The Thanatophoric Dysplasia Type II Mutation Hampers Complete Maturation of Fibroblast Growth Factor Receptor 3 (FGFR3), Which Activates Signal Transducer and Activator of Transcription 1 (STAT1) from the Endoplasmic Reticulum

University of Verona, Verona, Veneto, Italy
Journal of Biological Chemistry (Impact Factor: 4.57). 06/2003; 278(19):17344-9. DOI: 10.1074/jbc.M212710200
Source: PubMed


The K650E substitution in the fibroblast growth factor receptor 3 (FGFR3) causes constitutive tyrosine kinase activity of the receptor and is associated to the lethal skeletal disorder, thanatophoric dysplasia type II (TDII). The underlying mechanisms of how the activated FGFR3 causes TDII remains to be elucidated. FGFR3 is a transmembrane glycoprotein, which is synthesized through three isoforms, with various degrees of N-glycosylation. We have studied whether immature FGFR3 isoforms mediate the abnormal signaling in TDII. We show that synthesis of TDII-FGFR3 presents two phosphorylated forms: the immature non-glycosylated 98-kDa peptides and the intermediate 120-kDa glycomers. The mature, fully glycosylated 130-kDa forms, detected in wild type FGFR3, are not present in TDII. Endoglycosidase H cleaves the sugars on TDII intermediates thus indicating their intracellular localization in the endoplasmic reticulum. Accordingly, TDII-FGFR3-GFP co-localizes with calreticulin in the endoplasmic reticulum. Furthermore, following TDII transfection, signal transducer and activator of transcription 1 (STAT1) is phosphorylated in the absence of FGFR3 ligand and brefeldin A does not inhibit its activation. On the contrary, the cell membrane-anchored FRS2alpha protein is not activated in TDII cells. The opposite situation is observed in stable TDII cell clones where, despite the presence of phosphorylated mature receptor, STAT1 is not activated whereas FRS2alpha is phosphorylated. We speculate that the selection process favors cells defective in STAT1 activation through the 120-kDa TDII-FGFR3, thus allowing growth of the TDII cell clones. Accordingly, apoptosis is observed following TDII-FGFR3 transfection. These observations highlight the importance of the immature TDII-FGFR3 proteins as mediators of an abnormal signaling in TDII.

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    • "Arrow indicates TAK1. Multiple FGFR3 bands represent various glycosylation intermediates and appear as previously published [45], [85]. Four independent experiments were performed for each panel. "
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    • "were performed as previously described [5] "
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    ABSTRACT: The kinase activity of the thanatophoric dysplasia type II-fibroblast growth factor receptor 3 mutant (TDII-FGFR3) hampers its maturation. As a consequence, the immature receptor activates extracellular regulated kinases (ERKs) from the endoplasmic reticulum (ER), which leads to apoptosis. On the other hand, in stable TDII-FGFR3 cells receptor biosynthesis is restored and ERKs are activated from the cell surface. To identify potential mediators of cell adaptation to the activated receptor we investigated gene products that are differently regulated in TDII and wild-type FGFR3 cells. cDNA representational difference analysis reveals Sprouty4 up regulation in the TDII-FGFR3 cells. Interestingly, Sprouty4 inhibits the TDII-FGFR3-mediated ERKs activation from the ER, but fails to suppress ERKs activation from cell surface. We conclude that cell adaptation to activated FGFR3 include Sprouty4 activity, which silences the premature receptor signaling and suppress apoptosis.
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    • "In a kinase assay, FGFR3 equally phosphorylated both STAT1 and FRS2 (Fig. 1D). In contrast to normal cells, TD chondrocytes may have different substrate selectivities since FGFR3-K650E is active in the endoplasmic reticulum unlike wt FGFR3, which is active at the cell membrane (Lievens and Liboi, 2003; Raffioni et al., 1998). It is thus likely that cytoplasmic STAT1 is phosphorylated predominantly by FGFR3-K650E whereas membrane-anchored FRS2 is a preferred substrate for wt FGFR3. "
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