Translational and Therapeutic Evaluation of RAS-GTP Inhibition by RMC-6236 in RAS-Driven Cancers

RAS-driven cancers comprise up to 30% of human cancers. RMC-6236 is a RAS(ON) multi-selective noncovalent inhibitor of the active, GTP-bound state of both mutant and wild-type variants of canonical RAS isoforms with broad therapeutic potential for the aforementioned unmet medical need. RMC-6236 exhibited potent anticancer activity across RAS-addicted cell lines, particularly those harboring mutations at codon 12 of KRAS. Notably, oral administration of RMC-6236 was tolerated in vivo and drove profound tumor regressions across multiple tumor types in a mouse clinical trial with KRASG12X xenograft models. Translational PK/efficacy and PK/PD modeling predicted that daily doses of 100 mg and 300 mg would achieve tumor control and objective responses, respectively, in patients with RAS-driven tumors. Consistent with this, we describe here objective responses in two patients (at 300 mg daily) with advanced KRASG12X lung and pancreatic adenocarcinoma, respectively, demonstrating the initial activity of RMC-6236 in an ongoing phase I/Ib clinical trial (NCT05379985).

SIGNIFICANCE

The discovery of RMC-6236 enables the first-ever therapeutic evaluation of targeted and concurrent inhibition of canonical mutant and wild-type RAS-GTP in RAS-driven cancers. We demonstrate that broad-spectrum RAS-GTP inhibition is tolerable at exposures that induce profound tumor regressions in preclinical models of, and in patients with, such tumors.

ALK upregulates POSTN and WNT signaling to drive neuroblastoma

Neuroblastoma is the most common extracranial solid tumor of childhood. While MYCN and mutant anaplastic lymphoma kinase (ALKF1174L) cooperate in tumorigenesis, how ALK contributes to tumor formation remains unclear. Here, we used a human stem cell-based model of neuroblastoma. Mis-expression of ALKF1174L and MYCN resulted in shorter latency compared to MYCN alone. MYCN tumors resembled adrenergic, while ALK/MYCN tumors resembled mesenchymal, neuroblastoma. Transcriptomic analysis revealed enrichment in focal adhesion signaling, particularly the extracellular matrix genes POSTN and FN1 in ALK/MYCN tumors. Patients with ALK-mutant tumors similarly demonstrated elevated levels of POSTN and FN1. Knockdown of POSTN, but not FN1, delayed adhesion and suppressed proliferation of ALK/MYCN tumors. Furthermore, loss of POSTN reduced ALK-dependent activation of WNT signaling. Reciprocally, inhibition of the WNT pathway reduced expression of POSTN and growth of ALK/MYCN tumor cells. Thus, ALK drives neuroblastoma in part through a feedforward loop between POSTN and WNT signaling.

GATOR2-dependent mTORC1 activity is a therapeutic vulnerability in FOXO1 fusion-positive rhabdomyosarcoma

Oncogenic FOXO1 gene fusions drive a subset of rhabdomyosarcoma (RMS) with poor survival; to date, these cancer drivers are therapeutically intractable. To identify new therapies for this disease, we undertook an isogenic CRISPR-interference screen to define PAX3-FOXO1-specific genetic dependencies and identified genes in the GATOR2 complex. GATOR2 loss in RMS abrogated aa-induced lysosomal localization of mTORC1 and consequent downstream signaling, slowing G1-S cell cycle transition. In vivo suppression of GATOR2 impaired the growth of tumor xenografts and favored the outgrowth of cells lacking PAX3-FOXO1. Loss of a subset of GATOR2 members can be compensated by direct genetic activation of mTORC1. RAS mutations are also sufficient to decouple mTORC1 activation from GATOR2, and indeed, fusion-negative RMS harboring such mutations exhibit aa-independent mTORC1 activity. A bisteric, mTORC1-selective small molecule induced tumor regressions in fusion-positive patient-derived tumor xenografts. These findings highlight a vulnerability in FOXO1 fusion-positive RMS and provide rationale for the clinical evaluation of bisteric mTORC1 inhibitors, currently in phase I testing, to treat this disease. Isogenic genetic screens can, thus, identify potentially exploitable vulnerabilities in fusion-driven pediatric cancers that otherwise remain mostly undruggable.

Abstract A015: Biochemical and preclinical anti-tumor activity of a bi-steric mTORC1-selective inhibitor in fusion positive rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Although FOXO1 fusions portend a dismal prognosis for patients with both localized (5-year survival 65%) and metastatic (19%) disease, current therapy is largely agnostic to fusion status. The allosteric mTORC1 inhibitor temsirolimus delays disease progression in relapsed RMS but does not improve survival. We hypothesized that this incomplete response is due to inadequate suppression of critical downstream effectors of the mTORC1 complex. Recently described bi-steric mTORC1-selective inhibitors combine allosteric and catalytic modes of kinase inhibition, and have shown promise in diverse preclinical cancer models with early evidence of clinical activity. We therefore set out to compare the activity of the bi-steric mTORC1-selective inhibitor RMC-6272 to the allosteric mTOR inhibitor rapamycin in cell line and xenograft models of fusion positive RMS. Methods: We compared the in vitro anti-proliferative activity of RMC-6272 and rapamycin in FOXO1 fusion positive RMS cell lines at 72 and 144 hours. We assessed the ability of these inhibitors to suppress mTORC1 outputs, using immunoblot to quantify phosphorylation of p70S6K, RPS6, and 4EBP1, and m7-GTP pulldowns to assess repression of cap-dependent translation. In vivo studies were conducted in patient-derived xenograft (PDX) models subcutaneously implanted in NSG mice. We tested preclinical anti-tumor activity of RMC-6272 at two doses and used immunoblots and m7-GTP assays in tumors harvested from treated mice to assess target engagement. Results: We observed superior anti-proliferative effects of RMC-6272 when compared to rapamycin in vitro, with the latter having only modest effect. Biochemically, treatment of RMS cell lines with either RMC-6272 or rapamycin diminished phosphorylation of p70S6K and RPS6. However, RMC-6272 blocked 4EBP1 phosphorylation and cap-dependent translation much more potently than rapamycin. Based on these findings, we conducted in vivo studies using weekly intraperitoneal injections of RMC-6272 in PDX-harboring mice. Both doses were associated with tumor regressions in two fusion positive RMS PDX models: 2 CR and 1 PR at 8 mg/kg, and 3 CR, 1 PR, 1 SD, and 2 PD at 6 mg/kg. NSG mice treated at 8 mg/kg demonstrated more weight loss than those at 6 mg/kg, suggesting strain-specific determinants of tolerability. We confirmed in vivo suppression of p70S6K phosphorylation and cap-dependent translation in tumors. Conclusions: The mTORC1 inhibitor RMC-6272 exhibits greater anti-tumor activity than the allosteric inhibitor rapamycin in vitro, and is capable of inducing complete remissions as monotherapy in FOXO1 fusion positive RMS PDX. Increased anti-tumor activity was associated with suppression of cap-dependent translation as assessed by m7-GTP assays, nominating dephosphorylation of 4EBP1 as a potential response biomarker. Preclinical studies are ongoing to assess mechanism-informed combination therapeutic strategies incorporating this highly active agent in FOXO1 fusion positive RMS.

Citation Format: Jacqueline Morales, Kristen Kwong, W. Clay Gustafson, Amit J. Sabnis. Biochemical and preclinical anti-tumor activity of a bi-steric mTORC1-selective inhibitor in fusion positive rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference: Sarcomas; 2022 May 9-12; Montreal, QC, Canada. Philadelphia (PA): AACR; Clin Cancer Res 2022;28(18_Suppl):Abstract nr A015.

Abstract 3597: Direct targeting of KRASG12X mutant cancers with RMC-6236, a first-in-class, RAS-selective, orally bioavailable, tri-complex RASMULTI(ON) inhibitor

Mutant RAS is common in pancreatic carcinoma (PDAC), non-small cell lung cancer (NSCLC) and colorectal cancer (CRC) and exists predominantly in the GTP-bound (RAS(ON)) state, leading to excessive downstream oncogenic signaling. KRASG12C(OFF) inhibitors have provided clinical proof of concept for targeting mutant KRAS. Preclinical data suggests inhibition of RAS(ON) may be a superior therapeutic strategy. In addition, KRASG12 mutations such as KRASG12D and KRASG12V remain unserved.RMC-6236 is a first-in-class, potent, oral tri-complex RASMULTI(ON) small molecule inhibitor designed to treat cancers driven by a variety of RAS mutations. RMC-6236 non-covalently binds to an abundant intracellular chaperone protein, cyclophilin A (CypA), resulting in a binary complex that engages RAS(ON) to form a high-affinity, RAS-selective tri-complex that sterically inhibits RAS binding to effectors. Exposure to RMC-6236 suppressed ERK phosphorylation and cell growth, and induced apoptosis in multiple human RAS-addicted cancer cell lines in vitro.RMC-6236 induced dose-dependent, deep, and durable suppression of RAS pathway activation up to 48 hours in preclinical xenograft models in vivo. Prolonged exposure in tumors was observed relative to blood and various healthy tissues, likely mediated by high affinity binding of RMC-6236 to tumor CypA.RMC-6236 at tolerable doses induced profound and durable tumor regressions in multiple cell line-derived (CDX) and patient-derived (PDX) RASMUT xenograft models, including NSCLC, CRC and PDAC. Anti-tumor activity was particularly notable in KRAS position 12 (G12X) mutant tumors, particularly KRASG12D, KRASG12V, and KRASG12R, with significant tumor regressions observed. Tumor growth inhibition was durable even in tumors that did not regress. Intermittent scheduling of RMC-6236 was active and permitted a higher dose intensity than daily dosing.RMC-6236 promoted anti-tumor immunity in vivo and was additive with anti-PD1 antibodies, driving durable complete responses and immunologic memory in a KRAS mutant CRC model. Furthermore, RMC-6236 treatment reversed oncogenic RAS-driven immune evasion mechanisms in a checkpoint blockade refractory KRAS mutant model, significantly transforming the tumor microenvironment in favor of anti-tumor immunity.These preclinical results support the inclusion of NSCLC, PDAC, and CRC patients in our planned clinical trial of RMC-6236 in patients with KRASG12X advanced solid tumors.

Citation Format: Elena S. Koltun, Meghan A. Rice, W. Clay Gustafson, David Wilds, Jingjing Jiang, Bianca J. Lee, Zhengping Wang, Stephanie Chang, Mike Flagella, Yunming Mu, Nuntana Dinglasan, Nicole Nasholm, James W. Evans, Yingyun Wang, Kyle Seamon, Yang Liu, Cristina Blaj, John Knox, Rebecca Freilich, Elsa Quintana, Jim Cregg, Alun Bermingham, Adrian L. Gill, Jacqueline Am Smith, Mallika Singh. Direct targeting of KRASG12X mutant cancers with RMC-6236, a first-in-class, RAS-selective, orally bioavailable, tri-complex RASMULTI(ON) inhibitor [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 3597.

The bi-steric mTORC1-selective inhibitor RMC-5552 in tumors with activation of mTOR signaling: Preclinical activity in combination with RAS(ON) inhibitors in RAS-addicted tumors, and initial clinical findings from a single agent phase 1/1b study

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Background: RMC-5552 is a potent bi-steric mTORC1-selective inhibitor that activates the downstream tumor suppressor 4EBP1, thereby inhibiting initiation of protein translation. This novel therapeutic moiety addresses a key limitation of rapalogs, which do not effectively inhibit phosphorylation of 4EBP1. RMC-5552 has previously demonstrated significant anti-tumor activity in preclinical models of human cancers with mTOR pathway activation. Additionally, mTOR signaling plays a key role in therapeutic response and resistance in RAS-addicted cancers, which represent a significant unmet medical need. Methods: We examined the combination of bi-steric mTORC1 inhibitors (RMC-5552 and the research tool compound RMC-6272) with direct inhibitors of active RAS (RAS(ON) inhibitors) in mutant KRAS-driven models. To enable the clinical testing of RMC-5552 as a companion inhibitor for RAS(ON) inhibitors, a Phase 1/1b dose-escalation trial of RMC-5552 monotherapy is currently testing a once-a-week IV schedule. Results: RMC-5552 and RMC-6272 demonstrated marked combinatorial anti-tumor activity with RAS(ON) inhibitors across a series of preclinical models of KRAS mutated non-small cell lung cancer. The combination enhanced tumor apoptosis and resulted in durable tumor regressions as compared to tumor growth inhibition resulting from single agents alone. As of 13 January 2022, a total of 14 patients with solid tumors have been evaluated in an ongoing Phase 1/1b trial over 5 dose levels ranging from 1.6 to 12 mg IV weekly. Median age was 62 years and the majority received ≥3 prior therapies. The most common (> 25%) drug-related adverse events were mucositis/stomatitis (43%) and decreased appetite (29%). The most common grade 3 drug-related adverse events were mucositis/stomatitis observed in 3 patients in dose levels ≥ 10 mg (21%) and were dose-limiting. The dose of 6 mg IV weekly was well tolerated. Plasma exposures of RMC-5552 were dose-proportionate at lower dose levels up to 6 mg but increased above dose proportionality with higher dose levels. Plasma exposures at 6 mg and above were consistent with those resulting in inhibition of tumor p4EBP1 in preclinical models. Of 5 patients evaluable for efficacy at doses of 6 mg and higher, one confirmed PR was observed in a patient with head and neck cancer with a pathogenic mutation in PTEN (ORR 20%) and 3 patients had a best response of SD. Dose-optimization is ongoing. Conclusions: RMC-5552 is clinically active in tumors with mTORC1 signaling activation at a tolerable dose and schedule and has the potential to be a companion inhibitor of choice for RAS(ON) inhibitors in RAS-addicted tumors. Clinical trial information: NCT04774952.

Direct targeting of RAS in pancreatic ductal adenocarcinoma with RMC-6236, a first-in-class, RAS-selective, orally bioavailable, tri-complex RASMULTI(ON) inhibitor

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Background: RAS proteins (such as KRAS, NRAS, HRAS) are small GTPases that drive cell proliferation and survival when bound to GTP. Mutant RAS proteins exist predominantly in the GTP-bound (RAS(ON)) state, leading to excessive downstream signaling via interaction with effectors such as RAF kinases. Oncogenic KRAS is required for the initiation, progression, and maintenance of pancreatic ductal adenocarcinoma (PDAC) (Hezel et al, 2006, Ying et al 2012). Although extinction of KRAS expression as well as pharmacological inhibition of RAS effectors clearly abrogate the growth of human PDAC models, clinical trials of drugs targeting key components of the RAS pathway have remained largely unsuccessful. Several factors contribute to these failures including redundancy in signaling surrogates downstream of KRAS and/or tumor complexity driven by co-occurring genomic alterations and intra-tumoral heterogeneity. Methods: RMC-6236 is a small molecule that binds to an intracellular chaperone protein, Cyclophilin A (CypA), resulting in an inhibitory binary complex that binds active, GTP-bound RAS to form a tri-complex and suppresses RAS signaling by disrupting interactions with effectors such as RAF kinases. Results: Here, we demonstrate that single agent RMC-6236, a first-in-class, orally bioavailable, RAS-selective tri-complex inhibitor of multiple RAS mutations and wild-type RAS (RASMULTI inhibitor) is highly efficacious in preclinical models of KRAS mutant PDAC (with marked activity in RAS-mutant colorectal cancer models described in Koltun et al, AACR 2021). RMC-6236 suppresses phosphorylation of ERK kinases, downstream effectors of RAS involved in cell proliferation, and induces growth suppression and apoptosis in multiple human cancer cell lines in vitro. Oral administration of RMC-6236 produces deep, durable, and dose-dependent suppression of tumor RAS pathway activation in vivo. An extended duration of tumor pharmacodynamic activity, relative to plasma exposure, is observed that likely reflects retention of RMC-6236 in tumor tissue due to high affinity binding to CypA. Daily dosing of RMC-6236 drives profound and durable tumor regressions in multiple cell line derived (CDX) and patient derived (PDX) xenograft models of KRAS mutant PDAC at doses that are well-tolerated. Conclusions: These results indicate that direct targeting of mutant and possibly wild-type RAS in PDAC, without inhibition of signaling nodes outside the canonical RAS pathway, has the potential to translate into clinical benefit for patients with pancreatic cancer harboring mutations in KRAS that may be superior to therapies aimed at upstream or downstream signaling elements within the RAS pathway. Our preclinical data strongly support the inclusion of PDAC patients in our planned clinical trial of RMC-6236 in patients with advanced solid tumors.

The synergy of BET inhibitors with aurora A kinase inhibitors in MYCN-amplified neuroblastoma is heightened with functional TP53

Amplification of MYCN is a poor prognostic feature in neuroblastoma (NBL) indicating aggressive disease. We and others have shown BET bromodomain inhibitors (BETi) target MYCN indirectly by downregulating its transcription. Here we sought to identify agents that synergize with BETi and to identify biomarkers of resistance. We previously performed a viability screen of ∼1,900 oncology-focused compounds combined with BET bromodomain inhibitors against MYCN-amplified NBL cell lines. Reanalysis of our screening results prominently identified inhibitors of aurora kinase A (AURKAi) to be highly synergistic with BETi. We confirmed the anti-proliferative effects of several BETi+AURKAi combinations in MYCN-amplified NBL cell lines. Compared to single agents, these combinations cooperated to decrease levels of N-myc. We treated both TP53-wild type and mutant, MYCN-amplified cell lines with the BETi JQ1 and the AURKAi Alisertib. The combination had improved efficacy in the TP53-WT context, notably driving apoptosis in both genetic backgrounds. JQ1+Alisertib combination treatment of a MYCN-amplified, TP53-null or TP53-restored genetically engineered mouse model of NBL prolonged survival better than either single agent. This was most profound with TP53 restored, with marked tumor shrinkage and apoptosis induction in response to combination JQ1+Alisertib. BETi+AURKAi in MYCN-amplified NBL, particularly in the context of functional TP53, provided anti-tumor benefits in preclinical models. This combination should be studied more closely in a pediatric clinical trial.

Drugging the "Undruggable" MYCN Oncogenic Transcription Factor: Overcoming Previous Obstacles to Impact Childhood Cancers

Effective treatment of pediatric solid tumors has been hampered by the predominance of currently "undruggable" driver transcription factors. Improving outcomes while decreasing the toxicity of treatment necessitates the development of novel agents that can directly inhibit or degrade these elusive targets. MYCN in pediatric neural-derived tumors, including neuroblastoma and medulloblastoma, is a paradigmatic example of this problem. Attempts to directly and specifically target MYCN have failed due to its similarity to MYC, the unstructured nature of MYC family proteins in their monomeric form, the lack of an understanding of MYCN-interacting proteins and ability to test their relevance in vivo, the inability to obtain structural information on MYCN protein complexes, and the challenges of using traditional small molecules to inhibit protein-protein or protein-DNA interactions. However, there is now promise for directly targeting MYCN based on scientific and technological advances on all of these fronts. Here, we discuss prior challenges and the reasons for renewed optimism in directly targeting this "undruggable" transcription factor, which we hope will lead to improved outcomes for patients with pediatric cancer and create a framework for targeting driver oncoproteins regulating gene transcription.

124I-MIBG PET/CT to Monitor Metastatic Disease in Children with Relapsed Neuroblastoma

The metaiodobenzylguanidine (MIBG) scan is one of the most sensitive noninvasive lesion detection modalities for neuroblastoma. Unlike ¹²³I-MIBG, ¹²⁴I-MIBG allows high-resolution PET. We evaluated ¹²⁴I-MIBG PET/CT for its diagnostic performance as directly compared with paired ¹²³I-MIBG scans. Methods: Before ¹³¹I-MIBG therapy, standard ¹²³I-MIBG imaging (5.2 MBq/kg) was performed on 7 patients, including whole-body (anterior-posterior) planar imaging, focused-field-of-view SPECT/CT, and whole-body ¹²⁴I-MIBG PET/CT (1.05 MBq/kg). After therapy, 2 of 7 patients also completed ¹²⁴I-MIBG PET/CT as well as paired ¹²³I-MIBG planar imaging and SPECT/CT. One patient underwent ¹²⁴I-MIBG PET/CT only after therapy. We evaluated all 8 patients who showed at least 1 ¹²³I-MIBG-positive lesion with a total of 10 scans. In 8 pairs, ¹²³I-MIBG and ¹²⁴I-MIBG were performed within 1 mo of each other. The locations of identified lesions, the number of total lesions, and the curie scores were recorded for the ¹²³I-MIBG and ¹²⁴I-MIBG scans. Finally, for 5 patients who completed at least 3 PET/CT scans after administration of ¹²⁴I-MIBG, we estimated the effective dose of ¹²⁴I-MIBG. Results: ¹²³I-MIBG whole-body planar scans, focused-field-of-view SPECT/CT scans, and whole-body ¹²⁴I-MIBG PET scans found 25, 32, and 87 total lesions, respectively. There was a statistically significant difference in lesion detection for ¹²⁴I-MIBG PET/CT versus ¹²³I-MIBG planar imaging (P < 0.0001) and ¹²³I-MIBG SPECT/CT (P < 0.0001). The curie scores were also higher for ¹²⁴I-MIBG PET/CT than for ¹²³I-MIBG planar imaging and SPECT/CT in 6 of 10 patients. ¹²⁴I-MIBG PET/CT demonstrated better detection of lesions throughout the body, including the chest, spine, head and neck, and extremities. The effective dose estimated for patient-specific ¹²⁴I-MIBG was approximately 10 times that of ¹²³I-MIBG; however, given that we administered a very low activity of ¹²⁴I-MIBG (1.05 MBq/kg), the effective dose was only approximately twice that of ¹²³I-MIBG despite the large difference in half-lives (100 vs. 13.2 h). Conclusion: The first-in-humans use of low-dose ¹²⁴I-MIBG PET for monitoring disease burden demonstrated tumor detection capability superior to that of ¹²³I-MIBG planar imaging and SPECT/CT.

Single-Cell Transcriptomics in Medulloblastoma Reveals Tumor-Initiating Progenitors and Oncogenic Cascades during Tumorigenesis and Relapse

Progenitor heterogeneity and identities underlying tumor initiation and relapse in medulloblastomas remain elusive. Utilizing single-cell transcriptomic analysis, we demonstrated a developmental hierarchy of progenitor pools in Sonic Hedgehog (SHH) medulloblastomas, and identified OLIG2-expressing glial progenitors as transit-amplifying cells at the tumorigenic onset. Although OLIG2⁺ progenitors become quiescent stem-like cells in full-blown tumors, they are highly enriched in therapy-resistant and recurrent medulloblastomas. Depletion of mitotic Olig2⁺ progenitors or Olig2 ablation impeded tumor initiation. Genomic profiling revealed that OLIG2 modulates chromatin landscapes and activates oncogenic networks including HIPPO-YAP/TAZ and AURORA-A/MYCN pathways. Co-targeting these oncogenic pathways induced tumor growth arrest. Together, our results indicate that glial lineage-associated OLIG2⁺ progenitors are tumor-initiating cells during medulloblastoma tumorigenesis and relapse, suggesting OLIG2-driven oncogenic networks as potential therapeutic targets.

Drugging MYCN Oncogenic Signaling through the MYCN-PA2G4 Binding Interface

MYCN is a major driver for the childhood cancer, neuroblastoma, however, there are no inhibitors of this target. Enhanced MYCN protein stability is a key component of MYCN oncogenesis and is maintained by multiple feedforward expression loops involving MYCN transactivation target genes. Here, we reveal the oncogenic role of a novel MYCN target and binding protein, proliferation-associated 2AG4 (PA2G4). Chromatin immunoprecipitation studies demonstrated that MYCN occupies the PA2G4 gene promoter, stimulating transcription. Direct binding of PA2G4 to MYCN protein blocked proteolysis of MYCN and enhanced colony formation in a MYCN-dependent manner. Using molecular modeling, surface plasmon resonance, and mutagenesis studies, we mapped the MYCN-PA2G4 interaction site to a 14 amino acid MYCN sequence and a surface crevice of PA2G4. Competitive chemical inhibition of the MYCN-PA2G4 protein-protein interface had potent inhibitory effects on neuroblastoma tumorigenesis in vivo. Treated tumors showed reduced levels of both MYCN and PA2G4. Our findings demonstrate a critical role for PA2G4 as a cofactor in MYCN-driven neuroblastoma and highlight competitive inhibition of the PA2G4-MYCN protein binding as a novel therapeutic strategy in the disease. SIGNIFICANCE: Competitive chemical inhibition of the PA2G4-MYCN protein interface provides a basis for drug design of small molecules targeting MYC and MYCN-binding partners in malignancies driven by MYC family oncoproteins.

Technical Note: Simplified and practical pretherapy tumor dosimetry - A feasibility study for 131 I-MIBG therapy of neuroblastoma using 124 I-MIBG PET/CT

PURPOSE

Radiation dose calculated on tumors for radiopharmaceutical therapy varies significantly from tumor to tumor and from patient to patient. Accurate estimation of radiation dose requires multiple time point measurements using radionuclide imaging modalities such as SPECT or PET. In this report, we show our technical development of reducing the number of scans needed for reasonable estimation of tumor and normal organ dose in our pretherapy imaging and dosimetry platform of ¹²⁴ I-metaiodobenzylguanidine (MIBG) positron emission tomography/computed tomography (PET/CT) for ¹³¹ I-MIBG therapy of neuroblastoma.

METHODS

We analyzed the simplest kinetic data, areas of two-time point data for five patients with neuroblastoma who underwent 3 or 4 times of ¹²⁴ I-MIBG PET/CT scan prior to ¹³¹ I-MIBG therapy. The data for which we derived areas were percent of injected activity (%IA) and standardized uptake value of tumors. These areas were correlated with time-integrated activity coefficients (TIACs) from full data (3 or 4 time points). TIACs are direct correlates with radiation dose as long as the volume and the radionuclide are known.

RESULTS

The areas of %IAs between data obtained from all the two-time points with time points 1 and 2 (day 0 and day 1), time points 2 and 3 (day 1 and day 2), and time points 1 and 3 (day 0 and day 2) showed reasonable correlation (Pearson's correlation coefficient |r| > 0.5) with not only tumor and organ TIACs but also tumor and organ absorbed doses. The tumor and organ doses calculated using %IA areas of time point 1 and time point 2 were our best fits at about 20% individual percent difference compared to doses calculated using 3 or 4 time points.

CONCLUSIONS

We could achieve reasonable accuracy of estimating tumor doses for subsequent radiopharmaceutical therapy using only the two-time point imaging sessions. Images obtained from these time points (within the 48-h after administration of radiopharmaceutical) were also viewed as useful for diagnostic reading. Although our analysis was specific to ¹²⁴ I-MIBG PET/CT pretherapy imaging data for ¹³¹ I-MIBG therapy of neuroblastoma and the number of imaging datasets was not large, this feasible methodology would generally be applicable to other imaging and therapeutic radionuclides with an appropriate data analysis similar to our analysis to other imaging and therapeutic radiopharmaceuticals.

CRISPR-Cas9 screen reveals a MYCN-amplified neuroblastoma dependency on EZH2

Pharmacologically difficult targets, such as MYC transcription factors, represent a major challenge in cancer therapy. For the childhood cancer neuroblastoma, amplification of the oncogene MYCN is associated with high-risk disease and poor prognosis. Here, we deployed genome-scale CRISPR-Cas9 screening of MYCN-amplified neuroblastoma and found a preferential dependency on genes encoding the polycomb repressive complex 2 (PRC2) components EZH2, EED, and SUZ12. Genetic and pharmacological suppression of EZH2 inhibited neuroblastoma growth in vitro and in vivo. Moreover, compared with neuroblastomas without MYCN amplification, MYCN-amplified neuroblastomas expressed higher levels of EZH2. ChIP analysis showed that MYCN binds at the EZH2 promoter, thereby directly driving expression. Transcriptomic and epigenetic analysis, as well as genetic rescue experiments, revealed that EZH2 represses neuronal differentiation in neuroblastoma in a PRC2-dependent manner. Moreover, MYCN-amplified and high-risk primary tumors from patients with neuroblastoma exhibited strong repression of EZH2-regulated genes. Additionally, overexpression of IGFBP3, a direct EZH2 target, suppressed neuroblastoma growth in vitro and in vivo. We further observed strong synergy between histone deacetylase inhibitors and EZH2 inhibitors. Together, these observations demonstrate that MYCN upregulates EZH2, leading to inactivation of a tumor suppressor program in neuroblastoma, and support testing EZH2 inhibitors in patients with MYCN-amplified neuroblastoma.

Phase 1 study of sirolimus in combination with oral cyclophosphamide and topotecan in children and young adults with relapsed and refractory solid tumors

Purpose

To determine the maximum tolerated dose (MTD), toxicities, and pharmacodynamics effects of sirolimus combined with oral metronomic topotecan and cyclophosphamide in a pediatric population.

Materials and Methods

Patients who were 1 to 30 years of age with relapsed/refractory solid tumors (including CNS) were eligible. Patients received daily oral sirolimus and cyclophosphamide (25-50 mg/m²/dose) on days 1-21 and oral topotecan (0.8 mg/m²/dose) on days 1-14 in 28-day cycles. Sirolimus steady-state plasma trough concentrations of 3-7.9 ng/mL and 8-12.0 ng/mL were evaluated, with dose escalation based on a 3+3 phase 1 design. Biomarkers of angiogenesis were also evaluated.

Results

Twenty-one patients were treated (median age 18 years; range 9-30). Dose-limiting toxicities included myelosuppression, ALT elevation, stomatitis, and hypertriglyceridemia. The MTD was sirolimus with trough goal of 8-12.0 ng/mL; cyclophosphamide 25 mg/m²/dose; and topotecan 0.8 mg/m²/dose. No objective responses were observed. Four patients had prolonged stable disease > 4 cycles (range 4-12). Correlative biomarker analyses demonstrated reductions in thrombospondin-1 (p=0.043) and soluble vascular endothelial growth factor receptor-2 plasma concentrations at 21 days compared to baseline.

Conclusions

The combination of oral sirolimus, topotecan, and cyclophosphamide was well tolerated and biomarker studies demonstrated modulation of angiogenic pathways with this regimen.

N-Myc Drives Neuroendocrine Prostate Cancer Initiated from Human Prostate Epithelial Cells

MYCN amplification and overexpression are common in neuroendocrine prostate cancer (NEPC). However, the impact of aberrant N-Myc expression in prostate tumorigenesis and the cellular origin of NEPC have not been established. We define N-Myc and activated AKT1 as oncogenic components sufficient to transform human prostate epithelial cells to prostate adenocarcinoma and NEPC with phenotypic and molecular features of aggressive, late-stage human disease. We directly show that prostate adenocarcinoma and NEPC can arise from a common epithelial clone. Further, N-Myc is required for tumor maintenance, and destabilization of N-Myc through Aurora A kinase inhibition reduces tumor burden. Our findings establish N-Myc as a driver of NEPC and a target for therapeutic intervention.

Vesicular monoamine transporter protein expression correlates with clinical features, tumor biology, and MIBG avidity in neuroblastoma: a report from the Children's Oncology Group

PURPOSE

Vesicular monoamine transporters 1 and 2 (VMAT1 and VMAT2) are thought to mediate MIBG uptake in adult neuroendocrine tumors. In neuroblastoma, the norepinephrine transporter (NET) has been investigated as the principal MIBG uptake protein, though some tumors without NET expression concentrate MIBG. We investigated VMAT expression in neuroblastoma and correlated expression with MIBG uptake and clinical features.

METHODS

We evaluated VMAT1 and VMAT2 expression by immunohistochemistry (IHC) in neuroblastoma tumors from 76 patients with high-risk metastatic disease treated in a uniform cooperative group trial (COG A3973). All patients had baseline MIBG diagnostic scans centrally reviewed. IHC results were scored as the product of intensity grading (0 - 3+) and percent of tumor cells expressing the protein of interest. The association between VMAT1 and VMAT2 scores and clinical and biological features was tested using Wilcoxon rank-sum tests.

RESULTS

Patient characteristics were typical of high-risk neuroblastoma, though the cohort was intentionally enriched in patients with MIBG-nonavid tumors (n = 20). VMAT1 and VMAT2 were expressed in 62% and 75% of neuroblastoma tumors, respectively. VMAT1 and VMAT2 scores were both significantly lower in MYCN amplified tumors and in tumors with high mitotic karyorrhectic index. MIBG-avid tumors had significantly higher VMAT2 scores than MIBG-nonavid tumors (median 216 vs. 45; p = 0.04). VMAT1 expression did not correlate with MIBG avidity.

CONCLUSION

VMAT1 and VMAT2 are expressed in the majority of neuroblastomas. Expression correlates with other biological features. The expression level of VMAT2 but not that of VMAT1 correlates with avidity for MIBG.

Downregulation of MYCN through PI3K Inhibition in Mouse Models of Pediatric Neural Cancer

The MYCN proto-oncogene is associated with poor outcome across a broad range of pediatric tumors. While amplification of MYCN drives subsets of high-risk neuroblastoma and medulloblastoma, dysregulation of MYCN in medulloblastoma (in the absence of amplification) also contributes to pathogenesis. Since PI3K stabilizes MYCN, we have used inhibitors of PI3K to drive degradation. In this study, we show PI3K inhibitors by themselves induce cell cycle arrest, with modest induction of apoptosis. In screening inhibitors of PI3K against MYCN, we identified PIK-75 and its derivative, PW-12, inhibitors of both PI3K and of protein kinases, to be highly effective in destabilizing MYCN. To determine the effects of PW-12 treatment in vivo, we analyzed a genetically engineered mouse model for MYCN-driven neuroblastoma and a model of MYCN-driven medulloblastoma. PW-12 showed significant activity in both models, inducing vascular collapse and regression of medulloblastoma with prominent apoptosis in both models. These results demonstrate that inhibitors of lipid and protein kinases can drive apoptosis in MYCN-driven cancers and support the importance of MYCN as a therapeutic target.

The genetics of splicing in neuroblastoma

Regulation of mRNA splicing, a critical and tightly regulated cellular function, underlies the majority of proteomic diversity and is frequently disrupted in disease. Using an integrative genomics approach, we combined both genomic data and exon-level transcriptome data in two somatic tissues (cerebella and peripheral ganglia) from a transgenic mouse model of neuroblastoma, a tumor that arises from the peripheral neural crest. Here, we describe splicing quantitative trait loci associated with differential splicing across the genome that we use to identify genes with previously unknown functions within the splicing pathway and to define de novo intronic splicing motifs that influence splicing from hundreds of bases away. Our results show that these splicing motifs represent sites for functional recurrent mutations and highlight novel candidate genes in human cancers, including childhood neuroblastoma.

SIGNIFICANCE

Somatic mutations with predictable downstream effects are largely relegated to coding regions, which comprise less than 2% of the human genome. Using an unbiased in vivo analysis of a mouse model of neuroblastoma, we have identified intronic splicing motifs that translate into sites for recurrent somatic mutations in human cancers.

A new "angle" on kinase inhibitor design: Prioritizing amphosteric activity above kinase inhibition

The MYCN oncoprotein has remained an elusive target for decades. We recently reported a new class of kinase inhibitors designed to disrupt the conformation of Aurora kinase A enough to block its kinase-independent interaction with MYCN, resulting in potent degradation of MYCN. These studies provide proof-of-principle for a new method of targeting enzyme activity-independent functions of kinases and other enzymes.

Abstract PR08: Drugging MYCN protein stability through an allosteric transition in Aurora kinase A

MYC family proteins represent a critical node in oncogenesis across a wide range of pediatric and adult cancers. This is particularly true in neuroblastoma, where amplification of MYCN confers an especially poor prognosis. Proteolytic degradation of MYCN protein is regulated in part by a kinase-independent function of Aurora Kinase A. We describe a class of allosteric inhibitors that disrupts the native conformation of Aurora A and causes degradation of MYCN protein across MYCN-expressing neuroblastoma cell lines. We compare co-crystal structures with structure-activity relationships across multiple inhibitors and chemotypes to delineate an Aurora A conformation-specific effect on proteolytic degradation of MYCN protein, which is driven markedly by conformation disrupting inhibitors, and impacted more modestly by current clinical Aurora Kinase A inhibitors, which disrupt the kinase structure less efficiently. Using functional assays of apoptosis, cell cycle, and proliferation we further define a distinct MYCN-specific effect distinguishing it from simple inhibition of mitosis by conventional Aurora kinase inhibitors. This new class of conformation disrupting inhibitors, which block stabilizing interactions between Aurora A and MYCN, represents a novel strategy to target MYCN-driven cancers.

This abstract is also presented as Poster B46.

Citation Format: W. Clay Gustafson, Justin G. Meyerowitz, Erin A. Nekritz, Elise Charron, Katherine K. Matthay, Nicholas T. Hertz, Martin Eilers, Kevan M. Shokat, William A. Weiss. Drugging MYCN protein stability through an allosteric transition in Aurora kinase A. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr PR08.

Mutational analysis reveals the origin and therapy-driven evolution of recurrent glioma

Tumor recurrence is a leading cause of cancer mortality. Therapies for recurrent disease may fail, at least in part, because the genomic alterations driving the growth of recurrences are distinct from those in the initial tumor. To explore this hypothesis, we sequenced the exomes of 23 initial low-grade gliomas and recurrent tumors resected from the same patients. In 43% of cases, at least half of the mutations in the initial tumor were undetected at recurrence, including driver mutations in TP53, ATRX, SMARCA4, and BRAF; this suggests that recurrent tumors are often seeded by cells derived from the initial tumor at a very early stage of their evolution. Notably, tumors from 6 of 10 patients treated with the chemotherapeutic drug temozolomide (TMZ) followed an alternative evolutionary path to high-grade glioma. At recurrence, these tumors were hypermutated and harbored driver mutations in the RB (retinoblastoma) and Akt-mTOR (mammalian target of rapamycin) pathways that bore the signature of TMZ-induced mutagenesis.

EGFR phosphorylates tumor-derived EGFRvIII driving STAT3/5 and progression in glioblastoma

EGFRvIII, a frequently occurring mutation in primary glioblastoma, results in a protein product that cannot bind ligand, but signals constitutively. Deducing how EGFRvIII causes transformation has been difficult because of autocrine and paracrine loops triggered by EGFRvIII alone or in heterodimers with wild-type EGFR. Here, we document coexpression of EGFR and EGFRvIII in primary human glioblastoma that drives transformation and tumorigenesis in a cell-intrinsic manner. We demonstrate enhancement of downstream STAT signaling triggered by EGFR-catalyzed phosphorylation of EGFRvIII, implicating EGFRvIII as a substrate for EGFR. Subsequent phosphorylation of STAT3 requires nuclear entry of EGFRvIII and formation of an EGFRvIII-STAT3 nuclear complex. Our findings clarify specific oncogenic signaling relationships between EGFR and EGFRvIII in glioblastoma.

Abstract 4622: Targeting MYCN in Neuroblastoma by BET Bromodomain Inhibition

Bromodomain inhibition comprises a promising therapeutic strategy in cancer, particularly for hematologic malignancies. To date, however, genomic biomarkers to direct clinical translation have been lacking. We conducted a cell-based screen of genetically-defined cancer cell lines using a prototypical inhibitor of BET (bromodomain and extra-terminal domain) bromodomains. Integration of genetic features with chemosensitivity data revealed a robust correlation between MYCN amplification and sensitivity to bromodomain inhibition. We characterized the mechanistic and translational significance of this finding in neuroblastoma, a childhood cancer with frequent amplification of MYCN. Genome-wide expression analysis demonstrated downregulation of the MYCN transcriptional program accompanied by suppression of MYCN transcription, and a BET Bromodomain inhibitor was found to displace BRD4 from the MYCN promoter region in neuroblastoma cell lines. Functionally, bromodomain-mediated inhibition of MYCN impaired growth and induced apoptosis in neuroblastoma. BRD4 knock-down phenocopied these effects, establishing BET bromodomains as transcriptional regulators of MYCN. BET inhibition conferred a significant survival advantage in three in vivo neuroblastoma models, providing a compelling rationale for developing BET bromodomain inhibitors in patients with neuroblastoma.

Citation Format: Alexandre Puissant, Stacey M. Frumm, Gabriela Alexe, Christopher F. Bassil, Jun Qi, Yvan H. Chanthery, Erin A. Nekritz, Rhamy Zeid, W. Clay Gustafson, Patricia Greninger, Matthew J. Garnett, Ultan McDermott, Cyril H. Benes, Andrew L. Kung, William A. Weiss, James E. Bradner, Kimberly Stegmaier. Targeting MYCN in Neuroblastoma by BET Bromodomain Inhibition. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4622. doi:10.1158/1538-7445.AM2013-4622

Paracrine signaling through MYCN enhances tumor-vascular interactions in neuroblastoma

Neuroblastoma, a tumor of peripheral neural crest origin, numbers among the most common childhood cancers. Both amplification of the proto-oncogene MYCN and increased neoangiogenesis mark high-risk disease. Because angiogenesis is regulated by phosphatidylinositol 3-kinase (PI3K), we tested a clinical PI3K inhibitor, NVP-BEZ235, in MYCN-dependent neuroblastoma. NVP-BEZ235 decreased angiogenesis and improved survival in both primary human (highly pretreated recurrent MYCN-amplified orthotopic xenograft) and transgenic mouse models for MYCN-driven neuroblastoma. Using both gain- and loss-of-function approaches, we demonstrated that the antiangiogenic efficacy of NVP-BEZ235 depended critically on MYCN in vitro and in vivo. Thus, clinical PI3K/mammalian target of rapamycin inhibitors drove degradation of MYCN in tumor cells, with secondary paracrine blockade of angiogenesis. Our data demonstrated significantly improved survival in treated animals and suggest that NVP-BEZ235 should be tested in children with high-risk, MYCN-amplified neuroblastoma.

Abstract 1558: Paracrine signaling through Mycn enhances tumor-vascular microenvironment in neuroblastoma

Neuroblastoma, a tumor of peripheral neural crest origin, numbers among the most common childhood cancers. Both amplification of the proto-oncogene MYCN and increased vascular density mark high-risk disease. Since angiogenesis is regulated by phosphatidylinositol-3’ kinase (PI3K), we tested a clinical PI3K inhibitor, NVP-BEZ235, in MYCN-dependent neuroblastoma. NVP-BEZ235 decreased angiogenesis in a primary human neuroblastoma (MYCN-amplified orthotopic xenograft), and improved survival in a transgenic mouse model for MYCN-driven neuroblastoma. Using both gain and loss of function approaches, we demonstrate that the anti-angiogenic efficacy of NVP-BEZ235 was critically dependent on Mycn, in vitro and in-vivo. Thus, clinical PI3K/mTOR inhibitors drive degradation of Mycn in tumor cells, with secondary paracrine blockade of tumor-vascular microenvironment. These observations argue that NVP-BEZ235 should be tested in children with high-risk, MYCN amplified neuroblastoma.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1558. doi:10.1158/1538-7445.AM2011-1558

Role of gastrointestinal hormones in neuroblastoma

As a neuroendocrine tumor, neuroblastoma expresses various gastrointestinal (GI) hormones, such as vasoactive intestinal peptide, gastrin-releasing peptide (GRP), neurotensin, and somatostatin, which exert diverse cellular functions in neuroblastoma. In particular, we have recently found that GRP and its cell surface receptor, GRP-R, are abundantly expressed in neuroblastomas. Moreover, more advanced-stage neuroblastomas demonstrated an increased level of GRP-R, suggesting an important role of GRP in aggressive tumor behavior. This review describes the role of several GI hormones commonly expressed in neuroblastoma and discusses in depth the mitogenic actions of GRP in neuroblastoma. In addition, the molecular mechanisms involved in the GRP-induced stimulation of neuroblastoma cell growth are discussed. Our study results demonstrate a role of GRP as an autocrine/paracrine growth factor and elucidate involvement of specific intracellular signaling, the phosphatidylinositol 3-kinase pathway, in the growth regulation of neuroblastoma.

Genomic Mechanisms of p210BCR-ABL Signaling

Chronic myelogenous leukemia (CML) results from a t(9,22) translocation, producing the p210(BCR-ABL) oncoprotein, a tyrosine kinase that causes transformation and chemotherapy resistance. To further understand mechanisms mediating chemotherapy resistance, we identified 556 differentially regulated genes in HL-60 cells stably expressing p210(BCR-ABL) versus those expressing an empty vector using cDNA macro- and oligonucleotide microarrays. These BCR-ABL-regulated gene products play diverse roles in cellular function including apoptosis, cell cycle regulation, intracellular signaling, transcription, and cellular adhesion. In particular, we identified up-regulation of the inducible form of heat shock protein 70 (Hsp70), and further explored the mechanism for its up-regulation. In HL-60/BCR-ABL and K562 cells (expressing p210(BCR-ABL)), abundant cytoplasmic Hsp70 expression was detected by immunoblot analysis. Moreover, cells isolated from bone marrow aspirates of patients in different stages of CML (chronic, aggressive, and blast crisis) express Hsp70. Expression of p210(BCR-ABL) in BCR-ABL negative cells induced transcription of the proximal Hsp70 promoter. Mutational analysis mapped the major p210(BCR-ABL) responsive element to a high affinity 5'(A/T)GATA(A/G)-3' "GATA" response element (GATA-RE) that binds GATA-1 in CML cells. The GATA-RE was sufficient to confer p210(BCR-ABL)- and p185(BCR-ABL)-mediated trans-activation to an inert promoter. Short interfering RNA mediated "knockdown" of Hsp70 expression in K562 cells induced marked sensitivity to paclitaxel-induced apoptosis. Together these findings indicate that BCR-ABL confers chemotherapeutic resistance through intracellular signaling to the GATA-RE element found in the promoter region of the anti-apoptotic Hsp70 protein. We suggest that down-regulation of the GATA-Hsp70 pathway may be useful in the treatment of chemotherapy-resistant CML.

Bcr-Abl regulates protein kinase Ciota (PKCiota) transcription via an Elk1 site in the PKCiota promoter

The protein kinase C (PKC) family of serine/threonine kinases plays an important role in numerous cancer signaling pathways, including those downstream of the bcr-abl oncogene. We demonstrated previously that atypical PKCiota is required for Bcr-Abl-mediated resistance of human K562 chronic myelogenous leukemia (CML) cells to Taxol-induced apoptosis. Here, we report that the pattern of PKC isozyme expression characteristic of CML cells is regulated by Bcr-Abl. When Bcr-Abl was expressed in Bcr-Abl-negative HL-60 promyelocytic leukemia cells, expression of the PKCbetaI, PKCbetaII, and PKCiota genes was induced, whereas expression of the PKCdelta gene was reduced to levels similar to those found in CML cells. Given the importance of PKCiota in Bcr-Abl-mediated transformation, we characterized the mechanism by which Bcr-Abl regulates PKCiota expression. A 1200-bp PKCiota promoter construct isolated from genomic DNA was highly active in Bcr-Abl-positive K562 cells and was activated when Bcr-Abl-negative cells were transfected with Bcr-Abl. Bcr-Abl-mediated induction of the PKCiota promoter was dependent upon MEK1/2 activity, but not phosphatidylinositol 3-kinase or p38 MAPK activity. Mutational analysis of the PKCiota promoter revealed a region between 97 and 114 bp upstream of the transcriptional start site that is responsible for Bcr-Abl-mediated regulation. Mutation of a consensus Elk1-binding site within this region abolished Bcr-Abl-mediated regulation. We conclude that Bcr-Abl regulates PKCiota expression through the MEK-dependent activation of an Elk1 element within the proximal PKCiota promoter. Our results indicate that Bcr-Abl-mediated transformation involves transcriptional activation of the PKCiota gene, which in turn is required for Bcr-Abl-mediated chemoresistance.

Comparison of GLUT1, GLUT3, and GLUT4 mRNA and the subcellular distribution of their proteins in normal human muscle

Basal, "insulin-independent" glucose uptake into skeletal muscle is provided by glucose transporters positioned at the plasma membrane. The relative amount of the three glucose transporters expressed in muscle has not been previously quantified. Using a combination of qualitative and quantitative ribonuclease protection assay (RPA) methods, we found in normal human muscle that GLUT1, GLUT3, and GLUT4 mRNA were expressed at 90 +/- 10, 46 +/- 4, and 156 +/- 12 copies/ng RNA, respectively. Muscle was fractionated by DNase digestion and differential sedimentation into membrane fractions enriched in plasma membranes (PM) or low-density microsomes (LDM). GLUT1 and GLUT4 proteins were distributed 57% to 67% in LDM, whereas GLUT3 protein was at least 88% in the PM-enriched fractions. These data suggest that basal glucose uptake into resting human muscle could be provided in part by each of these three isoforms.

Overexpression of protein kinase C betaII induces colonic hyperproliferation and increased sensitivity to colon carcinogenesis

Protein kinase C betaII (PKC betaII) has been implicated in proliferation of the intestinal epithelium. To investigate PKC betaII function in vivo, we generated transgenic mice that overexpress PKC betaII in the intestinal epithelium. Transgenic PKC betaII mice exhibit hyperproliferation of the colonic epithelium and an increased susceptibility to azoxymethane-induced aberrant crypt foci, preneoplastic lesions in the colon. Furthermore, transgenic PKC betaII mice exhibit elevated colonic beta-catenin levels and decreased glycogen synthase kinase 3beta activity, indicating that PKC betaII stimulates the Wnt/adenomatous polyposis coli (APC)/beta-catenin proliferative signaling pathway in vivo. These data demonstrate a direct role for PKC betaII in colonic epithelial cell proliferation and colon carcinogenesis, possibly through activation of the APC/beta-catenin signaling pathway.

Nucleolar protein p120 contains an arginine-rich domain that binds to ribosomal RNA

Human proliferation-associated protein p120 has previously been shown to localize to the nucleolus, and several functional domains of p120 have been elucidated. By using a nitrocellulose filter binding assay and a Northwestern blotting procedure this study shows that recombinant p120 binds to an rRNA fragment in vitro with a dissociation constant of 4 nM. The specific RNA-binding region of p120 (residues 1-57) was identified with glutathione S-transferase-fused p120 deletion constructs and Northwestern blotting procedures. This RNA-binding region of p120, which includes the nucleolar localization signal of p120, is similar to the arginine-rich RNA-binding regions found in other RNA-binding proteins such as HIV Rev and Tat. Experiments in vivo with HeLa cell nucleolar extracts showed that p120 was associated with the 60-80S pre-ribosomal particles. This association is disrupted by treatment with either RNase A or buffer of high ionic strength. These results suggest that p120 might be involved in rRNA/ribosome maturation, consistent with the role of the yeast homologue Nop2p in rRNA biogenesis.

Overexpression of human nucleolar proteins in insect cells: characterization of nucleolar protein p120

Nucleolar p120 is a proliferation-associated protein, which becomes detectable early in the G1 phase of the cell cycle and peaks early in the S phase. A variety of human malignant tumor cells contain much higher levels of p120 than normal resting cells. The cellular functions of p120 are unknown, and little information is available on the structural characteristics of the human p120 protein. For biochemical characterization, human p120 protein was expressed in a baculovirus system and purified to approximately 95% purity. By indirect immunofluorescence, most of the recombinant human p120 as well as recombinant human B23, C23, or fibrillarin were localized to insect cell nucleoli and to large globular nuclear inclusions. Like endogenous p120 in HeLa cells, recombinant p120 expressed in insect cells was phosphorylated. On sucrose density gradients, p120 from HeLa cells sedimented in the 60-80S region, in which preribosomal particles sedimented using similar extraction and centrifugation procedures. The sedimentation of p120 shifted to the 5-10S region by treatment with 1 M KCl or with RNAse which suggests that p120 is bound to RNA.