The unfolded protein response in retinal vascular diseases: implications and therapeutic potential beyond protein folding

Abstract

Angiogenesis is a complex, step-wise process of new vessel formation that is involved in both normal embryonic development as well as postnatal pathological processes, such as cancer, cardiovascular disease, and diabetes. Aberrant blood vessel growth, also known as neovascularization, in the retina and the choroid is a major cause of vision loss in severe eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinopathy of prematurity, and central and branch retinal vein occlusion. Yet, retinal neovascularization is causally and dynamically associated with vasodegeneration, ischemia, and vascular remodeling in retinal tissues. Understanding the mechanisms of retinal neovascularization is an urgent unmet need for developing new treatments for these devastating diseases. Accumulating evidence suggests a vital role for the unfolded protein response (UPR) in regulation of angiogenesis, in part through coordinating the secretion of pro-angiogenic growth factors, such as VEGF, and modulating endothelial cell survival and activity. Herein, we summarize current research in the context of endoplasmic reticulum (ER) stress and UPR signaling in retinal angiogenesis and vascular remodeling, highlighting potential implications of targeting these stress response pathways in the prevention and treatment of retinal vascular diseases that result in visual deficits and blindness.

Keywords: Angiogenesis; Endoplasmic reticulum stress; Endothelial cells; Retina; Unfolded protein response; VEGF.

Figure 1. Implications of the IRE1/XBP1 pathway in angiogenesis

Upon ER stress, IRE1 is activated by autophosphorylation and oligomerization. Activated IRE splices the mRNA encoding XBP1 to generate spliced XBP1 (XBP1s). XBP1s induces UPR target genes, thereby restoring ER homeostasis and promoting cell survival. XBP1s can bind to the VEGF promoter to regulate VEGF transcription. In addition, activated IRE1 recruits TRAF2, which, in turn, results in the activation of stress kinases, such as JNK and IKK; this leads to induction of inflammatory cytokines. Hyperactivated IRE1 degrades mRNA, such as micR-17 and netrin-1, and leads to increased pro-angiogenic factor expression.

Figure 2. Signaling pathways of the PERK branch of the UPR in angiogenesis

The PERK branch of the UPR regulates angiogenesis mainly through induction of ATF4. During ER stress, PERK is activated by autophosphorylation and oligomerization. Activated PERK phosphorylates the translational initiation factor eIF2α, resulting in the global arrest of protein synthesis. Enhanced eIF2α phosphorylation preferentially increases the translation of the transcription factor ATF4; in turn, ATF4 induces the expression of inflammatory factors (MCP-1, IL-6, IL-8 and CXCL3) and VEGF, thereby causing increased inflammation and promoting angiogenesis. Additionally, ATF4 is a major inducer of CHOP, whose activation modulates pro- and anti-apoptotic pathways, resulting in apoptosis and cell death, and also contributes to increased oxidative stress and dysregulated calcium homeostasis.

Figure 3. Activation of the ATF6 pathway and angiogenesis

Upon ER stress, ATF6 disassociates from GRP78 and translocates to the Golgi apparatus, where it is cleaved by proteases. Cleaved ATF6 is an active bZIP transcription factor: it translocates to the nucleus and binds to the promoters of UPR target genes, such as BIP/GRP78, CRYAB, and XBP1, and VEGF. These genes directly and indirectly regulate the angiogenic process in various ways.

Figure 4. Potential role of ER stress in hematopoietic stem cell and progenitor cell dysfunction in diabetic vascular degeneration

In normal physiological conditions HSPCs reside in the stem cell niche of the bone marrow. The hypoxic environment in the niche maintains the quiescent state of HSPCs and sustains their self-renewal capacity through induction of HIF-1α. Release of HSPCs from the niche is mediated by MMP-9. Once in the bone marrow, HSPCs proliferate and differentiate into multipotent progenitor (MPP) cells, and mobilize into the circulation. The mobilization process is facilitated by cytokines and proteins, including CXCL-12, VEGF, NO, GM-CSF, and CXCL-8. In diabetes mellitus, increased levels of reactive oxyen species (ROS) in the bone marrow leads to decreased hypoxia, thereby disrupting the stem cell niche. This, in turn, leads to ER stress, resulting in apoptosis of HPSCs, hence disturbing the equilibrium between self-renewal and differentiation. Decreased levels of CXCL-12, VEGF, NO, GM-CSF, and CXCL-8 impair mobilization of cells from the bone marrow into the circulation.