Filament formation drives catalysis by glutaminase enzymes important in cancer progression

The glutaminase enzymes GAC and GLS2 catalyze the hydrolysis of glutamine to glutamate, satisfying the 'glutamine addiction' of cancer cells. They are the targets of anti-cancer drugs; however, their mechanisms of activation and catalytic activity have been unclear. Here we demonstrate that the ability of GAC and GLS2 to form filaments is directly coupled to their catalytic activity and present their cryo-EM structures which provide a view of the conformational states essential for catalysis. Filament formation guides an 'activation loop' to assume a specific conformation that works together with a 'lid' to close over the active site and position glutamine for nucleophilic attack by an essential serine. Our findings highlight how ankyrin repeats on GLS2 regulate enzymatic activity, while allosteric activators stabilize, and clinically relevant inhibitors block, filament formation that enables glutaminases to catalyze glutaminolysis and support cancer progression.

PTEN loss in glioma cell lines leads to increased extracellular vesicles biogenesis and PD-L1 cargo in a PI3K-dependent manner

Phosphatase and Tensin Homologue (PTEN) is one of the most frequently lost tumor suppressors in cancer and the predominant negative regulator of the PI3K/AKT signaling axis. A growing body of evidence has highlighted the loss of PTEN with immuno-modulatory functions including the upregulation of the programmed death ligand-1 (PD-L1), an altered tumor derived secretome that drives an immunosuppressive tumor immune microenvironment (TIME), and resistance to certain immunotherapies. Given their roles in immunosuppression and tumor growth, we examined whether the loss of PTEN would impact the biogenesis, cargo, and function of extracellular vesicles (EVs) in the context of the anti-tumor associated cytokine interferon-γ (IFN-γ). Through genetic and pharmacological approaches, we show that PD-L1 expression is regulated by JAK/STAT signaling, not PI3K signaling. Instead, we observe that PTEN loss positively upregulates cell surface levels of PD-L1 and enhances the biogenesis of EVs enriched with PD-L1 in a PI3K-dependent manner. We demonstrate that because of these changes, EVs derived from glioma cells lacking PTEN have a greater ability to suppress T cell receptor (TCR) signaling. Taken together, these findings provide important new insights into how the loss of PTEN can contribute to an immunosuppressive TIME, facilitate immune evasion, and highlight a novel role for PI3K signaling in the regulation of EV biogenesis and the cargo they contain.

A multiprotein signaling complex sustains AKT and mTOR/S6K activity necessary for the survival of cancer cells undergoing stress

Cancer cells encounter stresses during tumor progression and metastatic spread, however, how they survive these challenges is not fully understood. We now identify a mechanism for cancer cell survival through the discovery of a multiprotein signaling complex that includes the GTPase Cdc42, the Cdc42 GEF/effector protein Dock7, AKT, mTOR and the mTORC1 regulatory partners TSC1, TSC2, and Rheb. This pro-survival signaling complex sustains the activated state of AKT by preventing its dephosphorylation at Ser473 during serum starvation, resulting in a low but critical activation of a Raptor-independent mTOR/S6K activity. We demonstrate that the Dock7 DHR1 domain, previously of unknown function, is responsible for preserving AKT phosphorylation through an interaction requiring its C2-like motif. Collectively, these findings help address long-standing questions of how Cdc42 signals mTOR activation by elucidating the unique functions of its signaling partner Dock7 as an AKT regulator necessary for resistance to anoikis and apoptosis in cancer cells.

Defining the conformational states that enable transglutaminase 2 to promote cancer cell survival versus cell death

Transglutaminase 2 (TG2) is a GTP-binding/protein-crosslinking enzyme that has been investigated as a therapeutic target for Celiac disease, neurological disorders, and aggressive cancers. TG2 has been suggested to adopt two conformational states that regulate its functions: a GTP-bound, closed conformation, and a calcium-bound, crosslinking-active open conformation. TG2 mutants that constitutively adopt an open conformation are cytotoxic to cancer cells. Thus, small molecules that maintain the open conformation of TG2 could offer a new therapeutic strategy. Here, we investigate TG2, using static and time-resolved small-angle X-ray scattering (SAXS) and single-particle cryoelectron microscopy (cryo-EM), to determine the conformational states responsible for conferring its biological effects. We also describe a newly developed TG2 inhibitor, LM11, that potently kills glioblastoma cells and use SAXS to investigate how LM11 affects the conformational states of TG2. Using SAXS and cryo-EM, we show that guanine nucleotide-bound TG2 adopts a monomeric closed conformation while calcium-bound TG2 assumes an open conformational state that can form higher order oligomers. SAXS analysis also suggests how a TG2 mutant that constitutively adopts the open state binds nucleotides through an alternative mechanism to wildtype TG2. Furthermore, we use time-resolved SAXS to show that LM11 increases the ability of calcium to drive TG2 to an open conformation, which is not reversible by guanine nucleotides and is cytotoxic to cancer cells. Taken together, our findings demonstrate that the conformational dynamics of TG2 are more complex than previously suggested and highlight how conformational stabilization of TG2 by LM11 maintains TG2 in a cytotoxic conformational state.

Probing the mechanism by which the retinal G protein transducin activates its biological effector PDE6

Phototransduction in retinal rods occurs when the G protein-coupled photoreceptor rhodopsin triggers the activation of phosphodiesterase 6 (PDE6) by GTP-bound alpha subunits of the G protein transducin (GαT). Recently, we presented a cryo-EM structure for a complex between two GTP-bound recombinant GαT subunits and native PDE6, that included a bivalent antibody bound to the C-terminal ends of GαT and the inhibitor vardenafil occupying the active sites on the PDEα and PDEβ subunits. We proposed GαT-activated PDE6 by inducing a striking reorientation of the PDEγ subunits away from the catalytic sites. However, questions remained including whether in the absence of the antibody GαT binds to PDE6 in a similar manner as observed when the antibody is present, does GαT activate PDE6 by enabling the substrate cGMP to access the catalytic sites, and how does the lipid membrane enhance PDE6 activation? Here, we demonstrate that 2:1 GαT-PDE6 complexes form with either recombinant or retinal GαT in the absence of the GαT antibody. We show that GαT binding is not necessary for cGMP nor competitive inhibitors to access the active sites; instead, occupancy of the substrate binding sites enables GαT to bind and reposition the PDE6γ subunits to promote catalytic activity. Moreover, we demonstrate by reconstituting GαT-stimulated PDE6 activity in lipid bilayer nanodiscs that the membrane-induced enhancement results from an increase in the apparent binding affinity of GαT for PDE6. These findings provide new insights into how the retinal G protein stimulates rapid catalytic turnover by PDE6 required for dim light vision.

Overcoming Barriers Associated with Oral Delivery of Differently Sized Fluorescent Core-Shell Silica Nanoparticles

Oral delivery, while a highly desirable form of nanoparticle-drug administration, is limited by challenges associated with overcoming several biological barriers. Here, the authors study how fluorescent and poly(ethylene glycol)-coated (PEGylated) core-shell silica nanoparticles sized 5 to 50 nm interact with major barriers including intestinal mucus, intestinal epithelium, and stomach acid. From imaging fluorescence correlation spectroscopy studies using quasi-total internal reflection fluorescence microscopy, diffusion of nanoparticles through highly scattering mucus is progressively hindered above a critical hydrodynamic size around 20 nm. By studying Caco-2 cell monolayers mimicking the intestinal epithelia, it is observed that ultrasmall nanoparticles below 10 nm diameter (Cornell prime dots, [C' dots]) show permeabilities correlated with high absorption in humans from primarily enhanced passive passage through tight junctions. Particles above 20 nm diameter exclusively show active transport through cells. After establishing C' dot stability in artificial gastric juice, in vivo oral gavage experiments in mice demonstrate successful passage through the body followed by renal clearance without protein corona formation. Results suggest C' dots as viable candidates for oral administration to patients with a proven pathway towards clinical translation and may generate renewed interest in examining silica as a food additive and its effects on nutrition and health.

Inhibiting Glutamine Metabolism Blocks Coronavirus Replication in Mammalian Cells

Developing therapeutic strategies against COVID-19 has gained widespread interest given the likelihood that new viral variants will continue to emerge. Here we describe one potential therapeutic strategy which involves targeting members of the glutaminase family of mitochondrial metabolic enzymes (GLS and GLS2), which catalyze the first step in glutamine metabolism, the hydrolysis of glutamine to glutamate. We show three examples where GLS expression increases during coronavirus infection of host cells, and another in which GLS2 is upregulated. The viruses hijack the metabolic machinery responsible for glutamine metabolism to generate the building blocks for biosynthetic processes and satisfy the bioenergetic requirements demanded by the 'glutamine addiction' of virus-infected host cells. We demonstrate how genetic silencing of glutaminase enzymes reduces coronavirus infection and that newer members of two classes of small molecule allosteric inhibitors targeting these enzymes, designated as SU1, a pan-GLS/GLS2 inhibitor, and UP4, which is specific for GLS, block viral replication in mammalian epithelial cells. Overall, these findings highlight the importance of glutamine metabolism for coronavirus replication in human cells and show that glutaminase inhibitors can block coronavirus infection and thereby may represent a novel class of anti-viral drug candidates.

Teaser

Inhibitors targeting glutaminase enzymes block coronavirus replication and may represent a new class of anti-viral drugs.

Filament formation drives catalysis by glutaminase enzymes important in cancer progression

The glutaminase enzymes GAC and GLS2 catalyze the hydrolysis of glutamine to glutamate, satisfying the 'glutamine addiction' of cancer cells. They are the targets of anti-cancer drugs; however, their mechanisms of activation and catalytic activity have been unclear. Here we demonstrate that the ability of GAC and GLS2 to form filaments is directly coupled to their catalytic activity and present their cryo-EM structures which provide an unprecedented view of the conformational states essential for catalysis. Filament formation guides an 'activation loop' to assume a specific conformation that works together with a 'lid' to close over the active site and position glutamine for nucleophilic attack by an essential serine. Our findings highlight how ankyrin repeats on GLS2 regulate enzymatic activity, while allosteric activators stabilize, and clinically relevant inhibitors block, filament formation that enables glutaminases to catalyze glutaminolysis and support cancer progression.

Proteomic analysis reveals microvesicles containing NAMPT as mediators of radioresistance in glioma

Tumor-initiating cells contained within the aggressive brain tumor glioma (glioma stem cells, GSCs) promote radioresistance and disease recurrence. However, mechanisms of resistance are not well understood. Herein, we show that the proteome-level regulation occurring upon radiation treatment of several patient-derived GSC lines predicts their resistance status, whereas glioma transcriptional subtypes do not. We identify a mechanism of radioresistance mediated by the transfer of the metabolic enzyme NAMPT to radiosensitive cells through microvesicles (NAMPT-high MVs) shed by resistant GSCs. NAMPT-high MVs rescue the proliferation of radiosensitive GSCs and fibroblasts upon irradiation, and upon treatment with a radiomimetic drug or low serum, and increase intracellular NAD(H) levels. Finally, we show that the presence of NAMPT within the MVs and its enzymatic activity in recipient cells are necessary to mediate these effects. Collectively, we demonstrate that the proteome of GSCs provides unique information as it predicts the ability of glioma to resist radiation treatment. Furthermore, we establish NAMPT transfer via MVs as a mechanism for rescuing the proliferation of radiosensitive cells upon irradiation.

Chaotic advection mixer for capturing transient states of diverse biological macromolecular systems with time-resolved small-angle X-ray scattering

Advances in time-resolved structural techniques, mainly in macromolecular crystallography and small-angle X-ray scattering (SAXS), allow for a detailed view of the dynamics of biological macromolecules and reactions between binding partners. Of particular promise, are mix-and-inject techniques, which offer a wide range of experimental possibility as microfluidic mixers are used to rapidly combine two species just prior to data collection. Most mix-and-inject approaches rely on diffusive mixers, which have been effectively used within crystallography and SAXS for a variety of systems, but their success is dependent on a specific set of conditions to facilitate fast diffusion for mixing. The use of a new chaotic advection mixer designed for microfluidic applications helps to further broaden the types of systems compatible with time-resolved mixing experiments. The chaotic advection mixer can create ultra-thin, alternating layers of liquid, enabling faster diffusion so that even more slowly diffusing molecules, like proteins or nucleic acids, can achieve fast mixing on timescales relevant to biological reactions. This mixer was first used in UV-vis absorbance and SAXS experiments with systems of a variety of molecular weights, and thus diffusion speeds. Careful effort was also dedicated to making a loop-loading sample-delivery system that consumes as little sample as possible, enabling the study of precious, laboratory-purified samples. The combination of the versatile mixer with low sample consumption opens the door to many new applications for mix-and-inject studies.

Alone and together: current approaches to targeting glutaminase enzymes as part of anti-cancer therapies

Metabolic reprogramming is a major hallmark of malignant transformation in cancer, and part of the so-called Warburg effect, in which the upregulation of glutamine catabolism plays a major role. The glutaminase enzymes convert glutamine to glutamate, which initiates this pathway. Inhibition of different forms of glutaminase (KGA, GAC, or LGA) demonstrated potential as an emerging anti-cancer therapeutic strategy. The regulation of these enzymes, and the molecular basis for their inhibition, have been the focus of much recent research. This review will explore the recent progress in understanding the molecular basis for activation and inhibition of different forms of glutaminase, as well as the recent focus on combination therapies of glutaminase inhibitors with other anti-cancer drugs.

Oncogenic RAS promotes MYC protein stability by upregulating the expression of the inhibitor of apoptosis protein family member Survivin

The small GTPase KRAS is frequently mutated in pancreatic cancer and its cooperation with the transcription factor MYC is essential for malignant transformation. The key to oncogenic KRAS and MYC working together is the stabilization of MYC expression due to KRAS activating the extracellular signal-regulated kinase 1/2, which phosphorylates MYC at serine 62 (Ser 62). This prevents the proteasomal degradation of MYC while enhancing its transcriptional activity. Here, we identify how this essential signaling connection between oncogenic KRAS and MYC expression is mediated by the inhibitor of apoptosis protein family member Survivin. This discovery stemmed from our finding that Survivin expression is downregulated upon treatment of pancreatic cancer cells with the KRAS^G12C inhibitor Sotorasib. We went on to show that oncogenic KRAS increases Survivin expression by activating extracellular signal-regulated kinase 1/2 in pancreatic cancer cells and that treating the cells either with siRNAs targeting Survivin or with YM155, a small molecule that potently blocks Survivin expression, downregulates MYC and strongly inhibited their growth. We further determined that Survivin protects MYC from degradation by blocking autophagy, which then prevents cellular inhibitor of protein phosphatase 2A from undergoing autophagic degradation. Cellular inhibitor of protein phosphatase 2A, by inhibiting protein phosphatase 2A, helps to maintain MYC phosphorylation at Ser 62, thereby ensuring its cooperation with oncogenic KRAS in driving cancer progression. Overall, these findings highlight a novel role for Survivin in mediating the cooperative actions of KRAS and MYC during malignant transformation and raise the possibility that targeting Survivin may offer therapeutic benefits against KRAS-driven cancers.

Evolving Experimental Techniques for Structure-Based Drug Design

Structure-based drug design (SBDD) is a prominent method in rational drug development and has traditionally benefitted from the atomic models of protein targets obtained using X-ray crystallography at cryogenic temperatures. In this perspective, we highlight recent advances in the development of structural techniques that are capable of probing dynamic information about protein targets. First, we discuss advances in the field of X-ray crystallography including serial room-temperature crystallography as a method for obtaining high-resolution conformational dynamics of protein-inhibitor complexes. Next, we look at cryogenic electron microscopy (cryoEM), another high-resolution technique that has recently been used to study proteins and protein complexes that are too difficult to crystallize. Finally, we present small-angle X-ray scattering (SAXS) as a potential high-throughput screening tool to identify inhibitors that target protein complexes and protein oligomerization.

Abstract 139: Dock7 regulates the AKT/mTOR pathway to promote survival and sustain the transformative properties of cancer cells

Akt is a well-known target of mitogenic signaling, which is activated by phosphoinositide 3-kinase, PDK1 and mTORC2, and is commonly regarded as a master regulator of cell survival. However, there is still a good deal to learn regarding how Akt can promote survival when cancer cells are faced by the multitude of challenges they need to overcome during the process of malignant transformation. While investigating the role of Dock7, a Cdc42 and Rac guanine nucleotide exchange factor of the Dock180 family, in tumorigenesis, we discovered an unexpected connection between Dock7, Akt and cell survival. Specifically, we found that Dock7 is necessary for cancer cells to survive under the stressful conditions that arise due to the loss of a substratum or serum deprivation, through its ability to stabilize a low, basal level of Akt kinase activity. Dock7 binds to Akt, as well as associates with the known Akt substrate, TSC2, promoting its phosphorylation. Since the phosphorylation of TSC2 eliminates its ability to work with its partner TSC1 to deactivate the Rheb GTPase, a direct activator of mTORC1, we probed whether mTORC1 function is also necessary for Dock7-dependent cancer cell survival. Indeed, we determined that Dock7, as well as its signaling partner Cdc42, is required to maintain a functional level of S6 kinase activity, a downstream target of mTORC1, which was prevented by the mTORC1 inhibitor rapamycin. We further show an association between mTOR, Rheb and Dock7, but not Raptor, the defining component of mTORC1. Additionally, Raptor is dispensable for the Dock7-dependent S6 kinase activity, despite its rapamycin sensitivity. Together, these findings elucidate a novel Dock7-Akt-mTOR signaling node which promotes cancer cell survival in the absence of classical mTORC1 function to allow cells to overcome stresses faced during the development of cancer.

Citation Format: Oriana Y. Teran, Miao-chong J. Lin, Matthew R. Zanotelli, Kristin F. Wilson, Richard A. Cerione. Dock7 regulates the AKT/mTOR pathway to promote survival and sustain the transformative properties of cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 139.

Exploring the Role of Transglutaminase in Patients with Glioblastoma: Current Perspectives

Tissue transglutaminase (tTG) is a rather unique GTP-binding/protein crosslinking enzyme that has been shown to play important roles in a number of cellular processes that impact both normal physiology and disease states. This is especially the case in the context of aggressive brain tumors, such as glioblastoma. The diverse roles played by tTG in cancer survival and progression have led to significant interest in recent years in using tTG as a therapeutic target. In this review, we provide a brief overview of the transglutaminase family, and then discuss the primary biochemical activities exhibited by tTG with an emphasis on the role it plays in glioblastoma progression. Finally, we consider current approaches to target tTG which might eventually have clinical impact.

Weakly migratory metastatic breast cancer cells activate fibroblasts via microvesicle-Tg2 to facilitate dissemination and metastasis

Cancer cell migration is highly heterogeneous, and the migratory capability of cancer cells is thought to be an indicator of metastatic potential. It is becoming clear that a cancer cell does not have to be inherently migratory to metastasize, with weakly migratory cancer cells often found to be highly metastatic. However, the mechanism through which weakly migratory cells escape from the primary tumor remains unclear. Here, utilizing phenotypically sorted highly and weakly migratory human breast cancer cells, we demonstrate that weakly migratory metastatic cells disseminate from the primary tumor via communication with stromal cells. While highly migratory cells are capable of single cell migration, weakly migratory cells rely on cell-cell signaling with fibroblasts to escape the primary tumor. Weakly migratory cells release microvesicles rich in tissue transglutaminase 2 (Tg2) which activate murine fibroblasts and lead weakly migratory cancer cell migration in vitro. These microvesicles also induce tumor stiffening and fibroblast activation in vivo and enhance the metastasis of weakly migratory cells. Our results identify microvesicles and Tg2 as potential therapeutic targets for metastasis and reveal a novel aspect of the metastatic cascade in which weakly migratory cells release microvesicles which activate fibroblasts to enhance cancer cell dissemination.

New insights into the molecular mechanisms of glutaminase C inhibitors in cancer cells using serial room temperature crystallography

Cancer cells frequently exhibit uncoupling of the glycolytic pathway from the TCA cycle (i.e. the "Warburg effect"), and as a result, often become dependent on their ability to increase glutamine catabolism. The mitochondrial enzyme Glutaminase C (GAC) helps to satisfy this 'glutamine addiction' of cancer cells by catalyzing the hydrolysis of glutamine to glutamate, which is then converted to the TCA-cycle intermediate α-ketoglutarate. This makes GAC an intriguing drug target, and spurred the molecules derived from bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl)ethyl sulfide (the so-called BPTES-class of allosteric GAC inhibitors), including CB-839, which is currently in clinal trials. However, none of the drugs targeting GAC are yet approved for cancer treatment and their mechanism of action is not well understood. Here, we shed new light on the underlying basis for the differential potencies exhibited by members of the BPTES/CB-839 family of compounds, which could not previously be explained with standard cryo-cooled X-ray crystal structures of GAC bound to CB-839 or its analogs. Using an emerging technique known as serial room temperature crystallography, we were able to observe clear differences between the binding conformations of inhibitors with significantly different potencies. We also developed a computational model to further elucidate the molecular basis of differential inhibitor potency. We then corroborated the results from our modeling efforts using recently established fluorescence assays that directly read out inhibitor binding to GAC. Together, these findings should aid in future design of more potent GAC inhibitors with better clinical outlook.

Abstract P093: Proteomic analysis reveals a mechanism of resistance to radiation mediated by microvesicles in glioma

Glioma accounts for 80% of malignant brain tumors. Standard treatment for glioma involves surgical resection followed by radiation therapy and chemotherapy, however, tumors recur leading to death in 12 to 18 months on average. Gliomas contain tumor-initiating cells (glioma stem cells, GSCs) that contribute causally to resistance to radiation therapy. GSCs shed large amounts of extracellular vesicles (EVs), which can potentiate growth, therapy resistance and invasiveness of less-aggressive cells present within the tumor microenvironment. However, how EVs mediate these effects is still not well-understood. To gain insights into mechanisms of resistance to radiation, we profiled the proteome of patient-derived GSCs that were either left untreated or treated with a standard therapeutic dose of ionizing radiation (IR). We identify a subset of patient-derived GSC lines that are resistant to radiation. Particularly, we find the cell line GSC-267 displaying a pronounced vesicular transport function. Microvesicles (MVs) but not exosomes shed by GSC-267 can strongly potentiate the proliferation of recipient cells. Subsequent proteomic analysis of EVs highlights elevated levels of nicotinamide phosphoribosyltransferase (NAMPT) within the MVs shed by GSC-267 as well as in the cells of origin. NAMPT is a central enzyme in NAD+ metabolism, is overexpressed in a subset of glioma patients, and correlates with poor patient survival. We find that the NAMPT protein is transferred to recipient cells via MVs derived from GSC-267. Furthermore, presence of NAMPT within MVs, and not solely overxpression within cells, is required for MVs to be able to rescue the proliferation of fibroblasts treated with a radio-mimetic compound. NAMPT inhibition ablates the proliferative gain induced by MVs derived from GSC-267. Supplementation with nicotinamide mononucleotide, the enzymatic product of NAMPT, does not enhance the effect of MVs derived from GSC-267, indicating a NAD+-dependent mechanism. MVs isolated from NAMPT knock-down GSC-267, which no longer carry the NAMPT protein, are not able to rescue the proliferation of recipient cells that were treated with IR. Finally, to more closely model the tumor microenvironment, we employ radiation-sensitive GSC-1079 as recipient cells. Transfer of MVs derived from GSC-267 is able to rescue the viability of GSC-1079 treated with a therapeutic dose of IR. In summary, MVs derived from the radiation-resistant GSC-267 are able to spread aggressive traits in the tumor microenvironment determining an overall shift towards a resistant phenotype. We identify the transfer of the enzyme NAMPT via MVs to be a causative mediator of resistance to radiation. Specific therapeutics targeting NAMPT have been tested in the context of other types of cancer but have shown a poor safety profile. Further analyses may indicate molecular mediators of the ability of NAMPT to confer resistance to radiation, which may lead to identifying novel targets for the treatment of glioma.

Citation Format: Elena Panizza, Brandon D. Regalado, Fangyu Wang, Robert J. Munroe, Nathaniel M. Vacanti, Marc A. Antonyak, Richard A. Cerione. Proteomic analysis reveals a mechanism of resistance to radiation mediated by microvesicles in glioma [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P093.

KRAS-dependent cancer cells promote survival by producing exosomes enriched in Survivin

Mutations in KRAS frequently occur in human cancer and are especially prevalent in pancreatic ductal adenocarcinoma (PDAC), where they have been shown to promote aggressive phenotypes. However, targeting this onco-protein has proven to be challenging, highlighting the need to further identify the various mechanisms used by KRAS to drive cancer progression. Here, we considered the role played by exosomes, a specific class of extracellular vesicles (EVs) derived from the endocytic cellular trafficking machinery, in mediating the ability of KRAS to promote cell survival. We found that exosomes isolated from the serum of PDAC patients, as well as from KRAS-transformed fibroblasts and pancreatic cancer cells, were all highly enriched in the cell survival protein Survivin. Exosomes containing Survivin, upon engaging serum-starved cells, strongly enhanced their survival. Moreover, they significantly compromised the effectiveness of the conventional chemotherapy drug paclitaxel, as well as a novel therapy that combines an ERK inhibitor with chloroquine, which is currently in clinical trials for PDAC. The survival benefits provided by oncogenic KRAS-derived exosomes were markedly reduced when depleted of Survivin using siRNA or upon treatment with the Survivin inhibitor YM155. Taken together, these findings demonstrate how KRAS mutations give rise to exosomes that provide a unique form of intercellular communication to promote cancer cell survival and therapy resistance, as well as raise interesting possibilities regarding their potential for serving as therapeutic targets and diagnostic markers for KRAS-dependent cancers.

Isolation and characterization of extracellular vesicles produced by cell lines

Cells produce two broad classes of extracellular vesicles (EVs), exosomes and microvesicles (MVs). Exosomes are 30–150 nm vesicles derived from multivesicular bodies, while MVs are 200–1,000 nm vesicles that pinch off from plasma membranes. Reliable isolation of EVs is crucial to understand their biochemical and functional properties. Here, we describe a protocol to isolate and characterize EVs from conditioned medium from mammalian cell lines. This protocol has been optimized for adherent cells but can also be adapted for suspension cells.

For complete details on the use and execution of this protocol, please refer to Latifkar et al. (2019).

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Cells produce two major classes of extracellular vesicles, exosomes and microvesicles

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Extracellular vesicles function as mediators of intercellular communication

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The contents of EVs promote phenotypic changes and can be used as diagnostic markers

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Here, we provide an optimized protocol for isolating exosomes and microvesicles

Pharmacological and genetic perturbation establish SIRT5 as a promising target in breast cancer

SIRT5 is a member of the sirtuin family of NAD⁺-dependent protein lysine deacylases implicated in a variety of physiological processes. SIRT5 removes negatively charged malonyl, succinyl, and glutaryl groups from lysine residues and thereby regulates multiple enzymes involved in cellular metabolism and other biological processes. SIRT5 is overexpressed in human breast cancers and other malignancies, but little is known about the therapeutic potential of SIRT5 inhibition for treating cancer. Here we report that genetic SIRT5 disruption in breast cancer cell lines and mouse models caused increased succinylation of IDH2 and other metabolic enzymes, increased oxidative stress, and impaired transformation and tumorigenesis. We therefore developed potent, selective, and cell permeable small molecule SIRT5 inhibitors. SIRT5 inhibition suppressed the transformed properties of cultured breast cancer cells and significantly reduced mammary tumor growth in vivo, in both genetically engineered and xenotransplant mouse models. Considering that Sirt5 knockout mice are generally normal, with only mild phenotypes observed, these data establish SIRT5 as a promising target for treating breast cancer. The new SIRT5 inhibitors provide useful probes for future investigations of SIRT5 and an avenue for targeting SIRT5 as a therapeutic strategy.

Cdc42 functions as a regulatory node for tumour-derived microvesicle biogenesis

Tumour-derived microvesicles (MVs) serve as critical mediators of cell-to-cell communication in the tumour microenvironment. So far, the underlying mechanisms of MV biogenesis, especially how key tumorigenesis signals such as abnormal EGF signalling regulates MV release, remain unclear. Here, we set out to establish reliable readouts for MV biogenesis and then explore the molecular mechanisms that regulate MV generation. We found that Rho family small G protein Cdc42 is a convergent node of multiple regulatory signals that occur in MV biogenesis. The binding of activated GTP-bound Cdc42 and its downstream effector, Ras GTPase-activating-like protein 1 (IQGAP1), is required for MV shedding. Activated Cdc42 maintains sustained EGF signalling by inhibiting the internalization of cell surface receptors, including EGFR and the VEGF oligomer, VEGF_90K, and then facilitates MV release. Subsequently, we further demonstrated that blocking these signalling pathways using the corresponding mutants effectively reduced MV shedding and significantly inhibited MV-promoted in vivo tumour angiogenesis. These findings reveal a complex regulation of MV shedding by tumour cells, shedding light on the regulatory mechanism of MV biogenesis, and potentially contributing to strategies that target MVs in cancer therapy.

Extracellular Vesicles and Their Roles in Cancer Progression

Extracellular vesicles (EVs) produced by cancer cells function as a unique form of intercellular communication that can promote cell growth and survival, help shape the tumor microenvironment, and increase invasive and metastatic activity. There are two major classes of EVs, microvesicles (MVs) and exosomes, and they differ in how they are formed. MVs are generated by the outward budding and fission of the plasma membrane. On the other hand, exosomes are derived as multivesicular bodies (MVBs) fuse with the plasma membrane and release their contents. What makes EVs especially interesting is how they mediate their effects. Both MVs and exosomes have been shown to contain a wide-variety of bioactive cargo, including cell surface, cytosolic, and nuclear proteins, as well as RNA transcripts, micro-RNAs (miRNAs), and even fragments of DNA. EVs, and their associated cargo, can be transferred to other cancer cells, as well as to normal cell types, causing the recipient cells to undergo phenotypic changes that promote different aspects of cancer progression. These findings, combined with those demonstrating that the amounts and contents of EVs produced by cancer cells can vary depending on their cell of origin, stage of development, or response to therapies, have raised the exciting possibility that EVs can be used for diagnostic purposes. Moreover, the pharmaceutical community is aggressively pursuing the use of EVs as a potential drug delivery platform. Here, in this chapter, we will highlight what is currently known about how EVs are generated, how they impact cancer progression, and the different ways they are being exploited for clinical applications.

Structure of the Visual Signaling Complex between Transducin and Phosphodiesterase 6

Heterotrimeric G proteins communicate signals from activated G protein-coupled receptors to downstream effector proteins. In the phototransduction pathway responsible for vertebrate vision, the G protein-effector complex is composed of the GTP-bound transducin α subunit (Gα_T·GTP) and the cyclic GMP (cGMP) phosphodiesterase 6 (PDE6), which stimulates cGMP hydrolysis, leading to hyperpolarization of the photoreceptor cell. Here we report a cryo-electron microscopy (cryoEM) structure of PDE6 complexed to GTP-bound Gα_T. The structure reveals two Gα_T·GTP subunits engaging the PDE6 hetero-tetramer at both the PDE6 catalytic core and the PDEγ subunits, driving extensive rearrangements to relieve all inhibitory constraints on enzyme catalysis. Analysis of the conformational ensemble in the cryoEM data highlights the dynamic nature of the contacts between the two Gα_T·GTP subunits and PDE6 that supports an alternating-site catalytic mechanism.

Liver-Type Glutaminase GLS2 Is a Druggable Metabolic Node in Luminal-Subtype Breast Cancer

Efforts to target glutamine metabolism for cancer therapy have focused on the glutaminase isozyme GLS. The importance of the other isozyme, GLS2, in cancer has remained unclear, and it has been described as a tumor suppressor in some contexts. Here, we report that GLS2 is upregulated and essential in luminal-subtype breast tumors, which account for >70% of breast cancer incidence. We show that GLS2 expression is elevated by GATA3 in luminal-subtype cells but suppressed by promoter methylation in basal-subtype cells. Although luminal breast cancers resist GLS-selective inhibitors, we find that they can be targeted with a dual-GLS/GLS2 inhibitor. These results establish a critical role for GLS2 in mammary tumorigenesis and advance our understanding of how to target glutamine metabolism in cancer.

Lukey et al. report that basal- and luminal-subtype breast cancers employ different strategies for glutamine catabolism, impacting their sensitivity profiles to glutaminase inhibitors. Elevated GLS2 expression in luminal-subtype cancers is driven in part by GATA3. Targeting GLS2 with the pan-glutaminase inhibitor 968 inhibits luminal-subtype breast cancer cell proliferation and tumorigenesis.

Abstract 1447: Elucidating the mechanism of Dock7-mediated cellular transformation

Akt is a serine/threonine protein kinase that receives signals from a plethora of external and internal stimuli in order to promote survival and regulate cellular growth and metabolism. Akt is part of the highly conserved PI3K/Akt signaling axis and is classically regulated by its interaction with the phosphoinositide, PIP3, and its subsequent phosphorylation at S473 and S308 by the mTOR complex 2 (mTORC2) and 3-Phosphoinositide-dependent protein kinase 1 (PDK1), respectively. Altered Akt activity has been linked to numerous diseases, such as cancer, diabetes and neurodegenerative diseases. Specifically, PI3K/Akt deregulation occurs in approximately one third of human cancers and ~40% of breast cancers.

My research focuses on Dock7, a Guanine Nucleotide Exchange Factor (GEF) for the small GTPases Cdc42 and Rac1, and a novel player in the regulation of Akt activity and stability. Dock7 belongs to the Dock family of atypical GEFs and contains two evolutionarily conserved domains, DHR1 (a putative lipid binding domain) and DHR2 (the catalytic GEF domain). The expression level of Dock7 correlates with an unfavorable prognosis in liver cancer patients, and it is highly expressed in triple-negative breast cancers. Studies in our laboratory showed Dock7 is essential for the transformed properties of several cancer cell lines, and that it interacts with Akt in co-immunoprecipitation assays. In order to decipher the mechanism by which Dock7 mediates transformation, I created a Crispr-Cas9 Dock7 Knock-out HeLa cell line. I proceeded to confirm that these cells are not able to grow in an anchorage independent manner nor in serum free conditions, two hallmarks of transformation. Furthermore, in comparison to the phosphorylation status of Akt in parental HeLa cells, I found that in Dock7 KO HeLa cells, Akt is not phosphorylated at the S473 site in the absence of growth factors, indicative of lower basal activity. Accordingly, phosphorylation of downstream effectors of Akt is also downregulated. Besides promoting the phosphorylation state of Akt, Dock7 plays a role in the stability of the Akt protein during certain cellular stresses, highlighting a versatile and important role for Dock7 in Akt regulation and a potential target for therapeutic intervention in tumors where Akt is deregulated.

Citation Format: Oriana Y. Teran Pumar, Kristin F. Wilson, Richard A. Cerione. Elucidating the mechanism of Dock7-mediated cellular transformation [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1447.

Lysine succinylation and SIRT5 couple nutritional status to glutamine catabolism

The metabolic microenvironment of tumors is characterized by fluctuating and limited nutrient availability. To survive these conditions, cancer cell-intrinsic mechanisms sense and signal nutritional status. We describe how glutaminase (GLS) is destabilized by lysine succinylation and stabilized by the NAD⁺-dependent desuccinylase sirtuin 5 (SIRT5), coupling nutrient levels to metabolic flux.

Identification of ALDH1A3 as a Viable Therapeutic Target in Breast Cancer Metastasis-Initiating Cells

The development of efficacious therapies targeting metastatic spread of breast cancer to the brain represents an unmet clinical need. Accordingly, an improved understanding of the molecular underpinnings of central nervous system spread and progression of breast cancer brain metastases (BCBM) is required. In this study, the clinical burden of disease in BCBM was investigated, as well as the role of aldehyde dehydrogenase 1A3 (ALDH1A3) in the metastatic cascade leading to BCBM development. Initial analysis of clinical survival trends for breast cancer and BCBM determined improvement of breast cancer survival rates; however, this has failed to positively affect the prognostic milestones of triple-negative breast cancer (TNBC) brain metastases (BM). ALDH1A3 and a representative epithelial-mesenchymal transition (EMT) gene signature (mesenchymal markers, CD44 or Vimentin) were compared in tumors derived from BM, lung metastases (LM), or bone metastases (BoM) of patients as well as mice after injection of TNBC cells. Selective elevation of the EMT signature and ALDH1A3 were observed in BM, unlike LM and BoM, especially in the tumor edge. Furthermore, ALDH1A3 was determined to play a role in BCBM establishment via regulation of circulating tumor cell adhesion and migration phases in the BCBM cascade. Validation through genetic and pharmacologic inhibition of ALDH1A3 via lentiviral shRNA knockdown and a novel small-molecule inhibitor demonstrated selective inhibition of BCBM formation with prolonged survival of tumor-bearing mice. Given the survival benefits via targeting ALDH1A3, it may prove an effective therapeutic strategy for BCBM prevention and/or treatment.

Starving the Devourer: Cutting Cancer Off from Its Favorite Foods

Increased consumption of glucose and glutamine are metabolic hallmarks of tumorigenesis. In this issue of Cell Chemical Biology, Reckzeh et al. (2019) describe the discovery of Glutor, a potent inhibitor of cellular glucose uptake. Combining Glutor with the glutaminase inhibitor CB-839 creates a metabolic crisis in cancer cells, synergistically impeding proliferation.

Purification of the Rhodopsin-Transducin Complex for Structural Studies

G protein-coupled receptors (GPCRs) comprise the largest family of transmembrane receptors and are targets for over 30% of all drugs on the market. Structural information of GPCRs and more importantly that of the complex between GPCRs and their signaling partner heterotrimeric G proteins is of great importance. Here we present a method for the large-scale purification of the rhodopsin-transducin complex, the GPCR-G protein signaling complex in visual phototransduction, directly from their native retinal membrane using native proteins purified from bovine retinae. Formation of the complex on native membrane is orchestrated in part by the proper engagement of lipid-modified rhodopsin and transducin (i.e., palmitoylation of the rhodopsin C-terminus, myristoylation and farnesylation of the α_T and γ₁, respectively). The resulting complex is of high purity and stability and has proved suitable for further biophysical and structural studies. The methods described here should be applicable to other recombinantly expressed receptors from insect cells or mamalian cells by forming stable, functional complexes directly on purified cell membranes.

The two splice variant forms of Cdc42 exert distinct and essential functions in neurogenesis

The small GTPase cell division cycle 42 (CDC42) plays essential roles in neurogenesis and brain development. Previously, using murine embryonic P19 cells as a model system, we showed that CDC42 stimulates mTOR complex 1 (mTORC1) activity and thereby up-regulates transcription factors required for the formation of neural progenitor cells. However, paradoxically, although endogenous CDC42 is required for both the initial transition of undifferentiated P19 cells to neural progenitors and their ultimate terminal differentiation into neurons, ectopic CDC42 overexpression promotes only the first stage of neurogenesis (i.e. the formation of neuroprogenitors) and not the second phase (differentiation into neurons). Here, using both P19 cells and mouse embryonic stem cells, we resolve this paradox, demonstrating that two splice variants of CDC42, differing only in nine amino acid residues in their very C-terminal regions, play distinct roles in neurogenesis. We found that a CDC42 splice variant that has a ubiquitous tissue distribution, termed here as CDC42u, specifically drives the formation of neuroprogenitor cells, whereas a brain-specific CDC42 variant, CDC42b, is essential for promoting the transition of neuroprogenitor cells to neurons. We further show that the specific roles of CDC42u and CDC42b in neurogenesis are due to their opposing effects on mTORC1 activity. Specifically, CDC42u stimulated mTORC1 activity and thereby induced neuroprogenitor formation, whereas CDC42b worked together with activated CDC42-associated kinase (ACK) in down-regulating mTOR expression and promoting neuronal differentiation. These findings highlight the remarkable functional specificities of two highly similar CDC42 splice variants in regulating distinct stages of neurogenesis.

The activation loop and substrate-binding cleft of glutaminase C are allosterically coupled

The glutaminase C (GAC) isoform of mitochondrial glutaminase is overexpressed in many cancer cells and therefore represents a potential therapeutic target. Understanding the regulation of GAC activity has been guided by the development of spectroscopic approaches that measure glutaminase activity in real time. Previously, we engineered a GAC protein (GAC(F327W)) in which a tryptophan residue is substituted for phenylalanine in an activation loop to explore the role of this loop in enzyme activity. We showed that the fluorescence emission of Trp-327 is enhanced in response to activator binding, but quenched by inhibitors of the BPTES class that bind to the GAC tetramer and contact the activation loop, thereby constraining it in an inactive conformation. In the present work, we took advantage of a tryptophan substitution at position 471, proximal to the GAC catalytic site, to examine the conformational coupling between the activation loop and the substrate-binding cleft, separated by ∼16 Å. Comparison of glutamine binding in the presence or absence of the BPTES analog CB-839 revealed a reciprocal relationship between the constraints imposed on the activation loop position and the affinity of GAC for substrate. Binding of the inhibitor weakened the affinity of GAC for glutamine, whereas activating anions such as P_i increased this affinity. These results indicate that the conformations of the activation loop and the substrate-binding cleft in GAC are allosterically coupled and that this coupling determines substrate affinity and enzymatic activity and explains the activities of CB-839, which is currently in clinical trials.

Extracellular vesicles and their roles in stem cell biology

Stem cells use a variety of mechanisms to help maintain their pluripotency and promote self-renewal, as well as, at the appropriate time, to differentiate into specialized cells. One such mechanism that is attracting significant attention from the stem cell, development, and regenerative medicine research communities involves a form of intercellular communication, specifically, the ability of cells to form and release nontraditional membrane-enclosed structures, referred to as extracellular vesicles (EVs). There are two major classes of EVs, microvesicles (MVs), which are generated through the outward budding and fission of the plasma membrane, and exosomes, which are formed as multivesicular bodies (MVBs) in the endo-lysosomal pathway that fuse with the cell surface to release their contents. Although they differ in how they are formed, both MVs and exosomes have been shown to contain a diverse array of bioactive cargo, such as proteins, RNA transcripts, microRNAs, and even DNA, which can be transferred to other cells and promote phenotypic changes. Here, we will describe what is currently known regarding EVs and the roles they play in stem cell biology and different aspects of early development. We will also highlight how the EVs produced by stem cells are being aggressively pursued for clinical applications, including their potential use as therapeutic delivery systems and for their regenerative capabilities.

Tissue transglutaminase 2 regulates tumor cell tensional homeostasis by increasing contractility

Abnormal tensional cellular homeostasis is now considered a hallmark of cancer. Despite this, the origin of this abnormality remains unclear. In this work, we investigated the role of tissue transglutaminase 2 (TG2, also known as TGM2), a protein associated with poor prognosis and increased metastatic potential, and its relationship to the EGF receptor in the regulation of the mechanical state of tumor cells. Remarkably, we observed a TG2-mediated modulation of focal adhesion composition as well as stiffness-induced FAK activation, which was linked with a distinctive increase in cell contractility, in experiments using both pharmacological and shRNA-based approaches. Additionally, the increased contractility could be reproduced in non-malignant cells upon TG2 expression. Moreover, the increased cell contractility mediated by TG2 was largely due to the loss of EGFR-mediated inhibition of cell contractility. These findings establish intracellular TG2 as a regulator of cellular tensional homeostasis and suggest the existence of signaling switches that control the contribution of growth factor receptors in determining the mechanical state of a cell.

Structures of the Rhodopsin-Transducin Complex: Insights into G-Protein Activation

Rhodopsin (Rho), a prototypical G-protein-coupled receptor (GPCR) in vertebrate vision, activates the G-protein transducin (G_T) by catalyzing GDP-GTP exchange on its α subunit (Gα_T). To elucidate the determinants of G_T coupling and activation, we obtained cryo-EM structures of a fully functional, light-activated Rho-G_T complex in the presence and absence of a G-protein-stabilizing nanobody. The structures illustrate how G_T overcomes its low basal activity by engaging activated Rho in a conformation distinct from other GPCR-G-protein complexes. Moreover, the nanobody-free structures reveal native conformations of G-protein components and capture three distinct conformers showing the Gα_T helical domain (αHD) contacting the Gβγ subunits. These findings uncover the molecular underpinnings of G-protein activation by visual rhodopsin and shed new light on the role played by Gβγ during receptor-catalyzed nucleotide exchange.

Matrix stiffness regulates microvesicle-induced fibroblast activation

Extracellular vesicles released by cancer cells have recently been implicated in the differentiation of stromal cells to their activated, cancer-supporting states. Microvesicles, a subset of extracellular vesicles released from the plasma membrane of cancer cells, contain biologically active cargo, including DNA, mRNA, and miRNA, which are transferred to recipient cells and induce a phenotypic change in behavior. While it is known that microvesicles can alter recipient cell phenotype, little is known about how the physical properties of the tumor microenvironment affect fibroblast response to microvesicles. Here, we utilized cancer cell-derived microvesicles and synthetic substrates designed to mimic the stiffness of the tumor and tumor stroma to investigate the effects of microvesicles on fibroblast phenotype as a function of the mechanical properties of the microenvironment. We show that microvesicles released by highly malignant breast cancer cells cause an increase in fibroblast spreading, α-smooth muscle actin expression, proliferation, cell-generated traction force, and collagen gel compaction. Notably, our data indicate that these phenotypic changes occur only on stiff matrices mimicking the stiffness of the tumor periphery and are dependent on the cell type from which the microvesicles are shed. Overall, these results show that the effects of cancer cell-derived microvesicles on fibroblast activation are regulated by the physical properties of the microenvironment, and these data suggest that microvesicles may have a more robust effect on fibroblasts located at the tumor periphery to influence cancer progression.

New insights into extracellular vesicle biogenesis and function

It is becoming increasingly evident that most cell types are capable of forming and releasing multiple distinct classes of membrane-enclosed packages, referred to as extracellular vesicles (EVs), as a form of intercellular communication. Microvesicles (MVs) represent one of the major classes of EVs and are formed by the outward budding of the plasma membrane. The second major class of EVs, exosomes, are produced as components of multivesicular bodies (MVBs) and are released from cells when MVBs fuse with the cell surface. Both MVs and exosomes have been shown to contain proteins, RNA transcripts, microRNAs and even DNA that can be transferred to other cells and thereby trigger a broad range of cellular activities and biological responses. However, EV biogenesis is also frequently de-regulated in different pathologies, especially cancer, where MVs and exosomes have been suggested to promote tumor cell growth, therapy resistance, invasion and even metastasis. In this Review, we highlight some of the recent advances in this rapidly emerging and exciting field of cell biology, focusing on the underlying mechanisms that drive MV and exosome formation and release, with a particular emphasis on how EVs potentially impact different aspects of cancer progression and stem cell biology.

Inhibition of cancer metabolism: a patent landscape

Cancer metabolism is currently a hot topic. Since it was first realized that cancer cells rely upon an altered metabolic program to sustain their rapid proliferation, the enzymes that support those metabolic changes have appeared to be good targets for pharmacological intervention. Here, we discuss efforts pertaining to targets in cancer metabolism, focusing upon the tricarboxylic acid cycle and the mechanisms which feed nutrients into it. We describe a broad landscape of small-molecule inhibitors, targeting a dozen different proteins, each implicated in cancer progression. We hope that this will serve as a reference both to the areas being most highly examined today and, relatedly, the areas that are still ripe for novel intervention.

Loss of Sirtuin 1 Alters the Secretome of Breast Cancer Cells by Impairing Lysosomal Integrity

The NAD⁺-dependent deacetylase Sirtuin 1 (SIRT1) is down-regulated in triple-negative breast cancer. To determine the mechanistic basis by which reduced SIRT1 expression influences processes related to certain aggressive cancers, we examined the consequences of depleting breast cancer cells of SIRT1. We discovered that reducing SIRT1 levels decreased the expression of one particular subunit of the vacuolar-type H+ ATPase (V-ATPase), which is responsible for proper lysosomal acidification and protein degradation. This impairment in lysosomal function caused a reduction in the number of multi-vesicular bodies (MVBs) targeted for lysosomal degradation and resulted in larger MVBs prior to their fusing with the plasma membrane to release their contents. Collectively, these findings help explain how reduced SIRT1 expression, by disrupting lysosomal function and generating a secretome comprising exosomes with unique cargo and soluble hydrolases that degrade the extracellular matrix, can promote processes that increase breast-cancer-cell survival and invasion.

Amorphous Quantum Nanomaterials

In quantum materials, macroscopic behavior is governed in nontrivial ways by quantum phenomena. This is usually achieved by exquisite control over atomic positions in crystalline solids. Here, it is demonstrated that the use of disordered glassy materials provides unique opportunities to tailor quantum material properties. By borrowing ideas from single-molecule spectroscopy, single delocalized π-electron dye systems are isolated in relatively rigid ultrasmall (<10 nm diameter) amorphous silica nanoparticles. It is demonstrated that chemically tuning the local amorphous silica environment around the dye over a range of compositions enables exquisite control over dye quantum behavior, leading to efficient probes for photodynamic therapy (PDT) and stochastic optical reconstruction microscopy (STORM). The results suggest that efficient fine-tuning of light-induced quantum behavior mediated via effects like spin-orbit coupling can be effectively achieved by systematically varying averaged local environments in glassy amorphous materials as opposed to tailoring well-defined neighboring atomic lattice positions in crystalline solids. The resulting nanoprobes exhibit features proven to enable clinical translation.

Reconstitution of the Rhodopsin-Transducin Complex into Lipid Nanodiscs

Transmembrane proteins, such as G protein-coupled receptors (GPCR), require solubilization in detergents prior to purification. The recent development of novel detergents has allowed for the stabilization of GPCRs, which typically have a high degree of structural flexibility and are otherwise subject to denaturation. However, the detergent micelle environment is still very different from the native lipid membrane and the activity of GPCRs can be profoundly affected by interactions with annular lipid molecules. Moreover, GPCRs are often palmitoylated at their intracellular side, and a lipid bilayer environment would allow for proper orientation of these lipid modifications. Therefore, a reconstituted lipid bilayer environment would best mimic the physiological receptor microenvironment for biophysical studies of GPCRs and nanodiscs provide a methodology to address this aim. Nanodiscs are lipid bilayer discs stabilized by amphipathic membrane scaffolding proteins (MSP) where detergent-solubilized transmembrane proteins can be incorporated into them through a self-assembly process. Here we present a method for reconstituting the purified detergent-solubilized rhodopsin-transducin complex, the GPCR-G protein complex in visual phototransduction, into nanodiscs. The resulting complex incorporated into lipid nanodiscs can be used in biophysical studies including small-angle X-ray scattering and electron microscopy. This method is applicable to integral membrane proteins that mediate protein lipidation, including the zDHHC-family of S-acyltransferases and membrane-bound O-acyltransferases.

Gain-of-function screen of α-transducin identifies an essential phenylalanine residue necessary for full effector activation

Two regions on the α subunits of heterotrimeric GTP-binding proteins (G-proteins), the Switch II/α2 helix (which changes conformation upon GDP-GTP exchange) and the α3 helix, have been shown to contain the binding sites for their effector proteins. However, how the binding of Gα subunits to their effector proteins is translated into the stimulation of effector activity is still poorly understood. Here, we took advantage of a reconstituted rhodopsin-coupled phototransduction system to address this question and identified a distinct surface and an essential residue on the α subunit of the G-protein transducin (α_T) that is necessary to fully activate its effector enzyme, the cGMP phosphodiesterase (PDE). We started with a chimeric G-protein α subunit (α_T*) comprising residues mainly from α_T and a short stretch of residues from the G_i1 α subunit (α_i1), which only weakly stimulates PDE activity. We then reinstated the α_T residues by systematically replacing the corresponding α_i1 residues within α_T* with the aim of fully restoring PDE stimulatory activity. These experiments revealed that the αG/α4 loop and a phenylalanine residue at position 283 are essential for conferring the α_T* subunit with full PDE stimulatory capability. We further demonstrated that this same region and amino acid within the α subunit of the G_s protein (α_s) are necessary for full adenylyl cyclase activation. These findings highlight the importance of the αG/α4 loop and of an essential phenylalanine residue within this region on Gα subunits α_T and α_s as being pivotal for their selective and optimal stimulation of effector activity.

A small molecule regulator of tissue transglutaminase conformation inhibits the malignant phenotype of cancer cells

The protein crosslinking enzyme tissue transglutaminase (tTG) is an acyltransferase which catalyzes transamidation reactions between two proteins, or between a protein and a polyamine. It is frequently overexpressed in several different types of human cancer cells, where it has been shown to contribute to their growth, survival, and invasiveness. tTG is capable of adopting two distinct conformational states: a protein crosslinking active (“open”) state, and a GTP-bound, crosslinking inactive (“closed”) state. We have previously shown that the ectopic expression of mutant forms of tTG, which constitutively adopt the open conformation, are toxic to cells. This raises the possibility that strategies directed toward causing tTG to maintain an open state could potentially provide a therapeutic benefit for cancers in which tTG is highly expressed. Here, we report the identification of a small molecule, TTGM 5826, which stabilizes the open conformation of tTG. Treatment of breast and brain cancer cell lines, as well as glioma stem cells, with this molecule broadly inhibits their transformed phenotypes. Thus, TTGM 5826 represents the lead compound for a new class of small molecules that promote the toxicity of cancer cells by stabilizing the open state of tTG.

Targeting Therapy Resistance: When Glutamine Catabolism Becomes Essential

Identifying contexts in which cancer cells become addicted to specific nutrients is critical for developing targeted metabolic therapies. In this issue of Cancer Cell, Momcilovic et al. report that suppressed glycolysis following mTOR inhibition is countered by adaptive glutamine catabolism in lung squamous cell carcinoma, sensitizing tumors to glutaminase inhibition.

Characterization of the interactions of potent allosteric inhibitors with glutaminase C, a key enzyme in cancer cell glutamine metabolism

Altered glycolytic flux in cancer cells (the "Warburg effect") causes their proliferation to rely upon elevated glutamine metabolism ("glutamine addiction"). This requirement is met by the overexpression of glutaminase C (GAC), which catalyzes the first step in glutamine metabolism and therefore represents a potential therapeutic target. The small molecule CB-839 was reported to be more potent than other allosteric GAC inhibitors, including the parent compound bis-2-(5-phenylacetamido-1,2,4-thiadiazol-2-yl)ethyl (BPTES), and is in clinical trials. Recently, we described the synthesis of BPTES analogs having distinct saturated heterocyclic cores as a replacement for the flexible chain moiety, with improved microsomal stability relative to CB-839 and BPTES. Here, we show that one of these new compounds, UPGL00004, like CB-839, more potently inhibits the enzymatic activity of GAC, compared with BPTES. We also compare the abilities of UPGL00004, CB-839, and BPTES to directly bind to recombinant GAC and demonstrate that UPGL00004 has a similar binding affinity as CB-839 for GAC. We also show that UPGL00004 potently inhibits the growth of triple-negative breast cancer cells, as well as tumor growth when combined with the anti-vascular endothelial growth factor antibody bevacizumab. Finally, we compare the X-ray crystal structures for UPGL00004 and CB-839 bound to GAC, verifying that UPGL00004 occupies the same binding site as CB-839 or BPTES and that all three inhibitors regulate the enzymatic activity of GAC via a similar allosteric mechanism. These results provide insights regarding the potency of these inhibitors that will be useful in designing novel small-molecules that target a key enzyme in cancer cell metabolism.

The diamond anniversary of tissue transglutaminase: a protein of many talents

Tissue transglutaminase (tTG) is capable of binding and hydrolyzing GTP, as well as catalyzing an enzymatic transamidation reaction that crosslinks primary amines to glutamine residues. tTG adopts two vastly different conformations, depending on whether it is functioning as a GTP-binding protein or a crosslinking enzyme. It has been shown to have important roles in several different aspects of cancer progression, making it an attractive target for therapeutic intervention. Here, we highlight many of the major findings involving tTG since its discovery 60 years ago, and describe recent drug discovery efforts that target specific activities or conformations of this unique protein.