Bcl-2 overexpression protects photooxidative stress-induced apoptosis of photoreceptor cells via NF-kappaB preservation

We recently showed that photooxidative stress on cultured photoreceptor cells results in down-modulation of NF-kappaB activity which then leads to apoptosis of cultured 661W photoreceptor cells. In an effort to further delineate the mechanism of photoreceptor cell death, we sought to determine the effects of Bcl-2 overexpression on cell survivability. Wild-type 661W cells were transfected with the plasmid construct pSFFV-neo-Bcl-2 and several clones were isolated. All clones demonstrated increased Bcl-2 mRNA and protein levels, with the B4 clone exhibiting the greatest enhancement. On exposure to visible light the B4 cells were protected from undergoing apoptosis when compared with the mock transfected cells, as ascertained by TUNEL apoptosis assay and formazan based estimation of cell viability. The Bcl-2 overexpressing cells also maintained a higher Bcl-2/Bax ratio, suggesting that this ratio is important in protection from photooxidative stress. Electrophoretic mobility shift assays for NF-kappaB demonstrated higher activity in both nuclear and cytosolic fractions of the B4 photoreceptors compared with the 661W wild-type cells at all light exposure time points. Furthermore, the findings of the gel shift assays were further supported by immunocytochemistry for NF-kappaB which revealed that protein levels of the RelA subunit of NF-kappaB were protected in the nucleus as well as in the cytoplasm of Bcl-2 overexpressing B4 cells exposed to light compared to the 661W cells. These results suggest that Bcl-2 overexpression protects NF-kappaB protein levels and activity in the nucleus, indicating that preservation of NF-kappaB binding activity in the nucleus may be essential for photoreceptor cells to survive photooxidative damage induced apoptosis.