Signal transduction by the CEACAM1 tumor suppressor. Phosphorylation of serine 503 is required for growth-inhibitory activity.

CEACAM1 is a cell-cell adhesion molecule that mediates homophilic cell adhesion. In addition, CEACAM1 was also shown to suppress the growth of prostate, breast, and colon tumors. Structural and functional analyses showed that the adhesion activity of CEACAM1 is mediated by its extracellular domain while its cytoplasmic domain is necessary and sufficient for growth-inhibitory activity. The signal pathways leading to CEACAM1-mediated growth suppression are not known. We studied the importance of phosphorylation of serine 503 in this growth-inhibitory signaling pathway. Full-length CEACAM1 was found to be phosphorylated in vivo in both tyrosine and serine residues. Mutation of tyrosine 488 to phenylalanine did not abolish the tumor-suppressive activity of CEACAM1, suggesting that phosphorylation at tyrosine 488 is not critical for CEACAM1's tumor-suppressive activity. Although expression of CEACAM1's cytoplasmic domain inhibited the growth of DU145 prostate cancer cells in vivo, mutation of serine 503 to alanine abolished the growth-inhibitory activity. In addition, the change of serine 503 to aspartic acid produced tumor-suppressive activity similar to that of the wild-type CEACAM1. These results suggested that phosphorylation at serine 503 is essential for CEACAM1's growth-inhibitory function in vivo.

The cytoplasmic domain of C-CAM1 tumor suppressor is necessary and sufficient for suppressing the tumorigenicity of prostate cancer cells.

We have previously shown that C-CAM1 cell adhesion molecule can suppress the growth of prostate cancer cells in vivo. In this study, we determined the minimal domain of C-CAM1 that is required for its tumor-suppressive activity. DU145 prostate cancer cells were infected with recombinant adenoviruses containing various C-CAM1 mutant genes, and the effects of the mutant C-CAM1 proteins on the growth of DU145 cells were assessed in a nude-mice xenograft model. Deletion of C-CAM1's cytoplasmic domain, which is not required for its adhesion activity, abolished the growth-suppressive activity, whereas deletion of the adhesion domain did not. This observation suggests that C-CAM1's extracellular domain may be not essential for its tumor suppressive activity. Indeed, we found that expression of the C-CAM1 cytoplasmic domain alone led to growth suppression of DU145 cells. These results suggest that the cytoplasmic domain of C-CAM1 is necessary and sufficient for its growth-suppressive function.