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Smad proteins play a key role in the intracellular signaling of the transforming growth factor beta (TGF-beta) superfamily of extracellular polypeptides that initiate signaling to regulate a wide variety of biological processes. The inhibitory Smad, Smad7, has been shown to function as intracellular antagonists of TGF-beta family signaling and is upregulated in several cancers. To determine the effect of Smad7-mediated blockade of TGF-beta signaling, we have stably expressed Smad7 in a TGF-beta-sensitive, well-differentiated, and non-tumorigenic cell line, FET, that was derived from human colon adenocarcinoma. Smad7 inhibits TGF-beta-induced transcriptional responses by blocking complex formation between Smad 2/3 and Smad4. While Smad7 has no effect on TGF-beta-induced activation of p38 MAPK and ERK, it blocks the phosphorylation of Akt by TGF-beta and enhances TGF-beta-induced phosphorylation of c-Jun. FET cells expressing Smad7 show anchorage-independent growth and enhance tumorigenicity in athymic nude mice. Smad7 blocks TGF-beta-induced growth inhibition by preventing TGF-beta-induced G1 arrest. Smad7 inhibits TGF-beta-mediated downregulation of c-Myc, CDK4, and Cyclin D1, and suppresses the expression of p21(Cip1). As a result, Smad7 inhibits TGF-beta-mediated downregulation of Rb phosphorylation. Furthermore, Smad7 inhibits the apoptosis of these cells. Together, Smad7 may increase the tumorigenicity of FET cells by blocking TGF-beta-induced growth inhibition and by inhibiting apoptosis. Thus, this study provides a mechanism by which a portion of human colorectal tumors may become refractory to tumor-suppressive actions of TGF-beta that might result in increased tumorigenicity.
Coronaviruses encode the largest replicase polyprotein of any known positive-strand RNA virus. Replicase protein precursors and mature products are thought to mediate the formation and function of viral replication complexes on the surfaces of intracellular double-membrane vesicles. However, the functions of only a few of these proteins are known. For the coronavirus mouse hepatitis virus (MHV), the first proteolytic processing event of the replicase polyprotein liberates an amino-terminal 28-kDa product (p28). While previous biochemical studies have suggested that p28 is associated with viral replication complexes, the intracellular localization and interactions of p28 with other proteins during the course of MHV replication have not been defined. We used immunofluorescence confocal microscopy to show that p28 localizes to viral replication complexes in the cytoplasm during early times postinfection. However, at late times postinfection, p28 localizes to sites of M accumulation distinct from the replication complex. Furthermore, by yeast two-hybrid and coimmunoprecipitation analyses, we demonstrate that p28 specifically binds to p10 and p15, two coronavirus replicase proteins of unknown function. Deletion mutagenesis experiments determined that the carboxy terminus of p28 is not required for its interactions with p10 and p15. These results suggest that p28 may play a part at the replication complex by interacting with p10 and p15. Moreover, our findings highlight a potential role for p28 at virion assembly sites.
Increased expression of the oncogenic transcription factor c-Myc causes unregulated cell cycle progression. c-Myc can also cause apoptosis, but it is not known whether the activation and/or repression of c-Myc target genes mediates these diverse functions of c-Myc. Because unchecked cell cycle progression leads to hyperproliferation and tumorigenesis, it is essential for tumour suppressors, such as p53 and p19ARF (ARF), to curb cell cycle progression in response to increased c-Myc (refs 2, 3). Increased c-Myc has previously been shown to induce ARF expression, which leads to cell cycle arrest or apoptosis through the activation of p53 (ref. 4). Here we show that ARF can inhibit c-Myc by a unique and direct mechanism that is independent of p53. When c-Myc increases, ARF binds with c-Myc and dramatically blocks c-Myc's ability to activate transcription and induce hyperproliferation and transformation. In contrast, c-Myc's ability to repress transcription is unaffected by ARF and c-Myc-mediated apoptosis is enhanced. These differential effects of ARF on c-Myc function suggest that separate molecular mechanisms mediate c-Myc-induced hyperproliferation and apoptosis. This direct feedback mechanism represents a p53-independent checkpoint to prevent c-Myc-mediated tumorigenesis.
A hallmark of signal transduction is the dynamic and inducible post-translational modification of proteins. In addition to the well characterized phosphorylation of proteins, other modifications have been shown to be regulatory, including O-linked beta-N-acetylglucosamine (O-GlcNAc). O-GlcNAc modifies serine and threonine residues on a myriad of nuclear and cytosolic proteins, and for several proteins there appears to be a reciprocal relationship between phosphorylation and O-GlcNAc modification. Here we report further evidence of this yin-yang relationship by demonstrating that O-GlcNAc transferase, the enzyme that adds O-GlcNAc to proteins, exists in stable and active complexes with the serine/threonine phosphatases PP1beta and PP1gamma, enzymes that remove phosphate from proteins. The existence of this complex highlights the importance of understanding the dynamic relationship between O-GlcNAc and phosphate in modulating protein function in many cellular processes and disease states such as Alzheimer's disease and type II diabetes.
BCL2 family members are subject to regulation at multiple levels, providing checks on their ability to contribute to tumorigenesis. However, findings on post-translational BCL2 phosphorylation in different systems have been difficult to integrate. Another antiapoptotic family member, MCL1, exhibits a difference in electrophoretic mobility upon phosphorylation induced by an activator of PKC (12-O-tetradecanoylphorbol 13-acetate; TPA) versus agents that act on microtubules or protein phosphatases 1/2A. A multiple pathway model is now presented, which demonstrates that MCL1 can undergo distinct phosphorylation events - mediated through separate signaling processes and involving different target sites - in cells that remain viable in the presence of TPA versus cells destined to die upon exposure to taxol or okadaic acid. Specifically, TPA induces phosphorylation at a conserved extracellular signal-regulated kinase (ERK) site in the PEST region (Thr 163) and slows turnover of the normally rapidly degraded MCL1 protein; however, okadaic acid and taxol induce ERK-independent MCL1 phosphorylation at additional discrete sites. These findings add a new dimension to our understanding of the complex regulation of antiapoptotic BCL2 family members by demonstrating that, in addition to transcriptional and post-transcriptional regulation, MCL1 is subject to multiple, separate, post-translational phosphorylation events, produced in living versus dying cells at ERK-inducible versus ERK-independent sites.
G1-specific transcriptional activation by Cln3/CDK initiates the budding yeast cell cycle. To identify targets of Cln3/CDK, we analyzed the SBF and MBF transcription factor complexes by multidimensional protein interaction technology (MudPIT). Whi5 was identified as a stably bound component of SBF but not MBF. Inactivation of Whi5 leads to premature expression of G1-specific genes and budding, whereas overexpression retards those processes. Whi5 inactivation bypasses the requirement for Cln3 both for transcriptional activation and cell cycle initiation. Whi5 associates with G1-specific promoters via SBF during early G1 phase, then dissociates coincident with transcriptional activation. Dissociation of Whi5 is promoted by Cln3 in vivo. Cln/CDK phosphorylation of Whi5 in vitro promotes its dissociation from SBF complexes. Mutation of putative CDK phosphorylation sites, at least five of which are phosphorylated in vivo, strongly reduces SBF-dependent transcription and delays cell cycle initiation. Like mammalian Rb, Whi5 is a G1-specific transcriptional repressor antagonized by CDK.
Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) is selectively expressed in islet beta cells and is a major autoantigen in a mouse model of type I diabetes. The analysis of IGRP-chloramphenicol acetyltransferase (CAT) fusion gene expression through transient transfection of islet-derived betaTC-3 cells revealed that a promoter region, located between -273 and -254, is essential for high IGRP-CAT fusion gene expression. The sequence of this promoter region does not match that for any known islet-enriched transcription factor. However, data derived from gel retardation assays, a modified ligation-mediated polymerase chain reaction in situ footprinting technique and a SDS-polyacrylamide separation/renaturation procedure led to the hypothesis that this protein might be Pax-6, a conclusion that was confirmed by gel supershift assays. Additional experiments revealed a second non-consensus Pax-6 binding site in the -306/-274 IGRP promoter region. Pax-6 binding to these elements is unusual in that it appears to require both its homeo and paired domains. Interestingly, loss of Pax-6 binding to the -273/ -246 element is compensated by Pax-6 binding to the -306/-274 element and vice versa. Gel retardation assays revealed that another islet-enriched transcription factor, namely Pdx-1, binds four non-consensus elements in the IGRP promoter. However, mutation of these elements has little effect on IGRP fusion gene expression. Although chromatin immunoprecipitation assays show that both Pax-6 and Pdx-1 bind to the IGRP promoter within intact cells, in contrast to the critical role of these factors in beta cell-specific insulin gene expression, IGRP gene transcription appears to require Pax-6 but not Pdx-1.
Activation of phosphoenolpyruvate carboxykinase (PEPCK) gene transcription in response to all-trans-retinoic acid (RA) or a glucocorticoid such as dexamethasone (Dex) requires a distinct arrangement of DNA-response elements and their cognate transcription activators on the gene promoter. Two of the accessory factor-binding elements involved in the Dex response (gAF1 and gAF3) coincide with the DNA-response elements involved in the RA response. We demonstrate here that the combination of Dex/RA has a synergistic effect on endogenous PEPCK gene expression in rat hepatocytes and H4IIE hepatoma cells. Reporter gene studies show that the gAF3 element and one of the two glucocorticoid receptor-binding elements (GR1) are most important for this effect. Chromatin immunoprecipitation assays revealed that when H4IIE cells were treated with Dex/RA, ligand-activated retinoic acid receptors (retinoic acid receptor/retinoid X receptor) and glucocorticoid receptors are recruited to this gene promoter, as are the transcription coregulators p300, CREB-binding protein, p/CIP, and SRC-1. Notably, the recruitment of p300 and RNA polymerase II to the PEPCK promoter is increased by the combined Dex/RA treatment compared with Dex or RA treatment alone. The functional importance of p300 in the Dex/RA response is illustrated by the observation that selective reduction of this coactivator, but not that of CREB-binding protein, abolishes the synergistic effect in H4IIE cells.
Epidermal growth factor (EGF) family ligands are derived by proteolytic cleavage of the ectodomains of integral membrane precursors. Previously, we established that tumor necrosis factor alpha-converting enzyme (TACE/ADAM17) is a physiologic transforming growth factor-alpha (TGF-alpha) sheddase, and we also demonstrated enhanced shedding of amphiregulin (AR) and heparin-binding (HB)-EGF upon restoration of TACE activity in TACE-deficient EC-2 fibroblasts. Here we extended these results by showing that purified soluble TACE cleaved single sites in the juxtamembrane stalks of mouse pro-HB-EGF and pro-AR ectodomains in vitro. For pro-HB-EGF, this site matched the C terminus of the purified human growth factor, and we speculate that the AR cleavage site is also physiologically relevant. In contrast, ADAM9 and -10, both implicated in HB-EGF shedding, failed to cleave the ectodomain or cleaved at a nonphysiologic site, respectively. Cotransfection of TACE in EC-2 cells enhanced phorbol myristate acetate-induced but not constitutive shedding of epiregulin and had no effect on betacellulin (BTC) processing. Additionally, soluble TACE did not cleave the juxtamembrane stalks of either pro-BTC or pro-epiregulin ectodomains in vitro. Substitution of the shorter pro-BTC juxtamembrane stalk or truncation of the pro-TGF-alpha stalk to match the pro-BTC length reduced TGF-alpha shedding from transfected cells to background levels, whereas substitution of the pro-BTC P2-P2' sequence reduced TGF-alpha shedding less dramatically. Conversely, substitution of the pro-TGF-alpha stalk or lengthening of the pro-BTC stalk, especially when combined with substitution of the pro-TGF-alpha P2-P2' sequence, markedly increased BTC shedding. These results indicate that efficient TACE cleavage is determined by a combination of stalk length and scissile bond sequence.
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.