Impairment of erbB1 receptor and fluid-phase endocytosis and associated mitogenic signaling by inositol hexaphosphate in human prostate carcinoma DU145 cells.

Recently, we observed that epidermal growth factor receptor (EGFR or erbB1) endocytosis and associated mitogenic signaling occur in human prostate cancer (PCA) cells, suggesting that erbB1 endocytosis might be involved in advanced and androgen-independent PCA growth. Based on these findings, and the fact that aberrant expression of erbB family members is common in human prostatic intraepithelial neoplasia and invasive PCA, we reasoned that impairment of erbB1 endocytosis and associated mitogenic signaling might inhibit PCA growth. Inositol hexaphosphate (IP6) interacts with plasma membrane clathrin-associated protein complex 2 (AP2) and inhibits phosphatidylinositol 3-kinase (PI3K). As these are essential components of receptor-mediated and fluid-phase endocytosis, respectively, we reasoned that IP6 might impair erbB1 endocytosis and associated signaling in human PCA cells, leading to their growth inhibition. IP6 strongly to completely inhibited (26-100%; P < 0.05) transforming growth factor alpha-induced binding of activated erbB1 to AP2 in human PCA DU145 cells, demonstrating the impairment of the initial step in ligand-induced erbB1 endocytosis. IP6 treatment of cells resulted in a dose-dependent increase (1.8- to 7. 7-fold compared with cells treated with ligand alone; P < 0.05) in levels of activated erbB1. These two findings suggest that the inhibitory effect of IP6 on receptor endocytosis is independent of its lack of effect on ligand-induced erbB1 activation. These effects of IP6, however, were associated with strong inhibition of ligand-induced Shc phosphorylation (77-84% decrease; P < 0.05) and its binding to erbB1 (58-100% decrease; P < 0.05). IP6 also significantly and dose-dependently inhibited fluid-phase endocytosis (19-52%; P < 0.05). It inhibited PI3K-AKT signaling pathway as an upstream response in its effect on the inhibition of fluid-phase endocytosis. The inhibition of erbB1 receptor and fluid-phase endocytosis, and associated signaling by IP6, was corroborated by very strong to complete inhibition (70-100%; P < 0.05) of extracellular signal-regulated protein kinase 1/2 activation by IP6. IP6 significantly (P < 0.05) inhibited anchorage-dependent and -independent inhibition (50-100% and 30-75%, respectively) in DU145 cells. Targeting the impairment of erbB1 endocytosis and associated mitogenic signaling by IP6 in advanced and androgen-independent human PCA DU145 cells could be a useful approach for treating PCA.

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