Dual-specificity MAP kinase phosphatases as targets of cancer treatment

The protein tyrosine phosphatase family (PTP) contains a group of dual-specificity phosphatases (DUSPs) that regulate the activivity of MAP kinases (MAPKs), which are key effectors in the control of cell growth and survival in physiological and pathological processes, including cancer. These phosphatases, named as MKP-DUSPs, include the MAPK phosphatases (MKPs) as well as a group of small-size atypical DUSPs structurally and functionally related to the MKPs. MKP-DUSPs, in most of the cases, are direct inactivators of MAPKs by dephosphorylation of both the Thr and the Tyr regulatory residues at the MAPKs catalytic loop. In some other cases, MKP-DUSPs regulate the activity of MAPKs indirectly, acting through upstream MAPK pathways components. The active involvement of MKP-DUSPs in oncogenesis or resistance to cancer therapies is now well documented, making the search and validation of MKP-DUSPs inhibitors a prominent area in clinical cancer research. Here, we review the current knowledge on the role of MKP-DUSPs in human cancer, the status of the preclinical development and validation of specific MKP-DUSP inhibitors, and the potential of MKP-DUSPs as targets for anti-cancer drugs.

Selective reduction of carbon dioxide to bis(silyl)acetal catalyzed by a PBP-supported nickel complex

The selective reduction of CO₂ to the formaldehyde level remains an important challenge and to date only a few catalysts have been developed for this reaction.

Dual-specificity phosphatases as molecular targets for inhibition in human disease

SIGNIFICANCE

The dual-specificity phosphatases (DUSPs) constitute a heterogeneous group of cysteine-based protein tyrosine phosphatases, whose members exert a pivotal role in cell physiology by dephosphorylation of phosphoserine, phosphothreonine, and phosphotyrosine residues from proteins, as well as other non-proteinaceous substrates.

RECENT ADVANCES

A picture is emerging in which a selected group of DUSP enzymes display overexpression or hyperactivity that is associated with human disease, especially human cancer, making feasible targeted therapy approaches based on their inhibition. A panoply of molecular and functional studies on DUSPs have been performed in the previous years, and drug-discovery efforts are ongoing to develop specific and efficient DUSP enzyme inhibitors. This review summarizes the current status on inhibitory compounds targeting DUSPs that belong to the MAP kinase phosphatases-, small-sized atypical-, and phosphatases of regenerating liver subfamilies, whose inhibition could be beneficial for the prevention or mitigation of human disease.

CRITICAL ISSUES

Achieving specificity, potency, and bioavailability are the major challenges in the discovery of DUSP inhibitors for the clinics. Clinical validation of compounds or alternative inhibitory strategies of DUSP inhibition has yet to come.

FUTURE DIRECTIONS

Further work is required to understand the dual role of many DUSPs in human cancer, their function-structure properties, and to identify their physiologic substrates. This will help in the implementation of therapies based on DUSPs inhibition.

Enantioselective carbohydrate recognition by synthetic lectins in water

Carbohydrate receptors with a chiral framework have been generated by combining a tetra-aminopyrene and a C3-symmetrical triamine via isophthalamide spacers bearing water-solubilising groups. These "synthetic lectins" are the first to show enantiodiscrimination in aqueous solution, binding N-acetylglucosamine (GlcNAc) with 16 : 1 enantioselectivity. They also show exceptional affinities. GlcNAc is bound with Ka up to 1280 M-1, more than twice that measured for previous synthetic lectins, and three times the value for wheat germ agglutinin, the lectin traditionally employed to bind GlcNAc in glycobiological research. Glucose is bound with Ka = 250 M-1, again higher than previous synthetic lectins. The results suggest that chirality can improve complementarity to carbohydrate substrates and may thus be advantageous in synthetic lectin design.

ETHQV6.3 is involved in melon climacteric fruit ripening and is encoded by a NAC domain transcription factor

Fruit ripening is divided into climacteric and non-climacteric types depending on the presence or absence of a transient rise in respiration rate and the production of autocatalytic ethylene. Melon is ideal for the study of fruit ripening, as both climacteric and non-climacteric varieties exist. Two introgressions of the non-climacteric accession PI 161375, encompassed in the QTLs ETHQB3.5 and ETHQV6.3, into the non-climacteric 'Piel de Sapo' background are able to induce climacteric ripening independently. We report that the gene underlying ETHQV6.3 is MELO3C016540 (CmNAC-NOR), encoding a NAC (NAM, ATAF1,2, CUC2) transcription factor that is closely related to the tomato NOR (non-ripening) gene. CmNAC-NOR was functionally validated through the identification of two TILLING lines carrying non-synonymous mutations in the conserved NAC domain region. In an otherwise highly climacteric genetic background, both mutations provoked a significant delay in the onset of fruit ripening and in the biosynthesis of ethylene. The PI 161375 allele of ETHQV6.3 is similar to that of climacteric lines of the cantalupensis type and, when introgressed into the non-climacteric 'Piel de Sapo', partially restores its climacteric ripening capacity. CmNAC-NOR is expressed in fruit flesh of both climacteric and non-climacteric lines, suggesting that the causal mutation may not be acting at the transcriptional level. The use of a comparative genetic approach in a species with both climacteric and non-climacteric ripening is a powerful strategy to dissect the complex mechanisms regulating the onset of fruit ripening.

Long-term outcome of insulin pump therapy: reduction of hypoglycaemia and impact on glycaemic control

AIMS

To determine the long-term outcome of continuous subcutaneous insulin infusion (CSII) in Type 1 diabetes according to Catalan National Health Service indications.

METHODS

Retrospective observational study including 178 patients with Type 1 diabetes who started CSII treatment in our centre (2003-2008). All patients were followed in our CSII programme for outpatients for at least 5 years. Data on annual HbA1c levels were collected, and the main indication for starting CSII was analysed.

RESULTS

Twenty-seven of 178 patients were excluded because of loss to follow-up or withdrawal from CSII, thus 151 patients (aged 37.4 ± 10.5 years, 64% women) were analysed. The main indications for starting CSII were suboptimal metabolic control (60.9%), severe hypoglycaemia/hypoglycaemia unawareness (25.5%) and others (13.6%). HbA1c was 64 ± 13 mmol/mol (8.0 ± 1.2%) at the start of CSII and 62 ± 13 mmol/mol (7.8 ± 1.2%) after 5 years in the total cohort (P = 0.1). The severe hypoglycaemia rates were 0.66 ± 1.61 and 0.17 ± 0.42 episodes/patient/year (P < 0.001). In patients with suboptimal metabolic control, HbA1c decreased from 68 ± 12 mmol/mol (8.4 ± 1.1%) to 64 ± 14 mmol/mol (8.0 ± 1.3%) (P = 0.016), with 37.4% of those in this group having an HbA1c ≤ 58 mmol/mol (7.5%) after 5 years. In patients starting CSII due to severe hypoglycaemia the problem was considered resolved in 93%, and in 64% of those starting CSII because of suboptimal glycaemic control, HbA1c improved significantly.

CONCLUSIONS

CSII therapy achieves and maintains its efficacy mainly in terms of reducing severe hypoglycaemia. In the whole group of patients, the reduction in HbA1c is transient and disappears after 5 years.

Orthophosphate and Sulfate Utilization for C-E (E = P, S) Bond Formation via Trichlorosilyl Phosphide and Sulfide Anions

Reduction of phosphoric acid (H₃PO₄) or tetra- n-butylammonium bisulfate ([TBA][HSO₄]) with trichlorosilane leads to the formation of the bis(trichlorosilyl)phosphide ([P(SiCl₃)₂]^-, 1) and trichlorosilylsulfide ([Cl₃SiS]^-, 2) anions, respectively. Balanced equations for the formation of the TBA salts of anions 1 and 2 were formulated based on the identification of hexachlorodisiloxane and hydrogen gas as byproducts arising from these reductive processes: i) [H₂PO₄]^- + 10HSiCl₃ → 1 + 4O(SiCl₃)₂ + 6H₂ for P and ii) [HSO₄]^- + 9HSiCl₃ → 2 + 4O(SiCl₃)₂ + 5H₂ for S. Hydrogen gas was identified by its subsequent use to hydrogenate an alkene ((-)-terpinen-4-ol) using Crabtree's catalyst ([(COD)Ir(py)(PCy₃)][PF₆], COD = 1,5-cyclooctadiene, py = pyridine, Cy = cyclohexyl). Phosphide 1 was generated in situ by the reaction of phosphoric acid and trichlorosilane and used to convert an alkyl chloride (1-chlorooctane) to the corresponding primary phosphine, which was isolated in 41% yield. Anion 1 was also prepared from [TBA][H₂PO₄] and isolated in 62% yield on a gram scale. Treatment of [TBA]1 with an excess of benzyl chloride leads to the formation of tetrabenzylphosphonium chloride, which was isolated in 61% yield. Sulfide 2 was used as a thionation reagent, converting benzophenone to thiobenzophenone in 62% yield. It also converted benzyl bromide to benzyl mercaptan in 55% yield. The TBA salt of trimetaphosphate ([TBA]₃[P₃O₉]·2H₂O), also a precursor to anion 1, was found to react with either trichlorosilane or silicon(IV) chloride to provide bis(trimetaphosphate)silicate, [TBA]₂[Si(P₃O₉)₂], characterized by NMR spectroscopy, X-ray crystallography, and elemental analysis. Trichlorosilane reduction of [TBA]₂[Si(P₃O₉)₂] also provided anion 1. The electronic structures of 1 and 2 were investigated using a suite of theoretical methods; the computational studies suggest that the trichlorosilyl ligand is a good π-acceptor and forms σ-bonds with a high degree of s character.

ERK2 shows a restrictive and locally selective mechanism of recognition by its tyrosine phosphatase inactivators not shared by its activator MEK1

The two regulatory residues that control the enzymatic activity of the mitogen-activated protein (MAP) kinase ERK2 are phosphorylated by the unique MAP kinase kinases MEK1/2 and dephosphorylated by several tyrosine-specific and dual specificity protein phosphatases. Selective docking interactions facilitate these phosphorylation and dephosphorylation events, controlling the specificity and duration of the MAP kinase activation-inactivation cycles. We have analyzed the contribution of specific residues of ERK2 in the physical and functional interaction with the ERK2 phosphatase inactivators PTP-SL and MKP-3 and with its activator MEK1. Single mutations in ERK2 that abrogated the dephosphorylation by endogenous tyrosine phosphatases from HEK293 cells still allowed efficient phosphorylation by endogenous MEK1/2. Discrete ERK2 mutations at the ERK2 docking groove differentially affected binding and inactivation by PTP-SL and MKP-3. Remarkably, the cytosolic retention of ERK2 by its activator MEK1 was not affected by any of the analyzed ERK2 single amino acid substitutions. A chimeric MEK1 protein, containing the kinase interaction motif of PTP-SL, bound tightly to ERK2 through its docking groove and behaved as a gain-of-function MAP kinase kinase that hyperactivated ERK2. Our results provide evidence that the ERK2 docking groove is more restrictive and selective for its tyrosine phosphatase inactivators than for MEK1/2 and indicate that distinct ERK2 residues modulate the docking interactions with activating and inactivating effectors.

Selective catalytic synthesis of amino-silanes at part-per million catalyst loadings

Platinum(ii) complex [Pt(I^tBu′)(I^tBu)][BAr^F₄] (1a) is a highly active and selective catalyst in the dehydrocoupling of amines and silanes at part-per-million catalyst loadings (up to 10 ppm, 0.001 mol%), achieving the highest TON and TOF numbers reported in the literature (up to 1 mmol scale).

A novel phosphatase family, structurally related to dual-specificity phosphatases, that displays unique amino acid sequence and substrate specificity

Members of the superfamily of protein tyrosine phosphatases (PTPs) share the presence of an evolutionarily conserved PTP catalytic domain. Among them, the dual-specificity phosphatases (DSPs) constitute a diverse group of enzymes in terms of substrate specificity, including nonprotein substrates. In recent years, an increasing number of novel DSPs, whose functions and biological substrates are not well defined, have been discovered in a variety of organisms. In this study, we define the structural and functional properties of evolutionarily related atypical DSPs from different phyla. Sets of conserved motifs were defined that (i) uniquely segregated mammalian atypical DSPs from closely related enzymes and (ii) exclusively characterised a novel family of atypical DSPs present in plants, fungi, and kinetoplastids [plant and fungi atypical (PFA)-DSPs]; despite having different sequence "fingerprints," the PTP tertiary structure of PFA-DSPs is conserved. Analysis of the catalytic properties of PFA-DSPs suggests the existence of a unique substrate specificity for these enzymes. Our findings predict characteristic functional motifs for the diverse members of the DSP families of PTPs and provide insights into the functional properties of DSPs of unknown function.

Accuracy of immunoassay and mass spectrometry urinary free cortisol in the diagnosis of Cushing's syndrome

PURPOSE

Urinary free cortisol (UFC) determination by highly specific methods as mass spectrometry instead of commercially available antibody-based immunoassays is increasingly recommended. However, clinical comparisons of both analytical approaches in the screening of Cushing's syndrome (CS) are not available. The aim of this study was to evaluate the diagnostic value of mass spectrometry versus immunoassay measurements of 24 h-UFC in the screening of CS.

METHODS

Cross-sectional study of 33 histologically confirmed CS patients: 25 Cushing's disease, 5 adrenal CS and 3 ectopic CS; 92 non-CS patients; and 35 healthy controls. UFC by immunoassay (UFCxIA) and mass spectrometry (UFCxMS), urinary free cortisone (UFCo) and UFC:UFCo ratio were measured, together with creatinine-corrected values. Sensitivity, specificity, AUC and Landis and Koch concordance index were determined.

RESULTS

AUC for UFCxIA and UFCxMS were 0.77 (CI 0.68-0.87) and 0.77 (CI 0.67-0.87) respectively, with a kappa coefficient 0.60 and strong Landis and Koch concordance index. The best calculated cutoff values were 359 nmol/24 h for UFCxIA (78 % sensitivity, 62 % specificity) and 258.1 nmol/24 h for UCFxMS (53 % sensitivity, 86 % specificity). The upper limit of UFCxIA and UCFxMS reference ranges were 344.7 and 169.5 nmol/24 h respectively. Sensitivity and specificity for CS diagnosis at these cutpoints were 84 and 56 % for UFCxIA and 81 and 54 % for UFCxMS.

CONCLUSIONS

According to our data, both methods present a very similar diagnostic value. However, results suggest that lower cutoff points for mass spectrometry may be necessary in order to improve clinical sensitivity.

Evolution of serum phosphate in long intermittent hemodialysis

BACKGROUND

Phosphate (P) control is crucial to prevent secondary hyperparathyroidism and extraosseous calcification. Hemodialysis techniques normally used do not remove the amount of P absorbed from an adequate protein intake. Our long hemodialysis (LHD) procedure produces greater P removal in a seven-hour session than in a conventional four-hour session. Our aim was to assess the evolution of predialysis serum P and Ca x P product in patients on intermittent LHD during the first two years on this dialysis schedule.

METHODS

Eight irregular compliant patients underwent LHD for 12 months; five of them completing 24 months. In the conventional hemodialysis (CHD) period, session time was 12 hours per week, while it was 21 hours per week in LHD. We compared P and Ca x P product throughout both periods; each patient was his or her own control.

RESULTS

Mean serum P improved from 7.1 +/- 1.1 mg/dL in CHD to 5.6 +/- 1.1 mg/dL during the second year on LHD (P < 0.05). Mean Ca x P product decreased from 64 +/- 12.7 mg2/dL2 during CHD to 50.9 +/- 10.3 mg2/dL2 at the end of the second year of LHD.

CONCLUSION

By prolonging dialysis time to 21 hours per week, better P serum control and lower Ca x P product were achieved in our patients.

Clearance of hepatitis C virus in HIV-infected patients with multiple chronic viral hepatitis

Viral interferences between hepatitis C (HCV) and hepatitis B (HBV) viruses were investigated in a case-control study conducted in 107 human immunodeficiency virus (HIV)-infected patients with HCV antibodies. Overall, 15 (68%) of 22 hepatitis B surface antigen (HBsAg)-positive patients had negative serum HCV-RNA while it occurred in only nine (10%) of 85 HBsAg-negative counterparts (P = 0.02). After adjusting for age, antiretroviral therapy, plasma HIV-RNA and CD4 counts, being HBsAg-positive was strongly associated with having negative serum HCV-RNA (odds ratio: 23; 95% confidence interval: 6-59; P < 0.001). Thus, HBV may favour the elimination of HCV in HIV-infected patients, which may influence liver disease and therapeutic decisions.

Dose Escalation with Three-dimensional Conformal Radiotherapy for Prostate Cancer. Is More Dose Really Better in High-risk Patients Treated with Androgen Deprivation?

AIMS

To determine the effect of radiation dose on biochemical control in prostate cancer patients treated in a single institution with three-dimensional conformal radiotherapy (3DCRT) and the additional effect of androgen deprivation in prostate cancer patients.

MATERIALS AND METHODS

In total, 363 men with T1-T3b prostate cancer treated in a sequential radiation dose-escalation trial from 66.0 to 84.1 Gy (International Commission Radiation Units and Measurement [ICRU] reference point) between 1995 and 2003, and with a minimum follow-up of 24 months, were included in the analysis. One hundred and forty-eight (41%) men were treated with 3DCRT alone; 74 (20%) men received neoadjuvant androgen deprivation (NAD) 4-6 months before and during 3DCRT; and 141 (39%) men received NAD and adjuvant androgen deprivation (AAD) 2 years after 3DCRT. Univariate, stratified and multivariate analyses were carried out separately for defined risk groups (low, intermediate and high) to determine the effect of radiation dose on biochemical control and its interaction with hormonal manipulation and clinical prognostic variables.

RESULTS

The median follow-up was 59 months (range 24-147 months). The actuarial biochemical disease-free survival (bDFS) at 5 years for all patients was 75% (standard error 3%). For low-risk patients, the bDFS was 82% (standard error 5%), for intermediate-risk patients it was 64% (standard error 6%) and for high-risk patients it was 77% (standard error 3%) (P = 0.031). In stratified and multivariate analyses, high-dose 3DCRT for all risk groups, and for high-risk patients, the use of long-term AAD vs NAD, contributed independently and significantly to improve the outcome of prostate cancer patients.

CONCLUSION

The present study indicates an independent benefit on biochemical outcome of high-dose 3DCRT for low-, intermediate- and high-risk patients and of long-term AAD in high-risk prostate cancer patients.

σ-Silane Platinum(II) Complexes as Intermediates in C-Si Bond-Coupling Processes

Platinum complexes [Pt(NHC')(NHC)][BAr^F ] (in which NHC' denotes a cyclometalated N-heterocyclic carbene ligand, NHC) react with primary silanes RSiH₃ to afford the cyclometalated platinum(II) silyl complexes [Pt(NHC-SiHR')(NHC)][BAr^F ] through a process that involves the formation of C-Si and Pt-Si bonds with concomitant extrusion of H₂ . Low-temperature NMR studies indicate that the process proceeds through initial formation of the σ-SiH complexes [Pt(NHC')(NHC)(HSiH₂ R)][BAr^F ], which are stable at temperatures below -10 °C. At higher temperatures, activation of one Si-H bond followed by a C-Si coupling reaction generates an agostic SiH platinum hydride derivative [Pt(H)(NHC'-SiH₂ R)(NHC)][BAr^F ], which undergoes a second Si-H bond activation to afford the final products. Computational modeling of the reaction mechanism indicates that the stereochemistry of the silyl/hydride ligands after the first Si-H bond cleavage dictates the nature of the products, favoring the formation of a C-Si bond over a C-H bond, in contrast to previous results obtained for tertiary silanes. Furthermore, the process involves a trans-to-cis isomerization of the NHC ligand before the second Si-H bond cleavage.

Enhancing the catalytic properties of well-defined electrophilic platinum complexes

Platinum complexes have been often considered as the least reactive of the group 10 triad metals. Slow kinetics are behind this lack of reactivity but, still, some industrially relevant catalytic process are dominated by platinum compounds and sometimes different selectivities can be found in comparison to Ni or Pd. Nevertheless, during the last years, it has been reported that the catalytic behaviour of well-defined platinum derivatives can be improved through a judicious choice of their electronic and steric properties, leading to highly electrophilic or low-electron count platinum systems. In this feature article, we highlight some catalytic processes in which well-defined electrophilic platinum complexes or coordinatively unsaturated systems play an important role in their catalytic activity.

Ambiphilic boryl groups in a neutral Ni(ii) complex: a new activation mode of H2

The concept of metal-ligand cooperation opens new avenues for the design of catalytic systems that may offer alternative reactivity patterns to the existing ones. Investigations of this concept with ligands bearing a boron center in their skeleton established mechanistic pathways for the activation of small molecules in which the boron atom usually performs as an electrophile. Here, we show how this electrophilic behavior can be modified by the ligand trans to the boron center, evincing its ambiphilic nature. Treatment of diphosphinoboryl (PBP) nickel-methyl complex 1 with bis(catecholato)diboron (B2Cat2) allows for the synthesis of nickel(ii) bis-boryl complex 3 that promotes the clean and reversible heterolytic cleavage of dihydrogen leading to the formation of dihydroborate nickel complex 4. Density functional theory analysis of this reaction revealed that the heterolytic activation of H2 is facilitated by the cooperation of both boryl moieties and the metal atom in a concerted mechanism that involves a Ni(ii)/Ni(0)/Ni(ii) process. Contrary to 1, the boron atom from the PBP ligand in 3 behaves as a nucleophile, accepting a formally protic hydrogen, whereas the catecholboryl moiety acts as an electrophile, receiving the attack from the hydride-like fragment. This manifests the dramatic change in the electronic properties of a ligand by tuning the substituent trans to it and constitutes an unprecedented cooperative mechanism that involves two boryl ligands in the same molecule operating differently, one as a Lewis acid and the other one as a Lewis base, in cooperation with the metal. In addition, reactivity towards different nucleophiles such as amines or ammonia confirmed the electrophilic nature of the Bcat moiety, allowing the formation of aminoboranes.

Robust dicopper(i) μ-boryl complexes supported by a dinucleating naphthyridine-based ligand

Copper boryl species have been widely invoked as reactive intermediates in Cu-catalysed C–H borylation reactions, but their isolation and study have been challenging. Use of the robust dinucleating ligand DPFN (2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine) allowed for the isolation of two very thermally stable dicopper(i) boryl complexes, [(DPFN)Cu₂(μ-Bpin)][NTf₂] (2) and [(DPFN)Cu₂(μ-Bcat)][NTf₂] (4) (pin = 2,3-dimethylbutane-2,3-diol; cat = benzene-1,2-diol). These complexes were prepared by cleavage of the corresponding diborane via reaction with the alkoxide [(DPFN)Cu₂(μ-O^tBu)][NTf₂] (3). Reactivity studies illustrated the exceptional stability of these boryl complexes (thermal stability in solution up to 100 °C) and their role in the activation of C(sp)–H bonds. X-ray diffraction and computational studies provide a detailed description of the bonding and electronic structures in these complexes, and suggest that the dinucleating character of the naphthyridine-based ligand is largely responsible for their remarkable stability.

Cu(i) boryl species have been widely invoked as reactive intermediates in Cu-catalysed C–H borylations, but their isolation has been challenging. In this work, thermally robust dicopper(I) boryl complexes have been synthesized and studied in detail.

Proteolytic processing of the receptor-type protein tyrosine phosphatase PTPBR7

The single-copy mouse gene Ptprr gives rise to different protein tyrosine phosphatase (PTP) isoforms in neuronal cells through the use of distinct promoters, alternative splicing, and multiple translation initiation sites. Here, we examined the array of post-translational modifications imposed on the PTPRR protein isoforms PTPBR7, PTP-SL, PTPPBSgamma42 and PTPPBSgamma37, which have distinct N-terminal segments and localize to different parts of the cell. All isoforms were found to be short-lived, constitutively phosphorylated proteins. In addition, the transmembrane isoform, PTPBR7, was subject to N-terminal proteolytic processing, in between amino acid position 136 and 137, resulting in an additional, 65-kDa transmembrane PTPRR isoform. Unlike for some other receptor-type PTPs, the proteolytically produced N-terminal ectodomain does not remain associated with this PTPRR-65. Shedding of PTPBR7-derived polypeptides at the cell surface further adds to the molecular complexity of PTPRR biology.

Multimerisation of receptor-type protein tyrosine phosphatases PTPBR7 and PTP-SL attenuates enzymatic activity

Dimerisation of receptor-type protein tyrosine phosphatases (RPTPs) represents an appealing mechanism to regulate their enzymatic activity. Studies thus far mostly concern the dimerisation behaviour of RPTPs possessing two tandemly oriented catalytic PTP domains. Mouse gene Ptprr encodes four different protein isoforms (i.e. PTPBR7, PTP-SL and PTPPBSgamma-42/37) that contain a single PTP domain. Using selective membrane permeabilisation we here demonstrate that PTP-SL, like PTPBR7, is a single membrane-spanning RPTP. Furthermore, these two receptor-type PTPs constitutively formed homo- and hetero-meric complexes as witnessed in chemical cross-linking and co-immunoprecipitation experiments, in sharp contrast to the cytosolic PTPPBSgamma-42 and PTPPBSgamma-37 PTPRR isoforms. This multimerisation occurs independently of the PTP domain and requires the transmembrane domain and/or the proximal hydrophobic region. Using overexpression of a PTPBR7 mutant that essentially lacks the intracellular PTP domain-containing segment, a monomer-mimicking state was forced upon full-length PTPBR7 immunoprecipitates. This resulted in a significant increase in the enzymatic activity of the PTPRR PTP domain, which strengthens the notion that multimerisation represents a general mechanism to tone down RPTP catalytic activity.

Reversible carbon-boron bond formation at platinum centers through σ-BH complexes

A reversible carbon-boron bond formation has been observed in the reaction of the coordinatively unsaturated, cyclometalated, Pt(ii) complex [Pt(I ^t BuⁱPr')(I ^t BuⁱPr)][BAr^F], 1, with tricoordinated boranes HBR₂. X-ray diffraction studies provided structural snapshots of the sequence of reactions involved in the process. At low temperature, we observed the initial formation of the unprecedented σ-BH complexes [Pt(HBR₂)(I ^t BuⁱPr')(I ^t BuⁱPr)][BAr^F], one of which has been isolated. From -15 to +10 °C, the σ-BH species undergo a carbon-boron coupling process leading to the platinum hydride derivative [Pt(H)(I ^t BuⁱPr-BR₂)(I ^t BuⁱPr)][BAr^F], 4. Surprisingly, these compounds are thermally unstable undergoing carbon-boron bond cleavage at room temperature that results in the 14-electron Pt(ii) boryl species [Pt(BR₂)(I ^t BuⁱPr)₂][BAr^F], 2. This unusual reaction process has been corroborated by computational methods, which indicate that the carbon-boron coupling products 4 are formed under kinetic control whereas the platinum boryl species 2, arising from competitive C-H bond coupling, are thermodynamically more stable. These findings provide valuable information about the factors governing productive carbon-boron coupling reactions at transition metal centers.