The HIF1α-PFKFB3 Pathway: A Key Player in Diabetic Retinopathy

Diabetic retinopathy (DR) is a slow-progressing disease recognized as the most common ocular microvascular complication of diabetes and the leading cause of vision loss (or distortion of visual images) in middle-aged adults (1).

The clinical progression of DR occurs in 2 stages: an early stage, called nonproliferative diabetic retinopathy (NPDR), and a more advanced one, known as proliferative diabetic retinopathy (PDR). Whereas NPDR, often asymptomatic, is characterized by increased ocular capillary occlusion and vascular permeability, PDR is manifested in progressive ischemia, hypoxia, and pathologic angiogenesis. As a result, proangiogenic factors perpetually enhance ischemia and immune dysfunction, leading to retinal neovascularization and, consequentially, bleeding in the vitreous, retinal detachment, or glaucoma, which severely affects patients’ daily activities (1).

In this context, DR is a growing global public health problem, and its management and prevention are of the utmost importance. Current treatment strategies for DR include vitreous surgery, laser photocoagulation, and intravitreal pharmacologic agents. However, a deeper understanding of the dynamics and pathways of DR pathogenesis could reveal novel targets for more effective therapies.

To date, hyperglycemia and oxidative stress seem to be the primary pathogenic causes of DR. Specifically, circulating high glucose levels induce intracellular overproduction of reactive oxygen species (ROS)—highly bioactive molecules that contribute to the impairment of mitochondrial function—antioxidant defense enzymes, and enhanced sensitivity of retinal cells to oxidative stress (2). Excessive ROS generation is mainly due to mitochondrial impairment, including mutations in mitochondrial DNA, changes in mitochondrial membrane permeability, and damage to the mitochondrial respiratory chain, all of which provoke an inappropriate compensatory response to oxidative stimuli. The subsequent activation of many alternative pathways of glucose metabolism, such as accumulation of advanced glycation end products (AGEs) and upregulation of their receptors (RAGEs), as well as polyol and protein kinase C pathway activation, induces retinal endothelial dysfunction, which, if prolonged over extended periods of time, can lead to neovascularization and microvascular occlusion (2).

Concurrently with impaired capillary function, an insufficient O₂ supply to the retina drives the production of proangiogenic growth factors and proinflammatory cytokines, which cause unregulated angiogenesis, increased vascular permeability, and retinal cell death (3). As a result, retinal hypoxia and its related complications obstruct the high energy demand required for an efficient and sensitive transduction of images to readable neuronal signals, thus leading to distorted or blurred vision. Unfortunately, the specific mechanisms that mediate impaired O₂ supply or contribute to O₂-dependent retinal disorders are not well understood.

In this context, and in a recent issue of Journal of Clinical Endocrinology & Metabolism, Min et al summarize the evidence supporting a critical role of the hypoxia-inducible factor 1α (HIF1α)-6-phosphofructo-2-kinase—fructose-2,6-bisphosphatase 3 (PFKFB3) pathway in the islet pathology of diabetes and DR (4).

All hypoxia-dependent conditions seem to share a common denominator: HIF1α, a key regulator of adaptive responses to hypoxia that maintains cellular O₂ homeostasis by controlling the delivery and utilization of O₂ as well as the transcription of genes involved in glucose regulation and angiogenesis (5). In this regard, Min and colleagues highlight the involvement of HIF1α in the pathological neurodegeneration and angiogenesis of the retina seen in DR, mainly through 2 mechanisms: an increase in O₂ delivery and a decrease in O₂ consumption. Specifically, HIF1α promotes angiogenesis by activating several crucial genes, including vascular endothelial growth factor (VEGF), erythropoietin, angiopoietin 2 (ANGPT2), placental growth factor (PGF), C-X-C motif chemokine 12 (CXCL12), and platelet-derived growth factor B (PDGFB) (5). Similarly, HIF1α-signaling alters mitochondrial activity and cell metabolism by increasing glucose uptake and switching energy production from oxidative phosphorylation to glycolysis, as well as reducing O₂ availability.

In this sense, oral administration of HIF1α inhibitors has been shown to prevent pathological neovascularization in an O₂-induced retinopathy model (OIR) (6). Moreover, the specific knockout of HIF1α in retinal glial cells, known as Müller cells, also reduces the build-up of adherent leukocytes and retinal vascular leakage in OIR and in the mouse model of streptozotocin-induced DR (7).

Among the key effectors of HIF1α-mediated metabolic remodeling, PFKFB3 is the most intriguing one. PFKFB3 is 1 of 4 closely related proteins of the PFKFB family, and is the predominantly expressed isoform in endothelial cells, playing a crucial role in glycolysis and both in physiological and pathological angiogenesis. As Min et al report in their review, dysregulation of PFKFB3 can contribute to a chronic and/or excessive increase in glycolytic flux, as well as the secretion of proinflammatory factors and neurotoxic mediators, thus leading to pathological angiogenesis and neurodegeneration (4). In this regard, data obtained in OIR mice have shown that both the application of a PFKFB3 inhibitor and the selective deletion of endothelial PFKFB3 markedly suppress retinal neovascularization. Similarly, in both in vitro and in vivo models, endothelial silencing of PFKFB3 was shown to promote impaired angiogenesis characterized by a reduction in endothelial sprouting. Min et al also demonstrated that the decrease in vessel sprouting observed on PFKFB3 depletion was partially due to reduced cell proliferation. Consistent with these studies, PFKFB3-driven glycolysis was found to be essential for endothelial cell migration during angiogenic sprouting, since inhibition of PFKFB3 interfered with vascular endothelial-cadherin, a specific adhesion molecule that drives junctional remodeling during sprouting (8). Unfortunately, the underlying mechanisms of PFKFB3 in these processes remain unclear.

In conclusion, the findings of Min and colleagues highlight the HIF1α-PFKFB3 pathway as an interesting avenue for exploring novel treatment targets in the prevention and management of DR, particularly at an early stage. However, as the authors note, many critical aspects need to be clarified, so future research is essential.

Glossary

Abbreviations

ANGPT2

angiopoietin 2

CXCL12

C-X-C motif chemokine 12

DR

diabetic retinopathy

HIF1α

hypoxia-inducible factor 1α

NPDR

nonproliferative diabetic retinopathy

OIR

O₂-induced retinopathy model

PDGFB

platelet-derived growth factor B

PDR

proliferative diabetic retinopathy

PFKFB3

6-phosphofructo-2-kinase—fructose-2,6-bisphosphatase 3;

PGF

placental growth factor

RAGEs

receptor for advanced glycation end products

ROS

reactive oxygen species

VEGF

vascular endothelial growth factor

Additional Information

Disclosures: The authors have nothing to disclose.

Data Availability

Data sharing is not applicable to this article because no data sets were generated or analyzed during the present study.