Integrin regulation of membrane domain trafficking and Rac targeting

Integrins are crucial regulators of essential cellular processes such as gene expression, cell proliferation and migration. Alteration of these processes is central to tumourigenesis. Integrin signals mediate anchorage dependence of cell growth, while growth of cancer cells is anchorage-independent. Integrins critically regulate Rho family GTPases, that are also involved in cell-cycle progression and oncogenesis. In addition to their effect on GTP loading, integrins independently control the translocation of GTP-bound Rac to the plasma membrane. This step is essential for Rac binding to effectors. Integrins increase membrane affinity for Rac, leading to RhoGDI dissociation and effector coupling locally, in the vicinity of activated/bound integrins. Integrin-regulated Rac binding sites are within CEMMs (cholesterol-enriched membrane microdomains). Integrins control Rac signalling by preventing the internalization of its binding sites in CEMMs. Integrin regulation of signalling pathways initiated in CEMMs may be important for the spatial control of cell migration and anchorage dependence of cell growth.

Activated c-Abl is degraded by the ubiquitin-dependent proteasome pathway

C-Abl is a nonreceptor tyrosine kinase that is tightly regulated in the cell. Genetic data derived from studies in flies and mice strongly support a role for Abl kinases in the regulation of the cytoskeleton (reviewed in [1,2]). C-Abl can be activated by several stimuli, including oxidative stress [3], DNA damage [4], integrin engagement [5], growth factors, and Src family kinases [6]. Structural alterations elicit constitutive activation of the c-Abl tyrosine kinase, leading to oncogenic transformation. While the mechanisms that activate c-Abl are beginning to be elucidated, little is known regarding the mechanisms that downregulate activated c-Abl. Here, we show for the first time that activated c-Abl is downregulated by the ubiquitin-dependent degradation pathway. Activated forms of c-Abl are more unstable than wild-type and kinase-inactive forms. Moreover, inhibition of the 26S proteasome leads to increased c-Abl levels in vitro and in cells, and activated c-Abl proteins are ubiquitinated in vivo. Significantly, inhibition of the 26S proteasome in fibroblasts increases the levels of tyrosine-phosphorylated, endogenous c-Abl. Our data suggest a novel mechanism for irreversible downregulation of activated c-Abl, which is critical to prevent the deleterious consequences of c-Abl hyperactivation in mitogenic and cytoskeletal pathways.

Inhibition of human immunodeficiency virus Rev and human T-cell leukemia virus Rex function, but not Mason-Pfizer monkey virus constitutive transport element activity, by a mutant human nucleoporin targeted to Crm1

The hypothesis that the cellular protein Crm1 mediates human immunodeficiency virus type 1 (HIV-1) Rev-dependent nuclear export posits that Crm1 can directly interact both with the Rev nuclear export signal (NES) and with cellular nucleoporins. Here, we demonstrate that Crm1 is indeed able to interact with active but not defective forms of the HIV-1 Rev NES and of NESs found in other retroviral nuclear export factors. In addition, we demonstrate that Crm1 can bind the Rev NES when Rev is assembled onto the Rev response element RNA target and that Crm1, like Rev, is a nucleocytoplasmic shuttle protein. Crm1 also specifically binds the Rev NES in vitro, although this latter interaction is detectable only in the presence of added Ran . GTP. Overexpression of a truncated, defective form of the nucleoporin Nup214/CAN, termed DeltaCAN, that retains Crm1 binding ability resulted in the effective inhibition of HIV-1 Rev or human T-cell leukemia virus Rex-dependent gene expression. In contrast, DeltaCAN had no significant affect on Mason-Pfizer monkey virus constitutive transport element (MPMV CTE)-dependent nuclear RNA export or on the expression of RNAs dependent on the cellular mRNA export pathway. As a result, DeltaCAN specifically blocked late, but not early, HIV-1 gene expression in HIV-1-infected cells. These data strongly validate Crm1 as a cellular cofactor for HIV-1 Rev and demonstrate that the MPMV CTE nuclear RNA export pathway uses a distinct, Crm1-independent mechanism. In addition, these data identify a novel and highly potent inhibitor of leucine-rich NES-dependent nuclear export.

Angelman syndrome caused by loss of a marker chromosome: cytogenetic and fluorescence in situ hybridization analysis

We report a cytogenetic and fluorescence in situ hybridization study of a family in which a female child showed all the main characteristics of Angelman syndrome. Her karyotype revealed a translocation between chromosomes 5 and 15 with a partial deletion from 15pter to the Angelman region. Several members of her family appeared to be carriers of the same translocation, but showed no symptoms. The karyotypes showed a marker chromosome, that was not present in the female with Angelman syndrome. Fluorescence in situ hybridization revealed that the marker chromosome corresponded to material from chromosome 15. The present study is in agreement with the suggestion that genomic imprinting is one of the mechanisms involved in Angelman syndrome.

The N-terminal Arg-rich region of human immunodeficiency virus types 1 and 2 and simian immunodeficiency virus Nef is involved in RNA binding

Comparison of the amino acid sequences of human immunodeficiency virus (HIV) Nef protein and several RNA-binding proteins shows similarities in some regions of these proteins. Thus, poliovirus protein 2C, an RNA-binding protein, shares with Nef the sequence YXQQ...MDD...DXXD. In addition, both proteins contain an Arg-rich motif that, in the case of poliovirus 2C, is involved in RNA-binding activity. Moreover, the RNA-binding, anti-terminator N proteins of lambda, phi21 and P22 phages show sequence similarities with HIV Nef at the Arg-rich motif. To assess the significance of this motif, native and deletion variants of Nef protein were assayed for RNA-binding activity. The N-terminal 35 amino acids of HIV-1 Nef that comprise the Arg-rich motif are sufficient for RNA binding. Point mutations engineered at the Arg-rich motif of HIV-1 Nef revealed that basic amino acid residues are essential for RNA-binding activity. The Nef proteins from HIV-2 and SIV can also interact with RNA, while the same proteins with the N-terminal Arg-rich domain truncated fail to interact with RNA. These findings indicate that all three Nef proteins from HIV-1, HIV-2 and simian immunodeficiency virus belong to the RNA-binding family of proteins. The three proteins contain an Arg-rich region at the N-terminus which is necessary to interact with RNA.

Human immunodeficiency virus (HIV) Nef is an RNA binding protein in cell-free systems

The function of human immunodeficiency virus nef gene product has been much debated but the precise activity of this protein in the HIV replication cycle remains unknown. HIV-1 Nef was obtained as a fusion protein with maltose binding protein (MBF), purified by amylose column chromatography and separated from MBP by cleavage with factor Xa. Purified HIV-1 Nef protein, but not the fusion protein MBP-Nef, binds to RNA in vitro as tested by three different assays, radioactive or non-radioactive. North-western analysis, UV cross-linking or band-shift analysis. This activity was lost in a deletion mutant lacking 22 amino acids from the amino terminus of HIV-1 Nef, while a deletion of 44 residues from the carboxy terminus of the protein does not impair the RNA binding activity. Moreover, a single amino acid replacement, Arg to Gly at position 22 produces a Nef variant deficient in its ability to interact with RNA. Different Nef proteins from HIV-1, HIV-2 or SIV were fused to MBP and cleaved with factor Xa. The different Nef proteins were all endowed with RNA-binding capacity. Sequence similarities between several RNA binding proteins, including picornavirus 2C and different Nef proteins are observed. The function of Nef during the HIV replication cycle is discussed on the basis of the present findings.

[Eosinophilic esophagitis: an underdiagnosed entity?]

Eosinophilic esophagitis is an exceptional entity, although the number of published cases has increased three times during the last 3 years. We report the case of an 18-year-old male with a long esophageal stenosis due to eosinophilic esophagitis. Clinical, radiologic, endoscopic and manometric outcome was satisfactory after prednisone therapy. Although isolated esophageal involvement may occur, disseminated eosinophilic esophago-gastro-intestinal disease is more frequent. The disease is usually diagnosed in young male patients, presenting with dysphagia and, to a lesser extent, chest pain. Previous allergic conditions are reported in 80% of cases and peripheral eosinophilia is present in 80% of patients. Esophageal stenosis is present in 72% of cases and manometric alterations are found in 47% of patients. Radiographic findings are nonspecific and include stenosis and rigidity. Endoscopic features are not specific either, but diagnosis may be achieved by endoscopic biopsy. Corticosteroids and sodium chromoglycate have proved to be useful, with good therapeutic response being reported in 90% of patients with this underestimated disease.

[Analysis of suggestive endoscopic characteristics in the preoperative diagnosis of gastric lymphoma]

We study the diagnostic value of some macroscopic features which might be considered suspicious of lymphoma (RESL): thickened gastric folds, diffuse involvement, shallow star shaped ulcers, net lesion limits, size > 6 cm., supracisural location and duodenal involvement. We analyze their overall incidence in 36 primary gastric lymphomas, considering possible differences of each feature between pre and post surgery diagnostic achievement. Preoperative diagnosis was related with the total number of RESL (p < 0.01). Diagnosis of lymphoma was highly suspicious when two or more of these features were present. Supracisural location (p = 0.0008) and shallow star shaped ulcers (p = 0.07) were the most significant RESL. Although RESL sensitivity has been low in the diagnosis of gastric lymphoma, we have found a high specificity for the presence of star shaped ulcers, diffuse involvement and duodenal involvement, which might be an aid in differential diagnosis.