Disabled-2 exerts its tumor suppressor activity by uncoupling c-Fos expression and MAP kinase activation

Disabled-2 (Dab2) is a putative tumor suppressor in breast and ovarian cancers. Its expression is lost in a majority of tumors, and homozygous deletions have been identified in a small percentage of tumors. Dab2 expression is absent or very low in the majority of breast and ovarian cancer cell lines, including MCF-7 and SK-Br-3 breast cancer cells. Transfection and expression of Dab2 in MCF-7 and SK-Br-3 cells suppress tumorigenicity. The cells reach a much lower saturation density and have reduced ability to form colonies on agar plates. In examining the signal transduction pathway of Dab2-transfected cells, we found that serum-stimulated c-Fos expression was greatly suppressed; however, the effects of Dab2 on MAPK family kinases were not as consistent. In MCF-7 and SK-Br-3 cells, although c-Fos expression was suppressed, the Erk1/2, JNK, and p38(MAPK) activities were unchanged or even increased. Serum-stimulated c-Fos expression is dependent on MAPK/Erk activity because the MEK inhibitor PD98059 suppresses Erk activity and c-Fos expression. Therefore, Dab2 appears to uncouple MAPK activation and c-fos transcription. Thus, we conclude that Dab2 re-expression suppresses tumorigenicity by reducing c-Fos expression at a site downstream of the activation of MAPK family kinases. Because Dab2 is frequently lost in cancer, the uncoupling of MAPK activation and c-Fos expression may be a favored target for inactivation in tumorigenicity.