The chum salmon IGF-II gene promoter is activated by hepatocyte nuclear factor 3beta

The molecular events whereby gastrin occupancy of G/CCK(B) receptors leads to phosphatidylinositol (PI) 3-kinase activation have been examined. We report here that this peptide promotes the association between two non-receptor tyrosine kinases, p60Src and p125FAK, and elicits a parallel increase in tyrosine phosphorylation and activity of both kinases. Gastrin-induced PI 3-kinase activity was coprecipitated with p60Src and p125FAK and was inhibited by herbimycin A, the selective Src inhibitor PP-2 or cytochalasin D, which disrupts the actin cytoskeleton and prevents p125FAK activity. These results indicate, for the first time, that a p60Src/p125FAK complex acts upstream of the gastrin-stimulated PI 3-kinase pathway.     

Vascular-specific expression of the bean GRP 1.8 gene is negatively regulated.

In French bean, the glycine-rich cell wall protein GRP 1.8 is specifically synthesized in the vascular tissue. To identify cis-acting sequences required for cell type-specific synthesis of GRP 1.8, expression patterns of fusion gene constructs were analyzed in transgenic tobacco. In these constructs, the uidA (beta-glucuronidase) gene was placed under control of 5' upstream deletions as well as internal deletions of the GRP 1.8 promoter. Four different cis-acting regulatory regions, SE1 and SE2 (stem elements), a negative regulatory element, and a root-specific element, were found to control the tissue-specific expression. Deletion of the negative regulatory element resulted in expression of the uidA gene in cell types other than vascular cells. The SE1 region was essential for expression in several cell types in the absence of further upstream regulatory sequences. Full-length promoters having insertions between the negative regulatory element and SE1 strongly expressed the gene in nonvascular cell types in stems and leaves. Thus, vascular-specific expression of the GRP 1.8 promoter is controlled by a complex set of positive and negative interactions between cis-acting regulatory regions. The disturbance of these interactions results in expression in additional cell types.     

Pleiotrophin gene expression is highly restricted and is regulated by platelet-derived growth factor.

Pleiotrophin (PTN) is a growth and neurite extension promoting polypeptide which is highly expressed in brain and in tissues derived from mesenchyme. The PTN gene is developmentally regulated and is closely related to the MK and RI-HB genes, both of which are developmentally regulated and induced by retinoic acid. We now have screened 17 cell lines and report that expression of the PTN gene in these cells is restricted to embryo fibroblasts and intestinal smooth muscle cells. However, NIH 3T3 cells stimulated by the platelet-derived growth factor (PDGF) express a marked increase in levels of PTN mRNA whereas retinoic acid failed to increase levels of PTN mRNA in NIH 3T3 cells or in F9 embryonal carcinoma cells within 72 hours of exposure. The results suggest that expression of the PTN gene is highly restricted and that the PTN gene is a new member of the PDGF-induced cytokine family.     

Annexin A3 expression increases in hepatocytes and is regulated by hepatocyte growth factor in rat liver regeneration

Annexin (Anx) A3 increases and plays important roles in the signalling cascade in hepatocyte growth in cultured hepatocytes. However, no information is available on its expression and role in rat liver regeneration. In the present study, AnxA3 expression was investigated to determine whether it also plays a role in the signalling cascade in rat liver regeneration. AnxA3 protein and mRNA level both increase in liver after administration of carbon tetrachloride (CCl4) or 70% partial hepatectomy. AnxA3 protein level increases in isolated parenchymal hepatocytes, but not in non-parenchymal liver cells, in these rat liver regeneration models. AnxA3 mRNA increases in hepatocytes after CCl4 administration. Anti-hepatocyte growth factor antibody suppresses this increase in AnxA3 mRNA level. These results demonstrate that AnxA3 expression increases in hepatocytes through a hepatocyte growth factor-mediated pathway in rat liver regeneration models, suggesting that AnxA3 plays an important role in the signalling cascade in rat liver regeneration.     

MEKK4 stimulation of p38 and JNK activity is negatively regulated by GSK3beta

The MAPK kinase kinase MEKK4 is required for neurulation and skeletal patterning during mouse development. MEKK4 phosphorylates and activates MKK4/MKK7 and MKK3/MKK6 leading to the activation of JNK and p38, respectively. MEKK4 is believed to be auto-inhibited, and its interaction with other proteins controls its dimerization and activation. TRAF4, GADD45, and Axin each bind and activate MEKK4, with TRAF4 and Axin binding to the kinase domain and GADD45 binding within the N-terminal regulatory domain. Here we show that similar to the interaction with TRAF4 and Axin, the kinase domain of MEKK4 interacts with the multifunctional serine/threonine kinase GSK3beta. GSK3beta binding to MEKK4 blocks MEKK4 dimerization that is required for MEKK4 activation, effectively inhibiting MEKK4 stimulation of the JNK and p38 MAPK pathways. Inhibition of GSK3beta kinase activity with SB216763 results in enhanced MEKK4 kinase activity and increased JNK and p38 activation, indicating that an active state of GSK3beta is required for binding and inhibition of MEKK4 dimerization. Furthermore, GSK3beta phosphorylates specific serines and threonines in the N terminus of MEKK4. Together, these findings demonstrate that GSK3beta binds to the kinase domain of MEKK4 and regulates MEKK4 dimerization. However, unlike TRAF4, Axin, and GADD45, GSK3beta inhibits MEKK4 activity and prevents its activation of JNK and p38. Thus, control of MEKK4 dimerization is regulated both positively and negatively by its interaction with specific proteins.     

Interferon regulatory factor 7 is negatively regulated by the Epstein-Barr virus immediate-early gene, BZLF-1

Virus infection stimulates potent antiviral responses; specifically, Epstein-Barr virus (EBV) infection induces and activates interferon regulatory factor 7 (IRF-7), which is essential for production of alpha/beta interferons (IFN-alpha/beta) and upregulates expression of Tap-2. Here we present evidence that during cytolytic viral replication the immediate-early EBV protein BZLF-1 counteracts effects of IRF-7 that are central to host antiviral responses. We initiated these studies by examining IRF-7 protein expression in vivo in lesions of hairy leukoplakia (HLP) in which there is abundant EBV replication but the expected inflammatory infiltrate is absent. This absence might predict that factors involved in the antiviral response are absent or inactive. First, we detected significant levels of IRF-7 in the nucleus, as well as in the cytoplasm, of cells in HLP lesions. IRF-7 activity in cell lines during cytolytic viral replication was examined by assay of the IRF-7-responsive promoters, IFN-alpha4, IFN-beta, and Tap-2, as well as of an IFN-stimulated response element (ISRE)-containing reporter construct. These reporter constructs showed consistent reduction of activity during lytic replication. Both endogenous and transiently expressed IRF-7 and EBV BZLF-1 proteins physically associate in cell culture, although BZLF-1 had no effect on the nuclear localization of IRF-7. However, IRF-7-dependent activity of the IFN-alpha4, IFN-beta, and Tap-2 promoters, as well as an ISRE promoter construct, was inhibited by BZLF-1. This inhibition occurred in the absence of other EBV proteins and was independent of IFN signaling. Expression of BZLF-1 also inhibited activation of IRF-7 by double-stranded RNA, as well as the activity of a constitutively active mutant form of IRF-7. Negative regulation of IRF-7 by BZLF-1 required the activation domain but not the DNA-binding domain of BZLF-1. Thus, EBV may subvert cellular antiviral responses and immune detection by blocking the activation of IFN-alpha4, IFN-beta, and Tap-2 by IRF-7 through the medium of BZLF-1 as a negative regulator.     

Akt is negatively regulated by the MULAN E3 ligase

The serine/threonine kinase Akt functions in multiple cellular processes, including cell survival and tumor development. Studies of the mechanisms that negatively regulate Akt have focused on dephosphorylation-mediated inactivation. In this study, we identified a negative regulator of Akt, MULAN, which possesses both a RING finger domain and E3 ubiquitin ligase activity. Akt was found to directly interact with MULAN and to be ubiquitinated by MULAN in vitro and in vivo. Other molecular assays demonstrated that phosphorylated Akt is a substantive target for both interaction with MULAN and ubiquitination by MULAN. The results of the functional studies suggest that the degradation of Akt by MULAN suppresses cell proliferation and viability. These data provide insight into the Akt ubiquitination signaling network.     

A novel gene expression pathway regulated by nuclear phosphoinositides

Star-PAP is a recently identified nuclear speckle localized non-canonical poly(A) polymerase that has a functional interaction with PIPKIα, and whose activity is modulated by the PIPKIα product, PI4,5P₂. Similar to other poly(A) polymerases, such as the canonical PAPα and the non-canonical GLD2 PAP, Star-PAP resides in a large complex of proteins involved in the 3′ end formation of mRNAs (Fig. 4). The Star-PAP complex shares components with the canonical PAPα complex though it contains unique associated proteins such as PIPKIα and CKIα. The Star-PAP complex assembles into a highly stable 3′ end processing machine upon oxidative stress induction. This assembled complex shows enhanced enzyme activity and hypersensitivity to exogenous PI4,5P₂, implying that an activated Star-PAP is distinctly modified and/or contains unique factors as compared to Star-PAP purified from resting cells.     

Expression of nuclear transport importins beta 1 and beta 3 is regulated during rodent spermatogenesis

Spermatogenic differentiation requires progressive gene expression changes, and proteins required for this must be transported into the nucleus. Many of these contain a nuclear localization signal and are likely to be transported by importin protein family members, each of which recognizes and transports distinct cargo proteins. We hypothesized that importins, as modulators of protein nuclear access, would display distinct expression profiles during spermatogenesis, indicating their potential to regulate key steps in cellular differentiation. This was tested throughout testicular development in rodents. Real-time PCR analysis of postnatal mouse testes revealed changing expression levels of Knpb1 (encoding importin beta 1) and Ranbp5 (encoding beta 3) mRNAs, with Knpb1 highest at 26 days postpartum and Ranbp5 highest in Day 26 and adult testis. Their distinctive cellular expression patterns visualized using in situ hybridization and immunohistochemistry were identical in mouse and rat testes where examined. Within the seminiferous epithelium, Knpb1 mRNA and importin beta1 protein were detected within mitotic Sertoli and germ cells during fetal and early postnatal development, becoming restricted to spermatogonia and spermatocytes in adulthood. Importin beta 3 protein in fetal germ cells displayed a striking difference in intracellular localization between male and female gonads. In adult testes, Ranbp5 mRNA was detected in round spermatids and importin beta 3 protein in elongating spermatids. This is the first comprehensive in situ demonstration of developmentally regulated synthesis of nuclear transport components. The contrasting expression patterns of importins beta 1 and 3 identify them as candidates for regulating nuclear access of factors required for developmental switches.