Posted January 13, 2015
Dean Y. Li, M.D., Ph.D., University of Utah, Salt Lake City
In the United States, the predominant mechanism of catastrophic vision loss is due to complications from vascular eye diseases. The hallmarks of vascular eye diseases such as age-related macular degeneration (AMD) and proliferative diabetic retinopathy (DR) are the excessive generation of new blood vessels and increased permeability or "leakiness" of the retinal vessels. Uncontrolled growth of new blood vessels eventually damages the retina, and fluid leaked from vessels can cloud the retina and cause vision to blur. With funding from a Fiscal Year 2008 Peer Reviewed Medical Research Program Advanced Technology/Therapeutic Development Award, Dr. Dean Li of the University of Utah found that pathways activated in angiogenesis and inflammation also stimulate increased blood vessel permeability and this can be blunted by the activation of the Slit/Robo4 signaling pathway. In his studies, Dr. Li came to understand that the Slit/Robo4 pathway interacts with other molecular pathways that are stimulated by both angiogenic signals and inflammatory cytokines and that all of these pathways lead to increased blood vessel leakage. He determined that the pathways converge at a regulator protein known as ARF6 and showed that stopping the action of ARF6 reduced fluid leak and damage to the eye in animal models of AMD and DR.
Dr. Li and his colleagues uncovered evidence that ARF6 is a central mediator that controls the degree of stability or instability of the endothelial barrier in the eye and in other organs. ARF6 is an intracellular GTP exchange protein, and increasing amounts of the GTP-bound form of ARF6 promote processes leading to epithelial instability and vascular proliferation. He found that inflammatory cytokines, such as interleukins, increase ARF6-GTP levels and induce vascular permeability. Others had shown that ARF6-GTP was responsible for release of ß-catenin in melanoma-related Wnt signaling, and Dr Li further showed that blocking ARF6 reduced metastasis in that model. Together, these findings suggest that ARF6-GTP is a convergence point for pathways involved in promoting angiogenesis and the endothelial instability seen in angiogenesis and metastasis.
Blocking ARF6 function was shown to have a therapeutic effect, mitigating the severity of inflammation, edema, and fibrosis in animal models of inflammatory conditions and reducing fluid leakage and retinal damage in mouse models of AMD and DR. A key finding is that targeting ARF6 actually blunts disruption of cytokine signaling at a critical convergence point that does not interfere with transcriptional programs known to be critical for the immune response. Moreover, blocking ARF6 was also shown to reduce fluid leakage and tissue damage in animal models of rheumatoid arthritis, suggesting that this approach may have broad implications for the treatment of other inflammatory diseases.
While no direct inhibitors of ARF6 have been identified, Dr. Li collaborated with a biotechnology company and used high-throughput screening to find lead candidate molecules that inhibit the ARNO molecule that converts ARF6 to the activated GTP form. Evaluation of these candidates indicated they are effective in treating multiple animal models of vascular eye diseases.
Dr. Li hopes that this work will lead to a novel and effective treatment for multiple vascular eye diseases and other inflammatory conditions. The next step in this research is to focus a medicinal chemistry effort on improving the pharmacological properties of current lead candidate drugs and to move the resulting therapeutics into preclinical testing to determine efficacy and safety in animal and other model systems.
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