RNAi screening reveals Wnt inhibitors can suppress signaling, halt proliferation in cancer cells

Integrated screening platform combining RNAi technology and high-throughput chemical genetic screening reveals three novel small molecules with cancer applications

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NEW YORK—Through the use of an integrated screening platform combining RNA interference (RNAi) technology and high-throughput chemical genetic screening, three novel small molecules with cancer applications have been identified by researchers from the Cancer Institute at NYU Lagone Medical Center. These molecules have been identified as inhibitors of the Wnt signaling pathway, a cellular communication pathway responsible for regulating several aspects of cell development and cancer, and have applicability in discovering therapies for diseases associated with abnormalities in the Wnt pathway.

"These  molecules hold a lot of promise towards future Wnt-based drug development for cancer treatments," says Dr. Ramanuj DasGupta, assistant professor of pharmacology at NYU School of Medicine and the NYU Cancer Institute, the scientific director of the NYU RNAi Core Screening Facility and an author of the study about the molecules published recently in Proceedings of the National Academy of Sciences (PNAS). "They may allow the compounds to be used for specific therapeutic purposes in humans to induce the death of Wnt-dependent or Wnt-addicted cancer cells and tumor tissues without affecting the growth and proliferation of normal healthy cells."

The molecules that the researchers discovered are capable of suppressing the activity of the Wnt signaling pathway in human colon cancers from biopsies, in colon cancer cells lines and in a mouse tumor-xenograft model. In each case, the inhibitors halted cancer cell proliferation in either the laboratory dish or the mouse.

The Wnt pathway controls several biological processes by promoting cell-to-cell communication. Previous studies have indicated that cancers in the skin, breast, liver and colon are associated with abnormal signaling activity within the Wnt pathway. Wnt genes bind to receptors found on the surface of cells, which provokes a reaction within the cells that allows various "downstream effector proteins" to become activated. One such protein is ß- catenin, which moves into the nucleus of a cell and is responsible for the activation of genes associated with cell proliferation.

"While more exploratory research of these promising compounds is needed, these small molecules identified in the RNAi screens can serve as prototypes for the development of future antitumor drugs targeting the Wnt signaling pathway in different Wnt-associated cancers," says DasGupta.

The researchers used their integrated screening platform to determine the potency of 14,977 compounds in terms of the effects they have on the activity of the Wnt pathway. The targeted screening method helped the researchers to identify the three novel inhibitors, all of which demonstrated the ability to block Wnt target genes in various mammalian cancer cells lines such as human colon and breast cancer cells. Dr. Foster Gonsalves, the first author of the study and a postdoctoral fellow in DasGupta's lab, helped to develop this screening technique.

"Similar RNAi-based integrated screening technology should be widely applicable to a variety of other signaling pathways implicated in human disease," says DasGupta.

The study's abstract notes that "misregulated ß- catenin responsive transcription (CRT) has been implicated in the genesis of various malignancies, including colorectal carcinomas, and it is a key therapeutic target in combating various cancers." It goes on to explain that despite the potential that CRT inhibitory therapeutics represent, clinical implementation of these therapeutics is difficult due to the issue of identifying compounds that can affect "the nuclear transcriptional activity of ß- catenin" without affecting its other functions.

Additionally, the abstract adds that the researchers' data "provide support for the specificity of these inhibitory compounds in antagonizing the transcriptional function of nuclear ß- catenin." The inhibitors "efficiently block Wnt/ß- catenin-induced target genes and phenotypes" and are "specifically cytotoxic to human colon tumor biopsy cultures as well as colon cancer cell lines that exhibit deregulated Wnt signaling."
Though abnormal signaling in the Wnt pathway is most commonly associated with colon cancer in particular, ß- catenin seems to play a part in various other issues as well. According to the abstract of a paper published in 2004 by the University of Chicago Medical Center, "although oncogenic mutations of ß- catenin have only been discovered in a small fraction of non-colon cancers, elevated levels of ß- catenin protein, a hallmark of activated canonical Wnt pathway, have been observed in most common forms of human malignancies." The paper, "Wnt/beta-catenin signaling pathway as a novel cancer drug target," also affirms in its abstract that "the pathway itself offers ample targeting nodal points for cancer drug development." (The paper can be found online through the online archive at PubMed.)

"To date, no therapies for the control of Wnt-driven tumors have been available for colon cancer, lung cancer, leukemia and other forms of the disease caused by mutations in the Wnt pathway," says Dr. Robert Nagourney of Rational Therapeutics in Long Beach, Calif., another author of the study. "The findings in our human tissue model give us real hope that these compounds will have important implications in future clinical therapy and the development of an effective Wnt inhibitor."

The study, titled "An RNAi-based chemical genetic screen identifies three small-molecule inhibitors of the Wnt/wingless signaling pathway," was published in the April 12, 2011 issue of the PNAS, and was also featured on the cover. The other authors of the paper include Keren Klein, Shauna Katz, Laura A. Ekas and Timothy Cardozo from NYU School of Medicine; Steven Evans from Rational Therapeutics; and Anthony M.C. Brown and Brittany B. Carson from Weill Cornell Medical College.

Funding for the study came from grants from the National Institutes of Health, the Department of Defense and The Helen L. and Martin S. Kimmel Center for Stem Cell Biology at NYU School of Medicine.

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