Multiple imprinted sense and antisense transcripts, differential methylation and tandem repeats in a putative imprinting control region upstream of mouse Igf2

The mouse insulin-like growth factor 2 (Igf2) locus is a complex genomic region that produces multiple transcripts from alternative promoters. Expression at this locus is regulated by parental imprinting. However, despite the existence of putative imprinting control elements in the Igf2 upstream region, imprinted transcriptional repression is abolished by null mutations at the linked H19 locus. To clarify the extent to which the Igf2 upstream region contains autonomous imprinting control elements we have performed functional and comparative analyses of the region in the mouse and human. Here we report the existence of multiple, overlapping imprinted (maternally repressed) sense and antisense transcripts that are associated with a tandem repeat in the mouse Igf2 upstream region. Regions flanking the repeat exhibit tissue-specific parental allelic methylation patterns, suggesting the existence of tissue-specific control elements in the upstream region. Studies in H19 null mice indicate that both parental allelic methylation and monoallelic expression of the upstream transcripts depends on an intact H19 gene acting in cis. The homologous region in human IGF2 is structurally conserved, with the significant exception that it does not contain a tandem repeat. Our results support the proposal that tandem repeats act to target methylation to imprinted genetic loci.

Increased IGF-II protein affects p57kip2 expression in vivo and in vitro: Implications for Beckwith–Wiedemann syndrome

In both human and mouse, the Igf2 gene, localized on chromosomes 11 and 7, respectively, is expressed from the paternally inherited chromosome in the majority of tissues. Insulin-like growth factor-II (IGF-II) plays an important role in embryonic growth, and aberrant IGF2 expression has been documented in several human pathologies, such as Beckwith–Wiedemann syndrome (BWS), and a wide variety of tumors. Human and mouse genetic data strongly implicate another gene, CDKN1C (p57kip2), located in the same imprinted gene cluster on human chromosome II, in BWS. p57KIP2 is a cyclin-dependent kinase inhibitor and is required for normal mouse embryonic development. Mutations in CDKN1C (p57kip2) have been identified in a small proportion of patients with BWS, and removal of the gene from mice by targeted mutagenesis produces a phenotype with elements in common with this overgrowth syndrome. Patients with BWS with biallelic expression of IGF2 or with a CDKN1C (p57kip2) mutation, as well as overlapping phenotypes observed in two types of mutant mice, the p57kip2 knockout and IGF-II-overexpressing mice, strongly suggest that the genes may act in a common pathway of growth control in situations where Igf2 expression is abnormal. Herein, we show that p57kip2 expression is reduced on IGF-II treatment of primary embryo fibroblasts in a dose-dependent manner. In addition, p57kip2 expression is down-regulated in mice with high serum levels of IGF-II. These data suggest that the effects of increased IGF-II in BWS may, in part, be mediated through a decrease in p57kip2 gene expression.

Low IGF-I suppresses VEGF-survival signaling in retinal endothelial cells: Direct correlation with clinical retinopathy of prematurity

Retinopathy of prematurity is a blinding disease, initiated by lack of retinal vascular growth after premature birth. We show that lack of insulin-like growth factor I (IGF-I) in knockout mice prevents normal retinal vascular growth, despite the presence of vascular endothelial growth factor, important to vessel development. In vitro, low levels of IGF-I prevent vascular endothelial growth factor-induced activation of protein kinase B (Akt), a kinase critical for endothelial cell survival. Our results from studies in premature infants suggest that if the IGF-I level is sufficient after birth, normal vessel development occurs and retinopathy of prematurity does not develop. When IGF-I is persistently low, vessels cease to grow, maturing avascular retina becomes hypoxic and vascular endothelial growth factor accumulates in the vitreous. As IGF-I increases to a critical level, retinal neovascularization is triggered. These data indicate that serum IGF-I levels in premature infants can predict which infants will develop retinopathy of prematurity and further suggests that early restoration of IGF-I in premature infants to normal levels could prevent this disease.

Human fetal enterocytes in vitro: modulation of the phenotype by extracellular matrix.

The differentiation of small intestinal epithelial cells may require stimulation by microenvironmental factors in vivo. In this study, the effects of mesenchymal and luminal elements in nonmalignant epithelia] cells isolated from the human fetus were studied in vitro. Enterocytes from the human fetus were cultured and microenvironmental factors were added in stages, each stage more closely approximating the microenvironment in vivo. Four stages were examined: epithelial cells derived on plastic from intestinal culture and grown as a cell clone, the same cells grown on connective tissue support, primary epithelial explants grown on fibroblasts with a laminin base, and primary epithelial explants grown on fibroblasts and laminin with n-butyrate added to the incubation medium. The epithelial cell clone dedifferentiated when grown on plastic; however, the cells expressed cytokeratins and villin as evidence of their epithelial cell origin. Human connective tissue matrix from Engelbreth-Holm-Swarm sarcoma cells (Matrigel) modulated their phenotype: alkaline phosphatase activity increased, microvilli developed on their apical surface, and the profile of insulin-like growth factor binding proteins resembled that secreted by differentiated enterocytes. Epithelial cells taken directly from the human fetus as primary cultures and grown as explants on fibroblasts and laminin expressed greater specific enzyme activities in brush border membrane fractions than the cell clone. These activities were enhanced by the luminal molecule sodium butyrate. Thus the sequential addition of connective tissue and luminal molecules to nonmalignant epithelia] cells in vitro induces a spectrum of changes in the epithelial cell phenotype toward full differentiation.

Imprinting of the gene encoding a human cyclin-dependent kinase inhibitor, p57KIP2, on chromosome 11p15.

Parental origin-specific alterations of chromosome 11p15 in human cancer suggest the involvement of one or more maternally expressed imprinted genes involved in embryonal tumor suppression and the cancer-predisposing Beckwith-Wiedemann syndrome (BWS). The gene encoding cyclin-dependent kinase inhibitor p57KIP2, whose overexpression causes G1 phase arrest, was recently cloned and mapped to this band. We find that the p57KIP2 gene is imprinted, with preferential expression of the maternal allele. However, the imprint is not absolute, as the paternal allele is also expressed at low levels in most tissues, and at levels comparable to the maternal allele in fetal brain and some embryonal tumors. The biochemical function, chromosomal location, and imprinting of the p57KIP2 gene match the properties predicted for a tumor suppressor gene at 11p15.5. However, as the p57KIP2 gene is 500 kb centromeric to the gene encoding insulin-like growth factor 2, it is likely to be part of a large domain containing other imprinted genes. Thus, loss of heterozygosity or loss of imprinting might simultaneously affect several genes at this locus that together contribute to tumor and/or growth- suppressing functions that are disrupted in BWS and embryonal tumors.

Reciprocal interactions between β1-integrin and epidermal growth factor receptor in three-dimensional basement membrane breast cultures: A different perspective in epithelial biology

Anchorage and growth factor independence are cardinal features of the transformed phenotype. Although it is logical that the two pathways must be coregulated in normal tissues to maintain homeostasis, this has not been demonstrated directly. We showed previously that down-modulation of β1-integrin signaling reverted the malignant behavior of a human breast tumor cell line (T4–2) derived from phenotypically normal cells (HMT-3522) and led to growth arrest in a three-dimensional (3D) basement membrane assay in which the cells formed tissue-like acini (14). Here, we show that there is a bidirectional cross-modulation of β1-integrin and epidermal growth factor receptor (EGFR) signaling via the mitogen-activated protein kinase (MAPK) pathway. The reciprocal modulation does not occur in monolayer (2D) cultures. Antibody-mediated inhibition of either of these receptors in the tumor cells, or inhibition of MAPK kinase, induced a concomitant down-regulation of both receptors, followed by growth-arrest and restoration of normal breast tissue morphogenesis. Cross-modulation and tissue morphogenesis were associated with attenuation of EGF-induced transient MAPK activation. To specifically test EGFR and β1-integrin interdependency, EGFR was overexpressed in nonmalignant cells, leading to disruption of morphogenesis and a compensatory up-regulation of β1-integrin expression, again only in 3D. Our results indicate that when breast cells are spatially organized as a result of contact with basement membrane, the signaling pathways become coupled and bidirectional. They further explain why breast cells fail to differentiate in monolayer cultures in which these events are mostly uncoupled. Moreover, in a subset of tumor cells in which these pathways are misregulated but functional, the cells could be “normalized” by manipulating either pathway.

Molecular cloning of a high-affinity receptor for the growth factor-like lipid mediator lysophosphatidic acid from Xenopus oocytes

Lysophosphatidic acid (1-acyl-2-lyso-sn-glycero-3-phosphate, LPA) is a multifunctional lipid mediator found in a variety of organisms that span the phylogenetic tree from humans to plants. Although its physiological function is not clearly understood, LPA is a potent regulator of mammalian cell proliferation; it is one of the major mitogens found in blood serum. In Xenopus laevis oocytes, LPA elicits oscillatory Cl− currents. This current, like other effects of LPA, is consistent with a plasma membrane receptor-mediated activation of G protein-linked signal transduction pathways. Herein we report the identification of a complementary DNA from Xenopus that encodes a functional high-affinity LPA receptor. The predicted structure of this protein of 372 amino acids contains features common to members of the seven transmembrane receptor superfamily with a predicted extracellular amino and intracellular carboxyl terminus. An antisense oligonucleotide derived from the first 5–11 predicted amino acids, selectively inhibited the expression of the endogenous high-affinity LPA receptors in Xenopus oocytes, whereas the same oligonucleotide did not affect the low-affinity LPA receptor. Expression of the full-length cRNA in oocytes led to an increase in maximal Cl− current due to increased expression of the high-affinity LPA receptor, but activation of the low-affinity receptor was, again, unaffected. Oocytes expressing cRNA prepared from this clone showed no response to other lipid mediators including prostaglandins, leukotrienes, sphingosine 1-phosphate, sphingosylphosphorylcholine, and platelet-activating factor, suggesting that the receptor is highly selective for LPA.

Molecular characteristics of fibroblast growth factor–fibroblast growth factor receptor–heparin-like glycosaminoglycan complex

Fibroblast growth factor (FGF) family plays key roles in development, wound healing, and angiogenesis. Understanding of the molecular nature of interactions of FGFs with their receptors (FGFRs) has been seriously limited by the absence of structural information on FGFR or FGF–FGFR complex. In this study, based on an exhaustive analysis of the primary sequences of the FGF family, we determined that the residues that constitute the primary receptor-binding site of FGF-2 are conserved throughout the FGF family, whereas those of the secondary receptor binding site of FGF-2 are not. We propose that the FGF–FGFR interaction mediated by the ‘conserved’ primary site interactions is likely to be similar if not identical for the entire FGF family, whereas the ‘variable’ secondary sites, on both FGF as well as FGFR mediates specificity of a given FGF to a given FGFR isoform. Furthermore, as the pro-inflammatory cytokine interleukin 1 (IL-1) and FGF-2 share the same structural scaffold, we find that the spatial orientation of the primary receptor-binding site of FGF-2 coincides structurally with the IL-1β receptor-binding site when the two molecules are superimposed. The structural similarities between the IL-1 and the FGF system provided a framework to elucidate molecular principles of FGF–FGFR interactions. In the FGF–FGFR model proposed here, the two domains of a single FGFR wrap around a single FGF-2 molecule such that one domain of FGFR binds to the primary receptor-binding site of the FGF molecule, while the second domain of the same FGFR binds to the secondary receptor-binding site of the same FGF molecule. Finally, the proposed model is able to accommodate not only heparin-like glycosaminoglycan (HLGAG) interactions with FGF and FGFR but also FGF dimerization or oligomerization mediated by HLGAG.

Activating mutations in the extracellular domain of the fibroblast growth factor receptor 2 function by disruption of the disulfide bond in the third immunoglobulin-like domain

Multiple human skeletal and craniosynostosis disorders, including Crouzon, Pfeiffer, Jackson–Weiss, and Apert syndromes, result from numerous point mutations in the extracellular region of fibroblast growth factor receptor 2 (FGFR2). Many of these mutations create a free cysteine residue that potentially leads to abnormal disulfide bond formation and receptor activation; however, for noncysteine mutations, the mechanism of receptor activation remains unclear. We examined the effect of two of these mutations, W290G and T341P, on receptor dimerization and activation. These mutations resulted in cellular transformation when expressed as FGFR2/Neu chimeric receptors. Additionally, in full-length FGFR2, the mutations induced receptor dimerization and elevated levels of tyrosine kinase activity. Interestingly, transformation by the chimeric receptors, dimerization, and enhanced kinase activity were all abolished if either the W290G or the T341P mutation was expressed in conjunction with mutations that eliminate the disulfide bond in the third immunoglobulin-like domain (Ig-3). These results demonstrate a requirement for the Ig-3 cysteine residues in the activation of FGFR2 by noncysteine mutations. Molecular modeling also reveals that noncysteine mutations may activate FGFR2 by altering the conformation of the Ig-3 domain near the disulfide bond, preventing the formation of an intramolecular bond. This allows the unbonded cysteine residues to participate in intermolecular disulfide bonding, resulting in constitutive activation of the receptor.

Different Tyrosine Autophosphorylation Requirements in Fibroblast Growth Factor Receptor-1 Mediate Urokinase-Type Plasminogen Activator Induction and Mitogenesis

Among the seven tyrosine autophosphorylation sites identified in the intracellular domain of tyrosine kinase fibroblast growth factor receptor-1 (FGFR1), five of them are dispensable for FGFR1-mediated mitogenic signaling. The possibility of dissociating the mitogenic activity of basic FGF (FGF2) from its urokinase-type plasminogen activator (uPA)-inducing capacity both at pharmacological and structural levels prompted us to evaluate the role of these autophosphorylation sites in transducing FGF2-mediated uPA upregulation. To this purpose, L6 myoblasts transfected with either wild-type (wt) or various FGFR1 mutants were evaluated for the capacity to upregulate uPA production by FGF2. uPA was induced in cells transfected with wt-FGFR1, FGFR1-Y463F, -Y585F, -Y730F, -Y766F, or -Y583/585F mutants. In contrast, uPA upregulation was prevented in L6 cells transfected with FGFR1-Y463/583/585/730F mutant (FGFR1–4F) or with FGFR1-Y463/583/585/730/766F mutant (FGFR1–5F) that retained instead a full mitogenic response to FGF2; however, preservation of residue Y730 in FGFR1-Y463/583/585F mutant (FGFR1–3F) and FGFR1-Y463/583/585/766F mutant (FGFR1–4Fbis) allows the receptor to transduce uPA upregulation. Wild-type FGFR1, FGFR1–3F, and FGFR1–4F similarly bind to a 90-kDa tyrosine-phosphorylated protein and activate Shc, extracellular signal-regulated kinase (ERK)2, and JunD after stimulation with FGF2. These data, together with the capacity of the ERK kinase inhibitor PD 098059 to prevent ERK2 activation and uPA upregulation in wt-FGFR1 cells, suggest that signaling through the Ras/Raf-1/ERK kinase/ERK/JunD pathway is necessary but not sufficient for uPA induction in L6 transfectants. Accordingly, FGF2 was able to stimulate ERK1/2 phosphorylation and cell proliferation, but not uPA upregulation, in L6 cells transfected with the FGFR1-Y463/730F mutant, whereas the FGFR1-Y583/585/730F mutant was fully active. We conclude that different tyrosine autophosphorylation requirements in FGFR1 mediate cell proliferation and uPA upregulation induced by FGF2 in L6 cells. In particular, phosphorylation of either Y463 or Y730, dispensable for mitogenic signaling, represents an absolute requirement for FGF2-mediated uPA induction.

Interactions between Growth Factors and Integrins: Latent Forms of Transforming Growth Factor-β Are Ligands for the Integrin αvβ1

The multipotential cytokine transforming growth factor-β (TGF-β) is secreted in a latent form. Latency results from the noncovalent association of TGF-β with its processed propeptide dimer, called the latency-associated peptide (LAP); the complex of the two proteins is termed the small latent complex. Disulfide bonding between LAP and latent TGF-β–binding protein (LTBP) produces the most common form of latent TGF-β, the large latent complex. The extracellular matrix (ECM) modulates the activity of TGF-β. LTBP and the LAP propeptides of TGF-β (isoforms 1 and 3), like many ECM proteins, contain the common integrin-binding sequence RGD. To increase our understanding of latent TGF-β function in the ECM, we determined whether latent TGF-β1 interacts with integrins. A549 cells adhered and spread on plastic coated with LAP, small latent complex, and large latent complex but not on LTBP-coated plastic. Adhesion was blocked by an RGD peptide, and cells were unable to attach to a mutant form of recombinant LAP lacking the RGD sequence. Adhesion was also blocked by mAbs to integrin subunits αv and β1. We purified LAP-binding integrins from extracts of A549 cells using LAP bound to Sepharose. αvβ1 eluted with EDTA. After purification in the presence of Mn2+, a small amount of αvβ5 was also detected. A549 cells migrated equally on fibronectin- and LAP-coated surfaces; migration on LAP was αvβ1 dependent. These results establish αvβ1 as a LAP-β1 receptor. Interactions between latent TGF-β and αvβ1 may localize latent TGF-β to the surface of specific cells and may allow the TGF-β1 gene product to initiate signals by both TGF-β receptor and integrin pathways.

Fibroblast Growth Factor (FGF) Soluble Receptor 1 Acts as a Natural Inhibitor of FGF2 Neurotrophic Activity during Retinal Degeneration

Fibroblast growth factors (FGF) 1 and 2 and their tyrosine kinase receptor (FGFR) are present throughout the adult retina. FGFs are potential mitogens, but adult retinal cells are maintained in a nonproliferative state unless the retina is damaged. Our work aims to find a modulator of FGF signaling in normal and pathological retina. We identified and sequenced a truncated FGFR1 form from rat retina generated by the use of selective polyadenylation sites. This 70-kDa form of soluble extracellular FGFR1 (SR1) was distributed mainly localized in the inner nuclear layer of the retina, whereas the full-length FGFR1 form was detected in the retinal Muller glial cells. FGF2 and FGFR1 mRNA levels greatly increased in light-induced retinal degeneration. FGFR1 was detected in the radial fibers of activated retinal Muller glial cells. In contrast, SR1 mRNA synthesis followed a biphasic pattern of down- and up-regulation, and anti-SR1 staining was intense in retinal pigmented epithelial cells. The synthesis of SR1 and FGFR1 specifically and independently regulated in normal and degenerating retina suggests that changes in the proportion of various FGFR forms may control the bioavailability of FGFs and thus their potential as neurotrophic factors. This was demonstrated in vivo during retinal degeneration when recombinant SR1 inhibited the neurotrophic activity of exogenous FGF2 and increased damaging effects of light by inhibiting endogenous FGF. This study highlights the significance of the generation of SR1 in normal and pathological conditions.

c-mos and cdc2 Cooperate in the Translational Activation of Fibroblast Growth Factor Receptor-1 during Xenopus Oocyte Maturation

During oocyte maturation in Xenopus, previously quiescent maternal mRNAs are translationally activated at specific times. We hypothesized that the translational recruitment of individual messages is triggered by particular cellular events and investigated the potential for known effectors of the meiotic cell cycle to activate the translation of the FGF receptor-1 (XFGFR) maternal mRNA. We found that both c-mos and cdc2 activate the translation of XFGFR. However, although oocytes matured by injection of recombinant cdc2/cyclin B translate normal levels of XFGFR protein, c-mos depletion reduces the level of XFGFR protein induced by cdc2/cyclin B injection. In oocytes blocked for cdc2 activity, injection of mos RNA induced low levels of XFGFR protein, independent of MAPK activity. Through the use of injected reporter RNAs, we show that the XFGFR 3′ untranslated region inhibitory element is completely derepressed by cdc2 alone. In addition, we identified a new inhibitory element through which both mos and cdc2 activate translation. We found that cdc2 derepresses translation in the absence of polyadenylation, whereas mos requires poly(A) extension to activate XFGFR translation. Our results demonstrate that mos and cdc2, in addition to functioning as key regulators of the meiotic cell cycle, cooperate in the translational activation of a specific maternal mRNA during oocyte maturation.

Fibroblast growth factors 1 and 2 are distinct in oligomerization in the presence of heparin-like glycosaminoglycans

Fibroblast growth factor (FGF) 1 and FGF-2 are prototypic members of the FGF family, which to date comprises at least 18 members. Surprisingly, even though FGF-1 and FGF-2 share more than 80% sequence similarity and an identical structural fold, these two growth factors are biologically very different. FGF-1 and FGF-2 differ in their ability to bind isoforms of the FGF receptor family as well as the heparin-like glycosaminoglycan (HLGAG) component of proteoglycans on the cell surface to initiate signaling in different cell types. Herein, we provide evidence for one mechanism by which these two proteins could differ biologically. Previously, it has been noted that FGF-1 and FGF-2 can oligomerize in the presence of HLGAGs. Therefore, we investigated whether FGF-1 and FGF-2 oligomerize by the same mechanism or by a different one. Through a combination of matrix-assisted laser desorption ionization mass spectrometry and chemical crosslinking, we show here that, under identical conditions, FGF-1 and FGF-2 differ in the degree and kind of oligomerization. Furthermore, an extensive analysis of FGF-1 and FGF-2 uncomplexed and HLGAG complexed crystal structures enables us to readily explain why FGF-2 forms sequential oligomers whereas FGF-1 forms only dimers. FGF-2, which possesses an interface capable of protein association, forms a translationally related oligomer, whereas FGF-1, which does not have this interface, forms only a symmetrically related dimer. Taken together, these data show that FGF-1 and FGF-2, despite their sequence homology, differ in their mechanism of oligomerization.