Opinion: Searching for the elusive targets of farnesyltransferase inhibitors

The Genomic, Transcriptomic, and Immunologic Landscape of HRAS Mutations in Solid Tumors

Tipifarnib is the only targeted therapy breakthrough for HRAS-mutant (HRASmt) recurrent or metastatic head and neck squamous cell carcinoma (HNSCC). The molecular profiles of HRASmt cancers are difficult to explore given the low frequency of HRASmt. This study aims to understand the molecular co-alterations, immune profiles, and clinical outcomes of 524 HRASmt solid tumors including urothelial carcinoma (UC), breast cancer (BC), non-small-cell lung cancer (NSCLC), melanoma, and HNSCC. HRASmt was most common in UC (3.0%), followed by HNSCC (2.82%), melanoma (1.05%), BC (0.45%), and NSCLC (0.44%). HRASmt was absent in Her2+ BC regardless of hormone receptor status. HRASmt was more frequently associated with squamous compared to non-squamous NSCLC (60% vs. 40% in HRASwt, p = 0.002). The tumor microenvironment (TME) of HRASmt demonstrated increased M1 macrophages in triple-negative BC (TNBC), HNSCC, squamous NSCLC, and UC; increased M2 macrophages in TNBC; and increased CD8+ T-cells in HNSCC (all p < 0.05). Finally, HRASmt was associated with shorter overall survival in HNSCC (HR: 1.564, CI: 1.16-2.11, p = 0.003) but not in the other cancer types examined. In conclusion, this study provides new insights into the unique molecular profiles of HRASmt tumors that may help to identify new targets and guide future clinical trial design.

Geranylgeranylated-SCFFBXO10 Regulates Selective Outer Mitochondrial Membrane Proteostasis and Function

E3-ubiquitin ligases (E3s) are main components of the ubiquitin-proteasome system (UPS), as they determine substrate specificity in response to internal and external cues to regulate protein homeostasis. However, the regulation of membrane protein ubiquitination by E3s within distinct cell membrane compartments or organelles is not well understood. We show that FBXO10, the interchangeable component of the SKP1/CUL1/F-box ubiquitin ligase complex (SCF-E3), undergoes lipid-modification with geranylgeranyl isoprenoid at Cysteine953 (C953), facilitating its dynamic trafficking to the outer mitochondrial membrane (OMM). FBXO10 polypeptide does not contain a canonical mitochondrial targeting sequence (MTS); instead, its geranylgeranylation at C953 and the interaction with two cytosolic factors, PDE6δ (a prenyl group-binding protein), and HSP90 (a mitochondrial chaperone) orchestrate specific OMM targeting of prenyl-FBXO10 across diverse membrane compartments. The geranylgeranylation-deficient FBXO10(C953S) mutant redistributes away from the OMM, leading to impaired mitochondrial ATP production, decreased mitochondrial membrane potential, and increased mitochondrial fragmentation. Phosphoglycerate mutase 5 (PGAM5) was identified as a potential substrate of FBXO10 at the OMM using comparative quantitative mass spectrometry analyses of enriched mitochondria (LFQ-MS/MS), leveraging the redistribution of FBXO10(C953S). FBXO10, but not FBXO10(C953S), promoted polyubiquitylation and degradation of PGAM5. Examination of the role of this pathway in a physiological context revealed that the loss of FBXO10 or expression of prenylation-deficient-FBXO10(C953S) inhibited PGAM5 degradation, disrupted mitochondrial homeostasis, and impaired myogenic differentiation of human iPSCs and murine myoblasts. Our studies identify a mechanism for selective E3-ligase mediated regulation of mitochondrial membrane proteostasis and metabolic health, potentially amenable to therapeutic intervention.

Pepticinnamins N, O, and P, Nonribosomal Peptides from the Soil-Derived Streptomyces mirabilis P8-A2

Cinnamoyl moiety containing nonribosomal peptides represented by pepticinnamin E are a growing family of natural products isolated from different Streptomyces species and possess diverse bioactivities. The soil bacterium Streptomyces mirabilis P8-A2 harbors a cryptic pepticinnamin biosynthetic gene cluster, producing azodyrecins as major products. Inactivation of the azodyrecin biosynthetic gene cluster by CRISPR-BEST base editing led to the activation and production of pepticinnamin E (1) and its analogues, pepticinnamins N, O, and P (2-4), the structures of which were determined by detailed NMR spectroscopy, HRMS data, and Marfey's reactions. These new compounds did not show a growth inhibitory effect against the LNCaP and C4-2B prostate cancer lines, respectively.

Physiological adventures in Candida albicans: farnesol and ubiquinones

SUMMARYFarnesol was first identified as a quorum-sensing molecule, which blocked the yeast to hyphal transition in Candida albicans, 22 years ago. However, its interactions with Candida biology are surprisingly complex. Exogenous (secreted or supplied) farnesol can also act as a virulence factor during pathogenesis and as a fungicidal agent triggering apoptosis in other competing fungi. Farnesol synthesis is turned off both during anaerobic growth and in opaque cells. Distinctly different cellular responses are observed as exogenous farnesol levels are increased from 0.1 to 100 µM. Reported changes include altered morphology, stress response, pathogenicity, antibiotic sensitivity/resistance, and even cell lysis. Throughout, there has been a dearth of mechanisms associated with these observations, in part due to the absence of accurate measurement of intracellular farnesol levels (Fi). This obstacle has recently been overcome, and the above phenomena can now be viewed in terms of changing Fi levels and the percentage of farnesol secreted. Critically, two aspects of isoprenoid metabolism present in higher organisms are absent in C. albicans and likely in other yeasts. These are pathways for farnesol salvage (converting farnesol to farnesyl pyrophosphate) and farnesylcysteine cleavage, a necessary step in the turnover of farnesylated proteins. Together, these developments suggest a unifying model, whereby high, threshold levels of Fi regulate which target proteins are farnesylated or the extent to which they are farnesylated. Thus, we suggest that the diversity of cellular responses to farnesol reflects the diversity of the proteins that are or are not farnesylated.

Optogenetic inhibition of actomyosin reveals mechanical bistability of the mesoderm epithelium during Drosophila mesoderm invagination

Apical constriction driven by actin and non-muscle myosin II (actomyosin) provides a well-conserved mechanism to mediate epithelial folding. It remains unclear how contractile forces near the apical surface of a cell sheet drive out-of-the-plane bending of the sheet and whether myosin contractility is required throughout folding. By optogenetic-mediated acute inhibition of actomyosin, we find that during Drosophila mesoderm invagination, actomyosin contractility is critical to prevent tissue relaxation during the early, ‘priming’ stage of folding but is dispensable for the actual folding step after the tissue passes through a stereotyped transitional configuration. This binary response suggests that Drosophila mesoderm is mechanically bistable during gastrulation. Computer modeling analysis demonstrates that the binary tissue response to actomyosin inhibition can be recapitulated in the simulated epithelium that undergoes buckling-like deformation jointly mediated by apical constriction in the mesoderm and in-plane compression generated by apicobasal shrinkage of the surrounding ectoderm. Interestingly, comparison between wild-type and snail mutants that fail to specify the mesoderm demonstrates that the lateral ectoderm undergoes apicobasal shrinkage during gastrulation independently of mesoderm invagination. We propose that Drosophila mesoderm invagination is achieved through an interplay between local apical constriction and mechanical bistability of the epithelium that facilitates epithelial buckling.

Components of the phosphatidylserine endoplasmic reticulum to plasma membrane transport mechanism as targets for KRAS inhibition in pancreatic cancer

KRAS is mutated in 90% of human pancreatic ductal adenocarcinomas (PDACs). To function, KRAS must localize to the plasma membrane (PM) via a C-terminal membrane anchor that specifically engages phosphatidylserine (PtdSer). This anchor-binding specificity renders KRAS-PM localization and signaling capacity critically dependent on PM PtdSer content. We now show that the PtdSer lipid transport proteins, ORP5 and ORP8, which are essential for maintaining PM PtdSer levels and hence KRAS PM localization, are required for KRAS oncogenesis. Knockdown of either protein, separately or simultaneously, abrogated growth of KRAS-mutant but not KRAS-wild-type pancreatic cancer cell xenografts. ORP5 or ORP8 knockout also abrogated tumor growth in an immune-competent orthotopic pancreatic cancer mouse model. Analysis of human datasets revealed that all components of this PtdSer transport mechanism, including the PM-localized EFR3A-PI4KIIIα complex that generates phosphatidylinositol-4-phosphate (PI4P), and endoplasmic reticulum (ER)-localized SAC1 phosphatase that hydrolyzes counter transported PI4P, are significantly up-regulated in pancreatic tumors compared to normal tissue. Taken together, these results support targeting PI4KIIIα in KRAS-mutant cancers to deplete the PM-to-ER PI4P gradient, reducing PM PtdSer content. We therefore repurposed the US Food and Drug Administration-approved hepatitis C antiviral agent, simeprevir, as a PI4KIIIα inhibitor In a PDAC setting. Simeprevir potently mislocalized KRAS from the PM, reduced the clonogenic potential of pancreatic cancer cell lines in vitro, and abrogated the growth of KRAS-dependent tumors in vivo with enhanced efficacy when combined with MAPK and PI3K inhibitors. We conclude that the cellular ER-to-PM PtdSer transport mechanism is essential for KRAS PM localization and oncogenesis and is accessible to therapeutic intervention.

RAS Function in cancer cells: translating membrane biology and biochemistry into new therapeutics

The three human RAS proteins are mutated and constitutively activated in ∼20% of cancers leading to cell growth and proliferation. For the past three decades, many attempts have been made to inhibit these proteins with little success. Recently; however, multiple methods have emerged to inhibit KRAS, the most prevalently mutated isoform. These methods and the underlying biology will be discussed in this review with a special focus on KRAS-plasma membrane interactions.

Discovery of Sulfonamide-Derived Agonists of SOS1-Mediated Nucleotide Exchange on RAS Using Fragment-Based Methods

The nucleotide exchange factor Son of Sevenless (SOS) catalyzes the activation of RAS by converting it from its inactive GDP-bound state to its active GTP-bound state. Recently, we have reported the discovery of small-molecule allosteric activators of SOS1 that can increase the amount of RAS-GTP in cells. The compounds can inhibit ERK phosphorylation at higher concentrations by engaging a feedback mechanism. To further study this process, we sought different chemical matter from an NMR-based fragment screen using selective methyl labeling. To aid this process, several Ile methyl groups located in different binding sites of the protein were assigned and used to categorize the NMR hits into different classes. Hit to lead optimization using an iterative structure-based design paradigm resulted in compounds with improvements in binding affinity. These improved molecules of a different chemical class increase SOS1^cat-mediated nucleotide exchange on RAS and display cellular action consistent with our prior results.

The Impact of Statin Therapy on the Survival of Patients with Gastrointestinal Cancer

Statins are 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors that may play an important role in the evolution of cancers, due to their effects on cancer cell metabolism. Statins affect several potential pathways, including cell proliferation, angiogenesis, apoptosis and metastasis. The number of trials assessing the putative clinical benefits of statins in cancer is increasing. Currently, there are several trials listed on the global trial identifier website clinicaltrials.gov. Given the compelling evidence from these trials in a variety of clinical settings, there have been calls for a clinical trial of statins in the adjuvant gastrointestinal cancer setting. However, randomized controlled trials on specific cancer types in relation to statin use, as well as studies on populations without a clinical indication for using statins, have elucidated some potential underlying biological mechanisms, and the investigation of different statins is probably warranted. It would be useful for these trials to incorporate the assessment of tumour biomarkers predictive of statin response in their design. This review summarizes the recent preclinical and clinical studies that assess the application of statins in the treatment of gastrointestinal cancers with particular emphasize on their association with cancer risk.

Characterization of the Spatial Organization of Raf Isoforms Interacting with K-Ras4B in the Lipid Membrane

Activation of Raf kinases by the membrane-anchored protein K-Ras4B is a key step of cellular signal regulation. As a predominant variant of the Ras family, K-Ras4B has been considered to be a major drug target in cancer therapy. Therefore, an integrated study of Raf interaction with membrane-associated K-Ras4B is essential. While the Ras-binding domain (RBD) of Raf contains the main binding interface to K-Ras4B, its cysteine-rich domain (CRD) is thought to be responsible for its association with the membrane interface. We applied time-lapse tapping-mode atomic force microscopy to visualize and characterize the interaction of these binding motifs of A-, B-, and C-Raf isoforms with K-Ras4B in a raft-like anionic model biomembrane. However, we found that the RBDs of the Raf isomers are readily recruited to K-Ras4B nanoclusters in the lipid membrane, with different efficiencies. Unexpectedly and different from A-Raf-RBD, B- and C-Raf-RBD are able to bind markedly also directly to the lipid membrane. We also found that Raf-RBD-CRD is readily recruited to the K-Ras4B forming nanoclusters in the fluid membrane phase, with the CRD domains binding to the lipid interface. The K-Ras4B-nanoclusters are likely to enhance Raf binding and activate signaling by enriching the Raf proteins and facilitating formation of Raf dimers. Interestingly, A-, B-, and C-Raf-RBD-CRD are also able to bind directly to the heterogeneous membrane surrounding the K-Ras4B nanoclusters, which could potentially enhance the overall affinity to K-Ras4B in a Raf-isoform-dependent manner. Overall, these results provide new insights into the spatial organization of the membrane-associated Raf-Ras signaling module for the various Raf isoforms, which is important for understanding the activation of Raf kinases and required for the development of drugs against cancers through targeting Raf-Ras interactions.

Targeting plasma membrane phosphatidylserine content to inhibit oncogenic KRAS function

The small GTPase KRAS, which is frequently mutated in human cancers, must be localized to the plasma membrane (PM) for biological activity. We recently showed that the KRAS C-terminal membrane anchor exhibits exquisite lipid-binding specificity for select species of phosphatidylserine (PtdSer). We, therefore, investigated whether reducing PM PtdSer content is sufficient to abrogate KRAS oncogenesis. Oxysterol-related binding proteins ORP5 and ORP8 exchange PtdSer synthesized in the ER for phosphatidyl-4-phosphate synthesized in the PM. We show that depletion of ORP5 or ORP8 reduced PM PtdSer levels, resulting in extensive mislocalization of KRAS from the PM. Concordantly, ORP5 or ORP8 depletion significantly reduced proliferation and anchorage-independent growth of multiple KRAS-dependent cancer cell lines, and attenuated KRAS signaling in vivo. Similarly, functionally inhibiting ORP5 and ORP8 by inhibiting PI4KIIIα-mediated synthesis of phosphatidyl-4-phosphate at the PM selectively inhibited the growth of KRAS-dependent cancer cell lines over normal cells. Inhibiting KRAS function through regulating PM lipid PtdSer content may represent a viable strategy for KRAS-driven cancers.

A Guanidyl-Based Bivalent Peptidomimetic Inhibits K-Ras Prenylation and Association with c-Raf

Unusual lipid modification of K-Ras makes Ras-directed cancer therapy a challenging task. Aiming to disrupt electrostatic-driven protein-protein interactions (PPIs) of K-Ras with FTase and GGTase I, a series of bivalent dual inhibitors that recognize the active pocket and the common acidic surface of FTase and GGTase I were designed. The structure-activity-relationship study resulted in 8 b, in which a biphenyl-based peptidomimetic FTI-277 was attached to a guanidyl-containing gallate moiety through an alkyl linker. Cell-based evaluation demonstrated that 8 b exhibited substantial inhibition of K-Ras processing without apparent interference with Rap-1A processing. Fluorescent imaging showed that 8 b disrupts localization of K-Ras to the plasma membrane and impairs interaction with c-Raf, whereas only FTI-277 was found to be inactive. These results suggest that targeting the PPI interface of K-Ras may provide an alternative method of inhibiting K-Ras.

Acylpeptide hydrolase is a novel regulator of KRAS plasma membrane localization and function

The primary site for KRAS signaling is the inner leaflet of the plasma membrane (PM). We previously reported that oxanthroquinone G01 (G01) inhibited KRAS PM localization and blocked KRAS signaling. In this study, we identified acylpeptide hydrolase (APEH) as a molecular target of G01. APEH formed a stable complex with biotinylated G01, and the enzymatic activity of APEH was inhibited by G01. APEH knockdown caused profound mislocalization of KRAS and reduced clustering of KRAS that remained PM localized. APEH knockdown also disrupted the PM localization of phosphatidylserine (PtdSer), a lipid critical for KRAS PM binding and clustering. The mislocalization of KRAS was fully rescued by ectopic expression of APEH in knockdown cells. APEH knockdown disrupted the endocytic recycling of epidermal growth factor receptor and transferrin receptor, suggesting that abrogation of recycling endosome function was mechanistically linked to the loss of KRAS and PtdSer from the PM. APEH knockdown abrogated RAS-RAF-MAPK signaling in cells expressing the constitutively active (oncogenic) mutant of KRAS (KRASG12V), and selectively inhibited the proliferation of KRAS-transformed pancreatic cancer cells. Taken together, these results identify APEH as a novel drug target for a potential anti-KRAS therapeutic.

Biochemical significance of limonene and its metabolites: future prospects for designing and developing highly potent anticancer drugs

Monocyclic monoterpenes have been recognized as useful pharmacological ingredients due to their ability to treat numerous diseases. Limonene and perillyl alcohol as well as their metabolites (especially perillic acid and its methyl ester) possess bioactivities such as antitumor, antiviral, anti-inflammatory, and antibacterial agents. These therapeutic properties have been well documented. Based on the aforementioned biological properties of limonene and its metabolites, their structural modification and development into effective drugs could be rewarding. However, utilization of these monocyclic monoterpenes as scaffolds for the design and developments of more effective chemoprotective agents has not received the needed attention by medicinal scientists. Recently, some derivatives of limonene metabolites have been synthesized. Nonetheless, there have been no thorough studies on their pharmacokinetic and pharmacodynamic properties as well as their inhibition against isoprenylation enzymes. In this review, recent research progress in the biochemical significance of limonene and its metabolites was summarized with emphasis on their antitumor effects. Future prospects of these bioactive monoterpenes for drug design and development are also highlighted.

An oxanthroquinone derivative that disrupts RAS plasma membrane localization inhibits cancer cell growth

Oncogenic RAS proteins are commonly expressed in human cancer. To be functional, RAS proteins must undergo post-translational modification and localize to the plasma membrane (PM). Therefore, compounds that prevent RAS PM targeting have potential as putative RAS inhibitors. Here we examine the mechanism of action of oxanthroquinone G01 (G01), a recently described inhibitor of KRAS PM localization. We show that G01 mislocalizes HRAS and KRAS from the PM with similar potency and disrupts the spatial organization of RAS proteins remaining on the PM. G01 also inhibited recycling of epidermal growth factor receptor and transferrin receptor, but did not impair internalization of cholera toxin, indicating suppression of recycling endosome function. In searching for the mechanism of impaired endosomal recycling we observed that G01 also enhanced cellular sphingomyelin (SM) and ceramide levels and disrupted the localization of several lipid and cholesterol reporters, suggesting that the G01 molecular target may involve SM metabolism. Indeed, G01 exhibited potent synergy with other compounds that target SM metabolism in KRAS localization assays. Furthermore, G01 significantly abrogated RAS-RAF-MAPK signaling in Madin-Darby canine kidney (MDCK) cells expressing constitutively activated, oncogenic mutant RASG12V. G01 also inhibited the proliferation of RAS-less mouse embryo fibroblasts expressing oncogenic mutant KRASG12V or KRASG12D but not RAS-less mouse embryo fibroblasts expressing oncogenic mutant BRAFV600E. Consistent with these effects, G01 selectively inhibited the proliferation of KRAS-transformed pancreatic, colon, and endometrial cancer cells. Taken together, these results suggest that G01 should undergo further evaluation as a potential anti-RAS therapeutic.

Sphingomyelin Metabolism Is a Regulator of K-Ras Function

K-Ras must localize to the plasma membrane (PM) for biological activity. We show here that multiple acid sphingomyelinase (ASM) inhibitors, including tricyclic antidepressants, mislocalized phosphatidylserine (PtdSer) and K-RasG12V from the PM, resulting in abrogation of K-RasG12V signaling and potent, selective growth inhibition of mutant K-Ras-transformed cancer cells. Concordantly, in nude mice, the ASM inhibitor fendiline decreased the rate of growth of oncogenic K-Ras-expressing MiaPaCa-2 tumors but had no effect on the growth of the wild-type K-Ras-expressing BxPC-3 tumors. ASM inhibitors also inhibited activated LET-60 (a K-Ras ortholog) signaling in Caenorhabditis elegans, as evidenced by suppression of the induced multivulva phenotype. Using RNA interference against C. elegans genes encoding other enzymes in the sphingomyelin (SM) biosynthetic pathway, we identified 14 enzymes whose knockdown strongly or moderately suppressed the LET-60 multivulva phenotype. In mammalian cells, pharmacological agents that target these enzymes all depleted PtdSer from the PM and caused K-RasG12V mislocalization. These effects correlated with changes in SM levels or subcellular distribution. Selected compounds, including sphingosine kinase inhibitors, potently inhibited the proliferation of oncogenic K-Ras-expressing pancreatic cancer cells. In conclusion, these results show that normal SM metabolism is critical for K-Ras function, which may present therapeutic options for the treatment of K-Ras-driven cancers.

Inhibition of Farnesyltransferase Potentiates NOTCH-Targeted Therapy against Glioblastoma Stem Cells

Accumulating evidence suggests that cancer cells with stem cell-like phenotypes drive disease progression and therapeutic resistance in glioblastoma (GBM). NOTCH regulates self-renewal and resistance to chemoradiotherapy in GBM stem cells. However, NOTCH-targeted γ-secretase inhibitors (GSIs) exhibited limited efficacy in GBM patients. We found that farnesyltransferase inhibitors (FTIs) significantly improved sensitivity to GSIs. This combination showed significant antineoplastic and radiosensitizing activities in GBM stem cells, whereas non-stem GBM cells were resistant. These combinatorial effects were mediated, at least partially, through inhibition of AKT and cell-cycle progression. Using subcutaneous and orthotopic GBM models, we showed that the combination of FTIs and GSIs, but not either agent alone, significantly reduced tumor growth. With concurrent radiation, this combination induced a durable response in a subset of orthotopic tumors. These findings collectively suggest that the combination of FTIs and GSIs is a promising therapeutic strategy for GBM through selectively targeting the cancer stem cell subpopulation.

•

NOTCH signaling is preferentially activated in glioblastoma stem cells

•

GSIs have limited activities against glioblastoma stem cells

•

FTIs improve response to GSIs in vitro and in vivo

•

The combination of FTIs and GSIs makes glioblastoma more sensitive to radiation

Cellular mechanisms of noise-induced hearing loss

Exposure to intense sound or noise can result in purely temporary threshold shift (TTS), or leave a residual permanent threshold shift (PTS) along with alterations in growth functions of auditory nerve output. Recent research has revealed a number of mechanisms that contribute to noise-induced hearing loss (NIHL). The principle cause of NIHL is damage to cochlear hair cells and associated synaptopathy. Contributions to TTS include reversible damage to hair cell (HC) stereocilia or synapses, while moderate TTS reflects protective purinergic hearing adaptation. PTS represents permanent damage to or loss of HCs and synapses. While the substrates of HC damage are complex, they include the accumulation of reactive oxygen species and the active stimulation of intracellular stress pathways, leading to programmed and/or necrotic cell death. Permanent damage to cochlear neurons can also contribute to the effects of NIHL, in addition to HC damage. These mechanisms have translational potential for pharmacological intervention and provide multiple opportunities to prevent HC damage or to rescue HCs and spiral ganglion neurons that have suffered injury. This paper reviews advances in our understanding of cellular mechanisms that contribute to NIHL and their potential for therapeutic manipulation.

A novel role of farnesylation in targeting a mitotic checkpoint protein, human Spindly, to kinetochores

The mitotic checkpoint protein Spindly is farnesylated in vivo and this modification is required for its interaction with the RZZ complex and its localization to kinetochores.

Kinetochore (KT) localization of mitotic checkpoint proteins is essential for their function during mitosis. hSpindly KT localization is dependent on the RZZ complex and hSpindly recruits the dynein–dynactin complex to KTs during mitosis, but the mechanism of hSpindly KT recruitment is unknown. Through domain-mapping studies we characterized the KT localization domain of hSpindly and discovered it undergoes farnesylation at the C-terminal cysteine residue. The N-terminal 293 residues of hSpindly are dispensable for its KT localization. Inhibition of farnesylation using a farnesyl transferase inhibitor (FTI) abrogated hSpindly KT localization without affecting RZZ complex, CENP-E, and CENP-F KT localization. We showed that hSpindly is farnesylated in vivo and farnesylation is essential for its interaction with the RZZ complex and hence KT localization. FTI treatment and hSpindly knockdown displayed the same mitotic phenotypes, indicating that hSpindly is a key FTI target in mitosis. Our data show a novel role of lipidation in targeting a checkpoint protein to KTs through protein–protein interaction.

Mechanisms of sensorineural cell damage, death and survival in the cochlea

The majority of acquired hearing loss, including presbycusis, is caused by irreversible damage to the sensorineural tissues of the cochlea. This article reviews the intracellular mechanisms that contribute to sensorineural damage in the cochlea, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. These data have primarily been generated in hearing loss not directly related to age. However, there is evidence that similar mechanisms operate in presbycusis. Moreover, accumulation of damage from other causes can contribute to age-related hearing loss (ARHL). Potential therapeutic interventions to balance opposing but interconnected cell damage and survival pathways, such as antioxidants, anti-apoptotics, and pro-inflammatory cytokine inhibitors, are also discussed.

Towards the systematic mapping and engineering of the protein prenylation machinery in Saccharomyces cerevisiae

Protein prenylation is a widespread and highly conserved eukaryotic post-translational modification that endows proteins with the ability to reversibly attach to intracellular membranes. The dynamic interaction of prenylated proteins with intracellular membranes is essential for their signalling functions and is frequently deregulated in disease processes such as cancer. As a result, protein prenylation has been pharmacologically targeted by numerous drug discovery programs, albeit with limited success. To a large extent, this can be attributed to an insufficient understanding of the interplay of different protein prenyltransferases and the combinatorial diversity of the prenylatable sequence space. Here, we report a high-throughput, growth-based genetic selection assay in Saccharomyces cerevisiae based on the Ras Recruitment System which, for the first time, has allowed us to create a comprehensive map of prenylatable protein sequences in S. cerevisiae. We demonstrate that potential prenylatable space is sparsely (6.2%) occupied leaving room for creation of synthetic orthogonal prenylatable sequences. To experimentally demonstrate that, we used the developed platform to engineer mutant farnesyltransferases that efficiently prenylate substrate motives that are not recognised by endogenous protein prenyltransferases. These uncoupled mutants can now be used as starting points for the systematic engineering of the eukaryotic protein prenylation machinery.

Targeting drivers of melanoma with synthetic small molecules and phytochemicals

Melanoma is the least common form of skin cancer, but it is responsible for the majority of skin cancer deaths. Traditional therapeutics and immunomodulatory agents have not shown much efficacy against metastatic melanoma. Agents that target the RAS/RAF/MEK/ERK (MAPK) signaling pathway - the BRAF inhibitors vemurafenib and dabrafenib, and the MEK1/2 inhibitor trametinib - have increased survival in patients with metastatic melanoma. Further, the combination of dabrafenib and trametinib has been shown to be superior to single agent therapy for the treatment of metastatic melanoma. However, resistance to these agents develops rapidly. Studies of additional agents and combinations targeting the MAPK, PI3K/AKT/mTOR (PI3K), c-kit, and other signaling pathways are currently underway. Furthermore, studies of phytochemicals have yielded promising results against proliferation, survival, invasion, and metastasis by targeting signaling pathways with established roles in melanomagenesis. The relatively low toxicities of phytochemicals make their adjuvant use an attractive treatment option. The need for improved efficacy of current melanoma treatments calls for further investigation of each of these strategies. In this review, we will discuss synthetic small molecule inhibitors, combined therapies and current progress in the development of phytochemical therapies.

Simvastatin interacts synergistically with tipifarnib to induce apoptosis in leukemia cells through the disruption of RAS membrane localization and ERK pathway inhibition

Tipifarnib, a farnesyltransferase inhibitor (FTI), was initially designed to disrupt RAS farnesylation and membrane localization necessary for RAS function. However, alternative geranylgeranylation has been postulated as an escape mechanism by which RAS bypasses the effect of FTI treatment. In this study, we demonstrate that simvastatin, an HMG-CoA reductase inhibitor, augments the cytotoxic effect of tipifarnib by blocking the alternative geranylgeranylation of RAS. Notably, this effect was accompanied by disruption of RAS membrane localization and ERK downregulation. In addition, the apoptotic effect of this combination was associated with downregulation of the antiapoptotic Mcl-1 protein and activation of the caspase cascade. These findings demonstrate that combining tipifarnib and simvastatin was successful in targeting RAS/ERK signaling and inducing apoptosis in leukemia cells. Both simvastatin and tipifarnib were used at clinically achievable doses, which make the combination promising for future clinical studies.

A RAS renaissance: emerging targeted therapies for KRAS-mutated non-small cell lung cancer

Of the numerous oncogenes implicated in human cancer, the most common and perhaps the most elusive to target pharmacologically is RAS. Since the discovery of RAS in the 1960s, numerous studies have elucidated the mechanism of activity, regulation, and intracellular trafficking of the RAS gene products, and of its regulatory pathways. These pathways yielded druggable targets, such as farnesyltransferase, during the 1980s to 1990s. Unfortunately, early clinical trials investigating farnesyltransferase inhibitors yielded disappointing results, and subsequent interest by pharmaceutical companies in targeting RAS waned. However, recent advances including the identification of novel regulatory enzymes (e.g., Rce1, Icmt, Pdeδ), siRNA-based synthetic lethality screens, and fragment-based small-molecule screens, have resulted in a "Ras renaissance," signified by new Ras and Ras pathway-targeted therapies that have led to new clinical trials of patients with Ras-driven cancers. This review gives an overview of KRas signaling pathways with an emphasis on novel targets and targeted therapies, using non-small cell lung cancer as a case example.

Rational design of small molecule inhibitors targeting RhoA subfamily Rho GTPases

Rho GTPases have been implicated in diverse cellular functions and are potential therapeutic targets. By virtual screening, we have identified a Rho-specific inhibitor, Rhosin. Rhosin contains two aromatic rings tethered by a linker, and it binds to the surface area sandwiching Trp58 of RhoA with a submicromolar Kd and effectively inhibits GEF-catalyzed RhoA activation. In cells, Rhosin specifically inhibited RhoA activity and RhoA-mediated cellular function without affecting Cdc42 or Rac1 signaling activities. By suppressing RhoA or RhoC activity, Rhosin could inhibit mammary sphere formation by breast cancer cells, suppress invasion of mammary epithelial cells, and induce neurite outgrowth of PC12 cells in synergy with NGF. Thus, the rational designed RhoA subfamily-specific small molecule inhibitor is useful for studying the physiological and pathologic roles of Rho GTPase.

Use of synthetic isoprenoids to target protein prenylation and Rho GTPases in breast cancer invasion

Dysregulation of Ras and Rho family small GTPases drives the invasion and metastasis of multiple cancers. For their biological functions, these GTPases require proper subcellular localization to cellular membranes, which is regulated by a series of post-translational modifications that result in either farnesylation or geranylgeranylation of the C-terminal CAAX motif. This concept provided the rationale for targeting farnesyltransferase (FTase) and geranylgeranyltransferases (GGTase) for cancer treatment. However, the resulting prenyl transferase inhibitors have not performed well in the clinic due to issues with alternative prenylation and toxicity. As an alternative, we have developed a unique class of potential anti-cancer therapeutics called Prenyl Function Inhibitors (PFIs), which are farnesol or geranyl-geraniol analogs that act as alternate substrates for FTase or GGTase. Here, we test the ability of our lead PFIs, anilinogeraniol (AGOH) and anilinofarnesol (AFOH), to block the invasion of breast cancer cells. We found that AGOH treatment effectively decreased invasion of MDA-MB-231 cells in a two-dimensional (2D) invasion assay at 100 µM while it blocked invasive growth in three-dimensional (3D) culture model at as little as 20 µM. Notably, the effect of AGOH on 3D invasive growth was phenocopied by electroporation of cells with C3 exotransferase. To determine if RhoA and RhoC were direct targets of AGOH, we performed Rho activity assays in MDA-MB-231 and MDA-MB-468 cells and found that AGOH blocked RhoA and RhoC activation in response to LPA and EGF stimulation. Notably, the geranylgeraniol analog AFOH was more potent than AGOH in inhibiting RhoA and RhoC activation and invasive growth. Interestingly, neither AGOH nor AFOH impacted 3D growth of MCF10A cells. Collectively, this study demonstrates that AGOH and AFOH dramatically inhibit breast cancer invasion, at least in part by blocking Rho function, thus, suggesting that targeting prenylation by using PFIs may offer a promising mechanism for treatment of invasive breast cancer.

New treatment strategies for malignant gliomas

Malignant gliomas are the most prevalent type of primary brain tumor in adults. Despite progress in brain tumor therapy, the prognosis of malignant glioma patients remains dismal. The median survival of patients with glioblastoma multiforme, the most common grade of malignant glioma, is 10-12 months. Conventional therapy of surgery, radiation and chemotherapy is largely palliative. Essentially, tumor recurrence is inevitable. Salvage treatments upon recurrence are palliative at best and rarely provide significant survival benefit. Therapies targeting the underlying molecular pathogenesis of brain tumors are urgently required. Common genetic abnormalities in malignant glioma specimens are associated with aberrant activation or suppression of cellular signal transduction pathways and resistance to radiation and chemotherapy. Several low molecular weight signal transduction inhibitors have been examined in preclinical and clinical malignant glioma trials. The efficacy of these agents as monotherapies has been modest, at best; however, small subsets of patients who harbor specific genetic changes in their tumors may display favorable clinical responses to defined small molecule inhibitors. Multitargeted kinase inhibitors or combinations of agents targeting different mitogenic pathways may overcome the resistance of tumors to single-agent targeted therapies. Well designed studies of small molecule kinase inhibitors will include assessment of safety, drug delivery, target inhibition and correlative biomarkers to define mechanisms of response or resistance to these agents. Predictive biomarkers will enrich for patients most likely to respond in future clinical trials. Additional clinical studies will combine novel targeted therapies with radiation, chemotherapies and immunotherapies.

Role of farnesyltransferase inhibitors in hematologic malignancies

The treatment of hematologic malignancies has progressed in the last few years. Identification of new pathways and target molecules in leukemia has ushered in a promising new era of therapy. Ras mutations have recently been implicated in the pathogenesis of acute leukemia, and inhibition of Ras signaling through the use of farnesyltransferase inhibitors (FTIs) has shown promise in early trials in acute myeloid leukemia (AML). Responses have not correlated with the presence of Ras mutations, suggesting that novel pathways are involved. In several early trials, FTIs have shown activity as single agents in poor-risk AML, suggesting a potential role in combination with standard chemotherapy. FTIs are now being tested in other clinical settings, such as myelodysplasia, chronic myelogenous leukemia and multiple myeloma, with encouraging preliminary activity.

Personalized treatment for advanced colorectal cancer: KRAS and beyond

Targeted therapies have improved the survival of patients with advanced colorectal cancer (CRC). However, further improvements in patient outcomes may be gained by the development of predictive biomarkers in order to select individuals who are most likely to benefit from treatment, thus personalizing treatment. Using the epidermal growth-factor receptor (EGFR) pathway, we discuss the existing and potential predictive biomarkers in clinical development for use with EGFR-targeted agents in metastatic CRC. The data and technological issues surrounding such biomarkers as expression of EGFR or its family members or ligands, KRAS-, NRAS-, and BRAF-mutation status, PI3K/PTEN expression, and imaging and clinical biomarkers, such as rash and hypomagnesemia, are summarized. Although the discovery of KRAS mutations has improved patient selection for EGFR-targeted treatments, further biomarkers are required, especially for those patients who exhibit KRAS mutations rather than the wild-type gene.

Farnesyltransferase inhibitor tipifarnib inhibits Rheb prenylation and stabilizes Bax in acute myelogenous leukemia cells

Although farnesyltransferase inhibitors have shown promising activity in relapsed lymphoma and sporadic activity in acute myelogenous leukemia, their mechanism of cytotoxicity is incompletely understood, making development of predictive biomarkers difficult. In the present study, we examined the action of tipifarnib in human acute myelogenous leukemia cell lines and clinical samples. In contrast to the Ras/MEK/ERK pathway-mediated Bim upregulation that is responsible for tipifarnib-induced killing of malignant lymphoid cells, inhibition of Rheb-induced mTOR signaling followed by dose-dependent upregulation of Bax and Puma occurred in acute myelogenous leukemia cell lines undergoing tipifarnib-induced apoptosis. Similar Bax and Puma upregulation occurred in serial bone marrow samples harvested from a subset of acute myelogenous leukemia patients during tipifarnib treatment. Expression of FTI-resistant Rheb M184L, like knockdown of Bax or Puma, diminished tipifarnib-induced killing. Further analysis demonstrated that increased Bax and Puma levels reflect protein stabilization rather than increased gene expression. In U937 cells selected for tipifarnib resistance, neither inhibition of signaling downstream of Rheb nor Bax and Puma stabilization occurred. Collectively, these results not only identify a pathway downstream from Rheb that contributes to tipifarnib cytotoxicity in human acute myelogenous leukemia cells, but also demonstrate that FTI-induced killing of lymphoid versus myeloid cells reflects distinct biochemical mechanisms downstream of different farnesylated substrates. (ClinicalTrials.gov identifier NCT00602771).

Prenylation and membrane localization of Cdc42 are essential for activation by DOCK7

The unconventional guanine nucleotide exchange factor (GEF) family comprising 11 DOCK180 related proteins is classified into four subfamilies, A through D, based on their relative GEF activity toward the closely related Rac and Cdc42 GTPases. DOCK proteins participate in the remodeling of the actin cytoskeleton and are key regulators of cell motility, phagocytosis, and adhesion. Here we show that the guanine nucleotide exchange domain of DOCK7, DHR2 (for DOCK homology region 2), is a potent GEF for prenylated Cdc42 and Rac1 in a model liposome system, demonstrating that the prenylation and membrane localization of Cdc42 or Rac1 are necessary for their activation by DOCK7. Additionally, we identify DOCK7 residues that confer GTPase GEF specificity. Finally, using our liposome reconstitution assay, we show that a more narrowly defined GEF domain of DHR2 (designated DHR2s) harbors an N-terminal site distinct from the GEF active site that binds preferentially to the active, GTP-bound forms of Cdc42 and Rac1 and thereby recruits free DHR2s from solution to the membrane surface. This recruitment results in a progressive increase in the effective concentration of DHR2s at the membrane surface that in turn provides for an accelerated rate of guanine nucleotide exchange on Cdc42. The positive cooperativity observed in our reconstituted system suggests that the action of DOCK7 in vivo may involve the coordinated integration of Cdc42/Rac signaling in the context of the membrane recruitment of a DOCK7 GEF complex.

Fendiline inhibits K-Ras plasma membrane localization and blocks K-Ras signal transmission

Ras proteins regulate signaling pathways important for cell growth, differentiation, and survival. Oncogenic mutant Ras proteins are commonly expressed in human tumors, with mutations of the K-Ras isoform being most prevalent. To be active, K-Ras must undergo posttranslational processing and associate with the plasma membrane. We therefore devised a high-content screening assay to search for inhibitors of K-Ras plasma membrane association. Using this assay, we identified fendiline, an L-type calcium channel blocker, as a specific inhibitor of K-Ras plasma membrane targeting with no detectable effect on the localization of H- and N-Ras. Other classes of L-type calcium channel blockers did not mislocalize K-Ras, suggesting a mechanism that is unrelated to calcium channel blockade. Fendiline did not inhibit K-Ras posttranslational processing but significantly reduced nanoclustering of K-Ras and redistributed K-Ras from the plasma membrane to the endoplasmic reticulum (ER), Golgi apparatus, endosomes, and cytosol. Fendiline significantly inhibited signaling downstream of constitutively active K-Ras and endogenous K-Ras signaling in cells transformed by oncogenic H-Ras. Consistent with these effects, fendiline blocked the proliferation of pancreatic, colon, lung, and endometrial cancer cell lines expressing oncogenic mutant K-Ras. Taken together, these results suggest that inhibitors of K-Ras plasma membrane localization may have utility as novel K-Ras-specific anticancer therapeutics.

Identification of potential small molecule binding pockets on Rho family GTPases

Rho GTPases are conformational switches that control a wide variety of signaling pathways critical for eukaryotic cell development and proliferation. They represent attractive targets for drug design as their aberrant function and deregulated activity is associated with many human diseases including cancer. Extensive high-resolution structures (>100) and recent mutagenesis studies have laid the foundation for the design of new structure-based chemotherapeutic strategies. Although the inhibition of Rho signaling with drug-like compounds is an active area of current research, very little attention has been devoted to directly inhibiting Rho by targeting potential allosteric non-nucleotide binding sites. By avoiding the nucleotide binding site, compounds may minimize the potential for undesirable off-target interactions with other ubiquitous GTP and ATP binding proteins. Here we describe the application of molecular dynamics simulations, principal component analysis, sequence conservation analysis, and ensemble small-molecule fragment mapping to provide an extensive mapping of potential small-molecule binding pockets on Rho family members. Characterized sites include novel pockets in the vicinity of the conformationaly responsive switch regions as well as distal sites that appear to be related to the conformations of the nucleotide binding region. Furthermore the use of accelerated molecular dynamics simulation, an advanced sampling method that extends the accessible time-scale of conventional simulations, is found to enhance the characterization of novel binding sites when conformational changes are important for the protein mechanism.

The nuclear envelope environment and its cancer connections

Because of the association between aberrant nuclear structure and tumour grade, nuclear morphology is an indispensible criterion in the current pathological assessment of cancer. Components of the nuclear envelope environment have central roles in many aspects of cell function that affect tumour development and progression. As the roles of the nuclear envelope components, including nuclear pore complexes and nuclear lamina, are being deciphered in molecular detail there are opportunities to harness this knowledge for cancer therapeutics and biomarker development. In this Review, we summarize the progress that has been made in our understanding of the nuclear envelope and the implications of changes in this environment for cancer biology.

Inhibition of Ras for cancer treatment: the search continues

The RAS oncogenes (HRAS, NRAS and KRAS) comprise the most frequently mutated class of oncogenes in human cancers (33%), thus stimulating intensive effort in developing anti-Ras inhibitors for cancer treatment. Despite intensive effort, to date, no effective anti-Ras strategies have successfully made it to the clinic. We present an overview of past and ongoing strategies to inhibit oncogenic Ras in cancer. Since approaches to directly target mutant Ras have not been successful, most efforts have focused on indirect approaches to block Ras membrane association or downstream effector signaling. While inhibitors of effector signaling are currently under clinical evaluation, genome-wide unbiased genetic screens have identified novel directions for future anti-Ras drug discovery.

Rational design of Rho GTPase-targeting inhibitors

Rho GTPases have been implicated in diverse cellular functions and are potential therapeutic targets in inflammation, cancer, and neurologic diseases. Virtual screening of compounds that fit into surface grooves of RhoA known to be critical for guanine nucleotide exchange factor (GEF) interaction produced chemical candidates with minimized docking energy. Subsequent screening for inhibitory activity of RhoA binding to the Rho-GEF, LARG, identified a Rho-specific inhibitor as a lead compound capable of blocking RhoA-LARG interaction and RhoA activation by LARG specifically and dose dependently. A microscale thermophoresis analysis was applied to directly quantify the binding interaction of the lead inhibitor with RhoA target. The lead inhibitor highlights the principle that rational targeting of subfamily members of Rho GTPases is feasible and potentially useful in future drug design effort.

Posttranslational lipid modification of Rho family small GTPases

The Rho family comprises a major branch of the Ras superfamily of small GTPases. A majority of Rho GTPases are synthesized as inactive, cytosolic proteins. They then undergo posttranslational modification by isoprenoid or fatty acid lipids, and together with additional carboxyl-terminal sequences target Rho GTPases to specific membrane and subcellular compartments essential for function. We summarize the use of biochemical and cellular assays and pharmacologic inhibitors instrumental for the study of the role of posttranslational lipid modifications and processing in Rho GTPase biology.

Optimal functional levels of activation-induced deaminase specifically require the Hsp40 DnaJa1

The enzyme activation-induced deaminase (AID) deaminates deoxycytidine at the immunoglobulin genes, thereby initiating antibody affinity maturation and isotype class switching during immune responses. In contrast, off-target DNA damage caused by AID is oncogenic. Central to balancing immunity and cancer is AID regulation, including the mechanisms determining AID protein levels. We describe a specific functional interaction between AID and the Hsp40 DnaJa1, which provides insight into the function of both proteins. Although both major cytoplasmic type I Hsp40s, DnaJa1 and DnaJa2, are induced upon B-cell activation and interact with AID in vitro, only DnaJa1 overexpression increases AID levels and biological activity in cell lines. Conversely, DnaJa1, but not DnaJa2, depletion reduces AID levels, stability and isotype switching. In vivo, DnaJa1-deficient mice display compromised response to immunization, AID protein and isotype switching levels being reduced by half. Moreover, DnaJa1 farnesylation is required to maintain, and farnesyltransferase inhibition reduces, AID protein levels in B cells. Thus, DnaJa1 is a limiting factor that plays a non-redundant role in the functional stabilization of AID.

Measurement of protein farnesylation and geranylgeranylation in vitro, in cultured cells and in biopsies, and the effects of prenyl transferase inhibitors

The importance of the post-translational lipid modifications farnesylation and geranylgeranylation in protein localization and function coupled with the critical role of prenylated proteins in malignant transformation has prompted interest in their biology and the development of farnesyl transferase and geranylgeranyl transferase inhibitors (FTIs and GGTIs) as chemical probes and anticancer agents. The ability to measure protein prenylation before and after FTI and GGTI treatment is important to understanding and interpreting the effects of these agents on signal transduction pathways and cellular phenotypes, as well as to the use of prenylation as a biomarker. Here we describe protocols to measure the degree of protein prenylation by farnesyl transferase or geranylgeranyl transferase in vitro, in cultured cells and in tumors from animals and humans. The assays use [(3)H]farnesyl diphosphate and [(3)H]geranylgeranyl diphosphate, electrophoretic mobility shift, membrane association using subcellular fractionation or immunofluorescence of intact cells, [(3)H]mevalonic acid labeling, followed by immunoprecipitation and SDS-PAGE, and in vitro transcription, translation and prenylation in reticulocyte lysates. These protocols require from 1 d (enzyme assays) to up to 3 months (autoradiography of [(3)H]-labeled proteins).

Identification of a novel class of farnesylation targets by structure-based modeling of binding specificity

Farnesylation is an important post-translational modification catalyzed by farnesyltransferase (FTase). Until recently it was believed that a C-terminal CaaX motif is required for farnesylation, but recent experiments have revealed larger substrate diversity. In this study, we propose a general structural modeling scheme to account for peptide binding specificity and recapitulate the experimentally derived selectivity profile of FTase in vitro. In addition to highly accurate recovery of known FTase targets, we also identify a range of novel potential targets in the human genome, including a new substrate class with an acidic C-terminal residue (CxxD/E). In vitro experiments verified farnesylation of 26/29 tested peptides, including both novel human targets, as well as peptides predicted to tightly bind FTase. This study extends the putative range of biological farnesylation substrates. Moreover, it suggests that the ability of a peptide to bind FTase is a main determinant for the farnesylation reaction. Finally, simple adaptation of our approach can contribute to more accurate and complete elucidation of peptide-mediated interactions and modifications in the cell.

Combination of farnesyltransferase and Akt inhibitors is synergistic in breast cancer cells and causes significant breast tumor regression in ErbB2 transgenic mice

The Akt activation inhibitor triciribine and the farnesyltransferase inhibitor tipifarnib have modest to little activity in clinical trials when used as single agents. In this article, preclinical data show that the combination is more effective than single agents both in cultured cells and in vivo. Combination index data analysis shows that this combination is highly synergistic at inhibiting anchorage-dependent growth of breast cancer cells. This synergistic interaction is also observed with structurally unrelated inhibitors of Akt (MK-2206) and farnesyltransferase (FTI-2153). The triciribine/tipifarnib synergistic effects are seen with several cancer cell lines including those from breast, leukemia, multiple myeloma and lung tumors with different genetic alterations such as K-Ras, B-Raf, PI3K (phosphoinositide 3-kinase), p53 and pRb mutations, PTEN, pRB and Ink4a deletions, and ErbB receptor overexpression. Furthermore, the combination is synergistic at inhibiting anchorage-independent growth and at inducing apoptosis in breast cancer cells. The combination is also more effective at inhibiting the Akt/mTOR/S6 kinase pathway. In an ErbB2-driven breast tumor transgenic mouse model, the combination, but not single agent, treatment with triciribine and tipifarnib induces significant breast tumor regression. Our findings warrant further investigation of the combination of farnesyltransferase and Akt inhibitors.

Tipifarnib in the treatment of newly diagnosed acute myelogenous leukemia

Farnesyltransferase inhibitors (FTIs) represent a new class of signal transduction inhibitors that block the processing of cellular polypeptides that have cysteine terminal residues and, by so doing, interdict multiple pathways involved in proliferation and survival of diverse malignant cell types. Tipifarnib is an orally bioavailable, nonpeptidomimetic methylquinolone FTI that has exhibited clinical activity in patients with myeloid malignancies including elderly adults with acute myelogenous leukemia (AML) who are not candidates for traditional cytotoxic chemotherapy, patients with high-risk myelodysplasia, myeloproliferative disorders, and imatinib-resistant chronic myelogenous leukemia. Because of its relatively low toxicity profile, tipifarnib provides an important alternative to traditional cytotoxic approaches for elderly patients who are not likely to tolerate or even benefit from aggressive chemotherapy. In this review, we will focus on the clinical development of tipifarnib for treatment of newly diagnosed AML, both as induction therapy for elderly adults with poor-risk AML and as maintenance therapy following achievement of first complete remission following induction and consolidation therapies for poor-risk AML. As with all other malignancies, the optimal approach is likely to lie in rational combinations of tipifarnib with cytotoxic, biologic and/or immunomodulatory agents with non-cross-resistant mechanisms of action. Gene expression profiling has identified networks of differentially expressed genes and gene combinations capable of predicting response to single agent tipifarnib. The clinical and correlative laboratory trials in progress and under development will provide the critical foundations for defining the optimal roles of tipifarnib and in patients with AMl and other hematologic malignancies.

Protein farnesylation inhibitors cause donut-shaped cell nuclei attributable to a centrosome separation defect

Despite the success of protein farnesyltransferase inhibitors (FTIs) in the treatment of certain malignancies, their mode of action is incompletely understood. Dissecting the molecular pathways affected by FTIs is important, particularly because this group of drugs is now being tested for the treatment of Hutchinson-Gilford progeria syndrome. In the current study, we show that FTI treatment causes a centrosome separation defect, leading to the formation of donut-shaped nuclei in nontransformed cell lines, tumor cell lines, and tissues of FTI-treated mice. Donut-shaped nuclei arise during chromatin decondensation in late mitosis; subsequently, cells with donut-shaped nuclei exhibit defects in karyokinesis, develop aneuploidy, and are often binucleated. Binucleated cells proliferate slowly. We identified lamin B1 and proteasome-mediated degradation of pericentrin as critical components in FTI-induced "donut formation" and binucleation. Reducing pericentrin expression or ectopic expression of nonfarnesylated lamin B1 was sufficient to elicit donut formation and binucleated cells, whereas blocking proteasomal degradation eliminated FTI-induced donut formation. Our studies have uncovered an important role of FTIs on centrosome separation and define pericentrin as a (indirect) target of FTIs affecting centrosome position and bipolar spindle formation, likely explaining some of the anticancer effects of these drugs.

The farnesyl transferase inhibitor lonafarnib inhibits mTOR signaling and enforces sorafenib-induced apoptosis in melanoma cells

Farnesyl transferase inhibitors (FTIs) inhibit the farnesylation of proteins, including RAS and RHEB (Ras homolog enriched in brain). RAS signals to the RAF-MEK-ERK (MAPK) and PI3K-AKT-mTOR (AKT) signaling pathways, which have a major role in melanoma progression. RHEB positively regulates mammalian target of rapamycin (mTOR). We investigated the effects of the FTI lonafarnib alone and in combination with MAPK (mitogen-activated protein kinase) or AKT (acutely transforming retrovirus AKT8 in rodent T-cell lymphoma) pathway inhibitors on proliferation, survival, and invasive tumor growth of melanoma cells. Lonafarnib alone did not sufficiently inhibit melanoma cell growth. Combinations of lonafarnib with AKT pathway inhibitors did not significantly increase melanoma cell growth inhibition. In contrast, combinations of lonafarnib with MAPK pathway inhibitors yielded additional growth-inhibiting effects. In particular, the combination of the FTI lonafarnib with the pan-RAF inhibitor sorafenib synergistically inhibited melanoma cell growth, significantly enhanced sorafenib-induced apoptosis, and completely suppressed invasive tumor growth in monolayer and organotypic cultures, respectively. Apoptosis induction was associated with upregulation of the endoplasmic reticulum stress-related transcription factors p8 and CHOP (CAAT/enhancer binding protein (C/EBP) homologous protein), and downregulation of the antiapoptotic Bcl-2 (B-cell lymphoma-2) family protein Mcl-1(myeloid cell leukemia 1). Lonafarnib did not affect MAPK and AKT but did affect mTOR signaling. Together, these findings suggest that the FTI lonafarnib inhibits mTOR signaling and enforces sorafenib-induced apoptosis in melanoma cells and may therefore represent an effective alternative for melanoma treatment.