IDENTIFICATION OF PUTATIVE C-MYC-RESPONSIVE GENES - CHARACTERIZATION OF RCL, A NOVEL GROWTH-RELATED GENE

The c-Myc protein is a helix-loop-helix leucine zipper oncogenic trans cription factor that participates in the regulation of cell proliferat ion, differentiation, and apoptosis. The biochemical function of c-Myc has been well described, yet the identities of downstream effecters a re just beginning to emerge. We describe the identification of a set o f c-Myc-responsive genes in the Rat1a fibroblast through the applicati on of cDNA representational difference analysis (RDA) to cDNAs isolate d from nonadherent Rat1a and Rat1a myc cells. In this system, c-Myc ov erexpression is sufficient to induce the transformed phenotype of anch orage-independent growth. We identified 20 differentially expressed cD NAs, several of which represent novel cDNA sequences. We further chara cterized one of the novel cDNAs identified in this screen, termed rcl. rcl expression is (i) directly stimulated by c-Myc; (ii) stimulated i n the in vivo growth system of regenerating rat liver, as is c-myc; an d (iii) elevated in human lymphoid cells that overexpress c-myc. By us ing an anti-Rcl antibody, immunoblot analysis, and immunofluorescence microscopy, the Rcl protein was found to be a 23-kDa nuclear protein. Ectopic expression of the protein encoded by the rcl cDNA induces anch orage-independent growth in Rat1a fibroblasts, albeit to a diminished extent compared to ectopic c-Myc expression. These data suggest a role for rcl during cellular proliferation and c-Myc-mediated transformati on.