The binding of polypeptide growth factors to their appropriate cell surface transmembrane receptors triggers numerous biochemical responses, including the transcriptional activation of specific genes. We have used a differential display approach to identify fibroblast growth factor-1-inducible genes in murine NIH 3T3 cells. Here, we report that the fibroblast growth factorinducible-14 (Fn14) gene is a growth factor-regulated, immediate-early response gene expressed in a developmental stage-and adult tissue-specific manner in vivo. This gene, located on mouse chromosome 17, is predicted to encode an 129-amino acid type Ia membrane protein with no significant sequence similarity to any known protein. We have used two experimental approaches, direct fluorescence microscopy and immunoprecipitation analysis of biotinylated cell surface proteins, to demonstrate that Fn14 is located on the plasma membrane. To examine the biological consequences of constitutive Fn14 expression, we isolated NIH 3T3 cell lines expressing variable levels of epitope-tagged Fn14 and analyzed their phenotypic properties in vitro. These experiments revealed that Fn14 expression decreased cellular adhesion to the extracellular matrix proteins fibronectin and vitronectin and also reduced serum-stimulated cell growth and migration. These results indicate that Fn14 is a novel plasma membrane-spanning molecule that may play a role in cell-matrix interactions.Complex cellular processes such as proliferation, migration, differentiation, and apoptosis are regulated in part by a diverse group of molecules known as polypeptide growth factors. These factors act by binding and thereby activating specific transmembrane receptor tyrosine kinases. The activation of cell surface receptors by polypeptide ligands triggers downstream intracellular events, including the stimulation of protein phosphorylation cascades and the transcriptional activation of numerous genes (1, 2). Many mitogen-inducible genes have been identified, and they encode a diverse group of proteins including transcription factors, protein kinases and phosphatases, cell cycle regulators, and cytoskeletal and extracellular matrix proteins (2, 3). A recent study using cDNA microarray technology has demonstrated that Ͼ500 genes are transcriptionally activated after serum stimulation of quiescent human fibroblasts and that a subset of these genes encode proteins implicated in the wound healing process in vivo (3).Our laboratory has been studying fibroblast growth factor-1 (FGF-1) 1 -regulated gene expression in murine NIH 3T3 cells. FGF-1 (also referred to as acidic FGF) is one of the most extensively characterized members of the FGF family of heparin-binding proteins (4 -6). It is a potent mitogenic, chemotactic, angiogenic, and neurotrophic factor both in vitro and in vivo. These cellular responses are mediated via high affinity binding to a family of related membrane-spanning tyrosine kinase receptors (4 -6). We have shown by Northern blot hybridization analysis that FGF-1 stimulation of quies...

Fibroblast growth factor (FGF)-1 mitogenic signal transduction is mediated in part by gene products that are specifically expressed in response to cell surface receptor binding and activation. We have used a targeted differential display method to identify FGF-1-inducible genes in murine NIH 3T3 fibroblasts. Here we report that one of these genes is predicted to encode a novel serine/threonine-specific protein kinase. This putative kinase has been named Fnk, for FGF-inducible kinase. The deduced Fnk amino acid sequence has 49, 36, 33, 32, and 22% overall identity to mouse serum-inducible kinase (Snk), mouse polo-like kinase (Plk), Drosophila polo, Saccharomyces Cdc5, and mouse Snk/Plk-akin kinase (Sak), respectively. These proteins are all members of the polo subfamily of structurally related serine/threonine kinases. The Plk, polo, Cdc5, and Sak kinases are required for cell division. FGF-1 induction of Fnk mRNA expression is first detected at 30 min after mitogen addition, reflects transcriptional activation, and does not require de novo protein synthesis. FGF-2, platelet-derived growth factor-BB, calf serum, or phorbol myristate acetate treatment of quiescent cells also induces fnk gene expression. Fnk mRNA is expressed in vivo in a tissue-specific manner, with relatively high levels detected in newborn and adult mouse skin. These results indicate that Fnk may be a transiently expressed protein kinase involved in the early signaling events required for growth factor-stimulated cell cycle progression.

Polypeptide growth factors stimulate mammalian cell proliferation by binding to specific cell surface receptors. This interaction triggers numerous biochemical responses including the activation of protein phosphorylation cascades and the enhanced expression of specific genes. We have identified several fibroblast growth factor (FGF)-inducible genes in murine NIH 3T3 cells and recently reported that one of them, the FGF-inducible 14 (Fn14) immediate-early response gene, is predicted to encode a novel, cell surfacelocalized type Ia transmembrane protein. Here, we report that the human Fn14 homolog is located on chromosome 16p13.3 and encodes a 129-amino acid protein with ϳ82% sequence identity to the murine protein. The human Fn14 gene, like the murine Fn14 gene, is expressed at elevated levels after FGF, calf serum or phorbol ester treatment of fibroblasts in vitro and is expressed at relatively high levels in heart and kidney in vivo. We also report that the human Fn14 gene is expressed at relatively low levels in normal liver tissue but at high levels in liver cancer cell lines and in hepatocellular carcinoma specimens. Furthermore, the murine Fn14 gene is rapidly induced during liver regeneration in vivo and is expressed at high levels in the hepatocellular carcinoma nodules that develop in the c-myc/transforming growth factor-␣-driven and the hepatitis B virus X protein-driven transgenic mouse models of hepatocarcinogenesis. These results indicate that Polypeptide mitogens such as fibroblast growth factor (FGF)-1 and platelet-derived growth factor-BB stimulate cell cycle progression by binding to specific receptor tyrosine kinases and thereby activating intracellular signal transduction pathways.1 The activation of cytoplasmic signaling molecules promotes changes in gene expression that are critical for the cellular growth response. Numerous growth factor-and/or serum-inducible genes have been identified and classified into one of three groups: immediate-early, delayed-early, or late response genes.2 Immediate-early response genes are rapidly and transiently expressed following mitogenic stimulation of quiescent cells and their transcriptional activation does not require de novo protein synthesis. Delayed-early response genes are first expressed a few hours later, in the early to middle portions of the G1 phase, and transcript levels often remain elevated for the remainder of the cell cycle. Late response genes are generally expressed only during the S phase of the cell cycle. Both delayed-early and late response genes require de novo protein synthesis for their transcriptional activation. Growth factor-inducible genes encode many types of proteins, including transcription factors, cell cycle regulators, extracellular matrix proteins and metabolic enzymes. [2][3][4]

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