Elevated Phosphate Activates N-ras and Promotes Cell Transformation and Skin Tumorigenesis

Abstract

Recent results suggest a paradigm shift from viewing inorganic phosphate as a passive requirement for basic cell functions to an active regulator of cell behavior. We have previously shown that elevated concentrations of phosphate increased cell proliferation and expression of protumorigenic genes such as Fra-1 and osteopontin in a preosteoblast cell line. Therefore, we hypothesized that elevated phosphate concentrations would promote cell transformation in vitro and tumorigenesis in vivo. Supplementation of medium with phosphate increased anchorage-independent transformation and proliferation of BALB/c mouse JB6 epidermal cells, activation of N-ras, ERK1/2, and activator protein-1, and increased gene expression of Fra-1, COX-2, and osteopontin in a dose-dependent manner. These in vitro results led to the hypothesis that varying the levels of dietary inorganic phosphate would alter tumorigenesis in the mouse model of skin carcinogenesis. Female FVB/N mice were treated with 7,12-dimethylbenz(a)anthracene/12-O-tetradecanoylphorbol-13-acetate and fed high- or low-phosphate diets (1.2% versus 0.2% of the diet) for 19 weeks. The high-phosphate diet increased skin papilloma number by approximately 50% without changing feed intake and body weights. High dietary phosphate increased serum concentrations of phosphate, parathyroid hormone, and osteopontin and decreased serum concentrations of calcium. Thus, we conclude that elevated phosphate promotes cell transformation and skin tumorigenesis partly by increasing the availability of phosphate for activation of N-ras and its downstream targets, which defines reducing dietary phosphate as a novel target for chemoprevention.