The results indicated that vitreous LCN2 and VEGF levels were significantly higher in the PDR group compared to the nondiabetic control group. To our knowledge, this is the first study in the literature evaluating vitreous LCN2 in patients with PDR. Although whether PDR in this study was in active phase depended on our clinical experience, there might be some mistakes, but it does not affect our conclusion that vitreous LCN2 in active PDR was significantly higher than that in inactive PDR. These findings suggest that LCN2 may be involved in the vascularization of PDR.
LCN2 has been identified as a potential biomarker for several common inflammatory diseases including acute kidney injury, lupus nephritis, cardiovascular diseases, or intestinal inflammation[18]. Although LCN2 is highly upregulated under a large number of inflammatory conditions, both pro- and anti-inflammatory properties of this adipokine have been reported. Hu et al. described that LCN2 upregulation protects hepatocytes from interleukin-1beta-induced inflammation[19]. Vichaya found that LCN2 is dispensable for sterile inflammation-induced sickness and depression-like behavior[16]. Recently, LCN2 was also found to be involved in the pathogenesis of Alzheimer's disease[20]. In terms of ophthalmic diseases, the work of Parmar et al. have revealed that LCN2 plays a regulatory role in retinal inflammation during retinal degeneration such as Stargardt, retinitis pigmentosa and age-related macular degeneration. They found that LCN2 induced expression of antioxidant enzymes heme oxygenase 1 and superoxide dismutase 2 in RPE cells and could inhibit the cytotoxic effects of H2O2 and LPS, and exacerbated inflammation following light exposure was observed in LCN2 knockout mice model of Stargardt disease and age-related macular degeneration[21]. In the study of the Yaran Koba group, they demonstrated that increased LCN2 levels in the eyes of patients with central retinal vein occlusion begin an intense neuroinflammatory process and cause iron accumulation in retinal cells which may lead to the complications such as macular edema, macular ischemia, and neovascularization[10]. In a recent study, it has been demonstrated that a positive correlation between vitreous levels of LCN2 and proliferative vitreoretinopathy grading in patients with rhegmatogenous retinal detachment, revealing a potential role in the pathogenesis and progression of proliferative vitreoretinopathy. They thought LCN2 seems to play a role in reactive gliosis and neuroinflammation, which are involved in PVR pathophysiology.
At present, VEGF is recognized as an important protein in the pathogenesis of PDR, and anti-VEGF therapy has achieved good results in clinical use. Not surprisingly, we also found a significant increase in intravitreal VEGF concentration in PDR patients. The mean concentration of vitreous VEGF in eyes with PDR in our study was similar with that in other studies[22, 23]. Previous studies have found that successful PRP can effectively reduce the concentration of VEGF in the vitreous. Our study found that the concentrations of LCN2 and VEGF in vitreous fluid of PDR patients who had completed PRP were significantly lower than those of PDR patients who had not completed PRP. This indicates that PRP might affects the production of LCN2 and VEGF simultaneously. Similar results were obtained when comparing the concentrations of LCN2 and VEGF in vitreous fluid of inactive PDR with active PDR. This suggests that both LCN2 and VEGF may be related to the activity of neovascularization. The exact mechanisms of PRP are unclear, but it is possible that the decreased area of retinal tissue leads to improved oxygenation and a reduction in the levels of VEGF. A reduction in levels of VEGF may be important in reducing the risk of harmful new vessels forming. In our study, PRP reduced LCN2 concentration in vitreous cavity, the reason may be that PRP destroyed retinal tissues and reduced LCN2 secretion.
The significant correlation between the vitreous LCN2 and VEGF levels in eyes with PDR suggests that there may be upstream-downstream relationship between the two proteins. Yang et al. previously reported that VEGF was significantly increased with LCN2 expression in MCF-7 human breast cancer cells[24]. They found LCN2-induced VEGF was mediated through hypoxia-inducible factor 1alpha (HIF-1α). Intravitreous VEGF and HIF-1α in diabetic patients with PDR were found increased and related mutually previously[25]. Moreover, the production of VEGF was diminished in a diabetic mice model lacking HIF-1α expression.[26] These findings clearly support the hypothesis that LCN2 upregulates VEGF expression through HIF-1α and promotes neovascularization in PDR.
The limitation of this study is that the number of eyes in the study group and the control group was unbalanced at baseline, but the statistical value P in our primary outcome was less than 0.001, which does not seem to affect our conclusion that the concentration of LCN2 and VEGF in the vitreous cavity of PDR patients is higher than that of the control group.