UV induced bystander signaling leading to apoptosis

Abstract 

Human keratinocytes (HaCaT) were exposed to UV (A+B) (UVA-350–400mJ/cm² and UVB-30mJ/cm²) which induces apoptosis as evidenced by MTT assay, DNA laddering, Bax and Fas up-regulation. UV induced apoptotic conditioned media (6h or earlier) did not cause apoptosis in unexposed cells. However, treatment with conditioned medium collected post UV exposure (1h) induced Bax in unexposed cells as observed by RT-PCR. The induction of cell death was initiated by conditioned medium collected 12h after UV exposure and the extent of death was increased progressively when conditioned medium collected 24 or 72h post UV exposure was used. Medium collected 24h after UV exposure also increased mitochondrial membrane permeability as determined by rhodamine uptake. Conditioned medium induced apoptosis did not involve reactive oxygen species (ROS) unlike UV induced apoptosis indicating that the apoptosis pathway could be different. Interestingly, at high dilution apototic conditioned medium did not induce apoptosis but actually protected cells from UV insult. The role of nerve growth factor (NGF) in UV induced bystander effects are also discussed.

Keywords: Apoptosis, UV, Keratinocytes, Secreted factors, Protection, Nerve growth factor (NGF), Bystander effect