The mTOR inhibitor rapamycin down-regulates the expression of the ubiquitin ligase subunit Skp2 in breast cancer cells.
BACKGROUND
Loss of the cyclin-dependent kinase inhibitor p27 is associated with poor prognosis in breast cancer. The decrease in p27 levels is mainly the result of enhanced proteasome-dependent degradation mediated by its specific ubiquitin ligase subunit S phase kinase protein 2 (Skp2). The mammalian target of rapamycin (mTOR) is a downstream mediator in the phosphoinositol 3' kinase (PI3K)/Akt pathway that down-regulates p27 levels in breast cancer. Rapamycin was found to stabilize p27 levels in breast cancer, but whether this effect is mediated through changes in Skp2 expression is unknown.


METHODS
The expression of Skp2 mRNA and protein levels were examined in rapamycin-treated breast cancer cell lines. The effect of rapamycin on the degradation rate of Skp2 expression was examined in cycloheximide-treated cells and in relationship to the anaphase promoting complex/Cdh1 (APC\C) inhibitor Emi1.


RESULTS
Rapamycin significantly decreased Skp2 mRNA and protein levels in a dose and time-dependent fashion, depending on the sensitivity of the cell line to rapamycin. The decrease in Skp2 levels in the different cell lines was followed by cell growth arrest at G1. In addition, rapamycin enhanced the degradation rate of Skp2 and down-regulated the expression of the APC\C inhibitor Emi1.


CONCLUSIONS
These results suggest that Skp2, an important oncogene in the development and progression of breast cancer, may be a novel target for rapamycin treatment.
Shapira
Ma'anit
M
Department of Surgery A, Rambam Medical Center, Haifa 31096, Israel. m_shapira@rambam.health.gov.il
Kakiashvili
Eli
E
Rosenberg
Tzur
T
Hershko
Dan D
DD
D001943:Breast Neoplasms / Q000378:metabolism* / Q000503:physiopathology; D045744:Cell Line, Tumor; D015536:Down-Regulation; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005260:Female; D006801:Humans; D008025:Ligases; D011494:Protein Kinases / Q000378:metabolism*; D044786:S-Phase Kinase-Associated Proteins / Q000096:biosynthesis*; D020123:Sirolimus / Q000494:pharmacology*; D058570:TOR Serine-Threonine Kinases; D025801:Ubiquitin / Q000378:metabolism*
D004791:Enzyme Inhibitors; D044786:S-Phase Kinase-Associated Proteins; D025801:Ubiquitin; D011494:Protein Kinases; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases; D008025:Ligases; D020123:Sirolimus