“These findings raise some serious questions about using marijuana, in any form, if you have a weakened immune system,” said lead study author Jerome E. Groopman, M.D., professor of medicine at Harvard Medical School. “While THC is best known as the main psychotropic part of marijuana, an analog of THC is the active ingredient of marinol, a drug frequently given to AIDS patients, among others, for increasing appetite and limiting chemotherapy-induced nausea and vomiting.”

While previous studies indicated that marijuana smoking was associated with Kaposi’s sarcoma, this is the first to demonstrate that THC itself can assist the virus in entering endothelial cells, which comprise skin and related tissue.

According to Dr. Groopman, the study illustrates the complicated role marijuana and other cannabinoids play in human health. Numerous types of cells display cannabinoid receptors on their outer surfaces, which act as switches that control cellular processes. Dr. Groopman’s laboratory had previously demonstrated that THC could have a protective effect against a certain form of invasive, drug-resistant lung cancer.

To study the combined effect of THC and KSHV, the researchers examined a culture of human skin cells, which are susceptible to infection and could provide a model of Kaposi’s sarcoma. These culture cells display many copies of two prominent cannabinoid receptors. Dr. Groopman and his colleagues found that by bonding to these receptors, low doses of THC activate two proteins responsible for maintaining a cell’s internal framework, or cytoskeleton. By altering the cytoskeleton, THC effectively opens the door for KSHV, allowing the virus to more easily enter and infect the cell. “We can take away that effect by using antagonists that block the two cannabinoid receptors, which adds evidence that THC is the culprit,” Dr. Groopman said.

Once a cell is infected, the presence of THC may also promote the cellular events that turn it cancerous, the researchers say. They found that THC also promotes the production of a viral receptor similar to one that attracts a cell-signaling protein called interleukin-8. Previous studies have noted that this receptor could trigger the cell to reproduce, causing Kaposi’s sarcoma-like lesions in mice. Indeed, the researchers saw that THC induced the infected cells to reproduce and form colonies in culture.

“Here we see both infection and malignancy going on in the presence of THC, offering some serious concerns about the safety of THC among those at risk,” Dr. Groopman said. “Of course, we still do not know the exact molecular events that are occurring here, but these results are just the first part of our ongoing research.”

The study was funded by the National Institutes of Health.

The mission of the American Association for Cancer Research is to prevent and cure cancer. Founded in 1907, AACR is the world's oldest and largest professional organization dedicated to advancing cancer research. The membership includes nearly 26,000 basic, translational, and clinical researchers; health care professionals; and cancer survivors and advocates in the United States and more than 70 other countries.

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