The Sam Yoon Lab: Research Overview

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My primary research interest involves ways to target the tumor microenvironment, particularly for sarcomas and gastroesophageal cancer. The efforts of my laboratory are directed toward translational research and clinical trials, and currently include three main projects.

Hypoxia-inducible factors (HIFs) and vascular endothelial growth factor (VEGF) in sarcoma progression, metastasis, and radiation response

Soft tissue sarcomas arise in nearly 10,000 persons in the United States each year, and about 40 percent of patients die of either locoregional recurrence or distant metastasis. As sarcomas and other solid tumors outgrow their blood supply, hypoxia (or oxygen deprivation) stabilizes hypoxia-inducible factors 1α and 2α (HIFs), which bind to ARNT (aka HIF-1ß) and drive the transcription of more than 150 genes. Some of these genes, including  vascular endothelial growth factor A (VEGF-A) mediate tumor angiogenesis, while others play a role in tumor invasion (e.g., c-MET) (4) and metastasis (e.g., FOXM1).

VEGF-A is likely the most important factor driving tumor angiogenesis. HIFs and VEGF-A are upregulated in sarcomas, and increasing levels of these proteins correlate with the extent of disease and metastasis. Anti-VEGF-A agents effectively suppress tumor angiogenesis and growth in many human cancers, but VEGF-A inhibition can also increase hypoxia, leading to enhanced tumor invasion and metastasis.  

Our collaborator David Kirsch, of the Duke University Medical Center, has developed a genetically engineered mouse model of sarcomas in which oncogenic Kras is expressed and p53 is deleted in a spatially and temporally restricted manner. (1) We found that combining VEGF-A inhibition with the drug sunitinib along with radiation therapy synergistically inhibits the growth of sarcomas in these mice. (2) In addition, we recently found in a phase II clinical trial that bevacizumab (an anti-VEGF antibody) can significantly augment the therapeutic efficacy of radiation therapy against sarcomas. (3) Gene expression analysis of tumors from this study revealed that upregulation of HIFs and HIF-target genes is a likely mode of resistance to bevacizumab.

This R01-funded project is designed to test the hypothesis that VEGF-A and HIFs play critical and interdependent roles in regulating sarcoma progression, metastasis, and radiation sensitivity. Our goals are to understand the complex role of HIFs in controlling tumor progression and metastasis, the changes in HIF regulation in response to VEGF-A pathway disruption, and the biological effects of HIF and VEGF-A inhibition on the radiation sensitivity of sarcomas.

Tumor angiogenesis in different organ environments: implications for anti-angiogenic therapy for gastroesophageal cancers

Gastroesophageal cancer is the second leading cause of cancer death worldwide. Most patients who succumb to these tumors do not die from their primary tumors but from metastatic disease, which commonly occurs in the liver, lung, and peritoneal cavity. Many of these patients do not have identifiable metastatic disease at the time of diagnosis, but harbor radiologically occult micrometastases that ultimately grow to lethal sizes.

These micrometastases require angiogenesis, or new blood vessel formation, to grow beyond a couple millimeters in size. Vascular endothelial growth factor A (VEGF-A) is a key angiogenic factor in both primary and metastatic tumors, and exerts most of its effects though VEGF receptors 1 and 2 (VEGFR-1, VEGFR-2). Additional pathways important for tumor angiogenesis include fibroblast growth factor 2 (FGF-2), epidermal growth factor (EGF), and hepatocyte growth factor (HGF).

Given the significant diversity in the vasculature of different organs, we have postulated that key distinctions exist in the angiogenic pathways used by metastases in different organs. The long-term goal of this project is to elucidate differences in angiogenic pathways utilized by metastases in various host organ environments as a prerequisite to the development of site-specific, anti-angiogenic strategies.

We have begun by examining the VEGF-A pathway. We have found that Dscr1-/- mice, which have defective VEGFR-2/calcineurin/NF-AT signaling, have delayed growth of lung metastases but normal growth of liver metastases. (4) When experimental metastases were further examined in wild-type mice, VEGFR-1 neutralizing antibody suppressed experimental liver metastases but not lung metastases, while VEGFR-2 neutralizing antibody suppressed experimental lung metastases but not liver metastases. (5) Based on these findings, we are further examining the variable effects of VEGF receptor inhibitors as well as inhibitors of the FGF-2, EGF, and HGF pathways in different organ environments.

Using targeted biological agents with radiation therapy for soft tissue sarcomas and gastroesophageal cancer

Radiation therapy is an essential component for the local control of many primary tumors including soft tissue sarcomas and gastroesophageal cancers. Chemotherapy can augment the effects of radiation therapy for gastroesophageal cancers, leading to complete pathologic response in about one-third of tumors. Anti-VEGF-A agents can enhance the effects of radiation for sarcomas with complete pathologic response in about 15 percent of tumors. The morbidity of surgical resection for some difficult sarcomas and most gastroesophageal tumors can be quite significant. It is conceivable that surgical resection would become unnecessary if we were able to increase the complete pathologic response rate of tumors to radiation with the addition of targeted biological agents.

For sarcomas, we recently found in a phase II clinical trial that bevacizumab (an anti-VEGF antibody) can significantly augment the therapeutic efficacy of radiation therapy against sarcomas. (6) Gene expression analysis of tumors from this study revealed that upregulation of HIFs and HIF-target genes is a likely mode of resistance to bevacizumab. Agents targeting HIF-1α are in various stages of clinical development, and the most commonly used chemotherapeutic drug for sarcomas, doxorubicin, was recently found to block HIF-1α binding to DNA at low metronomic doses. We are currently developing a clinical trial combining bevacizumab, metronomic doxorubicin, and radiation (trimodality therapy) in the neoadjuvant treatment of resectable sarcomas.

For gastroesophageal cancers, the Hedgehog (HH) pathway is a key regulator of cell growth and differentiation during development. The HH pathway is inactive in most normal adult tissues, but HH pathway reactivation has been implicated in the pathogenesis of several cancers.

Berman and colleagues demonstrated increased HH pathway activity in esophageal and stomach cancers, and found suppression of cell growth in vitro as well as xenograft tumors in vivo using the HH pathway antagonist cyclopamine. Many studies have demonstrated that a small subset of cells within a tumor termed tumor-initiating cells or tumor stem cells are resistant to radiation therapy. HH signaling is critical in these tumor-initiating cells, and inhibition of HH signaling may increase the sensitivity of these tumor-initiating cells to radiation. We are currently examining the combination of HH inhibitors and radiation therapy for gastroesophageal cancers.