1. p21 Waf1/Cip1 Is a Novel Downstream Target of 40S Ribosomal S6 Kinase 2.
- Author
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Basu, Alakananda and Xuan, Zhenyu
- Subjects
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PROTEINS , *DRUG resistance in cancer cells , *RESEARCH funding , *BREAST tumors , *APOPTOSIS , *CELLULAR signal transduction , *RNA , *GENE expression profiling , *WESTERN immunoblotting , *DOXORUBICIN , *MTOR inhibitors , *RIBOSOMAL proteins , *PHOSPHOTRANSFERASES , *SIGNAL peptides , *SEQUENCE analysis , *PHARMACODYNAMICS - Abstract
Simple Summary: The majority of breast cancers express estrogen receptors (ER) and are treated with antiestrogens. However, many patients relapse and do not respond to further treatment. Therefore, an understanding of which molecules/pathways are affected in breast cancer is important. The Akt/mTOR (mechanistic target of rapamycin) pathway is often altered in breast cancers and is an important target for cancer therapy. The ribosomal S6 kinase 2 (S6K2) acts downstream of mTOR and has been associated with ER-positive breast cancers, but little is known about how S6K2 functions in these cancers. The aim of our present study was to identify potential downstream effectors of S6K2. We identified p21/CDKN1A as a novel downstream target of S6K2 in ER-positive breast cancer cells and showed that S6K2 acts via Akt and JNK signaling pathways to regulate p21 and an upregulation of p21 contributes to chemoresistance. Thus, targeting components of the S6K2/p21 signaling pathway could reverse chemoresistance. Background/Objectives: The ribosomal S6 kinase 2 (S6K2) acts downstream of the mechanistic target of rapamycin complex 1 and is a homolog of S6K1 but little is known about its downstream effectors. The objective of this study was to use an unbiased transcriptome profiling to uncover how S6K2 promotes breast cancer cell survival. Methods: RNA-Seq analysis was performed to identify novel S6K2 targets. Cells were transfected with siRNAs or plasmids containing genes of interest. Western blot analyses were performed to quantify total and phosphorylated proteins. Apoptosis was monitored by treating cells with different concentrations of doxorubicin. Results: Silencing of S6K2, but not S6K1, decreased p21 in MCF-7 and T47D breast cancer cells. Knockdown of Akt1 but not Akt2 decreased p21 in MCF-7 cells whereas both Akt1 and Akt2 knockdown attenuated p21 in T47D cells. While Akt1 overexpression enhanced p21 and partially reversed the effect of S6K2 deficiency on p21 downregulation in MCF-7 cells, it had little effect in T47D cells. S6K2 knockdown increased JUN mRNA and knockdown of cJun enhanced p21. Low concentrations of doxorubicin increased, and high concentrations decreased p21 levels in T47D cells. Silencing of S6K2 or p21 sensitized T47D cells to doxorubicin via c-Jun N-terminal kinase (JNK)-mediated downregulation of Mcl-1. Conclusions: S6K2 knockdown enhanced doxorubicin-induced apoptosis by downregulating the cell cycle inhibitor p21 and the anti-apoptotic protein Mcl-1 via Akt and/or JNK. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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