RB regulates pancreas development by stabilizing Pdx1.

RB is a key substrate of Cdks and an important regulator of the mammalian cell cycle. RB either represses E2Fs that promote cell proliferation or enhances the activity of cell-specific factors that promote differentiation, although the mechanism that facilitates this dual interaction is unclear. Here, we demonstrate that RB associates with and stabilizes pancreatic duodenal homeobox-1 (Pdx-1) that is essential for embryonic pancreas development and adult ß-cell function. Interestingly, Pdx-1 utilizes a conserved RB-interaction motif (RIM) that is also present in E2Fs. Point mutations within the RIM reduce RB-Pdx-1 complex formation, destabilize Pdx-1 and promote its proteasomal degradation. Glucose regulates RB and Pdx-1 levels, RB/Pdx-1 complex formation and Pdx-1 degradation. RB occupies the promoters of ß-cell-specific genes, and knockdown of RB results in reduced expression of Pdx-1 and its target genes. Further, RB-deficiency in vivo results in reduced pancreas size due to decreased proliferation of Pdx-1(+) pancreatic progenitors, increased apoptosis and aberrant expression of regulators of pancreatic development. These results demonstrate an unanticipated regulatory mechanism for pancreatic development and ß-cell function, which involves RB-mediated stabilization of the pancreas-specific transcription factor Pdx-1.

Lentiviral reporter constructs for fluorescence tracking of the temporospatial pattern of Smad3 signaling.

Canonical TGF-beta is involved in cell differentiation, tissue maintenance, and wound healing, but also plays a central role in the pathogenesis of diseases such as cancer Here we describe a lentivirus-based reporter vector system expressing green fluorescent protein (GFP) or red fluorescent protein (RFP) under the control of a Smad3-responsive element (CAGA)12 that allows observation of the temporospatial pattern of endogeneous Smad3-mediated signaling on a cellular level. Use of this method will be valuable to identify cells with active Smad3 signaling and investigate the role of endogenous Smad3 signaling in complex systems such as co-cultures in vitro, or in tumors during tumor cell invasion and metastasis in vivo.

Inhibition of p38MAP kinase suppresses fibrogenic reaction in conjunctiva in mice.

PURPOSE: To examine the effects of blocking p38 mitogen-activated protein kinase (MAPK) on post-injury conjunctival scarring in mice. Its effects on the behaviors of cultured subconjunctival fibroblasts were also investigated. METHODS: An in vivo study was conducted using an adenoviral vector carrying a dominant-negative (DN)-p38MAPK gene. A circumferential incision was made in the equatorial conjunctiva by scissors in the right eye of generally anesthetized adult C57BL/6 mice. DN-p38MAPK-expressing adenoviral vector was topically applied. The left control eye received non-functioning adenoviral vector. At 2, 5, and 7 days (each, n=22) the eyes were processed for histological or immunohistochemical examination to evaluate the tissue scarring. The expressions of type-I collagen and growth factors were evaluated by real time-reverse transcriptase-polymerase chain reaction. The effects of p38MAPK inhibitor on the proliferation, migration, and fibrogenic gene/protein expression of cultured human fibroblasts were also studied. RESULTS: The in vivo DN-p38MAPK gene introduction blocked the phospho-p38 expression with reduction of myofibroblast generation and suppression of mRNA expression of connective tissue growth factor (CTGF) and monocyte/macrophage chemoattractant protein-1 (MCP-1) in the mouse-injured conjunctiva. Blocking p38MAPK signal in the fibroblasts by a chemical inhibitor counteracted TGFbeta1's enhancement of expressions of type-I collagen, fibronectin, and CTGF. It also retarded cell migration, but cell proliferation was unchanged. CONCLUSIONS: Inhibiting p38MAPK signal impairs the fibrogenic reaction induced by the subconjunctival fibroblasts in vivo and in vitro, suggesting its potential effectiveness in preventing excessive scarring following glaucoma filtering surgery.

Protection from obesity and diabetes by blockade of TGF-ß/Smad3 signaling.

Imbalances in glucose and energy homeostasis are at the core of the worldwide epidemic of obesity and diabetes. Here, we illustrate an important role of the TGF-ß/Smad3 signaling pathway in regulating glucose and energy homeostasis. Smad3-deficient mice are protected from diet-induced obesity and diabetes. Interestingly, the metabolic protection is accompanied by Smad3(-)(/-) white adipose tissue acquiring the bioenergetic and gene expression profile of brown fat/skeletal muscle. Smad3(-/-) adipocytes demonstrate a marked increase in mitochondrial biogenesis, with a corresponding increase in basal respiration, and Smad3 acts as a repressor of PGC-1α expression. We observe significant correlation between TGF-ß1 levels and adiposity in rodents and humans. Further, systemic blockade of TGF-ß signaling protects mice from obesity, diabetes, and hepatic steatosis. Together, these results demonstrate that TGF-ß signaling regulates glucose tolerance and energy homeostasis and suggest that modulation of TGF-ß activity might be an effective treatment strategy for obesity and diabetes.

Transforming growth factor-beta/Smad3 signaling regulates insulin gene transcription and pancreatic islet beta-cell function.

Pancreatic islet beta-cell dysfunction is a signature feature of Type 2 diabetes pathogenesis. Consequently, knowledge of signals that regulate beta-cell function is of immense clinical relevance. Transforming growth factor (TGF)-beta signaling plays a critical role in pancreatic development although the role of this pathway in the adult pancreas is obscure. Here, we define an important role of the TGF-beta pathway in regulation of insulin gene transcription and beta-cell function. We identify insulin as a TGF-beta target gene and show that the TGF-beta signaling effector Smad3 occupies the insulin gene promoter and represses insulin gene transcription. In contrast, Smad3 small interfering RNAs relieve insulin transcriptional repression and enhance insulin levels. Transduction of adenoviral Smad3 into primary human and non-human primate islets suppresses insulin content, whereas, dominant-negative Smad3 enhances insulin levels. Consistent with this, Smad3-deficient mice exhibit moderate hyperinsulinemia and mild hypoglycemia. Moreover, Smad3 deficiency results in improved glucose tolerance and enhanced glucose-stimulated insulin secretion in vivo. In ex vivo perifusion assays, Smad3-deficient islets exhibit improved glucose-stimulated insulin release. Interestingly, Smad3-deficient islets harbor an activated insulin-receptor signaling pathway and TGF-beta signaling regulates expression of genes involved in beta-cell function. Together, these studies emphasize TGF-beta/Smad3 signaling as an important regulator of insulin gene transcription and beta-cell function and suggest that components of the TGF-beta signaling pathway may be dysregulated in diabetes.

Inhibition of metastatic outgrowth from single dormant tumor cells by targeting the cytoskeleton.

Metastatic breast cancer may emerge from latent tumor cells that remain dormant at disseminated sites for many years. Identifying mechanisms regulating the switch from dormancy to proliferative metastatic growth has been elusive due to the lack of experimental models of tumor cell dormancy. We characterized the in vitro growth characteristics of cells that exhibit either dormant (D2.0R, MCF-7, and K7M2AS1.46) or proliferative (D2A1, MDA-MB-231, and K7M2) metastatic behavior in vivo. Although these cells proliferate readily in two-dimensional culture, we show that when grown in three-dimensional matrix, distinct growth properties of the cells were revealed that correlate to their dormant or proliferative behavior at metastatic sites in vivo. In three-dimensional culture, cells with dormant behavior in vivo remained cell cycle arrested with elevated nuclear expression of p16 and p27. The transition from quiescence to proliferation of D2A1 cells was dependent on fibronectin production and signaling through integrin beta1, leading to cytoskeletal reorganization with filamentous actin (F-actin) stress fiber formation. We show that phosphorylation of myosin light chain (MLC) by MLC kinase (MLCK) through integrin beta1 is required for actin stress fiber formation and proliferative growth. Inhibition of integrin beta1 or MLCK prevents transition from a quiescent to proliferative state in vitro. Inhibition of MLCK significantly reduces metastatic outgrowth in vivo. These studies show that the switch from dormancy to metastatic growth may be regulated, in part, through epigenetic signaling from the microenvironment, leading to changes in the cytoskeletal architecture of dormant cells. Targeting this process may provide therapeutic strategies for inhibition of the dormant-to-proliferative metastatic switch.

Role of Rho/ROCK and p38 MAP kinase pathways in transforming growth factor-beta-mediated Smad-dependent growth inhibition of human breast carcinoma cells in vivo.

TGF-beta is a multifunctional cytokine known to exert its biological effects through a variety of signaling pathways of which Smad signaling is considered to be the main mediator. At present, the Smad-independent pathways, their interactions with each other, and their roles in TGF-beta-mediated growth inhibitory effects are not well understood. To address these questions, we have utilized a human breast cancer cell line MCF10CA1h and demonstrate that p38 MAP kinase and Rho/ROCK pathways together with Smad2 and Smad3 are necessary for TGF-beta-mediated growth inhibition of this cell line. We show that Smad2/3 are indispensable for TGF-beta-mediated growth inhibition, and that both p38 and Rho/ROCK pathways affect the linker region phosphorylation of Smad2/3. Further, by using Smad3 mutated at the putative phosphorylation sites in the linker region, we demonstrate that phosphorylation at Ser203 and Ser207 residues is required for the full transactivation potential of Smad3, and that these residues are targets of the p38 and Rho/ROCK pathways. We demonstrate that activation of the p38 MAP kinase pathway is necessary for the full transcriptional activation potential of Smad2/Smad3 by TGF-beta, whereas activity of Rho/ROCK is necessary for both down-regulation of c-Myc protein and up-regulation of p21waf1 protein, directly interfering with p21waf1 transcription. Our results not only implicate Rho/ROCK and p38 MAPK pathways as necessary for TGF-beta-mediated growth inhibition, but also demonstrate their individual contributions and the basis for their cooperation with each other.

Activation of myelin genes during transdifferentiation from melanoma to glial cell phenotype.

Induction of myelin genes occurs around birth in the last stage of Schwann cells differentiation and is reactivated in case of nerve injury. Previous studies showed that activation of the gp130 receptor system, using as ligand interleukin-6 fused to its soluble receptor (IL6RIL6), causes induction of myelin genes such as myelin basic protein (MBP) and myelin protein zero (Po) in embryonic dorsal root ganglia Schwann cells. We also reported that in murine melanoma B16/F10.9 cells, IL6RIL6 causes a shut-off of melanogenesis mediated by a down-regulation of the paired-homeodomain factor Pax3. The present work demonstrates that these IL6RIL6-treated F10.9 cells undergo transdifferentiation to a myelinating glial phenotype characterized by induction of the transcriptional activities of both Po and MBP promoters and accumulation of myelin gene products. For both Po and MBP promoters, a repression by Pax3 and stimulation by Sox10 can be demonstrated. Because after IL6RIL6-treatment, Pax3 disappears from the F10.9 cells (as it does in mature myelinating Schwann cells) whereas the level of Sox10 rather increases, we modulated the relative level of these factors and show their involvement in the induction of myelin gene expression by IL6RIL6. In addition, however, we show that a C/G-rich CACC box in the Po promoter is required for activation by IL6RIL6, as well as by ectopic Sox10, and identify a Kruppel-type zinc finger factor acting through this CACC box, which stimulates Po promoter activity.

C/EBP-delta induction by gp130 signaling. Role in transition to myelin gene expressing phenotype in a melanoma cell line model.

Expression of genes encoding structural myelin proteins marks the inception of the myelinating Schwann cell (SC) phenotype. Earlier embryonic SC as well as adult non-myelinating SC produce the intermediate filament glial fibrillary acid protein (GFAP), which disappears from the myelinating SC. We previously observed that triggering of the gp130 receptor system by the IL6RIL6 ligand, comprising interleukin-6 (IL-6) fused to the soluble IL-6 receptor, induces myelin gene expression in rat embryonic dorsal root ganglia (DRG) cultures as well as in the murine melanoma cell line B16/F10.9. Study of target genes regulated by IL6RIL6 indicates a strong and selective induction of the transcriptional regulator C/EBP-delta in DRG cultures and in the F10.9 cell line. As shown here, silencing of C/EBP-delta mRNA and protein expression by introduction of small interference RNA-producing plasmids in the F10.9 cells prevented the induction of myelin protein zero (P0) and myelin basic protein (MBP) mRNAs by IL6RIL6. Doxycycline-regulated overexpression of C/EBP-delta was sufficient to induce accumulation of P0 and MBP mRNAs, the effect being selective, because C/EBP-delta did not affect several other genes strongly regulated by IL6RIL6. Interestingly, GFAP was inhibited by C/EBP-delta overexpression, leading to a modulation of the ratio between myelin gene products versus GFAP and suggesting that C/EBP-delta plays a role in the switch to a myelinating phenotype. The down-regulation of Pax3, also typical of the transition to myelinating cells, was observed after C/EBP-delta expression in correlation to P0 induction and to decrease of melanogenesis and cell growth. In cultures of dissociated cells of embryonic rat DRG, where we knocked-down the C/EBP-delta mRNA, we found an inhibition of P0 mRNA induction by IL6RIL6, showing that the role of C/EBP-delta on this myelin gene is not unique to the melanoma system.