Aberrant expression and phosphorylation of 4E-BP1, a main target of mTOR signaling, in rat mammary carcinomas: an association with etiology.
OBJECTIVE
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.


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.


CONCLUSIONS
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.
Radiobiology for Children's Health Program, Research Center for Radiation Protection, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan.
Masaru
M
Takabatake
Kazuhiro
K
Daino
Tatsuhiko
T
Imaoka
Mayumi
M
Nishimura
Takamitsu
T
Morioka
Masahiro
M
Fukushi
Yoshiya
Y
Shimada
D000818:Animals; D015153:Blotting, Western; D002352:Carrier Proteins / Q000378:metabolism*; D005260:Female; D007150:Immunohistochemistry; D047908:Intracellular Signaling Peptides and Proteins; D008325:Mammary Neoplasms, Experimental / Q000209:etiology / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism*
D002352:Carrier Proteins; C499526:Eif4ebp1 protein, rat; D047908:Intracellular Signaling Peptides and Proteins; D010750:Phosphoproteins; C546845:mTOR protein, rat; D058570:TOR Serine-Threonine Kinases