Scaffold association factor B (SAFB) is required for expression of prenyltransferases and RAS membrane association.

Inhibiting membrane association of RAS has long been considered a rational approach to anticancer therapy, which led to the development of farnesyltransferase inhibitors (FTIs). However, FTIs proved ineffective against KRAS-driven tumors. To reveal alternative therapeutic strategies, we carried out a genome-wide CRISPR-Cas9 screen designed to identify genes required for KRAS4B membrane association. We identified five enzymes in the prenylation pathway and SAFB, a nuclear protein with both DNA and RNA binding domains. Silencing SAFB led to marked mislocalization of all RAS isoforms as well as RAP1A but not RAB7A, a pattern that phenocopied silencing FNTA, the prenyltransferase α subunit shared by farnesyltransferase and geranylgeranyltransferase type I. We found that SAFB promoted RAS membrane association by controlling FNTA expression. SAFB knockdown decreased GTP loading of RAS, abrogated alternative prenylation, and sensitized RAS-mutant cells to growth inhibition by FTI. Our work establishes the prenylation pathway as paramount in KRAS membrane association, reveals a regulator of prenyltransferase expression, and suggests that reduction in FNTA expression may enhance the efficacy of FTIs.

GGTase3 is a newly identified geranylgeranyltransferase targeting a ubiquitin ligase.

Protein prenylation is believed to be catalyzed by three heterodimeric enzymes: FTase, GGTase1 and GGTase2. Here we report the identification of a previously unknown human prenyltransferase complex consisting of an orphan prenyltransferase α-subunit, PTAR1, and the catalytic ß-subunit of GGTase2, RabGGTB. This enzyme, which we named GGTase3, geranylgeranylates FBXL2 to allow its localization at cell membranes, where this ubiquitin ligase mediates the polyubiquitylation of membrane-anchored proteins. In cells, FBXL2 is specifically recognized by GGTase3 despite having a typical carboxy-terminal CaaX prenylation motif that is predicted to be recognized by GGTase1. Our crystal structure analysis of the full-length GGTase3-FBXL2-SKP1 complex reveals an extensive multivalent interface specifically formed between the leucine-rich repeat domain of FBXL2 and PTAR1, which unmasks the structural basis of the substrate-enzyme specificity. By uncovering a missing prenyltransferase and its unique mode of substrate recognition, our findings call for a revision of the 'prenylation code'.

Targeting RAS - will GPR31 deliver us a new path forward?

Effective anti-rat sarcoma viral oncogene (RAS) therapies have remained the holy grail of cancer treatment. Mutant Kirsten rat sarcoma viral oncogene homolog (KRAS) sustains tumorigenesis when linked to the plasma membrane (PM). The G protein-coupled receptor 31 (GPR31) is now identified to mediate KRAS membrane association and is crucial for proliferation, survival and macropinocytosis of KRAS-dependent cancer cells, suggesting that GPR31 is a druggable target for anti-RAS therapy.

The G protein-coupled receptor GPR31 promotes membrane association of KRAS.

The product of the KRAS oncogene, KRAS4B, promotes tumor growth when associated with the plasma membrane (PM). PM association is mediated, in part, by farnesylation of KRAS4B, but trafficking of nascent KRAS4B to the PM is incompletely understood. We performed a genome-wide screen to identify genes required for KRAS4B membrane association and identified a G protein-coupled receptor, GPR31. GPR31 associated with KRAS4B on cellular membranes in a farnesylation-dependent fashion, and retention of GPR31 on the endoplasmic reticulum inhibited delivery of KRAS4B to the PM. Silencing of GPR31 expression partially mislocalized KRAS4B, slowed the growth of KRAS-dependent tumor cells, and blocked KRAS-stimulated macropinocytosis. Our data suggest that GPR31 acts as a secretory pathway chaperone for KRAS4B.

Phosphorylated K-Ras limits cell survival by blocking Bcl-xL sensitization of inositol trisphosphate receptors.

K-Ras4B is targeted to the plasma membrane by a farnesyl modification that operates in conjunction with a polybasic domain. We characterized a farnesyl-electrostatic switch whereby protein kinase C phosphorylates K-Ras4B on serine 181 in the polybasic region and thereby induces translocation from the plasma membrane to internal membranes that include the endoplasmic reticulum (ER) and outer mitochondrial membrane. This translocation is associated with cell death. Here we have explored the mechanism of phospho-K-Ras4B toxicity and found that GTP-bound, phosphorylated K-Ras4B associates with inositol trisphosphate receptors on the ER in a Bcl-xL-dependent fashion and, in so doing, blocks the ability of Bcl-xL to potentiate the InsP3 regulated flux of calcium from ER to mitochondria that is required for efficient respiration, inhibition of autophagy, and cell survival. Thus, we have identified inositol trisphosphate receptors as unique effectors of K-Ras4B that antagonize the prosurvival signals of other K-Ras effectors.

ErbB2-driven breast cancer cell invasion depends on a complex signaling network activating myeloid zinc finger-1-dependent cathepsin B expression.

Aberrant ErbB2 receptor tyrosine kinase activation in breast cancer is strongly linked to an invasive disease. The molecular basis of ErbB2-driven invasion is largely unknown. We show that cysteine cathepsins B and L are elevated in ErbB2 positive primary human breast cancer and function as effectors of ErbB2-induced invasion in vitro. We identify Cdc42-binding protein kinase beta, extracellular regulated kinase 2, p21-activated protein kinase 4, and protein kinase C alpha as essential mediators of ErbB2-induced cysteine cathepsin expression and breast cancer cell invasiveness. The identified signaling network activates the transcription of cathepsin B gene (CTSB) via myeloid zinc finger-1 transcription factor that binds to an ErbB2-responsive enhancer element in the first intron of CTSB. This work provides a model system for ErbB2-induced breast cancer cell invasiveness, reveals a signaling network that is crucial for invasion in vitro, and defines a specific role and targets for the identified serine-threonine kinases.

Single-walled carbon nanotubes deliver peptide antigen into dendritic cells and enhance IgG responses to tumor-associated antigens.

We studied the feasibility of using single-wall carbon nanotubes (SWNTs) as antigen carriers to improve immune responses to peptides that are weak immunogens, a characteristic typical of human tumor antigens. Binding and presentation of peptide antigens by the MHC molecules of antigen presenting cells (APCs) is essential to mounting an effective immune response. The Wilm's tumor protein (WT1) is upregulated in many human leukemias and cancers and several vaccines directed at this protein are in human clinical trials. WT1 peptide 427 induces human CD4 T cell responses in the context of multiple human HLA-DR.B1 molecules, but the peptide has a poor binding affinity to BALB/c mouse MHC class II molecules. We used novel, spectrally quantifiable chemical approaches to covalently append large numbers of peptide ligands (0.4 mmol/g) onto solubilized SWNT scaffolds. Peptide-SWNT constructs were rapidly internalized into professional APCs (dendritic cells and macrophages) within minutes in vitro, in a dose dependent manner. Immunization of BALB/c mice with the SWNT-peptide constructs mixed with immunological adjuvant induced specific IgG responses against the peptide, while the peptide alone or peptide mixed with the adjuvant did not induce such a response. The conjugation of the peptide to SWNT did not enhance the peptide-specific CD4 T cell response in human and mouse cells, in vitro. The solubilized SWNTs alone were nontoxic in vitro, and we did not detect antibody responses to SWNT in vivo. These results demonstrated that SWNTs are able to serve as antigen carriers for delivery into APCs to induce humoral immune responses against weak tumor antigens.

FKBP12 binds to acylated H-ras and promotes depalmitoylation.

A cycle of palmitoylation/depalmitoylation of H-Ras mediates bidirectional trafficking between the Golgi apparatus and the plasma membrane, but nothing is known about how this cycle is regulated. We show that the prolyl isomerase (PI) FKBP12 binds to H-Ras in a palmitoylation-dependent fashion and promotes depalmitoylation. A variety of inhibitors of the PI activity of FKBP12, including FK506, rapamycin, and cycloheximide, increase steady-state palmitoylation. FK506 inhibits retrograde trafficking of H-Ras from the plasma membrane to the Golgi in a proline 179-dependent fashion, augments early GTP loading of Ras in response to growth factors, and promotes H-Ras-dependent neurite outgrowth from PC12 cells. These data demonstrate that FKBP12 regulates H-Ras trafficking by promoting depalmitoylation through cis-trans isomerization of a peptidyl-prolyl bond in proximity to the palmitoylated cysteines.

Ras/MAPK signaling from endomembranes.

Signal transduction along the Ras/MAPK pathway has been generally thought to take place at the plasma membrane. It is now evident that the plasma membrane is not the only platform capable of Ras/MAPK signal induction. Fusion of Ras with green fluorescent protein and the development of genetically encoded fluorescent probes for Ras activation have revealed signaling events on a variety of intracellular membranes including endosomes, the Golgi apparatus and the endoplasmic reticulum. Thus, the Ras/MAPK pathway is spatially compartmentalized within cells and this may afford greater complexity of signal output.

Intracellular signaling: peripatetic Ras.

Ras proteins traffic between the plasma membrane and endomembranes and signal from the cytosolic face of a variety of organelles. Palmitoylated N-Ras and H-Ras signal from early endosomes. A recent study reports that K-Ras resides on and signals from various types of endosomes, including late endosomes/lysosomes and multivesicular bodies.

Sensitization to the lysosomal cell death pathway by oncogene-induced down-regulation of lysosome-associated membrane proteins 1 and 2.

Expression and activity of lysosomal cysteine cathepsins correlate with the metastatic capacity and aggressiveness of tumors. Here, we show that transformation of murine embryonic fibroblasts with v-H-ras or c-src(Y527F) changes the distribution, density, and ultrastructure of the lysosomes, decreases the levels of lysosome-associated membrane proteins (LAMP-1 and LAMP-2) in an extracellular signal-regulated kinase (ERK)- and cathepsin-dependent manner, and sensitizes the cells to lysosomal cell death pathways induced by various anticancer drugs (i.e., cisplatin, etoposide, doxorubicin, and siramesine). Importantly, K-ras and erbb2 elicit a similar ERK-mediated activation of cysteine cathepsins, cathepsin-dependent down-regulation of LAMPs, and increased drug sensitivity in human colon and breast carcinoma cells, respectively. Notably, reconstitution of LAMP levels by ectopic expression or by cathepsin inhibitors protects transformed cells against the lysosomal cell death pathway. Furthermore, knockdown of either lamp1 or lamp2 is sufficient to sensitize the cells to siramesine-induced cell death and photo-oxidation-induced lysosomal destabilization. Thus, the transformation-associated ERK-mediated up-regulation of cysteine cathepsin expression and activity leads to a decrease in the levels of LAMPs, which in turn contributes to the enhanced sensitivity of transformed cells to drugs that trigger lysosomal membrane permeabilization. These data indicate that aggressive cancers with high cysteine cathepsin levels are especially sensitive to lysosomal cell death pathways and encourage the further development of lysosome-targeting compounds for cancer therapy.

Anti-cancer agent siramesine is a lysosomotropic detergent that induces cytoprotective autophagosome accumulation.

A sigma-2 receptor ligand siramesine induces lysosomal leakage and cathepsin-dependent death of cancer cells in vitro and displays potent anti-cancer activity in vivo. The mechanism by which siramesine destabilizes lysosomes is, however, unknown. Here, we show that siramesine induces a rapid rise in the lysosomal pH that is followed by lysosomal leakage and dysfunction. The rapid accumulation of siramesine into cancer cell lysosomes, its ability to destabilize isolated lysosomes, and its chemical structure as an amphiphilic amine indicate that it is a lysosomotropic detergent. Notably, siramesine triggers also a substantial Atg6- and Atg7-dependent accumulation of autophagosomes that is associated with a rapid and sustained inhibition of mammalian target of rapamycin complex 1 (mTORC1; an inhibitor of autophagy). Siramesine fails, however, to increase the degradation rate of long-lived proteins. Thus, the massive accumulation of autophagosomes is likely to be due to a combined effect of activation of autophagy signaling and decreased autophagosome turnover. Importantly, pharmacological and RNA interference-based inhibition of autophagosome formation further sensitizes cancer cells to siramesine-induced cytotoxicity. These data identify siramesine as a lysosomotropic detergent that triggers cell death via a direct destabilization of lysosomes and cytoprotection by inducing the accumulation of autophagosomes. Threrefore, the combination of siramesine with inhibitors of autophagosome formation appears as a promising approach for future cancer therapy.

Control of macroautophagy by calcium, calmodulin-dependent kinase kinase-beta, and Bcl-2.

Macroautophagy is an evolutionary conserved lysosomal pathway involved in the turnover of cellular macromolecules and organelles. In spite of its essential role in tissue homeostasis, the molecular mechanisms regulating mammalian macroautophagy are poorly understood. Here, we demonstrate that a rise in the free cytosolic calcium ([Ca(2+)](c)) is a potent inducer of macroautophagy. Various Ca(2+) mobilizing agents (vitamin D(3) compounds, ionomycin, ATP, and thapsigargin) inhibit the activity of mammalian target of rapamycin, a negative regulator of macroautophagy, and induce massive accumulation of autophagosomes in a Beclin 1- and Atg7-dependent manner. This process is mediated by Ca(2+)/calmodulin-dependent kinase kinase-beta and AMP-activated protein kinase and inhibited by ectopic Bcl-2 located in the endoplasmatic reticulum (ER), where it lowers the [Ca(2+)](ER) and attenuates agonist-induced Ca(2+) fluxes. Thus, an increase in the [Ca(2+)](c) serves as a potent inducer of macroautophagy and as a target for the antiautophagy action of ER-located Bcl-2.

Apoptosome-independent activation of the lysosomal cell death pathway by caspase-9.

The apoptosome, a heptameric complex of Apaf-1, cytochrome c, and caspase-9, has been considered indispensable for the activation of caspase-9 during apoptosis. By using a large panel of genetically modified murine embryonic fibroblasts, we show here that, in response to tumor necrosis factor (TNF), caspase-8 cleaves and activates caspase-9 in an apoptosome-independent manner. Interestingly, caspase-8-cleaved caspase-9 induced lysosomal membrane permeabilization but failed to activate the effector caspases whereas apoptosome-dependent activation of caspase-9 could trigger both events. Consistent with the ability of TNF to activate the intrinsic apoptosis pathway and the caspase-9-dependent lysosomal cell death pathway in parallel, their individual inhibition conferred only a modest delay in TNF-induced cell death whereas simultaneous inhibition of both pathways was required to achieve protection comparable to that observed in caspase-9-deficient cells. Taken together, the findings indicate that caspase-9 plays a dual role in cell death signaling, as an activator of effector caspases and lysosomal membrane permeabilization.

Effective tumor cell death by sigma-2 receptor ligand siramesine involves lysosomal leakage and oxidative stress.

Acquired resistance to classic caspase-mediated apoptosis is a common problem for the treatment of human cancer. Here, we show that siramesine, a novel sigma-2 receptor ligand, effectively induces caspase-independent programmed cell death in immortalized and transformed cells of various origins. Siramesine-treated tumor cells displayed increased levels of reactive oxygen species, lysosomal membrane permeabilization, chromatin condensation, and shrinkage and detachment of cells. Lipid antioxidants (alpha-tocopherol and gamma-tocopherol), but not other tested antioxidants (butylated hydroxyanisol or N-acetyl cysteine), effectively inhibited siramesine-induced morphologic changes and cell death. Cathepsin B inhibitors (CA-074-Me and R-2525) conferred similar, but less pronounced protection, whereas ectopic expression of antiapoptotic protein Bcl-2, lack of wild-type p53 as well as pharmacologic inhibitors of caspases (zVAD-fmk, DEVD-CHO, and LEHD-CHO), calpains (PD150606), and serine proteases (N-tosyl-L-phenylalanine chloromethyl ketone and pefabloc) failed to protect cells against siramesine-induced death. Importantly, transformation of murine embryonic fibroblasts with activated c-src or v-Ha-ras oncogenes greatly sensitized them to siramesine-induced cytotoxicity. Furthermore, p.o. administration of well-tolerated doses of siramesine had a significant antitumorigenic effect in orthotopic breast cancer and s.c. fibrosarcoma models in mice. These results present siramesine as a promising new drug for the treatment of tumors resistant to traditional therapies.

Lysosomes as targets for cancer therapy.

Tumor invasion and metastasis are associated with altered lysosomal trafficking and increased expression of the lysosomal proteases termed cathepsins. Emerging experimental evidence suggests that such alterations in lysosomes may form an "Achilles heel" for cancer cells by sensitizing them to death pathways involving lysosomal membrane permeabilization and the release of cathepsins into the cytosol. Here, we highlight recent results on cancer-related changes in the composition and function of lysosomes, focusing on possible implications for the development of novel cancer therapeutics that target tumor cell lysosomes.

Sensitization to the lysosomal cell death pathway upon immortalization and transformation.

Tumorigenesis is associated with several changes that alter the cellular susceptibility to programmed cell death. Here, we show that immortalization and transformation sensitize cells in particular to the cysteine cathepsin-mediated lysosomal death pathway. Spontaneous immortalization increased the susceptibility of wild-type murine embryonic fibroblasts (MEFs) to tumor necrosis factor (TNF)-mediated cytotoxicity >1000-fold, whereas immortalized MEFs deficient for lysosomal cysteine protease cathepsin B (CathB) retained the resistant phenotype of primary cells. This effect was specific for cysteine cathepsins, because also lack of cathepsin L (a lysosomal cysteine protease), but not that of cathepsin D (a lysosomal aspartyl protease) or caspase-3 (the major executioner protease in classic apoptosis) inhibited the immortalization-associated sensitization of MEFs to TNF. Oncogene-driven transformation of immortalized MEFs was associated with a dramatic increase in cathepsin expression and additional sensitization to the cysteine cathepsin-mediated death pathway. Importantly, exogenous expression of CathB partially reversed the resistant phenotype of immortalized CathB-deficient MEFs, and the inhibition of CathB activity by pharmacological inhibitors or RNA interference attenuated TNF-induced cytotoxicity in immortalized and transformed wild-type cells. Thus, tumorigenesis-associated changes in lysosomes may counteract cancer progression and enhance therapeutic responses by sensitizing cells to programmed cell death.

Heat shock protein 70 promotes cell survival by inhibiting lysosomal membrane permeabilization.

Heat shock protein 70 (Hsp70) is a potent survival protein whose depletion triggers massive caspase-independent tumor cell death. Here, we show that Hsp70 exerts its prosurvival function by inhibiting lysosomal membrane permeabilization. The cell death induced by Hsp70 depletion was preceded by the release of lysosomal enzymes into the cytosol and inhibited by pharmacological inhibitors of lysosomal cysteine proteases. Accordingly, the Hsp70-mediated protection against various death stimuli in Hsp70-expressing human tumor cells as well as in immortalized Hsp70 transgenic murine fibroblasts occurred at the level of the lysosomal permeabilization. On the contrary, Hsp70 failed to inhibit the cytochrome c-induced, apoptosome-dependent caspase activation in vitro and Fas ligand-induced, caspase-dependent apoptosis in immortalized fibroblasts. Immunoelectron microscopy revealed that endosomal and lysosomal membranes of tumor cells contained Hsp70. Permeabilization of purified endo/lysosomes by digitonin failed to release Hsp70, suggesting that it is physically associated with the membranes. Finally, Hsp70 positive lysosomes displayed increased size and resistance against chemical and physical membrane destabilization. These data identify Hsp70 as the first survival protein that functions by inhibiting the death-associated permeabilization of lysosomes.