Description of Research Expertise

Research Interests
Our laboratory studies stem cells, angiogenesis, tumorigenesis, cancer cell metabolism, and cellular responses to oxygen deprivation.

Key words: stem cells, angiogenesis, hematopoiesis, cancer, metabolism, hypoxia, tumor suppressors, mouse models of human malignancy.

Description of Research
The maintenance, differentiation, and function of embryonic and adult stem cells are influenced by numerous complex signals provided by their immediate microenvironment. One important microenvironmental factor for stem/multipotent progenitor cells is actually molecular oxygen (O2). Our laboratory has conclusively shown that a protein complex called “hypoxia-inducible factor” (HIF) regulates how cells adapt to changes in O2 availability, and we continue to unravel the role of HIF in multiple developmental processes.
By deriving HIF-mutant embryonic stem (ES) cells and mice, we have previously established that HIF is essential for normal blood cell, blood vessel, placental, and cardiac development during embryogenesis. More recently, we have determined that HIF also directly regulates stem cell quiescence, proliferation, and differentiation. Furthermore, we have shown that O2 levels and HIFs modulate critical stem cell pathways, including Oct-4, c-Myc, Wnt and Notch. We have genetically ablated two related HIFs, HIF-1α and HIF-2α, in mice to evaluate their role in the biology of stem cells within the hematopoietic system, germline, skin, muscle, and hair follicles. Based on these studies, we hypothesize that some long-lived stem cells occupy relatively O2-starved regions to maintain quiescence and protect the integrity of their genome. Other short-lived stem and transit-amplifying cells are closely aligned with blood vessels, and likely to be well-oxygenated instead. Moreover, stem/progenitor cells migrate through natural O2 gradients as they adopt distinct cell fates and are therefore influenced by HIFs. However, the effects of O2 availability and HIFs on normal stem cells may be co-opted in tumors dependent on so-called “cancer stem cells”.
Because a solid tumor cannot grow unless it acquires new blood vessels from surrounding host tissues, the HIFs are necessary for tumor progression. Mutations in multiple tumor suppressor genes lead to HIF stimulation, tumor growth, and tumor angiogenesis. Our studies show that the HIFs are also clearly important for tumor metastasis. By identifying the underlying molecular mechanisms for the promotion of blood vessel growth and tumor cell survival by HIF, we hope to develop procedures that will “cut off” the tumor’s O2 supply, and diminish the cancer cell’s ability to metastasize. We have created conditional alleles of Hif-1α, Hif-2α, and the gene encoding their common dimerization partner, known as Arnt, to determine how they affect tumor growth in the lung, liver, kidney, and intestine. This will allow us to genetically dissect the role of HIFs in all phases of tumor progression, including, latency, size, altered cell metabolism, vascularity, and metastasis. Cancer cells must adapt to the combined stresses of O2, nutrient, and growth factor deprivation typical of the tumor microenvironment. We are therefore delineating a role for HIFs in cancer cell metabolic adaptations, hoping to exploit these pathways for therapeutic benefit. In addition to studying the role of HIFs in mouse models, we are also evaluating their role in human patient samples, focusing on specimens acquired from individuals with renal clear cell carcinoma, glioblastoma multiformae, and neuroblastoma.
Finally, though progress has been made in understanding the transcriptional mechanisms activated during hypoxia, the fundamental underlying mechanisms of O2 sensing by mammalian cells are not completely understood. We have shown that hypoxia activates HIF via a mitochondrial-dependent signaling process involving increased Reactive Oxygen Species (ROS). These results lead us to believe that mitochondria act as O2 sensors by increasing the generation of ROS during hypoxia. We are currently studying how this mitochondrial activity during hypoxia directly affects HIF stabilization and activity.

Research Techniques
Generation of standard and conditional knockout and transgenic mice, ES cell technology, stem cell biology, embryology, ex vivo developmental models of organogenesis, mouse models of human cancer, gene expression profiling using Affymatrix microarrays, array cGH (the Illumina platform), molecular biology, biochemistry, histology, immunohistochemistry, in situ hybridization, animal surgery, electron microscopy, confocal microscopy, and cell imaging.

Rotation Projects:
Continued analysis of the role of hypoxia-inducible factors and tumor suppressors in stem cell function, angiogenesis, hematopoiesis, and tumor progression. Other projects will focus on cellular oxygen sensing and distinct adaptations provided by HIF-1 versus HIF-2.

Lab Personnel:

Tim Cash, Ph.D. Student
Vijay Dondeti, Ph.D. Student
Brian Keith, Adjunct Professor
Bryan Krock (Postdoctoral Fellow)
Amar Majmundar, M.D., Ph.D. Student
Lijoy Mathew, HHMI Associate (Postdoctoral Fellow)
Jolly Mazumdar, HHMI Associate (Postdoctoral Fellow)
Guoliang Qing (Postdoctoral Fellow)
Shetal Patel, M.D., Ph.D. Student
Theresa Richardson, Research Specialist
Carmella Romeo, Research Specialist
Davesh Shah, Undergraduate Research Assistant
Nicolas Skuli, HHMI Associate (Postdoctoral Fellow)
James Webb (Postdoctoral Fellow)
Emily Williams, HHMI Medical Fellow
Waihay Wong, M.D., Ph.D. Student
Gina Young, Research Associate (Postdoctoral Fellow)
Hongewei Yu, Research Specialist
Hongxia Zhang, Research Associate (Postdoctoral Fellow)