Linear combinations of docking affinities explain quantitative differences in RTK signaling

Receptor tyrosine kinases (RTKs) process extracellular cues by activating a broad array of signaling proteins. Paradoxically, they often use the same proteins to elicit diverse and even opposing phenotypic responses. Binary, ‘on–off' wiring diagrams are therefore inadequate to explain their differences. Here, we show that when six diverse RTKs are placed in the same cellular background, they activate many of the same proteins, but to different quantitative degrees. Additionally, we find that the relative phosphorylation levels of upstream signaling proteins can be accurately predicted using linear models that rely on combinations of receptor-docking affinities and that the docking sites for phosphoinositide 3-kinase (PI3K) and Shc1 provide much of the predictive information. In contrast, we find that the phosphorylation levels of downstream proteins cannot be predicted using linear models. Taken together, these results show that information processing by RTKs can be segmented into discrete upstream and downstream steps, suggesting that the challenging task of constructing mathematical models of RTK signaling can be parsed into separate and more manageable layers

Protein Kinase C α Is a Central Signaling Node and Therapeutic Target for Breast Cancer Stem Cells

The epithelial-mesenchymal transition program becomes activated during malignant progression and can enrich for cancer stem cells (CSCs). We report that inhibition of protein kinase C α (PKCα) specifically targets CSCs but has little effect on non-CSCs. The formation of CSCs from non-stem cells involves a shift from EGFR to PDGFR signaling and results in the PKCα-dependent activation of FRA1. We identified an AP-1 molecular switch in which c-FOS and FRA1 are preferentially utilized in non-CSCs and CSCs, respectively. PKCα and FRA1 expression is associated with the aggressive triple-negative breast cancers, and the depletion of FRA1 results in a mesenchymal-epithelial transition. Hence, identifying molecular features that shift between cell states can be exploited to target signaling components critical to CSCs.National Cancer Institute (U.S.) (Grant P01-CA080111)National Institutes of Health (U.S.) (Grant R01-CA078461

Exact Nonlinear Excitations in Double-Degenerate Plasmas

In this work we use the conventional hydrodynamics (HD) formalism and incorporate the Chew-Goldberger-Low (CGL) double-adiabatic theory to evaluate the nonlinear electrostatic ion excitations in double-degenerate (electron spin-orbit degenerate) magnetized quantum plasmas. Based on the Sagdeev pseudopotential method an exact general pseudopotential is calculated which leads to the allowed Mach-number range criteria for such localized density structures in an anisotropic magnetized plasma. We employ the criteria on the Mach-number range for diverse magnetized quantums plasma with different equations of state (EoS). It is remarked that various plasma fractional parameters such as the system dimensionality, ion-temperature, relativistic-degeneracy, Zeeman-energy, and plasma composition are involved in the stability of an obliquely propagating nonlinear ion-acoustic wave in a double-degenerate quantum plasma. Current study is most appropriate for nonlinear wave analysis in the dense astrophysical magnetized plasma environments such as white-dwarfs and neutron-star crusts where the strong magnetic fields can be present

On the variation of the gauge couplings during inflation

It is shown that the evolution of the (Abelian) gauge coupling during an inflationary phase of de Sitter type drives the growth of the two-point function of the magnetic inhomogeneities. After examining the constraints on the variation of the gauge coupling arising in a standard model of inflationary and post-inflationary evolution, magnetohydrodynamical equations are generalized to the case of time evolving gauge coupling. It is argued that large scale magnetic fields can be copiously generated. Other possible implications of the model are outlined.Comment: 5 pages in RevTex style, one figur

High- and Low-Affinity Epidermal Growth Factor Receptor-Ligand Interactions Activate Distinct Signaling Pathways

Signaling mediated by the Epidermal Growth Factor Receptor (EGFR) is crucial in normal development, and aberrant EGFR signaling has been implicated in a wide variety of cancers. Here we find that the high- and low-affinity interactions between EGFR and its ligands activate different signaling pathways. While high-affinity ligand binding is sufficient for activation of most canonical signaling pathways, low-affinity binding is required for the activation of the Signal transducers and activators of transcription (Stats) and Phospholipase C-gamma 1 (PLCγ1). As the Stat proteins are involved in many cellular responses including proliferation, migration and apoptosis, these results assign a function to low-affinity interactions that has been omitted from computational models of EGFR signaling. The existence of receptors with distinct signaling properties provides a way for EGFR to respond to different concentrations of the same ligand in qualitatively different ways

Stat proteins and PLCγ1 cannot be activated by low EGF concentrations in A431 cells.

<p>Serum-starved A431 cells were treated with either 1 nM or 32 nM EGF. Phosphorylation of EGFR and downstream signaling proteins was monitored over the course of 30 minutes by quantitative immunoblotting. PLCγ1 and the Stat proteins were only activated at the high concentration of EGF (32 nM). All other signaling proteins were activated at both high (32 nM) and low (1 nM) concentrations of EGF. Phosphorylation levels were scaled relative to the maximum signal observed for each antibody. Error bars indicate the range of two biological replicates.</p

Distinct subsets of signaling proteins are activated by different concentrations of EGF.

<p><i>A</i>–<i>E.</i> Serum-starved A431 cells were treated for five minutes with different concentrations of EGF, ranging from 250 pM to 32 nM. Phosphorylation levels were determined by immunoblotting with phosphospecific antibodies and scaled relative to the maximum level observed for each antibody. <i>A</i>. All 12 signaling proteins, as well as two sites of phosphorylation on EGFR. Error bars indicate the standard error of the mean (SEM) of three biological replicates. Representative immunoblots are shown for each antibody. <i>B–D.</i> Proteins shown in panel <i>A</i> were divided into three subsets. EGFR tyrosine phosphorylation is shown in each plot for comparison. Error bars have been omitted for clarity. <i>B</i>. Proteins that are phosphorylated at low concentrations of EGF. <i>C</i>. Proteins that require high concentrations of EGF to be phosphorylated. <i>D</i>. Proteins with atypical responses. <i>E</i>. A subset of the data from panel <i>A</i> is shown, highlighting the lowest concentrations of EGF. <i>F.</i> Serum-starved A431 cells were treated for five minutes with different concentrations of EGF, ranging from 31 pM to 32 nM. Phosphorylation levels were plotted on a log scale to illustrate responses at low EGF concentrations. <i>G</i>. A saturation-binding curve (inset) was generated for EGF binding to A431 cells. Bound EGF is scaled relative to maximum binding. A Scatchard plot of EGF binding to A431 cells was generated by plotting the ratio of bound-to-free EGF as a function of bound EGF. Error bars indicate the SEM of five biological replicates.</p

Activation of low-affinity EGFR alters cellular adhesion properties.

<p>Phase-contrast images of A431 cells treated with different concentrations of EGF for 12 hours. <i>A.</i> Serum-starved cells. <i>B.</i> Cells grown in 10% serum. Onset of the cell clumping phenotype coincides with Stat phosphorylation.</p

Distinct subsets of signaling proteins are activated by different concentrations of both EGF and TGFα in multiple cell lines.

<p>Serum-starved cells were treated for five minutes with different concentrations of EGF or TGFα, ranging from 250 pM to 32 nM. Phosphorylation levels were determined by immunoblotting with phosphospecific antibodies and scaled relative to the maximum level observed for each antibody.</p