Modulation of the proteostasis network promotes tumor resistance to oncogenic KRAS inhibitors

Despite substantial advances in targeting mutant KRAS, tumor resistance to KRAS inhibitors (KRASi) remains a major barrier to progress. Here, we report proteostasis reprogramming as a key convergence point of multiple KRASi-resistance mechanisms. Inactivation of oncogenic KRAS down-regulated both the heat shock response and the inositol-requiring enzyme 1α (IRE1α) branch of the unfolded protein response, causing severe proteostasis disturbances. However, IRE1α was selectively reactivated in an ER stress-independent manner in acquired KRASi-resistant tumors, restoring proteostasis. Oncogenic KRAS promoted IRE1α protein stability through extracellular signal-regulated kinase (ERK)-dependent phosphorylation of IRE1α, leading to IRE1α disassociation from 3-hydroxy-3-methylglutaryl reductase degradation (HRD1) E3-ligase. In KRASi-resistant tumors, both reactivated ERK and hyperactivated AKT restored IRE1α phosphorylation and stability. Suppression of IRE1α overcame resistance to KRASi. This study reveals a druggable mechanism that leads to proteostasis reprogramming and facilitates KRASi resistance.

Proteogenomic Approaches for the Identification of NF1/Neurofibromin-depleted Estrogen Receptor-positive Breast Cancers for Targeted Treatment

NF1 is a key tumor suppressor that represses both RAS and estrogen receptor-α (ER) signaling in breast cancer. Blocking both pathways by fulvestrant (F), a selective ER degrader, together with binimetinib (B), a MEK inhibitor, promotes tumor regression in NF1-depleted ER⁺ models. We aimed to establish approaches to determine how NF1 protein levels impact B+F treatment response to improve our ability to identify B+F sensitive tumors. We examined a panel of ER⁺ patient-derived xenograft (PDX) models by DNA and mRNA sequencing and found that more than half of these models carried an NF1 shallow deletion and generally have low mRNA levels. Consistent with RAS and ER activation, RET and MEK levels in NF1-depleted tumors were elevated when profiled by mass spectrometry (MS) after kinase inhibitor bead pulldown. MS showed that NF1 can also directly and selectively bind to palbociclib-conjugated beads, aiding quantification. An IHC assay was also established to measure NF1, but the MS-based approach was more quantitative. Combined IHC and MS analysis defined a threshold of NF1 protein loss in ER⁺ breast PDX, below which tumors regressed upon treatment with B+F. These results suggest that we now have a MS-verified NF1 IHC assay that can be used for patient selection as a complement to somatic genomic analysis.

Significance

A major challenge for targeting the consequence of tumor suppressor disruption is the accurate assessment of protein functional inactivation. NF1 can repress both RAS and ER signaling, and a ComboMATCH trial is underway to treat the patients with binimetinib and fulvestrant. Herein we report a MS-verified NF1 IHC assay that can determine a threshold for NF1 loss to predict treatment response. These approaches may be used to identify and expand the eligible patient population.

Elevated NRAS expression during DCIS is a potential driver for progression to basal-like properties and local invasiveness

BACKGROUND

Ductal carcinoma in situ (DCIS) is the most common type of in situ premalignant breast cancers. What drives DCIS to invasive breast cancer is unclear. Basal-like invasive breast cancers are aggressive. We have previously shown that NRAS is highly expressed selectively in basal-like subtypes of invasive breast cancers and can promote their growth and progression. In this study, we investigated whether NRAS expression at the DCIS stage can control transition from luminal DCIS to basal-like invasive breast cancers.

METHODS

Wilcoxon rank-sum test was performed to assess expression of NRAS in DCIS compared to invasive breast tumors in patients. NRAS mRNA levels were also determined by fluorescence in situ hybridization in patient tumor microarrays (TMAs) with concurrent normal, DCIS, and invasive breast cancer, and association of NRAS mRNA levels with DCIS and invasive breast cancer was assessed by paired Wilcoxon signed-rank test. Pearson's correlation was calculated between NRAS mRNA levels and basal biomarkers in the TMAs, as well as in patient datasets. RNA-seq data were generated in cell lines, and unsupervised hierarchical clustering was performed after combining with RNA-seq data from a previously published patient cohort.

RESULTS

Invasive breast cancers showed higher NRAS mRNA levels compared to DCIS samples. These NRAShigh lesions were also enriched with basal-like features, such as basal gene expression signatures, lower ER, and higher p53 protein and Ki67 levels. We have shown previously that NRAS drives aggressive features in DCIS-like and basal-like SUM102PT cells. Here, we found that NRAS-silencing induced a shift to a luminal gene expression pattern. Conversely, NRAS overexpression in the luminal DCIS SUM225 cells induced a basal-like gene expression pattern, as well as an epithelial-to-mesenchymal transition signature. Furthermore, these cells formed disorganized mammospheres containing cell masses with an apparent reduction in adhesion.

CONCLUSIONS

These data suggest that elevated NRAS levels in DCIS are not only a marker but can also control the emergence of basal-like features leading to more aggressive tumor activity, thus supporting the therapeutic hypothesis that targeting NRAS and/or downstream pathways may block disease progression for a subset of DCIS patients with high NRAS.

Abstract 2979: ElevatedNRASexpression as a potential driver of DCIS progression to basal-like invasive breast cancer

Background: Ductal carcinoma in situ (DCIS) is the most common type of in situ premalignant breast cancers. What drives a subset of luminal DCIS to transition to basal invasive breast cancers is unclear. Basal-like invasive breast cancers are aggressive and associated with the shortest overall survival and time to distant metastasis. We have previously shown that N-Ras is highly expressed selectively in the basal-like subtypes of invasive breast cancers and can promote their growth and progression. In this study, we investigated whether NRAS expression at the DCIS stage can control transition from luminal DCIS to basal-like invasive breast cancers.

Methods: Wilcoxon rank-sum test was conducted to assess the relative expression of NRAS in DCIS compared to invasive breast cancers in a patient cohort (GSE59248). NRAS levels were also determined by fluorescence in situ hybridization (FISH) in a collection of 21 patient tumor microarrays (TMAs) with concurrent normal, DCIS, and invasive breast cancer, and NRAS’s association with DCIS and invasive breast cancer was assessed by Wilcoxon signed rank test. How NRAS levels correlated with basal biomarkers in the TMAs, as well as in patient datasets GSE59248 and GSE3369 was determined by Pearson correlation. Gene expression changes in cell line were assessed by RNA-seq.

Results: Invasive breast cancers showed higher NRAS mRNA levels, as compared to DCIS samples. These NRAShigh lesions were also enriched with basal-like features, such as basal gene expression signature, lower ER, higher p53 protein levels, and higher Ki67 levels. We have shown previously that N-Ras is a driver for tumor growth in SUM102 cells, which is a DCIS-like cell line model displaying basal-like features. Here we found that NRAS-silencing in these cells induced a shift to a luminal gene expression pattern as determined by PAM50.

Conclusions: These data suggest that the rise of N-RAS levels in DCIS can not only mark but also control the emergence of basal-like features leading to more aggressive tumor activities. Targeting N-Ras and/or its downstream pathway may prevent the emergence of basal invasive breast cancers.

Citation Format: Ze-yi Zheng, Hanan Elsarraj, Yang Hong, Jonathan T. Lei, Meenakshi Anurag, Yichao Shen, Flora Lo, Long Feng, Zifan Zhao, Xiang H. Zhang, Fariba Behbod, Eric C. Chang. ElevatedNRASexpression as a potential driver of DCIS progression to basal-like invasive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2979.

Abstract 2490: Optimizing treatment strategy for NF1-depleted estrogen receptor positive breast cancer

Background: Germline mutations in the NF1 gene are responsible for neurofibromatosis type 1, which is the world's most common genetic disorder. NF1 is also a key tumor suppressor gene that is frequently somatically mutated in a wide range of cancers. Approximately 80% of breast cancer is driven by the estrogen receptor α (ER), encoded by the ESR1 gene, and ER-positive (ER+) breast cancer can be treated by endocrine therapy targeting the ER transcriptional pathway. NF1 encodes neurofibromin, which is best known as a GTPase Activating Protein (GAP) for repressing Ras signaling. However, in a recent study we presented evidence supporting the model that NF1 has a GAP-independent activity by also acting as a transcriptional co-repressor for ER. NF1 loss enhanced ER transcription causing resistance to tamoxifen and aromatase inhibition.

Approach and Results: In this study, we examined patient data from TCGA cohort and found that low NF1 mRNA levels associated with recurrence in luminal breast cancer, particularly in the luminal B subtype. Using purified components, we showed that full-length NF1 can directly bind ER. The ESR1-pE380Q mutation is a recurrent event in metastatic ER+ breast cancer. Our two-hybrid data showed that NF1 interacted less with the ER-E380Q than wild type ER, which agrees with structural analysis predicting co-repressors binding to be mediated by the ER-E380 residue. To assess how NF1-loss impacts ER-dependent gene expression in ER+ breast cancer cells, our ER ChIP-seq data showed that in the presence of estradiol, NF1-depletion promoted global ER recruitment to estrogen response elements (EREs) on chromatin. Expression of ERE-bound genes showed concordant expression changes by RNA-seq, confirming genome-wide transcriptional dysregulation of ER targeted genes by NF1 loss. ER+ NF1-depleted breast cancer cells responded initially to a selective ER degrader (SERD), such as fulvestrant and an oral SERD AZD9496, but acquired resistance with prolonged treatment. Resistance may be dependent on CDK4/6, a common growth pathway controlled by both Ras and ER. We showed that fulvestrant together with a CDK4/6 inhibitor Palbociclib can efficiently inhibit the growth of ER+ NF1-depleted breast cancer leading to tumor regression in a patient derived xenograft model.

Conclusion: The loss of the full length NF1 can stimulate both ER and Ras signaling, and it is possible to efficiently treat ER+ NF1-depleted breast cancer by a SERD, in combination with CDK4/6 inhibitor.

Citation Format: Zeyi Zheng, Jonathan T. Lei, Meenakshi Anurag, Long Feng, Purba Singh, Hilda Kennedy, Jin Cao, Xi Chen, Matthew J. Ellis, Eric C. Chang. Optimizing treatment strategy for NF1-depleted estrogen receptor positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2490.

Abstract PS19-15: N-RAS as a marker for DCIS progression

Background: Ductal carcinoma in situ (DCIS) is a key premalignant stage. Approximately 50% of the DCIS cases will advance to invasive ductal carcinoma (IDC); however, they are usually universally treated leading to overtreatment. While earlier premalignant breast lesions, such as atypical ductal hyperplasia, are nearly all luminal and positive for estrogen receptor α (ER+), up to 8% DCIS are basal-like, which is more aggressive. We have previously shown that N-Ras, which is highly expressed selectively in the basal-like subtype, can promote the growth and transforming activity of basal-like breast cancer (PMID 26166574). In this study we investigate whether NRAS expression at DCIS can promote basal-like properties and the progression to invasiveness.

Methods: NRAS mRNA levels in normal, DCIS, and IDC patient-derived microarray databases and tissue microarrays (TMAs) were compared using student t-test. Correlation between NRAS mRNA levels and basal-like properties was assessed by Pearson Correlation. ER protein levels were assessed by IHC. NRAS was expressed in a luminal DCIS cell line model SUM225PE cells and the resulting cells were injected intraductally into mice to assess expression of basal-like markers and invasiveness in vivo. Conversely, NRAS expression was silenced in a DCIS-like basal-like cell line SUM102 and increased expression of luminal markers was assessed by RNA-seq and western blot.

Results: In microarray databases, DCIS samples with higher NRAS mRNA levels are enriched with basal-like gene expression signature; furthermore, NRAS mRNA levels appear to increase progressively from normal to DICS and from DCIS to IDC. A similar correlation between high NRAS levels and invasiveness and high Ki67 levels in TMAs was observed by RNA FISH. High levels of NRAS mRNAs also correlated with low ER levels.

Conclusions: How to identify the subset of DCIS cases that will become invasive is critical for addressing the current problem of overtreatment. Our data support the hypothesis that high N-RAS levels in DCIS mark invasiveness, by presumably inducing basal-like more aggressive tumor activities. On-going studies will further delineate whether N-Ras is also responsible for driving DCIS to the invasive state.

Citation Format: Eric C Chang, Zeyi Zheng, Hanan Elsarraj, Yan Hong, Yichao Shen Shen, Flora Lo, Long Feng, Xian H.-F. Zhang, Fariba Behbod. N-RAS as a marker for DCIS progression [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS19-15.

Neurofibromin Is an Estrogen Receptor-α Transcriptional Co-repressor in Breast Cancer

We report that neurofibromin, a tumor suppressor and Ras-GAP (GTPase-activating protein), is also an estrogen receptor-α (ER) transcriptional co-repressor through leucine/isoleucine-rich motifs that are functionally independent of GAP activity. GAP activity, in turn, does not affect ER binding. Consequently, neurofibromin depletion causes estradiol hypersensitivity and tamoxifen agonism, explaining the poor prognosis associated with neurofibromin loss in endocrine therapy-treated ER⁺ breast cancer. Neurofibromin-deficient ER⁺ breast cancer cells initially retain sensitivity to selective ER degraders (SERDs). However, Ras activation does play a role in acquired SERD resistance, which can be reversed upon MEK inhibitor addition, and SERD/MEK inhibitor combinations induce tumor regression. Thus, neurofibromin is a dual repressor for both Ras and ER signaling, and co-targeting may treat neurofibromin-deficient ER⁺ breast tumors.

Abstract P6-21-06: Target N-Ras for degradation by flunarizine to treat basal-like breast cancer

Background:Basal-like breast cancer (BLBC) is an aggressive form of breast cancer that are usually triple negative for ER, PR, and HER2. There is no effective targeted therapy for BLBC due to the lack of a druggable driver. Ras GTPases are powerful drivers for tumorigenesis. We have shown that wild type N-Ras, but not K- or H-Ras, is overexpressed in BLBC and driving itsgrowth and transforming activities. However, there is currently no treatment that directly target Ras. This study thus screened existing pharmacologically active and approved compounds for the new ability to induce N-Ras degradation in BLBC.

Methods:Compounds in the LOPAC library were screened by an automated microscopy system for the ability to reduce GFP-N-Ras signals in the cells. Isolated compounds were then examined to identify those that can degrade endogenous N-Ras in BLBC cells without impacting levels of other Ras proteins. Final candidate compounds were further examined to determine by which proteolytic pathway N-Ras is induced to be degraded. The potentials of the identified compound to treat BLBC were assessed by examining cell growth and soft agar colony formation in vitroand tumor growth in vivo.

Results:We identified flunarizine (FLN), previously approved for treating migraine and epilepsy. The FLN-induced N-Ras degradation was not affected by a 26S-proteasome inhibitor. Rather, it was blocked by autophagy inhibitors. Furthermore, N-Ras can be seen co-localized with active autophagosomes upon FLN treatment, suggesting that FLN alters the autophagy pathway to degrade N-Ras. Importantly, FLN treatment recapitulated the effect of N-RASsilencingin vitroby selectively inhibiting the growth of BLBC cells, but not that of breast cancer cells of other subtypes. In addition, in vivoFLN inhibited tumor growth of a BLBC xenograft model.

Conclusion:This proof-of-principle study presents evidence that the autophagy pathway can be coerced by small molecule inhibitors, such as FLN, to degrade Ras as a strategy to treat cancer. FLN has low toxicity and should be further investigated to enrich the toolbox of cancer therapeutics against BLBC.

Citation Format: Zheng Z-Y, Li J, Li F, Zhu Y, Cui K, Wong ST, Chang EC, Liao Y-h. Target N-Ras for degradation by flunarizine to treat basal-like breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-21-06.

Abstract P5-04-30: Developing an immunohistochemistry protocol to detect neurofibromin as an effective, simple, and rapid method to identify NF1-negative breast cancer patients

Background:Neurofibromin is a key tumor suppressor, well-known as a GTPase-Activating-Protein (GAP) to attenuate Ras signaling. It is encoded by the NF1gene, so named because its inactivation was first discovered to cause Neurofibromatosis type 1, an autosomal dominant genetic disorder. We have recently reported thatnonsense (NS) and frameshift (FS) mutations, but not missense mutations, in NF1are associated with a markedly higher risk of relapse and death in early stage ER+breast cancer after adjuvant tamoxifen monotherapy (Griffith et al. in press). Surprisingly, despite being best known as a GAP, no missense mutations inactivating NF1's GAP activity were found in our cohort. We have evidence that these NF1NS/FS mutations cause the resulting mRNAs to be degraded, leading to depletion of the entire NF1 protein. In a separate study that was presented here last year, we showed that NF1 is also an ER co-repressor, which partially explains why the loss of the single tumor suppressor NF1 is so detrimental — because this turns on two potent oncogenic pathways. Thus far there is no effective means to assess loss of NF1 protein in cancer. The objective of this project is to identify these aggressive NF1-negative breast cancers by establishing an immunohistochemistry (IHC) protocol with a valid NF1 antibody in order to properly find and treat them in the future.

Methods:A monoclonal antibody was raised against the C-terminus of NF1. Immunostaining as well as IHC was performed using a set of breast cancer cell lines with varying degrees of NF1 protein levels, including several NF1 null-like cell lines as negative controls. To assess whether the IHC protocol can be used on patients, NF1+and NF1–PDXs as well as patient biopsies were examined.

Results: We have purified a monoclonal antibody against NF1 (m376). By immunostaining, a strong NF1 signal can be seen in T47D cells, which have four copies of the NF1gene. In contrast, there was barely any signal in MDA-MB-175VII cells, which lack detectable NF1 due to NF1FS mutations. While NF1 appears mostly cytoplasmic, 10-15% can be seen in the nucleus. Nuclear NF1 levels can be further increased by the nuclear export blocker leptomycin-B, suggesting that NF1 is shuttled in and out of the nucleus. IHC staining confirmed these features of NF1. In addition, a weak nuclear signal can be seen in cancer cells carrying NF1FS mutations. Experiments are on-going to assess how to analyze tumor samples for NF1 loss and whether NF1FS mutations cause expression of truncated proteins that are nuclear.

Conclusion: Our results suggest that the m376 antibody has the potential to be used for IHC, provided that samples known to be NF1+or NF1–are included as controls. The success of this project will have particular clinical impact in telling us who should notbe treated by tamoxifen. Furthermore, we have an approved clinical trial protocol to assess the concept that these NF1–patients should instead be treated by combining a Ras pathway inhibitor and a SERD.

Citation Format: Peng J, Zheng Z-y, Cakar B, Li J, Singh P, Szafrain AT, Stossi F, Dubrulle J, Mancini MA, Mao R, Miles G, Ellis MJ, Chang EC. Developing an immunohistochemistry protocol to detect neurofibromin as an effective, simple, and rapid method to identify NF1-negative breast cancer patients [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-04-30.

Induction of N-Ras degradation by flunarizine-mediated autophagy

Ras GTPases are powerful drivers for tumorigenesis, but directly targeting Ras for treating cancer remains challenging. The growth and transforming activity of the aggressive basal-like breast cancer (BLBC) are driven by N-Ras. To target N-Ras in BLBC, this study screened existing pharmacologically active compounds for the new ability to induce N-Ras degradation, which led to the identification of flunarizine (FLN), previously approved for treating migraine and epilepsy. The FLN-induced N-Ras degradation was not affected by a 26S-proteasome inhibitor. Rather, it was blocked by autophagy inhibitors. Furthermore, N-Ras can be seen co-localized with active autophagosomes upon FLN treatment, suggesting that FLN alters the autophagy pathway to degrade N-Ras. Importantly, FLN treatment recapitulated the effect of N-RAS silencing in vitro by selectively inhibiting the growth of BLBC cells, but not that of breast cancer cells of other subtypes. In addition, in vivo FLN inhibited tumor growth of a BLBC xenograft model. In conclusion, this proof-of-principle study presents evidence that the autophagy pathway can be coerced by small molecule inhibitors, such as FLN, to degrade Ras as a strategy to treat cancer. FLN has low toxicity and should be further investigated to enrich the toolbox of cancer therapeutics.

Abstract 1814: NF1 as an estrogen receptor-α co-repressor in breast cancer

NF1 has been best known as a GAP (GTPase Activating Protein) that inactivates Ras. However, we are now finding evidence that it also functions as an ER co-repressor, whose loss leads to endocrine therapy resistance. Sequencing tumor DNA from >600 ER+ breast cancers treated by tamoxifen adjuvant monotherapy, we found that frameshift (FS) and nonsense (NS) NF1 mutations, which can create an NF1-null state, strongly correlate with relapse risk (HR=2.6, submitted). Surprisingly, no recurrent missense NF1 mutations inactivating GAP activity were found in our cohort, and such mutations are rare in primary cancers in general. We thus posulated that complete loss of NF1 protein (e.g., caused by NS/FS mutations), but not GAP inactivation alone, is required to drive endocrine therapy resistance.

Here we demonstrate that NF1 loss (by gene silencing) in ER+ breast cancer cells greatly enhances ligand-dependent ER transcriptional activity in vitro and in vivo, causing estradiol (E2) hypersensitivity and tamoxifen agonism. Mechanistically we show that NF1 can bind directly to ER, an interaction enhanced by tamoxifen but not by E2. Binding is mediated by leucine/isoleucine-rich motifs in NF1, analogous to other ER co-repressors. Mutations in these motifs (some of which are targeted by somatic mutation in cancer) inhibit ER binding and transcriptional activity without impacting GAP activity; conversely, inactivating GAP activity does not impact ER binding and repression. To validate NF1 as an ER co-repressor, we examined proteomic data from >100 breast cancer patients in the CPTAC data base and found that proteins whose levels are positively correlated with NF1 are highly enriched with factors known to bind nuclear receptors; by contrast, levels of another GAP, p120, which lacks ER binding sites, are negatively correlated with these molecules. Importantly, preclinical treatment studies indicate that while NF1-deficient ER+ breast cancer should not be treated by tamoxifen or aromatase inhibitors, fulvestrant, which degrades ER, remains effective. However, fulvestrant monotherapy can activate the Ras-MAP pathway, which may promote cell survival and acquired fulvestrant resistance unless combined with dabrafinib and trametinib to inhibit Raf and MEK —a clinical trial for this combination is in development.

Our data suggest that NF1 is a dual negative regulator at the intersection of two potent oncogenic signaling pathways, Ras and ER. Combination therapy targeting both the ER and the Ras-Raf pathways should be investigated for NF1-deficient cancers driven by ER.

Citation Format: Eric C. Chang, zeyi Zheng, Meenakshi Anurag, Jin Gao, Burcu Cakar, Xinhui Du, Jing Li, Philip Lavere, Jonathan T. Lei, Purba Singh, Sinem Seker, Wei Song, Jianheng Peng, Tiffany Nguyen, Doug Chan, Xi Chen, Kimberly C. Banks, Richarad B. Lanman, Maryam Shafaee, Susan Hilsenbeck, Charles Foulds, Matthew J. Ellis. NF1 as an estrogen receptor-α co-repressor in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1814.

Abstract GS2-02: Direct regulation of estrogen receptor-α (ER) transcriptional activity by NF1

Inactivating germline mutations in the NF1 gene (encoding neurofibromin) cause neurofibromatosis type 1. In addition to peripheral nervous system tumors, NF1 patients are at higher risk for other cancers, including breast cancer. Tumor exome-sequencing studies demonstrate that approximately 20% of all human cancers have somatic NF1 mutations. NF1 has been best known for its ability to inactivate Ras as a GAP (GTPase Activating Protein). However, this function is served by a small GAP domain in a very large protein. Recurrent missense mutations inactivating the GAP activity are infrequent. In contrast, it is common to detect frameshift (FS) and nonsense (NS) NF1 mutations, which can create an NF1-null state deleting not only GAP, but also, potentially, undefined NF1 functions whose loss could also drive tumorigenesis.

As we reported at SABCS previously, in 600+ patients treated by tamoxifen adjuvant monotherapy, we found that FS/NS NF1 mutations independently correlate with relapse risk (HR=2.6, p=0.03). To explore this finding, we silenced NF1 in preclinical models of ER+ breast cancer, which markedly enhanced ER transcriptional activities, causing estradiol (E2) hypersensitivity and converted tamoxifen into an agonist (in vitro and in vivo). Most important, these activities depend on ER, but not on NF1's GAP activity. These findings readily explain the poor patient outcomes associated with NS/FS NF1 mutations, and reveal a previously unrecognized function for NF1 in ER regulation.

In the presence of an agonist, liganded ER repels co-repressors and recruits co-activators, while the reverse is true with an antagonist such as tamoxifen. Many co-regulators contain leucine/isoleucine rich motifs, which bind directly to the ligand-binding domain (LBD) in ER. NF1 has several of these motifs that are much more highly conserved in species with a functional ER pathway, and some of these are mutated in cancers (e.g., in our patient cohort). Furthermore, we found that NF1 canbind directly to ER, and that this binding is mediated between the ER LBD and the NF1 leucine-rich regions. Like a classic co-repressor, wildtype NF1 (but not mutants lacking GAP activity or the Leu-rich motif) binds to ER, and is recruited by ER to the ERE in the presence of tamoxifen, but not E2.

Further preclinical treatment studies indicate that while NF1-deficient ER+ breast cancer should not be treated by tamoxifen or AIs, fulvestrant remains effective. Furthermore, when fulvestrant is combined with dabrafinib and trametinib to inhibit Ras effectors Raf and MEK, apoptosis is induced in vitro, and tumor regression is observed in vivo. In conclusion, we have demonstrated that NF1 is a dual negative regulator at the intersection of two potent oncogenic signaling pathways, Ras and ER, and that NF1-deficient ER+ breast cancer patients may be more effectively treated by co-targeting the Ras and ER signaling. These patients, up to 10% of those with advanced ER+ breast cancer, can be readily identified for treatment by ctDNA analysis. A clinical trial is under development.

Citation Format: Chang EC, Zheng Z, Philip L, Burcu C, Lei J, Singh P, Anurag M, Chan D, Li JD, Du XP, Shafaee MN, Banks K, Sacker S, Song W, Nguyen T, Cao J, Chen X, Haricharan S, Kavuri M, Kim B-J, Zhang B, Gutmann DH, Lanman RB, Foulds C, Ellis M. Direct regulation of estrogen receptor-α (ER) transcriptional activity by NF1 [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr GS2-02.

Abstract P1-08-01: Regulation of estrogen receptor-α by NF1

Background. Although great strides have been made in targeting the ER pathway for treating ER+ breast cancer, relapse and death is common and is closely linked to resistance to ER-targeting agents. As a result, the majority of deaths from breast cancer still come from ER+ tumors. To discover drivers for endocrine resistance, we have sequenced tumor DNAs from a cohort of >600 patients treated with 5-year tamoxifen (Tam) monotherapy with a median 10.4 years follow up. Our preliminary data show that the worst outcome mutations (Hazard Ratio of ∼3 for relapse) were mostly those of the Neurofibromatosis type 1 (NF1) gene (encoding Neurofibromin), with nonsense/frame shift mutations creating early stop codons.

Germline NF1mutations cause neurofibromatosis type 1, a common inherited disorder that predisposes individuals to both benign and malignant tumors of the nervous system, as well as an increased risk for breast cancer. Analysis of DNA sequencing data has also shown that the NF1 gene is mutated in a wide range of common cancers (e.g., melanoma, lymphoma, and cancers of the lung, breast, and colon). Thus, NF1-deficiency underlies the formation and/or progression of a large number of cancers, so that the development of therapies targeted to NF1-deficient malignancies would have broad impact.

These observations support the hypothesis that NF1 gene inactivation is associated with aggressive tumor behaviors, such as endocrine therapy resistance in breast cancer. The key focus of this study is to define how the NF1 protein neurofibromin, regulates endocrine therapy resistance. Although neurofibromin is best known as a negative regulator for Ras, our data show that it may have other functions.

Method. Our data suggest that many of the identified nonsense/frame shift create a NF1 null state; thus, we have used gene-silencing to recapitulate the effects of such NF1 mutations on the activities of ER+ breast cancer cells. NF1+ and NF1– ER+ breast cancer cells were grown in defined media to measure how estradiol (E2) and Tam impact their growth, transforming activities, and gene expression. The binding between neurofibromin and components of the ER transcriptional pathway was measured biochemically and using the mammalian two-hybrid system.

Results. Our data showed that NF1-silenced cells use Tam as an agonist and can grow with very little E2, and these activities are driven by enhanced recruitment of ER to the ERE, leading to efficient expression of many classic ER-responsive genes. Expressing the NF1-GAP domain does not restore normal responses to Tam and E2 in NF1-silenced cells, suggesting that neurofibromincan regulate ER activity in a Ras-independent manner. To investigate the possibility that neurofibromin can directly regulate ER, we found that it can bind ER; furthermore, neurofibromin was more strongly recruited to the ERE by Tam than by E2.

Conclusion. Our data support a model whereby neurofibromin acts like a co-repressor for ER. As such,NF1 loss may result in more aggressive tumor behaviors by activating, not only the Ras pathways, but also the ER transcriptional pathways. Simultaneous activation of two powerful oncogenic pathways by the loss of a single tumor suppressor may explain why neurofibromin is such a potent tumor suppressor lost in a wide range of cancers.

Citation Format: Zheng Z-Y, Cakar B, Lavere P, Cao J, Yao J, Singh P, Lei JT, Toonen JA, Haricharan S, Anurag M, Shah K, Kavuri M, Chan DW, Chen X, Gutmann DH, Foulds CE, Ellis MJ, Chang EC. Regulation of estrogen receptor-α by NF1 [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-08-01.

Abstract P2-06-11: Wild type N-Ras, overexpressed in basal-like breast cancer, promotes tumor formation by inducing IL8 secretion via JAK2 activation

"Basal-like" breast cancer (BLBC) is a very aggressive subtype of breast cancer. BLBC has very poor prognosis — median time to distant recurrence is just 2.6 years vs. 5 years overall, and survival time from diagnosis of distant metastatic disease is 9 months vs. 22 months. BLBC tumors usually do not express ER, Her2, or progesterone receptor. As such, they cannot be treated by the current targeted therapies, which target these molecules. What drive the formation and progression of BLBCs is largely unclear.

Ras GTPases are best known for mediating growth factor signaling. Oncogenic mutations in the RAS genes, K-RAS in particular, are found in more than 30% of human tumors. Surprisingly, oncogenic RAS mutations are rare in breast cancer. However, we found that wild-type N-RAS is overexpressed in BLBCs, possibly partly via promoter demethylation, but not in other breast cancer subtypes. Repressing N-RAS inhibits transformation and tumor growth, while overexpressing it enhances these processes even in preinvasive BLBC cells. In contrast, in breast cancer cells of other subtypes, repressing N-RAS expression does not affect growth and transforming activities. We identified N-Ras-responsive genes, most of which encode chemokines and cytokines, e.g., IL8. High expression levels of these N-Ras-responsive genes as well as of N-RAS itself in tumors correlate with poor patient outcome. N-Ras, but not K-Ras, induces IL8 by binding and activating the cytoplasmic pool of JAK2; IL8 then acts on both the cancer cells and stromal fibroblasts.

In conclusion, N-Ras drives BLBC by promoting transformation in epithelial cells, which may in turn remodel the tumor microenvironment to create a proinvasive state. Although oncogenic mutations affecting RAS are common in many other human cancers, tumorigenesis in an important subset of breast cancers is driven instead by increasing activity of wild-type N-Ras. Thus, to fully assess the impact of Ras on tumorigenesis, the role of wild-type as well as mutant Ras proteins must be carefully examined.

Citation Format: Zheng Z-Y, Bu W, Tian L, Fan C, Gao X, Zhang X, Yu C, Wang H, Liao Y-H, Li Y, Lewis MT, Edwards D, Zwaka TP, Hilsenbeck SG, Medina D, Perou CM, Creighton CJ, Zhang XH, Chang EC. Wild type N-Ras, overexpressed in basal-like breast cancer, promotes tumor formation by inducing IL8 secretion via JAK2 activation. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P2-06-11.

Abstract 2139: Wild type N-Ras, overexpressed in basal-like breast cancer, promotes tumor formation by inducing IL8 secretion via JAK2 activation

“Basal-like” breast cancer (BLBC) is the most aggressive breast cancer subtype, with a very poor prognosis _ median time to distant recurrence is just 2.6 years vs. 5 years overall, and survival time from diagnosis of distant metastatic disease is 9 months vs. 22 months. BLBC tumors usually do not express ER, Her2, or progesterone receptor. As such, they cannot be treated by the current targeted therapies, which target these molecules. What drive the formation and progression of BLBCs is largely unclear.

Ras GTPases are best known for mediating growth factor signaling, and oncogenic mutations in the RAS genes, and K-RAS in particular, are found in more than 30% of human tumors. Surprisingly, oncogenic RAS mutations are rare in breast cancer. However, we found that wild-type N-RAS is overexpressed in BLBCs, partly via promoter demethylation, but not in other breast cancer subtypes. Repressing N-RAS inhibits transformation and tumor growth, while overexpression enhances these processes even in preinvasive BLBC cells. In contrast, repressing N-RAS expression does not affect growth and transforming activities of breast cancer cells of other subtypes. We identified N-Ras-responsive genes, most of which encode chemokines, e.g., IL8. High expression levels of these N-Ras-responsive genes as well as of N-RAS itself in tumors correlate with poor patient outcome. N-Ras, but not K-Ras, induces IL8 by binding and activating the cytoplasmic pool of JAK2; IL8 then acts on both the cancer cells and stromal fibroblasts. Thus N-Ras drives BLBC by promoting transformation in epithelial cells, which in turn remodels the tumor microenvironment to create a proinvasive state.

These results illustrate an N-Ras-dependent convergence between the growth factor and cytokine pathways to enable efficient transformation of epithelial cells, which then orchestrate the remodeling of the tumor microenvironment. Although oncogenic mutations affecting RAS are common in many other human cancers, tumorigenesis in an important subset of breast cancers is driven instead by increasing activity of wild-type N-Ras. Thus, to fully assess the impact of Ras on tumorigenesis, the role of wild-type as well as mutant Ras proteins must be carefully examined.

Citation Format: Ze-Yi Zheng, Wen Bu, Lin Tian, Cheng Fan, Xia Gao, Cuijuan Yu, Liu Jun, Xiaomei Zhang, Yi-Hua Liao, Yi Li, Michael T. Lewis, Dean Edwards, Susan G. Hilsenbeck, Daniel Medina, Thomas P. Zwaka, Charles M. Perou, Chad J. Creighton, Xiang Zhang, Eric C. Chang. Wild type N-Ras, overexpressed in basal-like breast cancer, promotes tumor formation by inducing IL8 secretion via JAK2 activation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2139. doi:10.1158/1538-7445.AM2015-2139

Abstract P1-07-07: Recurrent ESR1-CCDC170 rearrangements in an aggressive subset of estrogen-receptor positive breast cancers

The crucial role of gene fusions in epithelial tumorigenesis has been recently appreciated by several milestone discoveries, but no significant recurrent translocations have yet been found in the vast majority of breast cancers that express the estrogen receptor (ER). While a majority of ER+ tumors can be effectively treated by endocrine therapy, tumors of the luminal B subtype are more aggressive and endocrine therapy-resistant. Further, the molecular blueprint of these aggressive tumors is poorly understood. Thus, characterizing the genetic alterations underlying the more aggressive forms of ER+ breast cancer is of critical significance in breast cancer management. In this study, by large-scale analyses of breast cancer transcriptome and copy number alterations, we identified recurrent rearrangements between estrogen receptor gene, ESR1 and its neighbor gene, CCDC170, which are enriched in the more aggressive and endocrine-resistant luminal-B tumors. Further screening of 200 ER+ breast tumor tissues by RT-PCR identified eight ESR1-CCDC170 positive tumors. The genomic rearrangements underlying these fusions were verified by genomic PCR and capillary sequencing. CCDC170 encodes a protein with unknown function. The observed fusion joins the 5’-untranslated region of ESR1 upstream to the coding region of CCDC170, enabling the expression of N-terminally truncated CCDC170 (ΔCCDC170) under the promoter of the ESR1 gene. Consistent with the behavior of luminal B tumors, forced expression of ΔCCDC170 in ER+ breast cancer cells leads to markedly increased cell motility, invasiveness and anchorage-independent growth, and reduced endocrine sensitivity in vitro, as well as enhanced xenograft tumor formation and reduced endocrine sensitivity of the tumors in vivo. When introduced into benign breast epithelial cells, ΔCCDC170 impairs acini morphogenesis and enhances cell motility and invasiveness. Further, Knockdown of the endogenous ESR1-CCDC170 fusion variant expressed in HCC1428 cells potently inhibited cancer cell proliferation and migration, which can be rescued by forced expression of this fusion. Mechanistic studies suggest that ΔCCDC170 engages Gab1 signalosome to potentiate growth factor signaling and enhance cell motility. The augmented growth factor signaling driven by ΔCCDC170 appears to be sustained even after withdrawal of serum, and is not affected by endocrine treatment. Together, this study identified neoplastic ESR1-CCDC170 fusions in a more aggressive subset of ER+ breast cancer, which also suggests a new role of ER in breast tumorigenesis by contributing its promoter to an oncogene.

Citation Format: Jamunarani Veeraraghavan, Ying Tan, Xi-Xi Cao, Jin-Ah Kim, Xian Wang, Dean P Edwards, Alejandro Contreras, Susan G Hilsenbeck, Eric C Chang, Rachel Schiff, Xiao-Song Wang. Recurrent ESR1-CCDC170 rearrangements in an aggressive subset of estrogen-receptor positive breast cancers [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P1-07-07.

Int6 reduction activates stromal fibroblasts to enhance transforming activity in breast epithelial cells

Background

The INT6 gene was first discovered as a site of integration in mouse mammary tumors by the mouse mammary tumor virus; however, INT6’s role in the development of human breast cancer remains largely unknown. By gene silencing, we have previously shown that repressing INT6 promotes transforming activity in untransformed human mammary epithelial cells. In the present study, guided by microarray data of human tumors, we have discovered a role of Int6 in stromal fibroblasts.

Results

We searched microarray databases of human tumors to assess Int6’s role in breast cancer. While INT6 expression levels, as expected, were lower in breast tumors than in adjacent normal breast tissue samples, INT6 expression levels were also substantially lower in tumor stroma. By immunohistochemistry, we determined that the low levels of INT6 mRNA observed in the microarray databases most likely occurs in stromal fibroblasts, because far fewer fibroblasts in the tumor tissue showed detectable levels of the Int6 protein. To directly investigate the effects of Int6 repression on fibroblasts, we silenced INT6 expression in immortalized human mammary fibroblasts (HMFs). When these INT6-repressed HMFs were co-cultured with breast cancer cells, the abilities of the latter to form colonies in soft agar and to invade were enhanced. We analyzed INT6-repressed HMFs and found an increase in the levels of a key carcinoma-associated fibroblast (CAF) marker, smooth muscle actin. Furthermore, like CAFs, these INT6-repressed HMFs secreted more stromal cell–derived factor 1 (SDF-1), and the addition of an SDF-1 antagonist attenuated the INT6-repressed HMFs’ ability to enhance soft agar colony formation when co-cultured with cancer cells. These INT6-repressed HMFs also expressed high levels of mesenchymal markers such as vimentin and N-cadherin. Intriguingly, when mesenchymal stem cells (MSCs) were induced to form CAFs, Int6 levels were reduced.

Conclusion

These data suggest that besides enhancing transforming activity in epithelial cells, INT6 repression can also induce fibroblasts, and possibly MSCs as well, via mesenchymal-mesenchymal transitions to promote the formation of CAFs, leading to a proinvasive microenvironment for tumorigenesis.

Magnetic Resonance Imaging of Asians with Multiple Sclerosis was Similar to that of the West

Magnetic resonance imaging (MRI) of the brain is the most important paraclinical diagnostic test in multiple sclerosis (MS). The appearance of MRI in Asians with MS is not well defined. We retrospectively surveyed the first brain and spinal cord MRI in patients diagnosed to have MS, according to Poser's criteria in seven regions throughout Asia to define the MRI changes among Asians with MS. There were 101 patients with first brain, and 86 with first spinal cord MRI, 66 of whom had both. The brain MRI showed a mean of 17 lesions per patient in T2 weighted images, mostly asymptomatic. Almost all the lesions were in the white matter, particularly in the juxtacortical, deep and periventricular white matter. A third of the lesions were greater than 5 mm, 14% enhanced with gadolinium. There were more supratentorial than infratentorial lesions at a ratio of 7.5: 1. Ninety five percent of the spinal cord lesions were in cervical and thoracic regions, 34% enhanced with gadolinium. The lesions extended over a mean of 3.6 +/- 3.3 vertebral bodies in length. Fifty (50%) of the brain and 54 (63%) of the spinal MRI patients had the optic-spinal form of MS. The MRI of the optic-spinal and classical groups of patients were similar in appearance and distribution, except that the optic-spinal MS patients have fewer brain but longer and more severe spinal cord lesions. In conclusion, the brain and spinal cord MRI of Asian patients with MS was similar to that of the West, although, in this study, Asian MS patients had larger spinal cord lesions.

Regulation of Ras localization and cell transformation by evolutionarily conserved palmitoyltransferases

Ras can act on the plasma membrane (PM) to mediate extracellular signaling and tumorigenesis. To identify key components controlling Ras PM localization, we performed an unbiased screen to seek Schizosaccharomyces pombe mutants with reduced PM Ras. Five mutants were found with mutations affecting the same gene, S. pombe erf2 (sp-erf2), encoding sp-Erf2, a palmitoyltransferase, with various activities. sp-Erf2 localizes to the trans-Golgi compartment, a process which is mediated by its third transmembrane domain and the Erf4 cofactor. In fission yeast, the human ortholog zDHHC9 rescues the phenotypes of sp-erf2 null cells. In contrast, expressing zDHHC14, another sp-Erf2-like human protein, did not rescue Ras1 mislocalization in these cells. Importantly, ZDHHC9 is widely overexpressed in cancers. Overexpressing ZDHHC9 promotes, while repressing it diminishes, Ras PM localization and transformation of mammalian cells. These data strongly demonstrate that sp-Erf2/zDHHC9 palmitoylates Ras proteins in a highly selective manner in the trans-Golgi compartment to facilitate PM targeting via the trans-Golgi network, a role that is most certainly critical for Ras-driven tumorigenesis.

Escorting Ras

Ras proteins are best known to function on the plasma membrane to mediate growth factor signaling. Controlling the length of time that Ras proteins stay on the plasma membrane is an effective way to properly modulate the intensity and duration of growth factor signaling. It has been shown previously that H- and N-Ras proteins in the GTP-bound state can be ubiquitylated via a K-63 linkage, which leads to endosome internalization and results in a negative-feedback loop for efficient signal attenuation. In a more recent study, two new Ras effectors have been isolated, CHMP6 and VPS4A, which are components of the ESCRT-III complex, best known for mediating protein sorting in the endosomes. Surprisingly, these molecules are required for efficient Ras-induced transformation. They apparently do so by controlling recycling of components of the Ras pathway back to the plasma membrane, thus creating a positive-feedback loop to enhance growth factor signaling. These results suggest the fates of endosomal Ras proteins are complex and dynamic — they can be either stored and/or destroyed or recycled. Further work is needed to decipher how the fate of these endosomal Ras proteins is determined.

An intraoperative multimodal neurophysiologic approach to successful resection of precentral gyrus epileptogenic lesions

Cortical dysplasias (CDs) are highly epileptogenic lesions with a good prognosis of seizure freedom, if totally resected. However, their accurate delineation and resection can be difficult, and depend on the extent of pathology and lesion location. Intraoperative neurophysiologic assessments are valuable in these situations. We present an illustrative case of intractable epilepsy where judicious use of intraoperative neurophysiologic-techniques guided resection of precentral CD, under general anesthesia and in the absence of preoperative electrophysiologic mapping data. Ictal onset was accurately delineated using electrocorticography (ECoG). Phase reversal of the median somatosensory-evoked potentials (MSSEPs) localized the central sulcus (CS). Motor evoked potentials (MEPs) triggered by high-frequency monopolar anodal electrical cortical stimulation at the primary motor cortex (PMC) threshold delineated the PMC. Using this technique, PMC and the corticospinal tract (CST) were continuously monitored during resection. No changes in MEPs from the preresection baseline were seen; no residual abnormal activity was present in the postresection ECoG. The patient emerged from surgery without deficits and has been seizure free during a 10-month follow-up. Staged multimodal intraoperative neurophysiology can be used successfully under general anesthesia to guide resection of epileptogenic lesions within the precentral gyrus, as an add-on or, in certain situations, as a viable alternative to preoperative electrophysiologic mapping.

CHMP6 and VPS4A mediate the recycling of Ras to the plasma membrane to promote growth factor signaling

While Ras is well-known to function on the plasma membrane (PM) to mediate growth factor signaling, increasing evidence suggests that Ras has complex roles in the cytoplasm. To uncover these roles, we screened a cDNA library and isolated H-Ras-binding proteins that also influence Ras functions. Many isolated proteins regulate trafficking involving endosomes; CHMP6/VPS20 and VPS4A, which interact with ESCRT-III, were chosen for further study. We showed that the binding is direct and occurs in endosomes. Furthermore, the binding is most efficient when H-Ras has a functional effector-binding-loop and is GTP-bound and ubiquitylated. CHMP6 and VPS4A also bound N-Ras, but not K-Ras. Repressing CHMP6 and VPS4A blocked Ras-induced transformation, which correlated with inefficient Ras localization to the PM as measured by cell fractionation and photobleaching. Moreover, silencing CHMP6 and VPS4A also blocked EGFR recycling. These data suggest that Ras interacts with key ESCRT-III components to promote recycling of itself and EGFR back to the PM to create a positive feedback loop to enhance growth factor signaling.

P2-02-08: Int6 Regulates Both Proteasomal Degradation and Translation Initiation and Is Critical for Proper Formation of Acini by Human Mammary Epithelium

The mammalian INT6/EIF3E gene has been implicated in breast tumor formation, but its functional activities remain poorly defined. Consistent with the fact that Int6 is frequently down-regulated in human breast tumors, we found that repressing INT6 expression induced transformed properties in normal human mammary epithelial cells (MCF10A) in both 2D and 3D cultures. Int6 contains a PCI domain, found in several components of both the proteasome and the translation initiation factor eIF3, suggesting that it can functionally interact with both. Indeed, our data show that Int6 associates with proteasomes in human cells, and that INT6 knockdown prevents proper assembly of active proteasomes. In addition, we show that down-regulation of INT6 reduces translation initiation (both polysome formation and global protein synthesis). However, Int6 regulates translation selectively, reducing translation from cap-dependent and Bcl2- IRES reporters, but stimulating translation from the CVB3 IRES. These data collectively suggest that Int6 controls both protein synthesis and protein degradation, and is thus capable of fine-tuning protein levels. Abnormal levels of regulatory proteins caused by Int6 abnormalities may thus be responsible for disrupting normal morphogenesis of mammary epithelial cells and causing transformed phenotypes.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-02-08.

Schizosaccharomyces pombe Arc3 is a conserved subunit of the Arp2/3 complex required for polarity, actin organization, and endocytosis

We characterized the Schizosaccharomyces pombe arc3 gene, whose product shares sequence homology with that of the budding yeast ARC18 and human ARPC3/p21 subunits of the Arp2/3 complex. Our data showed that Arc3p co-localizes with F-actin patches at the cell ends, but not with F-actin cables or the equatorial actin ring, and binds other subunits of the Arp2/3 complex. Gene deletion analysis showed that arc3 is essential for viability. When arc3 expression was repressed, F-actin patches became dispersed throughout the cell with greatly reduced mobility. Furthermore, in arc3-repressed cells, endocytosis was also inhibited. Human ARPC3 rescued the viability of the Sz. pombe arc3 null mutant; in addition, ARPC3 also localized to F-actin patches in human cells. These data suggest that Arc3p is an evolutionarily conserved subunit of the Arp2/3 complex required for proper F-actin organization and efficient endocytosis.

Proteasome nuclear import mediated by Arc3 can influence efficient DNA damage repair and mitosis in Schizosaccharomyces pombe

Proteasomes must efficiently remove their substrates throughout the cells in a timely manner as many of these proteins can be toxic. This study shows that proteasomes can do so efficiently because they are highly mobile. Furthermore this study uncovers that proteasome mobility requires functional Arc3, a subunit of the Arp2/3 complex.

Proteasomes must remove regulatory molecules and abnormal proteins throughout the cell, but how proteasomes can do so efficiently remains unclear. We have isolated a subunit of the Arp2/3 complex, Arc3, which binds proteasomes. When overexpressed, Arc3 rescues phenotypes associated with proteasome deficiencies; when its expression is repressed, proteasome deficiencies intensify. Arp2/3 is best known for regulating membrane dynamics and vesicular transport; thus, we performed photobleaching experiments and showed that proteasomes are readily imported into the nucleus but exit the nucleus slowly. Proteasome nuclear import is reduced when Arc3 is inactivated, leading to hypersensitivity to DNA damage and inefficient cyclin-B degradation, two events occurring in the nucleus. These data suggest that proteasomes display Arc3-dependent mobility in the cell, and mobile proteasomes can efficiently access substrates throughout the cell, allowing them to effectively regulate cell-compartment–specific activities.

Abstract 5074: Identification of Ras compartment-specific interacting proteins

Ras proteins are powerful molecular switches that control a wide range of functions, including cell growth, differentiation, apoptosis, cytoskeleton remodeling, etc. In humans, there are three RAS genes encode structurally highly similar Ras proteins that appear to have different functions in the cell. We have previously reported that a given Ras protein can signal from different cell compartments to control distinct functions in yeast. This present project aims to identify proteins that bind Ras in a cell compartment-specific manner in human cells to define how compartmentalized Ras signaling impacts tumor formation. We used the biomolecular fluorescence complementation (BiFC) method to screen a human cDNA library. Sixty two proteins were thus identified that bound the constitutively activated H-Ras (H-RasG12V), but not the GST control. These proteins also bound weakly to H-RasG12V whose effector binding motif has been mutated. To determine if these proteins can alter Ras functions, we first overexpressed them and then tested for their impacts on Ras-induced cell transformation and transcription. Only 25 of the isolated proteins could alter Ras signaling in these assays. Many of these 25 proteins are known to either associate with cellular membrane and/or regulate protein transportation, suggesting that as a group they highlight the importance of regulating Ras trafficking in the control of cell transformation. In particular, a number of isolated proteins may regulate endosome sorting, and we thus focused on two such proteins, CHMP6/VPS20 and VPS4A, for further investigation. We first validated the Ras binding by co-immunoprecipitation; furthermore the microscopy data showed that the binding is readily detectable in endosomes. Moreover, up- or down-regulating of level of either CHMP6/VPS20 or VPS4A dramatically inhibited Ras-induced transformation of NIH-3T3 cells. To determine how these molecules regulate Ras activities, we first examined whether they modulate Ras protein intracellular transport. Cell fractionation using sucrose gradients has shown that VPS4A knock-down could alter both wide-type and constitutively activated Ras membrane localization. In summary, we indentified Ras compartment-specific binding proteins by a cell-based approach. Many of the proteins are trafficking regulating proteins. In particular, two endosome sorting proteins CHMP6/VPS20 and VPS4A have been shown to regulate the oncogenic effects of Ras, possibly by localizing Ras proteins into proper cell compartments. We believe that the study of compartment-specific Ras interacting proteins is indispensable for better understanding the complex roles of Ras in oncogenesis of breast cancer as well as many other cancers.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5074.

Int6 and Moe1 interact with Cdc48 to regulate ERAD and proper chromosome segregation

Int6/eIF3e is implicated in tumorigenesis, but its molecular functions remain unclear. We have studied its fission yeast homolog Yin6, reporting that it regulates proteolysis by controlling the assembly/localization of proteasomes, and binds directly to another conserved protein, Moe1. In the present study, we isolated Cdc48 as a Moe1-binding protein from a yeast two-hybrid screen, and confirmed biochemically that they form a stable complex in fission yeast. Overexpressing Moe1 or Yin6 partially rescued phenotypes of cdc48 mutants; conversely, overexpressing Cdc48 partially rescued phenotypes of moe1 or yin6 mutants. Mutants defective in both Cdc48 and the Yin6-Moe1 complex showed growth defects that were far more severe than either alone. These double mutants were severely deficient in endoplasmic reticulum associated degradation (ERAD), as they were hypersensitive to accumulation of misfolded proteins. In addition, their chromosomes showed frequent defects in spindle attachment and segregation--these mitotic defects correlated with Ase1 and Bir1/survivin mislocalization. These results suggest that Cdc48, Yin6 and Moe1 act in the same protein complex to concertedly control ERAD and chromosome segregation. Many of these properties are evolutionarily conserved in humans, since human Cdc48 rescued the lethality of the yeast cdc48Delta mutant, and Int6 and Moe1/eIF3d bind Cdc48 in human cells.

Water-assisted CO(2) laser ablated glass and modified thermal bonding for capillary-driven bio-fluidic application

The glass-based microfluidic chip has widely been applied to the lab-on-a-chip for clotting tests. Here, we have demonstrated a capillary driven flow chip using the water-assisted CO(2) laser ablation for crackless fluidic channels and holes as well as the modified low-temperature glass bonding with assistance of adhesive polymer film at 300 degrees Celsius. Effect of water depth on the laser ablation of glass quality was investigated. The surface hydrophilic property of glass and polymer film was measured by static contact angle method for hydrophilicity examination in comparison with the conventional polydimethylsiloxane (PDMS) material. Both low-viscosity deionized water and high-viscosity whole blood were used for testing the capillary-driving flow behavior. The preliminary coagulation testing in the Y-channel chip was also performed using whole blood and CaCl(2) solution. The water-assisted CO(2) laser processing can cool down glass during ablation for less temperature gradient to eliminate the crack. The modified glass bonding can simplify the conventional complex fabrication procedure of glass chips, such as high-temperature bonding, long consuming time and high cost. Moreover, the developed fluidic glass chip has the merit of hydrophilic behavior conquering the problem of traditional hydrophobic recovery of polymer fluidic chips and shows the ability to drive high-viscosity bio-fluids.

The eIF3 interactome reveals the translasome, a supercomplex linking protein synthesis and degradation machineries

eIF3 promotes translation initiation, but relatively little is known about its full range of activities in the cell. Here, we employed affinity purification and highly sensitive LC-MS/MS to decipher the fission yeast eIF3 interactome, which was found to contain 230 proteins. eIF3 assembles into a large supercomplex, the translasome, which contains elongation factors, tRNA synthetases, 40S and 60S ribosomal proteins, chaperones, and the proteasome. eIF3 also associates with ribosome biogenesis factors and the importins-beta Kap123p and Sal3p. Our genetic data indicated that the binding to both importins-beta is essential for cell growth, and photobleaching experiments revealed a critical role for Sal3p in the nuclear import of one of the translasome constituents, the proteasome. Our data reveal the breadth of the eIF3 interactome and suggest that factors involved in translation initiation, ribosome biogenesis, translation elongation, quality control, and transport are physically linked to facilitate efficient protein synthesis.

Isolation of the Schizosaccharomyces pombe proteasome subunit Rpn7 and a structure-function study of the proteasome-COP9-initiation factor domain

Proper assembly of the 26 S proteasome is required to efficiently degrade polyubiquitinated proteins. Many proteasome subunits contain the proteasome-COP9-initiation factor (PCI) domain, thus raising the possibility that the PCI domain may play a role in mediating proteasome assembly. We have previously characterized the PCI protein Yin6, a fission yeast ortholog of the mammalian Int6 that has been implicated in breast oncogenesis, and demonstrated that it binds and regulates the assembly of the proteasome. In this study, we isolated another PCI proteasome subunit, Rpn7, as a high copy suppressor that rescued the proteasome defects in yin6 null cells. To better define the function of the PCI domain, we aligned protein sequences to identify a conserved leucine residue that is present in nearly all known PCI domains. Replacing it with aspartate in yeast Rpn7, Yin6, and Rpn5 inactivated these proteins, and mutant human Int6 mislocalized in HeLa cells. Rpn7 and Rpn5 bind Rpn9 with high affinity, but their mutant versions do not. Our data suggest that this leucine may interact with several hydrophobic amino acid residues to influence the spatial arrangement either within the N-terminal tandem alpha-helical repeats or between these repeats and the more C-terminal winged helix subdomain. Disruption of such an arrangement in the PCI domain may substantially inactivate many PCI proteins and block their binding to other proteins.

Spatial segregation of Ras signaling: new evidence from fission yeast

The Ras GTPases act as binary switches for signal transduction pathways that are important for growth regulation and tumorigenesis. Despite the biochemical simplicity of this switch, Ras proteins control multiple pathways, and the functions of the four mammalian Ras proteins are not overlapping. This raises an important question--how does a Ras protein selectively regulate a particular activity? One recently emerging model suggests that a single Ras protein can control different functions by acting in distinct cellular compartments. A critical test of this model is to identify pathways that are selectively controlled by Ras when it is localized to a particular compartment. A recent study has examined Ras signaling in the fission yeast Schizosaccharomyces pombe, which expresses only one Ras protein that controls two separate evolutionarily conserved pathways. This study demonstrates that whereas Ras localized to the plasma membrane selectively regulates a MAP kinase pathway to mediate mating pheromone signaling, Ras localized to the endomembrane activates a Cdc42 pathway to mediate cell polarity and protein trafficking. This study has provided unambiguous evidence for compartmentalized signaling of Ras.

Compartmentalized signaling of Ras in fission yeast

Compartment-specific Ras signaling is an emerging paradigm that may explain the multiplex outputs from a single GTPase. The fission yeast, Schizosaccharomyces pombe, affords a simple system in which to study Ras signaling because it has a single Ras protein, Ras1, that regulates two distinct pathways: one that controls mating through a Byr2-mitogen-activated protein kinase cascade and one that signals through Scd1-Cdc42 to maintain elongated cell morphology. We generated Ras1 mutants that are restricted to either the endomembrane or the plasma membrane. Protein binding studies showed that each could interact with the effectors of both pathways. However, when examined in ras1 null cells, endomembrane-restricted Ras1 supported morphology but not mating, and, conversely, plasma membrane-restricted Ras1 supported mating but did not signal to Scd1-Cdc42. These observations provide a striking demonstration of compartment-specific Ras signaling and indicate that spatial specificity in the Ras pathway is evolutionarily conserved.

Motor patterns and propulsion in the rat intestine in vivo recorded by spatio-temporal maps

We have used spatio-temporal maps derived from video images to investigate propagated contractions of the rat small intestine in vivo. The abdomen, including an exteriorized segment of jejunum, was housed in a humid chamber with a viewing window. Video records were converted to spatio-temporal maps of jejunal diameter changes. Intraluminal pressure and fluid outflow were measured. Contractions occupied 3.8 +/- 0.2 cm of intestine and propagated anally at 3.1 +/- 0.2 mm s(-1) when baseline pressure was 4 mmHg. Contractions at any one point lasted 8.7 +/- 0.6 s. Contractions often occurred in clusters; within cluster frequencies were 2.28 +/- 0.04 min(-1). Pressure waves, with amplitudes greater than about 9 mmHg, expelled fluid when the baseline pressure was 4 mmHg. In the presence of L-NAME, circular muscle contractions occurred at a high frequency, but they were not propagated. We conclude that video recording methods give good spatio-temporal resolution of intestinal movement when applied in vivo. They reveal neurally-mediated propulsive contractions, similar to those previously recorded from intestinal segments in vitro. The propagated contractions had speeds of propagation that were slower and frequencies of occurrence that were less than speeds and frequencies of slow waves in the rat small intestine.

Schizosaccharomyces pombe Ras1 effector, Scd1, interacts with Klp5 and Klp6 kinesins to mediate cytokinesis

Fission yeast Scd1 is an exchange factor for Cdc42 and an effector of Ras1. In a screen for scd1 interacting genes, we isolated klp5 and klp6, which encode presumptive kinesins. Klp5 and Klp6 form a complex to control the same processes, which so far include microtubule dynamics and chromosome segregation. We showed that klp5 or klp6 inactivation in combination with the scd1 deletion (scd1delta) created a synthetic temperature-dependent growth defect. Further genetic analysis demonstrated that Klp5 and Klp6 interacted specifically with the Ras1-Scd1 pathway, but not with the Ras1-Byr2 pathway. In addition, Klp5 and Klp6 can stably associate with Scd1 and Cdc42. A deletion in the Scd1 C terminus, which contains the PB1 domain, prevented Scd1 binding to Klp5/6 and caused a growth defect in Klp5/6 mutant cells that is indistinguishable from that induced by scd1delta. Analysis of the double-mutant phenotype indicated that at the nonpermissive temperature, cells failed to undergo cytokinesis efficiently. These cells contained abnormal contractile rings in which F-actin and Mid1, a key regulator of F-actin ring formation and positioning, are mispositioned and fragmented. These data suggest that Klp5/6 cooperate with the Ras1-Scd1 pathway to influence proper formation of the contractile ring for cytokinesis.

Rpn5 is a conserved proteasome subunit and required for proper proteasome localization and assembly

Proper function of the 26 S proteasome requires assembly of the regulatory complex, which is composed of the lid and base subcomplexes. We characterized Rpn5, a lid subunit, in fission yeast. We show that Rpn5 associates with the proteasome rpn5. Deletion (rpn5Delta) exacerbates the growth defects in proteasome mutants, leading to mitotic abnormalities, which correlate with accumulation of polyubiquitinated proteins, such as Cut2/securin. Rpn5 expression is tightly controlled; both overexpression and deletion of rpn5 impair proteasome functions. The proteasome is assembled around the inner nuclear membrane in wild-type cells; however, in rpn5Delta cells, proteasome subunits are improperly assembled and/or localized. In the lid mutants, Rpn5 is mislocalized in the cytosol, while in the base mutants, Rpn5 can enter the nucleus, but is left in the nucleoplasm, and not assembled into the nuclear membrane. These results suggest that Rpn5 is a dosage-dependent proteasome regulator and plays a role in mediating proper proteasome assembly. Moreover, the Rpn5 assembly may be a cooperative process that involves at least two steps: 1) nuclear import and 2) subsequent assembly into the nuclear membrane. The former step requires other components of the lid, while the latter requires the base. Human Rpn5 rescues the phenotypes associated with rpn5Delta and is incorporated into the yeast proteasome, suggesting that Rpn5 functions are highly conserved.

Schizosaccharomyces pombe Int6 and Ras homologs regulate cell division and mitotic fidelity via the proteasome

Yin6 is a yeast homolog of Int6, which is implicated in tumorigenesis. We show that Yin6 binds to and regulates proteasome activity. Overexpression of Yin6 strengthens proteasome function while inactivation weakens and causes the accumulation of polyubiquitinated proteins including securin/Cut2 and cyclin/Cdc13. Yin6 regulates the proteasome by preferentially interacting with Rpn5, a conserved proteasome subunit, and affecting its localization/assembly. We showed previously that Yin6 cooperates with Ras1 to mediate chromosome segregation; here, we demonstrate that Ras1 similarly regulates the proteasome via Rpn5. In yeast, human Int6 binds Rpn5 and regulates its localization. We propose that human Int6, either alone or cooperatively with Ras, influences proteasome activities via Rpn5. Inactivating Int6 can lead to accumulation of mitotic regulators affecting cell division and mitotic fidelity.

Two ras pathways in fission yeast are differentially regulated by two ras guanine nucleotide exchange factors

How a given Ras prreotein coordinates multiple signaling inputs and outputs is a fundamental issue of signaling specificity. Schizosaccharomyces pombe contains one Ras, Ras1, that has two distinct outputs. Ras1 activates Scd1, a presumptive guanine nucleotide exchange factor (GEF) for Cdc42, to control morphogenesis and chromosome segregation, and Byr2, a component of a mitogen-activated protein kinase cascade, to control mating. So far there is only one established Ras1 GEF, Ste6. Paradoxically, ste6 null (ste6 Delta) mutants are sterile but normal in cell morphology. This suggests that Ste6 specifically activates the Ras1-Byr2 pathway and that there is another GEF capable of activating the Scd1 pathway. We thereby characterized a potential GEF, Efc25. Genetic data place Efc25 upstream of the Ras1-Scd1, but not the Ras1-Byr2, pathway. Like ras1 Delta and scd1 Delta, efc25 Delta is synthetically lethal with a deletion in tea1, a critical element for cell polarity control. Using truncated proteins, we showed that the C-terminal GEF domain of Efc25 is essential for function and regulated by the N terminus. We conclude that Efc25 acts as a Ras1 GEF specific for the Scd1 pathway. While ste6 expression is induced during mating, efc25 expression is constitutive. Moreover, Efc25 overexpression renders cells hyperelongated and sterile; the latter can be rescued by activated Ras1. This suggests that Efc25 can recruit Ras1 to selectively activate Scd1 at the expense of Byr2. Reciprocally, Ste6 overexpression can block Scd1 activation. We propose that external signals can partly segregate two Ras1 pathways by modulating GEF expression and that GEFs can influence how Ras is coupled to specific effectors.

Optimism, pessimism, and positive and negative affectivity in middle-aged adults: a test of a cognitive-affective model of psychological adjustment

This study attempted to address limitations in the understanding of optimism and pessimism among middle-aged adults. Specifically, a model of affectivity as a mediator of the link between outcome expectancies and psychological adjustment (life satisfaction and depressive symptoms) was presented and examined in a sample of 237 middle-aged adults. Consistent with a mediation model, results of path analyses indicated that optimism and pessimism (particularly the former) had significant direct and indirect links (by means of positive and negative affectivity) with depressive symptoms and life satisfaction. These results add to the small but growing literature identifying optimism and pessimism as important concomitants of psychological adjustment in more mature adults.

Moe1 and spInt6, the fission yeast homologues of mammalian translation initiation factor 3 subunits p66 (eIF3d) and p48 (eIF3e), respectively, are required for stable association of eIF3 subunits

The protein encoded by the fission yeast gene, moe1(+) is the homologue of the p66/eIF3d subunit of mammalian translation initiation factor eIF3. In this study, we show that in fission yeast, Moe1 physically associates with eIF3 core subunits as well as with 40 S ribosomal particles as a constituent of the eIF3 protein complex that is similar in size to multisubunit mammalian eIF3. However, strains lacking moe1(+) (Deltamoe1) are viable and show no gross defects in translation initiation, although the rate of translation in the Deltamoe1 cells is about 30-40% slower than wild-type cells. Mutant Deltamoe1 cells are hypersensitive to caffeine and defective in spore formation. These phenotypes of Deltamoe1 cells are similar to those reported previously for deletion of the fission yeast int6(+) gene that encodes the fission yeast homologue of the p48/Int6/eIF3e subunit of mammalian eIF3. Further analysis of eIF3 subunits in Deltamoe1 or Deltaint6 cells shows that in these deletion strains, while all the eIF3 subunits are bound to 40 S particles, dissociation of ribosome-bound eIF3 results in the loss of stable association between the eIF3 subunits. In contrast, eIF3 isolated from ribosomes of wild-type cells are associated with one another in a protein complex. These observations suggest that Moe1 and spInt6 are each required for stable association of eIF3 subunits in fission yeast.

Optimism, pessimism, and positive and negative affectivity in middle-aged adults: a test of a cognitive-affective model of psychological adjustment

This study attempted to address limitations in the understanding of optimism and pessimism among middle-aged adults. Specifically, a model of affectivity as a mediator of the link between outcome expectancies and psychological adjustment (life satisfaction and depressive symptoms) was presented and examined in a sample of 237 middle-aged adults. Consistent with a mediation model, results of path analyses indicated that optimism and pessimism (particularly the former) had significant direct and indirect links (by means of positive and negative affectivity) with depressive symptoms and life satisfaction. These results add to the small but growing literature identifying optimism and pessimism as important concomitants of psychological adjustment in more mature adults.

Cultural variations in optimistic and pessimistic bias: do Easterners really expect the worst and Westerners really expect the best when predicting future life events?

The authors compared levels of optimistic and pessimistic bias in the prediction of positive and negative life events between European Americans and Japanese. Study 1 showed that European Americans compared with Japanese were more likely to predict positive events to occur to self than to others. The opposite pattern emerged in the prediction of negative events. Study 2 replicated these cultural differences. Furthermore, positive associations emerged between predictions and occurrence of life events 2 months later for both European Americans and Japanese. Across both studies, results of within-groups analyses indicated that both groups expected negative events to be more likely to occur to others than to self (optimistic bias). In addition, Japanese expected positive events to be more likely to occur to others than to self (pessimistic bias). However, European Americans failed to show the expected optimistic bias for positive events.

Planning for an annual episodic mass gathering: emergency department and clinic utilization in Yellowstone

OBJECTIVE

Planning and providing emergency and primary care for a large transient population of visitors and employees in a national park can be problematic. Furthermore, planning for emergency and primary health care needs of visitors and itinerant workers in a wilderness area national park has not been well documented. A study was performed to analyze emergency and primary health care utilization in a national park.

METHODS

Data was gathered from all patients presenting to Lake Hospital Emergency Department in Yellowstone in 1995, and a retrospective chart review was performed.

RESULTS

Two distinct populations with different health care needs were identified.

CONCLUSION

Utilization analysis revealed differences between conventional mass gatherings and the mass gatherings in Yellowstone. Because of the unique conditions and populations found in a wilderness area, conventional mass gathering emergency medical service models may not be an appropriate model for planning health care in a national park. Analysis of utilization data can help plan resources for emergency and primary health care for a park population.

Use of gene gun for genetic immunotherapy : in vitro and in vivo methods

A major thrust in the application of gene transfer technology for cancer therapy has been the modulation of the immune response. There has been a veritable explosion of information regarding the components of the immune response that are required to generate a meaningful cellular response to tumorassociated antigens (TAAs) capable of eliciting rejection of established tumor. Many of the preclinical and clinical immunogenetic studies have focused on melanoma. Historically, melanoma has been an immunoresponsive tumor for which several melanoma TAAs have been identified.

Yin6, a fission yeast Int6 homolog, complexes with Moe1 and plays a role in chromosome segregation

The INT6 gene has been implicated in human breast cancer formation, but its function is unknown. We isolated an Int6 homolog from fission yeast, Yin6, by its binding to a conserved protein in the Ras pathway, Moe1. Yin6 and Moe1 converge on the same protein complex to promote microtubule instability/disassembly. Yin6 and Moe1 interact cooperatively: when either protein is absent, the other becomes mislocalized with decreased protein levels. Furthermore, whereas full-length human Int6 rescues the phenotypes of the yin6-null (yin6Delta) mutant cells and binds human Moe1, truncated Int6 proteins found in tumors do not. Importantly, yin6Delta alone impairs chromosome segregation weakly, but yin6Delta together with ras1Delta causes severe chromosome missegregation. These data support a model in which INT6 mutations in humans either alone or together with additional mutations, such as a RAS mutation, may contribute to tumorigenesis by altering genome stability.

A conserved interaction between Moe1 and Mal3 is important for proper spindle formation in Schizosaccharomyces pombe

Moe1 is a conserved fission yeast protein that negatively affects microtubule stability/assembly. We conducted a two-hybrid screen to search for Moe1-binding proteins and isolated Mal3, a homologue of human EB1. We show that Moe1 and Mal3 expressed in bacteria form a complex and that Moe1 and Mal3 expressed in fission yeast cosediment with microtubules. Deletion of either moe1 or mal3 does not result in lethality; however, deletion of both moe1 and mal3 leads to cell death in the cold. The resulting cells appear to die of chromosome missegregation, which correlates with the presence of abnormal spindles. We investigated the cause for the formation of monopolar spindles and found that only one of the two spindle pole bodies (SPBs) contains gamma-tubulin, although both SPBs appear to be equal in size and properly inserted in the nuclear membrane. Moreover, the moe1 mal3 double null mutant in the cold contains abnormally short and abundant interphase microtubule bundles. These data suggest that Moe1 and Mal3 play a role in maintaining proper microtubule dynamics/integrity and distribution of gamma-tubulin to the SPBs during mitosis. Finally, we show that human Moe1 and EB1 can each rescue the phenotype of the moe1 mal3 double null mutant and form a complex, suggesting that these proteins are part of a well-conserved mechanism for regulating spindle functioning.