Comparison of gene expression profiles of normal and POAG TM revealed that POAG TM tissues have a unique glycogene expression pattern that defines them as a group distinct from normal. Indeed, some of the genes that we found to be differentially expressed were found in model systems as well. For example, mimecan and activinA, which were found to be elevated in POAG in the present study, have been reported to be upregulated by treatment of TM with TGFβ.
4 In contrast, clusterin which has been shown to be elevated in response to dexamethasone treatment
2 was found to be downregulated in all POAG specimens analyzed in the present study. This is not surprising, considering that there is no indication that the POAG samples used in the present study were derived from patients incurring glaucoma as a result of corticosteroid exposure. In addition, corticosteroid exposure may not induce the same results in situ and in culture. For instance, dexamethasone treatment of cultured rat thymocytes causes elevation in clusterin expression
23 but in vivo dexamethasone does not affect clusterin expression in the rat thymus.
24 Another gene, MCP-1 which has been shown to be downregulated in TM cells treated with TGFβ,
4 was found to be upregulated in all three POAG specimens analyzed in this study. This too is not surprising, because TGFβ has been shown to inhibit NFκB signaling in nonocular systems.
25 Because MCP-1 is a target of NFκB, it is reasonable to expect that TGFβ treatment of normal cells would result in decreased expression of MCP-1. Based on the same reasoning, if NFκB is constitutively activated in POAG TM, as has been shown before,
15 an increased expression of NFκB targets such as MCP-1 would be expected. Significantly, our study has identified a novel set of genes that are reported herein for the first time to be differentially expressed between normal and POAG TM. Although it is beyond the scope of this report to highlight the potential significance of each altered gene, some genes are of unique interest and warrant specific mention. It has previously been reported that the glycosaminoglycan (GAG) content of TM extracellular matrix is altered in POAG. The keratan sulfate (KS) GAG content is reduced in POAG TM, and the chondroitin sulfate (CS) GAG content is elevated.
26 Our results suggest a putative feedback mechanism responding to this aberrant GAG composition. We found that the expression of lumican, the core protein of KS proteoglycan and of α 1,6 fucosyltransferase (FUT8), an enzyme responsible for core fucosylation of
N-glycans of keratan sulfate as well as other oligosaccharide chains, is upregulated in POAG TM. Lumican has been shown to induce apoptosis of keratocytes, and it has been shown that corneal keratocytes derived from lumican-null mice show a significant increase in proliferation and a decreased rate of apoptosis in situ and in culture compared with keratocytes derived from wild-type mice. These effects are reversed when cultured cells are made to express recombinant human lumican and keratocyte proliferation is reduced in the presence of exogenous recombinant human lumican in culture media.
27 Thus, it is logical to hypothesize that increased lumican expression in TM cells of POAG eyes may, at least in part, be responsible for reduced TM cell density that is known to be associated with POAG.
28