Altered regulation of PDK4 expression promotes antiestrogen resistance in human breast cancer cells
© Walter et al. 2015
Received: 14 October 2015
Accepted: 19 October 2015
Published: 10 November 2015
Acquired or de novo resistance to the selective estrogen receptor modulators tamoxifen and fulvestrant (ICI) is a major barrier to successful treatment of breast cancer. Gene expression patterns in tamoxifen resistant (TamR-MCF-7) cells were compared to their parental cells (MCF-7L) to identify an aberrantly regulated metabolic pathway. TamR-MCF-7 cells are cross resistant to ICI and doxorubicin, and have increased mitochondrial DNA. A small subset of genes had altered expression in TamR-MCF-7 relative to MCF-7L cells. One of the genes, pyruvate dehydrogenase kinase-4 (PDK4), phosphorylates pyruvate dehydrogenase (PDH). PDK4 expression was elevated in TamR-MCF-7 cells; this result was also observed in a second model of acquired antiestrogen resistance. PDK4 expression is controlled in part by glucocorticoid response elements in the PDK4 gene promoter. In MCF-7L cells, PDK4 mRNA expression was insensitive to glucocorticoid receptor agonists, while dexamethasone dramatically increased PDK4 expression in TamR-MCF-7 cells. Using siRNA to knock down PDK4 expression increased TamR-MCF-7 sensitivity to ICI; in contrast adapting cells to growth in glucose depleted media did not affect ICI sensitivity. Despite TamR-MCF-7 cells high levels of PDK4 mRNA relative to MCF-7L, TamR-MCF-7 cells have increased PDH activity. Wild type MCF-7 cells are reported to be heterozygous for a G to A mutation that results in a substitution of threonine for alanine near PDK4′s catalytic site. We found loss of heterozygosity in TamR-MCF-7 cells; TamR-MCF-7 are homozygous for the wild type allele. These data support a role for altered regulation of PDH by PDK4 and altered substrate utilization in the development of drug resistance in human breast cancer cells.
Therapies that target the estrogen receptor-α (ER) have resulted in significant improvement in clinical outcomes for breast cancer patients (Early Breast Cancer Trialists’ Collaborative G 2005; Strasser-Weippl et al. 2013). These endocrine therapies include selective estrogen receptor modulators (SERMs) such as tamoxifen (Jordan and Koerner 1975), molecules that downregulate ER (e.g. fulvestrant Wakeling et al. 1991), and aromatase inhibitors that reduce expression of the endogenous ER ligand estradiol (Wells et al. 1978; Lipton et al. 1995). Despite these gains, recurrence of breast cancer after endocrine therapy is a major barrier to eliminating breast cancer mortality (Jordan and O’Malley 2007; Arpino et al. 2009; Moy et al. 2015). Mechanisms proposed for the development of resistance to ER targeted drugs include altered splicing or mutation of ER (Han et al. 2004; Scott et al. 1991; Banerji et al. 2012), reduced ER expression (Bayliss et al. 2007; Chu et al. 2007; Oh et al. 2001), aberrant tamoxifen metabolism (Osborne 1993; Osborne et al. 1992), altered function of cell cycle proteins regulating the G1 to S phase progression (Lehn et al. 2011; Abukhdeir et al. 2008; Varma et al. 2007; Mukherjee and Conrad 2005; Wilcken et al. 1997), altered interactions of scaffold proteins with downstream signaling proteins (Wallez et al. 2014; Brinkman et al. 2000), deregulated growth factor signaling (Fagan et al. 2012; Brockdorff et al. 2003), upregulated NF-kB signaling (Riggins et al. 2005), androgen receptor upregulation (Fujii et al. 2014), and activated Myc signaling (Mukherjee and Conrad 2005; Miller et al. 2011; Shajahan-Haq et al. 2014). Despite the many routes to endocrine therapy resistance in in vitro models, the clinical significance is less clear and has not led to predictive biomarkers or effective therapies to prevent or overcome resistance (Droog et al. 2013).
In the last two decades there has been a resurgence of interest in tumor metabolism as an exploitable target for cancer therapy, and metabolic abnormalities are now in the canon of hallmarks of cancer (Hanahan and Weinberg 2011; DeBerardinis et al. 2008). Cancers are generally more glycolytic than normal tissue, and this was originally interpreted as a compensatory response to defects in respiration (Cook and Higuchi 2012). One consequence of increased glycolysis is extracellular acidification, and this has been shown to cause insensitivity to endocrine therapies (Yang et al. 2013). Oncogenes and cell cycle regulatory proteins have been shown to regulate the metabolism of fuels in addition to cellular proliferation, with consequences for endocrine therapy in breast cancer cells (Shajahan-Haq et al. 2014; Lee et al. 2014; Wang et al. 2006). Cancer metabolism is diverse, and clinically the mitochondrial DNA (mtDNA) content of breast cancer varies with stage, with early and advanced stage cancers having higher mtDNA than those diagnosed at intermediate stage (Bai et al. 2011). This observation suggests that as cancers progress, they become more reliant on mitochondrial function. Consistent with that notion, malignant breast cancer cells are metabolically coupled with tumor associated fibroblasts, with fibroblasts fermenting substrates and providing lactate for oxidation by cancer cells; this relationship promotes SERM resistance (Martinez-Outschoorn et al. 2011). Taken together, these observations suggest that fuel metabolism is plastic in breast cancer cells, and that utilization of fuels impacts response to endocrine therapy.
A major determinant of fuel choice is the pyruvate dehydrogenase complex (PDH), an enzyme that converts pyruvate and coenzyme A to acetylCoA and carbon dioxide, reducing one NAD+ to NADH in the process. Different PDH subunits perform the decarboxylation, acyl transfer and oxidation/reduction reactions, and each subunit has cofactor requirements specific for their function. In addition to the abundance of substrate and product, PDH activity is also regulated by phosphorylation and dephosphorylation of the E1 subunit with inactivation of enzymatic activity by phosphorylation. The balance of kinase and phosphatase activities determines the PDH phosphorylation status. Four isoforms of pyruvate dehydrogenase kinase (PDK) are known, each active in response to different intracellular and extracellular conditions. PDK1 is activated by hypoxia (Mora et al. 2003; Kim et al. 2006); PDK2 is activated by the PDH products acetyl CoA and NADH and is inhibited by ADP and pyruvate (Hiromasa et al. 2006); and PDK3 is activated by ATP (Kato et al. 2005). Pyruvate dehydrogenase kinase-isoform 4 (PDK4), in contrast, is activated transcriptionally by hormonal signals such as retinoic acid and glucocorticoids, and transcriptionally repressed by insulin (Kwon et al. 2004). Thus while regulation of PDK1-3 reflects the immediate energy demands of the cell, PDK4 is reflective of whole organism energy balance and is upregulated in metabolic conditions such as hibernation (Buck et al. 2002), sustained exercise (Wang and Sahlin 2012; Pilegaard and Neufer 2004), and diabetes (Wu et al. 1998).
We previously reported that mitochondrial DNA copy number and mitochondrial superoxide were amplified in LCC9 cells, a cell line sequentially selected from parental MCF-7 cells for estrogen independence and fulvestrant resistance (Brunner et al. 1997; Skildum et al. 2011). We reasoned that mitochondrial amplification was a response to altered fuel metabolism, and that SERM resistant breast cancer cells would have altered expression of genes that regulate carbohydrate metabolism. In an independent model of acquired SERM resistance using cells selected directly from parental MCF-7 for growth in the presence of 4-hydroxytamoxifen (Fagan et al. 2012), we used quantitative PCR (qPCR) arrays to compare the expression of genes that regulate glucose metabolism. PDK4 expression was increased in tamoxifen resistant (TamR-MCF-7) cells, and the regulation of PDK4 by glucocorticoid receptor ligands was increased in TamR-MCF-7 cells. Paradoxically, PDH activity was also elevated in TamR cells. We found that parental MCF-7L cells were heterozygous for a mutation which causes an amino acid substitution near the PDK4 active site, and the TamR-MCF-7 cells lost heterozygosity and only expressed wild type PDK4. Decreased PDK4 expression partially restored antiestrogen sensitivity in these cells. We conclude that regulatory circuits that control oxidation of carbon derived from glucose are altered during selection for antiestrogen resistance.
Cell lines and reagents
Cell line name
Parental cell line
Source of cells
University of Minnesota Masonic Cancer Center
Proliferation in the presence of 4-hydroxytamoxifen in vitro
University of Minnesota Masonic Cancer Center
Wallez et al. (2014)
Georgetown University Lombardi Cancer Center
Estrogen independent tumorigenesis in vivo; Proliferation in the presence of ICI in vitro
Georgetown University Lombardi Cancer Center
Pilegaard and Neufer (2004)
American Type Culture Collection
Brunner et al. (1997)
Proliferation in 4.5 mM galactose with undetectable glucose
Generated at University of Minnesota Medical School, Duluth
MCF-7-GAL cells were generated by adapting ATCC-MCF-7 to progressively increased concentrations of galactose and decreased concentrations of glucose. Initially cells were cultured in DMEM (Sigma catalog number D5030) supplemented with 2 mM glutamine, 1 mM pyruvate, 5 % FBS, 1× penicillin/streptomycin, 12.5 mM d-glucose and 12.5 mM d-galactose; the concentration of glucose in this media is half that in MCF-7′s normal media. Over the course of 2 months, cells were allowed to grow to confluence, then split into new plates with progressively lower concentration of glucose and higher concentrations of galactose, until a population of cells was derived (MCF-7-GAL) that grew as rapidly in 25 mM galactose media as their parental cells grew in 25 mM glucose. In the 25 mM galactose media, some glucose was provided by FBS, however, the glucose concentration in this media was below the detection limit of a glucose oxidase based colorimetric assay (data not shown).
d-glucose, d-galactose, dexamethsone, mifepristone, 4-hydroxytamoxifen, and ICI were purchased from Sigma.
Cell cycle assay
MCF-7L and TamR-MCF-7 cells were plated at 106 per 10 cm dish in MCF-7L’s normal growth media (permissive media for both cell lines). After attaching overnight, 1 nm ICI or vehicle (DMSO) was added to the cells without changing media. After 48 h, cells were harvested by trypsinization and fixed in ethanol. Cells were then centrifuged and the fixative removed by aspiration. The cell pellet was resuspended in 0.4 mL fluorescence-activated cell sorting buffer (1.37 M NaCl, 27 mM KCl, 43 mM Na2HPO4, 14.7 mM KH2PO4, 1 mg/mL ribonuclease A, 0.5 mM EDTA, 0.1 % Triton X-100, and 0.2 mg/mL propidium iodide), pipetted several times to ensure a uniform single-cell suspension, and transferred to a polystyrene tube. Cells were incubated at 4 °C in the dark for 30 min before analysis.
Cells were analyzed on a FACS Caliber flow cytometer, and data were collected using CellQuest Pro software (Becton–Dickinson). Cells were gated on forward and side scatter to eliminate debris and on the width versus area of the red fluorescent voltage pulse to eliminate cell aggregates. The area of the red fluorescence voltage pulse for the gated cells is proportional to their DNA content, and the cell cycle profile for each sample was estimated using ModFit LT software (Verity Software House). A minimum of 10,000 gated cells were analyzed for each sample, and triplicate parallel cultures were analyzed for each treatment.
siRNA transfection and sulforhodamine B assays
TamR-MCF-7 cells were plated at 10,000 cells per well in 24 well plates in MCF-7L cells’ normal growth media. After overnight attachment, cells were transfected with one of two PDK4 siRNAs (Qiagen Flexitube Hs_PDK4_6 and Hs_PDK4_7) or a non-specific control siRNA using Lipofectamine 2000 (Invitrogen) according to the vendor’s protocol.
To validate efficacy of PDK4 knockdown, cells were treated with 1 nM dexamethasone or ethanol vehicle 1 day after transfection. Two days after treatment, total RNA was collected, reverse transcribed to cDNA, and used as template for qPCR amplification of PDK4 and actin using the methods described above.
One day after transfection cells were treated with 1 and 5 nM ICI or DMSO vehicle to test the effects of PDK4 knockdown on antiestrogen sensitivity. Four days later, cells were washed with PBS, then fixed in 10 % trichloroacetic acid for 30 min at 4 °C. The fixative was removed by aspiration and washing gently with tap water; the 24-well plates were then air dried. Each well was then stained with 0.4 % sulforhodamine B (SRB) in 1 % acetic acid at room temperature for 30 min, then washed thrice in 1 % acetic acid, and air dried. Stained proteins were then solubilized in 0.5 mL 10 mM tris base. A 0.1 mL aliquot was then transferred to a 96-well microtitre plate. SRB absorbance at 565 nm and background absorbance at 690 nm were measured spectrophotometrically.
To measure doxorubicin sensitivity, cells were plated at 5000 cells per well in 24 well plates and allowed to attach overnight. Cells were then treated ±0.02 micromolar doxorubicin (Sigma) from freshly prepared stocks diluted in water. Cells were cultured for 4 days, and cellular abundance was measured using SRB staining as described above.
Quantitative PCR arrays
To compare gene expression patterns in parental MCF-7L cells and TamR-MCF-7 cells selected for antiestrogen resistance, we employed qPCR arrays containing primer sets targeting 81 genes that encode enzymes that control different aspects of carbohydrate metabolism (RT2 Profiler PCR Array Human Glucose Metabolism, Qiagen). Samples of MCF-7L and TamR-MCF-7 cells were generated by thawing ampules of each cell line and growing to sub-confluence in T75 flasks. Cells were then trypsinized and plated at 500,000 cells per 10 cm dish and allowed to attach overnight. Cells were then cultured for 2 days. Total RNA was isolated from cells using the Qiagen RNeasy kit. Using this protocol, four independent samples from each cell line were prepared on different days to account for biological variability in gene expression. Three sample sets were used for qPCR arrays, while the fourth set was used as a validation set.
RNA quantity and quality were measured using an Agilent Bioanalyzer, a core service provided by the University of Minnesota Genomics Center. The RNA integrity number (RIN) scores ranged from 8.60 to 9.10, indicating good quality of the RNA preparations. RNA was reverse transcribed and genomic DNA eliminated using the RT2 First Strand kit (Qiagen) according to the manufacturer’s instructions, and the products were used as template for qPCR arrays. The qPCR array plates were run on a Light Cycler 480 thermocycler (Roche), and data were analyzed using Microsoft Excel templates supplied by Qiagen.
Pyruvate dehydrogenase activity assays
To measure PDH activity the Pyruvate Dehydrogenase Activity Colorimetric Assay Kit from BioVision was used. On three consecutive days, identically prepared aliquots of cells were thawed from liquid nitrogen and plated on T75 tissue culture plates. Cells were grown to sub-confluence, then split into 10 cm tissue culture dishes and grown for 2 days.
Assays were conducted on three consecutive days. On the day of each assay, cells were harvested by scraping cells and counted on a hemocytometer. 1x106 cells were Dounce homogenized with 100 μL ice cold PDH assay buffer. The samples were kept on ice for 10 min before being centrifuged at 10,000×g for 5 min. The supernatant was then transferred to a fresh tube. Protein concentration in each sample was determined with the BioVision BCA Protein Assay Kit II using bovine serum albumin standards according to the manufacturer’s protocols. Equal amounts of protein were added from each sample and the reaction volume was adjusted to 50 μL with PDH Assay Buffer and transferred to a microtitre plate. Triplicate assays were performed on each sample, and blank reactions were included as negative controls. The plate was immediately measured with the plate reader in kinetic mode at 450 nm for 30 min at 37 °C. Separately, an external NADH standard curve was prepared using the BioVision protocol where 0, 2.5, 5.0, 7.5, 10 and 12.5 nmol/well of NADH standard was adjusted to 50 μL/well with PDH assay buffer. 50 μL of the reaction mix was added and immediately measured with the plate reader in equilibrium mode at 450 nm at 37 °C. The slopes of the kinetic measurements were used to calculate a rate of NADH produced per minute per microgram of protein.
mtDNA copy number assay
To measure mitochondrial genome abundance relative to the nuclear genome, MCF-7L and TamR-MCF-7 cells were plated at 500,000 cells per 10 cm tissue culture dish, allowed to attach overnight, washed with PBS, and then treated with 10 nM 4-hydroxytamoxifen or ethanol vehicle for 4 days in phenol red free IMEM with 5 % steroid hormone depleted serum. Cells were then harvested by scraping after exposure to lysis buffer, and total RNase treated DNA was isolated using the Qiagen DNeasy kit according to the manufacturer’s instructions. 1 μg of DNA was used as template for qPCR amplification of mitochondrial DNA (a 125 bp portion of the cytochrome B coding sequence) and nuclear DNA (a 158 bp intron/exon boundary spanning fragment of the pyruvate kinase gene) as described above.
Pairwise comparisons were subjected to Student T-tests using Microsoft Excel. Significant differences in experiments with multiple comparisons were evaluated using analysis of variance (ANOVA) followed by Tukey–Kramer Honest Significant Difference tests; this analysis was performed using JMP Pro Version 11.
To test whether resistance to endocrine and chemotherapy may result from or cause a change in metabolic capacity, we next compared mitochondrial gene dose by quantitative PCR (qPCR) measurement of pyruvate kinase, encoded in nuclear DNA, and cytochrome B, encoded in mitochondrial DNA (mtDNA) (Fig. 2c). We found that TamR-MCF-7 cells had threefold higher mtDNA copy number than parental MCF-7L cells. The mtDNA copy number was not altered by short term treatment with 4-hydroxytamoxifen in either cell line.
The elevated mtDNA copy number of TamR-MCF-7 cells suggests that a metabolic abnormality may be associated with the multi drug resistant phenotype in this model. To identify potential mediators of altered metabolism, expression of a panel of 81 genes that encode enzymes involved in carbohydrate metabolism was compared. MCF-7L and TamR-MCF-7 cells were serum starved, then treated with 1 nM 17β-estradiol for 1 day. Total RNA was isolated and reverse transcribed to cDNA, which was used as template in pathway targeted qPCR arrays. The expression of three genes exceeded a two-fold difference with statistical significance (p < 0.05) in TamR-MCF-7 cells: Glucokinase was expressed at lower levels in TamR-MCF-7, while phosphoglucomutase and PDK4 were expressed at higher levels in TamR-MCF-7 cells (Additional file 1: Figure S1). PDK4 encodes a regulatory kinase that phosphorylates and inhibits pyruvate dehydrogenase (PDH), a major determinant of substrate utilization in cells, and is the subject of the current study; the significance of glucokinase and phosphoglucomutase expression in this model will be explored separately.
18 s rRNA for qPCR
5′ TCA ACT TTC GAT GGT AGT CGC CGT 3′
5′ TCC TTG GAT GTG GTA GCC GTT TCT 3′
β-actin mRNA for qPCR
5′ GCC GCC AGC TCA CCA TGG AT 3′
5′ CAC CAT CAC GCC CTG GTG CC 3′
PDK4 mRNA for qPCR
5′ CCT GTG AGA CTC GCC AAC A 3′
5′ TCC ACC AAA TCC ATC AGG CTC 3′
Cytochrome B genomic for qPCR
5′-TGA TAT TTC CTA TTC GCC TAC ACA-3′
5′-TGT TGT TTG GAT ATA TGG AGG ATG-3′
Pyruvate kinase genomic for qPCR
5′-GTC GAT CCA GGA GAA CAT ATC AT-3
5′-CTC CTA GTT TTC ACC CTC ATT TTC-3′
PDK4 genomic for sequencing
5′ CAC TGA GAA TGT GAC CCG CT 3′
5′ AGC CTT GTG TGA AGT AAC CTT AG 3′
PDK4 cDNA for sequencing
5′ ATC CTC CCG ACC CAA TTA GT 3′
5′ ACC CAC TGC TAC CAC ATC ACA 3′
The PDK4 promoter contains multiple glucocorticoid response elements. To test whether PDK4 expression was dependent on glucocorticoid receptor (GR) activity, we treated MCF-7L and TamR-MCF-7 with the GR agonist dexamethasone and the antagonist mifepristone in serum free media. To account for biological variability, triplicate cultures were prepared. In the cells treated with vehicle control (EtOH), we found elevated expression of PDK4 in TamR-MCF-7 cells (Fig. 3b), confirming our previous observations. While the GR ligands had no effect on PDK4 expression in MCF-7L cells, dexamethasone treatment resulted in an approximately five fold increase in PDK4 mRNA in TamR-MCF-7 cells. Mifepristone treatment alone caused a decrease in PDK4 expression in TamR-MCF-7 cells, but the change was not statistically significant.
To test whether increased PDK4 mRNA expression is a general phenomenon or is specific to the TamR-MCF-7 model of acquired antiestrogen resistance, we tested the dexamethasone induction of PDK4 mRNA in an independent model. In contrast to TamR-MCF-7 cells direct selection for growth in the presence of 4-hydroxytamoxien in vitro, LCC9 cells were selected from MCF-7 cells first for estrogen independence in ovariectomized nude mice, then selected for ICI resistance in vitro (Brunner et al. 1997). Relative to their parental LCC-MCF-7 cells, LCC9 cells exhibited higher basal PDK4 mRNA expression; dexamethasone treatment resulted in a ~tenfold increase in PDK4 mRNA abundance when treated in serum free media (Fig. 3c).
PDK4 is known to inhibit PDH, resulting in decreased oxidation of glucose derived carbon in the tricarboxylic acid cycle. Physiologically this results in conservation of glucose as cells switch to oxidation of fatty acids and amino acids. We reasoned that by limiting the ability of cells to gain ATP through glycolysis by substituting galactose for glucose (Frey 1996), we could increase expression of PDK4 and test whether increased PDK4 expression resulted in altered sensitivity to SERMs and doxorubicin. MCF-7L and TamR-MCF-7 cells were cultured for 2 days in media containing fetal bovine serum and either 4.5 mg/mL glucose or 4.5 mg/mL galactose. PDK4 mRNA expression was measured by qPCR, and PDK4 protein was measured by western blot. PDK4 mRNA expression was not altered by limiting the availability of glucose in either cell line (Fig. 1a). In contrast to the consistently elevated PDK4 mRNA observed in TamR-MCF-7, we found no difference in PDK4 protein detected by Western blot (Fig. 1b); Western blotting was challenging due to the consistent presence of non-specific background bands (indicated by black arrows in Fig. 1b). PDK4 protein was increased by culturing cells in galactose in both cell lines, but with a greater magnitude of change in drug sensitive MCF-7L cells.
In contrast, adaptation to galactose resulted in decreased sensitivity to doxorubicin. Parental MCF-7 and MCF-7-GAL cells were treated with doxorubicin for 2 days, after which cell mass was measured by SRB staining. Galactose adapted cells had a smaller decrease in cell mass after treatment with either 0.15 or 0.6 μM doxorubicin (Fig. 4b).
Because TamR-MCF-7 cells had, paradoxically, elevated PDK4 mRNA expression and elevated PDH activity relative to their parental cells, we wondered if the TamR cells expressed an altered form of PDK4. The Catalogue of Somatic Mutations In Cancer (COSMIC) database (Forbes et al. 2008, 2015) reports that MCF-7 cells are heterozygous for a point mutation that results in a non-conservative alanine to threonine substitution at amino acid position 144. This position is in alpha helix #7, near the PDK4 catalytic site (Wynn et al. 2008). We designed sequencing primers around this site (Table 2) and sequenced the region using cDNA and genomic DNA as template. We confirmed the heterozygous genotype reported in the COSMIC database in MCF-7L cells (Additional file 1: Figure S3). We found that TamR-MCF-7 cells had loss of heterozygosity at this position; surprisingly TamR-MCF-7 were homozygous for the wild type PDK4 sequence.
TamR-MCF-7 cells were then transfected with PDK4 or control siRNA and treated with 1 or 5 nM ICI for 4 days, after which cellular abundance was measured by SRB staining. A small but reproducible and statistically significant increase in ICI sensitivity was observed with PDK4 siRNA relative to control siRNA at the highest 5 nm dose (Figs. 6b), suggesting PDK4 expression can mediate the responsiveness to SERMs. This observation was reproduced using a second siRNA targeting a different region of the PDK4 mRNA 3′ untranslated region (Additional file 1: Figure S4).
The data presented above point to altered regulation of pyruvate dehydrogenase as mediating breast cancer cell sensitivity to estrogen receptor ligands such as tamoxifen and ICI, and reveal an important role for the serine threonine kinase PDK4 in acquired SERM resistance. PDK4 is reported to phosphorylate and inhibit PDH, preventing the oxidation of carbon derived from glucose. When PDH is inactive, cells generate ATP through glycolysis, oxidation of amino acids and/or fatty acid oxidation. Rapidly dividing cells such as cancer cells require not only ATP but also carbon for biosynthesis, and the inhibition of PDH may allow glucose derived carbon to be conserved for biosynthesis of ribose sugars, amino acids, and glycerol, and the reduction of NADP+ to NADPH. We show that sensitivity to SERMs is not affected by replacing glucose with galactose, a non-fermentable substrate. In contrast, galactose adapted cells showed resistance to the chemotherapy agent doxorubicin; this phenomenon has been reported previously in liver cancer cells (Marroquin et al. 2007).
PDK4 expression is transcriptionally regulated by numerous promoter elements, and its elevated mRNA in TamR-MCF-7 cells could indicate disruption of several signal transduction pathways. In a glioblastoma, Kim et al. describe PDK4 regulation through NF-κB activation of PGC1α (Kim et al. 2015), and NF-κB signaling is altered in the LCC9 model of acquired SERM resistance (Riggins et al. 2005). Hypoxic signaling through estrogen related receptor gamma (ERRγ) also activates PDK4 transcription (Lee et al. 2012; Zhang et al. 2006), and ERRγ is directly inhibited by 4-hydroxytamoxifen, the most biologically potent tamoxifen metabolite (Coward et al. 2001; Tremblay et al. 2001). Knutson et al. report that PDK4 expression is dependent on progesterone receptor sumoylation in MCF-7 cells (Knutson et al. 2012), and progesterone receptor regulates breast cancer cell proliferation (Skildum et al. 2005). Mifepristone is an antagonist of both glucocorticoid receptors and progesterone receptors (Beck et al. 1993), though we did not observe statistically significant regulation of PDK4 mRNA by mifepristone in either MCF-7L or TamR-MCF-7 (Fig. 2b). Interestingly, TamR-MCF-7 express reduced progesterone receptor mRNA relative to their parental cells (Fagan et al. 2012).
PDK4 was first identified in a linkage study of loci associated with Type 2 diabetes mellitus in Native Americans (Rowles et al. 1996), and insulin is known to repress PDK4 transcription (Wu et al. 1998; Majer et al. 1998). Interestingly, in serum free media we show that the regulation of PDK4 by the glucocorticoid receptor agonist dexamethasone only occurs in drug resistant breast cancer cells (Fig. 2c), while the dexamethasone induction is dramatically blunted when TamR-MCF-7 cells are treated in the presence of fetal bovine serum (Fig. 7a; Additional file 1: Figure S4a). Insulin inhibits glucocorticoid dependent expression of PDK4 expression by preventing FOXO1 translocation to the nucleus (Kwon et al. 2004). Together these results suggest that in the absence of insulin and other growth factors in serum, parental MCF-7 cells have increased FOXO1 activity compared to TamR-MCF-7 cells, but that serum growth factors are still capable of blunting PDK4 expression by dexamethasone in these cells.
The consequences of PDK4 expression on cancer progression and response to therapy are likely to be context dependent. PDK4 was shown to be abundant in patient glioblastoma compared to normal tissue, and indirectly reducing its expression through shRNA targeting the RelA subunit of NF-kB resulted in reduced growth of xenografts in nude mice (Kim et al. 2015). Similarly, reducing PDK4 directly through shRNA resulted in increased tumor free survival of mice bearing prostate cancer cell xenografts (Liu et al. 2014). In contrast, in lung cancer cells decreasing PDK4 expression both prevented the antiproliferative action of a PPARγ agonist by blocking its action at the G1 → S phase checkpoint (Srivastava et al. 2014) and promoted epithelial to mesenchymal transition and chemotherapy resistance. The latter effects may be due to PDK4 physically interacting with the mitochondrial apoptosis inducing factor (AIF) rather than its phosphorylation of PDH (Sun et al. 2014; Cande et al. 2002).
Although PDK4 mRNA is overexpressed in TamR-MCF-7 cells relative to their tamoxifen naïve parental cells, PDK4 protein levels are similar and PDH activity is higher in TamR-MCF-7. These results suggest that PDK4 in TamR-MCF-7 is not acting as an inhibitor of PDH. Increased mRNA without increased protein level is consistent with decreased translation and/or increased protein turnover. If PDK4 protein turnover is increased in TamR-MCF-7 cells, the increased PDH activity in these cells may be result because PDK4 does not have time to be appropriately post-translationally modified or transported to the mitochondria. PDK4 has several sites identified as phosphorylated in proteomic screens (Franchin et al. 2015; Hornbeck et al. 2012), but the significance of these modifications with respect to PDK4 or PDH activity have not been characterized. Newly synthesized PDK4 may have reduced PDH inhibitory activity, and/or may act through targets other than PDH (e.g. AIF). These possibilities, and the significance of PDK4 polymorphisms, are currently under study.
WW: qPCR, PDH assays, sequencing. JT: cell cycle assays, siRNA transfections, LCC9 PDK4 expression. JB: characterize galactose adapted cells. DHF: made and characterized TamR-MCF-7 model. CZ: qPCR arrays. DY: made TamR-MCF-7 model. AS: SRB assays, Western blotting, lab PI, manuscript preparation. All authors read and approved the final manuscript.
LCC-MCF-7 and LCC9 cells were a gift from Dr. Robert Clarke of the Lombardi Cancer Center. The qPCR array experiment was supported by a Grant In Aid to A. S. from the University of Minnesota Office of the Vice President for Research. W. W. and C. Z. were supported by the University of Minnesota, Duluth Undergraduate Research Opportunities Program. J. T. and J. B. were supported by the University of Minnesota Medical School, Duluth Pathways to Advanced Degrees in Life Sciences program. The authors thank Dr. Lester Drewes and Mr. Jon Bredeson for helpful review of the manuscript.
The authors declare that they have no competing interests.
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- Abukhdeir AM, Vitolo MI, Argani P, De Marzo AM, Karakas B, Konishi H, Gustin JP, Lauring J, Garay JP, Pendleton C et al (2008) Tamoxifen-stimulated growth of breast cancer due to p21 loss. Proc Natl Acad Sci USA 105(1):288–293View ArticleGoogle Scholar
- Arpino G, De Angelis C, Giuliano M, Giordano A, Falato C, De Laurentiis M, De Placido S (2009) Molecular mechanism and clinical implications of endocrine therapy resistance in breast cancer. Oncology 77(Suppl 1):23–37View ArticleGoogle Scholar
- Bai RK, Chang J, Yeh KT, Lou MA, Lu JF, Tan DJ, Liu H, Wong LJ (2011) Mitochondrial DNA content varies with pathological characteristics of breast cancer. J Oncol 2011:496189View ArticleGoogle Scholar
- Banerji S, Cibulskis K, Rangel-Escareno C, Brown KK, Carter SL, Frederick AM, Lawrence MS, Sivachenko AY, Sougnez C, Zou L et al (2012) Sequence analysis of mutations and translocations across breast cancer subtypes. Nature 486(7403):405–409View ArticleGoogle Scholar
- Bayliss J, Hilger A, Vishnu P, Diehl K, El-Ashry D (2007) Reversal of the estrogen receptor negative phenotype in breast cancer and restoration of antiestrogen response. Clin Cancer Res 13(23):7029–7036View ArticleGoogle Scholar
- Beck CA, Estes PA, Bona BJ, Muro-Cacho CA, Nordeen SK, Edwards DP (1993) The steroid antagonist RU486 exerts different effects on the glucocorticoid and progesterone receptors. Endocrinology 133(2):728–740Google Scholar
- Brinkman A, van der Flier S, Kok EM, Dorssers LC (2000) BCAR1, a human homologue of the adapter protein p130Cas, and antiestrogen resistance in breast cancer cells. J Natl Cancer Inst 92(2):112–120View ArticleGoogle Scholar
- Brockdorff BL, Heiberg I, Lykkesfeldt AE (2003) Resistance to different antiestrogens is caused by different multi-factorial changes and is associated with reduced expression of IGF receptor Ialpha. Endocr Relat Cancer 10(4):579–590View ArticleGoogle Scholar
- Brunner N, Boysen B, Jirus S, Skaar TC, Holst-Hansen C, Lippman J, Frandsen T, Spang-Thomsen M, Fuqua SA, Clarke R (1997) MCF7/LCC9: an antiestrogen-resistant MCF-7 variant in which acquired resistance to the steroidal antiestrogen ICI 182,780 confers an early cross-resistance to the nonsteroidal antiestrogen tamoxifen. Cancer Res 57(16):3486–3493Google Scholar
- Buck MJ, Squire TL, Andrews MT (2002) Coordinate expression of the PDK4 gene: a means of regulating fuel selection in a hibernating mammal. Physiol Genom 8(1):5–13View ArticleGoogle Scholar
- Cande C, Cohen I, Daugas E, Ravagnan L, Larochette N, Zamzami N, Kroemer G (2002) Apoptosis-inducing factor (AIF): a novel caspase-independent death effector released from mitochondria. Biochimie 84(2–3):215–222View ArticleGoogle Scholar
- Chu I, Arnaout A, Loiseau S, Sun J, Seth A, McMahon C, Chun K, Hennessy B, Mills GB, Nawaz Z et al (2007) Src promotes estrogen-dependent estrogen receptor alpha proteolysis in human breast cancer. J Clin Investig 117(8):2205–2215View ArticleGoogle Scholar
- Cook CC, Higuchi M (2012) The awakening of an advanced malignant cancer: an insult to the mitochondrial genome. Biochim Biophys Acta 1820(5):652–662View ArticleGoogle Scholar
- Coward P, Lee D, Hull MV, Lehmann JM (2001) 4-Hydroxytamoxifen binds to and deactivates the estrogen-related receptor gamma. Proc Natl Acad Sci USA 98(15):8880–8884View ArticleGoogle Scholar
- DeBerardinis RJ, Lum JJ, Hatzivassiliou G, Thompson CB (2008) The biology of cancer: metabolic reprogramming fuels cell growth and proliferation. Cell Metab 7(1):11–20View ArticleGoogle Scholar
- Droog M, Beelen K, Linn S, Zwart W (2013) Tamoxifen resistance: from bench to bedside. Eur J Pharmacol 717(1–3):47–57View ArticleGoogle Scholar
- Early Breast Cancer Trialists’ Collaborative G (2005) Effects of chemotherapy and hormonal therapy for early breast cancer on recurrence and 15-year survival: an overview of the randomised trials. Lancet 365(9472):1687–1717View ArticleGoogle Scholar
- Fagan DH, Uselman RR, Sachdev D, Yee D (2012) Acquired resistance to tamoxifen is associated with loss of the type I insulin-like growth factor receptor: implications for breast cancer treatment. Cancer Res 72(13):3372–3380View ArticleGoogle Scholar
- Forbes SA, Bhamra G, Bamford S, Dawson E, Kok C, Clements J, Menzies A, Teague JW, Futreal PA, Stratton MR (2008) The catalogue of somatic mutations in cancer (COSMIC). Curr Protoc Hum Genet Chapter 10:Unit 10 11Google Scholar
- Forbes SA, Beare D, Gunasekaran P, Leung K, Bindal N, Boutselakis H, Ding M, Bamford S, Cole C, Ward S et al (2015) COSMIC: exploring the world’s knowledge of somatic mutations in human cancer. Nucleic Acids Res 43(Database issue):D805–D811Google Scholar
- Franchin C, Cesaro L, Salvi M, Millioni R, Iori E, Cifani P, James P, Arrigoni G, Pinna L (2015) Quantitative analysis of a phosphoproteome readily altered by the protein kinase CK2 inhibitor quinalizarin in HEK-293T cells. Biochim Biophys Acta 1854(6):609–623View ArticleGoogle Scholar
- Frey PA (1996) The Leloir pathway: a mechanistic imperative for three enzymes to change the stereochemical configuration of a single carbon in galactose. FASEB J 10(4):461–470Google Scholar
- Fujii R, Hanamura T, Suzuki T, Gohno T, Shibahara Y, Niwa T, Yamaguchi Y, Ohnuki K, Kakugawa Y, Hirakawa H et al (2014) Increased androgen receptor activity and cell proliferation in aromatase inhibitor-resistant breast carcinoma. J Steroid Biochem Mol Biol 144(Pt B):513–522Google Scholar
- Han F, Miksicek R, Clarke R, Conrad SE (2004) Expression of an estrogen receptor variant lacking exon 3 in derivatives of MCF-7 cells with acquired estrogen independence or tamoxifen resistance. J Mol Endocrinol 32(3):935–945View ArticleGoogle Scholar
- Hanahan D, Weinberg RA (2011) Hallmarks of cancer: the next generation. Cell 144(5):646–674View ArticleGoogle Scholar
- Hiromasa Y, Hu L, Roche TE (2006) Ligand-induced effects on pyruvate dehydrogenase kinase isoform 2. J Biol Chem 281(18):12568–12579View ArticleGoogle Scholar
- Hornbeck PV, Kornhauser JM, Tkachev S, Zhang B, Skrzypek E, Murray B, Latham V, Sullivan M (2012) PhosphoSitePlus: a comprehensive resource for investigating the structure and function of experimentally determined post-translational modifications in man and mouse. Nucleic Acids Res 40(Database issue):D261–D270Google Scholar
- Jordan VC, Koerner S (1975) Tamoxifen (ICI 46,474) and the human carcinoma 8S oestrogen receptor. Eur J Cancer 11(3):205–206View ArticleGoogle Scholar
- Jordan VC, O’Malley BW (2007) Selective estrogen-receptor modulators and antihormonal resistance in breast cancer. J Clin Oncol 25(36):5815–5824View ArticleGoogle Scholar
- Kato M, Chuang JL, Tso SC, Wynn RM, Chuang DT (2005) Crystal structure of pyruvate dehydrogenase kinase 3 bound to lipoyl domain 2 of human pyruvate dehydrogenase complex. EMBO J 24(10):1763–1774View ArticleGoogle Scholar
- Kim JW, Tchernyshyov I, Semenza GL, Dang CV (2006) HIF-1-mediated expression of pyruvate dehydrogenase kinase: a metabolic switch required for cellular adaptation to hypoxia. Cell Metab 3(3):177–185View ArticleGoogle Scholar
- Kim MY, Koh DI, Choi WI, Jeon BN, Jeong DY, Kim KS, Kim K, Kim SH, Hur MW (2015) ZBTB2 increases PDK4 expression by transcriptional repression of RelA/p65. Nucleic Acids Res 43(3):1609–1625View ArticleGoogle Scholar
- Knutson TP, Daniel AR, Fan D, Silverstein KA, Covington KR, Fuqua SA, Lange CA (2012) Phosphorylated and sumoylation-deficient progesterone receptors drive proliferative gene signatures during breast cancer progression. Breast Cancer Res 14(3):R95View ArticleGoogle Scholar
- Kwon HS, Huang B, Unterman TG, Harris RA (2004) Protein kinase B-alpha inhibits human pyruvate dehydrogenase kinase-4 gene induction by dexamethasone through inactivation of FOXO transcription factors. Diabetes 53(4):899–910View ArticleGoogle Scholar
- Lee JH, Kim EJ, Kim DK, Lee JM, Park SB, Lee IK, Harris RA, Lee MO, Choi HS (2012) Hypoxia induces PDK4 gene expression through induction of the orphan nuclear receptor ERRgamma. PLoS One 7(9):e46324View ArticleGoogle Scholar
- Lee Y, Dominy JE, Choi YJ, Jurczak M, Tolliday N, Camporez JP, Chim H, Lim JH, Ruan HB, Yang X et al (2014) Cyclin D1-Cdk4 controls glucose metabolism independently of cell cycle progression. Nature 510(7506):547–551View ArticleGoogle Scholar
- Lehn S, Ferno M, Jirstrom K, Ryden L, Landberg G (2011) A non-functional retinoblastoma tumor suppressor (RB) pathway in premenopausal breast cancer is associated with resistance to tamoxifen. Cell Cycle 10(6):956–962View ArticleGoogle Scholar
- Lipton A, Demers LM, Harvey HA, Kambic KB, Grossberg H, Brady C, Adlercruetz H, Trunet PF, Santen RJ (1995) Letrozole (CGS 20267). A phase I study of a new potent oral aromatase inhibitor of breast cancer. Cancer 75(8):2132–2138View ArticleGoogle Scholar
- Liu Z, Chen X, Wang Y, Peng H, Wang Y, Jing Y, Zhang H (2014) PDK4 protein promotes tumorigenesis through activation of cAMP-response element-binding protein (CREB)-Ras homolog enriched in brain (RHEB)-mTORC1 signaling cascade. J Biol Chem 289(43):29739–29749View ArticleGoogle Scholar
- Majer M, Popov KM, Harris RA, Bogardus C, Prochazka M (1998) Insulin downregulates pyruvate dehydrogenase kinase (PDK) mRNA: potential mechanism contributing to increased lipid oxidation in insulin-resistant subjects. Mol Genet Metab 65(2):181–186View ArticleGoogle Scholar
- Marroquin LD, Hynes J, Dykens JA, Jamieson JD, Will Y (2007) Circumventing the Crabtree effect: replacing media glucose with galactose increases susceptibility of HepG2 cells to mitochondrial toxicants. Toxicol Sci 97(2):539–547View ArticleGoogle Scholar
- Martinez-Outschoorn UE, Goldberg A, Lin Z, Ko YH, Flomenberg N, Wang C, Pavlides S, Pestell RG, Howell A, Sotgia F et al (2011) Anti-estrogen resistance in breast cancer is induced by the tumor microenvironment and can be overcome by inhibiting mitochondrial function in epithelial cancer cells. Cancer Biol Ther 12(10):924–938View ArticleGoogle Scholar
- Miller TW, Balko JM, Ghazoui Z, Dunbier A, Anderson H, Dowsett M, Gonzalez-Angulo AM, Mills GB, Miller WR, Wu H et al (2011) A gene expression signature from human breast cancer cells with acquired hormone independence identifies MYC as a mediator of antiestrogen resistance. Clin Cancer Res 17(7):2024–2034View ArticleGoogle Scholar
- Mora A, Davies AM, Bertrand L, Sharif I, Budas GR, Jovanovic S, Mouton V, Kahn CR, Lucocq JM, Gray GA et al (2003) Deficiency of PDK1 in cardiac muscle results in heart failure and increased sensitivity to hypoxia. EMBO J 22(18):4666–4676View ArticleGoogle Scholar
- Moy B, Specht MC, Lanuti M, Rafferty EA, Lerwill MF (2015) Case records of the Massachusetts General Hospital. Case 1-2015. A 66-year-old woman with metastatic breast cancer after endocrine therapy. N Engl J Med 372(2):162–170View ArticleGoogle Scholar
- Mukherjee S, Conrad SE (2005) c-Myc suppresses p21WAF1/CIP1 expression during estrogen signaling and antiestrogen resistance in human breast cancer cells. J Biol Chem 280(18):17617–17625View ArticleGoogle Scholar
- Oh AS, Lorant LA, Holloway JN, Miller DL, Kern FG, El-Ashry D (2001) Hyperactivation of MAPK induces loss of ERalpha expression in breast cancer cells. Mol Endocrinol 15(8):1344–1359Google Scholar
- Osborne CK (1993) Mechanisms for tamoxifen resistance in breast cancer: possible role of tamoxifen metabolism. J Steroid Biochem Mol Biol 47(1–6):83–89View ArticleGoogle Scholar
- Osborne CK, Wiebe VJ, McGuire WL, Ciocca DR, DeGregorio MW (1992) Tamoxifen and the isomers of 4-hydroxytamoxifen in tamoxifen-resistant tumors from breast cancer patients. J Clin Oncol 10(2):304–310Google Scholar
- Pilegaard H, Neufer PD (2004) Transcriptional regulation of pyruvate dehydrogenase kinase 4 in skeletal muscle during and after exercise. Proc Nutr Soc 63(2):221–226View ArticleGoogle Scholar
- Riggins RB, Zwart A, Nehra R, Clarke R (2005) The nuclear factor kappa B inhibitor parthenolide restores ICI 182,780 (Faslodex; fulvestrant)-induced apoptosis in antiestrogen-resistant breast cancer cells. Mol Cancer Ther 4(1):33–41View ArticleGoogle Scholar
- Rowles J, Scherer SW, Xi T, Majer M, Nickle DC, Rommens JM, Popov KM, Harris RA, Riebow NL, Xia J et al (1996) Cloning and characterization of PDK4 on 7q21.3 encoding a fourth pyruvate dehydrogenase kinase isoenzyme in human. J Biol Chem 271(37):22376–22382View ArticleGoogle Scholar
- Scott GK, Kushner P, Vigne JL, Benz CC (1991) Truncated forms of DNA-binding estrogen receptors in human breast cancer. J Clin Investig 88(2):700–706View ArticleGoogle Scholar
- Shajahan-Haq AN, Cook KL, Schwartz-Roberts JL, Eltayeb AE, Demas DM, Warri AM, Facey CO, Hilakivi-Clarke LA, Clarke R (2014) MYC regulates the unfolded protein response and glucose and glutamine uptake in endocrine resistant breast cancer. Mol Cancer 13:239View ArticleGoogle Scholar
- Skildum A, Faivre E, Lange CA (2005) Progesterone receptors induce cell cycle progression via activation of mitogen-activated protein kinases. Mol Endocrinol 19(2):327–339View ArticleGoogle Scholar
- Skildum A, Dornfeld K, Wallace K (2011) Mitochondrial amplification selectively increases doxorubicin sensitivity in breast cancer cells with acquired antiestrogen resistance. Breast Cancer Res Treat 129(3):785–797View ArticleGoogle Scholar
- Soule HD, Vazguez J, Long A, Albert S, Brennan M (1973) A human cell line from a pleural effusion derived from a breast carcinoma. J Natl Cancer Inst 51(5):1409–1416Google Scholar
- Srivastava N, Kollipara RK, Singh DK, Sudderth J, Hu Z, Nguyen H, Wang S, Humphries CG, Carstens R, Huffman KE et al (2014) Inhibition of cancer cell proliferation by PPARgamma is mediated by a metabolic switch that increases reactive oxygen species levels. Cell Metab 20(4):650–661View ArticleGoogle Scholar
- Strasser-Weippl K, Badovinac-Crnjevic T, Fan L, Goss PE (2013) Extended adjuvant endocrine therapy in hormone-receptor positive breast cancer. Breast 22(Suppl 2):S171–S175View ArticleGoogle Scholar
- Sun Y, Daemen A, Hatzivassiliou G, Arnott D, Wilson C, Zhuang G, Gao M, Liu P, Boudreau A, Johnson L et al (2014) Metabolic and transcriptional profiling reveals pyruvate dehydrogenase kinase 4 as a mediator of epithelial-mesenchymal transition and drug resistance in tumor cells. Cancer Metab 2(1):20View ArticleGoogle Scholar
- Tremblay GB, Bergeron D, Giguere V (2001) 4-Hydroxytamoxifen is an isoform-specific inhibitor of orphan estrogen-receptor-related (ERR) nuclear receptors beta and gamma. Endocrinology 142(10):4572–4575Google Scholar
- Varma H, Skildum AJ, Conrad SE (2007) Functional ablation of pRb activates Cdk2 and causes antiestrogen resistance in human breast cancer cells. PLoS One 2(12):e1256View ArticleGoogle Scholar
- Wakeling AE, Dukes M, Bowler J (1991) A potent specific pure antiestrogen with clinical potential. Cancer Res 51(15):3867–3873Google Scholar
- Wallez Y, Riedl SJ, Pasquale EB (2014) Association of the breast cancer antiestrogen resistance protein 1 (BCAR1) and BCAR3 scaffolding proteins in cell signaling and antiestrogen resistance. J Biol Chem 289(15):10431–10444View ArticleGoogle Scholar
- Wang L, Sahlin K (2012) The effect of continuous and interval exercise on PGC-1alpha and PDK4 mRNA in type I and type II fibres of human skeletal muscle. Acta Physiol 204(4):525–532View ArticleGoogle Scholar
- Wang C, Li Z, Lu Y, Du R, Katiyar S, Yang J, Fu M, Leader JE, Quong A, Novikoff PM et al (2006) Cyclin D1 repression of nuclear respiratory factor 1 integrates nuclear DNA synthesis and mitochondrial function. Proc Natl Acad Sci USA 103(31):11567–11572View ArticleGoogle Scholar
- Wells SA Jr, Santen RJ, Lipton A, Haagensen DE Jr, Ruby EJ, Harvey H, Dilley WG (1978) Medical adrenalectomy with aminoglutethimide: clinical studies in postmenopausal patients with metastatic breast carcinoma. Ann Surg 187(5):475–484View ArticleGoogle Scholar
- Wilcken NR, Prall OW, Musgrove EA, Sutherland RL (1997) Inducible overexpression of cyclin D1 in breast cancer cells reverses the growth-inhibitory effects of antiestrogens. Clin Cancer Res 3(6):849–854Google Scholar
- Wu P, Sato J, Zhao Y, Jaskiewicz J, Popov KM, Harris RA (1998) Starvation and diabetes increase the amount of pyruvate dehydrogenase kinase isoenzyme 4 in rat heart. Biochem J 329(Pt 1):197–201View ArticleGoogle Scholar
- Wynn RM, Kato M, Chuang JL, Tso SC, Li J, Chuang DT (2008) Pyruvate dehydrogenase kinase-4 structures reveal a metastable open conformation fostering robust core-free basal activity. J Biol Chem 283(37):25305–25315View ArticleGoogle Scholar
- Yang N, Liu C, Peck AR, Girondo MA, Yanac AF, Tran TH, Utama FE, Tanaka T, Freydin B, Chervoneva I et al (2013) Prolactin-Stat5 signaling in breast cancer is potently disrupted by acidosis within the tumor microenvironment. Breast Cancer Res 15(5):R73View ArticleGoogle Scholar
- Zhang Y, Ma K, Sadana P, Chowdhury F, Gaillard S, Wang F, McDonnell DP, Unterman TG, Elam MB, Park EA (2006) Estrogen-related receptors stimulate pyruvate dehydrogenase kinase isoform 4 gene expression. J Biol Chem 281(52):39897–39906View ArticleGoogle Scholar