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Mammalian Target of Rapamycin and S6 Kinase 1 Positively Regulate 6-thioguanine-Induced Autophagy

Department of Radiation Oncology, Case Western Reserve University and the Case Integrative Cancer Biology Program, Case Comprehensive Cancer Center/University Hospitals Case Medical Center, Cleveland Ohio

Requests for reprints: Timothy J. Kinsella, Department of Radiation Oncology, LTR6068, University Hospitals Case Medical Center, 11100 Euclid Avenue, Cleveland, OH 44106-6068. Phone: 216-844-2530; Fax: 216-844-4799; E-mail: timothy.kinsella{at}uhhospitals.org.

Key Words: 6-thioguanine • autophagy • mTOR

DNA mismatch repair (MMR) ensures the fidelity of DNA replication and is required for activation of cell cycle arrest and apoptosis in response to certain classes of DNA damage. We recently reported that MMR is also implicated in initiation of an autophagic response after MMR processing of 6-thioguanine (6-TG). It is now generally believed that autophagy is negatively controlled by mammalian target of rapamycin (mTOR) activity. To determine whether mTOR is involved in 6-TG–induced autophagy, we used rapamycin, a potential anticancer agent, to inhibit mTOR activity. Surprisingly, we find that rapamycin cotreatment inhibits 6-TG–induced autophagy in MMR-proficient human colorectal cancer HCT116 (MLH1⁺) and HT29 cells as measured by LC3 immunoblotting, GFP-LC3 relocalization, and acridine orange staining. Consistently, short interfering RNA silencing of the 70-kDa ribosomal S6 kinase 1 (S6K1), the downstream effector of mTOR, markedly reduces 6-TG–induced autophagy. Furthermore, we show that inhibition of mTOR by rapamycin induces the activation of Akt as shown by increased Akt phosphorylation at Ser⁴⁷³ and the inhibition of 6-TG–induced apoptosis and cell death. Activated Akt is a well-known inhibitor of autophagy. In conclusion, our data indicate that mTOR-S6K1 positively regulates autophagy after MMR processing of 6-TG probably through its negative feedback inhibition of Akt. [Cancer Res 2008;68(7):2384–90]