Modulation of TNF-alpha-induced apoptosis in corneal fibroblasts by transcription factor NF-kappaB

Previous studies have suggested no role for tumor necrosis factor (TNF)-alpha in the modulation of apoptosis in corneal fibroblasts. However, recent investigations have demonstrated that nuclear factor (NF)-kappaB activation by TNF-alpha mediates negative apoptotic effects that must be blocked to unmask the apoptotic effects of TNF-alpha in vitro. The purpose of this study was to investigate the role of transcription factor NF-kappaB in the suppression of TNF-alpha-induced apoptosis of corneal fibroblasts.

mRNA was detected by reverse transcription-polymerase chain reaction (RT-PCR) and RNase protection assay. Proteins were detected by immunocytochemistry and immunoprecipitation with Western blot analysis. Cell death was evaluated by trypan blue exclusion assay in corneal fibroblasts treated with TNF-alpha in presence or absence of the specific inhibitor of NF-kappaB activation, SN50, actinomycin D, or actinomycin D with dexamethasone, ketorolac tromethamine, or diclofenac sodium. Apoptosis was monitored by trypan blue exclusion, colorimetric cell assay, CPP32 activation assay, DNA fragmentation assay, and transmission electron microscopy. NF-KB activation was monitored using electrophoretic gel shift assay.

TNF-alpha, TNF receptor (R)I, and TNFRII mRNAs were detected in all three cultured corneal cell types and in ex vivo corneal epithelium using RT-PCR. TNF-alpha mRNA was also detected in ex vivo corneal epithelium, corneal epithelial cells, and stromal fibroblasts with the RNase protection assay. TNF-alpha, TNFRI, and TNFRII proteins were detected by immunocytochemistry in all three major corneal cell types in human corneal tissue. TNF-alpha protein was also detected in ex vivo corneal epithelium, primary corneal epithelial cells, and primary stromal fibroblasts using immunoprecipitation and Western blot analysis. TNF-alpha stimulated corneal fibroblast cell death when NF-kappaB activation was blocked with actinomycin D or SN50. Enhanced cell death was noted with dexamethasone, ketorolac tromethamine, or diclofenac sodium when used in the presence, but not in the absence, of actinomycin D. A gel shift assay revealed induction of NF-KB by TNF-alpha and suppression of induction in the presence of actinomycin D or SN50, but not by the control peptide SN50M.

The TNF-alpha receptor system is expressed in the cornea, and NF-kappaB activation is an important regulator of TNF-alpha-mediated corneal fibroblast apoptosis. Nonsteroidal anti-inflammatory agents or corticosteroids may potentiate corneal fibroblast apoptosis in response to cytokine stimulation.