Muller Glial Cells Inhibit Proliferation of Retinal Endothelial Cells via TGF-β2 and Smad Signaling

Neovascularization is a sight-threatening complication of ischemic proliferative retinopathies. Transforming growth factor (TGF)-beta, a cytokine with multiple functions in the retina, participates in the control of pathological angiogenesis and neovascularization. Retinal glial (Muller) cells produce TGF-beta 2 under physiological and post-ischemic conditions. To characterize glial cell-derived mediators of angiogenesis regulation in glial-endothelial interactions in the retina, we co-cultured primary Muller cells and bovine microvascular retinal endothelial cells (BRECs). Muller cell-derived TGF-beta 2 was bound by the BRECs, which were found to express serine/threonine kinase TGF-beta receptors, and stimulated TGF-beta-dependent anti-proliferative signaling pathways. The proliferation of BRECs was attenuated by exogenous TGF-beta 2 as well as by the presence of Muller cell culture media. The following intracellular signaling mechanisms were found to be involved in the anti-angiogenic action of Muller cell-derived TGF-beta 2: (i) binding of TGF-beta 2 to BRECs is mediated by the type-II TGF-beta receptor, leading to (ii) activation and phosphorylation of receptor-activated Smads; (iii) Muller cell-derived TGF-beta 2 activates Smad2 and Smad3 to (iv) attenuate the phosphorylation state of the MAP kinases, extracellular signal-regulated kinase (ERK)-1/-2. Neutralizing TGF-beta or TGF-beta type-II receptor or blocking the activation of Smads partially abrogated the effect of Muller cell-conditioned media on BRECs. Together, our data suggest that Muller cells release TGF-beta 2, inhibiting the proliferation of retinal endothelial cells via activation of Smad2/Smad3 and attenuation of ERK signaling. Given the context-dependent action of TGF-beta 2 on angiogenesis, our results may have implications for understanding the pathogenesis of retinal angiopathies, such as diabetic retinopathy, and the anti-angiogenic role of TGF-beta therein.