In several pathological conditions, epithelial cells demonstrate a breakdown of barrier function and acquire an invasive phenotype. Endothelial cells in particular are maintained in a mesenchymal state during the cell invasion phase of angiogenesis. We show here that tyrosine phosphorylation of the adherens junction protein VE-cadherin at two critical tyrosines, Tyr-658 and Tyr-731, via tyrosine kinase activation or phosphatase inactivation was sufficient to prevent the binding of p120-and ␤-catenin, respectively, to the cytoplasmic tail of VE-cadherin. In fact, phosphorylation at either site led to the inhibition of cell barrier function. Cells expressing wild-type VE-cadherin showed decreased cell migration compared with cells lacking VE-cadherin, whereas expression of VE-cadherin with a simple phosphomimetic tyrosine-to-glutamic acid mutation of Y658E or Y731E was sufficient to restore the migratory response. These findings demonstrate that a single phosphorylation event within the VE-cadherin cytoplasmic tail is sufficient to maintain cells in a mesenchymal state.

Neuroblastoma, the most common paediatric solid tumour, arises from defective neural crest cells. Genetic alterations occur frequently in the most aggressive neuroblastomas. In particular, deletion or suppression of the proapoptotic enzyme caspase-8 is common in malignant, disseminated disease, although the effect of this loss on disease progression is unclear. Here we show that suppression of caspase-8 expression occurs during the establishment of neuroblastoma metastases in vivo, and that reconstitution of caspase-8 expression in deficient neuroblastoma cells suppressed their metastases. Caspase-8 status was not a predictor of primary tumour growth; rather, caspase-8 selectively potentiated apoptosis in neuroblastoma cells invading the collagenous stroma at the tumour margin. Apoptosis was initiated by unligated integrins by means of a process known as integrin-mediated death. Loss of caspase-8 or integrin rendered these cells refractory to integrin-mediated death, allowed cellular survival in the stromal microenvironment, and promoted metastases. These findings define caspase-8 as a metastasis suppressor gene that, together with integrins, regulates the survival and invasive capacity of neuroblastoma cells.

In current views, translation-coupled ribosome binding to the endoplasmic reticulum (ER) membrane is transient, with association occurring via the signal recognition particle pathway and dissociation occurring upon the termination of protein synthesis. Recent studies indicate, however, that ribosomal subunits remain membrane-bound following the termination of protein synthesis. To define the mechanism of post-termination ribosome association with the ER membrane, membrane-bound ribosomes were detergent-solubilized from tissue culture cells at different stages of the protein synthesis cycle, and the composition of the ribosomeassociated membrane protein fraction was determined. We report that ribosomes reside in stable association with the Sec61␣-translocon following the termination stage of protein synthesis. Additionally, in vitro experiments revealed that solubilized, gradient-purified ribosome-translocon complexes were able to initiate the translation of secretory and cytosolic proteins and were functional in assays of signal sequence recognition. Using this experimental system, synthesis of signal sequence-bearing polypeptides yielded a tight ribosometranslocon junction; synthesis of nascent polypeptides lacking a signal sequence resulted in a disruption of this junction. On the basis of these data, we propose that in situ, ribosomes reside in association with the translocon throughout the cycle of protein synthesis, with membrane release occurring upon translation of proteins lacking topogenic signals.

In current models, protein translocation in the endoplasmic reticulum (ER) occurs in the context of two cycles, the signal recognition particle (SRP) cycle and the ribosome cycle. Both SRP and ribosomes bind to the ER membrane as a consequence of the targeting process of translocation. Whereas SRP release from the ER membrane is regulated by the GTPase activities of SRP and the SRP receptor, ribosome release from the ER membrane is thought to occur in response to the termination of protein synthesis. We report that ER-bound ribosomes remain membrane-bound following the termination of protein synthesis and in the bound state can initiate the translation of secretory and cytoplasmic proteins. Two principal observations are reported. 1) Membrane-bound ribosomes engaged in the synthesis of proteins lacking a signal sequence are released from the ER membrane as ribosome-nascent polypeptide complexes. 2) Membrane-bound ribosomes translating secretory proteins can access the translocon in an SRP receptor-independent manner. We propose that ribosome release from the ER membrane occurs in the context of protein translation, with release occurring by default in the absence of productive nascent polypeptide-membrane interactions.

No abstract

Ovarian cancer patients are typically treated with carboplatin and paclitaxel, but suffer a high rate of relapse with recalcitrant disease. This challenge has fostered the development of novel approaches to treatment, including antagonists of the ‘inhibitor of apoptosis proteins' (IAPs), also called SMAC mimetics, as apoptosis-inducing agents whose action is opposed by caspase inhibitors. Surprisingly, IAP antagonist plus caspase inhibitor (IZ) treatment selectively induced a tumor necrosis factor-α (TNFα)-dependent death among several apoptosis-resistant cell lines and patient xenografts. The induction of necroptosis was common in ovarian cancer, with expression of catalytically active receptor-interacting protein kinase-3 (RIPK3) necessary for death, and in fact sufficient to compromise survival of RIPK3-negative, necroptosis-resistant ovarian cancer cells. The formation of a necrosome-like complex with a second critical effector, receptor-interacting serine–threonine kinase-1 (RIPK1), was observed. RIPK1, RIPK3 and TNFα were required for the induction of death, as agents that inhibit the function of any of these targets prevented cell death. Abundant RIPK3 transcript is common in serous ovarian cancers, suggesting that further evaluation and targeting of this RIPK3-dependent pathway may be of clinical benefit.

In this study, the contributions of membrane-bound ribosomes to the regulation of endoplasmic reticulum translocon composition and Sec61␣ conformation were examined. Following solubilization of rough microsomes (RM) with digitonin, ribosomes co-sedimented in complexes containing the translocon proteins Sec61␣, ribophorin I, and TRAP␣, and endoplasmic reticulum phospholipids. Complexes of similar composition were identified in digitonin extracts of ribosome-free membranes, indicating that the ribosome does not define the composition of the digitonin-soluble translocon. Whereas in digitonin solution a highly electrostatic ribosome-translocon junction is observed, no stable interactions between ribosomes and Sec61␣, ribophorin I, or TRAP␣ were observed following solubilization of RM with lipid-derived detergents at physiological salt concentrations. Sec61␣ was found to exist in at least two conformational states, as defined by mild proteolysis. A protease-resistant form was observed in RM and detergent-solubilized RM. Removal of peripheral proteins and ribosomes markedly enhanced the sensitivity of Sec61␣ to proteolysis, yet the readdition of inactive ribosomes to salt-washed membranes yielded only modest reductions in protease sensitivity. Addition of sublytic concentrations of detergents to salt-washed RM markedly decreased the protease sensitivity of Sec61␣, indicating that a protease-resistant conformation of Sec61␣ can be conferred in a ribosome-independent manner.