Cysteines 153 and 154 of transmembrane transforming growth factor-alpha are palmitoylated and mediate cytoplasmic protein association

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

From wavy hair to naked proteins: the role of transforming growth factor alpha in health and disease

Since its discovery in 1978 and cloning in 1984, transforming growth factor-alpha (TGF-α, TGFA) has been one of the most extensively studied EGF receptor (EGFR) ligands. In this review, we provide a historical perspective on TGFA-related studies, highlighting what we consider important advances related to its function in normal and disease states.

Ectodomain shedding and remnant peptide signalling of EGFRs and their ligands

Both receptor tyrosine kinases epidermal growth factor receptors (EGFRs) and their ligands are transmembrane proteins. It has been known that ligand binding activates cytoplasmic tyrosine kinase domains of EGFRs, resulting in the transduction of signals for cell proliferation, migration, differentiation or survival. In an EGFRs-ligands system, however, signal transduction occurs not only unidirectionally but also bidirectionally, which is regulated by cell-cell contact and proteolytic cleavage. Recent studies of proteolytic cleavage 'ectodomain shedding' of EGFRs and their ligands mediated by membrane-type metalloproteinases, a disintegrin and metalloproteinases have been unveiling novel functions and molecular mechanism of their remnant peptides. In addition, the study of the remnant peptide signalling would be essential for understanding the physiological and pathological relevance of anti-shedding therapeutic strategies for diseases such as cancer.

Sequential and gamma-secretase-dependent processing of the betacellulin precursor generates a palmitoylated intracellular-domain fragment that inhibits cell growth

Betacellulin (BTC) belongs to the family of epidermal growth factor (EGF)-like growth factors that are expressed as transmembrane precursors and undergo proteolytic ectodomain shedding to release soluble mature ligands. BTC is a dual-specificity ligand for ErbB1 and ErbB4 receptors, and can activate unique signal-transduction pathways that are beneficial for the function, survival and regeneration of pancreatic beta-cells. We have previously shown that BTC precursor (proBTC) is cleaved by ADAM10 to generate soluble ligand and a stable, transmembrane remnant (BTC-CTF). In this study, we analyzed the fate of the BTC-CTF in greater detail. We demonstrated that proBTC is cleaved by ADAM10 to produce BTC-CTF, which then undergoes intramembrane processing by presenilin-1- and/or presenilin-2-dependent gamma-secretase to generate an intracellular-domain fragment (BTC-ICD). We found that the proBTC cytoplasmic domain is palmitoylated and that palmitoylation is not required for ADAM10-dependent cleavage but is necessary for the stability and gamma-secretase-dependent processing of BTC-CTF to generate BTC-ICD. Additionally, palmitoylation is required for nuclear-membrane localization of BTC-ICD, as demonstrated by the redistribution of non-palmitoylated BTC-ICD mutant to the nucleoplasm. Importantly, a novel receptor-independent role for BTC-ICD signaling is suggested by the ability of BTC-ICD to inhibit cell growth in vitro.

Emerging concepts in the regulation of membrane-type 1 matrix metalloproteinase activity

Pericellular proteolysis mediated by membrane-type 1 matrix metalloproteinase (MT1-MMP) represents an essential component of the cellular machinery involved in the dissolution and penetration of ECM barriers by tumor cells. Although most studies on the proinvasive properties of MT1-MMP have focused on its unusually broad proteolytic activity towards several ECM components and cell surface receptors, recent evidence indicate that the cytoplasmic domain of the enzyme also actively participates in tumor cell invasion by regulating the cell surface localization of MT1-MMP as well as the activation of signal transduction cascades. The identification of the molecular events by which the intracellular domain of MT1-MMP links proteolysis of the surrounding matrix by the enzyme to modification of cell function may thus provide important new information on the mechanisms by which this enzyme controls the invasive behavior of neoplastic cells in vivo.

Substitution of methionine 435 with leucine, isoleucine, and serine in tumor necrosis factor alpha converting enzyme inactivates ectodomain shedding activity

Tumor necrosis factor alpha (TNF-alpha) converting enzyme (TACE) is a zinc metalloprotease that has emerged as a general sheddase, which is responsible for ectodomain release of numerous membrane proteins, including the proinflammatory cytokine TNF-alpha, the leukocyte adhesin L-selectin and epidermal growth factor receptor ligand-transforming growth factor alpha (TGF-alpha), and related family members. Structurally, TACE belongs to a large clan of proteases, designated the metzincins, because TACE possesses a conserved methionine (Met435), frequently referred to as the met-turn residue, in its active site. A vital role of this residue in the function of TACE is supported by the fact that cells expressing the M435I TACE variant are defective in ectodomain shedding. However, the importance of Met435 in TACE appears to be uncertain, since another metzincin, matrix metalloprotease-2, has been found to be enzymatically fully active with either leucine or serine in place of its met-turn residue. We constructed TACE mutants with leucine or serine in place of Met435 to further examine the role of the met-turn residue in TACE-mediated ectodomain cleavage. Similar to the M435I TACE mutant, both the M435L and M435S constructs are defective in cleaving transmembrane TNF-alpha, TGF-alpha, and L-selectin. Comparative modeling and dynamic computation detected structural perturbations, which resulted in higher energy, in the catalytic zinc complexes of the Met435 TACE mutants compared with that in the wild-type enzyme. Thus, Met435 serves to maintain the stability of the catalytic center of TACE for the hydrolysis of peptide bonds in substrates.

Matrix metalloproteinases in inflammatory bowel disease: boon or a bane?

Matrix metalloproteinases (MMPs) are a family of Zn(2+)-dependent extracellular matrix (ECM) degrading endopeptidases that share common functional domains, activation mechanisms, and collectively have the capacity to degrade all types of ECM proteins. In addition to playing a central role in ECM turnover, MMPs proteolytically activate or degrade a variety of nonmatrix substrates including chemokines, cytokines, growth factors, and junctional proteins. Thus, they are increasingly recognized as critical players in inflammatory response. Indeed, accumulating data from several studies indicate that they are the predominant proteases involved in the pathogenesis of inflammatory bowel disease (IBD) via their influence on the function and migration of inflammatory cells, mucosal ulceration, as well as matrix deposition and degradation. Some MMPs are constitutively expressed and play a protective role in IBD through their effect on cellular homeostasis, while others are induced during inflammation-mediated tissue damage. This article focuses on the role of the various MMPs in IBD, discussing their physiologic and pathogenetic role in the context of intestinal defense, mucosal inflammatory response, and immune cell-epithelial interaction.

Altered expression of matrix-related molecules in the development of chronic Thy1.1 nephritis

BACKGROUND/AIM

Matrix production and degradation are critically important in chronic nephritis. Our aim was to investigate the precise expression of matrix-related molecules which is essential for understanding the pathogenesis of renal disease.

METHODS

Chronic nephritis was induced by a single injection of anti-Thy1.1 antibody to unilaterally nephrectomized rats. RNA was extracted from renal cortex and isolated glomeruli 4, 7, and 10 weeks after the antibody injection. Matrix-related gene expressions were measured by polymerase chain reaction. The expression of alpha1(IV) and alpha3(IV) collagens was studied by immunohistochemistry. The gelatinolytic activity in the glomeruli was assayed by gelatin zymography.

RESULTS

Polymerase chain reaction revealed an increase of alpha1(IV) in both glomeruli and renal cortex from nephritic rats. In contrast, the expression of alpha3(IV), normally a component of the glomerular basement membrane, was decreased in nephritic animals. Immunohistochemistry confirmed the finding that alpha1(IV) and alpha3(IV) were up- and downregulated, respectively, in the glomeruli. Gene expression and activity of matrix metalloproteinase 2 were enhanced, while those of matrix metalloproteinase 9 were clearly suppressed in nephritis.

CONCLUSIONS

Downregulation of alpha3(IV) and enhancement of the matrix metalloproteinase-2 activity in the glomeruli may contribute to the glomerular damage by altering the glomerular basement membrane components. Impairment of the glomerular basement membrane integrity may possibly be implicated in irreversible renal dysfunction.

Recycling of MUC1 is dependent on its palmitoylation

MUC1 is a mucin-like transmembrane protein expressed on the apical surface of epithelia, where it protects the cell surface. The cytoplasmic domain has numerous sites for phosphorylation and docking of proteins involved in signal transduction. In a previous study, we showed that the cytoplasmic YXXphi motif Y20HPM and the tyrosine-phosphorylated Y60TNP motif are required for MUC1 clathrin-mediated endocytosis through binding AP-2 and Grb2, respectively (Kinlough, C. L., Poland, P. A., Bruns, J. B., Harkleroad, K. L., and Hughey, R. P. (2004) J. Biol. Chem. 279, 53071-53077). Palmitoylation of transmembrane proteins can affect their membrane trafficking, and the MUC1 sequence CQC3RRK at the boundary of the transmembrane and cytoplasmic domains mimics reported site(s) of S-palmitoylation. [3H]Palmitate labeling of Chinese hamster ovary cells expressing MUC1 with mutations in CQC3RRK revealed that MUC1 is dually palmitoylated at the CQC motif independent of RRK. Lack of palmitoylation did not affect the cold detergent solubility profile of a chimera (Tac ectodomain and MUC1 transmembrane and cytoplasmic domains), the rate of chimera delivery to the cell surface, or its half-life. Calculation of rate constants for membrane trafficking of wild-type and mutant Tac-MUC1 indicated that the lack of palmitoylation blocked recycling, but not endocytosis, and caused the chimera to accumulate in a EGFP-Rab11-positive endosomal compartment. Mutations CQC/AQA and Y20N inhibited Tac-MUC1 co-immunoprecipitation with AP-1, although mutant Y20N had reduced rates of both endocytosis and recycling, but a normal subcellular distribution. The double mutant chimera AQA+Y20N had reduced endocytosis and recycling rates and accumulated in EGFP-Rab11-positive endosomes, indicating that palmitoylation is the dominant feature modulating MUC1 recycling from endosomes back to the plasma membrane.

Palmitoylation of the EGFR Ligand Spitz by Rasp Increases Spitz Activity by Restricting Its Diffusion

Lipid modifications such as palmitoylation or myristoylation target intracellular proteins to cell membranes. Secreted ligands of the Hedgehog and Wnt families are also palmitoylated; this modification, which requires the related transmembrane acyltransferases Rasp and Porcupine, can enhance their secretion, transport, or activity. We show here that rasp is also essential for the developmental functions of Spitz, a ligand for the Drosophila epidermal growth factor receptor (EGFR). In cultured cells, Rasp promotes palmitate addition to the N-terminal cysteine residue of Spitz, and this cysteine is required for Spitz activity in vivo. Palmitoylation reduces Spitz secretion and enhances its plasma membrane association, but does not alter its ability to activate the EGFR in vitro. In vivo, overexpressed unpalmitoylated Spitz has an increased range of action but reduced activity. These data suggest a role for palmitoylation in restricting Spitz diffusion, allowing its local concentration to reach the threshold required for biological function.

Proteolytic processing of TGFα redirects its mitogenic activity: the membrane-anchored form is autocrine, the secreted form is paracrine

Wild-type transforming growth factor alpha (TGFalpha) expression in lactotrope cells in the pituitary gland led to lactotrope-specific pituitary hyperplasia and adenomata. To indicate whether the EGF receptor is involved in this TGFalpha-mediated phenotype, we bred TGFalpha mice with mice expressing the cytoplasmic truncated-EGF receptor (EGFR-tr), which is dominant-negative in other models. These bitransgenic mice developed pituitary pathology despite expression of the dominant-negative receptor. To further characterize this observation, we generated two lineages of transgenic mice that overexpress mutant forms of TGFalpha: a processed soluble form (s TGFalpha) and a cytoplasmic-deleted form (TGFalphaDeltaC). While sTGFalpha expression in lactotrope cells failed to induce autocrine lactotrope hyperplasia, the pituitary became very enlarged due to proliferation of neighboring interstitial cells. In contrast, the TGFalphaDeltaC mice did not develop a phenotype, although the mRNA and protein were present in the pituitary and this form of TGFalpha was confirmed to be biologically active and targeted properly to the plasma membrane of cultured CHO cells. The results suggest that the cytoplasmic domain of TGFalpha is required for autocrine parenchymal tumor formation in the pituitary gland. This signal cannot be inhibited by the EGFR-tr. Conversely, the released form of TGFalpha appears to have primarily paracrine activity.

Identification of MAGI-3 as a transforming growth factor-alpha tail binding protein

The cytoplasmic domain of the transforming growth factor-alpha precursor (proTGFalpha) contains a C-terminal PSD-95/SAP90, Discs Large, and Zona Occludens-1 (PDZ) recognition motif (TVV). By yeast two-hybrid screening of a mouse embryo library, we have found that a third member of a family of PDZ-containing proteins, membrane associated guanylate kinase inverted-3 (MAGI-3), binds to TGFalpha's TVV. MAGI-3 is widely expressed in multiple mouse tissues, including brain. Immunolocalization showed that MAGI-3 and TGFalpha were colocalized in neurons in the cortex and dentate gyrus, as well as in ependymal cells and some astrocytes. In vitro, proTGFalpha bound the PDZ-1 domain of MAGI-3 and MAGI-2, but not MAGI-1. MAGI-3 and the 17-kDa cell surface form of proTGFalpha interact transiently in MDCK cells stably transfected with both MAGI-3 and human proTGFalpha cDNAs. MAGI-3 and wild-type proTGFalpha colocalize at the cell surface. In contrast, MAGI-3 forms a stable complex with membrane-fixed TGFalpha early in the secretory pathway and interacts with immature and cell surface forms of membrane-fixed TGFalpha. Overexpression of MAGI-3 resulted in increased levels of TGFalpha in the basolateral medium of polarized MDCK cells, suggesting that MAGI-3 has a role in efficient trafficking of TGFalpha to the cell surface in polarized epithelial cells.

Src-mediated tyrosine phosphorylation of caveolin-1 induces its association with membrane type 1 matrix metalloproteinase

We have recently shown that stimulation of endothelial cells with vascular endothelial growth factor (VEGF) induces dissociation of caveolin-1 from the VEGFR-2 receptor, followed by Src family kinase-dependent tyrosine phosphorylation of the protein (Labrecque, L., Royal, I., Surprenant, D. S., Patterson, C., Gingras, D., and Beliveau, R. (2003) Mol. Biol. Cell 14, 334-347). In this study, we provide evidence that the VEGF-dependent tyrosine phosphorylation of caveolin-1 induces interaction of the protein with the membrane-type 1 matrix metalloproteinase (MT1-MMP). This interaction requires the phosphorylation of caveolin-1 on tyrosine 14 by members of the Src family of protein kinases, such as Src and Fyn, because it is completely abolished by expression of a catalytically inactive Src mutant or by site-directed mutagenesis of tyrosine 14 of caveolin-1. Most interestingly, the association of MT1-MMP with phosphorylated caveolin-1 induced the recruitment of Src and a concomitant inhibition of the kinase activity of the enzyme, suggesting that this complex may be involved in the negative regulation of Src activity. The association of MT1-MMP with phosphorylated caveolin-1 occurs in caveolae membranes and involves the cytoplasmic domain of MT1-MMP because it was markedly reduced by mutation of Cys574 and Val582 residues of the cytoplasmic tail of the enzyme. Most interestingly, the reduction of the interaction between MT1-MMP and caveolin-1 by using these mutants also decreases MT1-MMP-dependent cell locomotion. Overall these results indicate that MT1-MMP associates with tyrosine-phosphorylated caveolin-1 and that this complex may play an important role in MT1-MMP regulation and function.

Myristoylated Naked2 escorts transforming growth factor alpha to the basolateral plasma membrane of polarized epithelial cells

The epidermal growth factor receptor ligands transforming growth factor alpha (TGF alpha) and amphiregulin are delivered to the basolateral surface of polarized epithelial cells where they are cleaved by TACE/ADAM17. Basolateral sorting information resides in their cytoplasmic tail domains, but tail-interacting proteins required for basolateral trafficking have not been identified. Naked (NKD)1 and NKD2 are mammalian homologs of Drosophila Naked Cuticle, which negatively regulates canonical Wnt signaling by binding Dishevelled. We present evidence that NKD2, but not NKD1, binds to basolateral sorting motifs in the cytoplasmic tail of TGF alpha. Processing and cell-surface delivery of TGF alpha are accelerated in NKD2-overexpressing Madin-Darby canine kidney cells. NKD2 is myristoylated on glycine, the second residue. On expression of myristoylation-defective (G2A) NKD2, neither NKD2 nor TGF alpha appears at the basolateral plasma membrane of polarized Madin-Darby canine kidney cells; however, membrane staining for TGF alpha is restored on silencing expression of this mutant NKD2. Amphiregulin does not interact with NKD2 and retains its basolateral localization in G2A-NKD2-expressing cells, as do Na(+), K(+) ATPase alpha 1 and E-cadherin. These data identify an unexpected function for NKD2, i.e., myristoylation-dependent escort of TGF alpha to the basolateral plasma membrane of polarized epithelial cells.

Loss of ectodomain shedding due to mutations in the metalloprotease and cysteine-rich/disintegrin domains of the tumor necrosis factor-alpha converting enzyme (TACE)

Tumor necrosis factor-alpha converting enzyme (TACE), a multidomain protease essential for development and disease, releases the ectodomains from many transmembrane proteins in a regulated fashion. To understand the mechanism underlying the regulation of TACE activity, we sought to identify the cause of ectodomain shedding deficiencies in two mutated CHO sublines designated M1 and M2. Transfection of expression vectors for human and mouse TACE restored ectodomain shedding of TNF-alpha and TGF-alpha, suggesting that defects in the TACE gene contribute to the phenotype of M1 and M2 cells. The overall levels of endogenous TACE forms in M1 cells were significantly lower than those found in their parental cells, whereas only TACE zymogen, but not its mature form, was detectable in M2 cells. Molecular analyses suggested that M1 cells contained only one expressible TACE allele encoding an M435I point mutation in the catalytic center of the protease, and M2 cells produced two TACE variants with distinct point mutations in the catalytic domain (C225Y) and the cysteinerich/disintegrin domain (C600Y). Overexpression of the C225Y and C600Y TACE by transient transfection largely compensated for maturation defects in the variants but failed to restore TNF-alpha and TGF-alpha release in the shedding-defective CHO cell lines and fibroblasts derived from TACE-null mouse embryo. Further mutagenesis and functional analyses demonstrated that Cys(600) was absolutely essential for ectodomain shedding, suggesting that Cys(600), similar to Cys(225), participates in disulfide bonding, which is critical for both the processing and catalysis of TACE.

Basolateral sorting of transforming growth factor-alpha precursor in polarized epithelial cells: characterization of cytoplasmic domain determinants

In polarized Madin-Darby canine kidney (MDCK) cells, newly synthesized transforming growth factor-alpha precursor (proTGFalpha) is directly sorted to the basolateral cell surface where it is sequentially cleaved and released into the basolateral conditioned medium (Dempsey, P.J., Coffey, R.J., J. Biol. Chem. 269 (1994) 16878-16889). In the present study, the role of the proTGFalpha cytoplasmic domain in basolateral sorting has been investigated using deletional and site-directed mutagenesis, as well as chimeric analyses of different TGFalpha constructs stably expressed in MDCK cells. The loss of polarized secretion of a proTGFalpha secretory mutant (TGFsec88) indicated that the proTGFalpha transmembrane and/or cytoplasmic domains contain essential basolateral sorting information. Using reporter chimeras with two apically sorted membrane proteins, p75 neurotrophin growth factor receptor and placental alkaline phosphatase, we show that the proTGFalpha cytoplasmic domain contains dominant basolateral sorting information. Analysis of proTGFalpha cytoplasmic domain truncation and internal deletion mutants, together with site-directed mutagenesis studies within the full-length proTGFalpha cytoplasmic domain, revealed redundant basolateral sorting motifs. Importantly, the C-terminal type I PDZ-binding motif was not required for basolateral sorting as determined by the integrity of basolateral sorting in deletion mutants lacking this motif. ProTGFalpha basolateral sorting may have important consequences for ligand presentation and spatial compartmentalization of epidermal growth factor receptor signaling networks in polarized epithelial cells.

Proteomics methods for probing molecular mechanisms in signal transduction1

mRNA splicing and various posttranslational modifications to proteins result in a larger number of proteins than genes. Assessing the dynamic nature of this proteome is the challenge of modern proteomics. Recent advances in high throughput methods greatly facilitate the analysis of proteins involved in signal transduction, their production, posttranslational modifications and interactions. Highly reproducible two dimensional polyacrylamide gel electrophoresis (2D-PAGE) methods, coupled with matrix assisted laser desorption-time of flight-mass spectrometry (MALDI-TOF-MS) allow rapid separation and identification of proteins. These methods, alone or in conjunction with other techniques such as immunoprecipitation, allow identification of various critical posttranslational modifications, such as phosphorylation. High throughput identification of important protein-protein interactions is accomplished by yeast two hybrid approaches. In vitro and in vivo pulldown assays, coupled with MALDI-TOF-MS, provide an important alternative to two hybrid approaches. Emerging advances in production of protein-based arrays promise to further increase throughput of proteomics-based approaches to signal transduction.

The position of cysteine relative to the transmembrane domain is critical for palmitoylation of H1, the major subunit of the human asialoglycoprotein receptor

The mammalian hepatic asialoglycoprotein receptor (ASGP-R) is an endocytic recycling receptor that mediates the internalization of desialylated glycoproteins and their delivery to lysosomes where they are degraded. The human ASGP-R is a hetero-oligomeric complex composed of two subunits designated H1 and H2. Both subunits are palmitoylated at the cytoplasmic Cys residues near their transmembrane domains (TMD). The cytoplasmic Cys(36) in H1 is located at a position that is five amino acids from the transmembrane junction. Because the sequences of subunits in all mammalian ASGP-R species are highly conserved especially at the region near the palmitoylated Cys, we sought to identify a recognition signal for the palmitoylation of H1. Various types of H1 mutants were created by site-directed or deletion mutagenesis including alteration of the amino acids surrounding Cys(36), replacing portions of the TMD with that of a different protein and partial deletion of the cytoplasmic domain as well as transposing the palmitoylated Cys to positions further away from the TMD. Mutant H1 cDNAs were transiently expressed in COS-7 cells, and the H1 proteins were analyzed after metabolic labeling with [(3)H]palmitate. The results indicate that neither the native amino acid sequence surrounding Cys(36) nor the majority of the cytoplasmic domain sequence is critical for palmitoylation. Palmitoylation was also not dependent on the native TMD of H1. In contrast, the attachment of palmitate was abolished if the Cys residue was transposed to a position that was 30 amino acids away from the transmembrane border. We conclude that the spacing of a Cys residue relative to the TMD in the primary protein sequence of H1 is the major determinant for successful palmitoylation.

Transmembrane transforming growth factor-alpha tethers to the PDZ domain-containing, Golgi membrane-associated protein p59/GRASP55

Transforming growth factor-alpha (TGF-alpha) and related proteins represent a family of transmembrane growth factors with representatives in flies and worms. Little is known about the transport of TGF-alpha and other transmembrane growth factors to the cell surface and its regulation. p59 was purified as a cytoplasmic protein, which at endogenous levels associates with transmembrane TGF-alpha. cDNA cloning of p59 revealed a 452 amino acid sequence with two PDZ domains. p59 is myristoylated and palmitoylated, and associates with the Golgi system, where it co-localizes with TGF-alpha. Its first PDZ domain interacts with the C-terminus of transmembrane TGF-alpha and select transmembrane proteins. p59 is the human homolog of GRASP55, which is structurally related to GRASP65. GRASP55 and GRASP65 have been shown to play a role in stacking of the Golgi cisternae in vitro. C-terminal mutations of transmembrane TGF-alpha, which decrease or abolish the interaction with p59, also strongly impair cell surface expression of TGF-alpha. Our observations suggest a role for membrane tethering of p59/GRASP55 to select transmembrane proteins, including TGF-alpha, in maturation and transport to the cell surface.

What role(s) for TGFalpha in the central nervous system?

Transforming growth factor alpha (TGFalpha) is a member of the epidermal growth factor (EGF) family with which it shares the same receptor, the EGF receptor (EGFR or erbB1). Identified since 1985 in the central nervous system (CNS), its functions in this organ have started to be determined during the past decade although numerous questions remain unanswered. TGFalpha is widely distributed in the nervous system, both glial and neuronal cells contributing to its synthesis. Although astrocytes appear as its main targets, mediating in part TGFalpha effects on different neuronal populations, results from different studies have raised the possibility for a direct action of this growth factor on neurons. A large array of experimental data have thus pointed to TGFalpha as a multifunctional factor in the CNS. This review is an attempt to present, in a comprehensive manner, the very diverse works performed in vitro and in vivo which have provided evidences for (i) an intervention of TGFalpha in the control of developmental events such as neural progenitors proliferation/cell fate choice, neuronal survival/differentiation, and neuronal control of female puberty onset, (ii) its role as a potent regulator of astroglial metabolism including astrocytic reactivity, (iii) its neuroprotective potential, and (iv) its participation to neuropathological processes as exemplified by astroglial neoplasia. In addition, informations regarding the complex modes of TGFalpha action at the molecular level are provided, and its place within the large EGF family is precised with regard to the potential interactions and substitutions which may take place between TGFalpha and its kindred.

Unique carboxyl-terminal sequences of wild type and alternatively spliced variant forms of transforming growth factor-alpha precursors mediate specific interactions with ErbB4 and ErbB2

We have previously reported that the human transforming growth factor-alpha (TGF-alpha) gene encodes three forms of TGF-alpha precursors, designated wild type (WT), variant I (VaI), and variant II (VaII), derived from alternative splicing. The two carboxyl-terminal valine residues of WT are replaced by 5 (GCRLY) or 4 (ATLG) amino acids in VaI or VaII, respectively. When overexpressed in Chinese hamster ovary (CHO) cells, VaI and ValI, but not WT, support autonomous growth. We detected tyrosine phosphorylation of ErbB2 in the absence of serum, in CHO cells expressing WT, VaI, or VaII, but not in mock transfectants. These observations prompted us to investigate possible interactions between the ErbBs and the TGF-alpha precursors in CHO cells. All TGF-alpha precursors were found to co-immunoprecipitate with the ErbBs, but with different specificity. WT co-immunoprecipitated with ErbB4, but not with ErbB1, ErbB2, or ErbB3. VaI and VaII co-immunoprecipitated with ErbB2, but not with ErbB1, ErbB3, or ErbB4. Confocal fluorescent microscopy analysis demonstrated that WT, VaI, and VaII all distribute equally to the cell surface while, as expected, a WT mutant lacking the two C-terminal valine residues does not. Point and deletion mutants involving the unique carboxyl-terminal residues of WT, VaI and VaII, indicated that the interactions between the three TGF-alpha precursors and the ErbBs were mediated by their carboxyl-terminal regions, which constitute distinct protein-binding motifs. A chimera of the intracellular domain of WT TGF-alpha linked to exogenous transmembrane and extracellular domains retained both the cell surface distribution and the specific interaction with ErbB4 of full-length WT, confirming that this interaction is mediated by the C-terminus of the TGF-alpha precursor. While interactions of WT and variant TGF-alpha with the ErbBs all result in ErbB2 activation, they produce different biological consequences, suggesting that the various TGF-alpha precursors differentially modulate ErbB signaling.

Acylation of myelin Po protein is required for adhesion

The extracellular domains of myelin Po protein interact homophilically and hence hold myelin compact at the intraperiod line. The cytoplasmic domain of Po, however, can also affect the interactions of its extracellular sequences. Po is acylated, mostly with palmitic acid, at Cys 153, just at the transmembrane:cytoplasmic domain interface. Here we show that Po mutated at Cys 153 to alanine (C153A), is not acylated and is not adhesive. Like wild-type Po, C153A Po clusters within the membrane and seems to interact with the cytoskeleton. On the other hand, the rate of turnover of C153A Po in transfected Chinese hamster ovary cells is almost 4 times faster than wild-type Po. The increased instability of C153A Po compared to wild-type Po may account for its loss of adhesion.

Participation of macrophages in glomerular sclerosis through the expression and activation of matrix metalloproteinases

In order to investigate the role of macrophages in glomeruli in the progression of glomerular sclerosis, methyl-cellulose (MC) was administered intraperitoneally to Wistar rats, in addition to intravenous injection of anti-thy1-1 antibody. In this group of rats (Thy-1 + MC group), many macrophages infiltrated in the lytic mesangium accompanied by rupture of capillary loops at an early stage and stayed with abundant deposition of mesangial matrices until day 35, whereas the proliferative lesions following mesangiolysis almost vanished in the rats treated with anti-thy1-1 antibody alone (Thy-1 group). In immunostaining, matrix metalloproteinase (MMP)-9 was expressed along regenerating capillaries of the Thy-1 group and in extracapillary lesions of the Thy-1 + MC group after day 7. In gelatin zymography, the gelatinolytic band for MMP-9 was expressed much more strongly in the Thy-1 + MC group than in the Thy-1 group at day 3, but it was expressed a little more strongly in the Thy-1 group than in the Thy-1 + MC group at day 7. The bands for an active form of MMP-2 were more strongly expressed in the Thy-1 + MC group than in the Thy-1 group throughout the experimental period. These results suggest that persistent accumulation of macrophages in mesangium induces glomerular sclerosis through expression and activation of MMP.

Transforming growth factor-alpha (TGFA): genomic structure, boundary sequences, and mutation analysis in nonsyndromic cleft lip/palate and cleft palate only

Transforming growth factor-alpha (TGFA) has been proposed as a candidate gene in the etiology of nonsyndromic cleft lip with or without cleft palate (NS-CL/P) and of nonsyndromic cleft palate only (NS-CPO). Biologic support for a role of TGFA arises from its presence at high levels in the epithelial tissue of the medial edge of the palatal shelves at the time of shelf fusion in mice. Genetic support for the role of TGFA in clefting comes from the reported association of TGFA alleles with human NS-CPO and NS-CL/P. In this study we report the sequence and structure of human genomic TGFA and the search for causal TGFA mutations in 250 individuals with NS-CL/P or NS-CPO by conformational analysis of the coding sequence, splice junctions, and a portion of the 3' untranslated region strongly homologous between human and mouse. We confirm that human TGFA is composed of six exons and here report several new sequence substitutions and their frequencies. Five variants in conserved segments may represent rare causes for clefting in humans and provide support for the role of TGFA in facial morphogenesis.

Human keratinocytes and tumor-derived cell lines express alternatively spliced forms of transforming growth factor-alpha mRNA, encoding precursors lacking carboxyl-terminal valine residues

The human transforming growth factor-alpha (TGF-alpha) gene is thought to contain five introns and six exons, encoding a transmembrane precursor (proTGF-alpha) from which the mature polypeptide is released by proteolytic cleavage. We identified a novel 32-nucleotide exon (exon alpha) within intron 5 and an alternative splice acceptor site in exon 6, splitting exon 6 into two segments: 6A and 6B. Therefore, in addition to wild type (wt) proTGF-alpha mRNA, which skips exon alpha, two novel proTGF-alpha variants are produced: Variant I (VaI), skipping exons alpha and 6A, and Variant II (VaII) which includes exon alpha and skips exon 6A. The only significant difference between variant and wt proTGF-alpha proteins is that the two wt carboxyl-terminal valines are replaced in the variants by five or four other amino acids, respectively. Both variant TGF-alpha mRNAs were readily detected in human keratinocytes and tumor-derived cell lines. Their protein products were cleaved as efficiently as wt TGF-alpha in response to the calcium ionophore A23187. However, both variants (but not wt) reduced serum requirements for proliferation in CHO cells. In addition, VaII-expressing CHO cells (not VaI or wt) formed foci in monolayer cultures. These results suggest that variant TGF-alpha precursors induce autonomous growth.

Broad antitumor and antiangiogenic activities of AG3340, a potent and selective MMP inhibitor undergoing advanced oncology clinical trials

We studied AG3340, a potent metalloproteinase (MMP) inhibitor with pM affinities for inhibiting gelatinases (MMP-2 and -9), MT-MMP-1 (MMP-14), and collagenase-3 (MMP-13) in many tumor models. AG3340 produced dose-dependent pharmacokinetics and was well tolerated after intraperitoneal (i.p.) and oral dosing in mice. Across human tumor models, AG3340 produced profound tumor growth delays when dosing began early or late after tumor implantation, although all established tumor types did not respond to AG3340. A dose-response relationship was explored in three models: COLO-320DM colon, MV522 lung, and MDA-MB-435 breast. Dose-dependent inhibitions of tumor growth (over 12.5-200 mg/kg given twice daily, b.i.d.) were observed in the colon and lung models; and in a third (breast), maximal inhibitions were produced by the lowest dose of AG3340 (50 mg/kg, b.i.d.) that was tested. In another model, AG3340 (100 mg/kg, once daily, i.p.) markedly inhibited U87 glioma growth and increased animal survival. AG3340 also inhibited tumor growth and increased the survival of nude mice bearing androgen-independent PC-3 prostatic tumors. In a sixth model, KKLS gastric, AG3340 did not inhibit tumor growth but potentiated the efficacy of Taxol. Importantly, AG3340 markedly decreased tumor angiogenesis (as assessed by CD-31 staining) and cell proliferation (as assessed by bromodeoxyuridine incorporation), and increased tumor necrosis and apoptosis (as assessed by hematoxylin and eosin and TUNEL staining). These effects were model dependent, but angiogenesis was commonly inhibited. AG3340 had a superior therapeutic index to the cytotoxic agents, carboplatin and Taxol, in the MV522 lung cancer model. In combination, AG3340 enhanced the efficacy of these cytotoxic agents without altering drug tolerance. Additionally, AG3340 decreased the number of murine melanoma (B16-F10) lesions arising in the lung in an intravenous metastasis model when given in combination with carboplatin or Taxol. These studies directly support the use of AG3340 in front-line combination chemotherapy in ongoing clinical trials in patients with advanced malignancies of the lung and prostate.

Matrix metalloproteinase inhibition. From the Jurassic to the third millennium

A brief historical introduction to the matrix metalloproteinase (MMP) field, which began in 1962, is followed by an overview of the inhibition of these proteases by natural inhibitors such as alpha 2 macroglobulin and the TIMPs (tissue inhibitors of metalloproteinases) and by synthetic inhibitors, which are largely chelating agents. The latter include thiol, alkylcarbonyl, phosponamidate and hydroxamate compounds, as well as the tetracyclines. A review of the most recent progress concludes with prognostications as to where the field may be going next.

Palmitoyl-protein thioesterase, an enzyme implicated in neurodegeneration, is localized in neurons and is developmentally regulated in rat brain

Palmitoyl-protein thioesterase (PPT) is an enzyme involved in cleavage of palmitate residues from acylated proteins. Mutations in PPT gene cause a severe neurodegenerative disorder, infantile neuronal ceroid-lipofuscinosis (INCL), characterized by loss of cortical neurons. In order to clarify the role of PPT and palmitoylation/depalmitoylation in the development of CNS and pathogenesis of infantile neuronal ceroid-lipofuscinosis (INCL), we studied the localization and expression of PPT in developing rats. Using immunohistochemical methods, we show for the first time that PPT is truly localized in neurons. Further, using RT-PCR and Western blotting, we show that expression of PPT in rat brain is developmentally regulated, with increasing expression during the maturation of CNS, reaching the maximum in young adulthood. The presented data support the view of PPT being essential for both development and maintenance of cortical neurons.

A role for a PDZ protein in the early secretory pathway for the targeting of proTGF-alpha to the cell surface

In general, plasma membrane integral proteins, such as the membrane-anchored growth factor proTGF-alpha, are assumed to be transported to the cell surface via a nonregulated, constitutive pathway. proTGF-alpha C-terminal mutants are retained in an early secretory compartment. Here, using a two-hybrid screen, we identify two TACIPs (proTGF-alpha cytoplasmic domain-interacting proteins) that contain PDZ domains and do not interact with proTGF-alpha C-terminal mutants. The binding specificity of one of them, TACIP18 (previously identified and named Syntenin or mda-9), coincides with that of the component that possibly mediates the normal trafficking of proTGF-alpha. TACIP18 colocalizes and interacts specifically with immature, intracellular forms of proTGF-alpha. Therefore, it appears that the interaction of TACIP18 with proTGF-alpha in the early secretory pathway is necessary for the targeting of the latter to the cell surface.

Evidence of juxtacrine signaling for transforming growth factor alpha in human endometrium

To determine the mechanism of signaling for transforming growth factor alpha (TGFalpha) in human endometrium, uterine luminal fluid proteins were retrieved by lavage followed by collection of the adjacent endometrium at hysterectomy. In the endometrium we observed the presence of the full-length transmembrane TGFalpha protein and the phosphorylation of its only known receptor, the epidermal growth factor receptor (EGFR), by immunoprecipitation-Western blot; TGFalpha mRNA via reverse transcription-polymerase chain reaction; and immunolocalization of TGFalpha to the surface endometrium adjacent to the uterine lumen. Despite this demonstration of TGFalpha in functional endometrium, we could not detect measurable amounts of TGFalpha in any of the 16 endometrial washings by either immunoprecipitation-Western blot or by ELISA. Recovery rate for intraluminal fluid spiked with TGFalpha control peptide was 93.4-97%. The inability to detect TGFalpha in intraluminal fluid despite its high concentration in cells directly adjacent to the uterine lumen, along with the absence of any cleaved TGFalpha species identified in the endometrium, suggests that TGFalpha signals its receptor as a transmembrane ligand. Since the EGFR is present in the endometrium and on the surface of embryos, these data are consistent with a juxtacrine mode of signaling for TGFalpha between endometrial cells, and between the luminal surface epithelium and preimplantation embryos.

Measles virus fusion protein is palmitoylated on transmembrane-intracytoplasmic cysteine residues which participate in cell fusion

[3H]palmitic acid was metabolically incorporated into the viral fusion protein (F) of Edmonston or freshly isolated measles virus (MV) during infection of human lymphoid or Vero cells. The uncleaved precursor F0 and the F1 subunit from infected cells and extracellular virus were both labeled, indicating that palmitoylation can take place prior to F0 cleavage and that palmitoylated F protein was incorporated into virus particles. [3H]palmitic acid was released from F protein upon hydroxylamine or dithiothreitol treatment, indicating a thioester linkage. In cells transfected with the cloned MV F gene, in which the cysteines located in the intracytoplasmic and transmembrane domains (Cys 506, 518, 519, 520, and 524) were replaced by serine, a major reduction of [3H]palmitic acid incorporation was observed for F mutated at Cys 506 and, to a lesser extent, at Cys 518 and Cys 524. We also observed incorporation of [3H]palmitic acid in the F1 subunit of canine distemper virus F protein. Cell fusion induced by cotransfection of cells with MV F and H (hemagglutinin) genes was significantly reduced after replacement of Cys 506 or Cys 519 with serine in the MV F gene. Transfection with the F gene with a mutation for Cys 518 abolished cell fusion, although less mutant protein was detected on the cell surface. These results suggest that the F protein transmembrane domain cysteines 506 and 518 participate in structures involved in cell fusion, possibly mediated by palmitoylation.

Transforming growth factor alpha up-regulates desmin expression during embryonic mouse tongue myogenesis

Myogenesis is determined by a set of myogenic differentiation factors that are, in turn, regulated by a number of peptide growth factors. During embryonic mouse tongue formation, transforming growth factor alpha (TGF alpha), epidermal growth factor (EGF), and their cognate receptor (EGFR) are co-expressed spatially and temporally with desmin, a muscle-specific structural protein. This investigation tested the hypothesis that TGF alpha directly regulates the myogenic program in developing tongue myoblasts. Mandibular processes from the first branchial arch of embryonic day 10.5 (E10.5) mouse embryos were microdissected and explanted into an organ culture system using serumless chemically defined medium. Exogenous TGF alpha at 10 and 20 ng/ml specifically increased the amount of desmin expression and the number of desmin-positive cells without affecting the general growth and development of the mandibles. This inductive response was detected as early as 2 days after treatment and sustained up to 9 days in culture. EGFR antisense oligonucleotides (30 microM) as well as tyrphostin (80 microM) were able to negate TGF alpha-induced up-regulation of desmin expression. These data indicate that autocrine and/or paracrine action of TGF alpha promotes tongue myogenesis, and that this action is mediated through functional kinase activity of the EGFR. We speculate that the myogenic program in the developing mouse tongue is dependent upon growth factor mediated cell-cell communication of mesenchymal cells originating from the occipital somites and ectomesenchymal cells originating from the cranial neural crest.

Transmembrane neuregulins interact with LIM kinase 1, a cytoplasmic protein kinase implicated in development of visuospatial cognition

The neuregulins are receptor tyrosine kinase ligands that play a critical role in the development of the heart, nervous system, and breast. Unlike many extracellular signaling molecules, such as the neurotrophins, most neuregulins are synthesized as transmembrane proteins. To determine the functions of the highly conserved neuregulin cytoplasmic tail, a yeast two-hybrid screen was performed to identify proteins that interact with the 157-amino acid sequence common to the cytoplasmic tails of all transmembrane neuregulin isoforms. This screen revealed that the neuregulin cytoplasmic tail interacts with the LIM domain region of the nonreceptor protein kinase LIM kinase 1 (LIMK1). Interaction between the neuregulin cytoplasmic tail and full-length LIMK1 was demonstrated by in vitro binding and co-immunoprecipitation assays. Transmembrane neuregulins with each of the three known neuregulin cytoplasmic tail isoforms interacted with LIMK1. In contrast, the cytoplasmic tail of TGF-alpha did not interact with LIMK1. In vivo, neuregulin and LIMK1 are co-localized at the neuromuscular synapse, suggesting that LIMK1, like neuregulin, may play a role in synapse formation and maintenance. To our knowledge, LIMK1 is the first identified protein shown to interact with the cytoplasmic tail of a receptor tyrosine kinase ligand.

Vac8p, a vacuolar protein with armadillo repeats, functions in both vacuole inheritance and protein targeting from the cytoplasm to vacuole

During each cell cycle, the yeast vacuole actively partitions between mother and daughter cells. This process requires actin, profilin, an unconventional myosin (Myo2p), and Vac8p. A mutant yeast strain, vac8, is defective in vacuole inheritance, specifically, in early vacuole migration. Vac8p is a 64-kD protein found on the vacuole membrane, a site consistent with its role in vacuole inheritance. Both myristoylation and palmitoylation are required for complete Vac8p localization. Interestingly, whereas myristoylation of Vac8p is not required for vacuole inheritance, palmitoylation is essential. Thus, palmitoylation appears to play a more direct role in vacuole inheritance. Most of the VAC8 sequence encodes 11 armadillo (Arm) repeats. Arm repeats are thought to mediate protein-protein interactions, and many Arm proteins have multiple functions. This is also true for Vac8p. In addition to its role in early vacuole inheritance, Vac8p is required to target aminopeptidase I from the cytoplasm to the vacuole. Mutant analysis demonstrates that Vac8p functions separately in these two processes. Vac8p cosediments with actin filaments. Vac8p is related to beta-catenin and plakoglobin, which connect a specific region of the plasma membrane to the actin cytoskeleton. In analogy, Vac8p may link the vacuole to actin during vacuole partitioning.

Specificity within the EGF family/ErbB receptor family signaling network

Recent years have witnessed tremendous growth in the epidermal growth factor (EGF) family of peptide growth factors and the ErbB family of tyrosine kinases, the receptors for these factors. Accompanying this growth has been an increased appreciation for the roles these molecules play in tumorigenesis and in regulating cell proliferation and differentiation during development. Consequently, a significant question has been how diverse biological responses are specified by these hormones and receptors. Here we discuss several characteristics of hormone-receptor interactions and receptor coupling that contribute to specificity: 1) a single EGF family hormone can bind multiple receptors; 2) a single ErbB family receptor can bind multiple hormones; 3) there are three distinct functional groups of EGF family hormones; 4) EGF family hormones can activate receptors in trans, and this heterodimerization diversifies biological responses; 5) ErbB3 requires a receptor partner for signaling; and 6) ErbB family receptors differentially couple to signaling pathways and biological responses.

The carboxyl-terminal valine residues of proTGF alpha are required for its efficient maturation and intracellular routing

Soluble forms of transforming growth factor-alpha (TGF alpha) are derived by proteolytic processing of an integral membrane glycoprotein precursor (pro TGF alpha). Previous studies indicated that phorbol ester-induced cleavage of pro TGF alpha in CHO cells is dependent on the presence of a valine residue located at the carboxyl terminus of the precursor's cytoplasmic domain. We reassessed this requirement with epitope-tagged constructs introduced into transformed rat liver epithelial cells that normally express and process TGF alpha. We found that pro TGF alpha mutants lacking the terminal valine residues showed greatly reduced maturation to the fully glycosylated form. Additionally, they were present at substantially reduced levels on the cell surface and, instead, accumulated in the endoplasmic reticulum. Consistent with these results, enzyme-linked immunosorbent assay (ELISA) and Western blot analyses revealed little or no soluble TGF alpha in medium conditioned by cells expressing the mutant constructs. Finally, a truncated pro TGF alpha mutant lacking most of the cytoplasmic domain but retaining a carboxyl-terminal valine was processed and cleaved in a near-normal manner. These results, some of which were reproduced in CHO cells, indicate that the predominant effect of the carboxyl-terminal valines is to ensure normal maturation and routing of the precursor.

Characterization of posttranslational modifications of human A33 antigen, a novel palmitoylated surface glycoprotein of human gastrointestinal epithelium

Monoclonal antibody (mAb) A33 recognizes a differentiation antigen (A33) expressed in normal human gastrointestinal epithelium and in 95% of human colon cancers. Murine mAb A33 shows specific targeting of colon cancer in humans and a humanized A33 antibody is currently being evaluated in the clinic. The cDNA for the human A33 antigen has recently been cloned, and sequence comparison indicated that the A33 antigen is a novel human cell surface molecule of the immunoglobulin superfamily. Because mAb A33 recognizes a conformational epitope, only a partial characterization of the A33 antigen has been carried out to date. In this report we show that the A33 antigen is (I) N-glycosylated, containing approximately 8 K of N-linked carbohydrate and there is no evidence for O-glycosylation, sialylation or glycophosphatidylinositol, and (ii) S-acylated in vitro, incorporating [3H] palmitic acid linked through a hydroxylamine-sensitive thioester bond. The S-palmitoylation may be involved in regulating the internalization process initiated by binding of mAb A33 to cell surface A33 antigen.

Membrane-type-2 matrix metalloproteinase can initiate the processing of progelatinase A and is regulated by the tissue inhibitors of metalloproteinases

Membrane-type-1 matrix metalloproteinase has been identified as an activator of the matrix metalloproteinase progelatinase A at cell surfaces. We report here that a soluble active form of membrane-type-2 matrix metalloproteinase can also process progelatinase A in a comparable fashion to the type-1 at rates which are dependent on the concentration of the proenzyme. Activation is inhibited by tissue inhibitors of metalloproteinases TIMP-2 and TIMP-3, but only partially by TIMP-1. These results suggest that cellular activation of progelatinase A may be initiated by different members of the membrane-type matrix metalloproteinase family depending on tissue distribution.