Instructor, Radiation Oncology - Radiation Therapy
Honors & Awards
ECFMG (US Medical Board) Certificate, Educational Commission for Foreign Medical Graduate (2014)
Postdoctoral fellowship, California Institute for Regenerative Medicine (10/2012 - 09/2015)
Dean's fellowship, Stanford University School of Medicine (01/2012 - 12/2012)
Predoctoral fellowship, American Heart Association (07/2008 - 06/2009)
Korean Honor Scholarship, Embassy of Korea (2008)
Doctor of Philosophy, Johns Hopkins University (2009)
Intern, Seoul National University Hospital, Medicine (2002)
Doctor of Medicine, Seoul National University, Medicine (2001)
Maximilian Diehn, Postdoctoral Faculty Sponsor
Inhibition of Mouse Breast Tumor-Initiating Cells by Calcitriol and Dietary Vitamin D
MOLECULAR CANCER THERAPEUTICS
2015; 14 (8): 1951-1961
The anti-cancer actions of vitamin D and its hormonally active form, calcitriol, have been extensively documented in clinical and pre-clinical studies. However, the mechanisms underlying these actions have not been completely elucidated. Here we examined the effect of dietary vitamin D and calcitriol on mouse breast tumor-initiating cells (TICs, also known as cancer stem cells). We focused on MMTV-Wnt1 mammary tumors, for which markers for isolating TICs have previously been validated. We confirmed that these tumors expressed functional vitamin D receptors (VDRs) and estrogen receptors (ERs) and exhibited calcitriol-induced molecular responses including ER down-regulation. Following orthotopic implantation of MMTV-Wnt1 mammary tumor cells into mice, calcitriol injections or a vitamin D-supplemented diet caused a striking delay in tumor appearance and growth while a vitamin D-deficient diet accelerated tumor appearance and growth. Calcitriol inhibited TIC tumor spheroid formation in a dose-dependent manner in primary cultures and inhibited TIC self-renewal in secondary passages. A combination of calcitriol and ionizing radiation inhibited spheroid formation more than either treatment alone. Further, calcitriol significantly decreased TIC frequency as evaluated by in vivo limiting dilution analyses. Calcitriol inhibition of TIC spheroid formation could be overcome by the overexpression of β-catenin, suggesting that the inhibition of Wnt/β-catenin pathway is an important mechanism mediating the TIC inhibitory activity of calcitriol in this tumor model. Our findings indicate that vitamin D compounds target breast TICs reducing tumor-initiating activity. Our data also suggest that combining vitamin D compounds with standard therapies will enhance anti-cancer activity and may improve therapeutic outcomes.
View details for DOI 10.1158/1535-7163.MCT-15-0066
View details for Web of Science ID 000359324600018
View details for PubMedID 25934710
Identification and genetic manipulation of human and mouse oesophageal stem cells.
Human oesophageal stem cell research is hampered by the lack of an optimal assay system to study self-renewal and differentiation. We aimed to identify and characterise human and mouse oesophageal stem/progenitor cells by establishing 3-dimensional organotypic sphere culture systems for both species.Primary oesophageal epithelial cells were freshly isolated and fluorescence-activated cell sorting (FACS)-sorted from human and mouse oesophagus and 3-dimensional organotypic sphere culture systems were developed. The self-renewing potential and differentiation status of novel subpopulations were assessed by sphere-forming ability, cell cycle analysis, immunostaining, qPCR and RNA-Seq.Primary human and mouse oesophageal epithelial cells clonally formed esophagospheres consisting of stratified squamous epithelium. Sphere-forming cells could self-renew and form esophagospheres for over 43 passages in vitro and generated stratified squamous epithelium when transplanted under the kidney capsule of immunodeficient mice. Sphere-forming cells were 10-15-fold enriched among human CD49f(hi)CD24(low) cells and murine CD49f(+)CD24(low)CD71(low) cells compared with the most differentiated cells. Genetic elimination of p63 in mouse and human oesophageal cells dramatically decreased esophagosphere formation and basal gene expression while increasing suprabasal gene expression.We developed clonogenic and organotypic culture systems for the quantitative analyses of human and mouse oesophageal stem/progenitor cells and identified novel cell surface marker combinations that enrich for these cells. Using this system, we demonstrate that elimination of p63 inhibits self-renewal of human oesophageal stem/progenitor cells. We anticipate that these esophagosphere culture systems will facilitate studies of oesophageal stem cell biology and may prove useful for ex vivo expansion of human oesophageal stem cells.
View details for DOI 10.1136/gutjnl-2014-308491
View details for PubMedID 25897018
Histone deacetylase isoforms regulate innate immune responses by deacetylating mitogen-activated protein kinase phosphatase-1
JOURNAL OF LEUKOCYTE BIOLOGY
2014; 95 (4): 651-659
The MAPK pathway mediates TLR signaling during innate immune responses. We discovered previously that MKP-1 is acetylated, enhancing its interaction with its MAPK substrates and deactivating TLR signaling. As HDACs modulate inflammation by deacetylating histone and nonhistone proteins, we hypothesized that HDACs may regulate LPS-induced inflammation by deacetylating MKP-1. We found that mouse macrophages expressed a subset of HDAC isoforms (HDAC1, HDAC2, and HDAC3), which all interacted with MKP-1. Genetic silencing or pharmacologic inhibition of HDAC1, -2, and -3 increased MKP-1 acetylation in cells. Furthermore, knockdown or pharmacologic inhibition of HDAC1, -2, and -3 decreased LPS-induced phosphorylation of the MAPK member p38. Also, pharmacologic inhibition of HDAC did not decrease MAPK signaling in MKP-1 null cells. Finally, inhibition of HDAC1, -2, and -3 decreased LPS-induced expression of TNF-α, IL-1β, iNOS (NOS2), and nitrite synthesis. Taken together, our results show that HDAC1, -2, and -3 deacetylate MKP-1 and that this post-translational modification increases MAPK signaling and innate immune signaling. Thus, HDAC1, -2, and -3 isoforms are potential therapeutic targets in inflammatory diseases.
View details for DOI 10.1189/jlb.1013565
View details for Web of Science ID 000335346300010
View details for PubMedID 24374966
Erythropoietin promotes breast tumorigenesis through tumor-initiating cell self-renewal
JOURNAL OF CLINICAL INVESTIGATION
2014; 124 (2): 553-563
Erythropoietin (EPO) is a hormone that induces red blood cell production. In its recombinant form, EPO is the one of most prescribed drugs to treat anemia, including that arising in cancer patients. In randomized trials, EPO administration to cancer patients has been associated with decreased survival. Here, we investigated the impact of EPO modulation on tumorigenesis. Using genetically engineered mouse models of breast cancer, we found that EPO promoted tumorigenesis by activating JAK/STAT signaling in breast tumor-initiating cells (TICs) and promoted TIC self renewal. We determined that EPO was induced by hypoxia in breast cancer cell lines, but not in human mammary epithelial cells. Additionally, we demonstrated that high levels of endogenous EPO gene expression correlated with shortened relapse-free survival and that pharmacologic JAK2 inhibition was synergistic with chemotherapy for tumor growth inhibition in vivo. These data define an active role for endogenous EPO in breast cancer progression and breast TIC self-renewal and reveal a potential application of EPO pathway inhibition in breast cancer therapy.
View details for DOI 10.1172/JCI69804
View details for Web of Science ID 000331413300017
Aldosterone activates endothelial exocytosis
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2009; 106 (10): 3782-3787
Although elevated levels of aldosterone are associated with vascular inflammation, the proinflammatory pathways of aldosterone are not completely defined. We now show that aldosterone triggers endothelial cell exocytosis, the first step in leukocyte trafficking. Exogenous aldosterone stimulates endothelial exocytosis of Weibel-Palade bodies, externalizing P-selectin and releasing von Willebrand factor. Spironolactone, a nonselective mineralocorticoid receptor (MR) blocker, antagonizes aldosterone-induced endothelial exocytosis. Knockdown of the MR also decreases exocytosis, suggesting that the MR mediates exocytosis. Aldosterone triggers exocytosis within minutes, and this effect is not inhibited by actinomycin D, suggesting a nongenomic effect of aldosterone. Aldosterone treatment of endothelial cells increases leukocyte adherence to endothelial cells in culture. Taken together, our data suggest that aldosterone activates vascular inflammation in part through nongenomic, MR-mediated pathways. Aldosterone antagonism may decrease vascular inflammation and cardiac fibrosis in part by blocking endothelial exocytosis.
View details for DOI 10.1073/pnas.0804037106
View details for Web of Science ID 000264036900027
View details for PubMedID 19223584