Mechanisms of hypoxia-induced tumor progression
The Huang laboratory is interested in the molecular mechanisms of hypoxia-induced tumor progression. Hypoxia—deficiency in oxygen supply—is tightly associated with tumorigenesis, as well as with cerebral and myocardial ischemia. When challenged by low oxygen tension, cells strive for oxygen homeostasis by activating hypoxia-inducible factor 1α (HIF-1α), a master regulator of oxygen homeostasis. Activated HIF-1α not only attempts to maintain intracellular energy production by inducing angiogenesis and glycolysis, but also limits energy consumption by means of inhibiting cell proliferation and DNA repair.1 Our research objectives are to identify novel pathways regulating HIF-1α activity, to elucidate the role of HIF-1α in tumor development, and to identify the molecular basis for therapeutic alteration of HIF-1α activity.
Our research focuses on the role of HIF-1α in tumorigenesis and the molecular basis for HIF-1α therapeutics. Solid tumors, including brain tumors, contain hypoxic regions that are often associated with diminished apoptotic potential and resistance to chemotherapy and radiation therapy. The role of HIF-1α in tumorigenesis has been implicated by elevated expression of HIF-1α and HIF-2α, a close member of the HIF-α family, by the correlation of their overexpression with poor diagnosis, and by their role in the induction of angiogenesis and genetic instability. We have recently proposed that the novel HIF-1α–Myc pathway is essential for the hypoxic induction of cell-cycle arrest2 and genetic instability.3 Based on our hypotheses, we are in the process of unraveling the mechanisms by which HIF-1α contributes to hypoxia-induced genetic instability and tumor progression. A variety of experimental approaches are used, including molecular biology, cell biology, genomics, animal models, and human specimens. Given the essential role of HIF-1α in tumor development, we also have a great interest in developing novel therapeutics targeting HIF-1α in both cell culture and animal models for the treatment of tumors and ischemic diseases. We believe that a comprehensive understanding of the biological functions of HIF-1α and HIF-2α is key to the successful development of potential therapeutics.4
- To, K. K., Koshiji, M., Hammer, S., Huang, L. E. Genetic instability: the dark side of the hypoxic response. Cell Cycle 4, 881-2 (2005).
- Koshiji, M. et al. HIF-1α induces cell cycle arrest by functionally counteracting Myc. EMBO J 23, 1949-56 (2004).
- Koshiji, M. et al. HIF-1α induces genetic instability by transcriptionally downregulating MutSα expression. Mol Cell 17, 793-803 (2005).
- Huang, L. E. Targeting HIF-α: when a magic arrow hits the bull's eye. Drug Discov Today 9, 869 (2004).
Genetic abnormalities that cause pediatric brain tumors
Medulloblastomas are malignant brain tumors that arise in the cerebellum in children. The cells of tumor origin are cerebellar neuron precursors, which proliferate rapidly during early postnatal development. Medulloblastomas result from mutations that perturb cell signaling pathways governing the growth and differentiation of neural stem cells. A barrier to improving patient treatment is collateral damage to the developing brain caused by radiation and chemotherapy. The overall objective of my research is to identify signaling molecules that induce medulloblastoma formation and to target these molecules therapeutically to maximize tumor growth suppression and minimize treatment-related neurotoxicity. To do this, we are using a mouse model of medulloblastoma that we developed using the RCAS/tv-a gene transfer system. This system utilizes a retroviral vector (RCAS), derived from avian leukosis virus (ALV), and a transgenic mouse line (Ntv-a) that produces TVA (the cell surface receptor for ALV) under control of the Nestin gene promoter. Nestin is an intermediate filament protein expressed by neural stem cells prior to their commitment to neuronal or glial differentiation. This system enables us to transfer and express exogenous genes in nestin-expressing neural stem cells inside the brain of live mice.
We showed that activation of the Sonic Hedgehog (Shh) signaling pathway in the developing cerebellum induces tumors in mice the closely resemble human medulloblastomas. Furthermore, we identified proteins that cooperate with Shh to enhance tumor formation. These enhancing factors are (a) Myc oncoproteins, which stimulate proliferation and survival of neural stem cells during normal development, (b) Bcl-2, which potently inhibits apoptosis, (c) insulin-like growth factor-II, which concomitantly stimulates proliferation and blocks apoptosis by activating the phosphatidylinositol 3-kinase signaling pathway, and (d) hepatocyte growth factor (HGF), a growth factor with pleiotropic effects on tumor growth.
We are also using this mouse model as a preclinical testing platform for therapies that specifically target Shh and HGF signaling. We showed that an HGF-neutralizing monoclonal antibody or a small molecule that inhibits Shh signaling significantly prolongs survival of mice, in which medulloblastomas are induced by RCAS-mediated transfer of Shh and HGF.
A new project in the lab is to discover genes that cause medulloblastomas to metastasize to the spine and distant organs, a condition that carries a grim prognosis for patients. Our approach is to use the RCAS/tv-a system to test candidate metastasis genes, which were identified via a mutagenesis strategy using the Sleeping Beauty transposon system.
Benign and malignant brain tumor angiogenesis and biology
The Jensen laboratory is interested in both benign and malignant brain tumor angiogenesis, biology, and developing novel treatment modalities for these tumors. The first emphasis is on the role of Hypoxia-Inducible Factor-1 in malignant brain tumor angiogenesis and growth.
We are currently using siRNA techniques to inhibit HIF-1 in mouse tumor models with hopes of initiating human clinical trials in the near future. A second emphasis is toward an understanding of the biology of and the treatment of unresectable or recurrent meningiomas. Current investigations include the use of calcium channel antagonists to potentiate common chemotherapeutic approaches to the treatment of meningiomas.
We have also examined the role of COX-2 in meningioma development and growth. We have successfully inhibited the growth of meningiomas with COX-2 inhibitors. To better understand meningioma biology, we have developed animal models and evaluated currently available cell lines. Our hope is to provide cutting edge approaches to treat both benign and malignant brain tumors and transition these to Phase I Human clinical trials.