27 April 1997 AUA97\PCABCL2.MS-- 500 words
bcl-2 gene makes PCa hormone-refractory
but RNA catalyst destroys it
New Orleans--A gene that makes prostate cancer cells hormone-refractive has been disabled by an enzyme-like RNA molecule.
"We believe that hormone resistance in the prostate cancer cell is linked to the expression of a protein called bcl-2," said Ralph Buttyan, MD, associate professor at Columbia University, New York, at the AUA's 92nd Annual Meeting.
"bcl-2 protects cells from dying," said Dr. Buttyan. "It's expressed at very high levels in hormone-refractory prostate cancer but it's expressed at very low levels in normal prostate cancer," he said.
bcl-2 regulates apoptosis. When it is completely absent, cells die. When it is present at normal levels, cells live normally, and die in response to apoptosis signals. When it is overexpressed, cells ignore apoptosis signals and become immortal.
One anti-cancer strategy is to inactivate the DNA of cancer-causing genes. Complementary or anti-sense DNA, for example, can bind to the messenger RNA (mRNA) of a specific protein and stop the cell from synthesizing that protein. A group from Royal Marsden Hospital, London, reported, in the April 19 Lancet, a phase I/IIa clinical trial with anti-sense bcl-2 DNA for failed non-Hodgkin's lymphoma. The National Cancer Institute is planning Phase I trials for anti-sense bcl-2 DNA in prostate cancer.
Dr. Buttyan's group has developed an agent called a ribozyme, which is an enzyme-like RNA molecule. "It has catalytic activity," he explained. "It can recognize, bind to, and specifically destroy bcl-2 mRNA in vitro and in vivo."
"The beauty of a ribozyme," added Eric Goluboff, MD, assistant professor of urology at Columbia, "is that a single molecule of a ribozyme can cleave hundreds or thousands of mRNAs, whereas an antisense DNA can only bind to one mRNA. Milligrams of mRNA can be cleaved by micrograms of ribozymes."
When the anti-bcl-2 ribozyme is transfected into normal LNCaP prostate cancer cells, with low bcl-2, it kills them, said Dr. Buttyan. When it is transfected into refractory prostate cancer cells with high bcl-2, it does not kill them. "But it restores their ability to respond to other chemotherapeutic agents," and kills the cells, he said.
RNA transfection is inefficient, "but we're increasingly learning ways of making it more efficient," said Dr. Buttyan. In this study, they simply added the ribozymes, encapsulated in a lipid-based transfection medium, to the cultured cells.
The next step is to work in mouse models in which bcl-2 expression causes either abnormal prostate growth, or hormone-resistant prostate cancer. "We're hoping to treat these animal models with an adenovirus vector," said Dr. Buttyan, "and effectively cure the hormone resistant prostate cancer in these mice." An adenovirus vector is a virus that carries an engineered gene.
The ribozyme kills the prostate cancer cells by removing their defenses, said Dr. Buttyan. "bcl-2 is a defense against chemotherapy, radiation therapy, and other forms of therapy. We remove it and the cells are now susceptible to the same agents as the early prostate cancers."