Expanding the ligandable proteome by paralog hopping with covalent probes

More than half of the ~20,000 protein-encoding human genes have at least one paralog. Chemical proteomics has uncovered many electrophile-sensitive cysteines that are exclusive to a subset of paralogous proteins. Here, we explore whether such covalent compound-cysteine interactions can be used to discover ligandable pockets in paralogs that lack the cysteine. Leveraging the covalent ligandability of C109 in the cyclin CCNE2, we mutated the corresponding residue in paralog CCNE1 to cysteine (N112C) and found through activity-based protein profiling (ABPP) that this mutant reacts stereoselectively and site-specifically with tryptoline acrylamides. We then converted the tryptoline acrylamide-N112C-CCNE1 interaction into a NanoBRET-ABPP assay capable of identifying compounds that reversibly inhibit both N112C- and WT-CCNE1:CDK2 complexes. X-ray crystallography revealed a cryptic allosteric pocket at the CCNE1:CDK2 interface adjacent to N112 that binds the reversible inhibitors. Our findings thus provide a roadmap for leveraging electrophile-cysteine interactions to extend the ligandability of the proteome beyond covalent chemistry.

Rapid adaptation to CDK2 inhibition exposes intrinsic cell-cycle plasticity

CDK2 is a core cell-cycle kinase that phosphorylates many substrates to drive progression through the cell cycle. CDK2 is hyperactivated in multiple cancers and is therefore an attractive therapeutic target. Here, we use several CDK2 inhibitors in clinical development to interrogate CDK2 substrate phosphorylation, cell-cycle progression, and drug adaptation in preclinical models. Whereas CDK1 is known to compensate for loss of CDK2 in Cdk2-/- mice, this is not true of acute inhibition of CDK2. Upon CDK2 inhibition, cells exhibit a rapid loss of substrate phosphorylation that rebounds within several hours. CDK4/6 activity backstops inhibition of CDK2 and sustains the proliferative program by maintaining Rb1 hyperphosphorylation, active E2F transcription, and cyclin A2 expression, enabling re-activation of CDK2 in the presence of drug. Our results augment our understanding of CDK plasticity and indicate that co-inhibition of CDK2 and CDK4/6 may be required to suppress adaptation to CDK2 inhibitors currently under clinical assessment.

Multimodal perturbation analyses of cyclin-dependent kinases reveal a network of synthetic lethalities associated with cell-cycle regulation and transcriptional regulation

Cell-cycle control is accomplished by cyclin-dependent kinases (CDKs), motivating extensive research into CDK targeting small-molecule drugs as cancer therapeutics. Here we use combinatorial CRISPR/Cas9 perturbations to uncover an extensive network of functional interdependencies among CDKs and related factors, identifying 43 synthetic-lethal and 12 synergistic interactions. We dissect CDK perturbations using single-cell RNAseq, for which we develop a novel computational framework to precisely quantify cell-cycle effects and diverse cell states orchestrated by specific CDKs. While pairwise disruption of CDK4/6 is synthetic-lethal, only CDK6 is required for normal cell-cycle progression and transcriptional activation. Multiple CDKs (CDK1/7/9/12) are synthetic-lethal in combination with PRMT5, independent of cell-cycle control. In-depth analysis of mRNA expression and splicing patterns provides multiple lines of evidence that the CDK-PRMT5 dependency is due to aberrant transcriptional regulation resulting in premature termination. These inter-dependencies translate to drug-drug synergies, with therapeutic implications in cancer and other diseases.

PARP1-SNAI2 transcription axis drives resistance to PARP inhibitor, Talazoparib

The synthetic lethal association between BRCA deficiency and poly (ADP-ribose) polymerase (PARP) inhibition supports PARP inhibitor (PARPi) clinical efficacy in BRCA-mutated tumors. PARPis also demonstrate activity in non-BRCA mutated tumors presumably through induction of PARP1-DNA trapping. Despite pronounced clinical response, therapeutic resistance to PARPis inevitably develops. An abundance of knowledge has been built around resistance mechanisms in BRCA-mutated tumors, however, parallel understanding in non-BRCA mutated settings remains insufficient. In this study, we find a strong correlation between the epithelial-mesenchymal transition (EMT) signature and resistance to a clinical PARPi, Talazoparib, in non-BRCA mutated tumor cells. Genetic profiling demonstrates that SNAI2, a master EMT transcription factor, is transcriptionally induced by Talazoparib treatment or PARP1 depletion and this induction is partially responsible for the emerging resistance. Mechanistically, we find that the PARP1 protein directly binds to SNAI2 gene promoter and suppresses its transcription. Talazoparib treatment or PARP1 depletion lifts PARP1-mediated suppression and increases chromatin accessibility around SNAI2 promoters, thus driving SNAI2 transcription and drug resistance. We also find that depletion of the chromatin remodeler CHD1L suppresses SNAI2 expression and reverts acquired resistance to Talazoparib. The PARP1/CHD1L/SNAI2 transcription axis might be therapeutically targeted to re-sensitize Talazoparib in non-BRCA mutated tumors.

Abstract LBA011: Discovery of combination targets of CDK inhibitors from CRISPR screens

CRISPR/Cas9 functional genetic screens are a powerful tool to discover drug targets in cancer. While many screens have been performed for identification of intrinsic cancer vulnerabilities, synthetic lethalities of targeted therapies remain largely unexplored, despite the fact that they may provide a potential targets and pathways for potential combination therapies, advance understanding of target biology, and hone patient selection strategies. To discover novel combination targets of Palbociclib, a CDK2 inhibitor, and a CDK2,4,6 inhibitor, we have screened a genome-wide CRISPR library in priority tumors with genetic background of interest, including ER+ breast cancer (HCC1428, MCF7), cyclin E amplified breast (HCC1806) and/or ovarian cancer (OVCAR4), and pancreatic cancer cell lines (MIAPACA2, Hs766T, SU8686). We analyzed the samples by condition and compared using MAGeCK-MLE for gene-level drop out and enrichment to identify common sensitizers and resistors to different CDK inhibitors. Differential genes, complexes, and pathways were assessed, and we have found that genes involved in G1/S transition (CyclinE/CDK2) are top Palbo sensitizers and CDK1/CyclinA, CDC25A/B, MuvB complex (MYBL2 and FOXM1) are major CDK2 sensitizing genes. In addition to sensitizers, we have identified genes with strong signals of resistance to CDK inhibitors. Furthermore, we highlighted major mechanisms influencing response to CDK2 inhibitors; 1) Regulation of DREAM may influence response to CDK inhibitors, 2) Regulation of cell cycle check points mediated by CDC25. Our targeted CRISPR KO screens in different CDKi combinations have identified common sensitizers and mediators of resistance to CDK inhibitors, and highlighted biology worthy of deeper consideration and offers opportunities to consider new directions for therapeutic intervention into the cancer cell cycle

Citation Format: Kimberly H Kim, Jonathan Almaden, vinu arunachalam, Todd VanArsdale, chaoting liu. Discovery of combination targets of CDK inhibitors from CRISPR screens [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr LBA011.

Real-World Data of Palbociclib in Combination With Endocrine Therapy for the Treatment of Metastatic Breast Cancer in Men

This report examined the benefits and risks of palbociclib plus endocrine therapy (ET) in men with hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (MBC). Palbociclib was evaluated using three independent data sources: real-world data from pharmacy and medical claims, a de-identified real-world data source derived from electronic health records (EHRs), and a global safety database. From medical and pharmacy records, 1,139 men with MBC were identified; in the first-line setting, median duration of treatment was longer with palbociclib plus ET (n = 37, 8.5 months, 95% confidence interval (CI), 4.4-13.0) than ET alone (n = 214, 4.3 months, 95% CI, 3.0-5.7) and specifically, was longer with palbociclib plus letrozole (n = 26, 9.4 months, 95% CI, 4.4-14.0) than letrozole alone (n = 63, 3.0 months, 95% CI, 1.8-4.8). In the EHR-derived database, 59 men received treatment for MBC; real-world response across all lines of therapy in the metastatic setting was reported in 4 of 12 patients (33.3%) in the palbociclib plus ET group vs. 1 of 8 (12.5%) patients in the ET group. Review of the global safety database did not identify any new safety signals in palbociclib-treated men. Real-world data indicated that men with MBC benefit from palbociclib plus ET, with a safety profile consistent with previous observations in women with MBC. Collective data on palbociclib in women and men in this report, including clinical trial data, real-world data, and a well-established risk/benefit profile, led to US approval of an expansion of the palbociclib indication to include men with MBC.

Expanding control of the tumor cell cycle with a CDK2/4/6 inhibitor

The CDK4/6 inhibitor, palbociclib (PAL), significantly improves progression-free survival in HR⁺/HER2^- breast cancer when combined with anti-hormonals. We sought to discover PAL resistance mechanisms in preclinical models and through analysis of clinical transcriptome specimens, which coalesced on induction of MYC oncogene and Cyclin E/CDK2 activity. We propose that targeting the G₁ kinases CDK2, CDK4, and CDK6 with a small-molecule overcomes resistance to CDK4/6 inhibition. We describe the pharmacodynamics and efficacy of PF-06873600 (PF3600), a pyridopyrimidine with potent inhibition of CDK2/4/6 activity and efficacy in multiple in vivo tumor models. Together with the clinical analysis, MYC activity predicts (PF3600) efficacy across multiple cell lineages. Finally, we find that CDK2/4/6 inhibition does not compromise tumor-specific immune checkpoint blockade responses in syngeneic models. We anticipate that (PF3600), currently in phase 1 clinical trials, offers a therapeutic option to cancer patients in whom CDK4/6 inhibition is insufficient to alter disease progression.

Abstract 1954: Identifying synthetic lethality with novel CDK2 small molecule inhibitors via integration of high-content microscopy and -omics level platforms

CDK4/6 small molecule inhibitors represent a milestone in treating hormone receptor-positive breast cancer. PALOMA (Palbociclib: Ongoing Trials in the Management of Breast Cancer) studies have confirmed prolonged progression-free survival after CDK4/6 inhibitor and anti-estrogen therapy. Notably, disease progression is observed in a subset of patients receiving Palbociclib-Fulvestrant treatment (Turner et al., NEJM 2018). Circulating tumor DNA from PALOMA-3 patients revealed genetic mutations correlating with patient response (O'Leary et al., Cancer Discov, 2018). Compensations in CDK4/6i resistance include circumvention of CDK4/6 substrate regulation, CCNE1/2 amplification (Herrera-Abreu et al., Cancer Res. 2016), and compensatory CDK2 activity. The importance of compensatory CDK2 cancer re-wiring is highlighted by recently reported, highly efficacious CDK2/4/6i (Freeman-Cook et al., in review). Thus, mechanistic understanding of cancer cell and compensatory CDK2 activity is imperative to improve patient outcome. Acute, temporal phenotypes in CDK2-signaling have been observed by live single-cell tracking microscopy in human breast epithelial and ER+ breast cancer cells (Arora & Moser et al., in review). In extending these studies, we utilize first-in-class small molecule inhibitors against CDK2 combined with high content imaging approaches to illuminate cancer cell CDK2 signaling and identify potential synthetic lethality. We report biomarkers parsed in cycling populations as quantifiable phenotypes and identify disengaged control of several processes including S- and G2/M licensing after CDK2i. Combined with internal -omics studies, CDK2i selective molecules support a positive regulatory role of CDK2 on CCNB1-PLK1 signaling before mitotic entry. We combine our high-content techniques with proximity labeling to quantify alternative CDK-Cyclin complex formation following pharmacologic perturbation. We hypothesize that dysregulated CDK-Cyclin complex formation elicits unintended CDK2 kinase activity, revealing potential synthetic lethality with inhibition of centrosome duplication pathways in ER+ breast cancers.

Citation Format: Tim Sen Wang, Jon Almaden, Chen Shen, John Lapek, Sherry Niessen, Nichol Miller, Stephen Dann, Todd VanArsdale. Identifying synthetic lethality with novel CDK2 small molecule inhibitors via integration of high-content microscopy and -omics level platforms [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 1954.

Abstract 2960: A novel mouse model of pancreatic cancer reveals new insights into cell cycle deregulation

Overcoming checkpoints to cell cycle control is the basis for tumorigenesis and malignant growth. Therefore, models that recapitulate clinically relevant cell cycle deregulation enhance our understanding of defined tumors subsets. Specifically, pancreatic ductal adenocarcinomas (PDAs) frequently delete the 9p21 locus which contains the cyclin dependent kinase (CDK) inhibitors CDKN2A (p16, p14) and CDKN2B (p15), as well as methylthioadenosine phosphorylase (MTAP), a metabolic gene required for methionine salvage from methylthioadenosine. No model currently exists that accurately represents loss of the entire locus in a relevant disease context. Moreover, the contribution of MTAP to tumor progression remains largely unknown. Therefore, we have developed a novel genetically engineered mouse model (GEMM) of PDA which combines loss of the orthologous murine 9p21 region (4qC4) with activated KRAS [Pdx-Cre; LSL-KrasG12D; 9p21L/L (K9C)], which results in rapid adenocarcinoma formation and subsequent mortality in mice homozygous for 9p21 deletion. Single-cell RNA sequencing revealed a remarkable level of inter- and intra-tumoral heterogeneity, including a significant immune and stromal component that contribute to tumor growth and progression. Additionally, K9C derived cell lines are responsive to Pfizer's first-in class CDK2/4/6 selective inhibitor while displaying de novo resistance to CDK4/6 inhibitor Palbociclib. Allograft and single-cell RNA sequencing experiments corroborated these findings and implicate Myc in contributing to CDK2/4/6i sensitivity. Furthermore, phenotypic-based screens revealed synthetic-lethal hits with 9p21 loss, indicating ample opportunities for combination strategies in this select patient population. Thus, we show that the K9C model recapitulates salient aspects of PDA and is amenable to novel therapeutic intervention strategies that may aid in improving the outcomes of patients with this precise genetic background.

*All procedures performed on animals were in accordance with regulations and established guidelines and were reviewed and approved by an Institutional Animal Care and use committee

Citation Format: Christina Adams, Lynn Wang, Tim S. Wang, Nichol Miller, Elizabeth McMillan, Monica Ramstetter, John Chionis, Koleen Eisele, Jonathan Almaden, Timothy Affolter, Smitha Pillai, Todd VanArsdale, Chris Dillon, Stephen G. Dann. A novel mouse model of pancreatic cancer reveals new insights into cell cycle deregulation [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 2960.

CDK4/6 Inhibitors Impair Recovery from Cytotoxic Chemotherapy in Pancreatic Adenocarcinoma

Inhibition of the cell-cycle kinases CDK4 and CDK6 is now part of the standard treatment in advanced breast cancer. CDK4/6 inhibitors, however, are not expected to cooperate with DNA-damaging or antimitotic chemotherapies as the former prevent cell-cycle entry, thus interfering with S-phase- or mitosis-targeting agents. Here, we report that sequential administration of CDK4/6 inhibitors after taxanes cooperates to prevent cellular proliferation in pancreatic ductal adenocarcinoma (PDAC) cells, patient-derived xenografts, and genetically engineered mice with Kras G12V and Cdkn2a-null mutations frequently observed in PDAC. This effect correlates with the repressive activity of CDK4/6 inhibitors on homologous recombination proteins required for the recovery from chromosomal damage. CDK4/6 inhibitors also prevent recovery from multiple DNA-damaging agents, suggesting broad applicability for their sequential administration after available chemotherapeutic agents.

Cisplatin exposure causes c-Myc-dependent resistance to CDK4/6 inhibition in HPV-negative head and neck squamous cell carcinoma

The loss of p16 is a signature event in Human Papilloma Virus (HPV)-negative head and neck squamous cell carcinoma (HNSCC) that leads to increased Cyclin Dependent Kinase 4/6 (CDK) signaling. Palbociclib, a CDK4/6 inhibitor, is active for the treatment of a subset of HNSCC. In this study, we analyzed patient response data from a phase I clinical trial of palbociclib in HNSCC and observed an association between prior cisplatin exposure and CDK inhibitor resistance. We studied the effects of palbociclib on cisplatin-sensitive and -resistant HNSCC cell lines. We found that while palbociclib is highly effective against chemo-naive HNSCC cell lines and tumor xenografts, prior cisplatin exposure induces intrinsic resistance to palbociclib in vivo, a relationship that was not observed in vitro. Mechanistically, in the course of provoking a DNA damage-resistance phenotype, cisplatin exposure upregulates both c-Myc and cyclin E, and combination treatment with palbociclib and the c-Myc bromodomain inhibitor JQ1 exerts a synergistic anti-growth effect in cisplatin-resistant cells. These data show the benefit of exploiting the inherent resistance mechanisms of HNSCC to overcome cisplatin- and palbociclib resistance through the use of c-Myc inhibition.

Abstract P6-20-06: Withdrawn

This abstract was withdrawn by the authors.

Citation Format: Dann S, Chionis J, Eisele K, Zhang Q, Liu C, Yuan J, Miller N, Murray B, Xu M, Solowiej J, Wei P, Weinrich S, Sutton S, Behenna D, Ninkovic S, Hoffman R, Freeman-Cook K, Jessen B, Huser N, Zhang C, Visswanathan R, Boras B, VanArsdale T, White MA. Withdrawn [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 P6-20-06.

Combining CDK4/6 inhibition with taxanes enhances anti-tumor efficacy by sustained impairment of pRB-E2F pathways in squamous cell lung cancer

The CDK4/6 inhibitor palbociclib reduces tumor growth by decreasing retinoblastoma (RB) protein phosphorylation and inducing cell cycle arrest at the G1/S phase transition. Palbociclib in combination with anti-hormonal therapy brings significant benefit to breast cancer patients. In this study, novel combination approaches and underlying molecular/cellular mechanisms for palbociclib were explored in squamous cell lung cancer (SqCLC), the second most common subtype of non-small cell lung cancer. While approximate 20% lung patients benefit from immunotherapy, most SqCLC patients who receive platinum-doublet chemotherapy as first-line treatment, which often includes a taxane, are still in need of more effective combination therapies. Our results demonstrated enhanced cytotoxicity and anti-tumor effect with palbociclib plus taxanes at clinically achievable doses in multiple SqCLC models with diverse cancer genetic backgrounds. Comprehensive gene expression analysis revealed a sustained disruption of pRB-E2F signaling by combination that was accompanied with enhanced regulation of pleiotropic biological effects. These included several novel mechanisms such as abrogation of G2/M and mitotic spindle assembly checkpoints, as well as impaired induction of hypoxia-inducible factor 1 alpha (HIF-1α). The decrease in HIF-1α modulated a couple key angiogenic and anti-angiogenic factors, resulting in an enhanced anti-angiogenic effect. This preclinical work suggests a new therapeutic opportunity for palbociclib in lung and other cancers currently treated with taxane based chemotherapy as standard of care.

Palbociclib and Fulvestrant Act in Synergy to Modulate Central Carbon Metabolism in Breast Cancer Cells

The aims of this study were to determine whether combination chemotherapeutics exhibit a synergistic effect on breast cancer cell metabolism. Palbociclib, is a selective inhibitor of cyclin-dependent kinases 4 and 6, and when patients are treated in combination with fulvestrant, an estrogen receptor antagonist, they have improved progression-free survival. The mechanisms for this survival advantage are not known. Therefore, we analyzed metabolic and transcriptomic changes in MCF-7 cells following single and combination chemotherapy to determine whether selective metabolic pathways are targeted during these different modes of treatment. Individually, the drugs caused metabolic disruption to the same metabolic pathways, however fulvestrant additionally attenuated the pentose phosphate pathway and the production of important coenzymes. A comprehensive effect was observed when the drugs were applied together, confirming the combinatory therapy’s synergism in the cell model. This study also highlights the power of merging high-dimensional datasets to unravel mechanisms involved in cancer metabolism and therapy.

A versatile drug delivery system targeting senescent cells

Senescent cells accumulate in multiple aging-associated diseases, and eliminating these cells has recently emerged as a promising therapeutic approach. Here, we take advantage of the high lysosomal β-galactosidase activity of senescent cells to design a drug delivery system based on the encapsulation of drugs with galacto-oligosaccharides. We show that gal-encapsulated fluorophores are preferentially released within senescent cells in mice. In a model of chemotherapy-induced senescence, gal-encapsulated cytotoxic drugs target senescent tumor cells and improve tumor xenograft regression in combination with palbociclib. Moreover, in a model of pulmonary fibrosis in mice, gal-encapsulated cytotoxics target senescent cells, reducing collagen deposition and restoring pulmonary function. Finally, gal-encapsulation reduces the toxic side effects of the cytotoxic drugs. Drug delivery into senescent cells opens new diagnostic and therapeutic applications for senescence-associated disorders.

Therapeutic relevance of the PP2A-B55 inhibitory kinase MASTL/Greatwall in breast cancer

PP2A is a major tumor suppressor whose inactivation is frequently found in a wide spectrum of human tumors. In particular, deletion or epigenetic silencing of genes encoding the B55 family of PP2A regulatory subunits is a common feature of breast cancer cells. A key player in the regulation of PP2A/B55 phosphatase complexes is the cell cycle kinase MASTL (also known as Greatwall). During cell division, inhibition of PP2A-B55 by MASTL is required to maintain the mitotic state, whereas inactivation of MASTL and PP2A reactivation is required for mitotic exit. Despite its critical role in cell cycle progression in multiple organisms, its relevance as a therapeutic target in human cancer and its dependence of PP2A activity is mostly unknown. Here we show that MASTL overexpression predicts poor survival and shows prognostic value in breast cancer patients. MASTL knockdown or knockout using RNA interference or CRISPR/Cas9 systems impairs proliferation of a subset of breast cancer cells. The proliferative function of MASTL in these tumor cells requires its kinase activity and the presence of PP2A-B55 complexes. By using a new inducible CRISPR/Cas9 system in breast cancer cells, we show that genetic ablation of MASTL displays a significant therapeutic effect in vivo. All together, these data suggest that the PP2A inhibitory kinase MASTL may have both prognostic and therapeutic value in human breast cancer.

Abstract A08: Mechanistic basis of Palbociclib combinatorial activity in ER+ breast cancer and non-breast indications

Phosphorylation of the retinoblastoma protein (Rb) by cyclin-dependent kinases 4 and 6 (CDK4/6) is a critical checkpoint for G1/S cell cycle progression and commitment to cellular proliferation. Human malignancies often subvert these control mechanisms through a range of genetic and biochemical adaptations. Accordingly, tumors that depend on CDK4/6 activity for proliferation and survival are particularly sensitive to inhibition of this pathway by palbociclib (Ibrance™), a highly selective inhibitor of CDK4/6 kinase activities. Treatment regimen of palbociclib with letrozole significantly improved progression-free survival in a randomized phase 2 study of women with advanced estrogen receptor-positive (ER+), HER2-negative breast cancer. Likewise, in ER+ breast cancer models palbociclib and estrogen antagonists combine for greater anti-proliferative activity, increased hallmarks of cellular senescence and prolonged durability of response following drug removal. Dual inhibition of CDK4/6 and ER signaling demonstrated robust anti-tumor activity in xenograft studies. The addition of Palbociclib to other targeted therapeutics elicits improved activity in pre-clinical models of several non-breast indications and these effects also manifest through modulation of cellular proliferation, senescence and growth arrest. Data will be presented on the molecular basis of combination benefit with Palbociclib in ER+ breast and other oncology indications.

Citation Format: Stephen Dann, John Chionis, Liu Choating, Enhong Chen, Ping Wei, Koleen Eisele, David J. Shields, Paul A. Rejto, Todd VanArsdale. Mechanistic basis of Palbociclib combinatorial activity in ER+ breast cancer and non-breast indications. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr A08.

Abstract A36: Assessment of effectiveness and molecular markers of CDK4/6 inhibitor Palbociclib in Pancreatic Ductal Adenocarcinomas

Pancreatic Ductal Adenocarcinoma (PDAC) continues to be the deadliest human cancer with a 5-year survival rate of 7%. One of the current standards of care for advanced PDAC is gemcitabine plus nab-paclitaxel, a regimen our group helped develop. Finding new agents to combine with this regimen remains necessary, particularly using drug targeting strategies. At the molecular level, genetic and genomic profiling identified CDKN2A as a very frequently disrupted gene in PDAC (<90%). This gene encodes the tumor suppressor p16, a cyclin dependent kinase 4 and 6 (CDK4/6) inhibitor, that controls cell cycle progression. Recently, the selective CDK4/6 inhibitor palbociclib was approved by the FDA for certain types of breast cancer patients. Whether palbociclib can also benefit patients with PDAC is unknown.

In the current study, we aimed to evaluate the effectiveness of palbociclib monotherapy and in combination with nab-paclitaxel and gemcitabine/nab-paclitaxel in a cohort of PDACs patient derived xenograft (PDX) models.

PDX models obtained from late stage PDAC patients were treated with palbociclib alone and in combination with gemcitabine/nab-paclitaxel. The majority of models displayed >50% tumor growth inhibition (TGI) following treatment with single agent palbociclib. Treatment with palbociclib plus nab-paclitaxel or palbociclib plus gemcitabine/nab-paclitaxel increased TGI to a greater degree than the gemcitabine/nab-paclitaxel combination and also increased the duration of response as compared to the standard of care therapy.

Moving forward, the palbociclib/nab-paclitaxel combination will be evaluated in a clinical trial for PDAC patients. We are currently expanding our analyses to a larger cohort of PDX models and performing molecular characterization of these models in order to gain insights on biomarkers and mechanism of action of the drug combinations in PDAC.

Citation Format: Beatriz Salvador, Pedro P. López-Casas, Camino Menéndez, Natalia Baños, Francesca Sarno, Yin Min-Jean, Peter Olson, Todd VanArsdale, David J. Shields, Manuel Hidalgo. Assessment of effectiveness and molecular markers of CDK4/6 inhibitor Palbociclib in Pancreatic Ductal Adenocarcinomas. [abstract]. In: Proceedings of the AACR Special Conference: Patient-Derived Cancer Models: Present and Future Applications from Basic Science to the Clinic; Feb 11-14, 2016; New Orleans, LA. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(16_Suppl):Abstract nr A36.

Abstract 2826: Sustained melanoma regression is achieved with continuous palbociclib and PLX4720 treatment but not with intermittent or sequential dosing

In melanoma the development of resistance to BRAF inhibitors limits clinical responses. Thus the search for novel single agent and combination therapies as well as sequencing strategies that overcome or delay the emergence of resistance is needed. The p16-CDK4- cyclinD-RB1 pathway (CDK4 pathway) is deregulated in approximately 90% of melanomas and most melanoma cell lines are sensitive to palbociclib a specific CDK4/6 inhibitor. We hypothesized that dual targeting the MAPK/ERK and CDK4 pathways would lead to robust inhibition of the CDK4/Cyclin D complex and consequently induce greater tumor regression than single agent treatment. Proliferation and colony formation assays demonstrated that sequential and intermittent treatment with PLX4720 and palbociclib was not as effective as continuous combinational dosing. Only the combination of PLX4720 and palbociclib overcame the development of resistance leading to sustained inhibition of proliferation, cell death and induction of senescence. In A375 and HT144 human tumor xenograft models, palbociclib and PLX4720 initially induced tumor regression and tumor stasis, respectively, however resistance eventually developed to both agents. In contrast, the combination treatment induced rapid and sustained tumor regression. Biomarker studies indicate resistance to single agent PLX4720 was due to reactivation MAPK/ERK pathway and resistance to palbociclib to partial restoration of phosphorylated RB1. Tumor regression in the combination treatment was associated with the infiltration of leukocytes including activated natural killer cells indicating that these cells were involved in tumor clearance. These data demonstrate that dual targeting CDK4/6 and mutant BRAF evades resistance to single agents and leads to sustained tumor regression

Citation Format: Karen E. Sheppard, Carleen Cullinane, Claire Martin, Laura Kirby, Nicole Haynes, Kelly Waldeck, Richard Young, Todd VanArsdale, Grant McArthur. Sustained melanoma regression is achieved with continuous palbociclib and PLX4720 treatment but not with intermittent or sequential dosing. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2826.

Abstract 2740: Mechanistic basis of Palbociclib combinatorial activity in ER+ breast cancer and non-breast indications

Phosphorylation of the retinoblastoma protein (Rb) by cyclin-dependent kinases 4 and 6 (CDK4/6) is a critical checkpoint for G1/S cell cycle progression and commitment to cellular proliferation. Human malignancies often subvert these control mechanisms through a range of genetic and biochemical adaptations. Accordingly, tumors that depend on CDK4/6 activity for proliferation and survival are particularly sensitive to inhibition of this pathway by palbociclib (IbranceTM), a highly selective inhibitor of CDK4/6 kinase activities. Treatment regimen of palbociclib with letrozole significantly improved progression-free survival in a randomized phase 2 study of women with advanced estrogen receptor-positive (ER+), HER2-negative breast cancer. Likewise, in ER+ breast cancer models palbociclib and estrogen antagonists combine for greater anti-proliferative activity, increased hallmarks of cellular senescence and prolonged durability of response following drug removal. Dual inhibition of CDK4/6 and ER signaling demonstrated robust anti-tumor activity in xenograft studies. The addition of Palbociclib to other targeted therapeutics elicits improved activity in pre-clinical models of several non-breast indications and these effects also manifest through modulation of cellular proliferation, senescence and growth arrest. Data will be presented on the molecular basis of combination benefit with Palbociclib in ER+ breast and other oncology indications.

Citation Format: Stephen Dann, Jing Yuan, John Chionis, Chaoting Liu, Tao Xie, Nathan V. Lee, Enhong Chen, Ping Wei, Paul A. Rejto, David J. Shields, Todd VanArsdale. Mechanistic basis of Palbociclib combinatorial activity in ER+ breast cancer and non-breast indications. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2740.

Abstract A43: RB pathway disruption by the CDK4/6 inhibitor palbociclib enhances responses to chemotherapy in squamous cell lung cancer

Lung cancer remains one of the leading causes of cancer-related mortality. Squamous cell lung cancer (SqCLC) is the second most common subtype of non-small cell lung cancer (NSCLC). Despite recent development of effective targeted therapeutic agents for lung adenocarcinoma, patients with SqCLC often receive conventional cytotoxic chemotherapy as this cancer subtype lacks genomic alterations that can be targeted by personalized medicine. Hence, novel approaches that enhance the efficacy of chemotherapy will benefit treatment outcomes in this patient population. CDK inhibitors comprise a class of drugs that targets the dysregulated cell cycle in malignant cells. Treatment of tumor cells with the CDK4/6 inhibitor palbociclib inhibits tumor growth by decreasing retinoblastoma (RB) protein phosphorylation and inducing cell cycle arrest at the G1/S phase transition. Based on promising clinical trial data, palbociclib in combination with letrozole was granted accelerated approval by the US FDA for the treatment of postmenopausal women with ER-positive, HER2-negative advanced breast cancer. In this preclinical study, we explored the effect of palbociclib on several chemotherapies (taxanes, platins, and antimetabolites) in preclinical models of SqCLC. Because the activity of chemotherapy generally requires cell cycle progression, careful combination/sequencing of this class of drugs with CDK inhibitors may be important to achieve synergy as well as avoid potential antagonism. To obtain optimal activity of palbociclib and chemotherapy combinations, we investigated several combination/sequencing regimens (concurrent, chemotherapy followed by palbociclib or the reverse sequence) in several SqCLC cell lines. We did not encounter antagonism of chemotherapy-mediated cytotoxicity by palbociclib in any of the tested regimens. Rather, we observed robust combinatorial anti-cancer cell activity in all settings. Combination of palbociclib with chemotherapy was associated with reduction of RB phosphorylation and FOXM1 protein levels, and the induction of p21. Our studies demonstrated that, while palbociclib partially antagonized chemotherapy-induced apoptosis, it significantly synergized with chemotherapy to induce cell cycle arrest as well as a senescence-like phenotype. Cells pretreated with palbociclib plus cisplatin or palbociclib plus docetaxel displayed less cell growth upon drug removal compared to those treated with monotherapies. Finally, palbociclib treatment that followed docetaxel, nab-paclitaxel or cisplatin treatment significantly enhanced the antitumor activity of the chemotherapies in several cell line-derived or patient-derived xenograft models. Our results suggest that treatment with optimal palbociclib and chemotherapy combination/sequencing could lead to better clinical outcomes for SqCLC patients.

Citation Format: Ping Wei, Joan Cao, Goldie Lui, Hui Wang, Konstantinos Tsaparikos, David Shields, Kim Arndt, Paul Rejto, Todd VanArsdale, James Hardwick. RB pathway disruption by the CDK4/6 inhibitor palbociclib enhances responses to chemotherapy in squamous cell lung cancer. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A43.

Abstract 940: Identification of Palbociclib response signature across indications

Cellular proliferation is dependent on an orderly movement through the various phases of cell cycle. Progression through the G1 phase in particular requires phosphorylation of the retinoblastoma (Rb) protein which in turn releases E2F transcription factors resulting in transcriptional activation of response gene necessary for progression into S-phase. The cyclin D-CDK4/6 signaling pathway represent a critical regulatory pathway controlling transition from G1 into S-phase and greater than 90% of human tumors have mutations in this pathway. Palbociclib is a potent, selective, and orally bioavailable inhibitor of Cdk4/6. In human tumor xenograft models, Palbociclib has significant antitumor activities. The clinical activities of Palbociclib have also be demonstrated in the phase II PALOMA-1 trial for dramatic efficacy of postmenopausal patients with locally advanced or newly diagnosed estrogen receptor (ER)-positive, HER2-negative metastatic breast cancer in combination with letrozole. To better understand the molecular mechanisms of Palbociclib response, we have identified a common set of gene signatures for ER+ BC as well as melanoma models. The physiological role of these genes regulated by Palbociclib is associated with DNA replication and repair, cell cycle, signal transduction, and Mitosis. Many of these genes have been previously identified as E2F signatures. In this study, we are aiming to expand this analysis to include two additional indications including Head and Neck squamous cell carcinoma (HNSCC) as well as squamous cell lung carcinoma (Sq lung) using Illumina RNASeq technology platform. Results from these studies will be presented and core Palbo-response signature will be discussed. In addition, we will also present the pharmacological effects of Letrozole on the gene expression changes for these core signatures in ER+ BC models.

Citation Format: Xianxian Zheng, Mark Ozeck, Zhou Zhu, Keith Ching, David Shields, James Hardwick, Paul Rejto, Todd VanArsdale. Identification of Palbociclib response signature across indications. [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 940. doi:10.1158/1538-7445.AM2015-940

Abstract B05: Targeting BRAF and CDK4 in BRAF mutant melanoma induces sustained tumor regression

Background: The p16-cyclinD-CDK4-RB1 pathway (CDK4 pathway) is deregulated in 90% of melanomas. The main outcome of CDK4 activation is the phosphorylation and thus, inhibition of RB1 leading to G1–S cell-cycle transition. In addition, CDK4 directly phosphorylates other proteins that promote cell-cycle progression and inhibit both cell senescence and apoptosis. The MAPK/ERK pathway is deregulated in 90% of melanoma and enhances CDK4 pathway signaling through increasing cyclin D1 expression. We have previously demonstrated in a panel of melanoma cell lines that the majority of cell lines were sensitive to a CDK4/6 inhibitor (Palbociclib) and consistent with the concept of CDK4 pathway-dependent oncogene addiction, cell lines with loss of functional p16INK4A were significantly more sensitive to Palbociclib than cells with wild-type p16INK4A. We hypothesized that dual targeting the MAPK/ERK and CDK4 pathways would lead to robust inhibition of the CDK4/Cyclin D complex and consequently induce greater tumor regression than single agent treatment.

Results: In vitro, PLX4720, Palbociclib or their combination inhibited BRAF mutant melanoma cell line proliferation and clonogenic survival, however, only with the combination treatment group was a rapid sustained inhibition achieved. Enlarged flattened cells and a significant increase in SA-β-galactosidase staining indicated that the dual treatment resulted in robust cellular senescence. In a BRAFV600E mutant A375 xenograft model, Palbociclib and PLX4720 initially induced tumor regression and tumor stasis respectively, but resistance eventually developed. In contrast, in the combination treatment, tumors regressed and sustained inhibition of tumor growth was maintained. Preliminary biomarker studies indicate resistance to PLX4720 was due to reactivation MAPK/ERK pathway and resistance to Palbociclib to partial restoration of phosphorylated RB1.

Conclusions: Combination therapy of MAPK/ERK pathway inhibitors with CDK4 inhibitors is likely to be an effective treatment for BRAF mutant melanoma and in addition delay resistance to single agent targeted therapy.

Citation Format: Karen E. Sheppard, Cullinane Carleen, Laura Kirby, Kelly Waldeck, Richard Young, Claire Martin, Todd VanArsdale, Sophia Randolph, Grant A. McArthur. Targeting BRAF and CDK4 in BRAF mutant melanoma induces sustained tumor regression. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Melanoma: From Biology to Therapy; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(14 Suppl):Abstract nr B05.

Molecular Pathways: Targeting the Cyclin D-CDK4/6 Axis for Cancer Treatment

Cancer cells bypass normal controls over mitotic cell-cycle progression to achieve a deregulated state of proliferation. The retinoblastoma tumor suppressor protein (pRb) governs a key cell-cycle checkpoint that normally prevents G1-phase cells from entering S-phase in the absence of appropriate mitogenic signals. Cancer cells frequently overcome pRb-dependent growth suppression via constitutive phosphorylation and inactivation of pRb function by cyclin-dependent kinase (CDK) 4 or CDK6 partnered with D-type cyclins. Three selective CDK4/6 inhibitors, palbociclib (Ibrance; Pfizer), ribociclib (Novartis), and abemaciclib (Lilly), are in various stages of development in a variety of pRb-positive tumor types, including breast cancer, melanoma, liposarcoma, and non-small cell lung cancer. The emerging, positive clinical data obtained to date finally validate the two decades-old hypothesis that the cyclin D-CDK4/6 pathway is a rational target for cancer therapy.

Efficacy of SERD/SERM Hybrid-CDK4/6 Inhibitor Combinations in Models of Endocrine Therapy-Resistant Breast Cancer

PURPOSE

Endocrine therapy, using tamoxifen or an aromatase inhibitor, remains first-line therapy for the management of estrogen receptor (ESR1)-positive breast cancer. However, ESR1 mutations or other ligand-independent ESR1 activation mechanisms limit the duration of response. The clinical efficacy of fulvestrant, a selective estrogen receptor downregulator (SERD) that competitively inhibits agonist binding to ESR1 and triggers receptor downregulation, has confirmed that ESR1 frequently remains engaged in endocrine therapy-resistant cancers. We evaluated the activity of a new class of selective estrogen receptor modulators (SERM)/SERD hybrids (SSH) that downregulate ESR1 in relevant models of endocrine-resistant breast cancer. Building on the observation that concurrent inhibition of ESR1 and the cyclin-dependent kinases 4 and 6 (CDK4/6) significantly increased progression-free survival in advanced patients, we explored the activity of different SERD- or SSH-CDK4/6 inhibitor combinations in models of endocrine therapy-resistant ESR1(+) breast cancer.

EXPERIMENTAL DESIGN

SERDs, SSHs, and the CDK4/6 inhibitor palbociclib were evaluated as single agents or in combination in established cellular and animal models of endocrine therapy-resistant ESR1(+) breast cancer.

RESULTS

The combination of palbociclib with a SERD or an SSH was shown to effectively inhibit the growth of MCF7 cell or ESR1-mutant patient-derived tumor xenografts. In tamoxifen-resistant MCF7 xenografts, the palbociclib/SERD or SSH combination resulted in an increased duration of response as compared with either drug alone.

CONCLUSIONS

A SERD- or SSH-palbociclib combination has therapeutic potential in breast tumors resistant to endocrine therapies or those expressing ESR1 mutations. See related commentary by DeMichele and Chodosh, p. 4999.

GLI2 inhibition abrogates human leukemia stem cell dormancy

BACKGROUND

Dormant leukemia stem cells (LSC) promote therapeutic resistance and leukemic progression as a result of unbridled activation of stem cell gene expression programs. Thus, we hypothesized that 1) deregulation of the hedgehog (Hh) stem cell self-renewal and cell cycle regulatory pathway would promote dormant human LSC generation and 2) that PF-04449913, a clinical antagonist of the GLI2 transcriptional activator, smoothened (SMO), would enhance dormant human LSC eradication.

METHODS

To test these postulates, whole transcriptome RNA sequencing (RNA-seq), microarray, qRT-PCR, stromal co-culture, confocal fluorescence microscopic, nanoproteomic, serial transplantation and cell cycle analyses were performed on FACS purified normal, chronic phase (CP) chronic myeloid leukemia (CML), blast crisis (BC) phase CML progenitors with or without PF-04449913 treatment.

RESULTS

Notably, RNA-seq analyses revealed that Hh pathway and cell cycle regulatory gene overexpression correlated with leukemic progression. While lentivirally enforced GLI2 expression enhanced leukemic progenitor dormancy in stromal co-cultures, this was not observed with a mutant GLI2 lacking a transactivation domain, suggesting that GLI2 expression prevented cell cycle transit. Selective SMO inhibition with PF-04449913 in humanized stromal co-cultures and LSC xenografts reduced downstream GLI2 protein and cell cycle regulatory gene expression. Moreover, SMO inhibition enhanced cell cycle transit and sensitized BC LSC to tyrosine kinase inhibition in vivo at doses that spare normal HSC.

CONCLUSION

In summary, while GLI2, forms part of a core HH pathway transcriptional regulatory network that promotes human myeloid leukemic progression and dormant LSC generation, selective inhibition with PF-04449913 reduces the dormant LSC burden thereby providing a strong rationale for clinical trials predicated on SMO inhibition in combination with TKIs or chemotherapeutic agents with the ultimate aim of obviating leukemic therapeutic resistance, persistence and progression.

Resistance to dual blockade of the kinases PI3K and mTOR in KRAS-mutant colorectal cancer models results in combined sensitivity to inhibition of the receptor tyrosine kinase EGFR

Targeted blockade of aberrantly activated signaling pathways is an attractive therapeutic strategy for solid tumors, but drug resistance is common. KRAS is a frequently mutated gene in human cancer but remains a challenging clinical target. Inhibitors against KRAS signaling mediators, namely, PI3K (phosphatidylinositol 3-kinase) and mTOR (mechanistic target of rapamycin), have limited clinical efficacy as single agents in KRAS-mutant colorectal cancer (CRC). We investigated potential bypass mechanisms to PI3K/mTOR inhibition in KRAS-mutant CRC. Using genetically engineered mouse model cells that had acquired resistance to the dual PI3K/mTOR small-molecule inhibitor PF-04691502, we determined with chemical library screens that inhibitors of the ERBB [epidermal growth factor receptor (EGFR)] family restored the sensitivity to PF-04691502. Although EGFR inhibitors alone have limited efficacy in reducing KRAS-mutant tumors, we found that PF-04691502 induced the abundance, phosphorylation, and activity of EGFR, ERBB2, and ERBB3 through activation of FOXO3a (forkhead box O 3a), a transcription factor inhibited by the PI3K to AKT pathway. PF-04691502 also induced a stem cell-like gene expression signature. KRAS-mutant patient-derived xenografts from mice treated with PF-04691502 had a similar gene expression signature and exhibited increased EGFR activation, suggesting that this drug-induced resistance mechanism may occur in patients. Combination therapy with dacomitinib (a pan-ERBB inhibitor) restored sensitivity to PF-04691502 in drug-resistant cells in culture and induced tumor regression in drug-resistant allografts in mice. Our findings suggest that combining PI3K/mTOR and EGFR inhibitors may improve therapeutic outcome in patients with KRAS-mutant CRC.

Combination of 4-1BB agonist and PD-1 antagonist promotes antitumor effector/memory CD8 T cells in a poorly immunogenic tumor model

Immunotherapies targeting the programmed death 1 (PD-1) coinhibitory receptor have shown great promise for a subset of patients with cancer. However, robust and safe combination therapies are still needed to bring the benefit of cancer immunotherapy to broader patient populations. To search for an optimal strategy of combinatorial immunotherapy, we have compared the antitumor activity of the anti-4-1BB/anti-PD-1 combination with that of the anti-PD-1/anti-LAG-3 combination in the poorly immunogenic B16F10 melanoma model. Pronounced tumor inhibition occurred only in animals receiving anti-PD-1 and anti-4-1BB concomitantly, while combining anti-PD-1 with anti-LAG-3 led to a modest degree of tumor suppression. The activity of the anti-4-1BB/anti-PD-1 combination was dependent on IFNγ and CD8(+) T cells. Both 4-1BB and PD-1 proteins were elevated on the surface of CD8(+) T cells by anti-4-1BB/anti-PD-1 cotreatment. In the tumor microenvironment, an effective antitumor immune response was induced as indicated by the increased CD8(+)/Treg ratio and the enrichment of genes such as Cd3e, Cd8a, Ifng, and Eomes. In the spleen, the combination treatment shaped the immune system to an effector/memory phenotype and increased the overall activity of tumor-specific CD8(+) CTLs, reflecting a long-lasting systemic antitumor response. Furthermore, combination treatment in C57BL/6 mice showed no additional safety signals, and only minimally increased severity of the known toxicity relative to 4-1BB agonist alone. Therefore, in the absence of any cancer vaccine, anti-4-1BB/anti-PD-1 combination therapy is sufficient to elicit a robust antitumor effector/memory T-cell response in an aggressive tumor model and is therefore a candidate for combination trials in patients.

Abstract SY34-03: Braking the cycle: Inhibition of the cyclin D-Cdk4/6 pathway in breast cancer

In spite of the dramatically improved outlook for patients with ER+ breast cancer, a subset of patients are intrinsically non-responsive to anti-hormonal therapy alone, and even the responders invariably acquire resistance to this treatment modality. In the setting of established hormone resistance, most of these patients will receive chemotherapy, with limited benefit and considerable impact on quality of life due to toxicity. Given the sheer numbers of patients affected (approximately two-thirds of all breast cancers are ER+), the unmet need for well-tolerated therapies that overcome resistance to anti-hormonal agents remains very high.

Strikingly, the vast majority (>90%) of ER+ breast cancers express a functional retinoblastoma protein (Rb), a tumor suppressor that represents a critical regulator of the G1 to S phase transition in mammalian cells. In its hypo-phosphorylated state, Rb suppresses the expression of proteins that are essential for commitment to S-phase entry and progression through the remainder of the eukaryotic cell cycle. The G1 cyclin-dependent kinases 4 and 6 (cdk4 and cdk6) which function in complexes with the D-type cyclins (collectively termed cyclin D) initiate the phosphorylation of Rb and override the repressive effects of Rb on cell-cycle progression. In ER-driven breast cancers, oncogenic signaling through the ER stimulates the cyclin D-cdk4/6 -dependent phosphorylation of Rb, and this proliferative stimulus is frequently augmented by amplification of cyclin D or loss of expression of the cyclin D-cdk4/6 inhibitor, p16 (encoded by the CDKN2A gene), two alterations that lead to elevated and dysregulated cyclin D-cdk4/6 activity. The requirement for cyclin D-cdk4/6 activity to circumvent the cell-cycle braking action of Rb suggested that ER+ breast cancer cells might be vulnerable to drug-induced inhibition of cdk4/6 activity.

PD-0332991 is an orally administered, selective inhibitor of cyclin D-cdk4/6 kinase activities. In preclinical models, this drug showed significant anti-proliferative activity in Rb+ breast cancer cell lines, the majority of which also expressed the ER. The most well established substrate for the cyclin D-cdk4/6 is Rb, which is modified at multiple phosphoacceptor sites (including Ser-780, Ser-795, Ser-807, and Ser-811) by these cdks. The phosphorylation status of the Ser-807 and Ser-811 residues in Rb proved to be the most robust pharmacodynamic readouts for PD-0332991 activity in vivo, and serve as appropriate biomarkers for target modulation by this drug under in vitro and in vivo study conditions. The IC50 for reduction of Rb phosphorylation at these sites in two breast cancer cell lines (MCF7 and T47D) was approximately 20 nM. Similarly, we observed that PD-0332991 inhibits S-phase entry (measured by incorporation of thymidine into newly synthesized DNA) and cellular proliferation in panel of Rb+ cancer cell lines, with IC50 values ranging from 40-170 nM. The antiproliferative effects of PD-0332991 were not limited to transformed cells, reinforcing the idea that this drug was capable of suppressing the proliferation of any human cell type that requires cyclin D-cdk4/cdk6 activity to bypass the G1 restriction point imposed by Rb. Conversely, cancer cells that have inactivated Rb through genetic or epigenetic mechanisms are highly resistant (IC50 > 3 uM) to the inhibitory effects of PD-0332991 on cell-cycle progression and cellular proliferation.

Xenograft experiments confirmed the favorable in vivo safety profile and antitumor activity of PD-0332991. The maximum tolerated dose (MTD; defined as the dose that induced <10% lethality) in SCID mice was 150 mg/kg/day when administered orally, once per day, for 14 days. Daily dosing at levels equal to or less than this MTD resulted in significant growth inhibition in many but not all Rb+ human tumor xenograft models. These studies in xenograft-bearing mice also confirmed that PD-0332991 was inactive at therapeutically obtainable concentrations against Rb-negative tumors.

These preclinical studies provided a launching pad for a clinical trial in post-menopausal women with locally advanced or metastatic ER+ (HER2-) breast cancer. In this Phase 2 study, PD-0332991 was given in combination with letrozole (an inhibitor of estrogen synthesis), with the control arm receiving letrozole only. An interim analysis showed that the PD-0332991 plus letrozole combination yielded a dramatic improvement in treatment outcomes, more than tripling progression-free survival relative to the letrozole-only treatment arm. The drug was generally well-tolerated, due in part to its high degree of selectivity for cdk4/6 relative to other members of the cyclin-dependent kinase family, as well as other members of the extended kinase superfamily. These positive results led to a Breakthrough Therapy designation in 2013 for PD-0332991 in the treatment of patients with ER+ breast cancer. This presentation will provide information on the mechanisms of antitumor action of PD-0332991, together with updates on the current status of clinical development of this new addition to the armentarium of breast cancer treatments.

Citation Format: Robert T. Abraham, Todd VanArsdale, David V. Shields, Nathan V. Lee, Maria Koehler, Kim Arndt. Braking the cycle: Inhibition of the cyclin D-Cdk4/6 pathway in breast cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr SY34-03. doi:10.1158/1538-7445.AM2014-SY34-03

Abstract LB-136: Mechanistic exploration of combined CDK4/6 and ER inhibition in ER-positive breast cancer

The de-regulation of the Retinoblastoma tumor suppressor is widespread in cancers occurring through the direct mutation or loss of RB1, or enhanced signaling through the Cyclin dependent kinases CDK4 and CDK6 via amplification and/or over expression of D-type cyclins, or loss of p16 (CDKN2A) function. Tumors retaining intact RB1 functions therefore rely on the activity of CyclinD-CDK4/6 complexes to inactivate RB1 and promote progression through the G1 restriction point into S phase. Hormone receptor positive breast cancers are one tumor type where RB1 remains intact in most tumors and deregulation of CDK4/6-Cyclin D signaling is common. As such, dependence on CDK4/6 signaling in ER positive breast cancers has been demonstrated using the specific CDK4/6 inhibitor, PD-0332991 (palbociclib), and the combination of palbociclib and letrozole has been shown to provide significant clinical activity in ER+ breast cancer patients. To determine the mechanism of action for this combination, we investigated the effects of palbociclib with anti-estrogen therapeutics such as letrozole, fulvestrant and tamoxifen on ER+ breast cancer cell lines (MCF7, CAMA1, and T47D). Mechanistic analyses reveal that the combination of palbociclib with fulvestrant or letrozole enhanced inhibition of Rb phosphorylation leading to significantly greater loss of E2F1, FoxM1 and downstream target genes such as PLK1, SKP2 and CCNE2, leading to greater inhibition of cell proliferation. The enhanced growth arrest of the ER+ breast cell lines treated with palbociclib and ER antagonists is accompanied by increased hallmarks of cell senescence, cell enlargement and SA-β-Galactosidase staining, significantly greater than either single agent inhibitor treatment. Also, the arrest of cells following drug removal is maintained significantly longer for cells treated with the combination. To explore these activities in vivo, an ER+ breast patient derived xenograft (PDX) model was studied using the combination of palbociclib and letrozole (aromatase inhibitor). These studies recapitulate in vitro model systems, showing greater tumor growth inhibition with combination therapy, enhanced dephosphorylation of RB1 and accompanied inhibition of downstream signaling. PDX tumors treated with the combination of palbociclib and letrozole displayed reduced KI67 staining compared to single agent treatments and β-galactosidase staining revealed that cell senescence is also a component of the functional response of ER+ breast tumors to the combined inhibition of CDK4/6 and ER signaling in vivo. As apoptotic cell death was not clearly evident in any of these model systems, the mechanism of combined CDK4/6 inhibition and ER antagonism is likely driven by cellular senescence and accompanying long term arrest of tumor cells to prevent disease progression.

Citation Format: Nathan V. Lee, Jing Yuan, Koleen Eisele, Joan Q. Cao, Cory L. Painter, John Chionis, Chaoting Liu, David J. Shields, Julie L.C. Kan, Kim Arndt, Todd VanArsdale. Mechanistic exploration of combined CDK4/6 and ER inhibition in ER-positive breast cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-136. doi:10.1158/1538-7445.AM2014-LB-136

Antitumor Efficacy of the Dual PI3K/mTOR Inhibitor PF-04691502 in a Human Xenograft Tumor Model Derived from Colorectal Cancer Stem Cells Harboring a PIK3CA Mutation

PIK3CA (phosphoinositide-3-kinase, catalytic, alpha polypeptide) mutations can help predict the antitumor activity of phosphatidylinositol-3-kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway inhibitors in both preclinical and clinical settings. In light of the recent discovery of tumor-initiating cancer stem cells (CSCs) in various tumor types, we developed an in vitro CSC model from xenograft tumors established in mice from a colorectal cancer patient tumor in which the CD133+/EpCAM+ population represented tumor-initiating cells. CD133+/EpCAM+ CSCs were enriched under stem cell culture conditions and formed 3-dimensional tumor spheroids. Tumor spheroid cells exhibited CSC properties, including the capability for differentiation and self-renewal, higher tumorigenic potential and chemo-resistance. Genetic analysis using an OncoCarta™ panel revealed a PIK3CA (H1047R) mutation in these cells. Using a dual PI3K/mTOR inhibitor, PF-04691502, we then showed that blockage of the PI3K/mTOR pathway inhibited the in vitro proliferation of CSCs and in vivo xenograft tumor growth with manageable toxicity. Tumor growth inhibition in mice was accompanied by a significant reduction of phosphorylated Akt (pAKT) (S473), a well-established surrogate biomarker of PI3K/mTOR signaling pathway inhibition. Collectively, our data suggest that PF-04691502 exhibits potent anticancer activity in colorectal cancer by targeting both PIK3CA (H1047R) mutant CSCs and their derivatives. These results may assist in the clinical development of PF-04691502 for the treatment of a subpopulation of colorectal cancer patients with poor outcomes.

Development of a colon cancer GEMM-derived orthotopic transplant model for drug discovery and validation

PURPOSE

Effective therapies for KRAS-mutant colorectal cancer (CRC) are a critical unmet clinical need. Previously, we described genetically engineered mouse models (GEMM) for sporadic Kras-mutant and non-mutant CRC suitable for preclinical evaluation of experimental therapeutics. To accelerate drug discovery and validation, we sought to derive low-passage cell lines from GEMM Kras-mutant and wild-type tumors for in vitro screening and transplantation into the native colonic environment of immunocompetent mice for in vivo validation.

EXPERIMENTAL DESIGN

Cell lines were derived from Kras-mutant and non-mutant GEMM tumors under defined media conditions. Growth kinetics, phosphoproteomes, transcriptomes, drug sensitivity, and metabolism were examined. Cell lines were implanted in mice and monitored for in vivo tumor analysis.

RESULTS

Kras-mutant cell lines displayed increased proliferation, mitogen-activated protein kinase signaling, and phosphoinositide-3 kinase signaling. Microarray analysis identified significant overlap with human CRC-related gene signatures, including KRAS-mutant and metastatic CRC. Further analyses revealed enrichment for numerous disease-relevant biologic pathways, including glucose metabolism. Functional assessment in vitro and in vivo validated this finding and highlighted the dependence of Kras-mutant CRC on oncogenic signaling and on aerobic glycolysis.

CONCLUSIONS

We have successfully characterized a novel GEMM-derived orthotopic transplant model of human KRAS-mutant CRC. This approach combines in vitro screening capability using low-passage cell lines that recapitulate human CRC and potential for rapid in vivo validation using cell line-derived tumors that develop in the colonic microenvironment of immunocompetent animals. Taken together, this platform is a clear advancement in preclinical CRC models for comprehensive drug discovery and validation efforts.

Abstract 4619: PF-04449913 reverts multi drug resistance (MDR) by a strong down-regulation of ABCA2 and BCL2 on leukemia stem cells in phase I acute myeloid leukemia and chronic myeloid leukemia treated patients

PF-04449913 is a selective and potent inhibitor of the Hh pathway and leukemia self-renewal and is currently being evaluated in Phase I clinical trials. We studied leukemia stem cell population (CD34+ subpopulation) collected before and after 28 days treatment in a phase I dose escalation protocol (Clinical Trial Gov. NTC00953758) enrolling selected hematological malignancies including MF, MDS, CML, CMML and AML. We collected and separated highly purified (98%) bone marrow hematopoietic progenitor cells (CD34+ populations) in 5 AML, 1 MF and 2 CML patients, by immunomagnetic separation, and analyzed them for gene expression profile (GEP) using Affimetrix HG-U133 Plus 2.0 platform. We have observed that 1197 genes were differentially expressed between CD34+ cells collected before and after 28 days of PF-04449913 dose finding oral therapy. We demonstrated a down regulation of Bcl2 (fold change –1.03004; p value= 0.01), ABCA2 (fold change –1.08966; p value=0.03), Bcl2l13 (fold change –1,04259; p-value=0,027642), Bcl2l2 (fold change –1,17214; p-value=0,000768), Casp4 (fold change –1,06551; p-value=0,032428), Casp7 (fold change –1,01569; p-value=0,006688), Casp10 (fold-change –1,3076; p-value=0,050431), ABCF1 (fold change –1,04999; p-value=0,07213). On the contrary, ABCB1 (fold change 1,46592) and ABCG2 (fold change –1,16103) are respectively up and down regulated, with a not statistically significant p-value. Bcl2 (B-cell lymphoma 2), Bcl2l2 (Bcl2-like protein 2) and Bcl2l13 (Bcl2-like 13) are the founding members of the Bcl-2 family of apoptosis regulator proteins. Recent studies showed that Hh signals upregulate Bcl2 to promote cellular survival. Casp 4,7,10 (Caspases) are a family of cysteine proteases that play essential roles in apoptosis, necrosis, and inflammation. ABCA2 (ATP-binding cassette sub-family A member 2), ABCF1 (ATP-binding cassette sub-family F member 1), ABCB1 (ATP-binding cassette sub-family B member 1, MDR1), ABCG2 (ATP-binding cassette sub-family G member 2) belong to the superfamily of adenosine triphosphate-binding cassette (ABC) transporters. One mechanism of MDR is the increased expression of ABC drug transporters, resulting in low intracellular drug concentrations. We evaluated Gli1 and Smo expression by GEP, comparing data before and after 28 days of treatment with PF-04449913 and we observed a down regulation of both genes (fold changes –1.0775 and –1.07702 respectively). PF-04449913 is able to revert MRD mechanisms of LSC by a strong down regulation of genes (Bcl-2, Bcl-2l13, Bcl-2l2, ABCA2, and ABCF1), which are critical for chemoresistance in acute and chronic leukemia patients. Acknowledgments. Pfizer, European LeukemiaNet, FIRB 2006, AIRC, AIL, COFIN, University of Bologna and BolognAIL.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4619. doi:1538-7445.AM2012-4619

Abstract 906: Gas1 and Kif27 genes are strongly up-regulated biomarkers of Hedgehog inhibition (PF-04449913) on leukemia stem cells in Phase I Acute Myeloid Leukemia and Chronic Myeloid Leukemia treated patients

Hedgehog (Hh) pathway activation contributes to leukemia development and growth, and that targeted pathway inhibition is likely to offer an efficient therapeutic opportunity. PF-04449913, a Hh pathway inhibitor, is a new selective and potent inhibitor of leukemia self-renewal and is currently being evaluated in phase I clinical trials. In order to identify new potential clinical biomarkers for the PF-04449913, we studied CD34+ leukemia stem cell population (LSC) collected before and after 28 days treatment in a phase I dose escalation protocol (Clinical Trial Gov. NTC00953758) enrolling selected hematological malignancies. This experimental clinical trial enrolled Myelofibrosis (MF), MDS, blastic phases CML, chronic myelomonocytic leukemia (CMML) and AML patients (pts). We were able to collect and separate highly purified (98%) bone marrow CD34+ cells from 5 AML, 1 MF and 2 CML pts by immunomagnetic separation, and analysed them for gene expression profile using Affimetrix HG-U133 Plus 2.0 platform. 1197 genes were differentially expressed between CD34+ cells collected before and after 28 days of PF-04449913 dose finding oral therapy. Clustering of their expression profiles showed that mostly genes differentially expressed are mainly related to Hh signaling, this providing further evidences that PF-04449913 really therapeutically targets the Hh pathway. Regarding genes involved in Hh signaling pathway, Gas1 and Kif27 were strongly upregulated (fold change 1.0947 and 1.12757 respectively; p-value 0.01 and 0.02 respectively) in CD34+ LSC after 28 days exposure to PF-04449913 as compared to baseline, suggesting these two genes have potential as new biomarkers of activity. GAS-1 is a Sonic Hedgehog (Shh)-binding protein; it acts to sequester Shh and inhibit the Shh signalling pathway. Kif27 mainly acts as a negative regulator in the Hh signaling pathway, and inhibits the transcriptional activator activity of Gli1 by inhibiting its nuclear translocation. Other genes were differentially expressed after ‘ex- vivo’ treatment with PF-04449913 as compared to baseline: we observed a down regulation of Bcl2 (fold change -1.03004), ABCA2 (fold change -1.08966), LEF1 (fold change -1.28457), Gli1 (fold change -1.0775), Smo (fold change -1.07702), and an upregulation of Gli2 (fold change 1.08191). Conclusions: This data demonstrates that PF-04449913 specifically targets the Hh Pathway in CD34+ cells, suggesting that Hh inhibition may impair leukemia stem cell maintenance. In addition, we identify several new potential biomarkers (e.g. Gas1 and KIF27). Taken together, these data may be useful for pts selection strategies and subsequent eradication of the LSC. Acknowledgments: Work supported by Pfizer, European LeukemiaNet, FIRB 2006, AIRC, AIL, COFIN, University of Bologna and BolognAIL.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 906. doi:1538-7445.AM2012-906

Abstract 974: A selective Notch1 mAb targets leukemia progenitor cells in T-ALL

Introduction: Difficulties in maintaining primary cultures of leukemia cells hampered efforts to investigate the biology of human T cell acute lymphoblastic leukemia (T-ALL) underscoring the need for a direct transplantation model to characterize leukemia progenitor cells (LPC) in vivo and as a paradigm for screening candidate drugs that inhibit self-renewal pathways active in T-ALL. Approximately, 50% of patients with T-ALL harbor NOTCH1 activating mutations that promote therapeutic resistance providing the impetus for developing selective NOTCH1-inhibitory therapeutic strategies. To investigate 1) whether a select subclone of T-ALL cells harbor a greater capacity to propagate disease in vivo than other clones, 2) to establish a humanized T-ALL LPC mouse model and 3) to test whether a selective NOTCH1-NRR/Fc (hN1) mAb inhibits LPC survival and self-renewal.

Experimental Procedures: To facilitate non-invasive in vivo monitoring of leukemic engraftment, lentiviral luciferase transduced LPC were intrahepatically transplanted into neonatal immune deficient mice to establish humanized T-ALL LPC mouse models. These models were treated with hN1 mAb or a control IgG1 mAb at the dose of 10 mg/kg every 4 days for 21 days, and another group was treated with mouse IgG1 isotype control at the same dosing plan. Mice were sacrificed one day after the last dose. Thymus, spleen, liver and bone marrow (BM) were collected and analyzed by FACS. Some BM were sectioned for CD45, NOTCH1 and active Caspase 3 examination by immunohistochemistry.

Results: Human CD34+ enriched cells maintained leukemic engraftment while an equivalent number of Lin+ cells did not. T-ALL CD34+ progenitors from 32 T-ALL LPC models established with NOTCH1 mutated T-ALL have a significant higher engraftment in BM when compared with those from 14 T-ALL LPC models established with Non-NOTCH1 mutated T-ALL. Human CD45+CD34+CD2+ population in serial transplant recipients was more prominent in NOTCH1 mutated samples. Human CD34+ populations were significantly reduced in both BM (p < 0.01, Student t test) and spleen (p < 0.05, Student t test) when the T-ALL LPC treated with hN1 mAb, while CD45+ populations were also significantly reduced in both BM and spleen (p < 0.05, Student t test). NOTCH1 expression level in human CD34+ cells in NOTCH1 mutated T-ALL was markedly reduced after hN1 mAb treatment when compared with IgG1 mAb treatment. NOTCH1 and CD45 positive cells were significantly reduced, while apoptosis was remarkably increased in the BM treated with therapeutic hN1 mAb when compared with those treated with IgG1 mAb. Intracellular domain of Notch1 was significantly reduced in the BM treated with hN1 mAb when compared with those treated with IgG1 mAb.

Conclusions:

1. Human T-ALL LPC have enhanced NOTCH1 expression.

2. Human self-renewing T-ALL LPC are enriched in the CD45+CD34+CD2+ population.

3. A selective hN1 mAb inhibits human T-ALL LPC survival and self-renewal in vivo.

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 974. doi:10.1158/1538-7445.AM2011-974

PF-03732010: a fully human monoclonal antibody against P-cadherin with antitumor and antimetastatic activity

PURPOSE

P-cadherin is a membrane glycoprotein that functionally mediates tumor cell adhesion, proliferation, and invasiveness. We characterized the biological properties of PF-03732010, a human monoclonal antibody against P-cadherin, in cell-based assays and tumor models.

EXPERIMENTAL DESIGN

The affinity, selectivity, and cellular inhibitory activity of PF-03732010 were tested in vitro. Multiple orthotopic and metastatic tumor models were used for assessing the antitumor and antimetastatic activities of PF-03732010. Treatment-associated pharmacodynamic changes were also investigated.

RESULTS

PF-03732010 selectively inhibits P-cadherin-mediated cell adhesion and aggregation in vitro. In the P-cadherin-overexpressing tumor models, including MDA-MB-231-CDH3, 4T1-CDH3, MDA-MB-435HAL-CDH3, HCT116, H1650, PC3M-CDH3, and DU145, PF-03732010 inhibited the growth of primary tumors and metastatic progression, as determined by bioluminescence imaging. Computed tomography imaging, H&E stain, and quantitative PCR analysis confirmed the antimetastatic activity of PF-03732010. In contrast, PF-03732010 did not show antitumor and antimetastatic efficacy in the counterpart tumor models exhibiting low P-cadherin expression. Mechanistic studies via immunofluorescence, immunohistochemical analyses, and 3'-[(18)F]fluoro-3'-deoxythymidine-positron emission tomography imaging revealed that PF-03732010 suppressed P-cadherin levels, caused degradation of membrane β-catenin, and concurrently suppressed cytoplasmic vimentin, resulting in diminished metastatic capacity. Changes in the levels of Ki67, caspase-3, and 3'-[(18)F]fluoro-3'-deoxythymidine tracer uptake also indicated antiproliferative activity and increased apoptosis in the tested xenografts.

CONCLUSIONS

These findings suggest that interrupting the P-cadherin signaling pathway may be a novel therapeutic approach for cancer therapy. PF-03732010 is presently undergoing evaluation in Phase 1 clinical trials.

Abstract 4483: Establishing patient-derived colorectal cancer stem cell models with a PIK3CA mutation for the development of inhibitory drugs as targeted therapies

Emerging evidences suggest that cancer stem cells (CSC) may be critically responsible for tumor initiation, progression, metastasis, and drug resistance. It becomes important to ask whether anti-cancer agents are able to target the tumor-initiating subpopulation in relevant CSC models. In this study, we first established xenograft tumors in NOD/SCID mice from a colorectal cancer patient specimen and demonstrated that CD133/EpCAM-expressing CSC population was highly tumorigenic. We then sought to propagate the CSC population under a serum-free condition. In culture, the tumor cells formed non-adherent, 3-dimensional spheroids, a fraction of which retained expression of the CSC markers. When exposed to a serum-containing medium, tumor spheroid cells differentiated into epithelial-like adherent cells with an increase in cell proliferation rate. In comparison with the differentiated progeny, tumor spheroid cells exhibited resistance to the standard-of-care agent oxaliplatin and, in limiting dilution assays in mice, displayed substantially higher tumorigenic potential. In contrast to the tumors originated from the differentiated cells, tumor spheroid cell-derived tumors recapitulated not only the CSC frequency marked by CD133/EpCAM expression, but also the histological characters of the original tumor. Similarly, only were the fragments of spheroid cell-derived xenograft tumors capable of regenerating highly proliferative tumors in secondary transplantation. Thus, the tumor spheroid culture is indeed enriched of drug resistant, self-renewing, and tumor-initiating CSC populations. Mutation profiling of frequently mutated oncogenes using Sequenom OncoCarta™ panel identified a mutation in the kinase domain of PIK3CA (H1047R) in the cultured CSCs. This mutation has been reported present in a large number of colon cancer patients and likely functions as an oncogene (Samuels et al., Science 304:554; 2004). We further demonstrated that a dual mammalian target of rapamycin (mTOR)/phosphoinositide 3-kinase (PI3K) inhibitor (PF-04691502) exhibited a more potent effect on inhibition of in vitro proliferation of the mutated CSCs compared to the chemotoxic agent oxaliplatin. Collectively, our findings suggest that CSC models provide a novel avenue to drug sensitivity and efficacy studies. The well-characterized CSC model systems may assist in the development of more effective therapy against the subpopulation of tumors driven by the CSCs bearing specific mutations.

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 4483.

TRAF family proteins interact with the common neurotrophin receptor and modulate apoptosis induction

The common neurotrophin receptor, p75(NTR), has been shown to signal in the absence of Trk tyrosine kinase receptors, including induction of neural apoptosis and activation of NF-kappaB. However, the mechanisms by which p75(NTR) initiates these intracellular signal transduction pathways are unknown. Here we report interactions between p75(NTR) and the six members of TRAF (tumor necrosis factor receptor-associated factors) family proteins. The binding of different TRAF proteins to p75(NTR) was mapped to distinct regions in p75(NTR). Furthermore, TRAF4 interacted with dimeric p75(NTR), whereas TRAF2 interacted preferentially with monomeric p75(NTR). TRAF2-p75(NTR), TRAF4-p75(NTR), and TRAF6-p75(NTR) interactions modulated p75(NTR)-induced cell death and NF-kappaB activation with contrasting effects. Coexpression of TRAF2 with p75(NTR) enhanced cell death, whereas coexpression of TRAF6 was cytoprotective. Furthermore, overexpression of TRAF4 abrogated the ability of dimerization to prevent the induction of apoptosis normally mediated by monomeric p75(NTR). TRAF4 also inhibited the NF-kappaB response, whereas TRAF2 and TRAF6 enhanced p75(NTR)-induced NF-kappaB activation. These results demonstrate that TRAF family proteins interact with p75(NTR) and differentially modulate its NF-kappaB activation and cell death induction.

Immunohistochemical analysis of in vivo patterns of TRAF-3 expression, a member of the TNF receptor-associated factor family

An immunohistochemical approach was used to explore the in vivo expression of TNF receptor-associated factor 3 (TRAF-3), a putative signaling protein that binds to the cytosolic domains of CD30, CD40, and lymphotoxin-beta receptors. TRAF-3 immunostaining was detected in many types of cells throughout the human body. TRAF-3 immunostaining was only rarely present in thymocytes but was found in the thymic epithelioreticular cells. Lymphocytes in the bone marrow were also typically TRAF-3 immunonegative, whereas myeloid progenitor cells and megakaryocytes were often TRAF-3 positive. Peripheral blood lymphocytes were mostly TRAF-3 immunonegative, while granulocytes were TRAF-3 immunopositive. Monocytes were strongly immunostained for TRAF-3, but macrophages in nodes typically contained little or no TRAF-3 immunoreactivity. Some lymphocytes within the germinal centers of secondary lymphoid follicles in normal and reactive nodes were TRAF-3 immunopositive, as were occasional interfollicular lymphocytes in the T cell regions of these organs, but most lymphocytes appeared to be TRAF-3 immunonegative or stained only weakly. Plasma cells, however, were strongly TRAF-3 positive. Stimulation of PBLs with anti-CD3 Ab induced marked increases in the steady state levels of TRAF-3 protein in vitro as determined by immunoblotting, while levels of TRAF-2 were unchanged, implying a dynamic regulation of TRAF-3 expression. The findings establish for the first time the cell type- and differentiation-specific patterns of expression of a member of the TRAF family of proteins.

Immunohistochemical analysis of in vivo patterns of TRAF-3 expression, a member of the TNF receptor-associated factor family

An immunohistochemical approach was used to explore the in vivo expression of TNF receptor-associated factor 3 (TRAF-3), a putative signaling protein that binds to the cytosolic domains of CD30, CD40, and lymphotoxin-beta receptors. TRAF-3 immunostaining was detected in many types of cells throughout the human body. TRAF-3 immunostaining was only rarely present in thymocytes but was found in the thymic epithelioreticular cells. Lymphocytes in the bone marrow were also typically TRAF-3 immunonegative, whereas myeloid progenitor cells and megakaryocytes were often TRAF-3 positive. Peripheral blood lymphocytes were mostly TRAF-3 immunonegative, while granulocytes were TRAF-3 immunopositive. Monocytes were strongly immunostained for TRAF-3, but macrophages in nodes typically contained little or no TRAF-3 immunoreactivity. Some lymphocytes within the germinal centers of secondary lymphoid follicles in normal and reactive nodes were TRAF-3 immunopositive, as were occasional interfollicular lymphocytes in the T cell regions of these organs, but most lymphocytes appeared to be TRAF-3 immunonegative or stained only weakly. Plasma cells, however, were strongly TRAF-3 positive. Stimulation of PBLs with anti-CD3 Ab induced marked increases in the steady state levels of TRAF-3 protein in vitro as determined by immunoblotting, while levels of TRAF-2 were unchanged, implying a dynamic regulation of TRAF-3 expression. The findings establish for the first time the cell type- and differentiation-specific patterns of expression of a member of the TRAF family of proteins.

Hepatitis C virus core protein interacts with the cytoplasmic tail of lymphotoxin-beta receptor

Hepatitis C virus (HCV) core protein is a multifunctional protein. We examined whether it can interact with cellular proteins, thus contributing to viral pathogenesis. Using the HCV core protein as a bait to screen a human liver cDNA library in a yeast two-hybrid screening system, we have isolated several positive clones encoding cellular proteins that interact with the HCV core protein. Interestingly, more than half of these clones encode the cytoplasmic domain of lymphotoxin-beta receptor (LT betaR), which is a member of the tumor necrosis factor receptor family. Their binding was confirmed by in vitro glutathione S-transferase fusion protein binding assay and protein-protein blotting assay to be direct and specific. The binding sites were mapped within a 58-amino-acid region of the cytoplasmic tail of LT betaR. The binding site in the HCV core protein was localized within amino acid residues 36 to 91 from the N terminus, corresponding to the hydrophilic region of the protein. In mammalian cells, the core protein was found to be associated with the membrane-bound LT betaR. Since the LT betaR is involved in germinal center formation and developmental regulation of peripheral lymphoid organs, lymph node development, and apoptotic signaling, the binding of HCV core protein to LT betaR suggests the possibility that this viral protein has an immunomodulating function and may explain the mechanism of viral persistence and pathogenesis of HCV.

The Epstein-Barr virus transforming protein LMP1 engages signaling proteins for the tumor necrosis factor receptor family

The cytoplasmic C-terminus of Epstein-Barr virus (EBV) latent infection membrane protein 1 (LMP1) is essential for B lymphocyte growth transformation and is now shown to interact with a novel human protein (LMP1-associated protein 1 [LAP1]). LAP1 is homologous to a murine protein, tumor necrosis factor receptor-associated factor 2 (TRAF2), implicated in growth signaling from the p80 TNFR. A second novel protein (EBI6), induced by EBV infection, is the human homolog of a second murine TNFR-associated protein (TRAF1). LMP1 expression causes LAP1 and EBI6 to localize to LMP1 clusters in lymphoblast plasma membranes, and LMP1 coimmunoprecipitates with these proteins. LAP1 binds to the p80 TNFR, CD40, and the lymphotoxin-beta receptor, while EBI6 associates with the p80 TNFR. The interaction of LMP1 with these TNFR family-associated proteins is further evidence for their role in signaling and links LMP1-mediated transformation to signal transduction from the TNFR family.