Ras-dependent recruitment of c-Myc for transcriptional activation of nucleophosmin/B23 in highly malignant U1 bladder cancer cells.

U1 bladder cancer cells of high malignancy exhibited higher proliferation capacity than U4 premalignant cells. Higher expression of Ras, c-Myc, and nucleophosmin/B23 and greater c-Myc transactivation and nucleophosmin/B23 promoter activities were detected in U1 cells compared with U4 cells. Moreover, c-Myc and nucleophosmin/B23 were increased in U1 but not in U4 cells upon serum stimulation from quiescence. Likewise, only in U1 cells could serum stimulate transcriptional activity of nucleophosmin/B23 promoter and c-Myc response element. The increase of nucleophosmin/B23 promoter activity could be abrogated by mitogen-activated protein kinase/extracellular signal-regulated kinase activating kinase inhibitor and was associated with recruitment of c-Myc to the promoter. U1 cells constitutively expressing dominant-negative Ras reduced the levels of Ras, nucleophosmin/B23, and p-ERK, and consequently abolished the serum-induced up-regulation of nucleophosmin/B23 promoter activity and c-Myc promoter recruitment. Our results indicate that Ras and c-Myc play important roles in the up-regulation of nucleophosmin/B23 during proliferation of cells associated with a high degree of malignancy, thus outlining a signaling cascade involving these factors in the cancer cells.