Ligand-dependent degradation of retinoid X receptors does not require transcriptional activity or coactivator interactions

Cells utilize ubiquitin-mediated proteolysis to regulate the activity of numerous proteins involved in signal transduction, cell cycle control, and transcriptional regulation. For a number of transcription factors, there appears to be a direct correlation between transcriptional activity and protein instability, suggesting that cells use targeted destruction as one method to down-regulate or attenuate gene expression. In this report we demonstrate that retinoid X receptors (RXRs) which function as versatile mediators of nuclear hormone-dependent gene expression are marked for destruction upon binding agonist ligands. Interestingly, when RXR serves as a heterodimeric partner for retinoic acid (RAR) or thyroid hormone (TR) receptors, binding of agonists by RAR or TR leads to degradation of both the transcriptionally active RAR or TR subunits as well as the transcriptionally inactive RXR subunit. Furthermore, using a series of mutants in the ligand-dependent activation domain (activation function 2), we demonstrate that agonist-stimulated degradation of RXR does not require corepressor release, coactivator binding, or transcriptional activity. Taken together, the data suggest a model for targeted destruction of transcription factors based on structural or conformational signals as opposed to functional coupling with gene transcription.

Pharmaceutical intervention in the JAK/STAT signaling pathway

Many cytokines exert their effect via the JAK/STAT signal transduction pathway. Due to the medical relevance of many of these cytokines, they are being exploited, either directly, or through antagonists, as therapeutics for a variety of serious diseases. Currently, these therapeutics consist almost entirely of protein products, with all of their attendant drawbacks. Delineation of the signaling mechanisms for the cytokines, however, has allowed the design and implementation of a variety of cell-based and biochemical screens for small molecule mimics or antagonists of these cytokines. Several successful assays will be described along with the advantages of each type of assay. Use of assays of this type should make it possible to discover numerous small molecule cytokine modulators with significant utility in the clinic. Oncogene (2000).

Design of conditionally active STATs: insights into STAT activation and gene regulatory function

The STAT (signal transducer and activator of transcription) signaling pathway is activated by a large number of cytokines and growth factors. We sought to design a conditionally active STAT that could not only provide insight into basic questions about STAT function but also serve as a powerful tool to determine the precise biological role of STATs. To this end, we have developed a conditionally active STAT by fusing STATs with the ligand-binding domain of the estrogen receptor (ER). We have demonstrated that the resulting STAT-ER chimeras are estrogen-inducible transcription factors that retain the functional and biochemical characteristics of the cognate wild-type STATs. In addition, these tools have allowed us to evaluate separately the contribution of tyrosine phosphorylation and dimerization to STAT function. We have for the first time provided experimental data supporting the model that the only apparent role of STAT tyrosine phosphorylation is to drive dimerization, as dimerization alone is sufficient to unmask a latent STAT nuclear localization sequence and induce nuclear translocation, sequence-specific DNA binding, and transcriptional activity.

A randomized controlled trial of prenatal pediatric visits for urban, low-income families

BACKGROUND

Prenatal pediatric visits have been recommended by the American Academy of Pediatrics to allow the pediatrician to counsel parents on infant care issues, establish a supportive relationship, and provide pediatric practice information to parents. We hypothesized that prenatal pediatric visits would have an impact on breastfeeding decisions, health care behaviors, health care utilization, and the doctor-patient relationship.

METHODS

We conducted a randomized controlled trial of prenatal pediatric visits for urban, low-income families to measure the impact on breastfeeding decisions, infant car safety seat use, circumcision, health maintenance, and emergency room visits and the pediatrician's perception that he/she would know the mother better. Pregnant women were recruited prenatally from the obstetrics clinic. Outcomes were measured by maternal interview prenatally and when the infant was 2 months old, in addition to review of the nursery record. Physicians were interviewed after the 2-month visit. Health care utilization was measured by chart review at 7 months.

RESULTS

A total of 156 pregnant women were enrolled and randomized, 81 to the intervention group and 75 to the control group. Of mothers who breastfed, 45% in the intervention group changed their mind in favor of breastfeeding after enrollment compared with 14% in the control group. Mothers in the intervention group compared with the control group were more likely to make fewer emergency room visits, 0.58 compared with 1.0. Pediatricians were more likely to think that they knew mothers in the intervention group well, 54% versus 29% in the control group, yet 67% of mothers in both groups agreed their pediatrician knew them well. There were no differences between groups in initiation or duration of breastfeeding at 30 or 60 days, infant car safety seat use, circumcision, or health maintenance visits.

CONCLUSIONS

Prenatal pediatric visits have potential impact on a variety of health care outcomes. Among urban, low-income mothers, we found beneficial effects on breastfeeding decisions, a decrease in emergency department visits, and an initial impact on the doctor-patient relationship. We suggest urban practices actively promote prenatal pediatric visits.

STAT protein complexes activated by interferon-gamma and gp130 signaling molecules differ in their sequence preferences and transcriptional induction properties

Activation of members of the STAT (signal transducers and activators of transcription) family of latent transcription factors is an early event following the binding of many cytokines to their cognate receptors. Although the patterns of STATs activated by different cytokines are well described, the consequences of differential STAT activation are less well studied. We show by mutational analysis that STAT binding elements (SBEs) exist that discriminate between STAT complexes containing STAT1 alpha, STAT3 or both, and that these elements show altered cytokine responsiveness. We also show that in the context of a minimal promoter, single and multiple SBEs exhibit strikingly different patterns of transcriptional activation in response to IFN-gamma, IL-6, OSM or LIF. These differences in transcriptional activation are correlated with the differential ability of these cytokines to activate STAT1 alpha, STAT3 or both. Our results show that the pattern of STATs activated by a cytokine and the arrangement and sequence of the SBEs in the responding promoter have a profound effect on the ability of the cytokine to elicit a transcriptional response.

Spacing of palindromic half sites as a determinant of selective STAT (signal transducers and activators of transcription) DNA binding and transcriptional activity

Signal transducers and activators of transcription (STAT proteins) bind to palindromic sequence elements related to interferon gamma (IFN-gamma) activation sites, which were first identified in the promoters of IFN-gamma-inducible genes. Although the sequences of the natural palindromic STAT-binding elements vary considerably, they conform to the general structure TT(N)5AA. We have systematically examined the effects of the spacing between the TT and AA core half sites on the binding of the STAT complexes activated by IFN-gamma, interleukin (IL) 6, granulocyte-macrophage colony-stimulating factor, and IL-4. We show that (i) as suggested earlier, a core palindromic TT--AA motif with a 5-bp spacing displays general STAT binding, (ii) a palindromic motif with a spacing of 4 bp selectively binds to complexes containing Stat3, and (iii) a motif with a 6-bp spacing selectively binds the STAT complexes activated by IL-4. We have examined natural elements in the promoters of cytokine-responsive genes that differ in half-site spacing and found that they display binding properties predicted from the synthetic binding sites. Furthermore, the observed differential selective binding characteristics for the most part correlate with the ability to mediate transcriptional activation of transfected test genes in response to the cytokines tested. Our results thus demonstrate that the specificity of STAT-directed transcription in response to particular cytokines or cytokine families depends in part on the spacing of half sites within the conserved response element sequence.

Rapid activation of the interferon-gamma signal transduction pathway by inhibitors of tyrosine phosphatases

Induction of gene expression by interferon-gamma involves the activation of a latent cytoplasmic transcription factor, p91, by phosphorylation on a single tyrosyl residue. This phosphorylation triggers dimerization, nuclear translocation, and the binding of p91 to interferon-gamma response elements present in the promoters of induced genes. Phosphorylation of p91 requires the activation of two tyrosine kinases, JAK1 and JAK2, that themselves become phosphorylated on tyrosyl residues shortly after interferon-gamma binds to its receptor. The importance of tyrosine phosphorylation in this pathway prompted us to investigate the role of protein tyrosine phosphatases in the regulation of the pathway. We find that in the absence of interferon-gamma, treatment of cells with an inhibitor of tyrosine phosphatases causes a rapid and potent activation of the components of the interferon-gamma signal transduction pathway and induces an interferon-gamma-responsive gene. This suggests that tyrosine phosphatases act both to repress the interferon-gamma signal transduction pathway in the absence of interferon-gamma and to downregulate the pathway after interferon-gamma induction.

Rapid activation of the STAT3 transcription factor by granulocyte colony-stimulating factor

Granulocyte colony-stimulating factor (G-CSF) is a glycoprotein that stimulates proliferation and differentiation of progenitor cells of neutrophils by signaling through its receptor (G-CSFR). Although the G-CSFR belongs to the cytokine receptor superfamily, which lacks an intracellular kinase domain, G-CSF-induced tyrosine phosphorylation of cellular proteins is critical for its biologic activities. We report here that JAK1 and JAK2 tyrosine kinases are tyrosine phosphorylated in response to G-CSF induction. We also demonstrate that the DNA-binding protein STAT3 (also called the acute-phase response factor [APRF], activated by interleukin-6) is an early target of G-CSF-induced tyrosine phosphorylation. G-CSF induces two DNA-binding complexes; the major complex contains tyrosine phosphorylated STAT3 protein and the minor complex appears to be a heterodimer of the STAT1 (previously p91, a component of DNA-binding complexes activated by interferons) and STAT3 proteins. Antiphosphotyrosine antibody interferes with the DNA binding activity of activated STAT3, indicating that tyrosine phosphorylation of STAT3 is important for the DNA binding activity. These results identify a signal transduction pathway activated in response to G-CSF and provide a mechanism for the rapid modulation of gene expression by G-CSF.

Medical education change: a detailed study of six medical schools

This article reports a comparative case study of six selected USA medical schools, undertaken to identify factors that facilitate or obstruct innovation in medical education. The findings suggest that the culture of each medical school results from a combination of intra-institutional and external factors. Together these forces influence substantially the fate of educational innovations. The institutional culture influences critical elements such as educational philosophy, leadership and resources provided in support of innovation. Equally important, the culture shapes the level and type of change a school considers and implements. The findings also suggest that the availability of resources and the creative impetus present in schools giving priority to research can benefit the educational goals and facilitate educational change.

Interaction of inhibitors with phosphoenolpyruvate mutase: implications for the reaction mechanism and the nature of the active site

The active site and mechanism of action of the enzyme phosphoenolpyruvate mutase have been probed using substrate and intermediate analogues as inhibitors of the mutase-catalyzed reaction. Smaller anions (e.g. sulfite, nitrate, phosphinate, and bicarbonate) are noncompetitive inhibitors of the mutase, while larger anions in the complementary series (sulfate, phosphonate, phosphate) inhibit competitively. Combining oxalate, an intermediate analogue that is a potent inhibitor of the mutase (Ki = 25 microM), with small, noncompetitive inhibitor anions results in synergistic inhibition of the mutase, suggesting that the combined presence of oxalate and anion creates a "bimolecular transition-state analogue". The phosphoenolpyruvate (PEP) mutase genes from Tetrahymena and Streptomyces are known, and these enzymes share significant amino acid sequence similarity to the isocitrate lyase gene from Ricinus. Despite their seeming structural unrelatedness to the substrates of PEP mutase, several isocitrate analogues are good inhibitors, suggesting that isocitrate lyase and PEP mutase are evolutionarily related. An active-site model has been developed that is in accord with the data presented, which are consistent with a mechanism involving the intermediacy of a phosphoenzyme.

Rapid activation of proteins that interact with the interferon gamma activation site in response to multiple cytokines

Many cytokines and growth factors trigger rapid changes in gene expression upon binding to their receptors. In many cases, the mechanism by which these changes are affected is unknown. In this report, we show that interleukin-2 (IL-2), IL-3, IL-4, IL-6, leukemia inhibitory factor (LIF), erythropoietin (Epo), and granulocyte-macrophage colony-stimulating factor (GM-CSF) treatment of cells causes rapid activation of DNA-binding activities that recognize a DNA sequence element previously implicated in regulation of gene expression by interferon gamma (IFN gamma). The IL-4-, IL-6-, and GM-CSF-induced complexes can be distinguished from the recently characterized IFN gamma-activated protein p91 on the basis of mobility in polyacrylamide gels, sequence preferences, and lack of reactivity with an anti-p91 antiserum. The IL-4- and GM-CSF-induced complexes react with antiphosphotyrosine antibodies, demonstrating the presence of phosphotyrosine-containing proteins in these DNA-binding complexes. Transcriptional activation of a reporter gene linked to a synthetic IFN gamma-responsive promoter is observed in response to IFN gamma, IL-6, and LIF. These data suggest a pathway by which cytokines induce rapid changes in gene expression.

Phosphonate biosynthesis: molecular cloning of the gene for phosphoenolpyruvate mutase from Tetrahymena pyriformis and overexpression of the gene product in Escherichia coli

The phosphoenolpyruvate mutase gene from Tetrahymena pyriformis has been cloned and overexpressed in Escherichia coli. To our knowledge, this is the first Tetrahymena gene to be expressed in E. coli, a task made more complicated by the idiosyncratic codon usage by Tetrahymena. The N-terminal amino acid sequence of phosphoenolpyruvate mutase purified from T. pyriformis has been used to generate a precise oligonucleotide probe for the gene, using in vitro amplification from total genomic DNA by the polymerase chain reaction. Use of this precise probe and oligo(T) as primers for in vitro amplification from a T. pyriformis cDNA library has allowed the cloning of the mutase gene. A similar amplification strategy from genomic DNA yielded the genomic sequence, which contains three introns. The sequence of the DNA that encodes 10 amino acids upstream of the N-terminal sequence of the isolated protein was found by oligonucleotide hybridization to a subgenomic library. These 10 N-terminal amino acids are cleanly removed in Tetrahymena in vivo. The full mutase gene sequence codes for a protein of 300 amino acids, and it includes two amber (TAG) codons in the open reading frame. In Tetrahymena, TAG codes for glutamine. When the two amber codons are each changed to a glutamine codon (CAG) that is recognized by E. coli and the gene is placed behind a promoter driven by the T7 RNA polymerase, expression in E. coli is observed. The mutase gene also contains a large number of arginine AGA codons, a codon that is very rarely used by E. coli. Cotransformation with a plasmid carrying the dnaY gene [which encodes tRNA(Arg)(AGA)] results in more than 4-fold higher expression. The mutase then comprises about 25% of the total soluble cell protein in E. coli transformants. The mutase gene bears significant similarity to one other gene in the available data bases, that of carboxyphosphonoenolpyruvate mutase from Streptomyces hygroscopicus, an enzyme that catalyzes a closely related transformation. Due to the large evolutionary distance between Tetrahymena and Streptomyces, this similarity can be interpreted as the first persuasive evidence that the biosynthesis of phosphonates is an ancient metabolic process.

Synthesis of 24-heteroatom-substituted cholestanols

Short syntheses of 24-thia-5 alpha,20 xi-cholestan-3 beta-ol, 24-methyl-24-aza-5 alpha,20 xi-cholestan-3 beta-ol, and 24-nor-5 alpha,20 xi-cholan-3 beta-ol from 3 beta-hydroxy-5 alpha-pregnan-20-one are described. The products and synthetic intermediates have been fully characterized by the results of proton NMR, infrared, and high and low resolution mass spectral studies.

Phosphonate biosynthesis: isolation of the enzyme responsible for the formation of a carbon-phosphorus bond

The first isolation of a naturally occurring phosphonate in 1959 led rapidly to the discovery of a variety of metabolites containing a phosphorus-carbon bond. Phosphonates have been found in bacteria, fungi, and higher organisms such as the snail schistosome vector Biomphalaria. The biosynthetic path to the P-C bond has, however, remained undefined. Thus although it was shown twenty years ago that the isotope label from [14C]glucose or from [32P]phosphoenolpyruvate is incorporated into 2-aminoethylphosphonate by the protozoan Tetrahymena pyriformis, the presumed stoichiometric transformation of phosphoenolpyruvate to phosphonopyruvate has never been demonstrated. Low conversions of phosphoenolpyruvate into 2-aminoethylphosphonate and the trapping of phosphonopyruvate from phosphoenolpyruvate have been reported, but these reactions have not proved reproducible, and the existence of the critical enzyme, phosphoenolpyruvate phosphonomutase, has remained notional. We now report experiments that resolve this enigma, and describe the isolation and characterization of the pure mutase from T. pyriformis.

The Johns Hopkins experience in Columbia, Maryland

A university-related prepaid group practice was established in Columbia, Maryland, in 1969, by the Johns Hopkins Medical Institutions with the cooperation of the Connecticut General Life Insurance Company. Objectives were based on the traditional triad of service, teaching and research. The effort continues. There has been visible success in the achievement of each of the objectives. It is evident, however, that the development of the Columbia Medical Plan would have been facilitated if there had been a larger population base in which to market membership in the Plan, if marketing had been under the direction of the health-care provider, and if the burden of the building and running of a hospital had not been assumed. Overall, the experience suggests that the Health Maintenance Organization movement has only modest ultimate potential as an option in health-care delivery in the current economic milieu of American medicine.

Career decisions of unaccepted applicants to Medical School

To investigate the admissions process to medical school and the post-rejection behavior of unaccepted applicants, a national sample of the 16,837 such applicants to the entering medical school class of 1971-1972 was studied in 1973. The majority of the 1,933 respondents had reapplied to medical school following rejection, and 27% had gained entrance to either US or foreign schools by the time of study. Of those still unaccepted, about half were studying or working in health-related fields of study or occupations at least 2 1/2 years following the initial rejection. We conclude that unaccepted applicants demonstrate considerable variance in postrejection behavior, which is associated with both personal and institutional factors.