The RUNX2 Transcription Factor Negatively Regulates SIRT6 Expression to Alter Glucose Metabolism in Breast Cancer Cells

Abstract

Activation of genes promoting aerobic glycolysis and suppression of mitochondrial oxidative phosphorylation is one of the hallmarks of cancer. The RUNX2 transcription factor mediates breast cancer (BC) metastasis to bone and is regulated by glucose availability. But, the mechanisms by which it regulates glucose metabolism and promotes an oncogenic phenotype are not known. RUNX2 expression in luminal BC cells correlated with lower estrogen receptor-α (ERα) levels, anchorage-independent growth, expression of glycolytic genes, increased glucose uptake, and sensitivity to glucose starvation, but not to inhibitors of oxidative phosphorylation. Conversely, RUNX2 knockdown in triple-negative BC cells inhibited mammosphere formation and glucose dependence. RUNX2 knockdown resulted in lower LDHA, HK2, and GLUT1 glycolytic gene expression, but upregulation of pyruvate dehydrogenase-A1 (PDHA1) mRNA and enzymatic activity, which was consistent with lower glycolytic potential. The NAD-dependent histone deacetylase, SIRT6, a known tumor suppressor, was a critical regulator of these RUNX2-mediated metabolic changes. RUNX2 expression resulted in elevated pAkt, HK2, and PDHK1 glycolytic protein levels that were reduced by ectopic expression of SIRT6. RUNX2 also repressed mitochondrial oxygen consumption rates (OCR), a measure of oxidative phosphorylation (respiration). Overexpression of SIRT6 increased respiration in RUNX2-positive cells, but knockdown of SIRT6 in cells expressing low RUNX2 decreased respiration. RUNX2 repressed SIRT6 expression at both the transcriptional and post-translational levels and endogenous SIRT6 expression was lower in malignant BC tissues or cell lines that expressed high levels of RUNX2. These results support a hypothesis whereby RUNX2-mediated repression of the SIRT6 tumor suppressor regulates metabolic pathways that promote BC progression.

Sirtuin 6 regulates glucose-stimulated insulin secretion in mouse pancreatic beta cells

“…However, the cause of the mitochondrial defects is unclear. Consistent with these finding, mitochondrial defects have been also observed in Sirt6 -knockout mouse ES cells and Sirt6 -knockout breast cancer cells [21, 42]. In SIRT6-deficient mouse ES cells, mitochondrial respiration and a number of intermediate metabolites in the tricarboxylic acid cycle, including citrate, isocitrate, succinate, fumarate and malate, are decreased [21].…”

“…In SIRT6-deficient mouse ES cells, mitochondrial respiration and a number of intermediate metabolites in the tricarboxylic acid cycle, including citrate, isocitrate, succinate, fumarate and malate, are decreased [21]. In Hs578t breast cancer cells, overexpression of SIRT6 increases OCR and knockout of SIRT6 decreases it [42]. Together, these data suggest that SIRT6 promotes mitochondrial respiration.…”

SIRT6: Novel Mechanisms and Links to Aging and Disease

“…Indeed, down-regulation of SIRT6 correlated with increased glycolytic gene expression in human pancreatic and colorectal carcinomas [5]. Other studies confirmed the role of SIRT6 in regulating glycolysis during tumor progression in additional cancers (breast, bladder, and prostate), and showed that the RUNX2 and E2F1 transcription factors can facilitate tumor progression by repressing SIRT6 (See Box 1) [55, 56]. …”

The role of SIRT6 in tumors

“…12 In breast cancer, the repression of SIRT6, mediated by runt-related transcription factor 2 (RUNX2), regulates metabolic pathways and promotes tumor development. 13 More specifically, Choe et al 13 showed that RUNX2 downregulates the SIRT6 expression at both mRNA and protein levels, and that endogenous SIRT6 expression is lower in the tumor breast tissue and cell lines expressing high levels of RUNX2 regulating the metabolic pathways. In addition, Han et al…”

“…More specifically, Choe et al 13 showed that RUNX2 downregulates the SIRT6 expression at both mRNA and protein levels, and that endogenous SIRT6 expression is lower in the tumor breast tissue and cell lines expressing high levels of RUNX2 regulating the metabolic pathways. In addition, Han et al…”