Platelet-type 12-lipoxygenase accelerates tumor promotion of mouse epidermal cells through enhancement of cloning efficiency

Accumulating evidence suggests that platelet-type 12-lipoxygenase (p12-LOX) plays an important role in tumor development. However, how p12-LOX contributes to tumorigenesis is still not understood. The role of p12-LOX was therefore examined in tumor promotion using mouse epidermal JB6 P+ cells that are sensitive to 12-O- tetradecanoylphorbol-13-acetate-induced transformation. The expression of p12-LOX was significantly higher in JB6 P+ cells than in JB6 P- cells that were resistant to transformation, and its expression was further increased by tumor necrosis factor (TNF)-α. Importantly, the inhibition of p 12-LOX in JB6 P+ cells by baicalein, a specific inhibitor or small interfering RNA significantly suppressed TPA-induced transformation. Moreover, treatment with 12(S)-hydroxyeicosatetraenoic acid (HETE), a metabolite of p12-LOX, enhanced TPA-induced neoplastic transformation either in the presence or absence of baicalein. These results indicate that p12-LOX is required for tumor promotion of epidermal cells and that 12(S)-HETE functions as a rate-limiting factor. Notably, treatment with baicalein significantly suppressed the proliferation of JB6 P+ cells when cells were seeded at a low density in a culture plate. Moreover, the cloning efficiency of JB6 P+ cells was dramatically decreased by inhibition of p12-LOX. In contrast, baicalein treatment did not affect the cloning efficiency of most malignant cancer cells. These results indicate that p12-LOX is induced by the inflammatory cytokine TNF-α in the early stage of tumorigenesis, and is required for tumor promotion through enhancing efficient proliferation of a small number of initiated cells. The present results suggest that the p12-LOX pathway may be an effective target of chemoprevention for skin carcinogenesis.