The early diagnosis of biliary tract cancer (BTC) remains challenging, and there are few effective therapies. This study investigated whether the M2 isotype of pyruvate kinase (M2-PK), which serves as the key regulator of cellular energy metabolism in proliferating cells, could play a role in the diagnosis and therapy of BTC.
Plasma and bile M2-PK concentrations were measured by enzyme-linked immunosorbent assay in 88 patients with BTC, 79 with benign biliary diseases, and 17 healthy controls. M2-PK expression was assayed in a BTC tissue array by immunohistochemistry. The role of M2-PK in tumor growth, invasion, and angiogenesis was evaluated in BTC cell lines by retrovirus-mediated M2-PK transfection and short hairpin RNA silencing techniques.
Sensitivity (90.3%) and specificity (84.3%) of bile M2-PK for malignancy were significantly higher than those for plasma M2-PK and serum carbohydrate antigen 19-9. M2-PK expression was specific for cancer cells and correlated with microvessel density. M2-PK positivity was a significant independent prognostic factor by multivariable analysis. Transfection of M2-PK in a negatively expressed cell line (HuCCT-1 cells) increased cell invasion, whereas silencing in an M2-PK–positive cell line (TFK cells) decreased tumor nodule formation and cellular invasion. A significant increase in endothelial tube formation was noted when supernatants from M2-PK–transfected cells were added to an in vitro angiogenesis assay, whereas supernatants from silenced cells negated endothelial tube formation.
Biliary tract cancer (BTC), comprising cholangiocarcinoma and the less common cancer of the gallbladder, is one of the deadliest malignancies after pancreatic cancer, with similar incidence and mortality rates. Surgical resection provides the best hope of survival when diagnosed at an early stage, but of the 10% to 30% of cases where surgery is feasible, only a few patients survive beyond 3 years.1, 2 In most patients, relief of biliary obstruction and palliative chemotherapy are the mainstays of therapy.
In patients presenting with biliary symptoms, the diagnosis is largely based on the clinical presentation of cholestasis, abdominal pain, and weight loss, together with cross-sectional imaging studies, but it is often difficult to differentiate between BTC and benign stricturing conditions such as primary sclerosing cholangitis (PSC). Endobiliary procedures such as forceps biopsy, or brush or aspiration cytology have high (95%-100%) specificity but are invasive and have a sensitivity of only 40% to 70%.3 Liver biochemical abnormalities and the standard tumor markers such as carbohydrate antigen 19-9 (CA19-9) and carcinoembryonic antigen (CEA) are of limited value. Serum levels of CA19-9 are elevated in up to 50% to 85% of patients with BTC, but CA19-9 elevation can also occur in obstructive jaundice without malignancy. Other tumor markers such as DU-PAN-2, cancer antigen 125, and RCAS1 (receptor binding cancer antigen expressed on SiSo cells) have performed similarly or less well than CA19-9 and are not part of routine clinical practice.4 A reliable tumor marker that could discriminate between benign biliary disease and BTC would be a useful addition to the current diagnostic work-up for BTC.
One such potential biomarker is the pyruvate kinase isoenzyme type M2 (M2-PK). Pyruvate kinase plays a vital role in the final step of glycolysis, which produces pyruvate and adenosine triphosphate from glucose degradation. The M2-PK is the characteristic pyruvate kinase isoenzyme of all proliferating cells including tumor cells. A unique feature of M2-PK is that this isoenzyme may appear in different conformations: a highly active tetrameric and a nearly inactive dimeric form. The dimeric form works as a dam to build-up the intermediary macromolecules upfront in the glycolytic pathway which are subsequently used as cell building blocks such as fatty acids, amino acids and nucleic acids for cell proliferation. In tumors, M2-PK is found to be mainly in the dimeric form and has therefore been termed “tumor M2-PK.”5 Currently, the dimeric form can be detected by a specific antibody raised against it which does not react with the tetrameric form. M2-PK can be detected in different body fluids, such as plasma, stool, and pleural fluid, and may have a role in the diagnosis of solid tumors including lung, ovary, cervix, breast, kidney, gastrointestinal tumors, and melanoma.6-10 As yet, however, the role of M2-PK in bile as a marker for BTC has not been investigated. In this study, for the quantification of the dimeric form of M2-PK in plasma and bile samples, a sandwich enzyme-linked immunosorbent assay (ELISA) technique was used, which is based on 2 monoclonal antibodies directed against the dimeric M2-PK. The antibody used for immunohistochemistry (clone DF4) was shown to discriminate between the dimeric and tetrameric forms of M2-PK and only stains the dimeric form. In western blots, the antibody stains total M2-PK protein and reflects M2-PK expression.
In this study, we determined both plasma M2-PK (pM2-PK) and bile M2-PK (bM2-PK) in BTC and correlated the results with clinicopathological characteristics and patient prognosis. We also used a tissue array platform to determine the M2-PK expression pattern in BTC and correlated the results with clinicopathological parameters, including neoangiogenesis. Finally, we performed a series of in vitro studies in BTC cell lines to characterize the role of M2-PK in cell proliferation, invasion, and angiogenesis, and as a potential therapeutic target.
MATERIALS AND METHODS
A total of 167 patients were included in this study: 1) patients with histologically/cytologically proven BTC (n = 88), and 2) patients with benign biliary conditions (BBC, n = 79) including obstructive jaundice secondary to biliary stones or benign biliary strictures, sphincter of Oddi dysfunction, autoimmune pancreatitis, and PSC. Patient characteristics are shown in Table 1. Seventeen healthy controls also donated blood samples for the study; however, data from healthy controls were not used in generating the receiver operating characteristic (ROC) curve. Informed consent was obtained from all subjects for use of clinical material for research purposes, and the protocol was approved by the Ethical Committee of the UCL Institute of Hepatology.
Table 1. Patient Characteristics
Includes hepatopancreatobiliary inflammatory conditions, including hepatitis of different causes with biliary stricture.
Measurement of M2-PK Concentrations in EDTA Plasma and Bile Samples by ELISA
An ELISA kit (kindly provided as a gift from ScheBo Biotech, Giessen, Germany) was used for measuring bM2-PK and pM2-PK. In brief, diluted samples were added to 96-well plates coated with anti–M2-PK antibody. After 60 minutes of incubation and several rinses, plates were incubated with a biotin-conjugated anti–M2-PK monoclonal antibody for 30 minutes. Bound M2-PK molecules were detected with a streptavidin-coupled horseradish peroxidase reaction, and the plates were read at 450 nm using a spectrophotometer. All samples were assayed in duplicate.
Immunohistochemistry of M2-PK and CD34
A commercially available tissue array (Stretton Scientific Ltd, UK), comprising 46 BTC samples and 2 normal biliary epithelial tissues in duplicate, was used for dual immunostaining to simultaneously localize M2-PK and microvessels. Immunostaining was done with a double staining kit according to the manufacturer's instructions (PicTrue kit, Invitrogen, UK). Sections were incubated simultaneously with anti–M2-PK antibody (anti-human M2-PK antibody, ScheBo Biotech UK Ltd) and anti-human CD34 antibody overnight at 4°C. Two distinct substrate/chromogen/enzyme systems were used: immunoglobulin G (IgG)–horseradish peroxidase produced brown color (M2-PK) whereas IgG–alkaline phosphatase with Fast Red produced red color (blood vessels). M2-PK staining was evaluated according to a scoring formula, and number of microvessel density (MVD) was counted as described.11
Four established human BTC cell lines (HuCCT, TFK, SKCHA, and SG231) were used for this study; they were purchased from Japan Health Sciences Foundation, Japan, and DSMZ Scientifica, Germany. All cell lines were maintained in Roswell Park Memorial Institute (RPMI) medium.
Transfection of M2-PK
For M2-PK transfection experiments, a lentivirus vector was used for stable transfection of the full-length M2-PK gene. The M2-PK gene incorporated into the pOTB7 plasmid was purchased from Gene Services Ltd, Cambridge, UK. An expression vector was constructed with the pSIN vector (gift from Yasuhiro Ikeda, Windeyer Institute) and the M2-PK gene driven by the SFFV promoter. Coexpression of enhanced green fluorescent protein (eGFP) was obtained by using an internal ribosomal entry site (IRES; gift from Chris Boshoff, University College London). HuCCT cells were infected with different clones of M2-PK recombinant virus for stable transfection and monitored by GFP expression. The clone with the highest expression of M2-PK (HuCCT-CM92) was selected for subsequent analysis. A control virus without M2-PK but with GFP insert was used as a negative control (HuCCT-RIG2). Expression of M2-PK was checked by both western blot and reverse transcription polymerase chain reaction (RT-PCR) and the strongest clone was selected for further experiments.
Silencing of M2-PK by Short Hairpin RNA
For silencing M2-PK expression, we used a commercially available genome-wide plasmid based short hairpin RNA (SureSilencing shRNA, Superarray Bioscience). According to the manufacturer, 4 distinct M2-PK–specific shRNA targeting sequences were designed using a proprietary algorithm and cloned into the pGeneClip hMGFP vector (Promega) to generate SureSilencing shRNA plasmids. Transfection-grade SureSilencing plasmids (0.8 mg) (4 gene-specific and 1 scramble) were delivered by using the transfection reagent SureFECT to approximately 80,000 TFK cells. Silencing was confirmed by both real-time PCR and western blot. Among the 4 shRNA clones (B, G, R, and Y), clone B did not inhibit M2-PK expression and was discarded. Cells transfected with the negative control plasmid were used as negative controls.
Western Blot Analysis
Approximately 20 μg of protein was run on precast gels and transferred onto polyvinylidene fluoride membranes (Amersham, Little Chalfont, UK). Following overnight blocking, the membrane was incubated with anti-human M2-PK antibody and then incubated with horseradish peroxidase–linked secondary antibody (Dako, Glostrup, Denmark). The antigen-antibody reaction was visualized using enhanced chemiluminescence. We used anti–β-actin antibody (Sigma-Aldrich, St. Louis, Mo) as protein load control.
Quantitative RT-PCR Analysis
Total RNA was extracted using the RNeasy Mini Kit (Qiagen, Santa Clarita, Calif) and reverse transcribed with the SuperScript III First-Strand Synthesis System (Invitrogen Ltd, Paisley, UK). Amplification by quantitative RT-PCR (qRT-PCR) was performed using an ABI 7500 real-time PCR machine following standard procedures. M2-PK gene-specific primers (Superarray Bioscience, Frederick, Md) were used for the qRT-PCR reaction.
Cell proliferation rate was determined by a colorimetric MTS assay (Promega, Madison, Wis). Approximately 3000 cells were added to each well in duplicate and incubated for predetermined periods before adding 20 μL of MTS Reagent to each well. Cells were incubated for another 3 hours, and the optical density was measured at 490 nm by means of spectrophotometry.
Three-Dimensional Cell Culture
A three-dimensional (3D) cell culture system (AlgiMatrix, Invitrogen, UK) incorporating a bioscaffold of alginate inside the 96-well plate was used. M2-PK was transfected, silenced, and the control cell lines were seeded within the scaffolds (2.5 × 104/well). Partial medium changes were done every 72 hours. At 3 weeks, cancer spheroids were counted from the surface and bottom of the plate in 10 randomly selected areas. Data were expressed as average number of spheroids per high-power field (×400).
Cell Migration Assay
Tumor cell invasion was assayed in an invasion chamber (Cell Biolabs, San Diego, Calif) with 8 μm porosity polycarbonate membrane. The insert coated with Matrigel matrix was placed in each well of a 24-well plate filled with 300 μL of warm medium (RPMI without fetal bovine serum). The upper well contained suspension of tumor cells (0.5 × 106 cells per milliliter of medium). After incubation at 37°C for 24 hours, the cells on the upper surface were gently scrubbed, and cells that had migrated through the filter were stained with AlamarBlue dye. Migrated cells were extracted with 200 μL of extraction solution and quantified by a spectrophotometer at 560 nm. All assays were performed in duplicate.
In Vitro Angiogenesis
We used 24-well plates containing early stage cocultures of human umbilical vascular endothelial cells and matrix-producing cells (TCS Angio Kit). Conditioned media from cell lines at different cultivation time points were added to the culture plate in appropriate wells and were replaced with fresh conditioned media on the fourth and seventh day of cultivation. Recombinant vascular endothelial growth factor (VEGF) and suramin were used as positive and negative controls, respectively.
Differences were compared by t test or Mann-Whitney U test as appropriate. For comparison of multiple groups, 1-way analysis of variance with Scheffe's post hoc test was used. The Spearman test was used as a measurement of correlation between continuous variables. A cutoff point for pM2-PK and bM2-PK was determined by using ROC analysis. Differences between categorical values were determined by chi-square test with Yate's correction. Survival was analyzed using the Kaplan-Meier method in combination with the log-rank test. Statistical significance was taken as P < .05 by SPSS software.
M2-PK Concentrations in Bile and Plasma
Comparison of pM2-PK concentrations revealed no difference between benign biliary conditions (BBC) and healthy controls (29.2 ± 35.9 vs 8.9 ± 9.7 U/mL, mean ± standard deviation (SD), respectively, P = .22, analysis of variance with Scheffe's test) (Fig. 1A). However, in patients with BTC, pM2-PK level was significantly higher than in those with BBC (70.2 ± 52.5 vs 29.2 ± 35.9 U/mL, respectively; P < .001) (Fig. 1A). The bM2-PK concentration was 109.8 ± 63.6 U/mL in patients with BTC compared with 12.3 ± 18.8 U/mL in all noncancerous conditions (P < .001, t test; Fig. 1B), and only 4.1 U/mL (range, 1-17 U/mL) when 3 extreme values (45, 54, and 66 U/mL) in gallbladder bile samples from BBC patients collected at the time of cholecystectomy were excluded. In this cohort, the 17 patients with PSC had an average bM2-PK level of 15.2 U/mL, which was nonsignificantly higher than in the whole Benign Biliary Diseases group (12.3 U/mL). A paired analysis between pM2-PK and bM2-PK values in patients with BTC revealed that bM2-PK levels were always higher when compared with the pM2-PK level except in 1 case (Fig. 1C). ROC curves generated for bM2-PK showed 90.3% sensitivity and 84.3% specificity with a cutoff value of 24.4 U/mL, whereas the sensitivity and specificity for pM2-PK was 71.0% and 69.9%, respectively, at a cutoff value of 31.7 U/mL. The sensitivity and specificity for serum CA19-9 alone at a standard cutoff value of 37 U/mL were 69.4% and 78%, respectively (Fig. 1D). When considered as continuous variables, there was a significant positive correlation between the CA19-9 and M2-PK levels (bM2-PK and Ca19-9, P = .004, r2 = 39.4; pM2-PK and Ca19-9, P = .000, r2 = 36.4, Spearman rank correlation test). When the data were dichotomized (positive/negative) according to the set cutoff points, although bM2-PK and pM2-PK had 68.75% and 62.6% concordance with the CA19-9, respectively, they were still discordant in almost one-third of cases. When both M2-PK and CA19-9 status were taken into account, this did not improve the diagnostic sensitivity and specificity over the M2-PK alone status.
Relationship Between M2-PK Status and Patient Survival
Using the aforementioned cutoff values, patients were dichotomized into high (bM2-PK > 24.4 U/mL; pM2-PK > 31.7 U/mL) and low (bM2-PK ≤ 24.4 U/mL; pM2-PK ≤ 31.7 U/mL) for M2-PK. A total of 75 of 88 (85%) patients with BTC had up-to-date follow-up and outcome data. The prevalence of M2-PK positivity was significantly (P = .035) higher in patients aged 65 years and older, and there was a trend toward increased positivity in patients with advanced tumor T stage (P = .078) (Table 2). Patients with high pM2-PK levels had a shorter overall survival (12.1 ± 1.9 months) than patients with low M2-PK concentrations (28.8 ± 6.1 months) (P = .01, log-rank test), whereas a significant correlation could not be attained when patients were stratified by CA19-9 positivity (P = .50) (Fig. 2). Multivariable analysis showed that a combination of bM2-PK and pM2-PK became an independent prognostic predictor (P = .04; hazard ratio = 2.483) when adjusted for tumor stage, CA19-9 positivity, bilirubin level, and patient age. Besides M2-PK, patient age was the only independent predictor of survival (hazard ratio = 2.038).
Table 2. Patient Characteristics Stratified by M2-PK Positivitya
M2-PK– Negative (n = 21)
M2-PK– positive (n = 54)
Numbers in parenthesis indicate percentages.
Patient age, y
Serum bilirubin (μmol/L)
CA19-9 level (U/mL)
T1 and T2
T3 and T4
Tissue Array: Correlation Between M2-PK and MVD
M2-PK expression was not detectable in normal biliary epithelium, whereas it was detectable as cytoplasmic staining in most of the cancers (Fig. 3). Although variable amounts of infiltrating immune-competent cells were detectable in the vicinity of tumor nests, most of these cells were negative for M2-PK. Of 46 BTC cases, 17 (36%) had a score of ≤3 and were classified as negative for M2-PK. M2-PK–positive tumors had significantly higher number of MVD when compared with M2-PK–negative ones (mean ± SD; 83.2 ± 37.4 vs 34.0 ± 17.3, respectively, P < .01, r2 = 71.3, P < .001, Spearman rank correlation test) (Fig. 3). Tissue M2-PK positivity was correlated with patient/tumor characteristics data provided with the tissue array slides by Stretton Scientific Ltd, UK. Lymph node metastasis was positive in 71% of patients with M2-PK–positive tumors compared with 46% in those with negative tumors (P = .18, chi-square test). Tumors with advanced stage (T3 and T4) had significantly higher positivity for M2-PK than those with lower stages (81% vs 27%, P < 0.01). The number of intratumoral infiltrating lymphocytes residing within the tumor nodules was counted in the 5 most densely populated areas under a high-power objective (×400) and the average used as the intratumoral lymphocyte count. There was a trend toward lower counts in M2-PK–positive tumors when compared with the negative tumors (9.5 ± 6.6 vs 14.3 ± 11.6, P = .191) (Table 3).
Table 3. Correlation of Clinicopathological Parameters With M2-PK Expression in Tissue Array
69.4 ± 9.2
67.2 ± 10.8
4.6 ± 2.1
5.7 ± 2.6
Lymph node status
1 and 2
3 and 4
34.0 ± 17.3
83.2 ± 37.4
Number of intratumoral lymphocytes
9.5 ± 6.6
14.3 ± 11.6
Correlation With Cell Proliferation and Growth
Strong M2-PK expression was noted in TFK, SKCHA, and SG231 cells, whereas HuCCT cells had very weak expression (Fig. 4A). For our transfection experiments, we selected TFK and HuCCT as BTC cell lines with high and low M2-PK expression, respectively. Subsequently, we transfected HuCCT cells with a full-length M2-PK gene in order to enhance M2-PK expression. For silencing of M2-PK expression in TFK cells, 4 different shRNA clones (G, R, Y, and B), which are directed against 4 different locations of the PKM gene, were used. Changes in M2-PK expression both at protein (Fig. 4B,C) and messenger RNA level (data not shown) were checked in all transfected cells. All clones of shRNA except clone B produced significant decrease in cell proliferation in TFK cells (Fig. 4E).
M2-PK transfection of the HuCCT cell line almost doubled cell proliferation, whereas silencing of M2-PK in the TFK cell line inhibited it by up to 70% of the controls on day 5 (Fig. 4D). In M2-PK–transfected HuCCT cells, there was a trend toward increased tumor nodule formation (P = .08; Fig. 5A,B,E), whereas silencing of M2-PK significantly inhibited tumor nodule formation when compared with the mock-treated cells (P < .01) (Fig. 5C,D,E).
M2-PK and Cellular Invasion
Representative photomicrographs demonstrate compact cellular growth and invasion on the under surface of the filter in transfected HuCCT cells (Fig. 6A), whereas only a few scattered cells were noted after silencing M2-PK in TFK cells (Fig. 6D). In HuCCT cells, M2-PK transfection induced a nearly 3-fold increase in the motility and invasion of the cells (P < .01; Fig. 6B). Silencing in TFK cells almost abolished the invasion through the Matrigel-reconstituted basement membrane (P < .01; Fig. 6E), indicating a crucial role of M2-PK in cellular motility and invasion.
M2-PK in Capillary Formation
Incubation of human umbilical vascular endothelial cells with physiological concentrations of recombinant VEGF resulted in robust formation of capillary network at 7 days after initiation of the culture, whereas suramin, an angiogenesis inhibitor, almost abolished angiogenesis (Fig. 7A). A comparable angiogenic effect to that of VEGF was noted in wells supplemented with culture supernatant from M2-PK–transfected HuCCT cells (Fig. 7D), which was significantly (P = .002) higher in comparison to wild-type HuCCT cells (Fig. 7C). Treatment with conditioned media from the M2-PK–silenced cells was associated with a significant decrease in capillary formation (P = .019; Fig. 7F,G).
The results of this study provide evidence for the clinical significance and biological relevance of M2-PK in cell proliferation, growth, angiogenesis, and invasion in BTC. We demonstrated that M2-PK in bile and plasma can be used as novel diagnostic and prognostic markers for BTC, opening up the potential for earlier diagnosis of BTC and for use of M2-PK as a novel therapeutic target. pM2-PK levels were significantly higher in patients with BTC than in healthy controls (Fig. 1A), consistent with a previous report.10 In our study, pM2-PK was higher in BBCs than in healthy controls but did not reach statistical significance (Fig. 1A). In the bile of patients with BTC, M2-PK levels were 9-fold higher in comparison to patients with BBC, including the PSC-only group (Fig. 1B), with a sensitivity and specificity of 90.3% and 84.3%, respectively (Fig. 1D). When bM2-PK and pM2-PK values were compared, almost all patients with PSC with high plasma levels of M2-PK had low bM2-PK. Conversely, patients with BTC with low pM2-PK had high bM2-PK concentrations (Fig. 1C), pointing to a high discriminating capacity of bM2-PK for BTC and BBC. In contrast to bM2-PK, the measurement of CA19-9 and CEA in bile does not confer any additional advantage over their corresponding plasma levels.12
M2-PK values have also been reported to be raised in inflammatory conditions such as arthritis.13 The higher pM2-PK values in inflammatory conditions may be due to the systemic inflammatory response rather than local production of M2-PK. Indeed, in this study, M2-PK was rarely expressed by the infiltrating lymphocytes in BTC sections. Accordingly, Oehler et al14 showed that M2-PK is expressed by neutrophils but not by lymphocytes, which may explain a higher level of circulatory M2-PK in inflammatory conditions.
The treatment of BTC remains a daunting challenge for clinicians. To date, decision-making regarding the type and extent of therapy in BTC relies predominantly on cross-sectional imaging and biliary brushings. The results of this study indicate that M2-PK could be a useful prognostic marker in BTC. Indeed, M2-PK became an independent prognostic marker when analyzed with other conventional prognosticators in BTC. Similarly, in melanoma patients, high M2-PK levels identified a group of patients with poorer survival.7 The poor outcome of tumors with high M2-PK content could be attributed to tumor progression and aggressive phenotype. In this study, M2-PK expression in tissue array was significantly raised in advanced tumor stages.
M2-PK expressed by the immune-competent cells in the tumor milieu may also play a role in the aggressive phenotype of the tumor. Indeed, Zhang et al15 very recently showed that SOCS3 (suppressor of cytokine signaling 3) specifically binds to M2-PK in dendritic cells, causing dendritic cell dysfunction. Impaired antigen presentation by the dysfunctional dendritic cells and a lack of production of tumor-specific lymphocytes were correlated with ineffective tumor vaccination in lung carcinoma. Whether a similar phenomenon is responsible for immune tolerance and aggressiveness of BTC warrants further investigation.
In the 4 BTC cell lines studied, we found high M2-PK expression in all cell lines except in HuCCT. Similarly, in our tissue array study, high M2-PK expression (score >3) was noted in 64% of the BTC samples, whereas the normal biliary epithelial cells were all negative. These findings are in agreement with a recently published article describing M2-PK as the second highest differentially expressed gene in intrahepatic cholangiocarcinoma.16 To address the exact role of M2-PK in cellular proliferation, invasion, and angiogenesis, we used TFK cells as a representative cell line with high M2-PK expression and used shRNA to silence M2-PK. Silencing of M2-PK in TFK cells by shRNA was accompanied by pronounced inhibition of cell proliferation (>70% on day 5 of culture). Conversely, M2-PK transfection of HuCCT cells causing high M2-PK expression increased cell proliferation by up to 50% on the third cultivation day.
Silencing of M2-PK inhibited tumor growth significantly in 3D cell culture assays, with almost 80% inhibition of growth of tumor nodules, whereas transfection in HuCCT cells increased the tumor growth with enhanced sprouting of daughter nodules in the 3D cell culture assay. In keeping with these findings, we showed previously that cotransfection of oncogenic gag-A-Raf fusion and wild-type M2-PK doubled colony formation in NIH 3T3 cells.17 Very recently, Christofk et al18, 19 showed that in almost 90% of instances, tumor cells expressing the M2 isoform successfully produced tumors in mice, compared to <50% when cells expressing the M1 isoform were used. Only recently, the clinical relevance of M2-PK in tumor development has been shown in patients with Bloom syndrome, where presence of missense mutations of M2-PK results in the early development of multiple tumors.20
In this study, transfection of M2-PK into HuCCT cells significantly increased cellular invasion, whereas silencing of M2-PK almost negated cell invasion. Similarly, a stepwise increase of pM2-PK concentrations from limited to extensive metastatic disease was noted in pancreatic carcinoma.21 Interestingly, a differential proteome analysis on 2 hepatocellular carcinoma cell lines with high and low metastatic potentials showed that M2-PK was 1 of the 6 highly expressed proteins in the metastatic cells.22
Angiogenesis is a hallmark of tumor aggressiveness. Thus far, the role of M2-PK in tumor angiogenesis has not been investigated. In our study, we found a significant correlation between M2-PK expression and the number of MVD. In the in vitro angiogenesis study, conditioned media from the M2-PK–transfected cells significantly increased the number of nascent vessels, whereas those from the silenced cell line almost abolished angiogenesis to the same level as seen with suramin, an angiogenesis inhibitor. Although any direct correlation between M2-PK and angiogenesis is yet to be established, Duan et al23 showed recently that M2-PK promotes angiogenesis by binding to tumor endothelial marker 8, a cell membrane protein predominantly expressed in tumor endothelium. M2-PK may also contribute to tumor angiogenesis in a paracrine fashion by mast cell degranulation and release of angiogenic factors.24 Interestingly, Terada et al25 showed that the density of mast cells was significantly higher in BTC than in normal liver and hepatocellular carcinoma.
Results of the shRNA-based treatment raise the possibility of designing a novel antitumor treatment by targeting M2-PK, either alone or in combination with chemotherapeutic agents shown to be effective in this condition. Very recently, a selective small molecule inhibitor that inhibits cell proliferation has been described.26 Moreover, a synergistic antitumor effect by a small molecule inhibitor raised against M2-PK, along with other anticancer agents such as cisplatin or gefitinib, has also been reported.27 As such, a strategy of M2-PK inhibition would be a potential therapeutic approach in BTC.
We are grateful to Ms Anja Döring of ScheBo Biotech AG, Germany, for her excellent technical support for setting up the M2-PK ELISA measurement.
This study was supported in part by US National Institutes of Health grant PO1CA84203, UK Medical Research Council grant (G0801588), and Charitable Research Fund East and North Herts National Health Service Trust. Dipok Kumar Dhar is a Jason Boas Research Fellow, and this work was partly funded by the Jason Boas Charitable Fund. The work was undertaken at UCL Hospital/UCL, which receives a proportion of funding from the UK Department of Health's National Institute for Health Research Biomedical Research Centres funding scheme.
CONFLICT OF INTEREST DISCLOSURE
Sybille Mazurak was an employee of Schebo Biotech AG until June 2012 and owns stock in the company.