Fibroblast growth factor receptor influences primary cilium length through an interaction with intestinal cell kinase.

Vertebrate primary cilium is a Hedgehog signaling center but the extent of its involvement in other signaling systems is less well understood. This report delineates a mechanism by which fibroblast growth factor (FGF) controls primary cilia. Employing proteomic approaches to characterize proteins associated with the FGF-receptor, FGFR3, we identified the serine/threonine kinase intestinal cell kinase (ICK) as an FGFR interactor. ICK is involved in ciliogenesis and participates in control of ciliary length. FGF signaling partially abolished ICK's kinase activity, through FGFR-mediated ICK phosphorylation at conserved residue Tyr15, which interfered with optimal ATP binding. Activation of the FGF signaling pathway affected both primary cilia length and function in a manner consistent with cilia effects caused by inhibition of ICK activity. Moreover, knockdown and knockout of ICK rescued the FGF-mediated effect on cilia. We provide conclusive evidence that FGF signaling controls cilia via interaction with ICK.

Multikinase activity of fibroblast growth factor receptor (FGFR) inhibitors SU5402, PD173074, AZD1480, AZD4547 and BGJ398 compromises the use of small chemicals targeting FGFR catalytic activity for therapy of short-stature syndromes.

Activating mutations in the fibroblast growth factor receptor 3 (FGFR3) cause the most common genetic form of human dwarfism, achondroplasia (ACH). Small chemical inhibitors of FGFR tyrosine kinase activity are considered to be viable option for treating ACH, but little experimental evidence supports this claim. We evaluated five FGFR tyrosine kinase inhibitors (TKIs) (SU5402, PD173074, AZD1480, AZD4547 and BGJ398) for their activity against FGFR signaling in chondrocytes. All five TKIs strongly inhibited FGFR activation in cultured chondrocytes and limb rudiment cultures, completely relieving FGFR-mediated inhibition of chondrocyte proliferation and maturation. In contrast, TKI treatment of newborn mice did not improve skeletal growth and had lethal toxic effects on the liver, lungs and kidneys. In cell-free kinase assays as well as in vitro and in vivo cell assays, none of the tested TKIs demonstrated selectivity for FGFR3 over three other FGFR tyrosine kinases. In addition, the TKIs exhibited significant off-target activity when screened against a panel of 14 unrelated tyrosine kinases. This was most extensive in SU5402 and AZD1480, which inhibited DDR2, IGF1R, FLT3, TRKA, FLT4, ABL and JAK3 with efficiencies similar to or greater than those for FGFR. Low target specificity and toxicity of FGFR TKIs thus compromise their use for treatment of ACH. Conceptually, different avenues of therapeutic FGFR3 targeting should be investigated.

Fibroblast growth factor and canonical WNT/ß-catenin signaling cooperate in suppression of chondrocyte differentiation in experimental models of FGFR signaling in cartilage.

Aberrant fibroblast growth factor (FGF) signaling disturbs chondrocyte differentiation in skeletal dysplasia, but the mechanisms underlying this process remain unclear. Recently, FGF was found to activate canonical WNT/ß-catenin pathway in chondrocytes via Erk MAP kinase-mediated phosphorylation of WNT co-receptor Lrp6. Here, we explore the cellular consequences of such a signaling interaction. WNT enhanced the FGF-mediated suppression of chondrocyte differentiation in mouse limb bud micromass and limb organ cultures, leading to inhibition of cartilage nodule formation in micromass cultures, and suppression of growth in cultured limbs. Simultaneous activation of the FGF and WNT/ß-catenin pathways resulted in loss of chondrocyte extracellular matrix, expression of genes typical for mineralized tissues and alteration of cellular shape. WNT enhanced the FGF-mediated downregulation of chondrocyte proteoglycan and collagen extracellular matrix via inhibition of matrix synthesis and induction of proteinases involved in matrix degradation. Expression of genes regulating RhoA GTPase pathway was induced by FGF in cooperation with WNT, and inhibition of the RhoA signaling rescued the FGF/WNT-mediated changes in chondrocyte cellular shape. Our results suggest that aberrant FGF signaling cooperates with WNT/ß-catenin in suppression of chondrocyte differentiation.

Instability restricts signaling of multiple fibroblast growth factors.

Fibroblast growth factors (FGFs) deliver extracellular signals that govern many developmental and regenerative processes, but the mechanisms regulating FGF signaling remain incompletely understood. Here, we explored the relationship between intrinsic stability of FGF proteins and their biological activity for all 18 members of the FGF family. We report that FGF1, FGF3, FGF4, FGF6, FGF8, FGF9, FGF10, FGF16, FGF17, FGF18, FGF20, and FGF22 exist as unstable proteins, which are rapidly degraded in cell cultivation media. Biological activity of FGF1, FGF3, FGF4, FGF6, FGF8, FGF10, FGF16, FGF17, and FGF20 is limited by their instability, manifesting as failure to activate FGF receptor signal transduction over long periods of time, and influence specific cell behavior in vitro and in vivo. Stabilization via exogenous heparin binding, introduction of stabilizing mutations or lowering the cell cultivation temperature rescues signaling of unstable FGFs. Thus, the intrinsic ligand instability is an important elementary level of regulation in the FGF signaling system.

Proteomic analyses of signalling complexes associated with receptor tyrosine kinase identify novel members of fibroblast growth factor receptor 3 interactome.

Receptor tyrosine kinases (RTKs) form multiprotein complexes that initiate and propagate intracellular signals and determine the RTK-specific signalling patterns. Unravelling the full complexity of protein interactions within the RTK-associated complexes is essential for understanding of RTK functions, yet it remains an understudied area of cell biology. We describe a comprehensive approach to characterize RTK interactome. A single tag immunoprecipitation and phosphotyrosine protein isolation followed by mass-spectrometry was used to identify proteins interacting with fibroblast growth factor receptor 3 (FGFR3). A total of 32 experiments were carried out in two different cell types and identified 66 proteins out of which only 20 (30.3%) proteins were already known FGFR interactors. Using co-immunoprecipitations, we validated FGFR3 interaction with adapter protein STAM1, transcriptional regulator SHOX2, translation elongation factor eEF1A1, serine/threonine kinases ICK, MAK and CCRK, and inositol phosphatase SHIP2. We show that unappreciated signalling mediators exist for well-studied RTKs, such as FGFR3, and may be identified via proteomic approaches described here. These approaches are easily adaptable to other RTKs, enabling identification of novel signalling mediators for majority of the known human RTKs.

The PTH/PTHrP-SIK3 pathway affects skeletogenesis through altered mTOR signaling.

Studies have suggested a role for the mammalian (or mechanistic) target of rapamycin (mTOR) in skeletal development and homeostasis, yet there is no evidence connecting mTOR with the key signaling pathways that regulate skeletogenesis. We identified a parathyroid hormone (PTH)/PTH-related peptide (PTHrP)-salt-inducible kinase 3 (SIK3)-mTOR signaling cascade essential for skeletogenesis. While investigating a new skeletal dysplasia caused by a homozygous mutation in the catalytic domain of SIK3, we observed decreased activity of mTOR complex 1 (mTORC1) and mTORC2 due to accumulation of DEPTOR, a negative regulator of both mTOR complexes. This SIK3 syndrome shared skeletal features with Jansen metaphyseal chondrodysplasia (JMC), a disorder caused by constitutive activation of the PTH/PTHrP receptor. JMC-derived chondrocytes showed reduced SIK3 activity, elevated DEPTOR, and decreased mTORC1 and mTORC2 activity, indicating a common mechanism of disease. The data demonstrate that SIK3 is an essential positive regulator of mTOR signaling that functions by triggering DEPTOR degradation in response to PTH/PTHrP signaling during skeletogenesis.

Nanodiamonds as "artificial proteins": Regulation of a cell signalling system using low nanomolar solutions of inorganic nanocrystals.

The blocking of specific protein-protein interactions using nanoparticles is an emerging alternative to small molecule-based therapeutic interventions. However, the nanoparticles designed as "artificial proteins" generally require modification of their surface with (bio)organic molecules and/or polymers to ensure their selectivity and specificity of action. Here, we show that nanosized diamond crystals (nanodiamonds, NDs) without any synthetically installed (bio)organic interface enable the specific and efficient targeting of the family of extracellular signalling molecules known as fibroblast growth factors (FGFs). We found that low nanomolar solutions of detonation NDs with positive ζ-potential strongly associate with multiple FGF ligands present at sub-nanomolar concentrations and effectively neutralize the effects of FGF signalling in cells without interfering with other growth factor systems and serum proteins unrelated to FGFs. We identified an evolutionarily conserved FGF recognition motif, ∼17 amino acids long, that contributes to the selectivity of the ND-FGF interaction. In addition, we inserted this motif into a de novo constructed chimeric protein, which significantly improved its interaction with NDs. We demonstrated that the interaction of NDs, as purely inorganic nanoparticles, with proteins can mitigate pathological FGF signalling and promote the restoration of cartilage growth in a mouse limb explant model. Based on our observations, we foresee that NDs may potentially be applied as nanotherapeutics to neutralize disease-related activities of FGFs in vivo.

The inositol phosphatase SHIP2 enables sustained ERK activation downstream of FGF receptors by recruiting Src kinases.

Sustained activation of extracellular signal-regulated kinase (ERK) drives pathologies caused by mutations in fibroblast growth factor receptors (FGFRs). We previously identified the inositol phosphatase SHIP2 (also known as INPPL1) as an FGFR-interacting protein and a target of the tyrosine kinase activities of FGFR1, FGFR3, and FGFR4. We report that loss of SHIP2 converted FGF-mediated sustained ERK activation into a transient signal and rescued cell phenotypes triggered by pathologic FGFR-ERK signaling. Mutant forms of SHIP2 lacking phosphoinositide phosphatase activity still associated with FGFRs and did not prevent FGF-induced sustained ERK activation, demonstrating that the adaptor rather than the catalytic activity of SHIP2 was required. SHIP2 recruited Src family kinases to the FGFRs, which promoted FGFR-mediated phosphorylation and assembly of protein complexes that relayed signaling to ERK. SHIP2 interacted with FGFRs, was phosphorylated by active FGFRs, and promoted FGFR-ERK signaling at the level of phosphorylation of the adaptor FRS2 and recruitment of the tyrosine phosphatase PTPN11. Thus, SHIP2 is an essential component of canonical FGF-FGFR signal transduction and a potential therapeutic target in FGFR-related disorders.

ARQ 087 inhibits FGFR signaling and rescues aberrant cell proliferation and differentiation in experimental models of craniosynostoses and chondrodysplasias caused by activating mutations in FGFR1, FGFR2 and FGFR3.

Tyrosine kinase inhibitors are being developed for therapy of malignancies caused by oncogenic FGFR signaling but little is known about their effect in congenital chondrodysplasias or craniosynostoses that associate with activating FGFR mutations. Here, we investigated the effects of novel FGFR inhibitor, ARQ 087, in experimental models of aberrant FGFR3 signaling in cartilage. In cultured chondrocytes, ARQ 087 efficiently rescued all major effects of pathological FGFR3 activation, i.e. inhibition of chondrocyte proliferation, loss of extracellular matrix and induction of premature senescence. In ex vivo tibia organ cultures, ARQ 087 restored normal growth plate architecture and eliminated the suppressing FGFR3 effect on chondrocyte hypertrophic differentiation, suggesting that it targets the FGFR3 pathway specifically, i.e. without interference with other pro-growth pathways. Moreover, ARQ 087 inhibited activity of FGFR1 and FGFR2 mutants associated with Pfeiffer, Apert and Beare-Stevenson craniosynostoses, and rescued FGFR-driven excessive osteogenic differentiation in mouse mesenchymal micromass cultures or in ex vivo calvarial organ cultures. Our data warrant further development of ARQ 087 for clinical use in skeletal disorders caused by activating FGFR mutations.

Inhibitor repurposing reveals ALK, LTK, FGFR, RET and TRK kinases as the targets of AZD1480.

Many tyrosine kinase inhibitors (TKIs) have failed to reach human use due to insufficient activity in clinical trials. However, the failed TKIs may still benefit patients if their other kinase targets are identified by providing treatment focused on syndromes driven by these kinases. Here, we searched for novel targets of AZD1480, an inhibitor of JAK2 kinase that recently failed phase two cancer clinical trials due to a lack of activity. Twenty seven human receptor tyrosine kinases (RTKs) and 153 of their disease-associated mutants were in-cell profiled for activity in the presence of AZD1480 using a newly developed RTK plasmid library. We demonstrate that AZD1480 inhibits ALK, LTK, FGFR1-3, RET and TRKA-C kinases and uncover a physical basis of this specificity. The RTK activity profiling described here facilitates inhibitor repurposing by enabling rapid and efficient identification of novel TKI targets in cells.

One reporter for in-cell activity profiling of majority of protein kinase oncogenes.

In-cell profiling enables the evaluation of receptor tyrosine activity in a complex environment of regulatory networks that affect signal initiation, propagation and feedback. We used FGF-receptor signaling to identify EGR1 as a locus that strongly responds to the activation of a majority of the recognized protein kinase oncogenes, including 30 receptor tyrosine kinases and 154 of their disease-associated mutants. The EGR1 promoter was engineered to enhance trans-activation capacity and optimized for simple screening assays with luciferase or fluorescent reporters. The efficacy of the developed, fully synthetic reporters was demonstrated by the identification of novel targets for two clinically used tyrosine kinase inhibitors, nilotinib and osimertinib. A universal reporter system for in-cell protein kinase profiling will facilitate repurposing of existing anti-cancer drugs and identification of novel inhibitors in high-throughput screening studies.

ABC transporters affect the detection of intracellular oxidants by fluorescent probes.

Intracellular production of reactive oxygen species (ROS) plays an important role in the control of cell physiology. For the assessment of intracellular ROS production, a plethora of fluorescent probes is commonly used. Interestingly, chemical structures of these probes imply they could be substrates of plasma membrane efflux pumps, called ABC transporters. This study tested whether the determination of intracellular ROS production and mitochondrial membrane potential by selected fluorescent probes is modulated by the expression and activity of ABC transporters. The sub-clones of the HL-60 cell line over-expressing MDR1, MRP1 and BCRP transporters were employed. ROS production measured by luminol- and L-012-enhaced chemiluminescence and cytochrome c reduction assay showed similar levels of ROS production in all the employed cell lines. It was proved that dihydrorhodamine 123, dihexiloxocarbocyanine iodide, hydroethidine, tetrachloro-tetraethylbenzimidazolocarbo-cyanine iodide and tetramethylrhodamine ethyl ester perchlorate are substrates for MDR1; dichlorodihydrofluoresceine, hydroethidine and tetramethylrhodamine ethyl ester perchlorate are substrates for MRP1; dichlorodihydrofluoresceine, dihydrorhodamine 123, hydroethidine and tetrachloro-tetraethylbenzimidazolocarbo-cyanine iodide are substrates for BCRP. Thus, the determination of intracellular ROS and mitochondrial potential by the selected probes is significantly altered by ABC transporter activities. The activity of these transporters must be considered when employing fluorescent probes for the assessment of ROS production or mitochondrial membrane potential.