properties (such as pH and reaction time) were determined. The reaction milieu has to be suitable for both, the enzymatic reaction and the mass spectrometric detection. We found that the modifier content and the pH value had to be changed after the enzymatic reaction occurred. Through the addition of methanol and acetic acid, the reaction stopped immediately and a more sensitive mass spectrometric detection could be obtained simultaneously. Furthermore, an inhibitor study was performed, testing the inhibition potency of three protein kinase A inhibitors (PKIs). IC50 values were determined and used to calculate the Ki values, that were 7.4, 19.0 and 340.0 nmol/L for PKI(6-22)amide, PKI(5-24)amide, and PKI(14-24)amide, respectively. These data vary between factor 4.4 (for PKI(6-22)amide) and 8.3 (for PKI(5-24)amide) compared to the Ki values described in literature. However, the Ki values are in good agreement with the data mainly obtained by fluorescence- or radioactivity-based methods. Nevertheless, our results indicate that ESI-MS is a realistic alternative to radioactivity and fluorescence detection in determining enzymatic activity. Furthermore we were able to illustrate its high potential as a quantitative detection method.

Cyclin-dependent kinase activity is required for progesterone receptor function: novel role for cyclin A/Cdk2 as a progesterone receptor coactivator

Our studies examining the role of the cell cycle-regulated kinase cyclin A/Cdk2 in progesterone receptor (PR) action have demonstrated that cyclin-dependent kinase activity is required for PR function and that cyclin A/Cdk2 functions as a PR coactivator. Although Cdk2 can phosphorylate PR, elimination of these phosphorylation sites has little effect on the ability of cyclin A/Cdk2 to stimulate PR activity. PR interacts with cyclin A and recruits cyclin A/Cdk2 to progestin-responsive promoters, stimulating transcription. Inhibition of Cdk2 activity abolishes progesterone-dependent activation of PR target genes in part through inhibition of PR-dependent recruitment of steroid receptor coactivator 1 (SRC-1) and subsequent histone H4 acetylation at the target promoter. In vitro studies revealed that the interaction between SRC-1 and PR is dependent upon phosphorylation of SRC-1. This heretofore-unknown mechanism provides a potential means for integrating the regulation of PR activity with cell cycle progression. Moreover, the ability of PR to recruit cyclin A/Cdk2 to target promoters provides locally elevated levels of kinase, which can preferentially facilitate phosphorylation-dependent interactions and enzymatic activities of coactivators at the target promoter.

Proliferation of rat granulosa cells during the periovulatory interval

Granulosa cell proliferation during luteinization and terminal differentiation has historically been assumed to decline rapidly after an ovulatory stimulus. In contrast, terminal differentiation in other cell types has recently been associated with a transient increase in proliferation, suggesting that this may occur in the ovarian follicle. The goal of the current study was to test the hypothesis that an ovulatory stimulus to rats results in additional granulosa cell proliferation before cell cycle arrest. Immature rats were given a single injection of pregnant mare serum gonadotropin (PMSG) followed by human chorionic gonadotropin (hCG) to initiate periovulatory events. The proportion of granulosa cells in S phase did not change until 12 h after hCG, although the majority of the post-hCG proliferation was localized to cumulus granulosa cells for up to 10 h after hCG. The expression of cyclin D2 mRNA did not decline until 12 h after hCG, although both cyclin-dependent kinase (Cdk)4 and Cdk6 mRNA increased at 6 h. Protein levels of cyclin D2 and Cdk4 did not change as a result of hCG, whereas cyclin E increased 6 h after hCG. Kinase activity of Cdk2 dropped markedly by 4 h after hCG, but a slight increase in activity was evident 6-8 h after hCG. These data suggest that cumulus granulosa cells continue to proliferate for up to 10 h after an ovulatory stimulus, possibly via cyclin E/Cdk2. It is concluded that proliferation is maintained in granulosa cells in the proximity of the oocyte during luteinization of the rat follicle.

Phosphorylation of progesterone receptor serine 400 mediates ligand-independent transcriptional activity in response to activation of cyclin-dependent protein kinase 2

Human progesterone receptors (PR) are phosphorylated by cyclin-dependent protein kinase 2 (CDK2) at multiple sites, including Ser400. Herein, we have addressed the significance of phosphorylation of this residue. PR phospho-Ser400-specific antibodies revealed regulated phosphorylation of Ser400 in response to progestins and mitogens, and this correlated with increased CDK2 levels and activity. Expression of cyclin E elevated CDK2 activity and downregulated PR independently of ligand. Similarly, overexpression of activated mutant CDK2 increased PR transcriptional activity in the absence and presence of progestin. Mutation of PR Ser400 to alanine (S400A) blocked CDK2-induced PR activity in the absence, but not in the presence, of progestin. PR was unresponsive to activated CDK2 in breast cancer cells with elevated p27, and RNA interference knock-down of p27 partially restored CDK2-induced ligand-independent PR activation. Similarly, in p27(-/-) mouse embryonic fibroblasts, elevated CDK2 activity increased wild-type (wt) but not S400A PR transcriptional activity in the absence of progestin. CDK2 induced nuclear localization of unliganded wt but not S400A PR; liganded S400A PR exhibited delayed nuclear accumulation. These studies demonstrate that CDK2 regulates PR in the absence of progestins via phosphorylation of Ser400, thus revealing a novel mechanism for upregulated PR transcriptional activity in human breast cancer cells expressing altered cell cycle regulatory molecules.

Progesterone receptors induce cell cycle progression via activation of mitogen-activated protein kinases

Progestins induce proliferation of breast cancer cells and are implicated in the development of breast cancer. The effects of progestins are mediated by progesterone receptors (PRs), although it is unclear whether proliferative effects are delivered through activities as ligand-activated transcription factors or via activation of cytoplasmic kinases. We report that progestin induces S phase entry of T47D cells stably expressing either wild-type (wt) PR-B or a transcriptionally impaired PR-B harboring a point mutation at Ser294, a ligand-dependent and MAPK consensus phosphorylation site (S294A). Both wt and S294A PR are capable of activating p42/p44 MAPKs and promoting proliferation. However, cells expressing wt, but not S294A PR, exhibited enhanced proliferation in response to combined epidermal growth factor and progestin. S phase progression correlated with up-regulation of cyclin D1. The PR antagonist RU486 also induced MAPK activation, increased cyclin D1 expression, and stimulated S phase entry, which was blocked by inhibition of either p42/p44 or p38 MAPKs, whereas proliferation induced by R5020 was sensitive only to p42/p44 MAPK inhibition. MCF-7 cells stably expressing a mutant PR unable to bind c-Src and activate MAPK failed to support progestin-induced proliferation. These data suggest that PR mediate cell cycle progression primarily through activation of cytoplasmic kinases and independently of direct regulation of transcription, whereas the coordinate regulation of both aspects of PR action are required for enhanced proliferation in response to progestins in the presence of growth factors. Targeting the ability of steroid receptors to activate MAPKs may be beneficial for breast cancer patients.

The origin and functions of multiple human glucocorticoid receptor isoforms

Glucocorticoid hormones are necessary for life and are essential in all aspects of human health and disease. The actions of glucocorticoids are mediated by the glucocorticoid receptor (GR), which binds glucocorticoid hormones and regulates gene expression, cell signaling, and homeostasis. Decades of research have focused on the mechanisms of action of one isoform of GR, GRa. However, in recent years, increasing numbers of human GR (hGR) isoforms have been reported. Evidence obtained from this and other laboratories indicates that multiple hGR isoforms are generated from one single hGR gene via mutations and/or polymorphisms, transcript alternative splicing, and alternative translation initiation. Each hGR protein, in turn, is subject to a variety of posttranslational modifications, and the nature and degree of posttranslational modification affect receptor function. We summarize here the processes that generate and modify various hGR isoforms with a focus on those that impact the ability of hGR to regulate target genes. We speculate that unique receptor compositions and relative receptor proportions within a cell determine the specific response to glucocorticoids. Unchecked expression of some isoforms, for example hGRbeta, has been implicated in various diseases.

Association of progesterone receptor polymorphism with recurrent abortions

OBJECTIVE

The current study sought for polymorphisms within the progesterone receptor (PR) gene. Allele and genotype frequencies of patients with repeated abortions were compared to a control group.

DESIGN

All exons of the PR of 42 women with repeated abortions and 40 controls were screened for single nucleotide polymorphisms (SNP). Determination of the DNA-sequences was performed.

RESULTS

Three SNPs were detected (exon 1: G1031C Ser344Thr; exon 4: G1978T Leu660Val, exon 5: C2310T His770His). These SNPs are linked. The more frequent wildtype (*1) allele and the rarer (*2) allele were found in the control group and in the study group at different frequencies (control group: *1/*1: 78%, *1/*2: 22%, *2/*2: 0%; patient group: *1/*1: 50%, *1/*2: 43%, *2/*2: 7%). The genotypes distributions differed significantly from each other (P=0.019, chi2=7.879).

CONCLUSIONS

The data suggest that the rarer PR allele may be associated with an increased likelihood of repeated miscarriages contributing to its multi-factorial causes.

Interleukin 4 Regulates Phosphorylation of Serine 756 in the Transactivation Domain of Stat6

Lymphokines interleukin-4 (IL4) and IL13 exert overlapping biological activities via the shared use of the IL4 receptor alpha-chain and signal transducer and activator of transcription 6 (Stat6). Stat6 is critical for T-helper 2 cell differentiation, B-cell Ig class switch, and allergic diseases; thus, understanding its regulation is of central importance. Phosphorylation is crucial for Stat activity. Whereas Stat6 is phosphorylated on Tyr(641), less is known about serine or threonine. We demonstrate in primary human T-cells (>95% CD3+) that IL4 and for the first time IL13 induce Stat6 serine but not threonine phosphorylation that closely paralleled early IL4 receptor alpha-chain activation (10 min). Stat6 uniquely fails to share a positionally conserved Stat serine phosphorylation sequence; however, known phosphoacceptor sites are proline-flanked. Alanine substitutions of these conserved residues revealed that the transactivation domain, which localized Ser(756) but not Ser(827) or Ser(176), is the IL4-regulated site based on phosphoamino acid analysis. Tyr(641) was dispensable for IL4-mediated serine phosphorylation, suggesting that dimerization is not preconditional. Only Stat6 Y641F variant showed a significant effect on IL4-inducible Cepsilon DNA-binding and reporter gene expression. Lastly, recent work has shown that protein phosphatase 2A negatively regulates Stat6. We propose this target residue(s) is distinct from Ser(756) and may be proximal to Tyr(641) at Thr(645), a residue conserved only among Stat6 members. The phosphomimic variants T645E or T645D ablated Stat6 activation, whereas polar uncharged substitutions (Gln or Asn) and additional mutants (Ala, Val, or Phe) showed no effect. These findings suggest that Stat6 has mechanisms of regulation distinct from other Stats.

Determination of the relative energies of activation for the dissociation of aromatic versus aliphatic phosphopeptides by ESI-FTICR-MS and IRMPD

Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (ESI-FTICR-MS) coupled with infrared multiphoton dissociation (IRMPD) is potentially a powerful method for rapid phosphopeptide mapping of complex proteolytic digests. The dissociation of deprotonated phosphopeptides by IRMPD is energetically favorable over unmodified deprotonated peptides because of a lower energy of activation and a higher internal energy under identical irradiation conditions. The energies of activation for dissociation are determined for model peptides phosphorylated on an aliphatic side chain (serine) and an aromatic side chain (tyrosine). The determination of phosphorylation location provides important biochemical information identifying the kinase involved in specific phosphorylation mechanisms. The data presented in this manuscript also support the theory that for phosphopeptides, the phosphate moiety's P-O stretch is in direct resonance with the infrared laser (10.6 microm), thus increasing the relative absorptivity of the modified species. A greater extinction coefficient affords more extensive photon absorption and subsequently a greater internal energy at the rapid exchange limit.

Making sense of cross-talk between steroid hormone receptors and intracellular signaling pathways: who will have the last word?

In classical models of nuclear steroid hormone receptor function, ligand binds receptor, heat shock proteins dissociate, and receptor dimers enter or are withheld in the nucleus and interact with coregulatory molecules to mediate changes in gene expression. The footnotes, "receptors become phosphorylated" and "dynamic nucleo-cytoplasmic shuttling occurs" describe well-accepted, but less well-understood aspects of receptor action. Recently, the idea that several protein kinases are activated in response to steroid hormone binding to cognate cytoplasmic or membrane-associated receptors has become fashionable. However, the precise role of steroid hormone receptor phosphorylation and our understanding of which cytoplasmic kinases are activated and their functional significance remain elusive. This review provides an overview of the primary ways in which steroid hormone receptor and growth factor cross-talk occurs, using the human progesterone receptor (PR) as a model. The functional consequences of PR phosphorylation by protein kinases classically activated in response to peptide growth factors and novel extranuclear or nongenomic functions of PR as potential independent initiators of signal transduction pathways are discussed. Intracellular protein kinases are emerging as key mediators of steroid hormone receptor action. Cross-talk between steroid receptor- and growth factor-initiated signaling events may explain how gene subsets are coordinately regulated by mitogenic stimuli in hormonally responsive normal tissues, and is suspected to play a role in their cancer biology.

Repressors of androgen and progesterone receptor action

Androgen and progesterone receptors (AR and PR) are two determining factors in gonadal differentiation that are highly expressed in developing and mature gonads. Loss of AR results in XY sex reversal and mutations causing reduced AR activity lead to varying degrees of defects in masculinization. Female PR knockout mice are infertile due to ovarian defects. While much has been discovered about positive regulation of these receptors by coactivators little is known about repression of the transcriptional activity of AR and PR in the presence of agonists. In this study we assessed the effect of SMRT and DAX-1 on AR and PR activity in the presence of both agonists and partial antagonists. We show that SMRT and DAX-1 repress agonist-dependent activity of both receptors, and the mechanism of repression includes disruption of the receptor dimer interactions rather than recruitment of histone deacetylases. We demonstrate that endogenous agonist-bound PR and DAX-1 in T47D breast cancer cells and endogenous AR and DAX-1 in LNCaP prostate cancer cells can be coimmunoprecipitated suggesting that the interaction is physiological. Surprisingly, although DAX-1 represses partial antagonist activity of AR, it was ineffective in repressing partial antagonist induced activity of PR. In contrast to most reported repressors, the expression of DAX-1 is restricted. We found that although DAX-1 is expressed in normal human prostate, its expression is strongly reduced in benign prostatic hyperplasia suggesting that DAX-1 plays a role in limiting AR activity in prostate.

Selective interactions of Kruppel-like factor 9/basic transcription element-binding protein with progesterone receptor isoforms A and B determine transcriptional activity of progesterone-responsive genes in endometrial epithelial cells

The Sp/KLF transcription factor basic transcription element-binding protein (BTEB1) regulates gene transcription by binding to GC-rich sequence motifs present in the promoters of numerous tissue-specific as well as housekeeping genes. Similar to other members of this family, BTEB1 can act as a transactivator or transrepressor depending on cell and promoter context, although the molecular mechanism underlying these distinct activities remains unclear. Here we report that BTEB1 can mediate signaling pathways involving the nuclear receptor for the steroid hormone progesterone in endometrial epithelial cells by its selective interaction with the progesterone receptor (PR) isoforms, PR-A and PR-B. Functional interaction with ligand-activated PR-B resulted in superactivation of PR-B transactivity, facilitated the recruitment of the transcriptional integrator CREB-binding protein within the PR-dimer, and was dependent on the structure of the ligand bound by PR-B. By contrast, BTEB1 did not influence agonist-bound PR-A transactivity, although it augmented PR-A inhibition of PR-B-mediated transactivation as well as potentiated ligand-independent PR-A transcriptional activity in the presence of CREB-binding protein. We also demonstrate similar positive modulatory actions of BTEB1-related family members Krüppel-like family (KLF) 13/FKLF2/BTEB3 and Sp1 on PR-B transactivity. Further, we provide support for the potential significance of the selective functional interactions of PR isoforms with BTEB1 in the peri-implantation uterus using mouse and pig models and in the breast cancer cell lines MCF-7 and T47D. Our results suggest a novel mechanism for the divergent physiological consequences of PR-A and PR-B on progesterone-dependent gene transcription in the uterus involving select KLF members.

MAP kinases couple multiple functions of human progesterone receptors: degradation, transcriptional synergy, and nuclear association

Breast cancers often have increased mitogen-activated protein kinase (MAPK) activity; this pathway influences breast cancer cell growth in part by targeting steroid hormone receptors. Bidirectional cross-talk between these two pathways is well documented; progestins increase the expression of type I growth factor receptors that couple to MAPK activation, and in turn, activation of p42 and p44 MAPKs increases ligand-dependent progesterone receptor (PR) transcriptional activity, and parodoxically, augments PR downregulation. Breast cancers that have become steroid hormone resistant often remain highly sensitive to growth factors. We believe that the mechanism of steroid hormone resistance is biochemically linked to the acquisition of growth factor responsiveness. Using in vitro models, we have established numerous regulatory links between signal transduction pathways elicited by peptide growth factors and PR. Of note is the role of phosphorylation of human PRs by MAPKs. Phosphorylation of PR on a key serine residue (Ser294) by MAPKs couples multiple receptor functions, including ligand-dependent PR downregulation by the ubiquitin-proteasome pathway, transcriptional synergy between progestins and growth factors, and nuclear localization of PR proteins. Linkage of these events suggests a mechanism for steroid hormone receptor "hypersensitivity" induced by growth factors. The uncoupling of these events during breast cancer progression is predicted to profoundly influence hormone responsiveness, as PR with altered stability may be driven primarily by upregulated growth factors.

Nuclear receptors: integration of multiple signalling pathways through phosphorylation

Nuclear receptors (NRs) orchestrate the transcription of specific gene networks in response to binding of their cognate ligand. They also act as mediators in a variety of signalling pathways through integrating diverse phosphorylation events. NR phosphorylation concerns all three major domains, the N-terminal activation function (AF-1), the ligand-binding and the DNA binding domains. Often, phosphorylation of NRs by kinases that are associated with general transcription factors (e.g. cdk7 within TFIIH), or activated in response to a variety of signals (MAPKs, Akt, PKA, PKC), facilitates the recruitment of coactivators or of components of the transcription machinery and, therefore, cooperates with the ligand to enhance transcription activation. But phosphorylation can also contribute to the termination of the ligand response through inducing DNA dissociation or NR degradation or through decreasing ligand affinity. These different modes of regulation reveal an unexpected complexity of the dynamics of NR-mediated transcription. In addition, deregulation of NR phosphorylation may impact their action in certain diseases or cancers.

Heregulin induces transcriptional activation of the progesterone receptor by a mechanism that requires functional ErbB-2 and mitogen-activated protein kinase activation in breast cancer cells

The present study addresses the capacity of heregulin (HRG), a ligand of type I receptor tyrosine kinases, to transactivate the progesterone receptor (PR). For this purpose, we studied, on the one hand, an experimental model of hormonal carcinogenesis in which the synthetic progestin medroxyprogesterone acetate (MPA) induced mammary adenocarcinomas in female BALB/c mice and, on the other hand, the human breast cancer cell line T47D. HRG was able to exquisitely regulate biochemical attributes of PR in a way that mimicked PR activation by progestins. Thus, HRG treatment of primary cultures of epithelial cells of the progestin-dependent C4HD murine mammary tumor line and of T47D cells induced a decrease of protein levels of PRA and -B isoforms and the downregulation of progesterone-binding sites. HRG also promoted a significant increase in the percentage of PR localized in the nucleus in both cell types. DNA mobility shift assay revealed that HRG was able to induce PR binding to a progesterone response element (PRE) in C4HD and T47D cells. Transient transfections of C4HD and T47D cells with a plasmid containing a PRE upstream of a chloramphenicol acetyltransferase (CAT) gene demonstrated that HRG promoted a significant increase in CAT activity. In order to assess the molecular mechanisms underlying PR transactivation by HRG, we blocked ErbB-2 expression in C4HD and T47D cells by using antisense oligodeoxynucleotides to ErbB-2 mRNA, which resulted in the abolishment of HRG's capacity to induce PR binding to a PRE, as well as CAT activity in the transient-transfection assays. Although the inhibition of HRG binding to ErbB-3 by an anti-ErbB-3 monoclonal antibody suppressed HRG-induced PR activation, the abolishment of HRG binding to ErbB-4 had no effect on HRG activation of PR. To investigate the role of mitogen-activated protein kinases (MAPKs), we used the selective MEK1/MAPK inhibitor PD98059. Blockage of MAPK activation resulted in complete abrogation of HRG's capacity to induce PR binding to a PRE, as well as CAT activity. Finally, we demonstrate here for the first time that HRG-activated MAPK can phosphorylate both human and mouse PR in vitro.

Mechanism of action of progesterone antagonists

The effects of progesterone on target tissues are mediated by progesterone receptors (PRs), which belong to a family of nuclear receptors and function as ligand-activated transcription factors to regulate the expression of specific sets of target genes. Progesterone antagonists repress the biological actions of progesterone by "actively" inhibiting PR activation. This work discusses the first clinically used progesterone antagonist RU486 and closely related compounds in terms of how these compounds inhibit progesterone action through heterodimerization and competition for DNA binding and by the recruitment of corepressors to promoters of target genes to repress transcription. We discuss cellular factors that may influence the activity of these compounds, such as the availability of coactivators and corepressors and the context of specific target promoters in any given cell type. We also discuss steroidal and nonsteroidal antagonist selectivity for PR versus other steroid hormone receptors and suggest that it may be possible to develop tissue/cell specific modulators of PR.

Regulation of actin dynamics by tyrosine phosphorylation: identification of tyrosine phosphorylation sites within the actin-severing domain of villin

We have previously shown that villin, an epithelial cell actin-binding protein, is tyrosine phosphorylated both in vitro and in vivo and that villin's actin-modifying functions are regulated by phosphorylation. Here as a first step toward understanding the role of villin tyrosine phosphorylation, we sought to identify the major phosphorylation site(s) in human villin and study its role in actin filament assembly. We generated a series of carboxyl-terminal truncation mutants of villin and cloned them in the prokaryotic expression vector pGEX-2T. Full-length villin and the truncation mutants were expressed in TKX1 cells, which carry an inducible tyrosine kinase gene. Using this approach, we identified a region in the amino-terminal actin-severing domain of villin as the site of phosphorylation (amino acids 1-261). Five phosphorylation sites were identified by direct mutation of candidate tyrosines (Y) to phenylalanine (F), namely, Y46, -60, -64, -81, and -256. Changing all of these sites to phenylalanine resulted in a villin mutant that neither was phosphorylated in TKX1 cells nor was a substrate for c-src kinase in an in vitro kinase assay. Using a pyrene actin-based fluorescence assay, we mapped the various phosphorylated tyrosine residues with the actin-nucleating and -depolymerizing functions of villin. Phosphorylation of any one of the identified sites inhibited the actin-nucleating function of villin, whereas phosphorylation at Y46 and/or Y60 increased the actin-severing activity of villin. Since there is significant homology between the amino-terminal end of villin and other actin-severing proteins, the results provide a structural basis for the actin-severing mechanism and help understand the relationship of phosphorylation with this function.

Characterization of transcription factors by mass spectrometry and the role of SELDI-MS

Over the last decade, much progress has been made in the field of biological mass spectrometry, with numerous advances in technology, resolution, and affinity capture. The field of genomics has also been transformed by the sequencing and characterization of entire genomes. Some of the next challenges lie in understanding the relationship between the genome and the proteome, the protein complement of the genome, and in characterizing the regulatory processes involved in progressing from gene to functional protein. In this new age of proteomics, development of mass spectrometry methods to characterize transcription factors promises to add greatly to our understanding of regulatory networks that govern expression. However, at this time, regulatory networks of transcription factors are mostly uncharted territory. In this review, we summarize the latest advances in characterization of transcription factors by mass spectrometry including affinity capture, identification of complexes of DNA-binding proteins, structural characterization, determination of protein-DNA and protein-protein interactions, assessment of modification sites and metal binding, studies of functional activity, and the latest chip technologies that use SELDI-MS that allow the rapid capture and identification of transcription factors.

The inhibitory function in human progesterone receptor N termini binds SUMO-1 protein to regulate autoinhibition and transrepression

Although most studies of progesterone receptors (PR) and their two isoforms, PR-A and PR-B, focus on transcriptional stimulation, the receptors exhibit important inhibitory properties. Autoinhibition refers to an inhibitory function located in the PR N terminus, whose deletion increases transcriptional activity at least 6-10-fold. Transrepression refers to the ability of PR-A to suppress the transcriptional activity of PR-B and other nuclear receptors, including estrogen receptors. Self-squelching refers to the observation in transient transfection assays that increasing receptor concentrations paradoxically decrease transcriptional activity. Using a series of N-terminal deletion mutants constructed in both PR isoforms, we have mapped their autoinhibitory and transrepressor activities to a small ubiquitin-like modifier (SUMO-1) protein consensus-binding motif, (387)IKEE, located in the N terminus upstream of AF1. Self-squelching does not involve this site. SUMO-1 binds PR covalently at (387)IKEE, but only if the C-terminal, liganded, hormone-binding domain is also present. A single point K388R mutation within the (387)IKEE motif in either PR-A or PR-B leads to a loss of autoinhibitory and transrepressor functions of the liganded, full-length receptors. We conclude that autoinhibition and transrepression involve N-terminal sumoylation combined with intramolecular N/C-terminal communication.

Contributions of mass spectrometry in the study of nucleic acid-binding proteins and of nucleic acid-protein interactions

Nucleic-acid-protein (NA-P) interactions play essential roles in a variety of biological processes-gene expression regulation, DNA repair, chromatin structure regulation, transcription regulation, RNA processing, and translation-to cite only a few. Such biological processes involve a broad spectrum of NA-P interactions as well as protein-protein (P-P) interactions. These interactions are dynamic, in terms of the chemical composition of the complexes involved and in terms of their mere existence, which may be restricted to a given cell-cycle phase. In this review, the contributions of mass spectrometry (MS) to the deciphering of these intricate networked interactions are described along with the numerous applications in which it has proven useful. Such applications include, for example, the identification of the partners involved in NA-P or P-P complexes, the identification of post-translational modifications that (may) regulate such complexes' activities, or even the precise molecular mapping of the interaction sites in the NA-P complex. From a biological standpoint, we felt that it was worth the reader's time to be as informative as possible about the functional significance of the analytical methods reviewed herein. From a technical standpoint, because mass spectrometry without proper sample preparation would serve no purpose, each application described in this review is detailed by duly emphasizing the sample preparation-whenever this step is considered innovative-that led to significant analytical achievements.

Androgen receptor phosphorylation. Regulation and identification of the phosphorylation sites

Activation of signal transduction kinase cascades has been shown to alter androgen receptor (AR) activity. Although it has been suggested that changes in AR phosphorylation might be directly responsible, the basal and regulated phosphorylations of the AR have not been fully determined. We have identified the major sites of AR phosphorylation on ARs expressed in COS-1 cells using a combination of peptide mapping, Edman degradation, and mass spectrometry. We describe the identification of seven AR phosphorylation sites, show that the phosphopeptides seen with exogenously expressed ARs are highly similar to those seen with endogenous ARs in LNCaP cells and show that specific agonists differentially regulate the phosphorylation state of endogenous ARs in LNCaP prostate cancer cells. Treatment of LNCaP cells with the synthetic androgen, R1881, elevates phosphorylation of serines 16, 81, 256, 308, 424, and 650. Ser-94 appears constitutively phosphorylated. Forskolin, epidermal growth factor, and phorbol 12-myristate 13-acetate increase the phosphorylation of Ser-650. The kinetics of phosphorylation of most sites in response to hormone or forskolin is temporally delayed, reaching a maximum at 2 h post-stimulation. The exception is Ser-81, which continues to display increasing phosphorylation at 6 h. These data provide a basis for analyzing mechanisms of cross-talk between growth factor signaling and androgen in prostate development, physiology, and cancer.

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 dynamic and elusive membrane estrogen receptor-alpha

Many studies have demonstrated the nuclear forms of steroid receptors and their activities, while fewer investigators have identified and described the membrane forms of these receptors. Our immuno-identification approaches for the qualitative and quantitative comparison of the membrane form of the estrogen receptor-alpha (mER alpha) to its nuclear counterpart now allow us to address questions about the comparative levels and regulation of these receptor forms. ER alpha-specific antisense oligonucleotides eliminate mER alpha expression, while only mildly reducing the nuclear ER alpha. Success of immuno-identification for the mER alpha is very sensitive to different fixation protocols, affecting cell permeability (and thus distinction from the intracellular form) and differential epitope preservation. All such identifications must be accompanied by proof of cell membrane integrity and focal plane assessments. The mER alpha expression on selected cells declines rapidly with cell passage number and cell density. Expression of mER alpha is enhanced by serum starvation and selection for specific phases of the cell cycle. The hinge region of the protein is sensitive to ligand-induced epitope masking and to antibody-induced changes in receptor-mediated responses. Responsive cells are often diluted within cell populations by loss of the membrane receptor form. The bimodality of the rapid estrogen action, with inhibitory doses between picomolar and nanomolar stimulatory concentrations, requires detailed dose-response curves. Finally, responsive cells can be lost from assays, as upon estrogen treatment they rapidly round up and leave the substrates to which they are attached. These regulatory phenomena demonstrate that levels of the membrane form of the estrogen receptor are very dynamic.

Mapping the unique activation function 3 in the progesterone B-receptor upstream segment. Two LXXLL motifs and a tryptophan residue are required for activity

Progesterone receptors (PR) contain three activation functions (AFs) that together define the extent to which they regulate transcription. AF1 and AF2 are common to the two isoforms of PR, PR-A and PR-B, whereas AF3 lies within the N-terminal 164 amino acids unique to PR-B, termed the "B-upstream segment" (BUS). To define the BUS regions that contribute to AF3 function, we generated a series of deletion and amino acid substitution mutants and tested them in three backgrounds as follows: BUS alone fused to the PR DNA binding domain (BUS-DBD), the entire PR-B N terminus linked to its DBD (NT-B), and full-length PR-B. Analyses of these mutants identified two regions in BUS whose loss reduces AF3 activity by more than 90%. These are associated with amino acids 54-90 (R1) and 120-154 (R2). R1 contains a consensus (55)LXXLL(59) motif (L1) identical to ones found in nuclear receptor co-activators. R2 is adjacent to a second nuclear receptor box (L2) at (115)LXXLL(119) and contains a conserved tryptophan (Trp-140). Their mutation completely disrupts AF3 activity in a promoter and cell type-independent manner. Critical mutations elicited similar effects on all three B-receptor backgrounds. This underscores the probability that these mutations alter a process linking BUS structure to the function of full-length PR-B in a fundamental way.

Analysis of phosphorylated proteins and peptides by mass spectrometry

Phosphorylation on serine, threonine and tyrosine residues is an extremely important modulator of protein function. Therefore, there is a great need for methods capable of accurately elucidating sites of phosphorylation. Although full characterization of phosphoproteins remains a formidable analytical challenge, mass spectrometry has emerged as an increasingly viable tool for this task. This review summarizes the methodologies currently available for the analysis of phosphoproteins by mass spectrometry, including enrichment of compounds of interest using immobilized metal affinity chromatography and chemical tagging techniques, detection of phosphopeptides using mass mapping and precursor ion scans, localization of phosphorylation sites by peptide sequencing, and quantitation of phosphorylation by the introduction of mass tags. Despite the variety of powerful analytical methods that are now available, complete characterization of the phosphorylation state of a protein isolated in small quantities from a biological sample remains far from routine.

Transcriptional hyperactivity of human progesterone receptors is coupled to their ligand-dependent down-regulation by mitogen-activated protein kinase-dependent phosphorylation of serine 294

Breast cancers often exhibit elevated expression of tyrosine kinase growth factor receptors; these pathways influence breast cancer cell growth in part by targeting steroid hormone receptors, including progesterone receptors (PR). To mimic activation of molecules downstream of growth factor-initiated signaling pathways, we overexpressed mitogen-activated protein kinase (MAPK; also known as extracellular signal-regulated kinase) kinase kinase 1 (MEKK1) in T47D human breast cancer cells expressing the B isoform of PR. MEKK1 is a strong activator of p42 and p44 MAPKs. MEKK1 expression increased progestin-mediated transcription 8- to 10-fold above normal PR-driven transcription levels. This was dependent on the presence of a progesterone response element and functional PR. PR protein levels were unchanged by MEKK1 alone but were extensively down-regulated by MEKK1 plus the progestin R5020. MEKK1 expression resulted in phosphorylation of PR on Ser294, a MAPK consensus site known to mediate ligand-dependent PR degradation. MEK inhibitors blocked phosphorylation of Ser294 and attenuated PR transcriptional hyperactivity in response to MEKK1 plus R5020; stabilization of PR by inhibition of the 26S proteasome produced similar results. T47D cells stably expressing mutant S294A PR, in which serine 294 is replaced by alanine, fail to undergo ligand-dependent down-regulation and are resistant to MEKK1-plus-R5020-induced transcriptional synergy but respond to progestins alone. Similarly, c-myc protein levels are synergistically increased by epidermal growth factor and R5020 in cells expressing wild-type PR, but not S294A PR. Thus, highly stable mutant PR are functional in response to progestins but are incapable of cross talk with MAPK-driven pathways. These studies demonstrate a paradoxical coupling between steroid receptor down-regulation and transcriptional hyperactivity. They also suggest a link between phosphorylation of PR by MAPKs in response to peptide growth factor signaling and steroid hormone control of breast cancer cell growth.