Abstract
Although energy restriction has been recognized as an important target for cancer prevention, the mechanism by which energy restriction-mimetic agents (ERMAs) mediate apoptosis remains unclear. By using a novel thiazolidinedione-derived ERMA, CG-12 (Wei, S., Kulp, S. K., and Chen, C. S. (2010) J. Biol. Chem. 285, 9780-9791), vis-à-vis 2-deoxyglucose and glucose deprivation, we obtain evidence that epigenetic activation of the tumor suppressor gene Kruppel-like factor 6 (KLF6) plays a role in ERMA-induced apoptosis in LNCaP prostate cancer cells. KLF6 regulates the expression of many proapoptotic genes, and shRNA-mediated KLF6 knockdown abrogated the ability of ERMAs to induce apoptosis. Chromatin immunoprecipitation analysis indicates that this KLF6 transcriptional activation was associated with increased histone H3 acetylation and histone H3 lysine 4 trimethylation occupancy at the promoter region. Several lines of evidence demonstrate that the enhancing effect of ERMAs on these active histone marks was mediated through transcriptional repression of histone deacetylases and H3 lysine 4 demethylases by down-regulating Sp1 expression. First, putative Sp1-binding elements are present in the promoters of the affected histone-modifying enzymes, and luciferase reporter assays indicate that site-directed mutagenesis of these Sp1 binding sites significantly diminished the promoter activities. Second, shRNA-mediated knockdown of Sp1 mimicked the repressive effect of energy restriction on these histone-modifying enzymes. Third, ectopic Sp1 expression protected cells from the repressive effect of CG-12 on these histone-modifying enzymes, thereby abolishing the activation of KLF6 expression. Together, these findings underscore the intricate relationship between energy restriction and epigenetic regulation of tumor suppressor gene expression, which has therapeutic relevance to foster novel strategies for prostate cancer therapy.
Highlights
A hallmark feature of tumorigenesis is the shift of cellular metabolism from oxidative phosphorylation to aerobic glycolysis, the so-called Warburg effect, which provides a growth advantage to cancer cells in the microenvironment [1, 2]
Transcriptional Activation of the Tumor Suppressor Gene Kruppel-like factor 6 (KLF6) Represents a Cellular Death Response to Energy Restriction—As part of our effort to understand the complex signaling network by which energy restriction-mimetic agents (ERMAs) suppress cancer cell proliferation, we examined the effect of CG-12 on tumor suppressor gene expression in androgen-responsive LNCaP cells
Quantitative real-time reverse-transcription polymerase chain reaction analysis indicates that 48-h exposure of LNCaP cells to 10 M CG-12 boosted KLF6 mRNA levels by 25-fold, accompanied by increases in its proapoptotic target genes, including activating transcription factor (ATF) 3 (28-fold), Noxa (16-fold), and death-associated protein kinase (DAPK) 2 (8-fold) (Fig. 1A, upper panel)
Summary
A hallmark feature of tumorigenesis is the shift of cellular metabolism from oxidative phosphorylation to aerobic glycolysis, the so-called Warburg effect, which provides a growth advantage to cancer cells in the microenvironment [1, 2]. Transcriptional Activation of the Tumor Suppressor Gene KLF6 Represents a Cellular Death Response to Energy Restriction—As part of our effort to understand the complex signaling network by which ERMAs suppress cancer cell proliferation, we examined the effect of CG-12 on tumor suppressor gene expression in androgen-responsive LNCaP cells.
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