Figure 1.

The inadequate balance between pro-inflammatory cytokines, proteolitic enzymes, protease inhibitors, inflammatory modulators, and antioxidants may lead to an altered corneal structure and function in keratoconus, triggering a vicious circle between oxidative stress, keratocyte apoptosis, and increased activity of metalloproteinases.^{74, 103, 105} On cell cultures of keratocytes from keratoconic corneas, basal PGE2 production was found to be 10 times greater than in normals.¹⁷ PGE2, whose release may be induced by TNF-α, has been shown to have inhibitory effects on collagen synthesis (CS) and to increase collagen degradation (CD). The tear film in keratoconus have shown increased levels of pro-inflammatory molecules: IL-1α, -4, -5, -6, -8, and -17, TNF-α, TGF-B1 (TGFβ-1), ICAM-1, and VCAM-1.^{18, 20, 21, 86, 103} β-Actin gene has been found to be downregulated and the protein absent in corneal buttons from keratoconus patients.¹⁶⁰ The elevated levels of IL-1-α and TNF-α, and low levels of β-actin have been related to triggering apoptosis of keratocytes.^{86, 151, 152, 160} In keratoconus, levels of proteases such as lysosomal cathepsin-B, -G, -K, and -S, and metalloproteinases (MMPs) are elevated. IL-6 and TNF-α can stimulate the production of several MMPs (-1, -2, -3, -7, -9, and -13) and CATS.^{18, 21, 23, 75, 93, 125} There is also a decrease in the levels of several antioxidant or anti-inflammatory molecules: SOD, glutathione, lactoferrin, IgA, and IL-10.^{19, 20, 106, 108, 109, 110, 144} An important decrease in the level of protease inhibitors such as cystatins (inhibitors of cysteine proteases) and TIMP-1 (inhibitor of MMPs) have also been reported.^{95, 96, 101, 129, 130, 131} The increased activity of several proteolytic enzymes results in higher concentrations of ROS, RNS, cytotoxic aldehydes (CAs) and peroxynitrates (Ps) (which decreases the activity of TIMP-1 and increase MMP-2),^{63, 143, 144, 146, 149} and given the lower production of SOD¹⁴³ possibly related to IL-1α,¹⁵⁸ an environment with high oxidative stress and low pH is formed, causing an increase in the activation of the caspases (caspase-9 and -12), mitochondrial dysfunction (MD), and DNA damage,¹⁵⁶ which eventually lead to increased apoptosis. All of these could probably be the result of a complex interaction of both genetic predisposition and environmental triggering factors, such as eye rubbing and contact lenses wear (the ‘two-hit hypothesis') in keratoconus.^{42, 52, 100, 113, 119, 120}