Covalent Targeting of Ras G12C by Rationally Designed Peptidomimetics

Protein-protein interactions (PPIs) play a critical role in fundamental biological processes. Competitive inhibition of these interfaces requires compounds that can access discontinuous binding epitopes along a large, shallow binding surface area. Conformationally defined protein surface mimics present a viable route to target these interactions. However, the development of minimal protein mimics that engage intracellular targets with high affinity remains a major challenge because mimicry of a portion of the binding interface is often associated with the loss of critical binding interactions. Covalent targeting provides an attractive approach to overcome the loss of noncovalent contacts but have the inherent risk of dominating noncovalent contacts and increasing the likelihood of nonselective binding. Here, we report the iterative design of a proteolytically stable α3β chimeric helix mimic that covalently targets oncogenic Ras G12C as a model system. We explored several electrophiles to optimize preferential alkylation with the desired C12 on Ras. The designed lead peptide modulates nucleotide exchange, inhibits activation of the Ras-mediated signaling cascade, and is selectively toxic toward mutant Ras G12C cancer cells. The relatively high frequency of acquired cysteines as missense mutations in cancer and other diseases suggests that covalent peptides may offer an untapped therapeutic approach for targeting aberrant protein interactions.

Plasma membrane V-ATPase controls oncogenic RAS-induced macropinocytosis

Oncogenic activation of Ras is associated with the acquisition of a unique set of metabolic dependencies that contribute to tumor cell fitness. Mutant Ras cells are endowed with the capability to internalize and degrade extracellular protein via a fluid–phase uptake mechanism termed macropinocytosis¹. There is a growing appreciation for the role of this Ras-dependent process in the generation of free amino acids that can be used to support tumor cell growth under nutrient limiting conditions². However, little is known about the molecular steps that mediate the induction of macropinocytosis by oncogenic Ras. Here we identify vacuolar ATPase (v-ATPase) as an essential regulator of Ras-induced macropinocytosis. Oncogenic Ras promotes the translocation of v-ATPase from intracellular membranes to the plasma membrane (PM) via a pathway that requires protein kinase A (PKA) activation by a bicarbonate-dependent soluble adenylate cyclase (sAC). PM accumulation of v-ATPase is necessary for the cholesterol-dependent association of Rac1 with the PM, a prerequisite for the stimulation of membrane ruffling and macropinocytosis. These observations identify a link between v-ATPase trafficking and nutrient supply by macropinocytosis that could be exploited to curtail the metabolic adaptation capacity of mutant Ras tumor cells.

The Tumor-suppressive Small GTPase DiRas1 Binds the Noncanonical Guanine Nucleotide Exchange Factor SmgGDS and Antagonizes SmgGDS Interactions with Oncogenic Small GTPases

The small GTPase DiRas1 has tumor-suppressive activities, unlike the oncogenic properties more common to small GTPases such as K-Ras and RhoA. Although DiRas1 has been found to be a tumor suppressor in gliomas and esophageal squamous cell carcinomas, the mechanisms by which it inhibits malignant phenotypes have not been fully determined. In this study, we demonstrate that DiRas1 binds to SmgGDS, a protein that promotes the activation of several oncogenic GTPases. In silico docking studies predict that DiRas1 binds to SmgGDS in a manner similar to other small GTPases. SmgGDS is a guanine nucleotide exchange factor for RhoA, but we report here that SmgGDS does not mediate GDP/GTP exchange on DiRas1. Intriguingly, DiRas1 acts similarly to a dominant-negative small GTPase, binding to SmgGDS and inhibiting SmgGDS binding to other small GTPases, including K-Ras4B, RhoA, and Rap1A. DiRas1 is expressed in normal breast tissue, but its expression is decreased in most breast cancers, similar to its family member DiRas3 (ARHI). DiRas1 inhibits RhoA- and SmgGDS-mediated NF-κB transcriptional activity in HEK293T cells. We also report that DiRas1 suppresses basal NF-κB activation in breast cancer and glioblastoma cell lines. Taken together, our data support a model in which DiRas1 expression inhibits malignant features of cancers in part by nonproductively binding to SmgGDS and inhibiting the binding of other small GTPases to SmgGDS.

Abstract 4443: The tumor suppressive small GTPase DiRas1 binds the RhoGEF SmgGDS and antagonizes RhoA activation

A number of malignancies are promoted by the activation of Rho GTPases. Regulators of RhoA activity, including the RhoGEF, SmgGDS, can also promote cancer development. Both RhoA and SmgGDS have been shown to enhance NF-ĸB activation in a number of tumors. Utilizing mass spectrometry, we identified the tumor suppressive small GTPase DiRas1 as a novel binding partner for SmgGDS. DiRas1 is a tumor suppressor in malignant gliomas and esophageal squamous cell carcinomas. Examining DiRas gene expression using the ONCOMINE mRNA microarray database (www.oncomine.org) revealed that DIRAS1 and DIRAS2 are markedly decreased in central nervous system tumors, including anaplastic astrocytomas and GBMs, when compared to normal tissue. The role of DiRas1 in other cancers is largely undetermined, and the ways in which DiRas1 may antagonize pro-oncogenic small GTPase signaling is largely unknown. To define how DiRas1 expression may affect SmgGDS functions, we tested the association of DiRas family proteins with SmgGDS. SmgGDS robustly bound wildtype DiRas1, and its closely-related family member DiRas2, but only weakly bound DiRas3 in vitro and in cell lines. In addition, DiRas1, DiRas2, or DiRas3 protein expression decreased proliferation of HEK293T cells. DiRas1 expression could promote less activated RhoA, as a low level of DiRas1 expression caused markedly decreased RhoA-SmgGDS binding in a number of cell lines. DiRas1 expression also abrogated SmgGDS- and RhoA-mediated NF-ĸB activation in a concentration-dependent manner. SmgGDS may exhibit GEF activity toward DiRas1, as a dominant negative DiRas1 (S21N) binds to SmgGDS more than the wildtype protein. However, DiRas1 is predominantly GTP-bound in cells, likely due to a high rate of guanine nucleotide dissociation and low intrinsic GTPase activity. Thus, DiRas1 expression may in part promote tumor suppression by non-productively binding SmgGDS, resulting in less RhoA activity. Taken together, these results suggest that the tumor suppressive small GTPase DiRas1 can antagonize RhoA signaling by competing for binding of the RhoGEF SmgGDS. Further characterizing these actions may lead to novel therapeutic targeting of RhoA activation in cancer malignancies.

Citation Format: Andrew D. Hauser, Kristen M. Barr, Anne C. Frei, Patrick Gonyo, Ellen L. Lorimer, Carol L. Williams, Carmen Bergom. The tumor suppressive small GTPase DiRas1 binds the RhoGEF SmgGDS and antagonizes RhoA activation. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4443. doi:10.1158/1538-7445.AM2014-4443

The chaperone protein SmgGDS interacts with small GTPases entering the prenylation pathway by recognizing the last amino acid in the CAAX motif

Ras family small GTPases localize at the plasma membrane, where they can activate oncogenic signaling pathways. Understanding the mechanisms that promote membrane localization of GTPases will aid development of new therapies to inhibit oncogenic signaling. We previously reported that SmgGDS splice variants promote prenylation and trafficking of GTPases containing a C-terminal polybasic region and demonstrated that SmgGDS-607 interacts with nonprenylated GTPases, whereas SmgGDS-558 interacts with prenylated GTPases in cells. The mechanism that SmgGDS-607 and SmgGDS-558 use to differentiate between prenylated and nonprenylated GTPases has not been characterized. Here, we provide evidence that SmgGDS-607 associates with GTPases through recognition of the last amino acid in the CAAX motif. We show that SmgGDS-607 forms more stable complexes in cells with nonprenylated GTPases that will become geranylgeranylated than with nonprenylated GTPases that will become farnesylated. These binding relationships similarly occur with nonprenylated SAAX mutants. Intriguingly, farnesyltransferase inhibitors increase the binding of WT K-Ras to SmgGDS-607, indicating that the pharmacological shunting of K-Ras into the geranylgeranylation pathway promotes K-Ras association with SmgGDS-607. Using recombinant proteins and prenylated peptides corresponding to the C-terminal sequences of K-Ras and Rap1B, we found that both SmgGDS-607 and SmgGDS-558 directly bind the GTPase C-terminal region, but the specificity of the SmgGDS splice variants for prenylated versus nonprenylated GTPases is diminished in vitro. Finally, we present structural homology models and data from functional prediction software to define both similar and unique features of SmgGDS-607 when compared with SmgGDS-558.

SmgGDS-558 regulates the cell cycle in pancreatic, non-small cell lung, and breast cancers

Oncogenic mutation or misregulation of small GTPases in the Ras and Rho families can promote unregulated cell cycle progression in cancer. Post-translational modification by prenylation of these GTPases allows them to signal at the cell membrane. Splice variants of SmgGDS, named SmgGDS-607 and SmgGDS-558, promote the prenylation and membrane trafficking of multiple Ras and Rho family members, which makes SmgGDS a potentially important regulator of the cell cycle. Surprisingly little is known about how SmgGDS-607 and SmgGDS-558 affect cell cycle-regulatory proteins in cancer, even though SmgGDS is overexpressed in multiple types of cancer. To examine the roles of SmgGDS splice variants in the cell cycle, we compared the effects of the RNAi-mediated depletion of SmgGDS-558 vs. SmgGDS-607 on cell cycle progression and the expression of cyclin D1, p27, and p21 in pancreatic, lung, and breast cancer cell lines. We show for the first time that SmgGDS promotes proliferation of pancreatic cancer cells, and we demonstrate that SmgGDS-558 plays a greater role than SmgGDS-607 in cell cycle progression as well as promoting cyclin D1 and suppressing p27 expression in multiple types of cancer. Silencing both splice variants of SmgGDS in the cancer cell lines produces an alternative signaling profile compared with silencing SmgGDS-558 alone. We also show that loss of both SmgGDS-607 and SmgGDS-558 simultaneously decreases tumorigenesis of NCI-H1703 non-small cell lung carcinoma (NSCLC) xenografts in mice. These findings indicate that SmgGDS promotes cell cycle progression in multiple types of cancer, making SmgGDS a valuable target for cancer therapeutics.

The SmgGDS splice variant SmgGDS-558 is a key promoter of tumor growth and RhoA signaling in breast cancer

Breast cancer malignancy is promoted by the small GTPases RhoA and RhoC. SmgGDS is a guanine nucleotide exchange factor that activates RhoA and RhoC in vitro. We previously reported that two splice variants of SmgGDS, SmgGDS-607, and SmgGDS-558, have different characteristics in binding and transport of small GTPases. To define the role of SmgGDS in breast cancer, we tested the expression of SmgGDS in breast tumors, and the role of each splice variant in proliferation, tumor growth, Rho activation, and NF-κB transcriptional activity in breast cancer cells. We show upregulated SmgGDS protein expression in breast cancer samples compared with normal breast tissue. In addition, Kaplan-Meier survival curves indicated that patients with high SmgGDS expression in their tumors had worse clinical outcomes. Knockdown of SmgGDS-558, but not SmgGDS-607, in breast cancer cells decreased proliferation, in vivo tumor growth, and RhoA activity. Furthermore, we found that SmgGDS promoted a Rho-dependent activation of the transcription factor NF-κB, which provides a potential mechanism to define how SmgGDS-mediated activation of RhoA promotes breast cancer. This study demonstrates that elevated SmgGDS expression in breast tumors correlates with poor survival, and that SmgGDS-558 plays a functional role in breast cancer malignancy. Taken together, these findings define SmgGDS-558 as a unique promoter of RhoA and NF-κB activity and a novel therapeutic target in breast cancer.

IMPLICATIONS

This study defines a new mechanism to regulate the activities of RhoA and NF-κB in breast cancer cells, and identifies SmgGDS-558 as a novel promoter of breast cancer malignancy.

Cyclic AMP regulates the migration and invasion potential of human pancreatic cancer cells

Aggressive dissemination and metastasis of pancreatic ductal adenocarcinoma (PDAC) results in poor prognosis and marked lethality. Rho monomeric G protein levels are increased in pancreatic cancer tissue. As the mechanisms underlying PDAC malignancy are little understood, we investigated the role for cAMP in regulating monomeric G protein regulated invasion and migration of pancreatic cancer cells. Treatment of PDAC cells with cAMP elevating agents that activate adenylyl cyclases, forskolin, protein kinase A (PKA), 6-Bnz-cAMP, or the cyclic nucleotide phosphodiesterase inhibitor cilostamide significantly decreased migration and Matrigel invasion of PDAC cell lines. Inhibition was dose-dependent and not significantly different between forskolin or cilostamide treatment. cAMP elevating drugs not only blocked basal migration, but similarly abrogated transforming-growth factor-β-directed PDAC cell migration and invasion. The inhibitory effects of cAMP were prevented by the pharmacological blockade of PKA. Drugs that increase cellular cAMP levels decreased levels of active RhoA or RhoC, with a concomitant increase in phosphorylated RhoA. Diminished Rho signaling was correlated with the appearance of thickened cortical actin bands along the perimeter of non-motile forskolin or cilostamide-treated cells. Decreased migration did not reflect alterations in cell growth or programmed cell death. Collectively these data support the notion that increased levels of cAMP specifically hinder PDAC cell motility through F-actin remodeling.

An adenosine-mediated signaling pathway suppresses prenylation of the GTPase Rap1B and promotes cell scattering

During metastasis, cancer cells acquire the ability to dissociate from each other and migrate, which is recapitulated in vitro as cell scattering. The small guanosine triphosphatase (GTPase) Rap1 opposes cell scattering by promoting cell-cell adhesion, a function that requires its prenylation, or posttranslational modification with a carboxyl-terminal isoprenoid moiety, to enable its localization at cell membranes. Thus, signaling cascades that regulate the prenylation of Rap1 offer a mechanism to control the membrane localization of Rap1. We identified a signaling cascade initiated by adenosine A2B receptors that suppressed the prenylation of Rap1B through phosphorylation of Rap1B, which decreased its interaction with the chaperone protein SmgGDS (small GTPase guanosine diphosphate dissociation stimulator). These events promoted the cytosolic and nuclear accumulation of nonprenylated Rap1B and diminished cell-cell adhesion, resulting in cell scattering. We found that nonprenylated Rap1 was more abundant in mammary tumors than in normal mammary tissue in rats and that activation of adenosine receptors delayed Rap1B prenylation in breast, lung, and pancreatic cancer cell lines. Our findings support a model in which high concentrations of extracellular adenosine, such as those that arise in the tumor microenvironment, can chronically activate A2B receptors to suppress Rap1B prenylation and signaling at the cell membrane, resulting in reduced cell-cell contact and promoting cell scattering. Inhibiting A2B receptors may be an effective method to prevent metastasis.

The role of Rac1 in the regulation of NF-κB activity, cell proliferation, and cell migration in non-small cell lung carcinoma

The small GTPase Rac1 regulates many cellular processes, including cytoskeletal reorganization, cell migration, proliferation, and survival. Additionally, Rac1 plays a major role in activating NF-κB-mediated transcription. Both Rac1 and NF-κB regulate many properties of the malignant phenotype, including anchorage-independent proliferation and survival, metastasis, and angiogenesis. Despite these findings, the roles of Rac1and NF-κB in non-small cell lung carcinoma, a leading cause of cancer deaths, have not been thoroughly investigated. Here, we compared the effects of Rac1 siRNA to that of the Rac1 inhibitor NSC23766 on multiple features of the NSCLC malignant phenotype, including NF-κB activity. We show that the siRNA-mediated silencing of Rac1 in lung cancer cells results in decreased cell proliferation and migration. The decrease in proliferation was observed in both anchorage-dependent and anchorage-independent assays. Furthermore, cells with decreased Rac1 expression have a slowed progression through the G 1 phase of the cell cycle. These effects induced by Rac1 siRNA correlated with a decrease in NF-κB transcriptional activity. Additionally, inhibition of NF-κB signaling with BAY 11-7082 inhibited proliferation; indicating that the loss of cell proliferation and migration induced by the silencing of Rac1 expression may be attributed in part to loss of NF-κB activity. Interestingly, treatment with the Rac1 inhibitor NSC23766 strongly inhibits cell proliferation, cell cycle progression, and NF-κB activity in lung cancer cells, to an even greater extent than the inhibition induced by Rac1 siRNA. These findings indicate that Rac1 plays an important role in lung cancer cell proliferation and migration, most likely through its ability to promote NF-κB activity, and highlight Rac1 pathways as therapeutic targets for the treatment of lung cancer.

Mitochondria-targeted nitroxides exacerbate fluvastatin-mediated cytostatic and cytotoxic effects in breast cancer cells

Mito-CP11, a mitochondria-targeted nitroxide formed by conjugating a triphenylphosphonium cation to a five-membered nitroxide, carboxy-proxyl (CP), has been used as a superoxide dismutase (SOD) mimetic. In this study, we investigated the antiproliferative and cytotoxic properties of submicromolar levels of Mito-CP11 alone and in combination with fluvastatin, a well known cholesterol lowering drug, in breast cancer cells. Mito-CP11, but not CP or CP plus the cationic ligand, methyl triphenylphosphonium (Me-TPP+), inhibited MCF-7 breast cancer cell proliferation. Mito-CP11 had only minimal effect on MCF-10A, non-tumorigenic mammary epithelial cells. Mito-CP11, however, significantly enhanced fluvastatin-mediated cytotoxicity in MCF-7 cells. Mito-CP11 alone and in combination with fluvastatin inhibited nuclear factor kappa-B activity mainly in MCF-7 cells. We conclude that mitochondria-targeted nitroxide antioxidant molecules (such as Mito-CP11) that are non-toxic to non-tumorigenic cells could enhance the cytostatic and cytotoxic effects of statins in breast cancer cells. This strategy of combining mitochondria-targeted non-toxic molecules with cytotoxic chemotherapeutic drugs may be successfully used to enhance the efficacy of antitumor therapies in breast cancer treatment.

Body cavity-based malignant lymphoma containing Kaposi sarcoma-associated herpesvirus in an HIV-negative man with previous Kaposi sarcoma

BACKGROUND

The role of Kaposi sarcoma-associated herpesvirus in the development of malignant lymphomas in patients negative for the human immunodeficiency virus (HIV) has not been established.

OBJECTIVE

To examine the possible role of Kaposi sarcoma-associated herpesvirus in a case of body cavity-based malignant lymphoma that occurred in an HIV-negative patient who had previously had Kaposi sarcoma.

DESIGN

Case study.

SETTING

Academic medical center.

PATIENT

A 94-year-old man with lymphomatous ascites.

MEASUREMENTS

Polymerase chain reaction (PCR) and Southern blot DNA analysis.

RESULTS

The body cavity-based lymphoma cells were positive for Kaposi sarcoma-associated herpesvirus by PCR and were negative for other herpesviruses, including Epstein-Barr virus, cytomegalovirus, and human herpesviruses 6 and 7. Southern blot analysis of lymphoma DNA showed high levels of Kaposi sarcoma-associated herpesvirus (> 40 to 80 genomes/cell). Clonal rearrangement of the immunoglobulin JH and JK genes was present, confirming the presence of a clonal B-cell proliferation.

CONCLUSIONS

Kaposi sarcoma-associated herpesvirus may be involved in the development of malignant lymphoma after Kaposi sarcoma in HIV-negative patients. This type of lymphoma, in contrast to body cavity-based lymphoma related to the acquired immunodeficiency syndrome, may have an indolent clinical course.