It turns out that the small but mighty inhibitors of gene expression have an achilles heel and can be served up a taste of their own effect. Qihong Huang, Alexander Deiters, and colleagues at The Wistar Institute, North Carolina State University, and the University of Pennsylvania School of Medicine discovered the first small molecule inhibitor of miRNA.
Because miRNAs like miR-21 have been implicated in many forms of cancer, the search is on for inhibitors that interfere with miRNA binding to and translational inhibition of target mRNA transcripts. Antisense nucleic acids have been the obvious choices for miRNA inhibitors but are often undesirable for therapeutic applications. The discovery points the way to the development of a new generation of cancer drugs.
Huang notes that one sizeable hurdle in harnessing the power of miRNAs is getting “the right molecule into the right place at the right time” to regulate their function. “In terms of developing therapeutic agents for cancer, for example, we need to identify small molecules that can get into the bloodstream and get into the cells,” he says. “The problem is, to date, no one has been able to show that such miRNA inhibitors exist.”
The researchers developed a screening assay to look for small molecules that selectively inhibit miR-21, an anti-apoptotic miRNA that is elevated in various cancers. In the assay, a firefly luciferase reporter gene linked to the DNA sequence encoding miR-21 was introduced to human cells. Translation of the resulting mRNA was inhibited in cells by the binding of miR-21. “The idea was that when we add small molecules that inhibit the function of miR-21, the light signal [from luciferase] will increase,” Huang says.
The scientists then screened a library of 1,000 compounds and found one molecule that inhibited miR-21 in the assay. The molecule, diazobenzene 2, decreased miR-21 levels by 80 percent and produced a nearly five-fold increase in the intensity of the light signal from the firefly protein.
Preliminary data from the researchers’ ongoing studies suggest that the inhibitor could be used in combination with other chemotherapy drugs to provide a synergistic effect, Huang says. The researchers also will evaluate its potential as a stand-alone cancer drug. Huang and his colleagues are now conducting studies in mice to assess the inhibitor’s effectiveness against brain, breast, and colon tumors, and they are working to modify the molecule to make it even more efficient. For all the details, check out Angewandte Chemie: International Edition, September 2008