Role of MMP-9 in the breakdown of barrier integrity of the corneal endothelium in response to TNF-α

- Barrier integrity of corneal endothelium is critical for maintenance of corneal transparency.
- TNF-alpha is a major cytokine in the anterior chamber during allograft rejection and uveitis.
- TNF-alpha breaks down the barrier integrity of the endothelium through the activation of p38 MAP kinase.
- This research shows that MMP-9 is a downstream target of p38 MAP kinase.

TNF-α induces loss of barrier integrity of the corneal endothelium through mechanisms involving the activation of p38 MAP kinase. This study has investigated the role of matrix metalloproteinase-9 (MMP-9), known to be activated by mechanisms downstream of p38 MAP kinase, on the breakdown of the barrier integrity. Experiments were performed with primary cultures of bovine corneal endothelium. Changes in the trans-endothelial electrical resistance (TER), a measure of barrier integrity, were measured by electric cell-substrate impedance sensing. The integrity of the apical junctional assembly was imaged by immunolocalization of ZO-1. MMP-9 activity in the conditioned medium of cells treated with TNF-α was visualized by gelatin zymography. Transcriptional activation of MMP-9 was assessed by real-time RT-PCR. Exposure to TNF-α led to significant disruption of ZO-1 and also caused a continuous decline in TER for more than 20 h. These effects were opposed by cycloheximide (protein synthesis inhibitor), GM-6001 (broad spectrum inhibitor of MMPs), minocycline (MMP-2 and MMP-9 inhibitor), and MMP-9 inhibitor I (selective MMP-9 inhibitor). Cycloheximide, GM-6001, and MMP-9 inhibitor I also attenuated the increase in permeability to FITC-dextran (10 kDa). In addition, TNF-α led to an increased MMP-9 activity in the conditioned medium as well as a nearly 20-fold increase in mRNA for MMP-9 but not for MMP-2. The functional activity and increase in mRNA levels of MMP-9 were blocked by SB-203580 (selective p38 MAP kinase inhibitor) and cycloheximide. In conclusion, transcriptional and translational activation of MMP-9, downstream of p38 MAP kinase signaling, is involved in the (TNF-α)-induced loss of corneal endothelial barrier integrity.