Ligand binding to the receptor tyrosine kinase fibroblast growth factor (FGF) receptor 1 (FGFR1) causes dimerization and activation by transphosphorylation of tyrosine residues in the kinase domain. FGFR1 is ubiquitylated by the E3 ligase NEDD4 (also known as NEDD4-1), which promotes FGFR1 internalization and degradation. Although phosphorylation of FGFR1 is required for NEDD4-dependent endocytosis, NEDD4 directly binds to a nonphosphorylated region of FGFR1. We found that activation of FGFR1 led to activation of c-Src kinase-dependent tyrosine phosphorylation of NEDD4, enhancing the ubiquitin ligase activity of NEDD4. Using mass spectrometry, we identified several FGF-dependent phosphorylated tyrosines in NEDD4, including Tyr(43) in the C2 domain and Tyr(585) in the HECT domain. Mutating these tyrosines to phenylalanine to prevent phosphorylation inhibited FGF-dependent NEDD4 activity and FGFR1 endocytosis and enhanced cell proliferation. Mutating the tyrosines to glutamic acid to mimic phosphorylation enhanced NEDD4 activity. Moreover, the NEDD4 C2 domain bound the HECT domain, and the presence of phosphomimetic mutations inhibited this interaction, suggesting that phosphorylation of NEDD4 relieves an inhibitory intra- or intermolecular interaction. Accordingly, activation of FGFR1 was not required for activation of NEDD4 that lacked its C2 domain. Activation of c-Src by epidermal growth factor (EGF) also promoted tyrosine phosphorylation and enhanced the activity of NEDD4. Thus, we identified a feedback mechanism by which receptor tyrosine kinases promote catalytic activation of NEDD4 and that may represent a mechanism of receptor crosstalk.

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ObjectivesEvaluate gender differences in authorship of COVID-19 articles in high-impact medical journals compared with other topics.DesignCross-sectional review.Data sourcesMedline database.Eligibility criteriaArticles published from 1 January to 31 December 2020 in the seven leading general medical journals by impact factor. Article types included primary research, reviews, editorials and commentaries.Data extractionKey data elements were whether the study topic was related to COVID-19 and names of the principal and the senior authors. A hierarchical approach was used to determine the likely gender of authors. Logistic regression assessed the association of study characteristics, including COVID-19 status, with authors’ likely gender; this was quantified using adjusted ORs (aORs).ResultsWe included 2252 articles, of which 748 (33.2%) were COVID-19-related and 1504 (66.8%) covered other topics. A likely gender was determined for 2138 (94.9%) principal authors and 1890 (83.9%) senior authors. Men were significantly more likely to be both principal (1364 men; 63.8%) and senior (1332 men; 70.5%) authors. COVID-19-related articles were not associated with the odds of men being principal (aOR 0.99; 95% CI 0.81 to 1.21; p=0.89) or senior authors (aOR 0.96; 95% CI 0.78 to 1.19; p=0.71) relative to other topics. Articles with men as senior authors were more likely to have men as principal authors (aOR 1.49; 95% CI 1.21 to 1.83; p<0.001). Men were more likely to author articles reporting original research and those with corresponding authors based outside the USA and Europe.ConclusionsWomen were substantially under-represented as authors among articles in leading medical journals; this was not significantly different for COVID-19-related articles. Study limitations include potential for misclassification bias due to the name-based analysis. Results suggest that barriers to women’s authorship in high-impact journals during COVID-19 are not significantly larger than barriers that preceded the pandemic and that are likely to continue beyond it.PROSPERO registration numberCRD42020186702.