HIV drug discovery efforts have met with little success in finding compounds that interact with an important HIV virulence factor, called Nef, because it lacks biochemical activity that can be directly measured, explained Thomas E. Smithgall, Ph.D., William S. McEllroy Professor and Chair, Department of Microbiology and Molecular Genetics, and senior author of the paper, which was published last week in the early, online version of ACS Chemical Biology.
To get around that problem, Dr. Smithgall’s team developed an assay to measure Nef function indirectly by coupling it to another protein, called Hck, which Nef activates in HIV-infected cells. Because Hck activity can be easily measured, the investigators were able to use it as a reporter for Nef activity in an automated high-throughput screening process. In collaboration with the University of Pittsburgh Drug Discovery Institute, they screened a library of 10,000 chemical compounds against the coupled proteins to see which ones influenced Nef-induced activation of Hck.
After further testing, they confirmed that three compounds inhibited the activity of the Nef-Hck complex and, more importantly, all of them also interfered with HIV replication. One compound was so effective that it suppressed HIV replication to undetectable levels in cell culture experiments.
“So we now have a way to rapidly and efficiently screen for inhibitors of Nef signaling through Hck,” Dr. Smithgall said. “But the surprise was that some of those inhibitors also showed strong antiviral activity in cell culture models.”
There is evidence that people infected with HIV variants that have mutations in the Nef gene take substantially longer to develop disease symptoms or AIDS, he said. In animal models, disrupting the production of Nef from the virus or its interaction with Hck also delays or prevents disease symptoms. The next challenge for the researchers will be to determine whether these compounds also interfere with progression of AIDS-like disease in animal models by blocking Nef function.
“Most current therapies for HIV infection use drugs that interfere with the function of viral enzymes such as reverse transcriptase or with the interaction of the virus and the host cell,” Dr. Smithgall said. “Targeting Nef represents an entirely new approach that could be useful to deal with issues such as drug-resistant HIV strains, and may slow the progression to AIDS.”
He added that Nef is just one of several so-called “accessory proteins” encoded by HIV which are important virulence factors in AIDS. Inhibitory compounds against some of the others might be revealed using a similar coupled protein approach for high throughput screening.
Co-authors of the paper include Lori Emert-Sedlak, Ph.D., Toshiaki Kodama, Ph.D., and Edwina C. Lerner, Ph.D., Department of Microbiology and Molecular Genetics, School of Medicine; Weixiang Dai, Ph.D., and Billy Day, Ph.D., Department of Pharmaceutical Sciences, School of Pharmacy; and John S. Lazo, Ph.D., Department of Pharmacology and Chemical Biology, School of Medicine. Drs. Day and Smithgall also are members of the Drug Discovery Institute, which is directed by Dr. Lazo.
The research was supported by grants from the National Institutes of Health.
As one of the nation’s leading academic centers for biomedical research, the University of Pittsburgh School of Medicine integrates advanced technology with basic science across a broad range of disciplines in a continuous quest to harness the power of new knowledge and improve the human condition. Driven mainly by the School of Medicine and its affiliates, Pitt has ranked among the top 10 recipients of funding from the National Institutes of Health since 1997 and now ranks fifth in the nation, according to preliminary data for fiscal year 2008. Likewise, the School of Medicine is equally committed to advancing the quality and strength of its medical and graduate education programs, for which it is recognized as an innovative leader, and to training highly skilled, compassionate clinicians and creative scientists well-equipped to engage in world-class research. The School of Medicine is the academic partner of UPMC, which has collaborated with the University to raise the standard of medical excellence in Pittsburgh and to position health care as a driving force behind the region’s economy. For more information about the School of Medicine, see www.medschool.pitt.edu.