The long-term goal of my research is to reveal the fundamentals of vascular biology in both physiological and pathophysiological settings, and to exploit this newfound knowledge for the detection and treatment of diseases. Innovative animal models and state of the art imaging techniques are essential to achieve this goal. I have been developing and utilizing such imaging techniques - including an in vivo fluorescent protein gene reporter system (Cell 94: 715, 1998), intravital multiphoton laser-scanning microscopy (Nat Med 7:864, 2001) and optical frequency domain imaging (Nat Med 15: 1219, 2009) in collaboration with world-renowned experts at MGH. With these cutting-edge in vivo imaging approaches we have been providing novel insights into the role of host-tumor interaction in angiogenesis, vascular function, tumor growth, and response to treatment (Nature 416: 279, 2002; PNAS 109: E3119, 2012).
Role of NO in tumor angiogenesis, vascular function and anti-tumor therapy: One of my research interests lie in the role of nitric oxide (NO) in the vasculature and tumors. NO is a highly reactive signaling molecule that mediates a variety of physiological and pathological functions. I discovered that NO derived from vascular endothelial cells not only mediates angiogenesis (PNAS 98: 2604, 2001), but also subsequent maturation of blood vessels (JCI 115: 1816, 2005). I postulated and proved that perivascular gradients of NO – spatial and temporal distribution of NO from vasculature – is crucial in these processes and the disruption of perivascular NO gradients by non-vascular sources of NO results in immature and dysfunctional blood and lymphatic vessels (Nat Med 14: 255, 2008; PNAS 108: 18784, 2011). On the other hand, the promotion of endothelial NO production potentiates vascular normalization and improves concomitantly administered cytotoxic therapies (JNCI 105: 1188, 2013). We are currently studying perivascular NO gradient-induced vascular normalization from the following two angles – dissecting underlining mechanisms and establishing potential means of the improvement.
Role of tumor-host interactions in angiogenesis, tumor growth, metastasis and treatments: Using transgenic mice harboring the green fluorescent protein (GFP) gene driven by vascular endothelial growth factor (VEGF) promoter we found that VEGF promoter of non-transformed stromal cells is strongly activated by the tumor microenvironment (Cell 94: 715, 1998). Using tumor cells carrying the same gene construct we found, for the first time, that hypoxia and low pH independently upregulate VEGF in vivo (Cancer Res 61: 6248, 2001; J Biol Chem 277: 11368, 2002). Using VEGF-/- and wild type ES cell derived tumors we found that the host cells contribute approximately half of total VEGF production in this model (Cancer Res 60: 6248, 2000). MPLSM revealed that VEGF expressing stromal cells are closely associated with angiogenic vessel in the tumor (Nature Med 7: 864, 2001). The association of VEGF-expressing stromal cells spatially correlates with the extravasation of nanoparticles (Nature Med 11: 678, 2005). Furthermore, various anti-tumor treatments result in increased expression of host stromal cell VEGF and thus, may contribute to treatment resistance (Nature 416: 279, 2002; PNAS 109:E3119, 2012). In fact, judicious blockade of VEGF signaling can transiently normalize tumor vasculature and potentiate radiation therapy (Cancer Cell 6: 553, 2004; PNAS 108: 1799, 2011). Anti-VEGF treatments prolong survival of brain tumor bearing animals by normalizing the vasculature – reducing edema – (J Clin Oncol 27: 2542, 2009). However, ectopic expression of Angiopietin-2 compromises this benefit (Clin Cancer Res 16: 3618, 2010). On the other hand, platelet derived growth factor-D normalizes tumor vessels and improves delivery and efficacy of chemotherapeutics (Clin Cancer Res 17: 3638, 2011). In case of medulloblastoma, PlGF/NRP-1 signaling axis is the critical target (Cell 152: 1065, 2013). Finally, anti-VEGF treatment can restore immune microenvironment in tumors and potentiate vaccine therapy (PNAS 109:17561, 2012). We have been further dissecting mechanisms of tumor escape from anti-VEGF treatment focusing on inflammatory cells and pathways that can aggravate tumor growth, metastasis and response to treatment. Our recent data indicate that stromal cells in the primary tumor travel with tumor cells and facilitate survival and growth of metastatic tumors (PNAS 107: 21677, 2010). However, VEGFR1+ bone marrow derived cells (BMDCs) are not required for spontaneous metastasis (Nature 461: E4, 2009; PLoS One 4: e6525, 2009). We found that CXCR4 promotes metastasis via Gr-1+ BMDC recruitment (PNAS 108: 302, 2011). Finally, we found that metastatic tumors induce focal hyper-permeability in the lungs, leading to regional accumulation of inflammatory cytokines, and creating preferential sites of metastatic “soil” (PNAS 108: 3725, 2011).
Probing tumor microenvironment using nanotechnology: To dissect how the tumor microenvironment hinders delivery and efficacy of nanotherapeutics, and to develop strategies to overcome these barriers, we have been collaborating with MIT groups (Moungi Bawendi). We found that a “one size fits all” approach does not work for nanotherapeutics (Nat Nanotech 7: 383, 2012). I proposed and proved a provocative concept of multistage nanoparticle delivery system – a nanotherapeutic, which is optimal for transvascular transport, that reduces its size upon exposure to the tumor microenvironment for more efficient interstitial transport (PNAS 108: 2426, 2011).
Role of obesity in angiogenesis, tumor growth and treatments: As we have developed a strong interest in the link between angiogenesis and obesity, we established novel in vivo system to investigate adipogenesis and angiogenesis, simultaneously, in real time. Then, we discovered a provocative reciprocal regulation between adipogenesis and angiogenesis – adipogenesis induces angiogenesis while angiogenesis promotes adipocyte differentiation (Circ Res 93: e88, 2003). In a subsequent study, we showed an inhibition of diet-induced obesity by anti-angiogenic treatment. Obesity has been linked to poor treatment response in cancer patients including those receiving anti-angiogenic therapy. Using my expertise in vascular biology and anti-angiogenic therapy, we have been revealing the mechanisms of and strategies to overcome obesity-induced resistance to anti-angiogenic treatment.
Engineering blood vessels: My expertise and interest in angiogenesis and vessel maturation have been naturally transformed into a tissue-engineered blood vessel project. We first established methodologies for the generation of tissue engineered blood vessels and longitudinal monitoring of their formation and function in vivo (Nature 428: 138, 2004). Then, we evaluated various sources of human derived vascular cell progenitor/stem cells including human endothelial progenitor cells, embryonic stem cells and mesenchymal stem cells. Most recently, we successfully generated both endothelial cells and perivascular cells from human induced pluripotent stem (iPS) cells and ultimately their blood vessels in vivo (PNAS 110: 12774, 2013). This study opens a door to a variety of new research avenues including biology of de novo vascular formation, tissue engineering and potentially vascular disease models using patients-derived iPS cells.
Teaching responsibilities: Significant portion of my time has also been devoted to teaching tumor angiogenesis and microenviornment as well as various experimental techniques through daily supervision of research fellows and graduate students. My trainee are very successful as evidenced by their outstanding publications in Nature, Nature Biotechnology, Nature Medicine, Nature Methods, Journal of Clinical Investigation, Blood and Journal of Clinical Oncology as well as their awards from NIH, DOD, Howard Hughes Medical Institute, American Association for Cancer Research, Institute for Cancer Research, Susan Komen Foundation, Massachusetts Biomedical Research Corporation, Deutsche Forschungsgemeinschaft, La Ligue Nationnale Contre le Cancer, and Japanese Ministry of Health and Welfare. I also provide multiple lectures both in local and international courses and workshops.