TAK1 is involved in the autophagy process in retinal pigment epithelial cells.

Autophagy is an evolutionarily conserved mechanism for degrading long-lived or malfunctioning proteins and organelles, such as those resulting from oxidative stress. Several publications have demonstrated the importance of the autophagy process in the pathophysiology of dry age-related macular degeneration (AMD). Still, the mechanism underlying this process and its involvement in dry AMD are not fully characterized. Investigating the autophagy process in retinal pigment epithelial (RPE) cells, we identified transforming growth factor ß activated kinase 1 (TAK1) as a key player in the process. We found increased TAK1 phosphorylation in ARPE-19 and D407 cells treated with different inducers of autophagy, such as oxidative stress and rapamycin. Moreover, utilizing TAK1 specific inhibitor prior to oxidative stress or rapamycin treatment, we found significant reduction in LC3A/B-II expression. These results point at the involvement of TAK1 in the regulation of autophagy in RPE cells. This study suggests that aberrant activity of this kinase impairs autophagy and subsequently leads to alterations in the vitality of RPE cells. Proper activity of TAK1 may be essential for efficient autophagy, and crucial for the ability of RPE cells to respond to stress and dispose of damaged organelles, thus preventing or delaying retinal pathologies.

TAK1 inhibition accelerates cellular senescence of retinal pigment epithelial cells.

PURPOSE: Oxidative stress and cellular senescence are known to contribute to the development of AMD; however, the mechanism is not fully understood. This study investigated the role of TGF-ß-activated kinase 1 (TAK1) in the senescence of RPE cells as a model for the development of dry AMD. METHODS: Cultured human RPE cells were treated with the TAK1 inhibitor 5Z-7-oxozeaenol for 1 hour, and then treated with 200 µM hydrogen peroxide for 1 hour. Human RPE cells that were not pretreated with TAK1 inhibitor for 1 hour served as controls. Senescence-associated ß-galactosidase (SA-ß-gal) activity was detected by histochemistry, and p53 expression by immunoblotting. Cell-cycle and apoptosis rate in RPE cells were determined by flow cytometry. RESULTS: The TAK1 expression in human RPE cells was high and was altered on oxidative stress. Transforming growth factor-ß-activated kinase 1 inhibition led to reduction in cell proliferation, cell-cycle arrest at G0/G1, and increased SA-ß-gal expression, all known to be features of cell senescence. Exposure of cells to oxidative stress combined with inhibition of TAK1 activity decreased the expression of apoptotic proteins, such as p53, and promoted cellular senescence. Aberrant TAK1 activity in RPE cells triggered their secretion of factors that induced hypertrophy and fibrotic changes in neighboring cells. CONCLUSIONS: The in vitro evidence indicated a role for TAK1 in the onset of senescence in RPE cells. The data shown hereby demonstrated that TAK1 activity is essential for maintaining normal function of RPE cells. Elucidation of its role in mechanisms underlying RPE cellular senescence induction may potentiate development of powerful tools for halting the development of dry AMD.