FIG. 4.

c-Myc represses FLIP expression in vivo. (A and B) c-myc-expressing tumor xenografts have diminished FLIP expression. Ki-ras- or Ki-ras/c-myc-transformed p53-null colonocytes were injected subcutaneously into the flanks of mice, and tumors were collected 15 days later. (A) The left panel shows FLIP expression in tumors (with or without c-myc) obtained in two separate experiments. The right panel shows FLIP expression in the cultured cells from one of these experiments prior to injection. (B) Representative sections subjected to immunohistochemical staining for FLIP (brown) from tumors shown in panel A. (C to E) Conditionally activated myc-ER-expressing tumors show reduced FLIP mRNA and protein levels. Myc OFF and “tamoxifen−” refer to small neoplastic lesions from tumor-bearing animals originally treated with but subsequently deprived of 4-hydroxytamoxifen. Myc ON and “tamoxifen +” refer to analogous lesions after restimulation with 4- hydroxytamoxifen. (C) Tumors from these mice were analyzed for FLIP mRNA expression using real-time RT-PCR. Actin levels are shown as controls. (D) Gel electrophoresis of PCR products from real-time RT-PCR analysis depicted in panel C. (E) Immunoblotting of tumor lysates corresponding to samples analyzed in panel C.