Escape from HER-family tyrosine kinase inhibitor therapy by the kinase-inactive HER3

Oncogenic tyrosine kinases have proved to be promising targets for the development of highly effective anticancer drugs. However, tyrosine kinase inhibitors (TKIs) against the human epidermal growth factor receptor (HER) family show only limited activity against HER2-driven breast cancers, despite effective inhibition of epidermal growth factor receptor (EGFR) and HER2 in vivo. The reasons for this are unclear. Signalling in trans is a key feature of this multimember family and the critically important phosphatidylinositol-3-OH kinase (PI(3)K)/Akt pathway is driven predominantly through transphosphorylation of the kinase-inactive HER3 (refs 9, 10). Here we show that HER3 and consequently PI(3)K/Akt signalling evade inhibition by current HER-family TKIs in vitro and in tumours in vivo. This is due to a compensatory shift in the HER3 phosphorylation-dephosphorylation equilibrium, driven by increased membrane HER3 expression driving the phosphorylation reaction and by reduced HER3 phosphatase activity impeding the dephosphorylation reaction. These compensatory changes are driven by Akt-mediated negative-feedback signalling. Although HER3 is not a direct target of TKIs, HER3 substrate resistance undermines their efficacy and has thus far gone undetected. The experimental abrogation of HER3 resistance by small interfering RNA knockdown restores potent pro-apoptotic activity to otherwise cytostatic HER TKIs, re-affirming the oncogene-addicted nature of HER2-driven tumours and the therapeutic promise of this oncoprotein target. However, because HER3 signalling is buffered against an incomplete inhibition of HER2 kinase, much more potent TKIs or combination strategies are required to silence oncogenic HER2 signalling effectively. The biologic marker with which to assess the efficacy of HER TKIs should be the transphosphorylation of HER3 rather than autophosphorylation.     

A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence

Plants sense potential microbial invaders by using pattern-recognition receptors to recognize pathogen-associated molecular patterns (PAMPs). In Arabidopsis thaliana, the leucine-rich repeat receptor kinases flagellin-sensitive 2 (FLS2) (ref. 2) and elongation factor Tu receptor (EFR) (ref. 3) act as pattern-recognition receptors for the bacterial PAMPs flagellin and elongation factor Tu (EF-Tu) (ref. 5) and contribute to resistance against bacterial pathogens. Little is known about the molecular mechanisms that link receptor activation to intracellular signal transduction. Here we show that BAK1 (BRI1-associated receptor kinase 1), a leucine-rich repeat receptor-like kinase that has been reported to regulate the brassinosteroid receptor BRI1 (refs 6,7), is involved in signalling by FLS2 and EFR. Plants carrying bak1 mutations show normal flagellin binding but abnormal early and late flagellin-triggered responses, indicating that BAK1 acts as a positive regulator in signalling. The bak1-mutant plants also show a reduction in early, but not late, EF-Tu-triggered responses. The decrease in responses to PAMPs is not due to reduced sensitivity to brassinosteroids. We provide evidence that FLS2 and BAK1 form a complex in vivo, in a specific ligand-dependent manner, within the first minutes of stimulation with flagellin. Thus, BAK1 is not only associated with developmental regulation through the plant hormone receptor BRI1 (refs 6,7), but also has a functional role in PRR-dependent signalling, which initiates innate immunity.     

Structural basis for synthesis of inflammatory mediators by human leukotriene C4 synthase

Cysteinyl leukotrienes are key mediators in inflammation and have an important role in acute and chronic inflammatory diseases of the cardiovascular and respiratory systems, in particular bronchial asthma. In the biosynthesis of cysteinyl leukotrienes, conversion of arachidonic acid forms the unstable epoxide leukotriene A4 (LTA4). This intermediate is conjugated with glutathione (GSH) to produce leukotriene C4 (LTC4) in a reaction catalysed by LTC4 synthase: this reaction is the key step in cysteinyl leukotriene formation. Here we present the crystal structure of the human LTC4 synthase in its apo and GSH-complexed forms to 2.00 and 2.15 A resolution, respectively. The structure reveals a homotrimer, where each monomer is composed of four transmembrane segments. The structure of the enzyme in complex with substrate reveals that the active site enforces a horseshoe-shaped conformation on GSH, and effectively positions the thiol group for activation by a nearby arginine at the membrane-enzyme interface. In addition, the structure provides a model for how the omega-end of the lipophilic co-substrate is pinned at one end of a hydrophobic cleft, providing a molecular 'ruler' to align the reactive epoxide at the thiol of glutathione. This provides new structural insights into the mechanism of LTC4 formation, and also suggests that the observed binding and activation of GSH might be common for a family of homologous proteins important for inflammatory and detoxification responses.     

Importance of rain evaporation and continental convection in the tropical water cycle

Atmospheric moisture cycling is an important aspect of the Earth's climate system, yet the processes determining atmospheric humidity are poorly understood. For example, direct evaporation of rain contributes significantly to the heat and moisture budgets of clouds, but few observations of these processes are available. Similarly, the relative contributions to atmospheric moisture over land from local evaporation and humidity from oceanic sources are uncertain. Lighter isotopes of water vapour preferentially evaporate whereas heavier isotopes preferentially condense and the isotopic composition of ocean water is known. Here we use this information combined with global measurements of the isotopic composition of tropospheric water vapour from the Tropospheric Emission Spectrometer (TES) aboard the Aura spacecraft, to investigate aspects of the atmospheric hydrological cycle that are not well constrained by observations of precipitation or atmospheric vapour content. Our measurements of the isotopic composition of water vapour near tropical clouds suggest that rainfall evaporation contributes significantly to lower troposphere humidity, with typically 20% and up to 50% of rainfall evaporating near convective clouds. Over the tropical continents the isotopic signature of tropospheric water vapour differs significantly from that of precipitation, suggesting that convection of vapour from both oceanic sources and evapotranspiration are the dominant moisture sources. Our measurements allow an assessment of the intensity of the present hydrological cycle and will help identify any future changes as they occur.