*4.4.1.1 Vascular endothelial growth factor (VEGF)*

It is known that VEGF is the principal target of pharmacologic intervention for proliferative diabetic retinopathy and studies have confirmed elevated levels of VEGF in vitreous samples in DR causing increased vascular permeability [31, 32], stimulates angiogenesis because of its mitogenic effect on endothelial cells, and enhances endothelial cell migration and survival [33, 34] and their production increases markedly under conditions of hypoxia [35]. Intercellular adhesion molecule-1 (ICAM-1) is involved in inflammation and acts as a local intensifying signal in the pathological processes associated with chronic eye inflammation. It has been seen that VEGF increases ICAM-1 and leukocyte adhesion to vessel wall and elevated ICAM-1 and cell adhesion molecule-1 synthesis in retina. It further increases ICAM-1 in endothelial cells and this in turn leads to activation and increased production of cytokines and leukocyte activation [36], these cytokines initiate and mediate the inflammatory response and stimulate further release of VEGF [37]. Various studies have strongly indicated that the increased level of ICAM-1 generally exists in the patients with DR and it may associated with the severity of DR [38]. Placental growth factor (PGF), member of VEGF family, binds to VEGF- and neuropilin-receptor sub-types. PGF induces a range of neural, glial and vascular cell responses that are distinct from VEGF-A. As its expression is associated with pathological angiogenesis and inflammation, its blockade does not affect the healthy vasculature [39]. High levels of PGF have been found in aqueous humour, vitreous and in retina of patients especially those with diabetic retinopathy (DR). Results suggest that anti-PGF therapy might have advantages over anti-VEGF treatment, and that it may have clinical applications as a standalone treatment or in combination with anti-VEGF [39]. Low concentrations of VEGF have been seen to rise with PGF stimulating endothelial cell proliferation, migration, and angiogenesis [40]. Higher levels of PGF in vitreous are seen in DR and these levels are correlated well with VEGF levels [41].

## *4.4.1.2 Tenascin-C (TNC)*

Tenascin-C is an extracellular matrix protein and plays an important role in cell growth and adhesion, playing an equally involved in angiogenesis, oncogenesis, wound repair and inlflammation [42, 43]. Studies have shown that TNC is involved in the pathogenesis of ischemic proliferative retinopathy. Elevated levels have been detected in PDR vitreous humour. mRNA and protein expression of TNC has been found in pre-retinal fibrovascular membranes excised from PDR patients [44]. Extracellular matrix (ECM) synthesis plays an important part in the pathogenesis of the intravitreal membranes and is thus characteristic of both proliferative vitreoretinopathy (PVR) and early stages of proliferative diabetic retinopathy (PDR). Hence it is clear that TNC plays a role in the development of epiretinal PVR and PDR membranes by controlling cell adhesion and regulating extracellular matrix formation formation [45].

#### *4.4.1.3 Insulin like growth factor*

It is known that IGF-1 is has a significant role in pathogenesis of DR as it is involved in regulation along with influencing growth, maturation and functioning of blood vessels. It also activates VEGF in human RPE cell and receptors which are actively involved in development of vitreo-retinal disorders [46, 47]. Insulin-like growth factor-I (IGF-I) is known to enhances insulin action in normal subjects and in both type 1 and 2 diabetes. It is associated with significant side effects in a high percentage of patients. Simultaneous administration of IGF binding protein-3 with IGF-I limits IGF-I inducible side effects, but it does not downgrade the ability of IGF-I to enhance protein synthesis and bone accretion [48]. Severity of DR in patients with

type 1 diabetes is inversely related to serum IGF-1 levels. Low IGF levels are an indicator for closer follow-up and strict management of diabetes and retinopathy [49].
