Canonical fibroblast growth factors (FGFs) activate FGF receptors (FGFRs) through paracrine or autocrine mechanisms in a process that requires cooperation with heparan sulfate proteoglycans, which function as co-receptors for FGFR activation. By contrast, endocrine FGFs (FGF19, FGF21 and FGF23) are circulating hormones that regulate critical metabolic processes in a variety of tissues. FGF19 regulates bile acid synthesis and lipogenesis, whereas FGF21 stimulates insulin sensitivity, energy expenditure and weight loss. Endocrine FGFs signal through FGFRs in a manner that requires klothos, which are cell-surface proteins that possess tandem glycosidase domains. Here we describe the crystal structures of free and ligand-bound β-klotho extracellular regions that reveal the molecular mechanism that underlies the specificity of FGF21 towards β-klotho and demonstrate how the FGFR is activated in a klotho-dependent manner. β-Klotho serves as a primary 'zip code'-like receptor that acts as a targeting signal for FGF21, and FGFR functions as a catalytic subunit that mediates intracellular signalling. Our structures also show how the sugar-cutting enzyme glycosidase has evolved to become a specific receptor for hormones that regulate metabolic processes, including the lowering of blood sugar levels. Finally, we describe an agonistic variant of FGF21 with enhanced biological activity and present structural insights into the potential development of therapeutic agents for diseases linked to endocrine FGFs.

The role of endocytosis in the control of EGF receptor (EGFR) activation and cell signaling was explored by using mouse fibroblasts in which dynamin was conditionally depleted. Dynamin is a GTPase shown to play an important role in the control clathrin mediated endocytosis of EGFR and other cell surface receptors. In this report, we demonstrate that EGF binding activity and the display of high and low affinity EGFRs on the cell surface are not affected by dynamin depletion. By contrast, dynamin depletion leads to a strong inhibition of EGFR endocytosis, robust enhancement of EGFR autophosphorylation and ubiquitination, and slower kinetics of EGFR degradation. Surprisingly, MAPK stimulation induced by either low or high EGF concentrations is not affected by dynamin depletion. While a similar initial Akt response is detected in control or dynamin depleted fibroblasts, a somewhat more sustained Akt stimulation is detected in the dynamin depleted cells. These experiments demonstrate that dynamin-mediated endocytosis leads to attenuation of EGFR activation and degradation and that stimulation of the MAPK response and Akt activation are primarily mediated by activated EGFR located in the plasma membrane.membrane receptors | tyrosine kinases

Large genomic sequencing analysis as part of precision medicine efforts revealed numerous activating mutations in receptor tyrosine kinases, including KIT. Unfortunately, a single approach is not effective for inhibiting cancer cells or treating cancers driven by all known oncogenic KIT mutants. Here, we show that each of the six major KIT oncogenic mutants exhibits different enzymatic, cellular, and dynamic properties and responds distinctly to different KIT inhibitors. One class of KIT mutants responded well to anti-KIT antibody treatment alone or in combination with a low dose of tyrosine kinase inhibitors (TKIs). A second class of KIT mutants, including a mutant resistant to imatinib treatment, responded well to a combination of TKI with anti-KIT antibodies or to anti-KIT toxin conjugates, respectively. We conclude that the preferred choice of precision medicine treatments for cancers driven by activated KIT and other RTKs may rely on clear understanding of the dynamic properties of oncogenic mutants. T he rapid growth in large cancer-sequencing efforts using exome and genome sequencing, as well as elucidation of copy number variations of many cancers, has provided important information about the genetic landscapes and somatic mutations that occur in most cancers. The various catalogs of somatic mutations, chromosomal translocations, and aberrant gene expressions have provided valuable insights about distinct patient populations exhibiting gain-of-function mutations that function as causal "driver" genes for subtypes of different cancers (1-3). These studies have revealed numerous oncogenic mutations in receptor tyrosine kinases (RTKs), which result in constitutive ligand-independent tyrosine kinase activation, cell transformation, and oncogenesis. Consequently, more than 20 kinase inhibitors and half a dozen therapeutic antibodies that block the action of RTKs or the action of critical components of their intracellular signaling pathways have been developed during the past decade and successfully applied in the clinic for treatment of many cancers.A variety of gain-of-function somatic mutations in the receptor tyrosine kinase KIT, including point mutations, in-frame deletions, and in-frame duplications, have been identified in human cancers, including gastro-intestinal-stromal tumors (GISTs), acute myeloid leukemia (AML), mast cell leukemia (MCL), and melanomas (4). Activation of the receptor tyrosine kinase KIT by its ligand stem cell factor (SCF) plays a central role in development of germ cells, hematopoietic cells, interstitial pacemaker cells, and other cells (5). Like other members of the type ΙΙΙ family of RTKs, the extracellular ligand binding domain of KIT contains five Ig-like modules (designated as D1, D2, D3, D4, and D5) connected to a single transmembrane helix, followed by a cytoplasmic region containing a regulatory juxtamembrane (JM) domain, a tyrosine kinase domain (TKD) with a kinase insert region and a C-terminal tail. Tyrosine autophosphorylation sites in the kinase insert region and the...

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