Distinct utilization of effectors and biological outcomes resulting from site-specific Ras activation: Ras functions in lipid rafts and Golgi complex are dispensable for proliferation and transformation

Spatio-temporal segregation of Ras signals: one ship, three anchors, many harbors

Dynamic assembly of spatially separated signaling platforms enables a cell to tune cellular outputs in response to different input stimuli. Understanding how a vast diversity in signaling responses can be generated from a limited protein repertoire requires knowledge of how cells maintain the segregation of proteins and thereby orchestrate their local activities. Ras proteins are subject to this type of precise regulation of localization, and thus activity, in space and time. A model emerges where different lipid anchors dynamically shuttle Ras between specific membrane compartments, where differences in the accessibility of signaling environments and in the residence time of Ras therein account for isoform-specific signaling responses.

Mechanisms of Disease: preclinical reports of antineoplastic synergistic action of bisphosphonates

For patients with malignant bone disease, bisphosphonate therapy is the standard treatment. Preclinical and preliminary clinical data suggest that bisphosphonates have direct or indirect antitumor effects: they affect growth-factor release, cancer-cell adhesion, invasion and viability, angiogenesis, and apoptosis of cancer cells. These effects might be enhanced through co-administration with chemotherapy agents, biological agents, or both. We survey the biochemical pathways and molecular targets of bisphosphonates, and discuss the molecular mechanisms of these antitumor effects, as well as the documented antineoplastic preclinical effects of bisphosphonates used in combination with cytotoxic and biological drugs. Moreover, the positive interactions between bisphosphonates and farnesyltransferase inhibitors, KIT receptor tyrosine kinase inhibitors (e.g. imatinib mesylate) and cyclo-oxygenase-2 inhibitors are discussed in relation to their potential synergistic and additive effects. We briefly discuss identification of new molecular targets of bisphosphonates from genomic and proteomic analysis, and highlight the cellular consequences of drug-related enzyme inhibition.

Localization of Rheb to the endomembrane is critical for its signaling function

Rheb, a small GTPase, has emerged as a key molecular switch that directly regulates the activity of the mammalian target of rapamycin (mTOR). Similar to other members of the Ras superfamily, Rheb has a C-terminal CaaX box that is subject to farnesylation. This study reports that farnesylation is a key determinant of Rheb's subcellular localization and directs its association with the endomembrane. Timed imaging of live cells expressing EGFP-Rheb reveals that following brief association with the ER, Rheb localizes to highly ordered, distinct structures within the cytoplasm that display characteristics of Golgi membranes. Failure of Rheb to localize to the endomembrane impairs its ability to interact with mTOR and activate downstream targets. Consistent with the notion that the endomembrane may serve as a platform for the assembly of a functional Rheb/mTOR complex, treatment of cells with brefeldin A interferes with transmission of Rheb signals to p70S6K.

Ras signaling on the Golgi

The discovery that Ras proteins are modified by enzymes restricted to the endoplasmic reticulum and Golgi apparatus and that, at steady state, a significant pool of Ras is localized on the Golgi has led to the hypothesis that Ras can become activated on and signal from intracellular membranes. Fluorescent probes capable of showing when and where in living cells Ras becomes activated together with studies of Ras proteins stringently tethered to intracellular membranes have confirmed this hypothesis. Thus, recent studies of Ras have contributed to the rapidly expanding field of compartmentalized signaling.

Thematic review series: lipid posttranslational modifications. CAAX modification and membrane targeting of Ras

Proteins that terminate with a consensus sequence known as CAAX undergo a series of posttranslational modifications that include polyisoprenylation, endoproteolysis, and carboxyl methylation. These modifications render otherwise hydrophilic proteins hydrophobic at their C termini such that they associate with membranes. Whereas prenylation occurs in the cytosol, postprenylation processing is accomplished on the cytoplasmic surface of the endoplasmic reticulum and Golgi apparatus. Among the numerous CAAX proteins encoded in mammalian genomes are many signaling molecules such as monomeric GTPases, including the Ras proteins that play an important role in cancer. In the course of their processing, nascent Ras proteins traffic from their site of synthesis in the cytosol to the endomembrane and then out to the plasma membrane (PM) by at least two pathways. Recently, retrograde pathways have been discovered that deliver mature Ras from the PM back to the Golgi. The Golgi has been identified as a platform upon which Ras can signal. Thus, the subcellular trafficking of Ras proteins has the potential to increase the complexity of Ras signaling by adding a spatial dimension. The complexity of Ras trafficking also affords a wider array of potential targets for the discovery of drugs that might inhibit tumors by interfering with Ras trafficking.

Molecular mechanisms involved in Ras inactivation: the annexin A6–p120GAP complex

In mammalian cells, a complex network of signaling pathways tightly regulates a variety of cellular processes, such as proliferation and differentiation. New insights from one of the most-important signaling cascades involved in oncogenesis, the Ras-Raf-MAPK pathway, suggest that the subcellular localisation and assembly of signaling modules of this pathway is crucial to control the biological response. This commonly requires membrane targeting events that are mediated by adaptor/scaffold proteins. Of particular interest is the translocation and complex formation of GTPase-activating proteins (GAPs), such as p120GAP, at the plasma membrane to inactivate Ras. Recent studies indicate that one member of the annexin family, annexin A6 acts as a targeting protein for p120GAP. This review discusses how annexin A6 modulates the involvement of negative regulators of the Ras-Raf-MAPK pathway contributing to Ras inactivation.