Department Affiliation: Primary: Pharmacology and Molecular Sciences; Secondary:  Neuroscience
Degree: Ph.D., Massachusetts Institute of Technology
Rank: Professor
Telephone Number: 410-955-4619
Fax Number: 410-955-4520
E-mail address: joliu@jhu.edu
School of Medicine Address: Room 516 Hunterian Building, 725 N. Wolfe Street, Baltimore, MD 21205

Chemical Biology and Molecular and Cellular Biology, Use of Small Molecules As Probes to Elucidate Mechanisms of Signal Transduction, Angiogenesis and Cell Proliferation.

We are interested in a molecular understanding of intracellular signal transduction pathways involved in T lymphocyte activation and apoptosis and regulation of endothelial cell proliferation that underlies angiogenesis. We take interdisciplinary approaches involving a combination of techniques from protein biochemistry, molecular and cell biology to synthetic organic chemistry.

• Mechanisms of intracellular signal transduction involved in T cell activation and apoptosis. Of all cell types of the immune system, T cells are arguably the most critical one, as they play key roles in both the initiation and execution of an effective immune response.  The life and death of T cells are controlled, in large part, by the activation states of the T cell receptor (TCR).  The TCR-mediated signal transduction pathway has been a fertile ground for discovering and developing the most effective immunosuppressive agents used in organ transplantation and other autoimmune diseases.  We have been engaged in the dissection of the intracellular TCR signaling pathway involved in both T cell activation and apoptosis.  Using various immunosuppressive natural products, we are attempting to discover new players in the intracellular signaling pathway involved in T cell activation.  Concurrently, we have also been studying how the signal generated by the second messenger calcium is transmitted from the cytosol to the nucleus to activate transcription of genes that trigger T cell activation or apoptosis.  In this connection, we recently discovered a novel calcium signaling module composed of the transcription factor myocyte enhancer factor (MEF)-2, and its transcriptional corepressors Cabin1(cain)/HDAC4, 5, 7 and 9.  The Cabin1/HDAC family of MEF2 transcriptional corepressors bind and inhibit MEF2 in a calcium-dependent manner.  They are bound to MEF2 while it is associated with promoters of its target genes in the absence of calcium signaling.  When there is an increase in intracellular calcium concentrations, Cabin1 and other MEF2 repressors are displaced from MEF2 by calmodulin, allowing for the association of MEF2 with histone acetylases such as p300 and activating transcription.  We have shown that the MEF2-Cabin1/HDAC signaling module play essential roles in the regulation of transcription of both the Nur77 family of pro-apoptotic genes during thymocyte apoptosis and the IL-2 gene during T cell activation.   We are continuing to identify other proteins involved in the regulation of Nur77 and IL-2 along with MEF2 and Cabin1.  We are also engaged in a large-scale screening for small molecule modulators of MEF2 family of transcription factors to facilitate the investigation of their physiological functions in both cellular and animal models.
• Molecular mechanisms of inhibition of angiogenesis by small molecule inhibitors.

Angiogenesis, formation of new blood vessels, is necessary for tumor growth and metastasis as well as several other human diseases. Inhibition of angiogenesis is emerging as a promising strategy for treating cancer. Fumagillin and ovalicin are structurally-related natural products that were serendipitously discovered as highly selective inhibitors of angiogenesis over a decade ago. The inhibition of angiogenesis by fumagillin and ovalicin has been attributed to the inhibition of cell cycle progression of endothelial cells, the central component of all blood vessels. We and others have identified a molecular target for fumagillin as the type 2 methionine aminopeptidase (MetAP2), revealing a key role of MetAP2 in endothelial cell proliferation. Recently, we found that TNP-470 inhibits cell cycle progression of endothelial cells by activating p53 that in turn induces p21, leading to the inhibition of CyclinE-Cdk2 activity, which is likely to account for cell cycle blockade by fumagillin.  We are currently investigating how inhibition of MetAP2 leads to activation of p53 by identifying its substrates in endothelial cells. We are also performing high-throughput screens for novel, reversible inhibitors of MetAP2 as candidates for anti-angiogenic drugs and tools to study MetAP2 function in vivo.  

• Development of genetic systems for detecting small ligand-protein interactions.

Small ligand-receptor interactions underlie many fundamental biological processes and form the basis for pharmacological treatment of human diseases, as the majority of drugs in use today are derived from small organic compounds (as opposed to proteins). As chemical diversity rapidly grows as a result of recent advances in both natural product chemistry and combinatorial synthetic chemistry, there is an increasing demand for general, simple and sensitive methods to detect interactions between small ligands and proteins. To exploit the power of yeast genetics, we have developed a general method to detect ligand-protein receptor interactions in yeast by combining a hybrid ligand with the yeast two-hybrid system, named yeast three-hybrid system. By creating heterodimers of two small organic ligands and exposing them to fusion proteins between the receptors for the individual ligands to a DNA binding domain and a transcription activation domain, respectively, we showed that hybrid ligands can activate reporter genes in yeast, leading to growth in selective medium or color change due to reporter gene activation.  We are developing mammalian versions of the three-hybrid system to further improve on the sensitivity of the system.  These three-hybrid systems offer many exciting opportunities for future research including identification of new protein targets for “orphan” organic ligands, and selection of proteins possessing novel binding and catalytic properties.

Representative Publications:

• Youn, H.-D., Sun, L., Prywes, R. and Liu, J. O.  Apoptosis of T cells mediated by Ca2+-induced release of the transcription factor MEF2.  Science, 286:790-793, 1999.  Pub Med Reference
• Griffith, E.C., Licitra, E. J. and Liu, J. O.   A yeast three-hybrid system for detecting ligand-receptor interactions. Meth. Enzymol., 328:89-103, 2000.  Pub Med Reference
• Zhang, Y., Griffith, E. C., Sage, J., Jacks, T., and Liu, J. O.  Cell cycle inhibition by the anti-angiogenic agent TNP-470 is mediated by p53 and p21WAF1/CIP1.  Proc. Natl. Acad. Sci. USA, 97:6427-6432, 2000.  Pub Med Reference
• Youn, H.-D., Grozinger, C.M., and Liu, J. O.   Calcium regulates transcriptional repression of myocyte enhancer factor 2 by histone deacetylase 4.  J. Biol. Chem., 275:22563-22567, 2000.  Pub Med Reference
• Youn, H.-D. and Liu, J. O.   Regulation of the MEF2 family of transcription factors by calcium-regulated recruitment of p300 and histone deacetylases.  Immunity, 13:85-94, 2000.  Pub Med Reference
  
• Youn, H.-D., Chatila, T., and Liu, J. O.   NFAT mediates integration of calcineurin and MEF2 signals by the Coactivator p300.  EMBO J., 19:4323-4331, 2000. Pub Med Reference
• Zhang, L.-H., and Liu, J. O.   Sanglifehrin A, a novel cyclophilin-binding immunosuppressant, inhibits IL-2 dependent T cell proliferation at the G1 phase of the cell cycle.  J. Immunol., 166:5611-5618, 2001.  Pub Med Reference
• Smulik, J. A., Divers, S. T., Pan, F., and Liu, J. O.  Synthesis of cyclosporin A-derived affinity reagents by olefin metathesis, Org. Lett., 4: 2051-2054, 2002.  Pub Med Reference
• Huai, Q., Kim, HY., Liu, Y., Zhao, Y., Chen, Y., Xia, Y., Mondragon, A., Liu, J. O. and Ke, H.  Three-dimensional structure of the cyclophilin-cyclosporin A-calcineurin complex,  Proc. Natl. Acad. Sci. USA, 99: 12037-12042, 2002.  Pub Med Reference
• Han, A., Pan, F., Stroud, J. C., Youn, H. –D., Liu, J. O. and Chen, L.  Structural basis of sequence-specific recruitment of transcription corepressor Cabin1 by myocyte enhancer factor-2, Nature, 422: 730-734, 2003.  Pub Med Reference

Other graduate programs in which Dr. Liu participates:

BCMB Program
Anti-Cancer Drug Development Program
Chemistry-Biology Interface Program (CBI)