Research Interests
My major research interest is the structures and functions of protein/ligand complexes that play key roles in major hormone signaling pathways. My secondary research interest is to explore the structural information with a goal of developing therapeutic agents for treating human disease, including cancer and diabetes. Research in my group currently focuses on three areas—nuclear hormone receptors, the Met tyrosine kinase receptor, and G protein–coupled receptors—because these proteins, beyond their fundamental roles in biology, are important drug targets. Our studies use multidisciplinary approaches, including molecular and cellular biology, biochemistry, animal physiology, and X-ray crystallography. In addition, we have extended our research to plant hormone abscisic acid (ABA) receptors. Some of our key results on ABA receptors have been published as a cover article in Nature and were also cited as one of top 10 Breakthroughs of 2009 by Science.
Nuclear hormone receptors
Nuclear hormone receptors form a large family comprising ligand-regulated and DNA-binding transcriptional factors, including receptors for classic steroid hormones such as estrogen, progesterone, androgens, and glucocorticoids, as well as receptors for peroxisome proliferator activators, vitamin D, vitamin A, and thyroid hormones. These classic receptors are among the most successful targets in the history of drug discovery: every receptor has one or more synthetic ligands currently being used as medicines. In the last five years, we have developed the following projects centering on the structural biology of nuclear receptors.
Peroxisome proliferator–activated receptors
The peroxisome proliferator–activated receptors (PPARα, δ, and γ) are key regulators of glucose and fatty acid homeostasis and as such are important therapeutic targets for treating cardiovascular disease, diabetes, and cancer. Millions of patients have benefited from treatment with the PPARγ ligands rosiglitazone and pioglitazone for type II diabetes. To understand the molecular basis of ligand-mediated signaling by PPARs, we have determined crystal structures of each PPAR’s ligand-binding domain (LBD) bound to many diverse ligands, including fatty acids, the lipid-lowering fibrates ,and a new generation of antidiabetis drugs, the glitazones. We have also determined the crystal structures of these receptors bound to coactivators or co-repressors, and that of PPARg bound to natural ligand-nitrated fatty acid. These structures provide a framework for understanding the mechanisms of PPAR agonists and antagonists, as well as the recruitment of coactivators and co-repressors. We have discovered a number of natural ligands of PPARγ. The specific plan of this project is to test the physiological roles of these PPAR ligands in glucose and insulin regulation, to unravel their molecular and structural mechanisms of action, and to develop them as therapeutics for diabetes and dislipidemia treatment.
Human glucocorticoid and mineralocorticoid receptors
The human glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) are classic steroid hormone receptors that have crucial effects on immune/inflammatory responses, metabolic homeostasis, and control of blood pressure. Both GR and MR are well-established drug targets, and drugs targeting these receptors are sold for more than $10 billion annually. GR ligands such as dexamethasone (Dex) and fluticasone propionate (FP) are used to treat asthma, leukemia, and autoimmune diseases; MR ligands such as spironolactone and eplerenone are used to treat hypertension and heart failure. However, the clinical use of these ligands is limited by undesirable side effects partly associated with their receptor cross-reactivity or low potency. Thus, the discovery of highly potent and more-selective ligands for GR (such ligands are called “dissociated glucocorticoids”, which can separate good effects from bad ones) remains an intensive goal of pharmaceutical research.
Recently we determined the structure of GR bound to deacylcortivazol (DAC), which binds to GR with 200-fold more potency than cortisol, the physiological glucocorticoid. The GR DAC structure reveals that the GR ligand binding pocket can be expanded dramatically, to twice its normal size. This new pocket provides a tremendous opportunity for drug design and screening. Using a computational screen, we have identified several nonsteroidal ligands that like dissociated glucocorticoids in our cell-based assay. We are now running animal studies to confirm the physiological activities of these novel nonsteroidal ligands, which could lead to new methods of treating inflammation and autoimmune diseases. In addition, we plan to study the molecular and structural mechanisms of the dissociated glucocorticoids identified by our research.
The human androgen receptor
The androgen receptor (AR) is the central molecule in the development and progression of prostate cancer. Mutations of the AR can alter the three-dimensional structure of the receptor in cancer cells and allow the cells to escape the repression of anti-androgen treatment. In this project, we intend to determine the structural basis of the mutant AR proteins. We have discovered that AR mutation in prostate cancers often results in enhanced binding to a particular coactivator, SRC-3 (which is also called “amplified in breast cancer 1”, or AIB1). We plan to study the structural and molecular mechanism of AR antagonists used in prostate cancer treatment and to determine a crystal structure of full-length AR bound to DNA and coactivator motifs.
Structural genomics of nuclear receptor ligand-binding domains
The LBDs of nuclear receptors contain key structural elements that mediate ligand-dependent regulation of nuclear receptors; as such, they have been the focus of intense structural study. In the past two years, we have focused on structural characterization of two orphan receptors: constitutive androstane receptor (CAR) and steroidogenic factor-1 (SF-1), and we have made significant progress in understanding their ligand binding relationships. In addition, we have identified retinoic acid as a low-affinity ligand for COUP-TF, which is one of the most conserved nuclear receptors and has essential roles in angiogenesis, heart development, CNS activity, and metabolic homeostasis. We plan to solve the structures of the remaining orphan receptors, of which there are only four left.
The Met tyrosine kinase receptor
The MET receptor is a tyrosine kinase that is activated by hepatocyte growth factor/scatter factor (HGF/SF). Aberrant activation of the Met receptor has been linked with development and metastasis of many types of solid tumors and has been correlated with poor clinical prognosis. In collaboration with George Vande Woude and Ermanno Gherardi, we plan to develop HGF-Met antagonists for treating solid tumors.
G protein–coupled receptors
GPCRs form the largest family of receptors in the human genome; they receive signals from photons, ions, small chemicals, peptides, and protein hormones. Although these receptors account for over 40% of drug targets, their structure remains a challenge because they are seven-transmembrane receptors. There are only a few crystal structures for class A GPCRs, and many important questions regarding GPCR ligand binding and activation remain unanswered. Currently my group is focused on Class B GPCRs, which includes receptors for parathyroid hormone (PTH), corticotropin-releasing factor (CRF), glucagon, and glucagon-like peptide 1. We have determined crystal structures of the ligand binding domain of the PTH and CRF receptors, and we are developing hormone analogs for treating osteoporosis, depression, and diabetes. In addition, we are developing a mammalian overexpression system and plan to use this system for expressing full-length GPCRs for crystallization and structure studies.
