15-Lipoxygenase-1 Activates Tumor Suppressor p53 Independent of Enzymatic Activity

15-LOX-1 and its metabolites are involved in colorectal cancer. Recently, we reported that 15-LOX-1 overexpression in HCT-116 human colorectal cancer cells inhibited cell growth by induction of p53 phosphorylation (4). To determine whether the 15-LOX-1 protein or its metabolites are responsible for phosphorylation of p53 in HCT-116 cells, we used HCT-116 cells that expressed a mutant 15-LOX-1. The mutant 15-LOX-1 enzyme, with a substitution of Leu at residue His361, was devoid of enzymatic activity. HCT-116 cells transiently transfected with either native or mutant 15-LOX-1 showed an increase in p53 phosphorylation and an increase in the expression of downstream genes. Thus 15-LOX-1 induces p53 phosphorylation independent of enzymatic activity. Treatment of A549 human lung carcinoma cells with IL-4 increased the expression of 15-LOX-1 and also increased the expression of downstream targets of p53. This confirmed that the activation of p53 was also observed in wild type cells expressing physiological 15-LOX-1. Immunoprecipitation experiments revealed that 15-LOX-1 interacts with, and binds to, DNA-dependent protein kinase (DNA-PK). The binding of 15-LOX-1 to DNA-PK caused an approximate 3.0 fold enhancement in kinase activity, resulting in increased p53 phosphorylation a