Research, led by scientists at the Wilmer Eye Institute, Johns Hopkins University School of Medicine, Baltimore, have uncovered a novel angiogenic protein involved in the pathogenesis of diabetic retinopathy. The finding may in part explain why not all patients benefit equally from available anti-VEGF treatments. Published in the journal, Proceedings of the National Academy of Sciences (doi/10.1073/pnas.1423765112), the research shows that angiopoietin-like 4 protein (ANGPTL4) acts as a potent angiogenic factor independent of VEGF concentration and appears to correlate with both the presence of diabetic eye disease and areas of retinal neovascularization. The results of the study may provide the basis for a biomarker diagnostic / prognostic tool and for a novel therapeutic target against a rapidly growing healthcare challenge.
The research study, led by Dr. Akrit Sodhi, examined levels of ANGPTL4 in the aqueous fluid of control and diabetic patients, with and without proliferative diabetic retinopathy (PDR). In particular, the study examined the ability of aqueous fluid to stimulate endothelial cell tubule formation as a proxy for angiogenesis. Results showed that aqueous fluid from control patients, and from diabetic patients without diabetic retinopathy, did not significantly impact tubule formation, while aqueous fluid from diabetic patients with PDR clearly stimulated tubule formation. So far so good however, the researchers found that 36% of patients with PDR had VEGF levels less than the mean value measured for the control (non-diabetic) patients. Such levels were also similar to the VEGF levels in PDR patients who had recently received anti-VEGF therapy. In fact there appeared to be no correlation between the VEGF concentration and stimulation of tubule formation leading researchers to the conclusion that some other factor was driving the tubule formation. To find the elusive factor, the research team exposed retinal Müller cells to hypoxia and analysed the expression pattern of mRNAs encoding 22 inflammatory cytokines, proteases and angiogenic cytokines. While most of the mRNAs showed a modest fold increase, only one – ANGPTL4 mRNA – showed a ramp up comparable to that of VEGF mRNA. Having located the smoking gun, the researchers then went on to show that ANGPTL4 was both necessary and sufficient to stimulate tubule formation and that its activity could be blocked through an RNAi route or through application of a monoclonal antibody.
The authors concluded their study by commenting that, “[u]ltimately, our observations provide the foundation for studies to assess inhibition of ANGPTL4 – alone or in combination with inhibition of VEGF – as a therapeutic approach for the treatment of PDR and other ocular neovascular diseases as well as for studies investigating the use of aqueous fluid ANGPTL4 as a diagnostic and/or therapeutic biomarker for these diseases”. Neeraj Agarwal, Ph.D., a program director in the NEI Division of Extramural Research, provided further commentary on the exciting results noting that, “[t]his certainly opens a new target(s) for possible drug discovery. Also, it may be useful to measure the levels of angiopoietin-like 4 protein before deciding to treat proliferative diabetic retinopathy with anti-VEGF therapy.”