An unusual H-Ras mutant isolated from a human multiple myeloma line leads to transformation and factor-independent cell growth

Activating KRAS, NRAS, and BRAF mutants enhance proteasome capacity and reduce endoplasmic reticulum stress in multiple myeloma

KRAS, NRAS, and BRAF mutations which activate p44/42 mitogen-activated protein kinase (MAPK) signaling are found in half of myeloma patients and contribute to proteasome inhibitor (PI) resistance, but the underlying mechanisms are not fully understood. We established myeloma cell lines expressing wild-type (WT), constitutively active (CA) (G12V/G13D/Q61H), or dominant-negative (DN) (S17N)-KRAS and -NRAS, or BRAF-V600E. Cells expressing CA mutants showed increased proteasome maturation protein (POMP) and nuclear factor (erythroid-derived 2)-like 2 (NRF2) expression. This correlated with an increase in catalytically active proteasome subunit β (PSMB)-8, PSMB9, and PSMB10, which occurred in an ETS transcription factor-dependent manner. Proteasome chymotrypsin-like, trypsin-like, and caspase-like activities were increased, and this enhanced capacity reduced PI sensitivity, while DN-KRAS and DN-NRAS did the opposite. Pharmacologic RAF or MAPK kinase (MEK) inhibitors decreased proteasome activity, and sensitized myeloma cells to PIs. CA-KRAS, CA-NRAS, and CA-BRAF down-regulated expression of endoplasmic reticulum (ER) stress proteins, and reduced unfolded protein response activation, while DN mutations increased both. Finally, a bortezomib (BTZ)/MEK inhibitor combination showed enhanced activity in vivo specifically in CA-NRAS models. Taken together, the data support the hypothesis that activating MAPK pathway mutations enhance PI resistance by increasing proteasome capacity, and provide a rationale for targeting such patients with PI/RAF or PI/MEK inhibitor combinations. Moreover, they argue these mutations promote myeloma survival by reducing cellular stress, thereby distancing plasma cells from the apoptotic threshold, potentially explaining their high frequency in myeloma.

An Rb1-dependent amplification loop between Ets1 and Zeb1 is evident in thymocyte differentiation and invasive lung adenocarcinoma

Background

Ras pathway mutation leads to induction and Erk phosphorylation and activation of the Ets1 transcription factor. Ets1 in turn induces cyclin E and cyclin dependent kinase (cdk) 2 to drive cell cycle progression. Ets1 also induces expression of the epithelial-mesenchymal transition (EMT) transcription factor Zeb1, and thereby links Ras mutation to EMT, which is thought to drive tumor invasion. Ras pathway mutations are detected by the Rb1 tumor suppression pathway, and mutation or inactivation of the Rb1 pathway is required for EMT.

Results

We examined linkage between Rb1, Ets1 and Zeb1. We found that an Rb1-E2F complex binds the Ets1 promoter and constitutively limits Ets1 expression. But, Rb1 repression of Zeb1 provides the major impact of Rb1 on Ets1 expression. We link Rb1 repression of Zeb1 to induction of miR-200 family members, which in turn target Ets1 mRNA. These findings suggest that Ets1 and Zeb1 comprise an amplification loop that is dependent upon miR-200 and regulated by Rb1. Thus, induction of Ets1 when the Rb1 pathway is lost may contribute to deregulated cell cycle progression through Ets1 induction of cyclin E and cdk2. Consistent with such an amplification loop, we correlate expression of Ets1 and Zeb1 in mouse and human lung adenocarcinoma. In addition we demonstrate that Ets1 expression in thymocytes is also dependent upon Zeb1.

Conclusions

Taken together, our results provide evidence of an Rb1-dependent Ets1-Zeb1 amplification loop in thymocyte differentiation and tumor invasion.

Electronic supplementary material

The online version of this article (doi:10.1186/s12867-015-0038-4) contains supplementary material, which is available to authorized users.

Targeted overexpression of an activated N-ras gene results in B-cell and plasma cell lymphoproliferation and cooperates with c-myc to induce fatal B-cell neoplasia

Multiple myeloma is an incurable malignant expansion of plasma cells in the bone marrow. Although there is no pathognomonic genetic lesion among multiple myeloma patients, activation of the ras gene has been identified as a common mutation. We have previously described the use of the 3' κ immunoglobulin light chain enhancer (3'KE) to target transgenic expression in murine B and plasma cells, resulting in bcl-X(L) and c-myc-driven murine models of multiple myeloma. In this report, we characterize the role of activated mutant N-ras in B and plasma cells in transgenic mice. We constructed transgenic mice that use 3'KE to direct expression of a mutant activated N-ras. We also crossed the N-ras mice with mice bearing a c-myc transgene to study the cooperative effects of the transgenic constructs. Mice were sacrificed when moribund or at specific time intervals and characterized by serology, light microscopy, and flow cytometry. The transgenic N-ras animals develop B- and plasma cell lymphoproliferation, and aged mice develop immunoglobulinemia, renal hyaline tubular casts, and microscopic foci of abnormal plasma cells in extramedullary sites, including the liver and kidney. Bitransgenic 3'KE/N-Ras V12 × Eμ-c-Myc mice develop fatal B-cell neoplasia, with a median survival of 10 weeks. These data indicate that activated N-ras can play a role in B- and plasma cell homeostasis and that activated N-Ras and c-Myc can cooperate to induce B-cell neoplasia.

Synthetic lethal interaction between oncogenic KRAS dependency and STK33 suppression in human cancer cells

An alternative to therapeutic targeting of oncogenes is to perform "synthetic lethality" screens for genes that are essential only in the context of specific cancer-causing mutations. We used high-throughput RNA interference (RNAi) to identify synthetic lethal interactions in cancer cells harboring mutant KRAS, the most commonly mutated human oncogene. We find that cells that are dependent on mutant KRAS exhibit sensitivity to suppression of the serine/threonine kinase STK33 irrespective of tissue origin, whereas STK33 is not required by KRAS-independent cells. STK33 promotes cancer cell viability in a kinase activity-dependent manner by regulating the suppression of mitochondrial apoptosis mediated through S6K1-induced inactivation of the death agonist BAD selectively in mutant KRAS-dependent cells. These observations identify STK33 as a target for treatment of mutant KRAS-driven cancers and demonstrate the potential of RNAi screens for discovering functional dependencies created by oncogenic mutations that may enable therapeutic intervention for cancers with "undruggable" genetic alterations.

Mutation analysis in Costello syndrome: functional and structural characterization of the HRAS p.Lys117Arg mutation

Costello syndrome is a mental retardation syndrome characterized by high birth weight, postnatal growth retardation, coarse face, loose skin, cardiovascular problems, and tumor predisposition. De novo heterozygous missense mutations in HRAS codon 12 and 13 disturbing the intrinsic GTP hydrolysis cause Costello syndrome. We report a patient with typical Costello syndrome and a novel heterozygous missense mutation in codon 117 (c.350A>G, p.Lys117Arg) of the HRAS gene, resulting in constitutive activation of the RAS/MAPK pathway similar to the typical p.Gly12Ser and p.Gly12Ala mutations. Recombinant HRAS p.Lys117Arg demonstrates normal intrinsic GTP hydrolysis and responsiveness to GTPase-activating proteins, but the nucleotide dissociation rate is increased 80-fold. Consistent with the biochemical data, the crystal structure of the p.Lys117Arg mutant indicates an altered interaction pattern of the side chain that is associated with unfavorable nucleotide binding properties. Together, these data show that a RAS mutation that only perturbs guanine nucleotide binding has similar functional consequences as mutations that impair GTP hydrolysis and causes human disease.

Identification of novel single amino acid changes that result in hyperactivation of the unique GTPase, Rheb, in fission yeast

Rheb GTPase is a key player in the control of growth, cell cycle and nutrient uptake that is conserved from yeast to humans. To further our understanding of the Rheb pathway, we sought to identify hyperactivating mutations in the Schizosaccharomyces pombe Rheb, Rhb1. Hyperactive forms of Rhb1 were found to result from single amino acid changes at valine-17, serine-21, lysine-120 or asparagine-153. Expression of these mutants confers resistance to canavanine and thialysine, phenotypes which are similar to phenotypes exhibited by cells lacking the Tsc1/Tsc2 complex that negatively regulates Rhb1. The thialysine-resistant phenotype of the hyperactive Rhb1 mutants is suppressed by a second mutation in the effector domain. Purified mutant proteins exhibit dramatically decreased binding of GDP, while their GTP binding is not drastically affected. In addition, some of the mutant proteins show significantly decreased GTPase activities. Thus the hyperactivating mutations are expected to result in an increase in the GTP-bound/GDP-bound ratio of Rhb1. By using the hyperactive mutant, Rhb1(K120R), we have been able to demonstrate that Rhb1 interacts with Tor2, one of the two S. pombe TOR (Target of Rapamycin) proteins. These fission yeast results provide the first evidence for a GTP-dependent association of Rheb with Tor.

Inhibition of Nuclear Translocation of Nuclear Factor-κB Despite Lack of Functional IκBα Protein Overcomes Multiple Defects in Apoptosis Signaling in Human B-Cell Malignancies

PURPOSE

Defective apoptosis signaling is a typical feature of classic Hodgkin's lymphoma, multiple myeloma, and activated B-cell-like diffuse large B-cell lymphoma. In these malignancies, the transcription factor nuclear factor-kappaB (NF-kappaB) is a critical mediator of apoptosis resistance and oncogenic growth, making it an attractive therapeutic target. Here, we sought to determine how to overcome apoptosis resistance experimentally in these malignancies by targeting NF-kappaB.

EXPERIMENTAL DESIGN

We investigated the effect of different inhibitors of NF-kappaB on classic Hodgkin's lymphoma, multiple myeloma, and activated B-cell-like diffuse large B-cell lymphoma cell lines harboring different molecular defects in apoptosis signaling both quantitatively and qualitatively.

RESULTS

The cyclopentenone prostaglandin, 15-deoxy-Delta12,14-prostaglandin J(2), a known inhibitor of NF-kappaB, induced caspase-dependent apoptosis; it restored mitochondrial apoptotic signaling by down-regulation of X-linked inhibitor of apoptosis protein and heat shock protein 27 and led to breakdown of the mitochondrial membrane potential and, finally, cleavage of caspase-3 irrespective of IkappaBalpha mutational status. Surprisingly, 15-deoxy-Delta12,14-prostaglandin J(2) and the IkappaB kinase inhibitor curcumin both reduced nuclear levels of p65 in cell lines lacking IkappaBalpha, suggesting that inhibition of nuclear translocation of NF-kappaB can occur in the absence of IkappaBalpha. Finally, a synthetic peptide that specifically abrogates the assembly of the IkappaB kinase complex killed IkappaBalpha-defective cells by induction of apoptosis, paralleled by reduction of nuclear NF-kappaB.

CONCLUSIONS

These results show that molecular defects in apoptotic signaling, such as IkappaBalpha mutations, can be circumvented by targeting NF-kappaB through inhibition of the IkappaB kinase complex followed by induction of apoptosis in classic Hodgkin's lymphoma, multiple myeloma, and activated B-cell-like diffuse large B-cell lymphoma. Thus, targeting IkappaB kinases may represent an attractive therapeutic approach against these malignancies regardless of the mutational status of IkappaBalpha.

Combining prenylation inhibitors causes synergistic cytotoxicity, apoptosis and disruption of RAS-to-MAP kinase signalling in multiple myeloma cells

The high incidence of activating RAS mutations, coupled with accumulating evidence linking RAS to multiple myeloma (MM) pathogenesis, indicate that novel therapies utilising inhibitors of RAS prenylation and signalling may be successful in the management of this disease. While preclinical studies investigating prenylation inhibitors, such as lovastatin, farnesyltransferase inhibitors (FTI) and geranylgeranyltransferase inhibitors (GGTI), have been promising, recent phase I/II clinical trials with FTI R115777 were disappointing, suggesting resistance to FTI monotherapy. To address this issue, the effects of FTI, GGTI and lovastatin alone and in combination were analysed in MM cell lines and primary cells. FTI treatment blocked H-RAS processing, but was ineffective at inhibiting K- and N-RAS prenylation because of alternative geranylgeranylation of these isoforms. However, combinations of FTI and GGTI or lovastatin were found to synergistically inhibit MM cell proliferation, migration, K- and N-RAS processing, RAS-to-mitogen-activated protein kinase signalling and to induce apoptosis. In contrast to FTI, lovastatin and some GGTI were found to cause intracellular accumulation of Rho proteins. Our results suggest that clinical efficacy of prenylation inhibitors in MM are limited by alternative prenylation of several small G-proteins, such as RhoB, K- and N-RAS. Furthermore, strategies combining FTI with GGTI or statins may provide greater efficacy in MM treatment.

Small Molecule Inhibitors of IκB Kinase Are Selectively Toxic for Subgroups of Diffuse Large B-Cell Lymphoma Defined by Gene Expression Profiling

Constitutive activation of the NF-κB pathway is required for survival of the activated B cell–like (ABC) subgroup of diffuse large B-cell lymphoma (DLBCL). Here we show that a small molecule IκB kinase (IKK) inhibitor, PS-1145, and related compounds are toxic for ABC DLBCL cell lines but not for cell lines derived from the other prevalent form of DLBCL, germinal center B cell–like DLBCL. Treatment of ABC lines with these inhibitors rapidly induced a series of gene expression changes that were attributable to cessation of constitutive IKK activity, similar to changes induced by acute expression of genetic inhibitors of NF-κB, confirming the effectiveness and specificity of this compound. Before cell death, inhibition of IKK also induced features of apoptosis and an arrest in the G1 phase of the cell cycle. To test further the specificity of this toxicity, an inducible form of NF-κB was created by fusing the p65 NF-κB subunit with the ligand-binding domain of the estrogen receptor (p65-ERD). In the presence of tamoxifen, p65-ERD reversed the toxicity of IKK inhibition and restored expression of many NF-κB target genes. Another subgroup of DLBCL, primary mediastinal B-cell lymphoma (PMBL), also expresses NF-κB target genes, and treatment of a PMBL cell line with an IKK inhibitor was toxic and induced gene expression changes of a distinct group of NF-κB target genes. These studies validate the NF-κB pathway as a promising therapeutic target in ABC DLBCL, PMBL, and other lymphomas that depend on the activity of NF-κB for survival and proliferation.

PML mediates IFN-alpha-induced apoptosis in myeloma by regulating TRAIL induction

Interferon (IFN) induces expression of proapoptotic genes and has been used in the clinical treatment of multiple myeloma. The promyelocytic leukemia (PML) gene is an IFN-induced target that encodes a tumor suppressor protein. PML protein is typically localized within discrete speckled nuclear structures termed PML nuclear bodies (NBs). Multiple myeloma cells demonstrate differential responses to IFN treatment, the mechanism of which is largely unknown. Herein, we show that growth inhibition effects of IFN-alpha in myeloma cells correlate with PML NBs and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) induction, whereas known IFN targets including signal transducer and activator of transcription-1 (STAT1), STAT3, p38, and Daxx cannot account for these differential responses. RNAi silencing of PML blocks IFN-alpha-induced apoptosis in myeloma cells and correspondingly down-regulates TRAIL expression. Similarly, stable expression of a dominant negative TRAIL receptor DR5 partially blocks IFN-induced cell death. These results demonstrate that PML and TRAIL play important roles in IFN-induced apoptosis and identify TRAIL as a novel downstream transcriptional target of PML. Identification of PML and PML NBs as effectors of IFN responses provides insights into mechanisms by which tumor cells exhibit resistance to this class of agents and may prove useful in assessing treatment regimens.

Inhibition of fibroblast growth factor receptor 3 induces differentiation and apoptosis in t(4;14) myeloma

We have previously shown that dysregulation of fibroblast growth factor receptor 3 (FGFR3) by the t(4;14) translocation is a primary event in multiple myeloma (MM) and that activating mutations of FGFR3 are acquired in some cases. We describe here inhibition of wild-type (WT) and constitutively activated mutant FGFR3 autophosphorylation by the small molecule inhibitor, PD173074. Inhibition of FGFR3 in human myeloma cell lines was associated with decreased viability and tumor cell growth arrest. Further, morphologic, phenotypic, and functional changes typical of plasma cell (PC) differentiation, including increase in light-chain secretion and expression of CD31, were observed and this was followed by apoptosis. Finally, using a mouse model of FGFR3 myeloma, we demonstrate a delay in tumor progression and prolonged survival of mice treated with PD173074. These results indicate that inhibition of FGFR3, even in advanced disease associated with multiple genetic changes, may allow the cell to complete its developmental program and render it sensitive to apoptotic signals. In addition, this represents the validation of a therapeutic target in MM that may benefit patients who have a very poor prognosis with currently available treatments. (Blood. 2004;103:3521-3528)

Genetics and cytogenetics of multiple myeloma: a workshop report

Much has been learned regarding the biology and clinical implications of genetic abnormalities in multiple myeloma. Because of recent advances in the field, an International Workshop was held in Paris in february of 2003. This summary describes the consensus recommendations arising from that meeting with special emphasis on novel genetic observations. For instance, it is increasingly clear that translocations involving the immunoglobin heavy-chain locus are important for the pathogenesis of one-half of patients. As a corollary, it also clear that the remaining patients, lacking IgH translocations, have hyperdiploidy as the hallmark of their disease. Several important genetic markers are associated with a shortened survival such as chromosome 13 monosomy, hypodiploidy, and others. The events leading the transformation of the monoclonal gammopathy of undetermined significance (MGUS) to myeloma are still unclear. One of the few differential genetic lesions between myeloma and MGUS is the presence of ras mutations in the latter. Gene expression platforms are capable of detecting many of the genetic aberrations found in the clonal cells of myeloma. Areas in need of further study were identified. The study of the genetic aberrations will likely form the platform for targeted therapy for the disease.