Hiroyuki Ikeda

Bio: Hiroyuki Ikeda is an academic researcher from University of Tokyo. The author has contributed to research in topics: Medicine & Receptor tyrosine kinase. The author has an hindex of 13, co-authored 28 publications receiving 1946 citations. Previous affiliations of Hiroyuki Ikeda include Chiba University & Japan International Cooperation Agency.

Identification of mutations in the coding sequence of the proto-oncogene c-kit in a human mast cell leukemia cell line causing ligand-independent activation of c-kit product.

Abstract: The c-kit proto-oncogene encodes a receptor tyrosine kinase. Binding of c-kit ligand, stem cell factor (SCF) to c-kit receptor (c-kitR) is known to activate c-kitR tyrosine kinase, thereby leading to autophosphorylation of c-kitR on tyrosine and to association of c-kitR with substrates such as phosphatidylinositol 3-kinase (PI3K). In a human mast cell leukemia cell line HMC-1, c-kitR was found to be constitutively phosphorylated on tyrosine, activated, and associated with PI3K without the addition of SCF. The expression of SCF mRNA transcript in HMC-1 cells was not detectable by means of PCR after reverse transcription (RT-PCR) analysis, suggesting that the constitutive activation of c-kitR was ligand independent. Sequencing of whole coding region of c-kit cDNA revealed that c-kit genes of HMC-1 cells were composed of a normal, wild-type allele and a mutant allele with two point mutations resulting in intracellular amino acid substitutions of Gly-560 for Val and Val-816 for Asp. Amino acid sequences in the regions of the two mutations are completely conserved in all of mouse, rat, and human c-kit. In order to determine the causal role of these mutations in the constitutive activation, murine c-kit mutants encoding Gly-559 and/or Val-814, corresponding to human Gly-560 and/or Val-816, were constructed by site-directed mutagenesis and expressed in a human embryonic kidney cell line, 293T cells. In the transfected cells, both c-kitR (Gly-559, Val-814) and c-kitR (Val-814) were abundantly phosphorylated on tyrosine and activated in immune complex kinase reaction in the absence of SCF, whereas tyrosine phosphorylation and activation of c-kitR (Gly-559) or wild-type c-kitR was modest or little, respectively. These results suggest that conversion of Asp-816 to Val in human c-kitR may be an activating mutation and responsible for the constitutive activation of c-kitR in HMC-1 cells.

IGFBP-4 is an inhibitor of canonical Wnt signalling required for cardiogenesis

Abstract: Insulin-like growth-factor-binding proteins (IGFBPs) modulate the actions of insulin-like growth factors (IGFs) but they also have functions independent of IGFs This study reports a new function for IGFBP-4 as a cardiogenic growth factor, showing that IGFBP-4 enhances cardiomyocyte differentiation in vitro, and knockdown of IGFBP-4 attenuated cardiomyogenesis both in vitro and in vivo The effect was independent of IGF binding, and was mediated by canonical Wnt signalling IGFBP-4 physically interacted with a Wnt receptor Frizzled 8 (Frz8) and a Wnt co-receptor low-density lipoprotein receptor-related protein 6 (LRP6), and inhibited Wnt3A binding to Frz8 and LRP6 This paper provides the first molecular link between IGF signalling and Wnt signalling The research could thus pave the way towards understanding how anomalies in this process give rise to congenital heart defects and could potentially yield new strategies to repair tissue damaged by heart attacks This study reports a function for IGFBP-4 as a cardiogenic growth factor, showing that IGFBP-4 enhances cardiomyocyte differentiation in vitro, and knockdown of IGFBP-4 attenuated cardiomyogenesis both in vitro and in vivo The effect was independent of IGF binding, and was mediated by canonical Wnt signalling IGFBP-4 physically interacted with a Wnt receptor Frizzled 8 (Frz8) and a Wnt co-receptor low-density lipoprotein receptor-related protein 6 (LRP6), and inhibited Wnt3A binding to Frz8 and LRP6 Insulin-like growth-factor-binding proteins (IGFBPs) bind to and modulate the actions of insulin-like growth factors (IGFs)1 Although some of the actions of IGFBPs have been reported to be independent of IGFs, the precise mechanisms of IGF-independent actions of IGFBPs are largely unknown1,2 Here we report a previously unknown function for IGFBP-4 as a cardiogenic growth factor IGFBP-4 enhanced cardiomyocyte differentiation in vitro, and knockdown of Igfbp4 attenuated cardiomyogenesis both in vitro and in vivo The cardiogenic effect of IGFBP-4 was independent of its IGF-binding activity but was mediated by the inhibitory effect on canonical Wnt signalling IGFBP-4 physically interacted with a Wnt receptor, Frizzled 8 (Frz8), and a Wnt co-receptor, low-density lipoprotein receptor-related protein 6 (LRP6), and inhibited the binding of Wnt3A to Frz8 and LRP6 Although IGF-independent, the cardiogenic effect of IGFBP-4 was attenuated by IGFs through IGFBP-4 sequestration IGFBP-4 is therefore an inhibitor of the canonical Wnt signalling required for cardiogenesis and provides a molecular link between IGF signalling and Wnt signalling

Excessive cardiac insulin signaling exacerbates systolic dysfunction induced by pressure overload in rodents

Abstract: Although many animal studies indicate insulin has cardioprotective effects, clinical studies suggest a link between insulin resistance (hyperinsulinemia) and heart failure (HF). Here we have demonstrated that excessive cardiac insulin signaling exacerbates systolic dysfunction induced by pressure overload in rodents. Chronic pressure overload induced hepatic insulin resistance and plasma insulin level elevation. In contrast, cardiac insulin signaling was upregulated by chronic pressure overload because of mechanical stretch-induced activation of cardiomyocyte insulin receptors and upregulation of insulin receptor and Irs1 expression. Chronic pressure overload increased the mismatch between cardiomyocyte size and vascularity, thereby inducing myocardial hypoxia and cardiomyocyte death. Inhibition of hyperinsulinemia substantially improved pressure overload-induced cardiac dysfunction, improving myocardial hypoxia and decreasing cardiomyocyte death. Likewise, the cardiomyocyte-specific reduction of insulin receptor expression prevented cardiac ischemia and hypertrophy and attenuated systolic dysfunction due to pressure overload. Conversely, treatment of type 1 diabetic mice with insulin improved hyperglycemia during pressure overload, but increased myocardial ischemia and cardiomyocyte death, thereby inducing HF. Promoting angiogenesis restored the cardiac dysfunction induced by insulin treatment. We therefore suggest that the use of insulin to control hyperglycemia could be harmful in the setting of pressure overload and that modulation of insulin signaling is crucial for the treatment of HF.

Gain-of-function mutations of c-kit in human gastrointestinal stromal tumors.

Abstract: Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors in the human digestive tract, but their molecular etiology and cellular origin are unknown. Sequencing of c-kit complementary DNA, which encodes a proto-oncogenic receptor tyrosine kinase (KIT), from five GISTs revealed mutations in the region between the transmembrane and tyrosine kinase domains. All of the corresponding mutant KIT proteins were constitutively activated without the KIT ligand, stem cell factor (SCF). Stable transfection of the mutant c-kit complementary DNAs induced malignant transformation of Ba/F3 murine lymphoid cells, suggesting that the mutations contribute to tumor development. GISTs may originate from the interstitial cells of Cajal (ICCs) because the development of ICCs is dependent on the SCF-KIT interaction and because, like GISTs, these cells express both KIT and CD34.