The IκB family member Bcl-3 stabilizes c-Myc in colorectal cancer.

The IkB family member Bcl-3 stabilizes c-Myc in colorectal cancer Dear Editor, The proto-oncogene c-myc has been thought to play a critical role during the tumor-initiating process in multiple human cancers. Among others, colorectal cancer (CRC) is particularly associated with deregulated expression of c-Myc (Meyer and Penn, 2008; Wilkins and Sansom, 2008). Physiologically, Myc mRNA and protein levels are tightly regulated, and the Myc protein is highly unstable. The high levels of Myc protein in human CRC could be attributed to the altered Myc turnover and aberrant transcriptional activation of the myc genes (Ikegaki et al., 1986; Welcker and Clurman, 2008). The atypical member of the IkB family Bcl-3 can bind to p50 and p52 homodimers on DNA, thereby positively or negatively regulating the expression of NF-kB target genes, depending on the context (Fujita et al., 1993; Wang et al., 2012). Recently, high levels of Bcl-3 have been noted in a variety of solid cancers including CRC (Puvvada et al., 2010; Maldonado and Melendez-Zajgla, 2011). However, the function of Bcl-3 in colorectal tumorigenesis remains to be elucidated. We found that human CRC tissues exhibited increased levels of Bcl-3 compared with colorectal normal tissues (Figure 1A and B). In order to explore the role of Bcl-3 in human colorectal tumorigenesis, we transduced the human CRC cell line HCT116 with a tet-on lentiviral vector containing the shRNA against bcl-3 gene to establish the doxycyc-line (DOX)-inducible Bcl-3 knockdown cell line (HCT116/shBcl-3) (Supplementary Figure S1A). The in vitro cell growth was significantly slower upon Bcl-3 knockdown in HCT116 cells (Figure 1C and Supplementary Figure S2A). Bcl-3 knockdown also significantly suppressed the ability of HCT116 cells to form colonies in plate and soft agar (Supplementary Figure S2B and C). The inhibition was associated with a block in the G1/S transition of cell cycle (Supplementary Figure S2D). The inhibitory effects of Bcl-3 knockdown on in vitro CRC cell growth were also observed in mouse CRC cell line CT26 WT (Supplementary Figures S1B, S3A – C), excluding that the growth inhibition was due to the off-target effects of Bcl-3 knockdown. These results indicate that Bcl-3 knockdown suppresses colorectal tumor cell growth in vitro. To elucidate the mechanism by which Bcl-3 regulated the cell cycle, we compared the expressions of cell cycle-related genes between wild-type and Bcl-3 knockdown cells. We found that the level of c-Myc protein, but not the mRNA, was markedly decreased upon Bcl-3 knockdown. Accordingly …