Figure 1. Biological characteristics of a conditional c-Myc mouse liver tumor cell line.

A) A tumor cell line derived from the bitransgenic animal containing the liver associated protein (LAP) promoter driving the tetracycline transactivating protein (tTa) and c-Myc under the control of the tetracycline-response promoter (Tet-O). Addition of doxycycline prevents tTa protein from activating the Tet-O promoter. Absence of doxycycline triggers a conformational change that enables Tet-O binding, activation, and c-Myc transcription. B) Polymerase chain reaction indicates that Tet-O-MYC cells are positive for both LAP-tTa and c-Myc transcripts. C) MYC-ON cells show higher expression of messenger RNA for c-Myc than MYC-OFF cells by quantitative real-time polymerase chain reaction. D) Western blot result shows c-Myc expression only in the MYC-ON cells. E) Morphologically, MYC-OFF cells are larger than MYC-ON cells and have larger nuclei, increased cytoplasm, inconspicuous nucleoli, decreased nuclear-cytoplasmic ratio and more cytoplasmic processes. Immunofluoresence staining demonstrates increased c-MYC expression in the nuclei of the MYC-ON cells compared to MYC-OFF cells (original magnification×63). F) The proliferative activity of Tet-O-MYC cells is controlled by c-Myc. Removal of oncogenic c-Myc expression significantly attenuates Tet-O-MYC cell proliferation.