A kinetic model of GPCRs: analysis of G protein activity, occupancy, coupling and receptor-state affinity constants.

CONTEXT: G protein-coupled receptors are vital macromolecules for a wide variety of physiological processes. Upon agonist binding, these receptors accelerate the exchange of GDP for GTP in G proteins coupled to them. The activated G protein interacts with effector proteins to implement downstream biological functions. OBJECTIVE: We present a kinetic, quaternary complex model, based on a system of coupled linear first-order differential equations, which accounts for the binding attributes of the ligand, receptor, G protein and two types of guanine nucleotide (GDP and GTP) as well as for GTPase activity. METHODS: We solved the model numerically to predict the extents of G protein activation, receptor occupancy by ligand and receptor coupling that result from varying the ligand concentration, presence of GDP and/or GTP, the ratio of G protein to receptor and the equilibrium constants governing receptor pre-coupling and constitutive activity. We also simulated responses downstream from G protein activation using a transducer function. RESULTS: Our model shows that agonist-induced G protein activation can occur with either a net decrease or increase in total receptor-G protein coupling. In addition, we demonstrate that affinity constants of the ligand for both the active and inactive states of the receptor can be derived to a close approximation from analysis of simulated responses downstream from receptor activation. DISCUSSION AND CONCLUSION: The latter result validates our prior methods for estimating the active state affinity constants of ligands, and our results on receptor coupling have relevance to studies investigating receptor-G protein interactions using fluorescence techniques.

Identification of methyllysine peptides binding to chromobox protein homolog 6 chromodomain in the human proteome.

Methylation is one of the important post-translational modifications that play critical roles in regulating protein functions. Proteomic identification of this post-translational modification and understanding how it affects protein activity remain great challenges. We tackled this problem from the aspect of methylation mediating protein-protein interaction. Using the chromodomain of human chromobox protein homolog 6 as a model system, we developed a systematic approach that integrates structure modeling, bioinformatics analysis, and peptide microarray experiments to identify lysine residues that are methylated and recognized by the chromodomain in the human proteome. Given the important role of chromobox protein homolog 6 as a reader of histone modifications, it was interesting to find that the majority of its interacting partners identified via this approach function in chromatin remodeling and transcriptional regulation. Our study not only illustrates a novel angle for identifying methyllysines on a proteome-wide scale and elucidating their potential roles in regulating protein function, but also suggests possible strategies for engineering the chromodomain-peptide interface to enhance the recognition of and manipulate the signal transduction mediated by such interactions.

Recognition of methylated peptides by Drosophila melanogaster polycomb chromodomain.

Lysine methylation is one of the important post-translational modifications (PTMs) that regulate protein functions. Up to now, proteomic identification of this PTM remains a challenge due to the lack of effective enrichment methods in mass spectrometry experiments. To address this challenge, we present here a systematic approach to predicting peptides in which lysine residues may be methylated to mediate protein-protein interactions. We used the chromodomain of the polycomb protein in Drosophila melanogaster as a model system to illustrate the success of this approach. We started with molecular dynamics simulations and free energy analyses on the histone peptides complexed with the polycomb chromodomain to understand how the binding specificity is achieved. We next conducted virtual mutagenesis to quantify each domain and peptide residue's contribution to the domain-peptide recognition, based on which scoring scheme was developed to evaluate the possibility of any lysine-containing peptides to be methylated and recognized by the chromodomain. A peptide microarray experiment on a panel of conserved histone peptides showed a satisfactory prediction accuracy of the scoring scheme. Next, we implemented a bioinformatics pipeline that integrates multiple lines of evidence including conservation, subcellular localization, and mass spectrometry data to scan the fly proteome for a systematic identification of possible methyllysine-containing peptides. These putative chromodomain-binding peptides suggest unknown functions of the important regulator protein polycomb and provide a list of candidate methylation events for follow-up investigations.

Cancer- and chemotherapy-induced anemia.

Anemia is prevalent in 30% to 90% of patients with cancer. Anemia can be corrected through either treating the underlying cause or providing supportive care through transfusion with packed red blood cells or administration of erythropoiesis-stimulating agents (ESAs), with or without iron supplementation. Recent studies showing detrimental health effects of ESAs sparked a series of FDA label revisions and a sea change in the perception of these once commonly used agents. In light of this, the NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines) for Cancer- and Chemotherapy-Induced Anemia underwent substantial revisions this year. The purpose of these NCCN Guidelines is twofold: 1) to operationalize the evaluation and treatment of anemia in adult cancer patients, with an emphasis on those who are receiving concomitant chemotherapy, and 2) to enable patients and clinicians to individualize anemia treatment options based on patient condition.

A Phase II intra-patient dose-escalation trial of weight-based darbepoetin alfa with or without granulocyte-colony stimulating factor in myelodysplastic syndromes.

This Phase II study evaluated darbepoetin alfa (DA) in 24 patients with predominantly low or intermediate-1 risk myelodysplastic syndrome (MDS). Intra-patient dose escalation of DA was undertaken in three 6-week dose cohorts until a major erythroid response was achieved: 4.5 mcg/kg/week, 9 mcg/kg/week, and 9 mcg/kg/week plus granulocyte-colony stimulating factor (G-CSF) 2.5 mcg/kg twice weekly. Patients with refractory anemia with ringed sideroblasts (RARS) commenced DA at 9 mcg/kg/week. The weight-based dosing regimen translated into a median starting DA dose of 390 mcg/week. Erythroid responses were observed in 16/24 patients (67%; 12 major and 4 minor), with a median response duration of 11 months in major responders. Addition of G-CSF generated a major erythroid response in 7/15 patients (47%) who suboptimally responded to DA alone. DA was well tolerated, except for worsening of baseline mild hypertension and renal insufficiency in one patient with diabetes. IPSS score <0.5 and RBC transfusions <2 units/month increased the probability of an erythroid response. A minority of subjects (12%) developed low-level non-neutralizing anti-DA antibodies. Our data indicate that weekly weight-based dosing of DA, with the addition of G-CSF in selected individuals, can be an effective erythropoietic option in a high proportion of lower-risk MDS patients.

Deciphering and engineering chromodomain-methyllysine peptide recognition.

Posttranslational modifications (PTMs) play critical roles in regulating protein functions and mediating protein-protein interactions. An important PTM is lysine methylation that orchestrates chromatin modifications and regulates functions of non-histone proteins. Methyllysine peptides are bound by modular domains, of which chromodomains are representative. Here, we conducted the first large-scale study of chromodomains in the human proteome interacting with both histone and non-histone methyllysine peptides. We observed significant degenerate binding between chromodomains and histone peptides, i.e., different histone sites can be recognized by the same set of chromodomains, and different chromodomains can share similar binding profiles to individual histone sites. Such degenerate binding is not dictated by amino acid sequence or PTM motif but rather rooted in the physiochemical properties defined by the PTMs on the histone peptides. This molecular mechanism is confirmed by the accurate prediction of the binding specificity using a computational model that captures the structural and energetic patterns of the domain-peptide interaction. To further illustrate the power and accuracy of our model, we used it to effectively engineer an exceptionally strong H3K9me3-binding chromodomain and to label H3K9me3 in live cells. This study presents a systematic approach to deciphering domain-peptide recognition and reveals a general principle by which histone modifications are interpreted by reader proteins, leading to dynamic regulation of gene expression and other biological processes.

High-dose chemotherapy followed by autologous stem cell transplantation for relapsed or refractory Hodgkin lymphoma: prognostic features and outcomes.

Between January 1990 and April 2001, 115 patients received high-dose chemotherapy (HDT) followed by autologous stem cell transplantation (ASCT) for relapsed or refractory Hodgkin lymphoma (HL). With a median follow-up of 58 months (range, 1 - 175 months), 5-year progression-free survival (PFS) and overall survival (OS) were 46% and 58%, respectively. Twelve patients with primary refractory disease had a 5-year PFS of 41% and OS of 58%, not significantly different from those of the remaining cohort. Early and overall regimen related mortality were 7% and 16%, respectively. Male gender (P = 0.04) and a time to relapse (TTR) < 12 months (P = 0.03) were associated with decreased OS by univariate analysis. In multivariate analysis, TTR < 12 months remained statistically significant (P = 0.04). We have confirmed that HDT and ASCT result in long-term survival for a proportion of patients with relapsed or refractory HL. All patients, including those with primary refractory disease, benefited from HDT and ASCT.

Estimation of the receptor-state affinity constants of ligands in functional studies using wild type and constitutively active mutant receptors: Implications for estimation of agonist bias.

We describe a method for estimating the affinities of ligands for active and inactive states of a G protein-coupled receptor (GPCR). Our protocol involves measuring agonist-induced signaling responses of a wild type GPCR and a constitutively active mutant of it under control conditions and after partial receptor inactivation or reduced receptor expression. Our subsequent analysis is based on the assumption that the activating mutation increases receptor isomerization into the active state without affecting the affinities of ligands for receptor states. A means of confirming this assumption is provided. Global nonlinear regression analysis yields estimates of 1) the active (Kact) and inactive (Kinact) receptor-state affinity constants, 2) the isomerization constant of the unoccupied receptor (Kq-obs), and 3) the sensitivity constant of the signaling pathway (KE-obs). The latter two parameters define the output response of the receptor, and hence, their ratio (Kq-obs/KE) is a useful measure of system bias. If the cellular system is reasonably stable and the Kq-obs and KE-obs values of the signaling pathway are known, the Kact and Kinact values of additional agonists can be estimated in subsequent experiments on cells expressing the wild type receptor. We validated our method through computer simulation, an analytical proof, and analysis of previously published data. Our approach provides 1) a more meaningful analysis of structure-activity relationships, 2) a means of validating in silico docking experiments on active and inactive receptor structures and 3) an absolute, in contrast to relative, measure of agonist bias.

The role of erythropoietin in the anemia of myelodysplastic syndrome.

Although erythropoietin (EPO) deficiency is not responsible for the anemia of myelodysplasia, pharmacologic doses of recombinant human EPO (rHuEPO, epoetin alfa) and epoetin beta have been studied extensively as treatment of anemia in myelodysplastic syndrome (MDS). When an epoetin is used as a single growth factor in patients with MDS, clinically meaningful responses occur in only a small minority of patients (16%). Patients who are transfusion-dependent are less likely to respond than patients who are transfusion-independent. Serum EPO level has a weak association with response rate and cannot be used to select or exclude patients from empirical trials of epoetins. The dose schedule commonly used as initial treatment 40,000 U/week, is consistent with clinical observations, but an optimal dose schedule has not been determined. The combination of an epoetin and granulocyte colony-stimulating factor (G-CSF) produces a higher erythroid response rate (36%) than the regimen of epoetin alone, but we have found no randomized trial data to support this point. However, the design of the clinical trials, which included adding G-CSF after epoetin alone had failed, supports the hypothesis that combined use of growth factors, rather than just higher doses of epoetin, is responsible for the high response rate observed with the combination of epoetin and G-CSF. Unfortunately, as in the case of epoetin alone, patients who are transfusion-dependent (> or =2 U red blood cells/month) are less likely to respond to combined growth factor therapy. Although the ability of patients with MDS to show an erythroid response to epoetin is of biologic interest, because of high costs and the limited response rate in transfusion-dependent patients, epoetin therapy, with or without G-CSF, cannot be regarded as a definitive therapy for the anemia of MDS.

Peripheral T-cell lymphomas: clinical features and prognostic factors of 92 cases defined by the revised European American lymphoma classification.

The purpose of this study was to better define the clinical features and natural history of peripheral T-cell lymphomas (PTCL) entities included in the Revised European American lymphoma (REAL) classification. Cases of PTCL were retrieved from the records of the Department of Pathology and classified according to the REAL classification. In addition, cases of anaplastic large cell lymphoma (ALCL) were divided into classical, small cell, and primary cutaneous subtypes, and immunostaining for the anaplastic large-cell kinase (ALK) protein was performed on all cases of ALCL. Clinical features, response to therapy and survival were abstracted. Ninety-two cases of PTCL with adequate clinical information were retrieved. There were 40 cases of ALCL (30 classical, 7 small cell variant, 3 primary cutaneous), 28 PTCL, unspecified, 13 angioimmunoblastic T-cell lymphoma and 11 with other entities. The patients had a median age of 48 years with a range of 6-84 and had an estimated overall survival (OS) of 49% and progression-free survival (PFS) of 22% at 5 years. The International Prognostic Index (IPI) was a significant prognostic factor for both progression-free and OS. Histology was a significant predictor of PFS with anaplastic large cell having the best prognosis. ALK expression was not associated with an improved progression-free or overall-survival in patients with systemic T-cell ALCL. In conclusion, the REAL classification describes distinct PTCL entities. The IPI is the most important predictor of progression-free and OS in patients with PTCL. ALK expression may not provide prognostic information for systemic ALCL.

The recognition specificity of the CHD1 chromodomain with modified histone H3 peptides.

Histone tail peptides comprise the flexible portion of chromatin, the substance which serves as the packaging for the eukaryotic genome. According to the histone code hypothesis, reader protein domains (chromodomains) can recognize modifications of amino acid residues within these peptides, regulating the expression of genes. We have performed simulations on models of chromodomain helicase DNA-binding protein 1 complexed with a variety of histone H3 modifications. Binding free energies for both the overall complexes and the individual residues within the protein and peptides were computed with molecular mechanics-generalized Born surface area. The simulation results agree well with experimental data and identify several chromodomain helicase DNA-binding protein 1 residues that play key roles in the interaction with each of the H3 modifications. We identified one class of protein residues that bind to H3 in all of the complexes (generally interacting hydrophobically), and a second class of residues that bind only to particular H3 modifications (generally interacting electrostatically). Additionally, we found that modifications of H3R2 and H3T3 have a dominant effect on the binding affinity; methylation of H3K4 has little effect on the interaction strength when H3R2 or H3T3 is modified. Our findings with regard to the specificity shown by the latter class of protein residues in their binding affinity to certain modifications of H3 support the histone code hypothesis.

Scaling analysis of dynamic heterogeneity in a supercooled Lennard-Jones liquid.

We have performed molecular dynamics computer simulations of a dense Lennard-Jones liquid mixture to study dynamic heterogeneity from normal liquid temperatures down to a supercooled temperature 15% above the previously identified mode-coupling temperature Tc of the model. A temperature-dependent correlation length associated with the correlation function of mobility fluctuations is calculated. The results are used to test two sets of scaling hypotheses for the dynamic heterogeneity. The results are in close agreement with the inhomogeneous mode-coupling theory of Biroli et al. [Phys. Rev. Lett. 97, 195701 (2006)] for both the alpha and beta relaxation regimes. Comparison with results for kinetically constrained models suggest that the Lennard-Jones mixture studied is more similar to models of fragile liquids than models of very strong liquids.