Oncogenic ras alleles are among the most common mutations found in patients with acute myeloid leukemia (AML). Previously, the role of oncogenic ras in cancer was assessed in model systems overexpressing oncogenic ras from heterologous promoters. However, there is increasing evidence that subtle differences in gene dosage and regulation of gene expression from endogenous promoters play critical roles in cancer pathogenesis. We characterized the role of oncogenic K-ras expressed from its endogenous promoter in the hematopoietic system using a conditional allele and IFN-inducible, Cre-mediated recombination. Mice developed a completely penetrant myeloproliferative syndrome characterized by leukocytosis with normal maturation of myeloid lineage cells; myeloid hyperplasia in bone marrow; and extramedullary hematopoiesis in the spleen and liver. Flow cytometry confirmed the myeloproliferative phenotype. Genotypic and Western blot analysis demonstrated Cre-mediated excision and expression, respectively, of the oncogenic K-ras allele. Bone marrow cells formed growth factor–independent colonies in methylcellulose cultures, but the myeloproliferative disease was not transplantable into secondary recipients. Thus, oncogenic K-ras induces a myeloproliferative disorder but not AML, indicating that additional mutations are required for AML development. This model system will be useful for assessing the contribution of cooperating mutations in AML and testing ras inhibitors in vivo.
Iris T. Chan, Jeffery L. Kutok, Ifor R. Williams, Sarah Cohen, Lauren Kelly, Hirokazu Shigematsu, Leisa Johnson, Koichi Akashi, David A. Tuveson, Tyler Jacks, D. Gary Gilliland
Avicins are proapoptotic and anti-inflammatory triterpene electrophiles isolated from an Australian desert tree, Acacia victoriae. The presence of two α,β unsaturated carbonyl groups (Michael reaction sites) in the side chain of the avicin molecule prompted us to study its effects on NF-E2–related factor 2 (Nrf2), a redox-regulated transcription factor that controls the expression of a battery of detoxification and antioxidant proteins via its binding to antioxidant response element (ARE). Avicin D–treated Hep G2 cells showed translocation of Nrf2 into the nucleus and a time-dependent increase in ARE activity. These properties were sensitive to DTT, suggesting that avicins affect one or more critical cysteine residues, probably on the Keap1 molecule. Downstream of ARE, an activation of a battery of stress-induced proteins occurred. The implications of these findings were evaluated in vivo in mouse skin exposed to an ancient stressor, UV light. Avicins inhibited epidermal hyperplasia, reduced p53 mutation, enhanced apoptosis, decreased generation of 8-hydroxy-2′-deoxyguanosine, and enhanced expression of NADPH:quinone oxidoreductase 1 and heme oxygenase-1. These data, combined with our earlier published work, demonstrate that avicins represent a new class of plant stress metabolites capable of activating stress adaptation and suppressing proinflammatory components of the innate immune system in human cells by redox regulation. The relevance for treatment of clinical diseases in which stress responses are dysfunctional or deficient is discussed.
Valsala Haridas, Margaret Hanausek, Goshi Nishimura, Holly Soehnge, Amos Gaikwad, Maciej Narog, Erick Spears, Robert Zoltaszek, Zbigniew Walaszek, Jordan U. Gutterman
Malignant cells often display defects in autophagy, an evolutionarily conserved pathway for degrading long-lived proteins and cytoplasmic organelles. However, as yet, there is no genetic evidence for a role of autophagy genes in tumor suppression. The beclin 1 autophagy gene is monoallelically deleted in 40–75% of cases of human sporadic breast, ovarian, and prostate cancer. Therefore, we used a targeted mutant mouse model to test the hypothesis that monoallelic deletion of beclin 1 promotes tumorigenesis. Here we show that heterozygous disruption of beclin 1 increases the frequency of spontaneous malignancies and accelerates the development of hepatitis B virus–induced premalignant lesions. Molecular analyses of tumors in beclin 1 heterozygous mice show that the remaining wild-type allele is neither mutated nor silenced. Furthermore, beclin 1 heterozygous disruption results in increased cellular proliferation and reduced autophagy in vivo. These findings demonstrate that beclin 1 is a haplo-insufficient tumor-suppressor gene and provide genetic evidence that autophagy is a novel mechanism of cell-growth control and tumor suppression. Thus, mutation of beclin 1 or other autophagy genes may contribute to the pathogenesis of human cancers.
Xueping Qu, Jie Yu, Govind Bhagat, Norihiko Furuya, Hanina Hibshoosh, Andrea Troxel, Jeffrey Rosen, Eeva-Liisa Eskelinen, Noboru Mizushima, Yoshinori Ohsumi, Giorgio Cattoretti, Beth Levine
The NF1 tumor suppressor gene encodes a GTPase-activating protein called neurofibromin that negatively regulates Ras signaling. Mutations in NF1 cause neurofibromatosis type 1 (NF1). The development of neurofibromas, which are complex tumors composed of multiple cell types, is a hallmark of NF1. Somatic inactivation of murine Nf1 in Schwann cells is necessary, but not sufficient, to initiate neurofibroma formation. Neurofibromas occur with high penetrance in mice in which Nf1 is ablated in Schwann cells in the context of a heterozygous mutant (Nf1+/–) microenvironment. Mast cells infiltrate neurofibromas, where they secrete proteins that can remodel the ECM and initiate angiogenesis. Thus, identification of mechanisms responsible for mast cell migration to tumor microenvironments is important for understanding tumorigenesis and for designing potential therapies. Here, we show that homozygous Nf1 mutant (Nf1–/–) Schwann cells secrete Kit ligand (KitL), which stimulates mast cell migration, and that Nf1+/– mast cells are hypermotile in response to KitL. Furthermore, we link hyperactivation of the Ras-class IA-PI3K-Rac2 pathway to increased Nf1+/– mast cell migration. Thus, these studies identify a novel interaction between Nf1–/– Schwann cells and Nf1+/– mast cells that is likely to be important in neurofibroma formation.
Yang Feng-Chun, David A. Ingram, Shi Chen, Cynthia M. Hingtgen, Nancy Ratner, Kelly R. Monk, Travis Clegg, Hilary White, Laura Mead, Mary Jo Wenning, David A. Williams, Reuben Kapur, Simon J. Atkinson, D. Wade Clapp
Chronic infection and associated inflammation are key contributors to human carcinogenesis. Ulcerative colitis (UC) is an oxyradical overload disease and is characterized by free radical stress and colon cancer proneness. Here we examined tissues from noncancerous colons of ulcerative colitis patients to determine (a) the activity of two base excision–repair enzymes , AAG, the major 3-methyladenine DNA glycosylase, and APE1, the major apurinic site endonuclease; and (b) the prevalence of microsatellite instability (MSI). AAG and APE1 were significantly increased in UC colon epithelium undergoing elevated inflammation and MSI was positively correlated with their imbalanced enzymatic activities. These latter results were supported by mechanistic studies using yeast and human cell models in which overexpression of AAG and/or APE1 was associated with frameshift mutations and MSI. Our results are consistent with the hypothesis that the adaptive and imbalanced increase in AAG and APE1 is a novel mechanism contributing to MSI in patients with UC and may extend to chronic inflammatory or other diseases with MSI of unknown etiology.
Lorne J. Hofseth, Mohammed A. Khan, Mark Ambrose, Olga Nikolayeva, Meng Xu-Welliver, Maria Kartalou, S. Perwez Hussain, Richard B. Roth, Xiaoling Zhou, Leah E. Mechanic, Irit Zurer, Varda Rotter, Leona D. Samson, Curtis C. Harris
Prostate cancer is one of the most diagnosed and mortal cancers in western countries. A major clinical problem is the development of androgen-independent prostate cancer (AIPC) during antihormonal treatment. The molecular mechanisms underlying the change from androgen dependence to independence of these tumors are poorly understood and represent a challenge to develop new therapies. Based on genetic data showing amplification of the c-myc gene in AIPC, we studied the ability of c-myc to confer AIPC cell growth. Human androgen-dependent prostate cancer cells overexpressing c-myc grew independently of androgens and presented tumorigenic properties in androgen-depleted conditions. Analysis of signalling pathways by pharmacological inhibitors of the androgen receptor (AR) or by RNA interference directed against AR or c-myc showed that c-myc acted downstream of AR through multiple growth effectors. Thus c-myc is required for androgen-dependent growth and following ectopic expression can induce androgen-independent growth. Moreover, RNA interference directed against c-myc showed that growth of human AIPC cells, AR-positive or -negative, required c-myc expression. Furthermore, we showed that c-myc–overexpressing cells retain a functional p53 pathway and thus respond to etoposide.
David Bernard, Albin Pourtier-Manzanedo, Jesús Gil, David H. Beach
Tumor-infiltrating blood vessels deviate morphologically and biochemically from normal vessels, raising the prospect of selective pharmacological targeting. Current antiangiogenic approaches focus mainly on endothelial cells, but recent data imply that targeting pericytes may provide additional benefits. Further development of these concepts will require deeper insight into mechanisms of pericyte recruitment and function in tumors. Here, we applied genetic tools to decipher the function of PDGF-B and PDGF-Rβ in pericyte recruitment in a mouse fibrosarcoma model. In tumors transplanted into PDGF-B retention motif–deficient (pdgf-bret/ret) mice, pericytes were fewer and were partially detached from the vessel wall, coinciding with increased tumor vessel diameter and hemorrhaging. Transgenic PDGF-B expression in tumor cells was able to increase the pericyte density in both WT and pdgf-bret/ret mice but failed to correct the pericyte detachment in pdgf-bret/ret mice. Coinjection of exogenous pericytes and tumor cells showed that pericytes require PDGF-Rβ for recruitment to tumor vessels, whereas endothelial PDGF-B retention is indispensable for proper integration of pericytes in the vessel wall. Our data support the notion that pericytes serve an important function in tumor vessels and highlight PDGF-B and PDGF-Rβ as promising molecular targets for therapeutic intervention.
Alexandra Abramsson, Per Lindblom, Christer Betsholtz
Tuberous sclerosis (TSC) is a familial tumor syndrome due to mutations in TSC1 or TSC2, in which progression to malignancy is rare. Primary Tsc2–/– murine embryo fibroblast cultures display early senescence with overexpression of p21CIP1/WAF1 that is rescued by loss of TP53. Tsc2–/–TP53–/– cells, as well as tumors from Tsc2+/– mice, display an mTOR-activation signature with constitutive activation of S6K, which is reverted by treatment with rapamycin. Rapamycin also reverts a growth advantage of Tsc2–/–TP53–/– cells. Tsc1/Tsc2 does not bind directly to mTOR, however, nor does it directly influence mTOR kinase activity or cellular phosphatase activity. There is a marked reduction in Akt activation in Tsc2–/–TP53–/– and Tsc1–/– cells in response to serum and PDGF, along with a reduction in cell ruffling. PDGFRα and PDGFRβ expression is markedly reduced in both the cell lines and Tsc mouse renal cystadenomas, and ectopic expression of PDGFRβ in Tsc2-null cells restores Akt phosphorylation in response to serum, PDGF, EGF, and insulin. This activation of mTOR along with downregulation of PDGFR PI3K-Akt signaling in cells lacking Tsc1 or Tsc2 may explain why these genes are rarely involved in human cancer. This is in contrast to PTEN, which is a negative upstream regulator of this pathway.
Hongbing Zhang, Gregor Cicchetti, Hiroaki Onda, Henry B. Koon, Kirsten Asrican, Natalia Bajraszewski, Francisca Vazquez, Christopher L. Carpenter, David J. Kwiatkowski
The TGF-β signaling network plays a complex role in carcinogenesis because it has the potential to act as either a tumor suppressor or a pro-oncogenic pathway. Currently, it is not known whether TGF-β can switch from tumor suppressor to pro-oncogenic factor during the course of carcinogenic progression in a single cell lineage with a defined initiating oncogenic event or whether the specific nature of the response is determined by cell type and molecular etiology. To address this question, we have introduced a dominant negative type II TGF-β receptor into a series of genetically related human breast–derived cell lines representing different stages in the progression process. We show that decreased TGF-β responsiveness alone cannot initiate tumorigenesis but that it can cooperate with an initiating oncogenic lesion to make a premalignant breast cell tumorigenic and a low-grade tumorigenic cell line histologically and proliferatively more aggressive. In a high-grade tumorigenic cell line, however, reduced TGF-β responsiveness has no effect on primary tumorigenesis but significantly decreases metastasis. Our results demonstrate a causal role for loss of TGF-β responsiveness in promoting breast cancer progression up to the stage of advanced, histologically aggressive, but nonmetastatic disease and suggest that at that point TGF-β switches from tumor suppressor to prometastatic factor.
Binwu Tang, Mary Vu, Timberly Booker, Steven J. Santner, Fred R. Miller, Miriam R. Anver, Lalage M. Wakefield
Cripto, a cell surface–associated protein belonging to the EGF-CFC family of growth factor–like molecules, is overexpressed in many human solid tumors, including 70–80% of breast and colon tumors, yet how it promotes cell transformation is unclear. During embryogenesis, Cripto complexes with Alk4 via its unique cysteine-rich CFC domain to facilitate signaling by the TGF-β ligand Nodal. We report, for the first time to our knowledge, that Cripto can directly bind to another TGF-β ligand, Activin B, and that Cripto overexpression blocks Activin B growth inhibition of breast cancer cells. This result suggests a novel mechanism for antagonizing Activin signaling that could promote tumorigenesis by deregulating growth homeostasis. We show that an anti–CFC domain antibody, A8.G3.5, both disrupts Cripto-Nodal signaling and reverses Cripto blockade of Activin B–induced growth suppression by blocking Cripto’s association with either Alk4 or Activin B. In two xenograft models, testicular and colon cancer, A8.G3.5 inhibited tumor cell growth by up to 70%. Both Nodal and Activin B expression was found in the xenograft tumor, suggesting that either ligand could be promoting tumorigenesis. These data validate that functional blockade of Cripto inhibits tumor growth and highlight antibodies that block Cripto signaling mediated through its CFC domain as an important class of antibodies for further therapeutic development.
Heather B. Adkins, Caterina Bianco, Susan G. Schiffer, Paul Rayhorn, Mohammad Zafari, Anne E. Cheung, Olivia Orozco, Dian Olson, Antonella De Luca, Ling Ling Chen, Konrad Miatkowski, Chris Benjamin, Nicola Normanno, Kevin P. Williams, Matthew Jarpe, Doreen LePage, David Salomon, Michele Sanicola