Background/aim: Breast cancer is a heterogeneous disease. Animal studies indicate that this heterogeneity is caused, in part, by the type of carcinogen which causes this disease. Recently, molecular-targeted drugs, such as rapamycin, are also reported to be heterogeneous in their therapeutic effects on breast tumors. The aim of this study was to clarify the activity of mammalian target of rapamycin (mTOR), as determined by the phosphorylation status of eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), in rat mammary carcinomas induced by several carcinogens to see if it is associated with the carcinogen species.
Materials and methods: The expression level of 4E-BP1 protein in its phosphorylated (Thr37/46) and unphosphorylated forms was assessed by Western blotting and immunohistochemistry in Sprague-Dawley rat mammary carcinomas induced by γ-rays, carbon ions, 1-methyl-1-nitrosourea (MNU), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), and γ-rays combined with MNU or PhIP, and in normal mammary glands.
Results: 4E-BP1 was composed of at least five isoforms whose expression varied among the carcinomas. Interestingly, loss of their expression, which has not been described previously, was observed in 7 out of 56 carcinomas (13%) regardless of the carcinogen used. Phosphorylation at Thr37/46 of 4E-BP1 was detected in the largest two isoforms in most carcinomas, but in smaller isoforms in carcinomas induced by γ-rays plus PhIP. Quantitative analysis revealed a significant decrease in phosphorylated 4E-BP1 levels in the carcinomas induced by MNU alone or MNU combined with γ-rays.
Conclusion: Expression of 4E-BP1 isoforms varied among rat mammary carcinomas, their phosphorylation level being low in MNU-induced carcinomas, suggesting an association of mTOR activity with cancer etiology.