Discovery of HC-7366: An Orally Bioavailable and Efficacious GCN2 Kinase Activator

A series of activators of GCN2 (general control nonderepressible 2) kinase have been developed, leading to HC-7366, which has entered the clinic as an antitumor therapy. Optimization resulted in improved permeability compared to that of the original indazole hinge binding scaffold, while maintaining potency at GCN2 and selectivity over PERK (protein kinase RNA-like endoplasmic reticulum kinase). The improved ADME properties of this series led to robust in vivo compound exposure in both rats and mice, allowing HC-7366 to be dosed in xenograft models, demonstrating that activation of the GCN2 pathway by this compound leads to tumor growth inhibition.

PERK Inhibition by HC-5404 Sensitizes Renal Cell Carcinoma Tumor Models to Antiangiogenic Tyrosine Kinase Inhibitors

PURPOSE

Tumors activate protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK, also called EIF2AK3) in response to hypoxia and nutrient deprivation as a stress-mitigation strategy. Here, we tested the hypothesis that inhibiting PERK with HC-5404 enhances the antitumor efficacy of standard-of-care VEGF receptor tyrosine kinase inhibitors (VEGFR-TKI).

EXPERIMENTAL DESIGN

HC-5404 was characterized as a potent and selective PERK inhibitor, with favorable in vivo properties. Multiple renal cell carcinoma (RCC) tumor models were then cotreated with both HC-5404 and VEGFR-TKI in vivo, measuring tumor volume across time and evaluating tumor response by protein analysis and IHC.

RESULTS

VEGFR-TKI including axitinib, cabozantinib, lenvatinib, and sunitinib induce PERK activation in 786-O RCC xenografts. Cotreatment with HC-5404 inhibited PERK in tumors and significantly increased antitumor effects of VEGFR-TKI across multiple RCC models, resulting in tumor stasis or regression. Analysis of tumor sections revealed that HC-5404 enhanced the antiangiogenic effects of axitinib and lenvatinib by inhibiting both new vasculature and mature tumor blood vessels. Xenografts that progress on axitinib monotherapy remain sensitive to the combination treatment, resulting in ∼20% tumor regression in the combination group. When tested across a panel of 18 RCC patient-derived xenograft (PDX) models, the combination induced greater antitumor effects relative to monotherapies. In this single animal study, nine out of 18 models responded with ≥50% tumor regression from baseline in the combination group.

CONCLUSIONS

By disrupting an adaptive stress response evoked by VEGFR-TKI, HC-5404 presents a clinical opportunity to improve the antitumor effects of well-established standard-of-care therapies in RCC.

Netrin-1 feedforward mechanism promotes pancreatic cancer liver metastasis via hepatic stellate cell activation, retinoid, and ELF3 signaling

The biology of metastatic pancreatic ductal adenocarcinoma (PDAC) is distinct from that of the primary tumor due to changes in cell plasticity governed by a distinct transcriptome. Therapeutic strategies that target this distinct biology are needed. We detect an upregulation of the neuronal axon guidance molecule Netrin-1 in PDAC liver metastases that signals through its dependence receptor (DR), uncoordinated-5b (Unc5b), to facilitate metastasis in vitro and in vivo. The mechanism of Netrin-1 induction involves a feedforward loop whereby Netrin-1 on the surface of PDAC-secreted extracellular vesicles prepares the metastatic niche by inducing hepatic stellate cell activation and retinoic acid secretion that in turn upregulates Netrin-1 in disseminated tumor cells via RAR/RXR and Elf3 signaling. While this mechanism promotes PDAC liver metastasis, it also identifies a therapeutic vulnerability, as it can be targeted using anti-Netrin-1 therapy to inhibit metastasis using the Unc5b DR cell death mechanism.

Abstract 4010: Inhibition of PERK by HC-5404 sensitizes clear cell renal cell carcinoma tumor models to anti-angiogenic tyrosine kinase inhibitors

Anti-angiogenic agents form the backbone of standard of care for advanced clear cell renal cell carcinoma (ccRCC), but their clinical impact is limited by primary and secondary resistance mechanisms that remain a critical problem. Furthermore, the approvals for VEGFR-targeting receptor tyrosine kinase inhibitors (VEGFR-TKIs), cabozantinib in second-line, and tivozanib in third-line RCC patients were based on modest objective response rates and median progression-free survival. There is an urgent need for novel mechanisms that target adaptive tumor responses that drive resistance to these agents, as well as combination drug partners that improve outcomes for patients.

As part of their mechanism, VEGFR-TKIs induce oxygen- and nutrient-deprivation that drives ER stress. Tumors can evade deleterious ER stress by activating PERK branch of the integrated stress response, which arrests global translation and restores homeostasis. We hypothesized that inhibiting PERK would enhance the anti-tumor activity of VEGFR-TKIs in vivo and tested this using HC-5404, a potent and selective PERK inhibitor currently in Ph1 clinical testing (NCT04834778). Here, we present preclinical evidence that supports combining HC-5404 with VEGFR-TKIs in ccRCC. We demonstrate that axitinib, cabozantinib, lenvatinib, and sunitinib all activate PERK in 786-O ccRCC xenografts in a dose-responsive manner. The addition of HC-5404 significantly enhanced the tumor growth inhibition (TGI) of VEGFR-TKIs across multiple ccRCC tumor models, resulting in tumor stasis or regression in combination groups. Expression profiling and IHC analysis of tumor sections revealed that HC-5404 enhanced the anti-angiogenic effects of axitinib and lenvatinib in 786-O tumors, highlighting the protective role of PERK in response to anti-angiogenics.

To evaluate whether the combination treatments could benefit a diverse patient population, sensitivity to HC-5404 and axitinib was evaluated across a panel of patient-derived xenograft (PDX) models. This experiment confirmed widespread responsiveness to the combination treatment that in some cases achieved >50% tumor regression. As tumor progression on VEGFR-TKIs limits the success of these agents in the clinic, we evaluated the effect of adding HC-5404 to tumors that have previously progressed on axitinib. In this study, 786-O xenografts were treated with axitinib for 2-weeks and non-responders were rerandomized into groups of either single agent or combination of HC-5404 and axitinib. The combination treatment significantly improved TGI relative to either monotherapy, resulting in tumor regression of ~20%. Taken together, these findings highlight that by disrupting an adaptive stress response evoked by VEGFR-TKIs, HC-5404 presents a clinical opportunity to enhance the anti-tumor effects of well-established standard of care therapies in ccRCC.

Citation Format: Michael E. Stokes, Veronica Calvo, Crissy Dudgeon, Sho Fujisawa, Sharon Huang, Leyi Shen, Nupur Ballal, Joe McGinley, David Liu, Mark J. Mulvihill, Alan C. Rigby, Nandita Bose, Eric S. Lightcap, David Surguladze. Inhibition of PERK by HC-5404 sensitizes clear cell renal cell carcinoma tumor models to anti-angiogenic tyrosine kinase inhibitors. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4010.

Abstract 6231: Activation of GCN2 by HC-7366 results in significant antitumor efficacy as monotherapy and in combination with multiple standard of care agents in various solid cancer models

The integrated stress response (ISR) is an adaptive signaling pathway that cells utilize to respond to a wide range of extrinsic and intrinsic stresses, which are important for tumorigenesis. Activation of ISR is suggested to play a dual role in cell fate decisions. While the ISR promotes survival, prolonged activation of ISR induces apoptosis. We are developing HC-7366, a first-in-class, first-in-human GCN2 activator, and are currently evaluating it in a phase 1 clinical trial in solid tumors (NCT05121948). In this study, we present the characterization of the antitumor effects of HC-7366 in solid tumors.

In vivo efficacy studies using HC-7366 montherapy showed significant tumor growth inhibition (TGI%) in preclinical cancer models of colorectal (78-95%), head and neck (33% regression), sarcoma (80%) and prostate (65%). HC-7366 activated the ISR in tumors from treated mice as evidenced by induction of the ATF4 target genes ASNS and PSAT1. Additionally, HC-7366 induced the proapoptotic protein PUMA and reduced HIF1⍺ and HIF2⍺ levels. Furthermore, HC-7366 showed significant benefit in colorectal models when combined with DC101 (anti-VEGFR2 antibody), 5-fluorouracil (chemotherapy), alpelisib (PI3Kα inhibitor), or trametinib (MEK1/2 inhibitor). Using GCN2 CRISPR-knockout cells, we confirmed that the HC-7366 mediated reduction of cell growth and induction of ISR markers was dependent on GCN2. We performed multi-omics analyses to further understand the mechanism of action. Metabolomics analysis of tumors treated with HC-7366 revealed that HC-7366 altered several metabolites involved in amino acid metabolism, oxidative stress, the urea cycle, and pyrimidine biosynthesis. Additionally, proteomics analysis showed that HC-7366 significantly reduced proteins involved in oxidative phosphorylation. Analysis of the transcriptome in tumors from treated mice demonstrated that HC-7366 reduced the activity of HIF and E2F1-driven transcription, including expression of metaphase-anaphase transition genes, consistent with decreased Ki67 staining in tumors. ATF4 and JUN transcriptional activity was enhanced with HC-7366 treatment consistent with activation of ISR. Collectively, our in vitro and in vivo results demonstrate that HC-7366 is a potent GCN2 activator with strong antitumor activity across multiple solid tumor models as a monotherapy or in combination with standard of care agents.

Citation Format: Feven Tameire, Paulina Wojnarowicz, Crissy Dudgeon, Sho Fujisawa, Sharon Huang, Owen B. Reilly, Nicholas Collette, Jeremy Drees, Kathryn Bieging-Rolett, Takashi O. Kangas, Weiyu Zhang, Maria Fumagalli, Iman Dewji, Yunfang Li, Anissa SH Chan, Xiaohong Qiu, Ben Harrison, Ashley LaCayo, Ricardo A. Cordova, Kirk A. Staschke, Alan C. Rigby, Savithri Ramurthy, Eric S. Lightcap, David Surguladze, Nandita Bose. Activation of GCN2 by HC-7366 results in significant antitumor efficacy as monotherapy and in combination with multiple standard of care agents in various solid cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6231.

Abstract 1287: DEC2, a circadian rhythm transcription factor, promotes growth, metastasis and dormancy by facilitating immune cell invasion through decreasing cell surface expression of MHC-1 in a novel model of pancreatic cancer dormancy

Latent recurrence following curative-intent pancreatic cancer surgery is a major clinical problem thought to be due to the reactivation of dormant tumor cells that disseminate before the primary tumor has been removed. How dormancy is established and ultimately reversed to drive recurrence is poorly understood. Here we introduce a novel mouse model of pancreatic cancer dormancy in which we establish distal pancreatic tumors by orthotopic injection, resect them four weeks later and then follow the mice for recurrence. We observed recurrence patterns and survival outcomes that mimic human patients undergoing surgery for pancreatic cancer where two thirds of the mice succumb to early metastatic recurrence (median survival 26 days) and one third of the mice (called dormant mice) live substantially long (median survival 554 days) without clinic evidence of disease yet harbor disseminated tumor cells in most organs of the body. Disseminated tumor cells isolated from the livers of dormant mice were quiescent, exhibited stem cell properties and upregulated the expression of Dec2, a circadian rhythm gene. Overexpression of Dec2 induced quiescence suggesting that it may be a driver of the dormant phenotype. Curiously, endogenous Dec2 actually increased the growth and metastasis of nondormant tumors. Dec2WT and Dec2 knockout cell lines were used to generate resectable orthotopic tumors in immune-competent mice, and loss of Dec2 decreased tumor growth, dissemination and metastasis, and the median survival of mice with DecWT tumors was 25 days whereas 70% of the mice with the otherwise isogenic Dec2KOs were still alive after 200 days. Notably, these differences in tumor size, dissemination, metastasis and survival were abrogated in nude mice, indicating an immune mediated mechanism. Dec2WT and Dec2KO cell lines expressed the same total amount of the antigen-presenting protein MHC-1, but much more of the MHC-1 was present on the surface of Dec2KO cell lines than on the Dec2WT cell lines. MHC-1 requires peptide loading before transport to the cell surface, and we see evidence for reduced production of small peptides in the DecKO cells. For instance the half-life of luciferase, a very potent immunogen in mice, was reduced in the Dec2KO cells, potentially providing small peptides for MHC-1 biosynthesis. Protein turnover may be upregulated to compensate for increases in translation, as ribosomal genes are more highly expressed in Dec2KO cells, including many c-Myc pathway genes. Indeed, Western blot analysis revealed upregulation of c-Myc protein in Dec2KO cells. These data suggest that Dec2 plays a novel role in the immune system by decreasing MHC-1 cell surface expression through decreasing peptide generation, protein translation and turnover.

Citation Format: Chris R. Harris, Crissy Dudgeon, Orjola Prela, Lan Wang, Anthony Casabianca, Juliana Cazarin de Menezes, Christina Davidson, Paula Vertino, Brian Altman, Darren Carpizo. DEC2, a circadian rhythm transcription factor, promotes growth, metastasis and dormancy by facilitating immune cell invasion through decreasing cell surface expression of MHC-1 in a novel model of pancreatic cancer dormancy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1287.

Abstract B021: The axon guidance receptor UNC5B is a driver of pancreatic cancer metastasis

Axon guidance pathways have often been linked to tumorigenesis by informatics analyses, but their roles in tumorigenesis are not well understood at a functional level. Here we show that axon guidance receptor UNC5B and its ligand, NTN1, promote the metastasis of pancreatic ductal adenocarcinomas (PDACs) by reducing anoikis and by promoting a mesenchymal phenotype. In vivo, removal of two floxed UNC5B alleles eliminated PDAC metastasis in the genetically engineered KPC mouse model, and also reduced metastasis by xenografted PDAC cell lines. PDAC metastasis also decreased upon treatment of animals with NP137, an antibody that targets NTN1. In vitro, deletion of UNC5B from pancreatic adenocarcinoma cell lines reduced invasiveness, and was sufficient to convert PDAC cell lines from an epithelial to a mesenchymal state. UNC5B also increased YAP1 expression but decreased anoikis; anoikis could then be restored by treatment with the YAP1 inhibitor CA3. Clinically, UNC5B expression associated with poor outcomes in patients with pancreatic adenocarcinomas. UNC5B and NTN1 expression increased upon truncation of the SWI/SNF component ARID1A, which is recurrently mutated in PDACs. Highly focal amplifications of UNC5B were also observed in some patient samples, and were mutually exclusive with mutations in ARID1A or ARID1B. NP137 is currently in clinical trials for various cancers, and may benefit pancreatic adenocarcinoma patients, particularly those with UNC5B amplifications or with ARID1A/ARID1B mutations.

Citation Format: Chris Harris, Anthony Casabianca, Zach Sechrist, Cory Shields, Wade Narrow, Tracy Withers, Crissy Dudgeon, Mike O'Dell, Aram Hezel, Darren Carpizo. The axon guidance receptor UNC5B is a driver of pancreatic cancer metastasis [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr B021.

Abstract B020: Retinoic acid produced by hepatic stellate cells facilitates Netrin-1 mediated pancreatic cancer metastasis

A hallmark of pancreatic ductal adenocarcinoma (PDAC) is its proclivity for metastasis as evidenced by the fact that 85% are stage IV at diagnosis. This highlights the need to better understand the biology of metastatic PDAC and identify novel therapies for this patient population. Axon guidance genes have been shown to be involved in PDAC progression, but their role is unclear. We have investigated the role of the axon guidance molecule Netrin-1 and its receptors Unc5b and DCC in PDAC. We found that in both murine and human samples that NTN1 expression is increased in metastatic PDAC and the quasi-mesenchymal subtype. Murine and TCGA data indicate that Unc5b is the dominant NTN1 receptor and genetic knock-down (KD) or knock-out (KO) of either Netrin-1 or Unc5b decreases migration, invasion, and cell survival in vitro and hepatic metastatic growth in vivo. The mechanism of Netrin-1 upregulation in metastatic PDAC is unknown. We found that hepatic stellate cell (HSC) secreted retinoic acid upregulates NTN1 through both an RXR/RAR and Elf mediated mechanism. To determine if NTN1 is involved in the process of HSC activation we found that recombinant NTN1 added to HSCs in vitro induced activation. We examined the livers of mice harboring orthotopic PDAC tumors using murine pancreatic cancer lines that were either NTN1 wild type (WT) or KO, and found that the NTN-expressing lines increased HSC activation providing evidence that NTN1 is important for long distance intercellular communication between primary pancreatic tumors and the pre-metastatic liver. We detected NTN1 within extracellular vesicles, and mice pre-conditioned with EVs from NTN1 KO cells demonstrated a decreased metastatic burden as compared mice preconditioned with NTN1 WT cells. Treatment of several murine PDAC models (autochthonous and metastatic) with a monoclonal antibody to NTN1 led to decreased metastases and increased survival. These studies reveal that NTN1 is upregulated in metastatic PDAC mediated by a novel mechanism that involves EVs, HSC activation and RXR/RAR signaling. These studies provide pre-clinical evidence to support a human clinical trial of anti-NTN1 therapy in PDAC.

Citation Format: Crissy Dudgeon, Anthony Casabianca, Chris Harris, Igor Astsaturov, Charline Ogier, Xiaoyang Su, Jason Pitarresi, Wade Narrow, Fady Soliman, Tracy Withers, Patrick Mehlen, Darren Carpizo. Retinoic acid produced by hepatic stellate cells facilitates Netrin-1 mediated pancreatic cancer metastasis [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr B020.

Abstract 916: Dec2, a circadian rhythm gene is necessary for disseminated tumor cell survival in a novel murine model of pancreatic cancer dormancy

Pancreatic ductal adenocarcinoma (PDAC) is a highly lethal malignancy with a proclivity for early metastasis. Resection remains the only chance for long term survival though most patients die from metastatic recurrence. While most patients recur within the first two years, many develop recurrence much later indicating they harbor dormant disseminated tumor cells (dDTCs). Currently there are no murine models which recapitulate this disease pattern and as a result pancreatic cancer dormancy is poorly understood. We sought to better understand PDAC dormancy using a novel murine model of human pancreatic cancer from which we investigate the genetic and molecular contributions to dormancy. A model of resected PDAC was developed using murine PDAC cells expressing luciferase/mCherry orthotopically implanted into FVB hosts. Primary tumors are resected after 4 weeks and mice followed for recurrence. dDTCs’ are harvested from the livers of mice without evidence of recurrence by fluorescence activated cell sorting (FACS) and subjected to transcriptomic profiling using single cell RNA sequencing. Dec2’s contribution to dormancy was evaluated by overexpression and knockout using CRISPR. Overall survival in the mouse model mirrored 41,552 stage I, II PDAC patients from the National Cancer Database with similar frequency and location of recurrence. One third of mice exhibited latent recurrence with median survival of 568 days before succumbing to recurrence. These mice were used to study dDTCs. Transcriptomic profiling of FACS sorted dDTCs revealed a distinct transcriptome from primary tumors and early recurrences, indicating cell plasticity. dDTCs exhibited decreased proliferation markers and upregulated genes involved in immune modulation, cell stemness, linoleic acid (LA) metabolism, and Dec2. dDTCs were resistant to chemotherapy. An in vitro model of dormancy was constructed in which LA treatment inhibited cell proliferation and increased Dec2. Dec2 overexpression resulted in cell quiescence, and knockout increased apoptosis with LA exposure. Dec2-KO cells implanted in the murine model dramatically improved survival compared to Dec-WT cells (Dec2-WT median survival 30 days, Dec2-KO median survival was not reached, p=0.013). Livers of mice in the resection model where Dec2-KO cells were used had decreased dDTC burden with fewer mice showing detectable luciferase gDNA (p<0.01) indicating that loss of Dec2 reduced survival of dDTC’s. This is the first murine model of pancreatic cancer dormancy that recapitulates outcomes of resected PDAC patients. These studies reveal pancreatic cancer dormancy is characterized by a distinct, plastic cellular state characterized by chemotherapy resistance. Dec2 appears required for dormancy, its absence leads to disseminated tumor cell apoptosis. This suggests Dec2 may be a therapeutic target in pancreatic cancer dormancy.

Citation Format: Anthony S. Casabianca, Chris Harris, Crissy Dudgeon, Darren R. Carpizo. Dec2, a circadian rhythm gene is necessary for disseminated tumor cell survival in a novel murine model of pancreatic cancer dormancy [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 916.

Abstract 121: A genetic signature of cancer dormancy: implications for human disease derived from a novel murine model

Pancreatic cancer (PC) is a highly lethal malignancy characterized by local tumor aggressiveness and early metastatic dissemination. While surgical resection is used to treat localized cancers, the majority of patients do recur suggesting the presence of disseminated tumor cells (DTC) at the time of surgery. These DTC's represent minimal residual disease. While most patients recur early, a subset recur late due to the reactivation of dormant tumor cells. Currently, the mechanisms of cancer dormancy are poorly understood in part due to a lack of animal models that reflect human disease. Here we describe a murine model of PC dormancy that mimics outcomes in resected human patients. Using single-cell transcriptomics and an assay for chromatin accessibility, we found dormancy is a distinct cellular state from pre or post-dormant cancer cells. Mechanisms of dormancy include increased expression of transcriptional repressor Dec2, which functionally drives quiescence, and monoallelic suppression of mutant KRAS by DNA methylation. Pathway analysis of dormant tumor cell transcriptomics indicated an up-regulation of enzymes involved in linoleic acid metabolism among others. Linoleic acid treatment of tumor cells resulted in inhibited cell proliferation, decreased tumor growth, and an increase in Dec2 expression. We have identified a dormancy gene signature that is distinct from non-dormant PC cells and found that this correlates with human resected patients with long term survivals, and inversely in patients with short term survivals. We demonstrate that dormancy is characterized by large scale transcriptomic changes and global chromatin remodeling. We developed methods for isolating DTCs from the livers of early stage PC patients undergoing resection. Using these methods we have found that the murine dormancy signature correlated with the gene expression of the DTCs and not primary tumor cells indicating this dormancy signature has human relevance with potential novel therapeutic avenues.

Citation Format: Anthony S. Casabianca, Crissy Dudgeon, Chris Harris, Subhajyoto De, Mihir Shah, Arthur Roberts, Eric Collisson, Vinod Balachandran, Darren Carpizo. A genetic signature of cancer dormancy: implications for human disease derived from a novel murine model [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 121.

U1 Adaptors Suppress the KRAS-MYC Oncogenic Axis in Human Pancreatic Cancer Xenografts

Targeting KRAS and MYC has been a tremendous challenge in cancer drug development. Genetic studies in mouse models have validated the efficacy of silencing expression of both KRAS and MYC in mutant KRAS-driven tumors. We investigated the therapeutic potential of a new oligonucleotide-mediated gene silencing technology (U1 Adaptor) targeting KRAS and MYC in pancreatic cancer. Nanoparticles in complex with anti-KRAS U1 Adaptors (U1-KRAS) showed remarkable inhibition of KRAS in different human pancreatic cancer cell lines in vitro and in vivo As a nanoparticle-free approach is far easier to develop into a drug, we refined the formulation of U1 Adaptors by conjugating them to tumor-targeting peptides (iRGD and cRGD). Peptides coupled to fluorescently tagged U1 Adaptors showed selective tumor localization in vivo Efficacy experiments in pancreatic cancer xenograft models showed highly potent (>90%) antitumor activity of both iRGD and (cRGD)₂-KRAS Adaptors. U1 Adaptors targeting MYC inhibited pancreatic cancer cell proliferation caused by apoptosis in vitro (40%-70%) and tumor regressions in vivo Comparison of iRGD-conjugated U1 KRAS and U1 MYC Adaptors in vivo revealed a significantly greater degree of cleaved caspase-3 staining and decreased Ki67 staining as compared with controls. There was no significant difference in efficacy between the U1 KRAS and U1 MYC Adaptor groups. Our results validate the value in targeting both KRAS and MYC in pancreatic cancer therapeutics and provide evidence that the U1 Adaptor technology can be successfully translated using a nanoparticle-free delivery system to target two undruggable genes in cancer. Mol Cancer Ther; 16(8); 1445-55. ©2017 AACR.

Abstract LB-351: The expression landscape of pancreatic cancer recurrence following resection

Pancreatic ductal adenocarcinoma (PDAC) is one of the most aggressive human malignancies with a 5-year survival rate for all stages of only 6%. While surgery remains the most effective treatment for pancreatic cancer, the majority of patients that undergo resection go on to die of pancreatic cancer recurrence. Pancreatic cancer undergoes early stage metastasis; wherein tumorigenic cells disseminate prior to the formation of invasive PDAC during the PanIN stage. The cellular and molecular mechanisms governing the processes of pancreatic cancer metastasis and recurrence following resection are poorly understood in part because there are no animal models of pancreatic cancer recurrence. To address this, we have adapted the Syngraft model, an orthotopic syngeneic mouse model using a pancreatic tumor cell line (Ink4a.1 luc/mcherry) derived from the genetically engineered conditional model (KrasG12D/+; Ink4a flox/flox) that has been modified with lentiviral vectors expressing mCherry and luciferase. Immunoproficient FVB mice were orthotopically injected with 100 Ink4a.1 cells and primary tumor growth was monitored by luciferase activity using the IVIS Spectrum. Mice underwent distal pancreatectomy with spleenectomy when the primary pancreatic tumor measured ∼107 photons/sec/cm2, with a mean time to surgical resection at 19 days. Metastatic recurrence to the liver (65%) was followed using the IVIS Spectrum, with a time to metastatic recurrence of 22 +/- 13 days. After 17 pancreatectomies, we observed the following recurrence patterns: 1) liver only (2/17), 2) liver and peritoneal (9/17), and 3) no recurrence (6/17) with a median follow time of 436 days. To investigate the molecular mechanisms of recurrence, we generated four metastatic cell lines from intrahepatic macrometastases. These metastatic cell lines and the control parental Ink4a.1 cell line were subjected to poly A RNA sequencing. Following data analysis by Tophat and Cufflinks, up and down regulation was determined by a 2-fold change in expression. We identified a total of 56 downregulated and 94 upregulated genes. Basp1, Vgll3, Cldn1, Gjb3, Gas6, Itm2a, and Tle3, were found to be downregulated while 26 genes, including Add2, Col6a3, Dpysl3, Ntn1, Selp, and Serpinf1, were upregulated in all four metastatic cell lines. Validation by Western blotting confirmed the upregulation of Netrin1 (Ntn1) as a strong candidate for the regulation of metastasis. Because the recurrence patterns following pancreatectomy recapitulate the human pancreatic cancer, we conclude the Syngraft model can be successfully adapted to study pancreatic cancer recurrence. Future work in the lab will characterize the function(s) of Netrin1 that contribute to the processes of pancreatic cancer metastasis.

Citation Format: Crissy Dudgeon, Jeffrey Rosenfeld, Matthew Habel, Eric Collisson, Darren Carpizo. The expression landscape of pancreatic cancer recurrence following resection. [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 LB-351.

Abstract B14: The expression landscape of pancreatic cancer recurrence following resection

This abstract is being presented as a short talk in the scientific program. A full abstract is printed in the Proffered Abstracts section (PR13) of the Conference Proceedings.

Citation Format: Crissy Dudgeon, Jeffrey Rosenfeld, Eric Collisson, Darren Carpizo. The expression landscape of pancreatic cancer recurrence following resection. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr B14.

Abstract PR13: The expression landscape of pancreatic cancer recurrence following resection

Pancreatic ductal adenocarcinoma (PDAC) is one of the most aggressive human malignancies with a 5-year survival rate for all stages of only 6%. While surgery remains the most effective treatment for pancreatic cancer, the majority of patients that undergo resection go on to die of pancreatic cancer recurrence. Pancreatic cancer undergoes early stage metastasis; wherein tumorigenic cells disseminate prior to the formation of invasive PDAC during the PanIN stage. The cellular and molecular mechanisms governing the processes of pancreatic cancer metastasis and recurrence following resection are poorly understood in part because there are no animal models of pancreatic cancer recurrence. To address this, we have adapted the Syngraft model, an orthotopic syngeneic mouse model using a pancreatic tumor cell line (Ink4a.1 luc/mcherry) derived from the genetically engineered conditional model (KrasG12D/+; Ink4aflox/flox) that has been modified with lentiviral vectors expressing mCherry and luciferase. Immunocompetent FVB mice were orthotopically injected with 100 Ink4a.1 cells and primary tumor growth was monitored by luciferase activity using the IVIS Spectrum. Mice underwent distal pancreatectomy with spleenectomy when the primary pancreatic tumor measured ~107 photons/sec/cm2, with a mean time to surgical resection at 19 days. Metastatic recurrence to the liver (65%) was followed using the IVIS Spectrum, with a time to metastatic recurrence of 22 +/- 13 days. After 17 pancreatectomies, we observed the following recurrence patterns: 1) liver only (2/17), 2) liver and peritoneal (9/17), and 3) no recurrence (6/17) with a median follow time of 370 days. To investigate the molecular mechanisms of recurrence, we generated four metastatic cell lines from intrahepatic macrometastases. These metastatic cell lines and the control parental Ink4a.1 cell line were subjected to poly A RNA sequencing. Following data analysis by Tophat and Cufflinks, up and down regulation was determined by a 2-fold change in expression. We identified a total of 45 down regulated and 95 up regulated genes. Furthermore, only 1 gene, Basp1, was found to be down regulated while 6 genes, Add2, Col6a3, Npnt, Ntn1, Selp, and Serpinf1, were up regulated in all four metastatic cell lines. Validation by Western blotting confirmed the upregulation of Netrin1 (Ntn1) as a strong candidate for the regulation of metastasis. Because the recurrence patterns following pancreatectomy recapitulate what is seen in human pancreatic cancer, we conclude the Syngraft model can be successfully adapted to study pancreatic cancer recurrence. Future work in the lab will characterize the function(s) of Netrin1 that contribute to the processes of pancreatic cancer metastasis.

Citation Format: Crissy Dudgeon, Jeffrey Rosenfeld, Eric Collisson, Darren Carpizo. The expression landscape of pancreatic cancer recurrence following resection. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr PR13.

The Evolution of Tumors in Mice and Humans with Germline p53 Mutations

Mice with a homozygous p53 gene deletion develop thymic lymphomas by 9 wk of age. Using the sequence of the rearranged T-cell receptor gene from each clone of cells in the thymus, one can determine the number of independent transformation events. These tumors are oligoclonal, occurring at a frequency of 0.13-0.8 new cancer clones per day. By 20 wk only a few clones are detected, indicating competition among transformed cell clones. DNA sequencing of these tumors demonstrates a point mutation frequency of one per megabase and many genes that are consistently amplified or deleted in independent tumors. The tumors begin with an inherited p53 gene deletion. Next is a PTEN mutation in a stem cell or progenitor cell, before the rearrangement of the T-cell receptor. After that, the T-cell clone selects gene amplifications in cyclin D and cdk-6, and in Ikaros in the Notch pathway. Humans heterozygous for the p53 mutant gene in the germline (Li-Fraumeni syndrome) develop cancers at an early age. The penetrance of heterozygous p53 mutations is ∼93% of individuals developing tumors over their lives. At older ages the remaining 7% of this Li-Fraumeni population actually have a lower risk of developing tumors than the population at large with wild-type p53 genes.

Abstract 1441: Adapting the Syngraft mouse model to study metastatic recurrence of pancreatic cancer

Pancreatic cancer is one of the most aggressive human malignancies with a 5-year survival rate for all stages of only 6%. While surgery remains the most effective treatment for pancreatic cancer, the majority of patients that undergo resection go on to die of pancreatic cancer recurrence. The biology of pancreatic recurrence is poorly understood due to the lack of sufficient mouse models to study this aspect of the disease. Our laboratory has adapted the Syngraft mouse model in order to study metastatic recurrence of pancreatic cancer. The Syngraft model is an orthotopic syngeneic mouse model using a pancreatic tumor cell line derived from the genetically engineered model (KrasG12D/+; Ink4a flox/flox) that has been modified with lentiviral vectors expressing mCherry and luciferase. After orthotopic injection, tumor formation was detected with the IVIS Spectrum using injected luciferin. Mice underwent distal pancreatectomy with spleenectomy when the primary pancreatic tumor measured ∼107 photons/sec/cm2, with a mean time to surgical resection at 19 days. Mice were then followed for metastatic recurrence to the liver using the IVIS Spectrum, with a mean time to recurrence of 23 days. We were able to establish a rate of metastatic recurrence in the Syngraft mouse model of 41% to the liver. Research is ongoing to enhance the metastatic efficiency of the injected cell lines and to further characterize the model for evidence of single cell and micrometastases in the liver. This model holds potential to be a useful tool to study the cell and molecular biology of pancreatic cancer recurrence.

Citation Format: Crissy Dudgeon, Ashley Tsang, Neil Campbell, Eric Collisson, Darren Carpizo. Adapting the Syngraft mouse model to study metastatic recurrence of pancreatic cancer. [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 1441. doi:10.1158/1538-7445.AM2015-1441

Pancreatic cancer modeling using retrograde viral vector delivery and in vivo CRISPR/Cas9-mediated somatic genome editing

Pancreatic ductal adenocarcinoma (PDAC) is a genomically diverse, prevalent, and almost invariably fatal malignancy. Although conventional genetically engineered mouse models of human PDAC have been instrumental in understanding pancreatic cancer development, these models are much too labor-intensive, expensive, and slow to perform the extensive molecular analyses needed to adequately understand this disease. Here we demonstrate that retrograde pancreatic ductal injection of either adenoviral-Cre or lentiviral-Cre vectors allows titratable initiation of pancreatic neoplasias that progress into invasive and metastatic PDAC. To enable in vivo CRISPR/Cas9-mediated gene inactivation in the pancreas, we generated a Cre-regulated Cas9 allele and lentiviral vectors that express Cre and a single-guide RNA. CRISPR-mediated targeting of Lkb1 in combination with oncogenic Kras expression led to selection for inactivating genomic alterations, absence of Lkb1 protein, and rapid tumor growth that phenocopied Cre-mediated genetic deletion of Lkb1. This method will transform our ability to rapidly interrogate gene function during the development of this recalcitrant cancer.

Abstract LB-80: Exome sequencing of murine p53-knockout T-cell lymphomas reveals Pten loss and Cdk6 amplification as key steps toward cellular transformation

Loss of the tumor suppressor, p53, occurs in over 50% of cancers. The ability for p53 to protect the cell from tumorigenesis is based on its ability to induce cell cycle arrest, apoptosis, and senescence following genotoxic stress. Furthermore, genetic deletion of p53 in mice gives rise to spontaneous thymic (T-cell) lymphomas and sarcomas. In this study, we used the p53-knockout (KO) mouse as a model for progression of tumorigenesis. We first analyzed genomic DNA of p53-KO and WT normal thymus at various ages and p53-KO thymic lymphomas using TCRβ sequencing, allowing us to determine the clonality of the T cell repertoire. These data suggest that p53-KO thymic lymphomas were mostly oligoclonal, with some tumors comprising of a dominant clone. We further analyzed tumors with dominant clones for genomic instability by exome sequencing. Sequencing analysis revealed that there were many non-synonymous mutations occurring in each tumor, but with no similarities between tumors. On the other hand, all of the murine tumors had deletion of Pten and amplification of Cdk6. This is in stark contrast to what is seen in most human T cell lymphoblastic leukemias, which have activating mutations in Notch1 and loss of Arf. Interestingly, loss of Pten not only occurred in all tumors, but also in most, if not all, clones in the tumor. This suggests that Pten loss is an early event driving T cell transformation in a p53-KO background. Using this data, we found the somatic mutation rate in the tumors to be 1000-fold greater than the germline mutation rate. Loss of p53 also caused chromosomal abnormalities as seen with increased copy number variations. Taken together, our work suggests that, with initial loss of p53, the path to T cell tumorigenesis is vastly different from that of human tumors. Without p53, the somatic genome increases in mutation rate and copy number variations, thus solidifying p53's role as guardian of the genome.

Citation Format: Crissy Dudgeon, Chang Chan, Yvonne Sun, Arnold J. Levine. Exome sequencing of murine p53-knockout T-cell lymphomas reveals Pten loss and Cdk6 amplification as key steps toward cellular transformation. [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-80. doi:10.1158/1538-7445.AM2014-LB-80

Aurora kinase inhibition induces PUMA via NF-κB to kill colon cancer cells

Aurora kinases play a key role in mitosis and are frequently overexpressed in a variety of tumor cells. Inhibition of aurora kinases results in mitotic arrest and death of cancer cells, and has been explored as an anticancer strategy. However, how aurora inhibition kills cancer cells is poorly understood. In this study, we found that inhibition of aurora kinases by siRNA or small-molecule inhibitors led to induction of p53 upregulated modulator of apoptosis (PUMA), a BH3-only Bcl-2 family protein, in colorectal cancer cells irrespective of p53 status. Deficiency in PUMA increased polyploidy, improved cell survival, and abrogated mitochondria-mediated apoptosis induced by aurora kinase inhibitors. In response to aurora kinase inhibition, PUMA was directly activated by p65 through the canonical NF-κB pathway following AKT inhibition. Furthermore, PUMA was necessary for the chemosensitization and in vivo antitumor effects of aurora kinase inhibitors in colon cancer cells. These results suggest that PUMA induction mediates the apoptotic response to mitotic arrest imposed by aurora kinase inhibition, and may be a useful indicator for the anticancer activity of aurora kinase inhibitors.

Genetic variants and mutations of PPM1D control the response to DNA damage

The Wip1 phosphatase is an oncogene that is overexpressed in a variety of primary human cancers. We were interested in identifying genetic variants that could change Wip1 activity. We identified 3 missense SNPs of the human Wip1 phosphatase, L120F, P322Q, and I496V confer a dominant-negative phenotype. On the other hand, in primary human cancers, PPM1D mutations commonly result in a gain-of-function phenotype, leading us to identify a hot-spot truncating mutation at position 525. Surprisingly, we also found a significant number of loss-of-function mutations of PPM1D in primary human cancers, both in the phosphatase domain and in the C terminus. Thus, PPM1D has evolved to generate genetic variants with lower activity, potentially providing a better fitness for the organism through suppression of multiple diseases. In cancer, however, the situation is more complex, and the presence of both activating and inhibiting mutations requires further investigation to understand their contribution to tumorigenesis.

Abstract 4650: Inhibiting oncogenic signaling by sorafenib activates PUMA via GSK3β and NF-κB to suppress tumor cell growth

Aberrant Ras/Raf/MEK/ERK signaling is one of the most prevalent oncogenic alterations and confers survival advantage to tumor cells. Inhibition of this pathway can effectively suppress tumor cell growth. For example, sorafenib, a multi-kinase inhibitor targeting B-Raf and other oncogenic kinases, has been used clinically for treating advanced liver and kidney tumors, and also has shown efficacy against other malignancies. However, how inhibition of oncogenic signaling by sorafenib and other drugs suppresses tumor cell growth remains unclear. In this study, we found that sorafenib kills cancer cells by activating PUMA, a p53 target and a BH3-only Bcl-2 family protein. Sorafenib treatment induces PUMA in a variety of cancer cells irrespective of their p53 status. Surprisingly, the induction of PUMA by sorafenib is mediated by IαB-independent activation of NF-κB, which directly binds to the PUMA promoter to activate its transcription. NF-κB activation by sorafenib requires GSK3β activation, subsequent to ERK inhibition. Deficiency in PUMA abrogates sorafenib-induced apoptosis and caspase activation, and renders sorafenib resistance in colony formation and xenograft tumor assays. Furthermore, the chemosensitization effect of sorafenib is dependent on PUMA, and involves concurrent PUMA induction through different pathways. Enhanced PUMA expression or BH3 mimetics potentiate the anticancer effects of sorafenib, and restore sorafenib sensitivity in resistant cells. We also developed a high-throughput screening assay for identifying anticancer compounds based on the activation of PUMA by sorafenib. Together, these results demonstrate a key role of PUMA-dependent apoptosis in therapeutic inhibition of Ras/Raf/MEK/ERK signaling. They provide a rationale for manipulating the apoptotic machinery to improve sensitivity and overcome resistance to the therapies that target oncogenic kinase signaling.

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 4650. doi:1538-7445.AM2012-4650

Abstract 860: The multi-targeted kinase inhibitor Sunitinib induces apoptosis in colon cancer cells via PUMA

Key words: Sunitinib; PUMA; FoxO3a; Bcl-2 family; colon cancer Purpose: Sunitinib is a FDA approved multi-targeted receptor tyrosine kinase (RTK) inhibitor for treating solid tumors. Inhibition of angiogenesis, immune modulation and induction of apoptosis has been suggested to mediate the anti-tumor effects of sunitinib. However, its cell killing mechanisms are not well-understood. In this study, we investigated the mechanisms and significance of sunitinib-mediated apoptosis in its antitumor activities using colon cancer cells and xenografts. Experimental Design: The expression of various Bcl-2 family proteins, apoptosis and long-term growth in response to sunitinib were examined in colon cancer cells. The mechanism of PUMA induction and effects of PUMA deficiency, BH3 mimetics and 5-FU on sunitinib-induced apoptosis were investigated. The effects of PUMA deficiency in responses to sunitinib were determined in xenografts. Results: We found that consistent PUMA induction mediated by transcription factor FoxO3a plays an important role in the apoptosis induced by sunitinib. PUMA deficiency led to impaired apoptosis and caspase activation in colon cancer cells with either wildtype or mutant p53. Small molecule BH3 mimetics or elevated PUMA levels sensitized colon cancer cells to sunitinib-induced apoptosis. PUMA deficiency significantly blocked apoptosis and therapeutic responses to sunitinib in xenografts. Conclusions: Our results establish a critical role of PUMA in mediating sunitinib sensitivity in colon cancer cells and lack of apoptosis induction as a potential resistance mechanism. Our study suggests modulations of Bcl-2 family proteins as potential biomarkers, and combination of sunitinib with BH3 mimetics or chemotherapeutic agents might improve the treatment efficacy in colon cancer patients.

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 860. doi:1538-7445.AM2012-860

Inhibiting oncogenic signaling by sorafenib activates PUMA via GSK3β and NF-κB to suppress tumor cell growth

Aberrant Ras/Raf/MEK/ERK signaling is one of the most prevalent oncogenic alterations and confers survival advantage to tumor cells. Inhibition of this pathway can effectively suppress tumor cell growth. For example, sorafenib, a multi-kinase inhibitor targeting c-Raf and other oncogenic kinases, has been used clinically for treating advanced liver and kidney tumors, and also has shown efficacy against other malignancies. However, how inhibition of oncogenic signaling by sorafenib and other drugs suppresses tumor cell growth remains unclear. In this study, we found that sorafenib kills cancer cells by activating PUMA, a p53 target and a BH3-only Bcl-2 family protein. Sorafenib treatment induces PUMA in a variety of cancer cells irrespective of their p53 status. Surprisingly, the induction of PUMA by sorafenib is mediated by IκB-independent activation of NF-κB, which directly binds to the PUMA promoter to activate its transcription. NF-κB activation by sorafenib requires GSK3β activation, subsequent to ERK inhibition. Deficiency in PUMA abrogates sorafenib-induced apoptosis and caspase activation, and renders sorafenib resistance in colony formation and xenograft tumor assays. Furthermore, the chemosensitization effect of sorafenib is dependent on PUMA, and involves concurrent PUMA induction through different pathways. BH3 mimetics potentiate the anticancer effects of sorafenib, and restore sorafenib sensitivity in resistant cells. Together, these results demonstrate a key role of PUMA-dependent apoptosis in therapeutic inhibition of Ras/Raf/MEK/ERK signaling. They provide a rationale for manipulating the apoptotic machinery to improve sensitivity and overcome resistance to the therapies that target oncogenic kinase signaling.

The multi-targeted kinase inhibitor sunitinib induces apoptosis in colon cancer cells via PUMA

Constitutive activation of pro-survival kinases has become a promising target of small molecules with an increasing interest in developing multi-targeted agents. The mechanisms underlying the responsiveness to most agents targeting cancer specific survival pathways are still poorly understood but critical for their clinical application. In this study, we found that sunitinib, a small molecule inhibitor of multiple tyrosine kinases including VEGFRs and PDGFRs induces apoptosis and inhibits cell growth in colon cancer cells in cell culture and xenograft models via the BH3-only protein PUMA. Sunitinib treatment induced PUMA transcription via the AKT/FoxO3a axis. PUMA, BH3 mimetics, or 5-Flurourical sensitized colon cancer cells to sunitinib-induced apoptosis. Furthermore, PUMA was induced by sunitinib treatment in xenograft tumors, and deficiency in PUMA significantly suppressed the anti-tumor effects of sunitinib. Our study suggests that PUMA-mediated apoptosis is important for the therapeutic responses to sunitinib, and activation of the mitochondrial pathway by BH3 mimetics or PUMA manipulation may be useful for improving the antitumor activity of sunitinib. Modulation of PUMA and selective Bcl-2 family members might be potential biomarkers for predicting sunitinib responses.

PUMA induction by FoxO3a mediates the anticancer activities of the broad-range kinase inhibitor UCN-01

Most targeted anticancer drugs are inhibitors of kinases that are aberrantly activated in cancer cells. However, the mechanisms by which kinase inhibitors suppress tumor growth remain unclear. In this study, we found that UCN-01, a staurosporine analogue and broad-range kinase inhibitor used in clinical trials, inhibits colon cancer cell growth by inducing apoptosis via PUMA, a BH3-only Bcl-2 family member and a p53 target. PUMA expression was markedly elevated in a p53-independent fashion following UCN-01 treatment. The induction of PUMA by UCN-01 was mediated by direct binding of FoxO3a to the PUMA promoter following inhibition of AKT signaling. Deficiency in PUMA abrogated UCN-01-induced apoptosis, caspase activation, and mitochondrial dysfunction, and rendered UCN-01 resistance in a clonogenic assay, whereas elevated PUMA expression or a BH3 mimetic sensitized UCN-01 induced apoptosis. Chemosensitization by UCN-01 seemed to involve simultaneous PUMA induction through both p53-dependent and p53-independent mechanisms. Furthermore, deficiency in PUMA suppressed the antitumor effects of UCN-01 in a xenograft model, concurrent with reduced apoptosis and caspase activation in vivo. These results suggest that PUMA-mediated apoptosis is pivotal for the anticancer activities of UCN-01, and possibly other clinically used kinase inhibitor drugs, and that PUMA manipulation may be useful for improving their anticancer activities.

Wip1 phosphatase modulates ATM-dependent signaling pathways

Deletion of Ppm1d, the gene encoding the Wip1 phosphatase, renders cells resistant to transformation and mice resistant to tumor development. Here, we report that deficiency of Wip1 resulted in activation of the ataxia-telangiectasia mutated (ATM) kinase. In turn, overexpression of Wip1 was sufficient to reduce activation of the ATM-dependent signaling cascade after DNA damage. Wip1 dephosphorylated ATM Ser1981, a site critical for ATM monomerization and activation, and was critical for resetting ATM phosphorylation as cells repaired damaged DNA. We propose that the Wip1 phosphatase is an integral component of an ATM-dependent signaling pathway.

Tumor susceptibility and apoptosis defect in a mouse strain expressing a human p53 transgene

Activation of apoptosis is believed to be critical for the role of p53 as a tumor suppressor. Here, we report a new mouse strain carrying a human p53 transgene in the mouse p53-null background. Expression of human p53 in these mice was comparable with wild-type murine p53; however, transactivation, induction of apoptosis, and G(1)-S checkpoint, but not transrepression or regulation of a centrosomal checkpoint, were deregulated. Although multiple functions of p53 were abrogated, mice carrying the human p53 transgene did not show early onset of tumors as typically seen for p53-null mice. In contrast, human p53 in the p53-null background did not prevent accelerated tumor development after genotoxic or oncogenic stress. Such behavior of human p53 expressed at physiologic levels in transgenic cells could be explained by unexpectedly high binding with Mdm2. By using Nutlin-3a, an inhibitor of the interaction between Mdm2 and p53, we were able to partially reconstitute p53 transactivation and apoptosis in transgenic cells. Our findings indicate that the interaction between p53 and Mdm2 controls p53 transcriptional activity in homeostatic tissues and regulates DNA damage- and oncogene-induced, but not spontaneous, tumorigenesis.