Scientists at Penn's Medical Center have discovered a novel pathway in the human immunodeficiency virus (HIV) life cycle, which may have relevance for several important aspects of HIV biology and pathology. The discovery also reveals a new avenue for development of drugs to treat AIDS, according to Dr. David B. Weiner, assistant professor of pathology and laboratory medicine and senior author of the study.
Weiner and students Yosef Refaeli and David N. Levy made their discovery as they searched for the cellular target of a viral protein known as Vpr. Earlier studies showed that Vpr alters growth characteristics of tumor cells, stops cell proliferation and increases viral replication. All of these observations suggested that Vpr interacts with an important regulatory pathway in cells. But it was a surprising observation about the fate of Vpr within cells that indicated what this pathway might be: when cells are treated with Vpr, the protein moves from the cytoplasm to the nucleus.
"It was the ability of Vpr to move into the nucleus that provided us with the most important clue. It immediately suggested a pathway that Vpr might be using, as well as an explanation for how Vpr could alter the genetic programming of the cell," Weiner said. "We asked ourselves what other intracellular pathways have this biology and move this way. One answer was that the glucocorticoid pathway does." Glucocorticoids are steroid hormones that have wide-ranging effects on the human body. They influence metabolism as well as the nervous, neuroendocrine and immune systems.
The Penn discovery was reported in the April 11 issue of the Proceedings of the National Academy of Sciences.
Through a series of experiments, the scientists showed that Vpr interacts with a cellular protein called rip-1 that appears to be part of the glucocorticoid receptor complex-a cellular shuttle system that transports glucocorticoids to the nucleus of the cell. The scientists' work also showed that transport of rip-1 to the nucleus could be facilitated by glucocorticosteroids, like hydrocortisone, as well as by Vpr; and that both glucocorticosteroids and Vpr stimulate production of virus in infected cells. Finally, they demonstrated that the glucocorticoid inhibitor, mifepristone (RU-486), blocked transport of both glucocorticoids and Vpr to the nucleus. As a result, mifepristone inhibited production of virus in infected cells by about 70 percent.
"This work supports the idea that Vpr turns on the glucocorticoid receptor pathway," Weiner explained. "This is a pathway that can affect cell proliferation, regulate gene transcription and play a role in immune suppression. The role of Vpr in relationship to these aspects of HIV disease is being investigated."
The ability of Vpr to pirate the glucocorticoid receptor provides new ground for speculation on the link between the virus and several pathological features of AIDS-muscle wasting and susceptibility to fungal infection, for example. These symptoms are reminiscent of some aspects of corticosteroid toxicity. Glucocorticoids also have been reported to induce apoptosis, or programmed cell death, in some T-cells. Apoptosis may be related to viral-induced T-cell depletion in HIV infection.
Weiner cautioned that inhibitors like RU-486 have not yet been proven to be useful clinically for the treatment of HIV infection. "This must be evaluated extremely carefully before being considered as a treatment strategy. Inhibition of this pathway has the potential to affect naturally-occurring cortisol levels, a highly regulated pathway, and thus could be very harmful and actually have the opposite of the desired effects."
Lisa Bain is a science writer in the Department of Public Affairs at the Medical Center.