Sustained ERK1/2 but not STAT1 or 3 activation is required for thanatophoric dysplasia phenotypes in PC12 cells

Dissecting biological activities of fibroblast growth factor receptors by the coiled-coil-mediated oligomerization of FGF1

Fibroblast growth factor receptors (FGFRs) are integral membrane proteins involved in various biological processes including proliferation, migration and apoptosis. There are a number of regulatory mechanisms of FGFR signaling, which tightly control the specificity and duration of transmitted signals. The effect of the FGFRs spatial distribution in the plasma membrane on receptor-dependent functions is still largely unknown. We have demonstrated that oligomerization of FGF1 with coiled-coil motifs largely improves FGF1 affinity for FGFRs and heparin. Set of developed FGF1 oligomers evoked prolonged activation of FGFR1 and receptor-downstream signaling pathways, as compared to the wild type FGF1. The majority of obtained oligomeric FGF1 variants showed increased stability, enhanced mitogenic activity and largely improved internalization via FGFR1-dependent endocytosis. Importantly, FGF1 oligomers with the highest oligomeric state exhibited reduced ability to stimulate FGFR-dependent glucose uptake, while at the same time remained hyperactive in the induction of cell proliferation. Our data implicate that oligomerization of FGF1 alters the biological activity of the FGF/GFR1 signaling system. Furthermore, developed FGF1 oligomers, due to improved stability and proliferative potential, can be applied in the regenerative medicine or as drug delivery vehicles in the ADC approach against FGFR1-overproducing cancers.

Oxidized low-density lipoprotein (ox-LDL) promotes cardiac differentiation of bone marrow mesenchymal stem cells via activating ERK1/2 signaling

BACKGROUND/AIMS

The differentiation efficiency of bone marrow mesenchymal stem cells (BM-MSCs) is low in vivo after transplantation. Therefore, it is necessary to look for effective reagents for enhancing cardiac differentiation of BM-MSCs. It has been reported that cardiac differentiation of stem cells depends on the activation of extracellular signal-regulated protein 1/2 (ERK1/2) signaling. Oxidized low-density lipoprotein (ox-LDL) is a potent reagent for ERK1/2 activation. This indicates that ox-LDL may be a potential reagent to stimulate cardiac differentiation of stem cells. In this study, we investigated the effect of ox-LDL on cardiac differentiation of BM-MSCs and its relationship with ERK1/2 signaling.

METHODS

BM-MSCs were isolated from mouse bone marrow, cultured in DMEM supplemented with 15% FBS, and passaged up to the 3rd passage. Following culture with 5 μg/mL ox-LDL for 3 weeks, the cardiac differentiation of the 3rd passage BM-MSCs was identified by immunostaining, Western blotting, and RT-PCR assays for measuring the expression of cardiac-specific markers. To further explore the role of ERK1/2 signaling in cardiac differentiation of BM-MSCs, we simultaneously exposed BM-MSCs to ERK1/2 inhibitor (U0126) and ox-LDL, and identified the cardiac differentiation again.

RESULTS

The expressions of cardiac-specific markers including α-cardiac actin, α-MHC, β-MHC, ANP, and BNP were markedly increased in BM-MSCs following treatment with ox-LDL (P < .05), which indicates a directional differentiation of BM-MSCs to cardiac cells. Further, ox-LDL could also activate ERK1/2 in BM-MSCs, and application of U0126 markedly inhibited ox-LDL-induced cardiac transformation of BM-MSCs.

CONCLUSIONS

Ox-LDL induces cardiac differentiation of BM-MSCs via activation of ERK1/2 signaling.

Tyrosine kinase inhibitor NVP-BGJ398 functionally improves FGFR3-related dwarfism in mouse model

Achondroplasia (ACH) is the most frequent form of dwarfism and is caused by gain-of-function mutations in the fibroblast growth factor receptor 3-encoding (FGFR3-encoding) gene. Although potential therapeutic strategies for ACH, which aim to reduce excessive FGFR3 activation, have emerged over many years, the use of tyrosine kinase inhibitor (TKI) to counteract FGFR3 hyperactivity has yet to be evaluated. Here, we have reported that the pan-FGFR TKI, NVP-BGJ398, reduces FGFR3 phosphorylation and corrects the abnormal femoral growth plate and calvaria in organ cultures from embryos of the Fgfr3Y367C/+ mouse model of ACH. Moreover, we demonstrated that a low dose of NVP-BGJ398, injected subcutaneously, was able to penetrate into the growth plate of Fgfr3Y367C/+ mice and modify its organization. Improvements to the axial and appendicular skeletons were noticeable after 10 days of treatment and were more extensive after 15 days of treatment that started from postnatal day 1. Low-dose NVP-BGJ398 treatment reduced intervertebral disc defects of lumbar vertebrae, loss of synchondroses, and foramen-magnum shape anomalies. NVP-BGJ398 inhibited FGFR3 downstream signaling pathways, including MAPK, SOX9, STAT1, and PLCγ, in the growth plates of Fgfr3Y367C/+ mice and in cultured chondrocyte models of ACH. Together, our data demonstrate that NVP-BGJ398 corrects pathological hallmarks of ACH and support TKIs as a potential therapeutic approach for ACH.

Recent research on the growth plate: Advances in fibroblast growth factor signaling in growth plate development and disorders

Skeletons are formed through two distinct developmental actions, intramembranous ossification and endochondral ossification. During embryonic development, most bone is formed by endochondral ossification. The growth plate is the developmental center for endochondral ossification. Multiple signaling pathways participate in the regulation of endochondral ossification. Fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling has been found to play a vital role in the development and maintenance of growth plates. Missense mutations in FGFs and FGFRs can cause multiple genetic skeletal diseases with disordered endochondral ossification. Clarifying the molecular mechanisms of FGFs/FGFRs signaling in skeletal development and genetic skeletal diseases will have implications for the development of therapies for FGF-signaling-related skeletal dysplasias and growth plate injuries. In this review, we summarize the recent advances in elucidating the role of FGFs/FGFRs signaling in growth plate development, genetic skeletal disorders, and the promising therapies for those genetic skeletal diseases resulting from FGFs/FGFRs dysfunction. Finally, we also examine the potential important research in this field in the future.

Structural mimicry of a-loop tyrosine phosphorylation by a pathogenic FGF receptor 3 mutation

The K650E gain-of-function mutation in the tyrosine kinase domain of FGF receptor 3 (FGFR3) causes Thanatophoric Dysplasia type II, a neonatal lethal congenital dwarfism syndrome, and when acquired somatically, it contributes to carcinogenesis. In this report, we determine the crystal structure of the FGFR3 kinase domain harboring this pathogenic mutation and show that the mutation introduces a network of intramolecular hydrogen bonds to stabilize the active-state conformation. In the crystal, the mutant FGFR3 kinases are caught in the act of trans-phosphorylation on a kinase insert autophosphorylation site, emphasizing the fact that the K650E mutation circumvents the requirement for A-loop tyrosine phosphorylation in kinase activation. Analysis of this trans-phosphorylation complex sheds light onto the determinants of tyrosine trans-phosphorylation specificity. We propose that the targeted inhibition of this pathogenic FGFR3 kinase may be achievable by small molecule kinase inhibitors that selectively bind the active-state conformation of FGFR3 kinase.

Genes and environment: novel, functional polymorphism in the human cathepsin L (CTSL1) promoter disrupts a xenobiotic response element (XRE) to alter transcription and blood pressure

BACKGROUND

Cathepsin L (CTSL1) catalyzes the formation of peptides that influence blood pressure (BP). Naturally occurring genetic variation or targeted ablation of the Ctsl1 locus in mice yield cardiovascular pathology. Here, we searched for genetic variation across the human CTSL1 locus and probed its functional effects, especially in the proximal promoter.

METHODS AND RESULTS

Systematic polymorphism discovery by re-sequencing across CTSL1 in 81 patients uncovered 38 genetic variants, five of which were relatively common (MAF >5%), creating a single linkage disequilibrium block in multiple biogeographic ancestries. One of these five common variants lay in a functional domain of the gene: promoter C-171A (rs3118869), which disrupts a predicted xenobiotic response element (XRE; match C>A). In transfected CTSL1 promoter/luciferase reporter plasmids, C-171A allele influenced transcription (C>A, P = 3.36E-6), and transcription was also augmented by co-exposure to the aryl hydrocarbon receptor (AHR) complex (AHR:ARNT) in the presence of their ligand dioxin (P = 6.81E-8); allele (C vs. A) and AHR:ARNT/dioxin stimulus interacted to control gene expression (interaction P = 0.033). Endogenous Ctsl1, Ahr, and Arnt transcripts were present in chromaffin cells. Promoter functional C-171A genotype also predicted hypertension (P = 1.0E-3), SBP (P = 4.0E-4), and DBP (P = 3.0E-3), in an additive pattern for diploid genotypes (A/A > C/A > C/C) in 868 patients, and the results were extended by validation analysis into an independent population sample of 986 patients.

CONCLUSION

We conclude that common genetic variation in the proximal CTSL1 promoter, especially at position C-171A, is functional in cells, and alters transcription so as to explain the association of CTSL1 with BP in vivo. At the XRE, endogenous genetic variation plus exogenous aryl hydrocarbon stimulation interact to control CTSL1 gene expression. These results unveil a novel control point whereby heredity and environment can intersect to control a complex trait, and point to new transcriptional strategies for intervention into transmitter biosynthesis and its cardiovascular consequences.

Genetic variation within a metabolic motif in the chromogranin a promoter: pleiotropic influence on cardiometabolic risk traits in twins

BACKGROUND

The cardiometabolic syndrome comprised of multiple correlated traits, but its origin is incompletely understood. Chromogranin A (CHGA) is required for formation of the catecholamine secretory pathway in sympathochromaffin cells. In twin pair studies, we found that CHGA traits aggregated with body mass index (BMI), as well as its biochemical determinant leptin. METHODS Here we used the twin method to probe the role of heredity in generating such risk traits, and then investigated the role of risk-trait-associated CHGA promoter genetic variation in transfected chromaffin cells. Trait heritability (h(2)) and shared genetic determination among traits (pleiotropy, genetic covariance, ρ(G)) were estimated by variance components in twin pairs.

RESULTS

CHGA, BMI, and leptin each displayed substantial h(2), and the traits also aggregated with several features of the metabolic syndrome (e.g., insulin resistance, blood pressure (BP), hypertension, catecholamines, and C-reactive protein (CRP)). Twin studies demonstrated genetic covariance (pleiotropy, ρ(G)) for CHGA, BMI, and leptin with other metabolic traits (insulin resistance, BP, and CRP). We therefore investigated the CHGA locus for mechanisms of codetermination with such metabolic traits. A common functional variant in the human CHGA promoter (G-462A, rs9658634, minor allele frequency ~21%) was associated with leptin and CRP secretion, as well as BMI, especially in women; marker-on-trait effects on BMI were replicated across twin populations on two continents. In CHGA promoter/luciferase reporter plasmids transfected into chromaffin cells, G-462A alleles differed markedly in reporter expression. The G-462A variant disrupted predicted transcriptional control by a PPARγ/RXRα motif and costimulation by PPARγ/RXRα and their cognate ligands, differentially activated the two alleles. During chromatin immunoprecipitation, endogenous PPARγ bound the motif.

CONCLUSIONS

Multiple features of the metabolic syndrome are thus under joint (pleiotropic) genetic determination, with CHGA as one such contributory locus: a common polymorphism in the promoter (G-462A) of CHGA predicts such heritable metabolic traits as BMI and leptin. CHGA promoter variant G-462A was not only associated with such metabolic traits but also disrupted a PPARγ/RXRα motif and responded differentially to characteristic trans-activators of that motif. The results suggest novel links between the catecholaminergic system and risk for the metabolic syndrome as well as systemic hypertension.

Sixteen years and counting: the current understanding of fibroblast growth factor receptor 3 (FGFR3) signaling in skeletal dysplasias

In 1994, the field of bone biology was significantly advanced by the discovery that activating mutations in the fibroblast growth factor receptor 3 (FGFR3) receptor tyrosine kinase (TK) account for the common genetic form of dwarfism in humans, achondroplasia (ACH). Other conditions soon followed, with the list of human disorders caused by FGFR3 mutations now reaching at least 10. An array of vastly different diagnoses is caused by similar mutations in FGFR3, including syndromes affecting skeletal development (hypochondroplasia [HCH], ACH, thanatophoric dysplasia [TD]), skin (epidermal nevi, seborrhaeic keratosis, acanthosis nigricans), and cancer (multiple myeloma [MM], prostate and bladder carcinoma, seminoma). Despite many years of research, several aspects of FGFR3 function in disease remain obscure or controversial. As FGFR3-related skeletal dysplasias are caused by growth attenuation of the cartilage, chondrocytes appear to be unique in their response to FGFR3 activation. However, the reasons why FGFR3 inhibits chondrocyte growth while causing excessive cellular proliferation in cancer are not clear. Likewise, the full spectrum of molecular events by which FGFR3 mediates its signaling is just beginning to emerge. This article describes the challenging journey to unravel the mechanisms of FGFR3 function in skeletal dysplasias, the extraordinary cellular manifestations of FGFR3 signaling in chondrocytes, and finally, the progress toward therapy for ACH and cancer.

Oncogenic role of fibroblast growth factor receptor 3 in tumorigenesis of urinary bladder cancer

Bladder cancer is the second most common genitourinary tumor and constitutes a very heterogeneous disease. Molecular and pathologic studies suggest that low-grade noninvasive and high-grade invasive urothelial cell carcinoma (UCC) arise via distinct pathways. Low-grade noninvasive UCC represent the majority of tumors at presentation. A high proportion of patients with low-grade UCC develop recurrences but usually with no progression to invasive disease. At presentation, a majority of the bladder tumors (70%-80%) are low-grade noninvasive (pTa). Several genetic changes may occur in bladder cancer, but activating mutations in the fibroblast growth factor receptor 3 (FGFR3) genes are the most common and most specific genetic abnormality in bladder cancer. Interestingly, these mutations are associated with bladder tumors of low stage and grade, which makes the FGFR3 mutation the first marker that can be used for diagnosis of noninvasive bladder tumors. Since the first report of FGFR3 involvement in bladder tumors, numerous studies have been conducted to understand its function and thereby confirm the oncogenic role of this receptor particularly in noninvasive groups. Efforts are on to exploit this receptor as a therapeutic target, which holds much promise in the treatment of bladder cancer, particularly low-grade noninvasive tumors. Further studies need to explore the potential use of FGFR3 mutations in bladder cancer diagnosis, prognosis, and in surveillance of patients with bladder cancer. This review focuses on the role of FGFR3 in bladder tumors in the backdrop of various studies published.

FGFR3 signaling induces a reversible senescence phenotype in chondrocytes similar to oncogene-induced premature senescence

Oncogenic activation of the RAS-ERK MAP kinase signaling pathway can lead to uncontrolled proliferation but can also result in apoptosis or premature cellular senescence, both regarded as natural protective barriers to cell immortalization and transformation. In FGFR3-related skeletal dyplasias, oncogenic mutations in the FGFR3 receptor tyrosine kinase cause profound inhibition of cartilage growth resulting in severe dwarfism, although many of the precise mechanisms of FGFR3 action remain unclear. Mutated FGFR3 induces constitutive activation of the ERK pathway in chondrocytes and, remarkably, can also cause both increased proliferation and apoptosis in growing cartilage, depending on the gestational age. Here, we demonstrate that FGFR3 signaling is also capable of inducing premature senescence in chondrocytes, manifested as reversible, ERK-dependent growth arrest accompanied by alteration of cellular shape, loss of the extracellular matrix, upregulation of senescence markers (alpha-GLUCOSIDASE, FIBRONECTIN, CAVEOLIN 1, LAMIN A, SM22alpha and TIMP 1), and induction of senescence-associated beta-GALACTOSIDASE activity. Our data support a model whereby FGFR3 signaling inhibits cartilage growth via exploiting cellular responses originally designed to eliminate cells harboring activated oncogenes.

New insight on FGFR3-related chondrodysplasias molecular physiopathology revealed by human chondrocyte gene expression profiling

Endochondral ossification is the process by which the appendicular skeleton, facial bones, vertebrae and medial clavicles are formed and relies on the tight control of chondrocyte maturation. Fibroblast growth factor receptor (FGFR)3 plays a role in bone development and maintenance and belongs to a family of proteins which differ in their ligand affinities and tissue distribution. Activating mutations of the FGFR3 gene lead to craniosynostosis and multiple types of skeletal dysplasia with varying degrees of severity: thanatophoric dysplasia (TD), achondroplasia and hypochondroplasia. Despite progress in the characterization of FGFR3-mediated regulation of cartilage development, many aspects remain unclear. The aim and the novelty of our study was to examine whole gene expression differences occurring in primary human chondrocytes isolated from normal cartilage or pathological cartilage from TD-affected fetuses, using Affymetrix technology. The phenotype of the primary cells was confirmed by the high expression of chondrocytic markers. Altered expression of genes associated with many cellular processes was observed, including cell growth and proliferation, cell cycle, cell adhesion, cell motility, metabolic pathways, signal transduction, cell cycle process and cell signaling. Most of the cell cycle process genes were down-regulated and consisted of genes involved in cell cycle progression, DNA biosynthesis, spindle dynamics and cytokinesis. About eight percent of all modulated genes were found to impact extracellular matrix (ECM) structure and turnover, especially glycosaminoglycan (GAG) and proteoglycan biosynthesis and sulfation. Altogether, the gene expression analyses provide new insight into the consequences of FGFR3 mutations in cell cycle regulation, onset of pre-hypertrophic differentiation and concomitant metabolism changes. Moreover, impaired motility and ECM properties may also provide clues about growth plate disorganization. These results also suggest that many signaling pathways may be directly or indirectly altered by FGFR3 and confirm the crucial role of FGFR3 in the control of growth plate development.

Increased p21 expression in chondrocytes of achondroplasic children independently from the presence of the G380R FGFR3 mutation

BACKGROUND

Achondroplasia (ACH) represents the major cause of dwarfism and is due to mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. The cellular mechanisms involved in the reduced growth have been mainly described for in vitro or in vivo models, but few data have been obtained for humans.

METHODS

Thirteen children with ACH were enrolled in the study; the presence of FGFR3 mutations was determined by restriction fragment length polymorphism analysis and sequencing, whereas protein expression in cartilage biopsy was assessed by immunohistochemistry.

RESULTS

Chondrocytes in cartilage biopsies of ACH children were characterized by the presence of growth arrest mediated by STAT activation (both STAT1 and STAT5) and increased expression of p21 and cyclin D1, whereas no expression of either p53 or cyclin D3 could be detected. This mechanism was present in ACH children carrying the G380R mutation but also in a patient in whom no mutation could be detected in the entire coding region of the FGFR3 gene.

CONCLUSIONS

These data thus demonstrate the presence of a common final mechanism involving p21 and possibly leading to a block in chondrocyte proliferation.

Analysis of STAT1 activation by six FGFR3 mutants associated with skeletal dysplasia undermines dominant role of STAT1 in FGFR3 signaling in cartilage

Activating mutations in FGFR3 tyrosine kinase cause several forms of human skeletal dysplasia. Although the mechanisms of FGFR3 action in cartilage are not completely understood, it is believed that the STAT1 transcription factor plays a central role in pathogenic FGFR3 signaling. Here, we analyzed STAT1 activation by the N540K, G380R, R248C, Y373C, K650M and K650E-FGFR3 mutants associated with skeletal dysplasias. In a cell-free kinase assay, only K650M and K650E-FGFR3 caused activatory STAT1(Y701) phosphorylation. Similarly, in RCS chondrocytes, HeLa, and 293T cellular environments, only K650M and K650E-FGFR3 caused strong STAT1 activation. Other FGFR3 mutants caused weak (HeLa) or no activation (293T and RCS). This contrasted with ERK MAP kinase activation, which was strongly induced by all six mutants and correlated with the inhibition of proliferation in RCS chondrocytes. Thus the ability to activate STAT1 appears restricted to the K650M and K650E-FGFR3 mutants, which however account for only a small minority of the FGFR3-related skeletal dysplasia cases. Other pathways such as ERK should therefore be considered as central to pathological FGFR3 signaling in cartilage.

Fibroblast growth factor inhibits interferon gamma-STAT1 and interleukin 6-STAT3 signaling in chondrocytes

Activation of fibroblast growth factor receptor 3 (FGFR3) leads to attenuation of cartilage growth. The members of the STAT family of transcription factors are believed to participate in FGFR3 signaling in cartilage, however the molecular mechanism of this action is poorly understood. Here, we demonstrate that a chronic FGF stimulus leads to accumulation of STAT1, 3, 5 and 6, evident in both in vitro chondrocyte model and murine limb explant cultures. Despite the accumulation, both endogenous and cytokine-induced activation of STAT1 and STAT3 is impaired by FGF, as demonstrated by imaging of active STAT nuclear translocation and analyses of STAT activatory phosphorylation and transcriptional activation. Further, we demonstrate that FGF induces expression of CIS, SOCS1 and SOCS3 inhibitors of gp130, a common receptor for the IL6-family of cytokines. Since cytokine-gp130 signaling represents an important positive regulator of cartilage, its inhibition may contribute to the growth-inhibitory effect of FGFR3 in cartilage.

STAT1 and STAT3 do not participate in FGF-mediated growth arrest in chondrocytes

Activating mutations in fibroblast growth factor receptor 3 (FGFR3) cause several human skeletal dysplasias as a result of attenuation of cartilage growth. It is believed that FGFR3 inhibits chondrocyte proliferation via activation of signal transducers and activators of transcription (STAT) proteins, although the exact mechanism of both STAT activation and STAT-mediated inhibition of chondrocyte growth is unclear. We show that FGFR3 interacts with STAT1 in cells and is capable of activating phosphorylation of STAT1 in a kinase assay, thus potentially serving as a STAT1 kinase in chondrocytes. However, as demonstrated by western blotting with phosphorylation-specific antibodies, imaging of STAT nuclear translocation, STAT transcription factor assays and STAT luciferase reporter assays, FGF does not activate STAT1 or STAT3 in RCS chondrocytes, which nevertheless respond to a FGF stimulus with potent growth arrest. Moreover, addition of active STAT1 and STAT3 to the FGF signal, by means of cytokine treatment, SRC-mediated STAT activation or expression of constitutively active STAT mutants does not sensitize RCS chondrocytes to FGF-mediated growth arrest. Since FGF-mediated growth arrest is rescued by siRNA-mediated downregulation of the MAP kinase ERK1/2 but not STAT1 or STAT3, our data support a model whereby the ERK arm but not STAT arm of FGF signaling in chondrocytes accounts for the growth arrest phenotype.

Increased achondroplasia mutation frequency with advanced age and evidence for G1138A mosaicism in human testis biopsies

OBJECTIVE

To evaluate the influence of aging on the achondroplasia mutation rate in the male germline.

DESIGN

Studies in sperm and testis biopsy DNA according to donor's age.

SETTING

University teaching hospital.

PATIENT(S)

Seventeen donors aged 30 to 65 years for sperm collection and 14 deceased donors aged 53 to 95 years for testis biopsies, all with normal stature.

INTERVENTION(S)

Testes were obtained from 14 deceased donors, and sperm was obtained from 17 patients who requested ART.

MAIN OUTCOME MEASURE(S)

Real-time polymerase chain reaction quantification of the G1138A mutation in sperm and testis biopsies.

RESULT(S)

The rate of G1138A mutation did not significantly vary with age in sperm, whereas in testis biopsies it increased markedly past the age of 70 years. Moreover, and for the first time, a mosaic for this mutation was detected in the testis of three subjects who were >80 years of age.

CONCLUSION(S)

These findings could contribute to providing a molecular explanation for the increased incidence of achondroplastic offspring with advanced paternal age.

The localization of FGFR3 mutations causing thanatophoric dysplasia type I differentially affects phosphorylation, processing and ubiquitylation of the receptor

Recurrent missense fibroblast growth factor receptor 3 (FGFR3) mutations have been ascribed to skeletal dysplasias of variable severity including the lethal neonatal thanatophoric dysplasia types I (TDI) and II (TDII). To elucidate the role of activating mutations causing TDI on receptor trafficking and endocytosis, a series of four mutants located in different domains of the receptor were generated and transiently expressed. The putatively elongated X807R receptor was identified as three isoforms. The fully glycosylated mature isoform was constitutively but mildly phosphorylated. Similarly, mutations affecting the extracellular domain (R248C and Y373C) induced moderate constitutive receptor phosphorylation. By contrast, the K650M mutation affecting the tyrosine kinase 2 (TK2) domain produced heavy phosphorylation of the nonglycosylated and mannose-rich isoforms that impaired receptor trafficking through the Golgi network. This resulted in defective expression of the mature isoform at the cell surface. Normal processing was rescued by tyrosine kinase inhibitor treatment. Internalization of the R248C and Y373C mutant receptors, which form stable disulfide-bonded dimers at the cell surface was less efficient than the wild-type, whereas ubiquitylation was markedly increased but apparently independent of the E3 ubiquitin-ligase casitas B-lineage lymphoma (c-Cbl). Constitutive phosphorylation of c-Cbl by the K650M mutant appeared to be related to the intracellular retention of the receptor. Therefore, although mutation K650M affecting the TK2 domain induces defective targeting of the overphosphorylated receptor, a different mechanism characterized by receptor retention at the plasma membrane, excessive ubiquitylation and reduced degradation results from mutations that affect the extracellular domain and the stop codon.

Human immortalized chondrocytes carrying heterozygous FGFR3 mutations: an in vitro model to study chondrodysplasias

Achondroplasia and thanatophoric dysplasia are human chondrodysplasias caused by mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. We have developed an immortalized human chondrocyte culture model to study the regulation of chondrocyte functions. One control and eight mutant chondrocytic lines expressing different FGFR3 heterozygous mutations were obtained. FGFR3 signaling pathways were modified in the mutant lines as revealed by the constitutive activation of the STAT pathway and an increased level of P21(WAF1/CIP1) protein. This model will be useful for the study of FGFR3 function in cartilage studies and future therapeutic approaches in chondrodysplasias.

Autocrine extracellular signal-regulated kinase activation in normal human keratinocytes is not interrupted by calcium triggering and is involved in the control of cell cycle at the early stage of calcium-induced differentiation

Normal human epidermal keratinocytes (NHEK) respond to the autocrine activated extracellular signal-regulated kinase (ERK) signaling pathway, which contributes to the survival of keratinocytes. However, during the condition of calcium-induced differentiation, how the autocrine ERK signaling is regulated and affected is poorly understood. The purpose of this study was to understand and to obtain clues to the possible function of the autocrine ERK activation during the calcium-induced differentiation of NHEK. We demonstrated that the autocrine activated ERK was not interrupted by calcium triggering and that it was sustained for at least one day after changing the medium. We also found that the autocrine ERK activation was associated with the expression of cyclin D1 and the cell cycle regulation at the early stage of calcium triggering by treating the cells with the mitogen-activated protein kinase inhibitor PD98059. However, the PD98059 treatment did not have a significant influence on the expression of involucrin and loricrin. In addition, we demonstrated that autocrine ERK activation was associated with protein kinase C and p38MAPK signaling. We suggest that the activation of autocrine ERK is not interrupted by calcium triggering and it might participate in cell growth during the early stage of calcium-induced differentiation in NHEK.

Bisindolylmaleimide I suppresses fibroblast growth factor-mediated activation of Erk MAP kinase in chondrocytes by preventing Shp2 association with the Frs2 and Gab1 adaptor proteins

Fibroblast growth factors (FGFs) inhibit chondrocyte proliferation via the Erk MAP kinase pathway. Here, we explored the role of protein kinase C in FGF signaling in chondrocytes. Erk activity in FGF2-treated RCS (rat chondrosarcoma) chondrocytes or human primary chondrocytes was abolished by the protein kinase C inhibitor bisindolylmaleimide I (Bis I). Bis I inhibited FGF2-induced activation of MEK, Raf-1, and Ras members of Erk signaling module but not the FGF2-induced tyrosine phosphorylation of Frs2 or the kinase activity of FGFR3, demonstrating that it targets the Erk cascade immediately upstream of Ras. Indeed, Bis I abolished the FGF2-mediated association of Shp2 tyrosine phosphatase with Frs2 and Gab1 adaptor proteins necessary for proper Ras activation. We also determined which PKC isoform is involved in FGF2-mediated activation of Erk. When both conventional and novel PKCs expressed by RCS chondrocytes (PKCalpha, -gamma, -delta, and -epsilon) were down-regulated by phorbol ester, cells remained responsive to FGF2 with Erk activation, and this activation was sensitive to Bis I. Moreover, treatment with PKClambda/zeta pseudosubstrate lead to significant reduction of FGF2-mediated activation of Erk, suggesting involvement of an atypical PKC.

MIP-1alpha (CCL3) is a downstream target of FGFR3 and RAS-MAPK signaling in multiple myeloma

Overexpression of fibroblast growth factor receptor 3 (FGFR3) is a hallmark of t(4;14) multiple myeloma (MM). To dissect the mechanism of FGFR3 oncogenesis in MM, we used 3 FGFR selective kinase inhibitors-CHIR258, PD173074, and SU5402-and FGFR3-specific siRNA to modulate FGFR3 activity. Conversely, the ligand FGF was used to stimulate FGFR3 function in human MM cells. The transcriptional response to FGFR3 modification was recorded, and gene expression changes common to all 5 modifiers were documented. Ten genes were commonly regulated. Macrophage inflammatory protein-1 alpha (MIP-1alpha) was the single most differentially altered gene. MIP-1 alpha promoter function, gene expression, and protein secretion were each down-regulated following inhibition of FGFR3 signaling. Down-regulation of MIP-1 alpha was not, however, observed following FGFR3 inhibition in MM cells with RAS mutations implicating RAS-MAPK in MIP-1 alpha regulation. As confirmation, inhibition of ERK1 also down-regulated MIP-1 alpha in FGFR3 inhibitor-resistant cells harboring RAS mutations. MIP-1 alpha is implicated in the survival and proliferation of MM cells and the pathogenesis of MM bone disease. Our observation is the first to directly link an initiating IgH translocation not only to MM-cell growth and survival but also to the disease-associated bone disease.

K644E/M FGFR3 mutants activate Erk1/2 from the endoplasmic reticulum through FRS2 alpha and PLC gamma-independent pathways

Fibroblast growth factor receptors 3 (FGFR3) with K644M/E substitutions are associated to the severe skeletal dysplasias: severe achondroplasia with developmental delay and achanthosis nigricans(SADDAN) and thanatophoric dysplasia(TDII). The high levels of kinase activity of the FGFR3-mutants cause uncompleted biosynthesis that results in the accumulation of the immature/mannose-rich, phosphorylated receptors in the endoplasmic reticulum (ER) and STATs activation. Here we report that FGFR3 mutants activate Erk1/2 from the ER through an FRS2-independent pathway: instead, a multimeric complex by directly recruiting PLCgamma, Pyk2 and JAK1 is formed. The Erk1/2 activation from the ER however, is PLCgamma-independent, since preventing the PLCgamma/FGFR3 interaction by the Y754F substitution does not inhibit Erks. Furthermore, Erk1/2 activation is abrogated upon treatment with the Src inhibitor PP2, suggesting a role played by a Src family member in the pathway from the ER. Finally we show that the intrinsic kinase activity by mutant receptors is required to allow signaling from the ER. Overall these results highlight how activated FGFR3 exhibits signaling activity in the early phase of its biosynthesis and how segregation in a sub-cellular compartment can affect the FGFR3 multi-faceted capacity to recruit specific substrates.