Alternatively spliced juxtamembrane domain of a tyrosine kinase receptor is a multifunctional regulatory site. Deletion alters cellular tyrosine phosphorylation pattern and facilitates binding of phosphatidylinositol-3-OH kinase to the hepatocyte growth factor receptor

RNA splicing alterations in lung cancer pathogenesis and therapy

RNA splicing alterations are widespread and play critical roles in cancer pathogenesis and therapy. Lung cancer is highly heterogeneous and causes the most cancer-related deaths worldwide. Large-scale multi-omics studies have not only characterized the mutational landscapes but also discovered a plethora of transcriptional and post-transcriptional changes in lung cancer. Such resources have greatly facilitated the development of new diagnostic markers and therapeutic options over the past two decades. Intriguingly, altered RNA splicing has emerged as an important molecular feature and therapeutic target of lung cancer. In this review, we provide a brief overview of splicing dysregulation in lung cancer and summarize the recent progress on key splicing events and splicing factors that contribute to lung cancer pathogenesis. Moreover, we describe the general strategies targeting splicing alterations in lung cancer and highlight the potential of combining splicing modulation with currently approved therapies to combat this deadly disease. This review provides new mechanistic and therapeutic insights into splicing dysregulation in cancer.

MET∆14 promotes a ligand-dependent, AKT-driven invasive growth

MET is an oncogene encoding the tyrosine kinase receptor for hepatocyte growth factor (HGF). Upon ligand binding, MET activates multiple signal transducers, including PI3K/AKT, STAT3, and MAPK. When mutated or amplified, MET becomes a "driver" for the onset and progression of cancer. The most frequent mutations in the MET gene affect the splicing sites of exon 14, leading to the deletion of the receptor's juxtamembrane domain (MET∆14). It is currently believed that, as in gene amplification, MET∆14 kinase is constitutively active. Our analysis of MET in carcinoma cell lines showed that MET∆14 strictly depends on HGF for kinase activation. Compared with wt MET, ∆14 is sensitive to lower HGF concentrations, with more sustained kinase response. Using three different models, we have demonstrated that MET∆14 activation leads to robust phosphorylation of AKT, leading to a distinctive transcriptomic signature. Functional studies revealed that ∆14 activation is predominantly responsible for enhanced protection from apoptosis and cellular migration. Thus, the unique HGF-dependent ∆14 oncogenic activity suggests consideration of HGF in the tumour microenvironment to select patients for clinical trials.

cMET Exon 14 Skipping: From the Structure to the Clinic

The abnormal stimulation of the multiple signal transduction pathways downstream of the receptor tyrosine kinase mesenchymal-epithelial transition factor (cMET) promotes cellular transformation, tumor motility, and invasion. Therefore, cMET has been the focus of prognostic and therapeutic studies in different tumor types, including non-small cell lung cancer. In particular, several cMET inhibitors have been developed as innovative therapeutic candidates and are currently under investigation in clinical trials. However, one of the challenges in establishing effective targeted treatments against cMET remains the accurate identification of biomarkers for the selection of responsive subsets of patients. Recently, splice site mutations have been discovered in cMET that lead to the skipping of exon 14, impairing the breakdown of the receptor. Patients with NSCLC who are carrying this splice variant typically overexpress the cMET receptor and show a response to small molecule inhibitors of cMET. Here, we review the main differences at the structural level between the wild-type and the splice variants of cMET and their influence on cMET signaling. We clarify the reason why this variant responds to small molecule inhibitors and their prognostic/predictive role.

Mutations Preventing Regulated Exon Skipping in MET Cause Osteofibrous Dysplasia

The periosteum contributes to bone repair and maintenance of cortical bone mass. In contrast to the understanding of bone development within the epiphyseal growth plate, factors that regulate periosteal osteogenesis have not been studied as intensively. Osteofibrous dysplasia (OFD) is a congenital disorder of osteogenesis and is typically sporadic and characterized by radiolucent lesions affecting the cortical bone immediately under the periosteum of the tibia and fibula. We identified germline mutations in MET, encoding a receptor tyrosine kinase, that segregate with an autosomal-dominant form of OFD in three families and a mutation in a fourth affected subject from a simplex family and with bilateral disease. Mutations identified in all families with dominant inheritance and in the one simplex subject with bilateral disease abolished the splice inclusion of exon 14 in MET transcripts, which resulted in a MET receptor (MET(Δ14)) lacking a cytoplasmic juxtamembrane domain. Splice exclusion of this domain occurs during normal embryonic development, and forced induction of this exon-exclusion event retarded osteoblastic differentiation in vitro and inhibited bone-matrix mineralization. In an additional subject with unilateral OFD, we identified a somatic MET mutation, also affecting exon 14, that substituted a tyrosine residue critical for MET receptor turnover and, as in the case of the MET(Δ14) mutations, had a stabilizing effect on the mature protein. Taken together, these data show that aberrant MET regulation via the juxtamembrane domain subverts core MET receptor functions that regulate osteogenesis within cortical diaphyseal bone.

Single-nucleotide polymorphism rs41736 located in MET was significantly associated with prognosis of small cell lung cancer patients

MET has been suggested to have an intimate relationship with small cell lung cancer (SCLC) and might be a promising therapeutic target. To date, relatively limited reports have been explored on MET mutational status in SCLC patients. To investigate the relationship between MET mutations and SCLC, 68 Chinese patients surgically treated for SCLC were enrolled. MET mutational analyses were performed in tumors, adjacent normal tissues as well as in lymph nodes with no metastasis nonadherent to tumor tissues using Sanger sequencing after PCR. The same mutation types were found in tumors, adjacent normal tissues as well as lymph nodes, including the only missense mutation N375S encoding semaphorin domain in exon 2 in 4 patients (5.9%), one single-nucleotide polymorphism (SNP) rs35775721: C>T also encoding semaphorin domain as heterozygous in 10 cases (14.7%) and another SNP rs41736: C>T encoding tyrosine kinase domain in exon 20 existing in 49 cases (72.1%). In survival analysis, the wild genotype CC-carriers of rs41736 conferred a significantly shorter progression-free survival (PFS) and overall survival (OS) compared to CT + TT-carriers (HR 0.455, 95% CI 0.229-0.904; HR 0.226, 95% CI 0.099-0.515, for PFS and OS, respectively) in limited-stage SCLC patients. We also found that the lymph node status was significantly associated with OS, and the shorter OS was present in positive group (HR 2.187, 95% CI 1.170-4.088). In this study, rs41736 polymorphism of MET was first found to be associated with prognosis of limited-stage SCLC patients and could be considered as a prognostic marker for limited-stage SCLC.

Voltage-dependent open-channel block of G protein-gated inward-rectifying K(+) (GIRK) current in rat atrial myocytes by tamoxifen

Tamoxifen (Tmx) is a nonsteroidal selective estrogen receptor antagonist and is frequently used in the treatment and prevention of breast cancer. The compound and its metabolites have been reported to inhibit functions of different classes of membrane proteins, including various ion channels. For members of the inward-rectifying K(+) (Kir) channel family, interference of Tmx with binding of phosphatidylinositol 4,5-bisphosphate (PIP(2)) has been suggested as the mechanism underlying such inhibition. We have studied the inhibition of G protein-activated K(+) (GIRK) current by Tmx in isolated myocytes from hearts of adult rats using whole-cell voltage clamp and experimental conditions for measuring K(+) currents as inward currents (E (K) -50 mV; holding potential -90 mV). Extracellular Tmx reversibly inhibited GIRK current activated by acetylcholine (I (K(ACh))) with an EC(50) of 7.4 × 10(-7) M. This inhibition was composed of two components, a basal reduction in peak current and a block that required opening of channels by ACh. The open-channel block was partially relieved by depolarizing voltage steps in a voltage- and time-dependent fashion. A voltage-dependent open-channel block was not observed when I (K(ACh)) was measured as outward current (E (K) -90 mV; holding potential -40 mV). Intracellular application of Tmx via the patch clamp pipette at a concentration (7 × 10(-6) M) that caused a rapid inhibition of I (K(ACh)) upon extracellular application did not affect the current. Intracellular application of the H(2)O-soluble PIP(2) analog diC(8)-PIP(2) reduced the voltage-independent component of inhibition but had no effect on voltage-dependent open-channel block. The effects of 4-hydroxy-Tmx, a major active metabolite, tested at 2 × 10(-6) M, had effects on I (K(ACh)) analogous to those of Tmx. Inhibition of constitutive inward-rectifying K(+) current (I (K1)) in ventricular myocytes, carried by Kir2 complexes, by Tmx was devoid of a voltage-dependent component. This study suggests the voltage-dependent open-channel block of GIRK inward current as a novel mechanism of Tmx action.

Low-molecular-weight heparins: differential characterization/physical characterization

Low-molecular-weight heparins (LMWHs), derived from unfractionated heparin (UFH) through different depolymerization processes, have advantages with respect to the parent heparin in terms of pharmacokinetics, convenience of administration, and reduced side effects. Each LMWH can be considered as an independent drug with its own activity profile, placing significance on their biophysical characterization, which will also enable a better understanding of their structure-function relationship. Several chemical and physical methods, some involving sample modification, are now available and are reviewed.

Characterization of an activating transcription factor 4 gene containing a consensus phosphorylation site for PKA in the gonads of Xenopus embryos

Activating transcription factor / cyclic-AMP response element-binding protein (ATF/CREB) has been implicated as a key regulator in the transcriptional control of many genes. In this study, we isolated and characterized a full-length cDNA that encodes a CRE-binding protein 2 (CREB2) called ATF4 in Xenopus embryos. Like other CREB 2 transcription factors, the 342-amino acid ATF4 protein contains a carboxyl terminal leucine-zipper motif, an adjacent basic domain, and an amino terminal leucine-zipper motif. Unlike other CREB2 (ATF4) proteins, the ATF4 isolated from the gonads of Xenopus embryos contains a consensus phosphorylation site for protein kinase A (PKA). In a gel shift analysis, ATF4 bound to a CLS sequence in the promoter of Xenopus aromatase.

Nucleotide excision repair and photolyase repair of UV photoproducts in nucleosomes: assessing the existence of nucleosome and non-nucleosome rDNA chromatin in vivo

The genome is organized into nuclear domains, which create microenvironments that favor distinct chromatin structures and functions (e.g., highly repetitive sequences, centromeres, telomeres, noncoding sequences, inactive genes, RNA polymerase II and III transcribed genes, and the nucleolus). Correlations have been drawn between gene silencing and proximity to a heterochromatic compartment. At the other end of the scale are ribosomal genes, which are transcribed at a very high rate by RNA polymerase I (~60% of total transcription), have a loose chromatin structure, and are clustered in the nucleolus. The rDNA sequences have 2 distinct structures: active rRNA genes, which have no nucleosomes; and inactive rRNA genes, which have nucleosomes. Like DNA transcription and replication, DNA repair is modulated by the structure of chromatin, and the kinetics of DNA repair vary among the nuclear domains. Although research on DNA repair in all chromosomal contexts is important to understand the mechanisms of genome maintenance, this review focuses on nucleotide excision repair and photolyase repair of UV photoproducts in the first-order packing of DNA in chromatin: the nucleosome. In addition, it summarizes the studies that have demonstrated the existence of the 2 rDNA chromatins, and the way this feature of the rDNA locus allows for direct comparison of DNA repair in 2 very different structures: nucleosome and non-nucleosome DNA.

Molecular characterization of the injury-induced aromatase transcript in the adult zebra finch brain

In the zebra finch (Taeniopygia guttata), the aromatase gene is transcribed from one of two promoters resulting in two transcripts constitutively expressed in brain or ovary. These transcripts differ only in Exon 1 which lies in the 5' un-translated region (UTR). An inducible form of aromatase is expressed following brain injury in glia. Towards characterizing this transcript, we (a) examined the up-regulation of amplicons within the aromatase transcript using quantitative PCR (qPCR), (b) performed 5' and 3' rapid amplification of cDNA ends (RACE) on injured brain RNA and (c) sequenced the injury-induced aromatase transcript. qPCR suggested that inducible aromatase may contain a novel 3'UTR. However, neither 3' nor 5' RACE revealed novel UTRs in the injured telencephalon. We then sequenced aromatase from injured entopallium, a region that lacks detectable constitutive aromatase. Inducible aromatase was identical in sequence to the known neural aromatase transcript. These data suggest that injury-induced aromatase differs from ovarian, but is indistinguishable from neuronal aromatase. We suggest that an injury-specific signal in glia may modulate aromatase transcription. Alternatively, injury-induced aromatase transcription may be silenced under constitutive conditions. To the best of our knowledge, this is the first report that documents the sequence of inducible aromatase in any vertebrate.

MET meet adaptors: functional and structural implications in downstream signalling mediated by the Met receptor

The tyrosin kinase Met receptor regulates multiple cellular events, ranging from cell motility and angiogenesis to morphological differentiation and tissue regeneration. To conduce these activities, the cytoplasmic C-terminal region of this receptor acts as a docking site for multiple protein substrates, including Grb 2, Gab 1, STAT 3, Shc, SHIP-1 and Src. These substrates are characterised by the presence of multiple domains, including the PH, PTB, SH 2 and SH 3 domains, which directly interact with the multisubstrate C-terminal region of Met. How this receptor recognises and binds a specific substrate in a space-temporal mode is a central question in cell signalling. The recently solved crystal structure of the tyrosine kinase domain of the Met receptor and that of domains of diverse Met substrates provides the molecular framework to understand Met substrate specificity. This structural information also gives new insights on the plasticity of Met signalling and the implications of Met deregulation in tumorigenic processes. In the light of these advances, the present work discusses the molecular basis of Met-substrate recognition and its functional implications in signalling events mediated by this pleiotropic receptor.

Altered exon usage in the juxtamembrane domain of mouse and human RON regulates receptor activity and signaling specificity

Alternative splicing of signaling proteins can contribute to the complexity of signaling networks. We find that expression of mouse RON, but not human RON, results in constitutive receptor autophosphorylation, ligand-independent activation of the mitogen-activated protein kinase pathway, and association of the receptor with c-Src. Using chimeric receptors, we mapped the region for this difference in signaling capacity of mouse and human RON to the juxtamembrane domain. Expression of these receptors in primary erythroid progenitor cells also demonstrated a functional difference in the ability of mouse and human RON to support erythropoietin-independent colony formation that mapped to the juxtamembrane domain. Splicing of the mouse RON receptor tyrosine kinase transcript results in the constitutive deletion of an exon used by all other known RON orthologs that encodes part of the juxtamembrane domain of the receptor. Mutational analysis indicated that the two tyrosines present in this region in human RON, one of which has been previously shown to be a c-Cbl binding site, are not responsible for this difference. However, deletion of this region in the context of human RON enhanced receptor phosphorylation, activation of mitogen-activated protein kinase, and association of c-Src at levels comparable with those observed with mouse RON. These data provide direct evidence that the divergence of exon usage among different species can generate a protein with novel activity and subsequently add to the complexity of cellular signaling regulation.

Proapoptotic function of the MET tyrosine kinase receptor through caspase cleavage

The MET tyrosine kinase, the receptor of hepatocyte growth factor-scatter factor (HGF/SF), is known to be essential for normal development and cell survival. We report that stress stimuli induce the caspase-mediated cleavage of MET in physiological cellular targets, such as epithelial cells, embryonic hepatocytes, and cortical neurons. Cleavage occurs at aspartic residue 1000 within the SVD site of the juxtamembrane region, independently of the crucial docking tyrosine residues Y1001 or Y1347 and Y1354. This cleavage generates an intracellular 40-kDa MET fragment containing the kinase domain. The p40 MET fragment itself causes apoptosis of MDCK epithelial cells and embryonic cortical neurons, whereas its kinase-dead version is impaired in proapoptotic activity. Finally, HGF/SF treatment does not favor MET cleavage and apoptosis, confirming the known survival role of ligand-activated MET. Our results show that stress stimuli convert the MET survival receptor into a proapoptotic factor.

Aromatase--a brief overview

There is growing awareness that androgens and estrogens have general metabolic roles that are not directly involved in reproductive processes. These include actions on vascular function, lipid and carbohydrate metabolism, as well as bone mineralization and epiphyseal closure in both sexes. In postmenopausal women, as in men, estrogen is no longer solely an endocrine factor but instead is produced in a number of extragonadal sites and acts locally at these sites in a paracrine and intracrine fashion. These sites include breast, bone, vasculature, and brain. Within these sites, aromatase action can generate high levels of estradiol locally without significantly affecting circulating levels. Circulating C19 steroid precursors are essential substrates for extragonadal estrogen synthesis. The levels of these androgenic precursors decline markedly with advancing age in women, possible from the mid-to-late reproductive years. This may be a fundamental reason why women are at increased risk for bone mineral loss and fracture, and possibly decline of cognitive function, compared with men. Aromatase expression in these various sites is under the control of tissue-specific promotors regulated by different cohorts of transcription factors. Thus in principle, it should be possible to develop selective aromatase modulators (SAMs) that block aromatase expression, for example, in breast, but allow unimpaired estrogen synthesis in other tissues such as bone.

Epidermal growth factor-induced phosphatidylinositol 3-kinase activation and DNA synthesis. Identification of Grb2-associated binder 2 as the major mediator in rat hepatocytes

In previous work we showed that the phosphatidylinositol 3-kinase (PI3-kinase), not the mitogen-activated protein kinase, pathway is necessary and sufficient to account for insulin- and epidermal growth factor (EGF)-induced DNA synthesis in rat hepatocytes. Here, using a dominant-negative p85, we confirmed the key role of EGF-induced PI3-kinase activation and sought to identify the mechanism by which this is effected. Our results show that EGF activates PI3-kinase with a time course similar to that of the association of p85 with three principal phosphotyrosine proteins (i. e. PY180, PY105, and PY52). We demonstrated that each formed a distinct p85-associated complex. PY180 and PY52 each constituted about 10% of EGF-activated PI3-kinase, whereas PY105 was responsible for 80%. PY105 associated with Grb2 and SHP-2, and although it behaved like Gab1, none of the latter was detected in rat liver. We therefore cloned a cDNA from rat liver, which was found to be 95% homologous to the mouse Grb2-associated binder 2 (Gab2) cDNA sequence. Using a specific Gab2 antibody, we demonstrated its expression in and association with p85, SHP-2, and Grb2 upon EGF treatment of rat hepatocytes. Gab2 accounted for most if not all of the PY105 species, since immunoprecipitation of Gab2 with specific antibodies demonstrated parallel immunodepletion of Gab2 and PY105 from the residual supernatants. We also found that the PI3-kinase activity associated with Gab2 was totally abolished by dominant negative p85. Thus, Gab2 appears to be the principal EGF-induced PY protein recruiting and activating PI3-kinase and mitogenesis.

Hepatocyte growth factor-induced differential activation of phospholipase cgamma 1 and phosphatidylinositol 3-kinase is regulated by tyrosine phosphatase SHP-1 in astrocytes

Hepatocyte growth factor (HGF) elicits pleiotropic effects on various types of cells through the c-Met receptor tyrosine kinase. However, the mechanisms underlying the diverse responses of cells remain unknown. We show here that HGF promoted chemokinesis of rat primary astrocytes through the activation of phosphatidylinositol 3 (PI3)-kinase without any influence on mitogenesis of the cells. Under the same condition, phospholipase Cgamma1 (PLCgamma1), which is another signal mediator of c-Met, was not tyrosine-phosphorylated during HGF stimulation. However, treatment of the cells with orthovanadate, a tyrosine phosphatase inhibitor, restored the HGF-induced tyrosine phosphorylation of PLCgamma1. A tyrosine phosphatase, SHP-1, was associated with both PI3-kinase and PLCgamma1 before HGF stimulation, but it was dissociated only from PI3-kinase after the stimulation. Furthermore, transfectants of catalytically inactive mutant of SHP-1 showed tyrosine phosphorylation of PLCgamma1 and mitogenic responses to HGF, and the mitogenic response was blocked with, an inhibitor of phosphatidylinositol-specific PLC, and calphostin C, an inhibitor of protein kinase C downstream of the PLCgamma1. These results indicate that PLCgamma1 is selectively prevented from being a signal mediator by constitutive association of SHP-1, and that this selective inhibition of PLCgamma1 may determine the cellular response of astrocytes to HGF.

Heparin reduces the alpha-helical content of cobra basic phospholipase A(2) and promotes its complex formation

The interaction of phospholipase A(2) (PLA(2)) with glycosaminoglycans (GAGs) has recently attracted attention in view of its implication on inflammation and cell proliferation. By using Fourier Transformed Infrared (FTIR) spectroscopic measurements, we demonstrate here that binding of cobra basic phospholipase A(2) from Naja nigricollis (N-PLA(2)) to heparin may induce a significant conformational change observed in the amide I region of the enzyme's alpha-helical and beta-sheet structure. It is observed that notable conformational change of N-PLA(2) due to heparin binding occurs only when heparin's chain length is at least an octasaccharide as evidenced by circular dichroism and optical density measurements. This correlation may be an important factor in the aggregation of N-PLA(2) and N-PLA(2)-heparin complexes. Heparin induced change in conformation of PLA(2) is suggested to be a notable link in understanding the diversity in PLA(2) activity when rendered to the extracellular matrix of cell membranes that is full of GAG molecules.

Hepatocyte growth factor in renal failure: promise and reality

Can science discover some secrets of Greek mythology? In the case of Prometheus, we can now suppose that his amazing hepatic regeneration was caused by a peptide growth factor called hepatocyte growth factor (HGF). Increasing evidence indicates that HGF acts as a multifunctional cytokine on different cell types. This review addresses the molecular mechanisms that are responsible for the pleiotropic effects of HGF. HGF binds with high affinity to its specific tyrosine kinase receptor c-met, thereby stimulating not only cell proliferation and differentiation, but also cell migration and tumorigenesis. The three fundamental principles of medicine-prevention, diagnosis, and therapy-may be benefited by the rational use of HGF. In renal tubular cells, HGF induces mitogenic and morphogenetic responses. In animal models of toxic or ischemic acute renal failure, HGF acts in a renotropic and nephroprotective manner. HGF expression is rapidly up-regulated in the remnant kidney of nephrectomized rats, inducing compensatory growth. In a mouse model of chronic renal disease, HGF inhibits the progression of tubulointerstitial fibrosis and kidney dysfunction. Increased HGF mRNA transcripts were detected in mesenchymal and tubular epithelial cells of rejecting kidney. In transplanted patients, elevated HGF levels may indicate renal rejection. When HGF is considered as a therapeutic agent in human medicine, for example, to stimulate kidney regeneration after acute injury, strategies need to be developed to stimulate cell regeneration and differentiation without an induction of tumorigenesis.

Loss of the exon encoding the juxtamembrane domain is essential for the oncogenic activation of TPR-MET

TPR-MET, a transforming counterpart of the c-MET proto-oncogene detected in experimental and human cancer, results from fusion of the MET kinase domain with a dimerization motif encoded by TPR. In this rearrangement the exons encoding the Met extracellular, transmembrane and juxtamembrane domains are lost. The juxtamembrane domain has been suggested to be a regulatory region endowed with negative feedback control. To understand whether its absence is critical for the generation of the Tpr-Met transforming potential, we produced a chimeric molecule (Tpr-juxtaMet) with a conserved juxtamembrane domain. The presence of the domain (aa 962-1009) strongly inhibited Tpr-Met dependent cell transformation. Cell proliferation, anchorage-independent growth, motility and invasion were also impaired. The enzymatic behavior of Tpr-Met and Tpr-juxtaMet was the same, while Tpr-juxtaMet ability to associate cytoplasmic signal transducers and to elicit downstream signaling was severely impaired. These data indicate that the presence of the juxtamembrane domain counterbalances the Tpr-Met transforming potential and therefore the loss of the exon encoding the juxtamembrane domain is crucial in the generation of the active TPR-MET oncogene.

Substrate specificity studies of Flavobacterium chondroitinase C and heparitinases towards the glycosaminoglycan--protein linkage region. Use of a sensitive analytical method developed by chromophore-labeling of linkage glycoserines using dimethylaminoazobenzenesulfonyl chloride

Bacterial chondroitinases and heparitinases are potentially useful tools for structural studies of chondroitin sulfate and heparin/heparan sulfate. Substrate specificities of Flavobacterium chondroitinase C, as well as heparitinases I and II, towards the glycosaminoglycan-protein linkage region -HexA-HexNAc-GlcA-Gal-Gal-Xyl-Ser (where HexA represents glucuronic acid or iduronic acid and HexNAc represents N-acetylgalactosamine or N-acetylglucosamine) were investigated using various structurally defined oligosaccharides or oligosaccharide-serines derived from the linkage region. In the case of oligosaccharide-serines, they were labeled with a chromophore dimethylaminoazobenzenesulfonyl chloride (DABS-Cl), which stably reacted with the amino group of the serine residue and rendered high absorbance for microanalysis. Chondroitinase C cleaved the GalNAc bond of the pentasaccharides or hexasaccharides derived from the linkage region of chondroitin sulfate chains and tolerated sulfation of the C-4 or C-6 of the GalNAc residue and C-6 of the Gal residues, as well as 2-O-phosphorylation of the Xyl residue. In contrast, it did not act on the GalNAc-GlcA linkage when attached to a 4-O-sulfated Gal residue. Heparitinase I cleaved the innermost glucosaminidic bond of the linkage region oligosaccharide-serines of heparin/heparan sulfate irrespective of substitution by uronic acid, whereas heparitinase II acted only on the glucosaminidic linkages of the repeating disaccharide region, but not on the innermost glucosaminidic linkage. These defined specificities of chondroitinase C, as well as heparitinases I and II, will be useful for preparation and structural analysis of the linkage oligosaccharides.

Modulation of the protein tyrosine kinase activity and autophosphorylation of the epidermal growth factor receptor by its juxtamembrane region

Using peptides epidermal growth factor receptor (EGFR)-13 and EGFR-14, which correspond to residues 645-657 and 679-692, respectively, in the juxtamembrane, cytosolic region of the epidermal growth factor receptor (EGFR) we have investigated the role of specific regions of the receptor in regulating its autophosphorylation and protein tyrosine kinase activity. EGFR-13, but not EGFR-14, increased autophosphorylation (by twofold) of the full-length and two truncated forms (Delta1022-1186 and a constitutively active receptor kinase domain) of the EGFR. EGFR-13 increased the stoichiometry of tyrosine phosphorylation of the full-length receptor from 4.2 to 10.1 mol Pi/mol EGFR and that of EGFRDelta1022-1186 from 1.0 to 2 mol Pi/mol receptor. Increased receptor autophosphorylation in the presence of EGFR-13 cannot solely be attributed to an increase in tyrosine kinase activity because EGFR-14 and polylysine increased tyrosine kinase activity of EGFRDelta1022-1186 and full-length EGFR, respectively, to the same extent as EGFR-13 without any effects on receptor autophosphorylation. Phosphorylation of EGFR-13 (P-EGFR-13) on the threonine residue corresponding to Thr654 in EGFR obliterated the ability of the peptide to increase autophosphorylation and markedly diminished its capacity to increase receptor tyrosine kinase activity. Additionally, EGFR-13, but not EGFR-14 or P-EGFR-13, decreased the migration of the receptor on nondenaturing gels, indicating that EGFR-13 induces some conformational change. Phosphopeptide maps of the EGFR phosphorylated in the presence of EGFR-13 or pp60(c-src) demonstrated that the additional sites phosphorylated in the presence of EGFR-13 were the same as those phosphorylated by pp60(c-src) (i.e., Y803, Y845, Y891, Y920, and Y1101). Thus, we conclude that EGFR-13, but not EGFR-14 or P-EGFR-13, competes to disrupt interactions between amino acids 645-657 and some other region(s) on the EGFR to either alleviate a conformational constraint or alter dimer conformation. This change increases the protein tyrosine kinase activity of the EGFR and provides access to additional tyrosine autophosphorylation sites in the receptor.

Ku autoimmune antigen is involved in placental regulation of rat P450c17 gene transcription

The steroidogenic enzyme P450c17 (17alpha hydroxylase/C17,20 lyase) regulates a key branchpoint in steroidogenesis, as its activity directs the steroid biosynthetic pathways toward glucocorticoid or sex hormone synthesis. Expression of the P450c17 gene is transcriptionally regulated in steroidogenic tissues by cAMP. We showed that DNA between -84 and -55 in the rat P450c17 gene was bound uniquely by steroidogenic factor-1 (SF-1), which regulated both basal and cAMP-stimulated transcription in mouse adrenocortical and Leydig cells. SF-1 gene ablation experiments in mice indicate that SF-1 is not mandatory for placental steroidogenesis. We studied P450c17 gene regulation in the placenta using human placental JEG-3 trophoblast cells. Transfection of reporter luciferase gene constructs containing serial deletions of the 5' flanking region of the rat P450c17 gene showed that DNA between -98 and +13 mediated basal and cAMP-regulated transcription in placental JEG-3 cells, as it did in adrenal and Leydig cells. DNase footprints further identified a region between -88 and the TATA box that was bound by protein. Transfection of luciferase reporter constructs containing -84 to -55 of the rat P450c17 DNA ligated to the minimal promoter of the thymidine kinase gene showed that this DNA increased both basal and cAMP-simulated luciferase activity. Gel mobility shift assays identified two DNA-protein complexes with JEG-3 cell nuclear extracts that were different from complexes formed with MA-10 cell extracts and did not involve SF-1. Mutational analysis of the -84/-55 DNA showed that JEG-3 nuclear proteins bound to a site containing, but not identical to, the SF-1 sequence. One complex involved Ku autoimmune antigen, which bound to DNA sequence specifically. Overexpression of Ku antigen in MA-10 cells stimulated rat P450c17 gene transcription, thus demonstrating a biologic effect of Ku. Ku also bound to a similar region of the human P450c17 gene, and the DNA region to which Ku bound was transcriptionally active in JEG-3 cells. Ku was also found in extracts from rat placenta and bound to the -84/-55 rat P450c17 DNA. These data demonstrate a role of Ku in regulating P450c17 gene expression. These data further indicate that although human P450c17 is not normally expressed in the placenta, factors that could activate this gene are indeed present.

Fractionation of rat hepatocyte subpopulations with varying metabolic potential, proliferative capacity, and retroviral gene transfer efficiency

The liver contains hepatocytes with varying ploidy and gene expression. To isolate cells on the basis of ploidy for analyzing mechanisms concerning cell proliferation and differentiation, we used Percoll gradients to separate F344 rat hepatocyte subpopulations. Specific fractions were enriched in polyploid (H2 fraction) or diploid (H3 and H4 fractions) hepatocytes containing glycogen and glucose-6-phosphatase. H4 cells were relatively smaller with greater nuclear/cytoplasmic ratios, less complex cytoplasm, and higher serum albumin or ceruloplasmin biosynthetic rates. H2 fraction cells were larger with lesser nuclear/cytoplasmic ratio, more complex cytoplasm, and more cytochrome P450 activity. Phenotypic marking showed that H4 cells originated in zone one and H2 cells in zones two or three of the liver lobule. H4 cells showed much greater mitogenic responsiveness to human hepatocyte growth factor. Retroviral gene transfer, which requires both viral receptors and cellular DNA synthesis, was significantly more efficient in H4 cells. The findings indicated that small diploid and large polyploid hepatocytes show unique biological differences. The ability to isolate hepatocytes of varying maturity is relevant for mechanisms concerning liver growth control and hepatic gene expression.

Interactions between scatter factors and their receptors: hints for therapeutic applications

The scatter factors, which include hepatocyte growth factor and macrophage stimulating protein, stand out from other cytokines because of their uncommon biological properties. In addition to promoting cell growth and protection from apoptosis, they are involved in the control of cell dissociation, migration into extracellular matrices, and a unique process of differentiation called 'branching morphogenesis'. Through the concerted regulation of these complex phenomena, scatter factors promote development, regeneration, and reconstruction of normal organ architecture. In transformed epithelia, scatter factors can mediate tumor invasive growth, a harmful feature of neoplastic progression in which cancer cells invade surrounding tissues, penetrate across the vascular walls, and eventually disseminate throughout the body, giving rise to systemic metastases. A much-debated issue in basic biology, which has strong implications for experimental medicine, is how to dissociate the favorable effects of growth factors from their adverse ones. Accordingly, to find agonists or antagonists with potential therapeutic applications is a crucial undertaking for current research. Domain-mapping analyses of growth factor molecules can help to isolate specific structural requirements for the induction of selective biological effects. Based on the observation that certain growth factors must undergo posttranslational modifications to exert a full response, it is possible to interfere with their activation mechanisms to modulate their functions. Finally, the identification of cell type-specific coreceptors able to potentiate their activity allows drawing of a functional body map, where some organs or tissues may be more responsive than others to growth factors. This review is focused on how, and to what extent, scatter factors can behave 'well' or 'badly' according to their molecular structure, the way they are activated, and the way they interact with cell surface receptors and coreceptors.

Polyamines and cerebral ischemia

It has been well established that alterations in polyamine metabolism are associated with animal models of global ischemia. Recently, this has been extended to include models of focal ischemia and traumatic brain injury. There is much evidence to support the idea that polyamines may play a multifaceted detrimental role following ischemia reperfusion. Due to the deficit of knowledge about their physiology in the CNS, the link between ischemia-induced alterations in polyamine metabolism and neuronal injury remains to be substantiated. With the recent revelation that polyamines are major intracellular modulators of inward rectifier potassium channels and certain types of NMDA and AMPA receptors, the long wait for the physiologic relevance of these ubiquitous compounds may be in sight. Therefore, it is now conceivable that the alterations in polyamines could have major effects on ion homeostasis in the CNS, especially potassium, and thus account for the observed injury after cerebral ischemia.

Effects of transforming growth factor alpha and hepatocyte growth factor on acid-induced damage in rat gastric epithelial cells

We examined the effects of transforming growth factor alpha (TGF-alpha) and hepatocyte growth factor (HGF) on acid-induced damage in a rat gastric epithelial cell line. Pretreatment of cells for 30 min with TGF-alpha, but not HGF, significantly prevented the acid-induced cell damage in a concentration-dependent manner. Genistein significantly reduced the protective effect of TGF-alpha. DNA synthesis in RGM1 cells was increased when the cells were incubated with TGF-alpha and HGF for 24 hr. We conclude that the protective effect of TGF-alpha against acid-induced damage seems to be caused by the activation of Na+/H+ exchangers and not by enhanced DNA synthesis.

Implications of atomic-resolution structures for cell adhesion

Molecules involved in cell adhesion processes are often both structurally and functionally modular, with subdomains that are members of large protein families. Recently, high-resolution structures have been determined for representative members of many of these families including fragments of integrins, cadherins, fibronectin-like domains, and immunoglobulin-like domains. These structures have enhanced our understanding of cell adhesion processes at several levels. In almost all cases, ligand-binding sites have been visualized and provide insight into how these molecules mediate biologically important interactions. Metal-binding sites have been identified and characterized, allowing assessment of the role of bound ions in cell adhesion processes. Many of these structures serve as templates for modeling homologous domains in other proteins or, when the structure of a fragment consisting of more than one domain is determined, the structure of multidomain arrays of homologous domains. Knowledge of atomic structure also allows rational design of drugs that either mimic or target specific binding sites. In many cases, high-resolution structures have revealed unexpected relationships that pose questions about the evolutionary origin of specific domains. This review briefly describes several recently determined structures of cell adhesion molecules, summarizes some of the main results of each structure, and highlights common features of different systems.

Function, regulation, pharmacology, and molecular structure of ATP-sensitive K+ channels in the cardiovascular system

ATP-sensitive K+ (K[ATP]) channels are inhibited by intracellular ATP and activated by intracellular nucleoside diphosphates, and thus provide a link between cellular metabolism and excitability. K(ATP) channels are widely distributed in various tissues and may be associated with diverse cellular functions. In the heart, the K(ATP) channel appears to be activated during ischemic or hypoxic conditions and may be responsible for the increase of K+ efflux and shortening of the action potential duration. Therefore, opening of this channel may result in cardioprotective as well as proarrhythmic effects. In the vascular smooth muscle, the K(ATP) channel is believed to mediate the relaxation of vascular tone. Thus, K(ATP) channels play important regulatory roles in the cardiovascular system. Furthermore, K(ATP) channels are the targets of two important classes of drugs, i.e., the antidiabetic sulfonylureas, which block the channels, and a series of vasorelaxants called "K+ channel openers," which tend to maintain the channels in an open conformation. Recently, the molecular structure of K(ATP) channels has been clarified. The K(ATP) channel in pancreatic beta-cells is a complex composed of at least two subunits, a member of inwardly rectifying K+ channels and a sulfonylurea receptor. Subsequently, two additional homologs of the sulfonylurea receptor, which form cardiac and smooth muscle type K(ATP) channels, respectively, have been reported. Further works are now in progress to understand the molecular mechanisms of K(ATP) channel function.

Existence of two nonlinear elimination mechanisms for hepatocyte growth factor in rats

Nonlinearity in the overall elimination of hepatocyte growth factor (HGF) was examined in rats. After intravenous administration, the plasma clearance (CLplasma) of HGF exhibited a dose-dependent biphasic reduction with high- and low-affinity components. If we consider our previous finding that both receptor-mediated endocytosis (RME) and a low-affinity uptake mechanism, probably mediated by heparan sulfate proteoglycan (HSPG), in the liver are major HGF clearance mechanisms, it may be that saturation of CLplasma at lower and higher doses represents saturation of RME and HSPG-mediated uptake, respectively. At an HGF dose (1.46 nmol/kg), which completely saturates the high-affinity component, CLplasma was almost completely reduced when HGF was premixed with heparin. However, CLplasma was reduced by heparin to, at most, one-fifth that after HGF alone in a dose near the linear range (3.66 pmol/kg). Saturation of CLplasma for HGF premixed with heparin was monophasic and nonlinear only at the lowest HGF doses. In vitro, high-affinity binding of [35S]heparin to HGF was found, showing that one HGF molecule binds to the penta- or hexasaccharide unit. Because mitogenic activity of HGF has been reported in the presence of heparin, these results suggest that heparin mainly inhibits low-affinity HGF uptake by complexing with HGF, whereas its effect on RME is relatively minor.

Inward rectifier potassium channels

The past three years have seen remarkable progress in research on the molecular basis of inward rectification, with significant implications for basic understanding and pharmacological manipulation of cellular excitability. Expression cloning of the first inward rectifier K channel (Kir) genes provided the necessary break-through that has led to isolation of a family of related clones encoding channels with the essential functional properties of classical inward rectifiers, ATP-sensitive K channels, and muscarinic receptor-activated K channels. High-level expression of cloned channels led to the discovery that classical inward so-called anomalous rectification is caused by voltage-dependent block of the channel by polyamines and Mg2+ ions, and it is now clear that a similar mechanism results in inward rectification of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-kainate receptor channels. Knowledge of the primary structures of Kir channels and the ability to mutate them also has led to the determination of many of the structural requirements of inward rectification.

A splicing variant of the RON transcript induces constitutive tyrosine kinase activity and an invasive phenotype

The Ron tyrosine kinase receptor shares with the members of its subfamily (Met and Sea) a unique functional feature: the control of cell dissociation, motility, and invasion of extracellular matrices (scattering). The mature Ron protein is a heterodimer of disulfide-linked alpha and beta chains, originated by proteolytic cleavage of a single-chain precursor of 185 kDa. In a human gastric cancer cell line (KATO-III), we found abnormal accumulation of an uncleaved single-chain protein (delta-Ron) of 165 kDa; this molecule is encoded by a transcript differing from the full-length RON mRNA by an in-frame deletion of 49 amino acids in the beta-chain extracellular domain. The deleted transcript originates by an alternatively spliced cassette exon of 147 bp, flanked by two short introns. The delta-Ron tyrosine kinase is constitutively activated by disulfide-linked intracellular oligomerization because it contains an uneven number of cysteine residues. Oligomerization and constitutive tyrosine phosphorylation of the full-size Ron was obtained by site-directed mutagenesis of a single cysteine residue in the region encoded by the cassette exon, mimicking that occurring in the delta-Ron isoform. Inhibition of thiol-mediated intermolecular disulfide bonding prevented delta-Ron oligomerization. The intracellular activation of Ron is followed by acquisition of invasive properties in vitro. These data (i) provide a novel molecular mechanism for posttranscriptional activation of a tyrosine kinase receptor protein and (ii) suggest a role for the Ron receptor in progression toward malignancy.

C-Met signalling in an HGF/SF-insensitive variant MDCK cell line with constitutive motile/invasive behaviour

The Met protein is a receptor tyrosine kinase for hepatocyte growth factor/scatter factor (HGF/SF), a multifunctional growth factor with mitogenic, motogenic and morphogenic properties. A morphologically altered variant of the MDCK cell line, MDCK-1, spontaneously exhibits a number of features associated with a partial HGF/SF-Met induced phenotype (less adhesive colonies in culture, enhanced invasion and motility, nascent tubule formation), but paradoxically does not respond to HGF/SF treatment. Although the overall cell surface expression and distribution of Met were found to be similar in parental MDCK cells and the MDCK-1 cell line, p145met autophosphorylation (+/ HGF/SF) was significantly reduced in MDCK-1 cells in vitro and in vivo when compared with parental MDCK cells. In contrast, EGF induced cell proliferation and EGF receptor autophosphorylation to similar levels in both cell lines. The basal levels of protein tyrosine phosphorylation were higher in MDCK-1 cells when compared with parental MDCK cells, including that of two prominent proteins with molecular masses of approximately 185 kDa and 220 kDa. Moreover, both p185 and p220 are present and tyrosine phosphorylated in Met immunoprecipitates from MDCK-1 cells (+/-HGF/SF), but not parental MDCK cells. In addition, Met immunocomplexes from MDCK-1 cells exhibited an approximately 3-fold increased tyrosine kinase activity in vitro when compared with MDCK cells, correlating with the higher basal levels of total phosphotyrosine. Treatment of MDCK-1 cells with the tyrosine kinase inhibitor herbimycin A reverted the cell phenotype to a more MDCK-like morphology in culture, with a concomitant reduction in the tyrosine phosphorylation predominantly of p220. Taken together these data suggest that aberrations in Met activity and associated signalling render MDCK-1 cells insensitive to HGF/SF, and may also mediate alterations in MDCK-1 cell behaviour.

Basic fibroblast growth factor controls the expression and molecular structure of heparan sulfate in corneal endothelial cells

Cultured bovine corneal endothelial cells express 5-8 ng basic fibroblast growth factor (bFGF)/mg cell protein and distribute it between the intracellular and pericellular compartment. Confluent cultures retain approximately 80% of the total bFGF intracellularly, whereas 20% is present in the pericellular (trypsin-releasable) compartment. No bFGF can be detected in the culture medium. The presence of 1-2 ng/ml medium of endogenous or exogenous (human recombinant) bFGF is sufficient to support cell growth. Simultaneously, cells incorporate [35S]sulfate and [3H]glucosamine into the sulfated proteoglycans associated with the cell layer at a rate that is three times higher than in the absence of bFGF. The enhanced proteoglycan synthesis is accompanied by a shift in proteoglycan distribution. In control cells, cell-associated heparan sulfate accounts for about 30% of the total glycosaminoglycans, whereas under the influence of bFGF the amount of heparan sulfate increases to approximately 60%. At the same time, the molecular structure of the heparan sulfate molecule undergoes bFGF-specific changes as indicated by the [35S]oligosaccharide pattern generated by heparitinase I degradation. The proportion of [35S]oligosaccharides with greater than six monosaccharides decreases on account of disaccharides and tetrasaccharides under the influence of bFGF. Pretreatment of bFGF with neutralizing antibodies against bFGF abolishes its biological activity. The results suggest a bFGF-dependent change in the rate of synthesis and structural features of the membrane-associated heparan sulfate in corneal endothelial cells. The modification of the heparan sulfate structure could influence its bFGF-binding and antiproliferative activity.