Genetic analyses of the role of RCE1 in RAS membrane association and transformation

Post-translational modification of KRAS: potential targets for cancer therapy

Aberrant activation of the RAS superfamily is one of the critical factors in carcinogenesis. Among them, KRAS is the most frequently mutated one which has inspired extensive studies for developing approaches to intervention. Although the cognition toward KRAS remains far from complete, mounting evidence suggests that a variety of post-translational modifications regulate its activation and localization. In this review, we summarize the regulatory mode of post-translational modifications on KRAS including prenylation, post-prenylation, palmitoylation, ubiquitination, phosphorylation, SUMOylation, acetylation, nitrosylation, etc. We also highlight the recent studies targeting these modifications having exhibited potent anti-tumor activities.

Real-Time In-Cell NMR Reveals the Intracellular Modulation of GTP-Bound Levels of RAS

The small guanosine triphosphatase (GTPase) RAS serves as a molecular switch in signal transduction, and its mutation and aberrant activation are implicated in tumorigenesis. Here, we perform real-time, in-cell nuclear magnetic resonance (NMR) analyses of non-farnesylated RAS to measure time courses of the fraction of the active GTP-bound form (fGTP) within cytosol of live mammalian cells. The observed intracellular fGTP is significantly lower than that measured in vitro for wild-type RAS as well as oncogenic mutants, due to both decrease of the guanosine diphosphate (GDP)-GTP exchange rate (k_ex) and increase of GTP hydrolysis rate (k_hy). In vitro reconstitution experiments show that highly viscous environments promote a reduction of k_ex, whereas the increase of k_hy is stimulated by unidentified cytosolic proteins. This study demonstrates the power of in-cell NMR to directly detect the GTP-bound levels of RAS in mammalian cells, thereby revealing that the k_hy and k_ex of RAS are modulated by various intracellular factors.

RCE1 deficiency enhances invasion via the promotion of epithelial-mesenchymal transition and predicts poor prognosis in hepatocellular carcinoma

Ras converting CAAX endopeptidase 1 (RCE1) is an integral membrane protease involved in cell proliferation, differentiation, and carcinogenesis. RCE1 plays opposite roles in different tumor types; however, the actual biological function of RCE1 in hepatocellular carcinoma (HCC) is unknown. Here, we aim to investigate the prognostic value and molecular function of RCE1 in HCC. We performed immunohistochemistry in 20 normal human liver, 216 HCC, and 216 adjacent non-tumorous tissues and analyzed the expression change and clinical value of RCE1. Additionally, in vitro and in vivo studies were performed to investigate the role of RCE1 in regulating HCC proliferation, invasion, and metastasis. We found decreased RCE1 expression in HCC tissues. Moreover, the RCE1 expression level was negatively correlated with pathological parameters characteristic of early recurrence (P < 0.044) and the serum alpha-fetoprotein (AFP) level (P < 0.018). Survival analysis indicated that reduced RCE1 expression was a predictor of poor outcomes in patients with HCC. Functional studies showed that the knockdown of RCE1 promoted proliferation, migration, and invasion of HCC cells, while RCE1 overexpression suppressed these effects. In vivo studies further confirmed that the stable knockdown of RCE1 resulted in more rapid tumor growth and an increased number of lung metastatic nodules. Mechanistically, we found that RCE1 deficiency induced epithelial-mesenchymal transition (EMT) via activation of the P38 signaling pathway. Collectively, these results indicate that RCE1 deficiency enhances invasion via promoting epithelial-mesenchymal transition. The downregulation of RCE1 in HCC tissues predicts an unsatisfactory prognosis for patients with HCC.

RASpecting the oncogene: New pathways to therapeutic advances

RAS is the most commonly mutated driver of tumorigenesis, seen in about 30% of all cancer cases. There is a subset of tumors termed RAS-driven cancers in which RAS mutation or overactivation is evident, including as much as 95% in pancreatic and 50% in colon cancer. RAS is a family of small membrane bound GTPases that act as a signaling node to control both normal and cancer biology. Since the discovery of RAS' overall prominence in many tumor types and specifically in RAS-dependent cancers, it has been an obvious therapeutic target for drug development. However, RAS has proved a very elusive target, and after a few prominent RAS targeted drugs failed in clinical trials after decades of research, RAS was termed "undruggable" and research in this field was greatly hampered. An increase in knowledge about basic RAS biology has led to a resurgence in the generation of novel therapeutics targeting RAS signaling utilizing various and distinct approaches. These new drugs target RAS activation directly, block downstream signaling effectors and inhibit proper post-translational processing and trafficking/recycling of RAS. This review will cover how these new drugs were developed and how they have fared in preclinical and early phase clinical trials.

Targeting Mitogen-Activated Protein Kinase Signaling in Mouse Models of Cardiomyopathy Caused by Lamin A/C Gene Mutations

The most frequently occurring mutations in the gene encoding nuclear lamin A and nuclear lamin C cause striated muscle diseases virtually always involving the heart. In this review, we describe the approaches and methods used to discover that cardiomyopathy-causing lamin A/C gene mutations increase MAP kinase signaling in the heart and that this plays a role in disease pathogenesis. We review different mouse models of cardiomyopathy caused by lamin A/C gene mutations and how transcriptomic analysis of one model identified increased cardiac activity of the ERK1/2, JNK, and p38α MAP kinases. We describe methods used to measure the activity of these MAP kinases in mouse hearts and then discuss preclinical treatment protocols using pharmacological inhibitors to demonstrate their role in pathogenesis. Several of these kinase inhibitors are in clinical development and could potentially be used to treat human subjects with cardiomyopathy caused by lamin A/C gene mutations.

HIV protease inhibitors block streptolysin S production

Streptolysin S (SLS) is a post-translationally modified peptide cytolysin that is produced by the human pathogen Streptococcus pyogenes. SLS belongs to a large family of azole-containing natural products that are biosynthesized via an evolutionarily conserved pathway. SLS is an important virulence factor during S. pyogenes infections, but despite an extensive history of study, further investigations are needed to clarify several steps of its biosynthesis. To this end, chemical inhibitors of SLS biosynthesis would be valuable tools to interrogate the various maturation steps of both SLS and biosynthetically related natural products. Such chemical inhibitors could also potentially serve as antivirulence therapeutics, which in theory may alleviate the spread of antibiotic resistance. In this work, we demonstrate that FDA-approved HIV protease inhibitors, especially nelfinavir, block a key proteolytic processing step during SLS production. This inhibition was demonstrated in live S. pyogenes cells and through in vitro protease inhibition assays. A panel of 57 nelfinavir analogs was synthesized, leading to a series of compounds with improved anti-SLS activity while illuminating structure-activity relationships. Nelfinavir was also found to inhibit the maturation of other azole-containing natural products, namely those involved in listeriolysin S, clostridiolysin S, and plantazolicin production. The use of nelfinavir analogs as inhibitors of SLS production has allowed us to begin examining the proteolysis event in SLS maturation and will aid in further investigations of the biosynthesis of SLS and related natural products.

Photoaffinity labeling of Ras converting enzyme using peptide substrates that incorporate benzoylphenylalanine (Bpa) residues: improved labeling and structural implications

Rce1p catalyzes the proteolytic trimming of C-terminal tripeptides from isoprenylated proteins containing CAAX-box sequences. Because Rce1p processing is a necessary component in the Ras pathway of oncogenic signal transduction, Rce1p holds promise as a potential target for therapeutic intervention. However, its mechanism of proteolysis and active site have yet to be defined. Here, we describe synthetic peptide analogues that mimic the natural lipidated Rce1p substrate and incorporate photolabile groups for photoaffinity-labeling applications. These photoactive peptides are designed to crosslink to residues in or near the Rce1p active site. By incorporating the photoactive group via p-benzoyl-l-phenylalanine (Bpa) residues directly into the peptide substrate sequence, the labeling efficiency was substantially increased relative to a previously-synthesized compound. Incorporation of biotin on the N-terminus of the peptides permitted photolabeled Rce1p to be isolated via streptavidin affinity capture. Our findings further suggest that residues outside the CAAX-box sequence are in contact with Rce1p, which has implications for future inhibitor design.

Evaluation of a cell penetrating prenylated peptide lacking an intrinsic fluorophore via in situ click reaction

Protein prenylation involves the addition of either a farnesyl (C(15)) or geranylgeranyl (C(20)) isoprenoid moiety onto the C-terminus of many proteins. This natural modification serves to direct a protein to the plasma membrane of the cell. A recently discovered application of prenylated peptides is that they have inherent cell-penetrating ability, and are hence termed cell penetrating prenylated peptides. These peptides are able to efficiently cross the cell membrane in an ATP independent, non-endocytotic manner and it was found that the sequence of the peptide does not affect uptake, so long as the geranylgeranyl group is still present [Wollack, J. W.; Zeliadt, N. A.; Mullen, D. G.; Amundson, G.; Geier, S.; Falkum, S.; Wattenberg, E. V.; Barany, G.; Distefano, M. D. Multifunctional Prenylated Peptides for Live Cell Analysis. J. Am. Chem. Soc.2009, 131, 7293-7303]. The present study investigates the effect of removing the fluorophore from the peptides and investigating the uptake by confocal microscopy and flow cytometry. Our results show that the fluorophore is not necessary for uptake of these peptides. This information is significant because it indicates that the prenyl group is the major determinant in allowing these peptides to enter cells; the hydrophobic fluorophore has little effect. Moreover, these studies demonstrate the utility of the Cu-catalyzed click reaction for monitoring the entry of nonfluorescent peptides into cells.