The RNA-binding protein IGF2BP1 (IGF-II mRNA binding protein 1) stabilizes the c-myc RNA by associating with the Coding Region instability Determinant (CRD). If and how other proteins cooperate with IGF2BP1 in promoting stabilization of the cmyc mRNA via the CRD remained elusive. Here, we identify various RNA-binding proteins that associate with IGF2BP1 in an RNA-dependent fashion. Four of these proteins (HNRNPU, SYNCRIP, YBX1, and DHX9) were essential to ensure stabilization of the c-myc mRNA via the CRD. These factors associate with IGF2BP1 in a CRD-dependent manner, co-distribute with IGF2BP1 in non-polysomal fractions comprising c-myc mRNA, and colocalize with IGF2BP1 in the cytoplasm. A selective shift of relative c-myc mRNA levels to the polysomal fraction is observed upon IGF2BP1 knockdown. These findings suggest that IGF2BP1 in complex with at least four proteins promotes CRD-mediated mRNA stabilization. Complex formation at the CRD presumably limits the transfer of c-myc mRNA to the polysomal fraction and subsequent translation-coupled decay.

The IMP (IGFII mRNA-binding protein) family comprises a group of three RNA-binding proteins involved in the regulation of cytoplasmic mRNA-fate. Recent studies identified IMP proteins as oncofetal factors in various neoplasias, but knowledge of a potential role in ovarian carcinomas is still lacking. The immunohistochemical analysis of 107 ovarian carcinomas, 30 serous borderline tumors of the ovary and five normal ovaries revealed de novo synthesis of IMP1 in 69% of ovarian carcinomas. Elevated IMP1 expression was observed preferentially in high-grade and high-stage cases and was a significant prognostic indicator for reduced recurrence-free and overall survival. Phenotypic studies in ovarian carcinoma-derived ES-2 cells demonstrated that IMP1 knockdown affects proliferation and cell survival. Reduced proliferation was associated with decreased c-myc mRNA half-life, suggesting IMP1 as an oncogenic factor that is involved in promoting elevated proliferation by stabilizing the c-myc mRNA in ovarian carcinoma cells.