Differential effects of carboxy-terminal sequence deletions on platelet-derived growth factor receptor signaling activities and interactions with cellular substrates

Protein Engineering of the Colony-stimulating Factor-1 Receptor Kinase Domain for Structural Studies

A parallel approach to designing crystallization constructs for the c-FMS kinase domain was implemented, resulting in proteins suitable for structural studies. Sequence alignment and limited proteolysis were used to identify and eliminate unstructured and surface-exposed domains. A small library of chimeras was prepared in which the kinase insert domain of FMS was replaced with the kinase insert domain of previously crystallized receptor-tyrosine kinases. Characterization of the newly generated FMS constructs by enzymology and thermoshift assays demonstrated similar activities and compound binding to the FMS full-length cytoplasmic domain. Two chimeras were evaluated for crystallization in the presence and absence of a variety of ligands resulting in crystal structures, and leading to a successful structure-based drug design project for this important inflammation target.

The association of NHERF adaptor proteins with g protein-coupled receptors and receptor tyrosine kinases

The sodium-hydrogen exchanger regulatory factors (NHERF-1 and NHERF-2) are a family of adaptor proteins characterized by the presence of two tandem PDZ protein interaction domains and a C-terminal domain that binds the cytoskeleton proteins ezrin, radixin, moesin, and merlin. The NHERF proteins are highly expressed in the kidney, small intestine, and other organs, where they associate with a number of transporters and ion channels, signaling proteins, and transcription factors. Recent evidence has revealed important associations between the NHERF proteins and several G protein-coupled receptors such as the beta2-adrenergic receptor, the kappa-opioid receptor, and the parathyroid hormone receptor, as well as growth factor tyrosine kinase receptors such as the platelet-derived growth factor receptor and the epidermal growth factor receptor. This review summarizes the emerging data on the biochemical mechanisms, physiologic outcomes, and potential clinical implications of the assembly and disassembly of receptor/NHERF complexes.

Substitution of C-terminus of VEGFR-2 with VEGFR-1 promotes VEGFR-1 activation and endothelial cell proliferation

VEGFR-1 is devoid of ligand-dependent tyrosine autophosphorylation and its activation is not associated with proliferation of endothelial cells. The molecular mechanism responsible for this characteristic of VEGFR-1 is not known. In this study, we show that VEGFR-1 is devoid of ligand-dependent downregulation and failed to stimulate intracellular calcium release, cell migration and angiogenesis in vitro. To understand the molecular mechanisms responsible for the poor tyrosine autophosphorylation of VEGFR-1, we have either deleted the carboxyl terminus of VEGFR-1 or exchanged it with the carboxyl terminus of VEGFR-2. The deletion of carboxyl terminus of VEGFR-1 did not reverse its defective ligand-dependent autophosphorylation. The carboxyl terminus-swapped VEGFR-1, however, displayed ligand-dependent autophosphorylation, downregulation and also conveyed strong mitogenic responses. Thus, the carboxyl tail of VEGFR-1 restrains the ligand-dependent kinase activation and downregulation of VEGFR-1 and its ability to convey the angiogenic responses in endothelial cells.

A ligand-inducible epidermal growth factor receptor/anaplastic lymphoma kinase chimera promotes mitogenesis and transforming properties in 3T3 cells

Oncogenic rearrangements of the anaplastic lymphoma kinase (ALK) gene, encoding a receptor type tyrosine kinase, are frequently associated with anaplastic large cell lymphomas. Such rearrangements juxtapose the intracellular domain of ALK to 5'-end sequences belonging to different genes and create transforming fusion proteins. To understand how the oncogenic versions of ALK contribute to lymphomagenesis, it is important to analyze the biological effects and the biochemical properties of this receptor under controlled conditions of activation. To this aim, we constructed chimeric receptor molecules in which the extracellular domain of the ALK kinase is replaced by the extracellular, ligand-binding domain of the epidermal growth factor receptor (EGFR). Upon transfection in NIH 3T3 fibroblasts, the EGFR/ALK chimera was correctly synthesized and transported to the cell surface, where it was fully functional in forming high versus low affinity EGF-binding sites and transducing an EGF-dependent signal intracellularly. Overexpression of the EGFR/ALK chimera in NIH 3T3 was sufficient to induce the malignant phenotype; the appearance of the transformed phenotype was, however, conditionally dependent on the administration of EGF. Moreover, the EGFR/ALK chimera was significantly more active in inducing transformation and DNA synthesis than the wild type EGFR when either was expressed at similar levels in NIH 3T3 cells. Comparative analysis of the biochemical pathways implicated in the transduction of mitogenic signals did not show any increased ability of the EGFR/ALK to phosphorylate PLC-gamma and MAPK compared with the EGFR. On the contrary, EGFR/ALK showed to have a consistently greater effect on phosphatidylinositol 3-kinase activity compared with the EGFR, indicating that this enzyme plays a major role in mediating the mitogenic effects of ALK in NIH 3T3 cells.

Ret oncogene signal transduction via a IRS-2/PI 3-kinase/PKB and a SHC/Grb-2 dependent pathway: possible implication for transforming activity in NIH3T3 cells

Multiple endocrine neoplasia 2A (MEN 2A) is an inherited disease caused by mutations of the Ret proto-oncogene. Although many different Ret mutations have been described, little is known about the signaling pathways triggered by the Ret oncogene. In this study, we have determined the signaling properties of a Ret-9bp duplication encoding amino acids 634-636, which was recently identified in a patient with all clinical features of the MEN 2A syndrome. The Ret-9bp duplication leads to constitutive activation of the Ret tyrosine kinase. Furthermore, Ret-9bp increased mitogenic and transforming activity demonstrated by thymidine incorporation as well as colony formation in soft agar. Studying intracellular signaling pathways, which may be involved in malignant transformation of Ret-9bp expressing NIH3T3 cells, we could demonstrate Ret-9bp dependent phosphorylation of insulin receptor substrate-2 (IRS-2) with consecutive activation of phosphatidylinositol 3-kinase (PI 3-kinase) and protein kinase B (PKB/AKT). Moreover, Ret-9bp induces phosphorylation of SHC resulting in growth factor receptor binding protein-2 (Grb-2) binding and activation of the mitogen activating protein (MAP) kinase pathway. In addition to these postreceptor cytoplasmic signaling events, we have studied nuclear signal by Ret-9bp and found activation of c-jun and jun-D, two members of the jun/AP-1 family of transcription factors. In summary, an oncogenic 9bp duplication of Ret causes Ret dimer formation and ligand independent activation of the tyrosine kinase. Besides the signaling steps leading to MAPK activation, we could demonstrate that Ret-9bp induced constitutive activation of a signaling pathway involving IRS-2, PI 3-kinase and PKB/AKT which could transduce the oncogenic Ret signal to increased gene transcription via activation of the jun/AP-1 transcription factor family.

Phosphoinositide-specific phospholipase C and mitogenic signaling

The importance of PLC activation in cell proliferation is evident from the fact that the hydrolysis of PtdIns(4,5)P2 is one of the early events that follow the interaction of many growth factors and mitogens with their respective receptors. However, the importance of PLC activation is not restricted to proliferation; it is one of the most common transmembrane signaling events elicited by receptors that regulate many other cellular processes, including differentiation, metabolism, secretion, contraction, and sensory perception. It is also clear that cell proliferation signaling does not always require PLC, as indicated by the fact that growth factors such as insulin and CSF-1 do not appear to elicit the hydrolysis of PtdIns(4,5)P2, even though the intracellular domains of their receptors carry a PTK domain and the receptors show topologies very similar to those of the PLC-activating growth factors PDGF, EGF, and FGF. The growth factor-dependent activation of PLC is initiated by the formation of a complex between the receptor PTK and PLC-gamma; the formation of this complex is mediated by a specific interaction between a tyrosine phosphate residue on the intracellular domain of PTK and the SH2 domain of PLC-gamma. The receptor PTK subsequently phosphorylates PLC-gamma, of which two distinct isozymes, PLC-gamma 1 and PLC-gamma 2, have been identified. Proliferation of T cells and B cells in response to the aggregation of their respective cell surface receptors is also accompanied by the activation of PLC-gamma isozymes at an early stage. Unlike growth factor receptors, the T cell and B cell receptors lack intrinsic PTK activity but associate with several non-receptor PTKs of the Src and Syk families. Although the specific kinases are not known, one or more of these enzymes phosphorylate and activate PLC-gamma 1 and PLC-gamma 2. Transduction of growth signals by G protein-coupled receptors such as those for thrombin or bombesin also requires PtdIns(4,5)P2 hydrolysis, which, in this instance, is mediated by PLC-beta isozymes. The PLC-beta subfamily consists of four distinct members: PLC-beta 1, PLC-beta 2, PLC-beta 3, and PLC-beta 4. Agonist interaction with specific G protein-coupled receptors causes the dissociation of Gq proteins into G alpha and G beta gamma subunits and the exchange of GDP bound to G alpha for GTP. The resulting GTP-bound G alpha subunit then activates PLC-beta isoforms by binding to the carboxyl-terminal region of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced activation of RAF-1 and MAP kinase by a fibroblast growth factor receptor mutant deficient in stimulation of phosphatidylinositol hydrolysis

Signaling via the fibroblast growth factor receptor 1 (FGFR1, flg) was analyzed in Ba/F3 hematopoietic cells expressing either wild-type or a mutant FGF receptor (Y766F) unable to activate phospholipase C-gamma (PLC-gamma) and stimulate phosphatidylinositol (PI) hydrolysis. Stimulation of cells expressing wild-type or mutant FGFR with acidic FGF (aFGF) caused similar activation of Ras. However, an approximately 3-fold reduced activation of Raf-1 and MAP kinase was observed in aFGF-stimulated cells expressing mutant receptors as compared to cells expressing wild-type FGF receptors. Comparison of phosphopeptide maps of Raf-1 immunoprecipitated from the two cell types activated by either aFGF or the phorbol ester (12-O-tetradecanoylphorbol-13-acetate) suggests that Raf-1 is phosphorylated by both Ras-dependent and PLC-gamma-dependent mechanisms. In spite of the differential effect on Raf-1 and MAP kinase activation, aFGF stimulated similar proliferation of cells expressing wild-type or mutant receptors indicating that Ras-dependent activation of Raf-1 and MAP kinase is sufficient for transduction of FGFR mitogenic signals. Ras may also activate signal transduction pathways that are complementary or parallel to the MAP kinase pathway to stimulate cell proliferation.

An SH3 domain is required for the mitogenic activity of microinjected phospholipase C-gamma 1

Phospholipase activity is elevated in dividing cells. In response to growth factor stimulation, phospholipase C-gamma (PLC-gamma) binds to activated tyrosine kinase receptors via SH2 binding domains, resulting in phosphorylation of PLC-gamma and activation of its enzyme activity. These observations suggest that PLC-gamma participates in the signal transduction pathway employed by growth factors to promote mitogenesis. Consistent with this hypothesis, microinjection of purified bovine PLC-gamma into quiescent fibroblasts has been previously reported to initiate a mitogenic response [Smith et al. (1989) Proc. Natl. Acad. Sci. 86, 3659]. We have reproduced this result using recombinant rat PLC-gamma protein. Surprisingly, however, a catalytically inactive mutant of PLC-gamma, H335Q, also elicited a full mitogenic response. The capacity to induce mitogenesis by microinjection of PLC-gamma was mapped to the 'Z' domain of the protein, which contains PLC-gamma's SH2 and SH3 motifs. Inactivation of the phosphorylated tyrosine binding properties of both SH2 domains had no effect on the mitogenic activity of the Z-domain peptide. However, deletion of the SH3 domain resulted in a complete loss of activity. These results suggest that PLC-gamma's mitogenic properties do not require the enzyme's phospholipase activity, but are instead mediated by a novel pathway for mitogenic stimulation which is dependent upon an intact SH3 domain.

Distinctive effects of the carboxyl-terminal sequence of the insulin-like growth factor I receptor on its signaling functions

We have shown previously that the extracellular sequences of the human insulin receptor (IR) and the insulin-like growth factor I receptor (IGFR) have an inhibitory effect on protein tyrosine kinase (PTK) activity and on the biological functions of their respective Gag-receptor fusion proteins. To study the role of IGFR carboxyl sequence in modulation of the Gag-IGFR PTK and biological activities, five mutants, CM1, CM2, CM3, CM4, and CM5, containing carboxyl deletions of 17, 27, 47, 67, and 88 amino acids (aa), respectively, were constructed from the parental virus UIGFR encoding the Gag-IGFR. Deletion of up to 27 aa had little effect on the cell-transforming and PTK activities of UIGFR. Deletions of 47 aa in CM3 abolished PTK and transforming activities. Surprisingly, a further deletion of 20 aa in CM4 beyond that in CM3 reactivated the kinase and transforming activities. CM5, containing a deletion of 20 aa beyond that in CM4, had only marginal transforming and PTK activities. We conclude that deletion of the carboxyl region of the Gag-IGFR inactivates, instead of activating as in the case with Gag-IR, its transforming activity and the amino acid sequence 1250 to 1310 is essential for PTK and transforming activities. Analysis of the ability of the full-length IGFR and its mutant receptors described above to associate with phosphatidylinositol 3 kinase indicated that the association required PTK activity and tyrosine phosphorylation of the receptors and correlated well with their transforming activities. The carboxyl 88 aa are not essential for the association.

Intracellular signalling mediated by protein-tyrosine kinases: networking through phospholipid metabolism

In recent years, it has become apparent that receptor-mediated intracellular signals are not linear cascades beginning at the plasma membrane and terminating with the production of a needed metabolite or the induction of gene expression. Instead, complex networks of interactive intracellular signals are activated in response to extracellular stimuli. Many responses to extracellular stimuli are mediated by protein-tyrosine kinases (PTKs). Activating PTKs leads to the recruitment of a variety of intracellular signalling molecules that execute a complex set of instructions. The response to PTK activity is dependent upon which PTK is activated and the cellular context in which the PTK exists. Several signalling molecules recruited by PTKs are involved in the metabolism of phospholipids. In this Mini Review, intracellular signalling networks activated by PTKs are discussed with an emphasis on the potential for generating highly specific and sophisticated responses to PTK activity through phospholipid metabolism.