Stem cells are the most important root cells in an organism. Cancer stem cells (CSCs) may be responsible for the tumor metastasis, relapse, and eventual death of most patients. These cells are often resistant to various commonly used therapies, which is a major obstacle for curative cancer therapy. Unfortunately, very little is known about the biology behind this therapy resistance or the unique property of stem cells in general. My laboratory has a long-standing interest in understanding the unique property as well as the regulation of stem cells. The story started with my identification of mutations in the Drosophila stat gene during my postdoctoral research in Dr. Norbert Perrimon's lab at the Howard Hughes Medical Institute (HHMI)/Harvard Medical School. The work was published in Cell (Hou et al., 1996) and opened the door for the field to use Drosophila genetics to study JAK-Stat signaling mechanisms and functions. Over the years, my lab at the National Cancer Institute (NCI) has made seminal contributions to understanding the biological functions of JAK-Stat signaling in Drosophila, including the identification of a receptor for the JAK-STAT signal transduction pathway (Chen et al., Genes Dev., 2002) and the discovery that the JAK-STAT pathway and Cyclin D/Cdk4 cooperatively regulate tumor development in fly blood and eyes (Chen et al., Dev. Cell, 2003). Additionally, the Drosophila anti-STAT and anti-CKA antibodies generated in my lab have been distributed to hundreds of labs and are the most widely used reagents in signal transduction research of Drosophila. Using the reporters of JAK-Stat signaling, we found that the signaling is mainly activated in stem cells and regulates stem cell proliferation in adult Drosophila. Taking advantage of this new finding, we identified adult kidney multipotent renal and nephric stem cells (RNSCs) in the adult Drosophila Malpighian tubules (MTs) (Singh et al., Cell Stem Cell, 2007) and gastric stem cells (GaSCs) in the adult Drosophila gastric and stomach organs (Singh et al., Cell Cycle, 2010). These were the first animal models to study stem cells in these two adult organs. My laboratory has been responsible for generating many of the most important tools and resources for stem cell research in Drosophila. To comprehensively understand the unique property of stem cells, we performed genome-wide RNAi screens in both the Drosophila intestinal (Zeng et al., Cell Reports, 2015) and male germline stem cells (Liu et al., Nature Communications, 2016). We further characterized several genes that regulate different aspects of stem cells, including: 1) that RapGEF/Rap signaling regulates stem cell anchoring to the niche by regulating E-cadherin-mediated cell adhesion (Wang et al., Dev. Cell, 2006), 2) that the mouse RapGEF/SCL pathway regulates the development of haematopoietic stem cells (Satya et al., Blood, 2010), 3) that Broad relays hormone signals to regulate stem cell differentiation in the Drosophila midgut during metamorphosis (Zeng and Hou, Development, 2012), 4) that the Osa-containing SWI/SNF chromatin-remodeling complex regulates stem cell commitment in the adult Drosophila intestine (Zeng et al., Development, 2013), 5) that enteroendocrine cells are generated from stem cells through a distinct progenitor in the adult Drosophila midgut (Zeng and Hou, Development, 2015), 6) that the nuclear matrix protein megator regulates stem cell asymmetric division through the mitotic checkpoint complex in Drosophila testes (Liu et al., PLoS Genetics, 2016), and 7) that the novel tumor suppressor Madm regulates stem cell competition in the Drosophila testis (Singh et al., Nature Communications, 2016). In addition to the work described above, we generated Dl-Gal4 (specifically expressed in intestinal stem cells), Su(H)GBE-Gal4 (specifically expressed in intestinal progenitor EB cells), and T-Trace technique tools (used for stem cell lineage tracing). These have been distributed to hundreds of labs and are the most widely used lines in Drosophila stem cell research. The RapGEF2flox/flox mice generated in my lab have been distributed to several labs. The most striking property of cancer stem cells (CSCs) is their resistance to various inductions of cell death. Using the reagents generated from our genome-wide RNAi screen, we investigated stem-cell death in the adult Drosophila digestive system and found that both normal and transformed stem cells are resistant to pro-apoptotic gene-induced cell death. However, in our landmark paper just published in Nature (Singh et al., 2016) we found that stem cells are metabolic unique, and like hibernating animals, they rely primarily on lipid reserves for energy. Consequently, blocking lipolysis starves them to death. We found that knockdowns of the COPI/Arf1 complex selectively killed quiescent and transformed stem cells through necrosis by attenuating the lipolysis pathway while sparing differentiated cells. The dying stem cells were engulfed by neighboring differentiated cells through a Draper-Mbc/Rac1-JNK-dependent autophagy pathway. Furthermore, Arf1 inhibitors selectively decreased cancer stem cells in human cancer cell lines. This finding may open a new avenue in stem cell research and lead to new therapies that eliminate stem cells in human cancers. We generated Arf1 CKO mouse and are following this new direction by both investigating the detail molecular mechanism of the COPI/Arf1-regulated lipolysis in stem cells in Drosophila/mouse models and identifying new anti-CSC drugs through targeting the lipolysis pathway.
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