pf-4708671 and Breast-Neoplasms

The unique metabolic profile of cancer (aerobic glycolysis) is an attractive therapeutic target for cancer. Dichloroacetate (DCA), an inhibitor of pyruvate dehydrogenase kinase, has been shown to reverse glycolytic phenotype and induce mitochondrion-dependent apoptosis. In the present study, we investigated the effects of S6 kinase 1 (S6K1) inhibition on DCA-induced cell death and the underlying mechanisms in breast cancer cells.. Cell death was evaluated by annexin V and PI staining. The synergistic effects of DCA and PF4708671 were assessed by isobologram analysis. Small interfering RNA (siRNA) was used for suppressing gene expression. The mRNA and protein levels were measured by RT-PCR and Western blot analysis, respectively.. PF4708671, a selective inhibitor of S6K1, and knockdown of S6K1 with specific siRNA enhanced DCA-induced cell death. Interestingly, a combination of DCA/PF4708671 markedly reduced protein expression of a glycolytic enzyme, hexokinase 2 (HK2). Suppression of HK2 activity using specific siRNA and 2-deoxyglucose (2-DG) further enhanced cell sensitivity to DCA/PF4708671. Overexpression of Myc-tagged HK2 rescued cell death induced by DCA/PF4708671.. Based on these findings, we propose that inhibition of S6K1, in combination with the glycolytic inhibitor, DCA, provides effective cancer therapy.

Topics: Antineoplastic Combined Chemotherapy Protocols; Breast Neoplasms; Cell Death; Cell Proliferation; Dichloroacetic Acid; Drug Synergism; Enzyme Inhibitors; Female; Humans; Imidazoles; MCF-7 Cells; Piperazines; Ribosomal Protein S6 Kinases, 70-kDa; RNA Interference; RNA, Small Interfering; Tumor Cells, Cultured

Topics: Animals; Breast Neoplasms; Cell Line, Tumor; Female; Humans; Imidazoles; Mice; Neoplasm Recurrence, Local; Piperazines; Ribosomal Protein S6 Kinases, 70-kDa

S6 kinase 1 (S6K1) was suggested to be a marker for endocrine therapy resistance in breast cancer. We examined whether tamoxifen's effect can be modulated by S6K1 inhibition. S6K1 inhibition by PF4708671, a selective inhibitor of S6K1, acts synergistically with tamoxifen in S6K1-high MCF-7 cells. Similarly, the knockdown of S6K1 with small interfering RNA (siRNA) significantly sensitized MCF-7 cells to tamoxifen. Inhibition of S6K1 by PF4708671 led to a marked decrease in the expression levels of the anti-apoptotic proteins Mcl-1 and survivin, which was not related to mRNA levels. In addition, suppression of Mcl-1 or survivin, using specific siRNA, further enhanced cell sensitivity to tamoxifen. These results showed that inhibition of S6K1 acts synergistically with tamoxifen, via translational modulation of Mcl-1 and survivin. Based on these findings, we propose that targeting S6K1 may be an effective strategy to overcome tamoxifen resistance in breast cancer.

Topics: Apoptosis; Breast Neoplasms; Cell Line, Tumor; Cell Survival; Drug Resistance, Neoplasm; Female; Gene Expression; Humans; Imidazoles; Inhibitor of Apoptosis Proteins; MCF-7 Cells; Myeloid Cell Leukemia Sequence 1 Protein; Piperazines; Receptors, Estrogen; Ribosomal Protein S6 Kinases, 70-kDa; RNA Interference; RNA, Small Interfering; Survivin; Tamoxifen

Nutrient-limiting conditions are frequently encountered by tumor cells in poorly vascularized microenvironments. These stress conditions may facilitate the selection of tumor cells with an inherent ability to decrease apoptotic potential. Therefore, selective targeting of tumor cells under glucose deprivation conditions may provide an effective alternative strategy for cancer therapy. In the present study, we investigated the effects of S6 kinase 1 (S6K1) inhibition on glucose deprivation-induced cell death and the underlying mechanisms in MCF-7 breast cancer cells. PF4708671, a selective inhibitor of S6K1, and knockdown of S6K1 with specific siRNA enhanced cell death induced under glucose deprivation conditions. Moreover, inhibition of S6K1 led to apoptosis in glucose-starved MCF-7 cells via downregulation of the anti-apoptotic proteins, Mcl-1 and survivin. Further experiments revealed that sorafenib, shown to be involved in Mcl-1 and survivin downregulation via mTOR/S6K1 inhibition significantly promotes cell death under glucose deprivation conditions. These findings collectively suggest that S6K1 plays an important role in tumor cell survival under stress conditions, and thus inhibition of S6K1 may be an effective strategy for sensitizing cells to glucose deprivation.

Topics: Apoptosis; Apoptosis Regulatory Proteins; Breast Neoplasms; Down-Regulation; Female; Gene Knockdown Techniques; Glucose; Humans; Imidazoles; Inhibitor of Apoptosis Proteins; MCF-7 Cells; Myeloid Cell Leukemia Sequence 1 Protein; Niacinamide; Phenylurea Compounds; Piperazines; Protein Kinase Inhibitors; Proto-Oncogene Proteins c-bcl-2; Ribosomal Protein S6 Kinases, 70-kDa; Sorafenib; Survivin; TOR Serine-Threonine Kinases