The study, presented at the annual meeting of the American Association for Cancer Research (AACR), used a modified form of RNAi to silence STAT3 in a permanent way. Typically, only a transient effect is achieved with RNAi before the tiny bits of genetic material are become inactive as the cell population continues to expand.
"We are a long way from using this technique in patients, but this study shows that that it may be possible to use RNAi in more than just experiments that silence genes temporarily," says the study's principal investigator, Ralph Arlinghaus, Ph.D., a professor and chair of the Department of Molecular Pathology. Details of the study appeared in the April 1 2005 issue of the journal Cancer Research.
"The technique is also providing some valuable insights into the role of STAT3 and its downstream targets," adds Arlinghaus, who also will discuss the work in a mini symposium at the AACR meeting.
RNAi has been employed as a laboratory tool to knock down expression of genes in a variety of cells and organisms. It works by introducing a small double-stranded RNA (RNAi) that specifically targets a gene's product, its messenger RNA. This action then blocks translation and production of the protein that the gene encodes.
In this study, the researchers used a lentivirus (a type of retrovirus) to deliver a specifically designed long-acting small interfering RNA (termed a short hairpin RNA, shRNA) for mouse STAT3 into a mouse breast cancer cell line. They chose STAT3 because when activated, it is involved in the formation of multiple types of tumors, including breast cancer. When hijacked by a cancer cell, the activated gene is believed to interfere with the ability of key immune cells to attack a growing tumor.
Investigators used the lentivirus to permanently insert the RNAi into the genome of the cancer cell. After a single exposure of this delivery system, they found 75 percent of laboratory breast cancer cells stopped expressing the STAT3 protein. The researchers also discovered that expression of a protein called TWIST that is known to be involved in cancer metastasis was drastically reduced in the STAT3 knockdown cells, thus greatly reducing the ability of these cancer cells to invade normal tissues like the lung. "Somehow STAT3 is controlling TWIST expression, and this is important to know with regards to activated STAT3 and its involvement in cancer metastasis," Arlinghaus says.
When the mouse breast cells transduced with STAT3, shRNA were then tested in immunocompetent mice, researchers found that the treated breast cancer cells were unable to form breast tumors either at the site of injection or at distant sites typically involved in metastatic breast cancer in this mouse model.
Arlinghaus points out that a human therapy based on these findings is not on the horizon because lentivirus delivery systems haven't been approved for human use yet, and because of the many problems associated with treating metastatic breast cancer. But he says that proof that RNAi can be used to permanently silence such critical genes as STAT3 "has potential application for treating breast cancer."