Tgf Beta 2-Hepcidin Feed-Forward Loop In The Trabecular Meshwork Implicates Iron In Glaucomatous Pathology

PURPOSE. Elevated levels of transforming-growth-factor (TGF)-beta 2 in the trabecular meshwork (TM) and aqueous humor are associated with primary open-angle glaucoma (POAG). The underlying mechanism includes alteration of extracellular matrix homeostasis through Smad-dependent and independent signaling. Smad4, an essential coSmad, upregulates hepcidin, the master regulator of iron homeostasis. Here, we explored whether TGF-beta 2 upregulates hepcidin, implicating iron in the pathogenesis of POAG.METHODS. Primary human TM cells and human and bovine ex vivo anterior segment organ cultures were exposed to bioactive TGF-beta 2, hepcidin, heparin (a hepcidin antagonist), or N-acetyl carnosine (an antioxidant), and the change in the expression of hepcidin, ferroportin, ferritin, and TGF-beta 2 was evaluated by semiquantitative RT-PCR, Western blotting, and immunohistochemistry. Increase in reactive oxygen species (ROS) was quantified with dihydroethidium, an ROS-sensitive dye.RESULTS. Primary human TM cells and bovine TM tissue synthesize hepcidin locally, which is upregulated by bioactive TGF-beta 2. Hepcidin downregulates ferroportin, its downstream target, increasing ferritin and iron-catalyzed ROS. This causes reciprocal upregulation of TGF-beta 2 at the transcriptional and translational levels. Heparin downregulates hepcidin, and reduces TGF-beta 2-mediated increase in ferritin and ROS. Notably, both heparin and N-acetyl carnosine reduce TGF-beta 2-mediated reciprocal upregulation of TGF-beta 2.CONCLUSIONS. The above observations suggest that TGF-beta 2 and hepcidin form a self-sustained feed-forward loop through iron-catalyzed ROS. This loop is partially disrupted by a hepcidin antagonist and an anti-oxidant, implicating iron and ROS in TGF-beta 2-ediated POAG. We propose that modification of currently available hepcidin antagonists for ocular use may prove beneficial for the therapeutic management of TGF-beta 2-associated POAG.