c-myc null cells misregulate cad and gadd45 but not other proposed c-Myc targets

Abstract

We report here that the expression of virtually all proposed c-Myc target genes is unchanged in cells containing a homozygous null deletion of c-myc. Two noteworthy exceptions are the gene cad, which has reduced log phase expression and serum induction in c-myc null cells, and the growth arrest gene gadd45, which is derepressed by c-myc knockout. Thus, cad and gadd45 are the only proposed targets of c-Myc that may contribute to the dramatic slow growth phenotype of c-myc null cells. Our results demonstrate that a loss-of-function approach is critical for the evaluation of potential c-Myc target genes.

Tumor promoter-induced ornithine decarboxylase gene expression occurs independently of AP-1 activation

“…Overexpressed c-Myc has been shown to activate ODC gene expression independent of growth factors (BelloFernandez et al, 1993). However, Bush et al (1998) reported no di erence in serum-induced ODC gene expression in ®broblasts that had c-myc knocked out compared to wild type ®broblasts when normalized to GADPH levels. Packham and Cleveland (1997) have postulated a biphasic model in which ODC gene expression is regulated by two independent pathways after interleukin-3 (IL-3) stimulation.…”

A genetic screen to identify genes that rescue the slow growth phenotype of c-myc null fibroblasts

“…It was shown that cmyc 7/7 cells have a reduced activity of all cyclin/CDK complexes, and CDK7 and p27 were implicated as downstream e ectors of c-Myc (Mateyak et al, 1999). Interestingly, when these cells were analysed for expression of several putative c-Myc-regulated genes, only the expression of cad and gadd45 appeared to be a ected, suggesting that many of the proposed c-Mycactivated genes do not contribute to the growth defect of the c-myc null cells (Bush et al, 1998). Although the mechanism of c-Myc-induced repression is still unclear, accumulating evidence suggests that gene repression may be su cient for many aspects of c-Myc function.…”

The conserved Myc box 2 and Myc box 3 regions are important, but not essential, for Myc function in vivo

“…Furthermore, an MB2-mutant Myc very inefficiently rescues the proliferation defect of rat fibroblasts lacking endogenous c-Myc (Bush et al, 1998;Xiao et al, 1998;Nikiforov et al, 2002;Oster et al, 2003). The MB2 motif is required for the efficient interaction with several co-factors such as TRRAP and associated HATs (McMahon et al, 1998;Bouchard et al, 2001;Frank et al, 2001;Nikiforov et al, 2002), the HAT complex component BAF53 , TIP48 & TIP49 , Skp2 (Kim et al, 2003;von der Lehr et al, 2003), and for the stimulation of pre-mRNA capping (Cowling and Cole, 2007).…”

“…Nevertheless, it has been shown that a Myc mutant lacking MB2 still efficiently activates the expression of some target genes and artificial reporters (Kato et al, 1990;BelloFernandez et al, 1993;Li et al, 1994;Brough et al, 1995;Lee et al, 1997;Xiao et al, 1998;Nikiforov et al, 2002), although many targets are only poorly activated by such a protein (Conzen et al, 2000;Hirst and Grandori, 2000;Bouchard et al, 2001;Nikiforov et al, 2002;Kenney et al, 2003;Cowling and Cole, 2007). Such a MycΔMB2 mutant also fails to repress the genes that are normally down-regulated by wild type Myc (Li et al, 1994;Lee et al, 1997), including the c-myc locus itself (Penn et al, 1990;Bush et al, 1998;Oster et al, 2003). Importantly, the biological function of the MB2 cannot be replaced by the strong transactivation domain of the viral transcription factor VP16 (Brough et al, 1995).…”