CRBP suppresses breast cancer cell survival and anchorage-independent growth

We showed earlier that cellular retinol-binding protein (CRBP) expression i s downregulated in a subset of human breast cancers. We have now investigat ed the outcome of ectopic CRBP expression in MTSV1-7 cells, a SV40 T antige n-transformed human breast epithelial cell line devoid of endogenous CRBP e xpression. We found that: (i) CRBP did not inhibit adherent cell growth but suppressed foci formation in post-confluent cultures and colony formation in soft agar; (ii) this effect was due to CRBP inhibition of cell survival, as demonstrated by viability and TUNEL assays of cells in soft-agar or pla ted on polyHEMA-coated dishes; (iii) CRBP inhibited protein kinase B/Akt ac tivation in cells in suspension but not in adherent cells and the CRBP supp ression of anchorage-independent growth was mimicked by cell treatment with the phosphatidylinositol-3 kinase (PI3K) inhibitor LY294002; (iv) CRBP enh anced retinyl ester formation and storage but did not regulate retinoic aci d synthesis or retinoic acid receptor activity. Ectopic CRBP-mediated inhib ition of anchorage-independent cell survival and colony formation in the ab sence of significantly altered responses to either retinol or retinoic acid was also documented in T47D human breast cancer cells. In conclusion, the data suggest two novel and linked CRBP functions in mammary epithelial cell s: inhibition of the PI3K/Akt survival pathway and suppression of anchorage -independent growth.