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Researcher Information

Bret A. Hassel, Ph.D.
Associate Professor

Microbiology and Immunology

UMGCC Research Program:
Tumor Immunology and Immunotherapy

College Degree: BS, University of Miami, Coral Gables, FL
Medical Degree: Ph.D.,The Johns Hopkins University, Baltimore, MD
Fellowship: Research Associate, The Cleveland Clinic Foundation, Cleveland, OH

Contact Information:

Mailing Address: The University of Maryland
Greenebaum Cancer Center
655 West Baltimore Street
9th floor BRB
Baltimore, MD 21201
Email: bhassel@som.umaryland.edu
Phone: 410-328-2344
Fax: 410-328-6559

Research Interests:
Interferons are part of the body's natural defense against infection by external microbes (such as viruses and bacteria) and against cancer, the uncontrolled proliferation of our own cells. To effectively use interferons in a therapeutic setting, the challenge is to first determine which pathways of interferon action mediate the clinically relevant activities, and then to selectively modulate these pathways to combat human diseases. My laboratory studies two pathways of interferon action that are involved in the antiviral and tumor suppressive activities of interferon.

In one pathway, a ribonuclease known as RNase-L inhibits virus replication by degrading viral RNAs. In the absence of virus, RNase-L functions as a tumor suppressor. Evidence of this antitumor activity comes from the finding that patients in which RNase-L is mutated exhibit an increased risk for hereditary prostate cancer. In most cells, RNase-L is present in an inactive form; our research focuses on how the activity of RNase-L is regulated in normal cells, and how this control is altered in cancer cells. Furthermore, we want to determine the mechanism by which RNase-L keeps cell proliferation in check. This information will help us predict the diseases in which interferon therapy will be beneficial, to evaluate the role of RNase-L in the response to interferon, and ultimately to modulate RNase-L activity to enhance its therapeutic activity.

The second pathway we study involves an ubiquitin-like protein (ubl) known as ISG15 that posttranslationally modifies cellular proteins (ISGylation), and is implicated in the antiviral and antitumor activities of IFN. Only a few of the hundreds of proteins thought to undergo ISGylation have been identified, however, we and others have recently cloned the enzymes that mediate ISG15 conjugation and deconjugation. We are now utilizing these tools in overexpression and siRNA knockdown approaches to manipulate ISG15 conjugation and determine its biologic functions. In addition to its intracellular functions as an ubl, free, unconjugated, ISG15 is released from cells where it exhibits immunomodulatory, cytokine-like activities. ISG15 is induced as one of the earliest and strongest responses to microbial challenge, being directly induced through Toll-Like Receptors independent of IFN. In addition, we have recently reported that ISG15 is induced by chemotherapeutic agents, where its immunomodulatory activities may contribute to the antitumor effects of these agents. Current studies are focused on developing an ELISA assay to measure extracellular ISG15 in vivo, and on identifying ISG15 responsive cells.

Castelli, J.C., Hassel, B.A., Wood, K.A., Li, X-L., Amemiyz, K., Dalakas, M.C., Torrence, P.F. and Youle, R.J. A study of the interferon antiviral mechanism: apoptosis activation by the 2-5A system. J. Ex. Med., 186:1-6. 1997.

Li, X-L., Blackford, J.A. and Hassel, B.A. RNase-L mediates the antiviral effect of interferon through a selective reduction of viral RNA during encephalomyocarditis virus infection. J. Virol., 72:2752-2759. 1998.

Li, X-L., Blackford, J.A., Judge, C.S., Liu, M., Xiao, W., Kalvakolanu, D.V. and Hassel, B.A. RNase-L-dependent destabilization of interferon-induced mRNAs: a role for the 2-5A system in attenuation of the interferon response. J. Biol. Chem., 275:8880-8888. 2000.

Hummer, B.T., Li, X-L., and Hassel, B.A. A role for p53 in gene induction by double-stranded RNA. J. Virol., 75:7774-7777. 2001.

Liu, M., Reimschuessel, R., and Hassel, B.A., Molecular cloning of the fish ISG15 orthologue: a ubiquitin-like gene induced by nephrotoxic damage. Gene. 298:129-139. 2002.

Liu, M., Li, X-L., and Hassel, B.A. Proteasome modulates conjugation to the ubiquitin-like protein, ISG15. J. Biol. Chem. 278:1594-602. 2003.

Pitha-Rowe, I., Hassel, B.A., and Dmitrovsky, E. 2003 UBE1L is required for ISG15 conjugation during retinoid-induced differentiation of Acute promyelocytic leukemia. J. Biol. Chem. 279:18178.

Liu, M., Hummer, B.T., Li, X-L. and Hassel, B.A. 2004 Camptothecin induces the ubiquitin-like protein, ISG15, and enhances ISG15 conjugation in response to interferon. Journal of Interferon and Cytokine Research. in press.