Abstract
Translationally controlled tumor protein (TCTP) is expressed in many tissues, particularly in human tumors. It plays a role in malignant transformation, apoptosis prevention, and DNA damage repair. The signaling mechanisms underlying TCTP regulation in cancer are only partially understood. Here, we investigated the role of mTORC1 in regulating TCTP protein levels, thereby modulating chemosensitivity, in human lung cancer cells and an A549 lung cancer xenograft model. The inhibition of mTORC1, but not mTORC2, induced ubiquitin/proteasome-dependent TCTP degradation without a decrease in the mRNA level. PLK1 activity was required for TCTP ubiquitination and degradation and for its phosphorylation at Ser46 upon mTORC1 inhibition. Akt phosphorylation and activation was indispensable for rapamycin-induced TCTP degradation and PLK1 activation, and depended on S6K inhibition, but not mTORC2 activation. Furthermore, the minimal dose of rapamycin required to induce TCTP proteolysis enhanced the efficacy of DNA-damaging drugs, such as cisplatin and doxorubicin, through the induction of apoptotic cell death in vitro and in vivo. This synergistic cytotoxicity of these drugs was induced irrespective of the functional status of p53. These results demonstrate a new mechanism of TCTP regulation in which the mTORC1/S6K pathway inhibits a novel Akt/PLK1 signaling axis and thereby induces TCTP protein stabilization and confers resistance to DNA-damaging agents. The results of this study suggest a new therapeutic strategy for enhancing chemosensitivity in lung cancers regardless of the functional status of p53.
Highlights
Controlled tumor protein (TCTP) is expressed in many tissues, in human tumors
In A549 cells, Raptor siRNA transfection (Fig. 1D, lanes 1 and 3) and PP242 treatment (Fig. 1E) reduced the level of Translationally controlled tumor protein (TCTP) protein, whereas Rictor knockdown did not (Fig. 1D, lanes 1 and 2). These results indicate that mTOR complex 1 (mTORC1), but not mTOR complex 2 (mTORC2), plays an important role in maintaining a high protein level of TCTP, which is commonly observed in lung cancer cells
TCTP protein degradation upon mTORC1 inhibition is accompanied by an increase in polo-like kinase 1 (PLK1) phosphorylation at T hr[210] and abolished by inhibiting PLK1 activation. (A) PLK1 phosphorylation and activation was induced in A549 cells after rapamycin treatment. (B,C) The decrease in TCTP protein levels caused by rapamycin treatment or raptor knockdown was abrogated by PLK1 inhibition. (B) Cells were pretreated with cyclapolin 9, a PLK1 inhibitor, for 2 h and treated for 12 h with 50 pM rapamycin (n = 4). (C) A549 cells were treated with 5 μM cyclapolin 9 beginning 4 h after transfection with 100 pM siRNA and harvested 48 h after transfection. (D) The decrease in TCTP protein levels by rapamycin treatment was abolished by PLK1 knockdown
Summary
Controlled tumor protein (TCTP) is expressed in many tissues, in human tumors. The decrease in TCTP protein levels caused by mTORC1 inhibition was abrogated by cyclapolin 9 pretreatment in A549, H1299, and H157 cells (Fig. 3B) and inhibition of PLK1 activity by the inhibitor was confirmed by a significant change in the phosphorylation of Cdc25C at Ser[198] (Fig. 3B, left panel and Fig. 3C).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
