*3.3.4. Matrix metalloproteinases 9*

Matrix metalloproteinases (MMPs) are a family of zinc ion-binding endopeptidases that degrade most of the extracellular matrix (ECM). MMPs regulate many cellular functions including apoptosis, wound healing, and angiogenesis. In angiogenesis, MMPs increase VEGF production and remove physical barriers to new vessel growth [58, 59]. MMPs are produced as a response to increased oxidative stress. Diabetic patients often have increased MMP, mainly MMP-9 and MMP-2 in the retina and vitreous. These are controlled by endogenous tissue inhibitors of metalloproteinases (TIMPs). TIMP-1 regulates MMP-9 and TIMP-2 regulates MMP-2 [59]. Several studies suggest that MMPs are responsible for many diabetic complications, including cardiomyopathy, nephropathy, and retinopathy. MMPs are suspected to facilitate apoptosis of retinal capillary cells during early stages leading to disruption of blood-retinal barrier integrity [58–60]. Kowluru et al. found an increase in MMP-9 and a decrease in TIMP-1 in the retina of DR patients [58]. Abu et al. found significant increases in vitreous zymography levels of MMP-9 in PDR patients (392.3 ± 253.6 scanning units) compared to non-diabetic control patients (168.2 ± 65.0 scanning units). However, the levels of vitreous MMP-2 in PDR patients (540.9 ± 185.6 scanning units) did not differ significantly from non-diabetic control patients (505.4 ± 216.1 scanning units) [60]. Inhibitors of MMPs have been used to treat several diseases, however, there have been no studies using these inhibitors to treat DR patients [59].

#### *3.3.5. Transforming growth factor β*

Transforming growth factor β (TGF-β) is a polypeptide responsible for controlling cell proliferation and differentiation. It is usually secreted in a latent phase and must be transformed to become a mature active form. In the human eye, there are three known TGF-β isoforms (TGF-β<sup>1</sup> , TGF-β2 , and TGF-β<sup>3</sup> ), where the posterior segment of the eye mainly contains TGF-β2 as the dominant form [61–63]. Hirase et al. found an increase in total vitreous TGF-β2 levels in PDR patients (2634 ± 1652 pg/mL) compared to control patients (1305 ± 972 pg/mL) [61]. This result is also consistent with a McAuley et al. study about vitreous biomarkers in diabetic retinopathy [62]. The mature active form of TGF-β2 levels are also increased in PDR patients. This increase correlates with the disease severity, suggesting that TGF-β2 angiogenesis properties play a role in the progression of PDR [61].

*In vitro* study of IL-6 reports its ability to increase endothelial cell and vascular cell permeability by rearranging actin filaments and by changing the shape of endothelial cells [3, 65]. Several studies state that IL-6 also plays an important role in angiogenesis by activating VEGF, and regulating expression of metalloproteinases [3, 64]. IL-8 is known to be a potent angiogenic factor and also a potent chemoattractant and activator of neutrophils and T lymphocytes [64, 84, 85]. Increase of IL-8 concentrations in PDR patients, suggest that they are upregulated in response to oxygen stress and contribute to triggering inflammatory reactions. Study by Takahashi et al. shows that there is a significant increase in IL-6 and IL-8 values in PDR patients (918.0 and 2168.0 ng/mL) compared to control patient (517.0 and 343.0 ng/mL) [85]. Elner et al. also found increased levels of IL-8 in active PDR patients (24.7 ± 4.5 ng/mL) compared to control patients (7.5 ± 2.3 ng/mL), however inactive PDR patients (11.6 ± 5.2 ng/mL) did not differ significantly from controls [79]. It is most likely that VEGF expression causes an increase of IL-8 [86]. On the other hand, IL-10 concentration is not increased in the vitreous of patients with PDR. IL-10 is another important immunoregulatory cytokine that is induced by cell hypoxia. IL-10 activates nitric oxide and increases vascular permeability during the development of PDR [3, 65, 84, 85].

Proliferative Diabetic Retinopathy: An Overview of Vitreous Immune and Biomarkers

http://dx.doi.org/10.5772/intechopen.74366

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Monokine induced by interferon-γ (Mig) attracts activated T cells and has potent angiostatic activity. Several studies suggest that Mig correlates with VEGF and contributes to the progression of neovascularization in DR patients. The main function of Mig in the progression of DR might be related to its leukostasis function [88, 89]. Wakabayashi et al. found significant increases in vitreous concentration of Mig in active (148 pg/mL) and inactive (82.3 pg/mL) DR patients compared with non-diabetic patients who had macular disease (21 pg/mL). However, there was no significant difference in serum Mig concentration between DR patients (85.9 pg/mL) and control subjects (70.4 pg/mL) [87]. Takeuchi et al. also found an increase in Mig vitreous concentration in PDR patients compared to epiretinal membrane patients, idiopathic macular

Receptor tyrosine kinase (c-kit) is expressed by bone marrow and involved in intracellular signaling. It plays an important role in cell proliferation, cell adhesion, cell survival, and neovascularization [89]. Several studies have shown that C-kit plays an important role in the angiogenic process of PDR. C-kit has a soluble form called s-kit that can be generated by proteolytic cleavage [90]. Abu et al. found an increase of c-kit expression in membranes from patients with active neovascularization (697.4 ± 1528.1 pg/mL) compared to patients with inactive PDR (205.3 ± 106.4 pg/mL) and control patients (87.5 ± 91.5 pg/mL). This demonstrates that an increase of c-kit expression is correlated to the progression of PDR [90]. However, Lee et al. found a slight decrease of c-kit values in the PDR group compared to NPDR group [91].

Chemokines are low molecular weight proteins that have many functions, including enhanced immune responses, regulation of homeostasis, and controlling angiogenesis [20, 92, 93].

*3.4.3. Monokine induced by interferon-γ*

hole patients, and uveitis patients [88].

*3.4.4. Receptor tyrosine kinase*

**3.5. Chemokine**
