A total of 98 spot urine samples were used to characterise liver-fluke induced CCA signatures using a MS-based metabonomics approach. Clinical information including the anatomical location of the tumours and the presence of obstructive jaundice were used to stratify the patients and explore the effect on the detected metabolite profiles. The presence of obstructive jaundice had a clear impact on bile acid urinary elimination, whereas no obvious difference was observed, based on the tumour anatomical location. Acylcarnitine perturbation was pronounced in urine metabolic profile from CCA patients. Environmental and physiological factors such as drug intake, were found to be the most influential confounding factors.
Despite its promising potential, metabolic profiling holds some intrinsic flaws. The biological and experimental limitations associated with such a system biology approach can challenge its biological interpretation and clinical implementation. Various factors can be a source of variability in the MS data matrix produced using the training and validation samples, which are generally divided into 1) pre-analytical and 2) instrumental or analytical-related (5).
Pre-analytical causes of variability in the detected metabolic profiles can affect the sample at any point from the sample selection and collection until the sample preparation for chromatographic analysis. The stability of human urine specimens can be potentially jeopardised because of sample-related factors, such as prolonged storage and freeze thaw cycles (7). Additionally, physiological factors, including age, gender, ethnicity and disease statues, and environmental factors (such as diet, drugs, physical activity, diurnal cycles, and the host-gut microbiota metabolic interactions) can increase inter-individual biological diversity and mask the true biomarker associations relative to the disease by the addition of noise (5). For example, the presence of drug-related analytes in a sample can affect the presence of metabolic features, as a result of sample saturation and can potentially challenge the interpretation of the results.
It is therefore critical to study these non-disease-specific factors which can significantly induce a high level of noise in the urinary metabolic profiles. The pre-analytical variability can be partially controlled for by matching the participants on demographic characteristics (such as age and gender), collection of sufficient metadata, and standardizing the dietary recording and sample handling protocols.
Sources of technical variability can be accounted for by standardising the analytical pipeline, including the use of the same instrument, column type, solvent batch and instrument parameters (such as mass resolution and collision energy). Small variations between MS experiments in retention times and accurate mass measurements are expected. Furthermore, it is also possible to experience variation across the same analytical run such as retention time drift over the run. Therefore, intermittent injection of quality control samples is routinely used to assess the analytical variation in the MS system (5).
Nevertheless, it is difficult to assess precisely what influences the reproducibility and selectivity of untargeted MS signal detection. Ion suppression and/or ion-enhancement due to the endogenous compounds within the sample (such as salts, proteins and lipids) and the presence of xenobiotic substances (such as drugs) are named ”matrix effects”. The suppression or enhancement response of an analyte results when an analyte present in a sample, other than the analyte of interest compete to be ionised. The volatility of compounds in a sample is another factor that can impact the number of ions detected (8). Furthermore, the degree of droplet evaporation in the ESI source alters the amount of charged ions in the gas phase (8).
Evidence of perturbation in bile acid homoeostasis and increased urinary elimination of excess bile acids in patients with obstructive jaundice have been acknowledged in the late 1960s (9). In the current study, a dramatic increase in urinary bile acid excretion was the most pronounced feature in jaundiced-CCA patients (fold change increase from 75 to 4), compared to non-jaundiced CCA patients. Taurine- and glycine-conjugated cholic acid (taurocholic acid, glycocholic acid), conjugated glycocholic acid and glycochenodeoxycholic were significantly increased in jaundiced-CCA patients, compared to non-jaundiced patients.
To a lesser extent, the urinary excretion of acylcarnitine species was also dysregulated in jaundiced patients. Two medium-chain hydroxylated acylcarnitines, acylcarnitine (C9-OH) and acylcarnitine (C10:2-OH), were elevated in jaundiced patients compared to the non-jaundiced group urine metabolome. Greater urinary concentrations of acylcarnitine (C9-OH) have been detected in Chinese colorectal cancer patients (10) and a rise in the plasma level of acylcarnitine (C10:2-OH) was found implicated in recovery after exercise (11). Acylcarnitine implication in CCA with jaundice is uncertain, but it is possibly due to compromised liver capacity to oxidise fatty acid in clinically-icteric patients (12).
A reduction in the urinary relative abundance of citrate and its isomer, isocitrate, was generally associated with CCA patients, but, it was more pronounced in patients with biliary obstruction. This was contrary to the observation found in a recent Chinese LC-MS metabolic profiling study of urine samples from extrahepatic CCA patients, where urinary citrate levels were higher in cancer patients (13). Shao and colleagues identified general reduction in urinary citrate in presence of liver disease, both benign (cirrhosis) and malignant (HCC), but the metabolite exhibited no significant difference between the two groups (14). An observation which was confirmed by Chen and colleagues, where citrate was only significantly reduced in HCC patients with cirrhosis and hepatitis. (15). The decrease in citrate excretion in urine is possibly related to TCA cycle perturbation, resulting from mitochondria dysfunction in liver disease (16).
A compound putatively identified as vanilpyruvate, a catecholamine and phenylpyruvate derivative, was significantly associated with jaundiced patients (VIP = 3.5, p < 0.0001 and FC = 2.56). Vanilpyruvate is reported to be increased in the urinary metabolic profile of individuals with aromatic l-amino acid decarboxylase deficiency, a disorder that impairs the synthesis of serotonin, dopamine and catecholamines (17). Several other compounds were found to be differential between jaundiced and non-jaundiced CCA urine samples, but not to a level of significance. Greater abundance of creatinine, creatine, proline betaine, citrate, hippurate and indoxylsulfate was found to be associated with jaundiced patients, whereas, 4-phenylbutanic acid-O-sulfate and butyryl-L-carnitine were down-regulated, compared to the non-jaundiced group.
A multivariate statistical model was computed using the spectral data of CCA patients (excluding jaundiced participants) versus healthy controls to identify a panel of urinary metabolic markers related to CCA-genesis. Several metabolites belonging to various metabolic pathways, particularly related to acylcarnitine and steroid metabolism, were found to be perturbed. Assessment the validity of these metabolites and their relevance to hepatobiliary disease was carefully reviewed before considering these for future evaluation as potential markers in CCA.
As an example, leucyl- or isoleucyl- proline was ruled out from our diagnostic panel: the dipeptide was higher in men and it is therefore possibly influenced by protein intake. Previous MS studies on animals have shown influence of disease and diet on leucyl-proline levels. The urinary clearance of the peptide decreased in rats with atherosclerosis (18). In another study, the plasma levels of leucyl-proline decreased after 7 weeks of hypercholesterolemic diet (19). Leucyl-proline was also significantly (p < 0.001) reduced in human MS urinary profiles in bladder cancer patients (20), but was found to be higher in HCC (21). Therefore, because of the ambiguous kinetics leading to these observations, leucyl-proline was not included as one of the diagnostic markers.
Compounds classified as gut microbial co-metabolites and/or dietary phenols (hippurate, phenylacetylglutamine, indoxyl sulfate, pyrocatechol sulfate, 4-phenylbutanic acid-O-sulphate, and phenylalanine) are influenced by a number of factors other than cancer. For instance, hippurate excretion has been observed to be higher in females, as reviewed by (22).
Lees and colleagues also reported reduction in hippurate urinary clearance in obesity, diabetes, inflammatory bowel disease, parasitic infection and cancer, as observed in numerous publications investigated using a metabonomic approach (22). Analysis of the diagnostic power of these markers generally showed poor performance (AUC < 70) and/or wide confidence intervals. These markers did not contribute greatly to the multi-panel marker classification.
The AUC values and the corresponding CIs of potential biomarker combinations yielded high values ranging from 0.942 using only 2 metabolites to 0.99 using 20 or 25 molecules. The best diagnostic accuracy, 93.4%, was achieved with a total of 10 metabolic features. The performance compares favourably to CA-19, the currently used clinical marker, which has been reported to have a sensitivity and specificity of 40–70% and 50–80%, respectively in CCA patients (23).
Acylcarnitine and steroid species yielded the best classification performance and were selected in the multipanel model. Accumulation of hydroxylacylcarnitine species (C9-OH and C10-OH) was a distinctive pattern in the CCA urine metabolome. Levels of saturated (C9) and unsaturated (C8:1, C10:1, C10:2 and C10:3) long-chain acylcarnitine species were significantly reduced in CCA, particularly in patients without obstructive jaundice. An acylcarnitine, 2-trans,4-cis-decadienoylcarnitine (C10:2), was shown to be the most frequently selected metabolite, based on a repeated random sub-sampling cross validation (n = 50 runs).
Steroid-related analytes also showed excellent diagnostic utility. Four steroid metabolites, tetrahydroaldosterone-3-glucuronide, 3b,16a-dihydroxyandrostenone sulfate, pregnanediol-3-glucuronide and steroid glucuronide, were selected among the diagnostic panel. Low urinary elimination of steroids was observed in all CCA patients, regardless of jaundice. Urine steroid metabonomics studies revealed a promising diagnostic utility for distinguishing early HCC from cirrhosis (24) and adrenocortical carcinoma from adrenocortical adenomas (25). The strong male predominance of HCC in males prompts the question on the involvement of sex hormones (particularly androgens) in hepatocarcinogenesis (26).
The role of various agents in modulating biliary carcinogenesis has been thoroughly studied in experimental models (27, 28). Steroid hormones have been shown to play a role in supporting the growth of the biliary epithelium (28). The bile epithelial network possess a number of biological functions (such as secretion, absorption, proliferation, regenerative processes and signalling) that is regulated by several agents including neuropeptides, steroid hormones, cytokines and growth factors (27, 29). In the presence of cholagiopathies, the maintenance of biliary functions is greatly affected as the balance between proliferation/loss of cholangiocytes is lost (28).
Oestrogens, for example, are well known carcinogenic agents in oestrogen-responsive tissues and have been recently shown to play a role in promoting CCA cellular growth and apoptosis (28). Cholangiocytes of normal liver do not express oestrogen receptors, but in presence of biliary malignancy, oestrogen receptors were positive in > 80% of cholangiocarcinoma cells (30). Oestrogens were found to favour the growth and proliferation of malignant mass by synergising the activities of growth factors, and by taking part at both receptor and post-receptor levels (31). Thus, they play a critical role in inducing neo-angiogenesis in oestrogen-sensitive cancers through the activities of vascular endothelial growth factor (VEGF) receptors, which in turn, mediate the proliferative effects of oestrogens (31). Mancinelli et al., suggested that CCA tumours could be consider oestrogen dependent and the use of anti-oestrogen drugs should control the growth of these tumours (32).
Interestingly, glycochenodeoxycholate-N-sulfate was also selected among the biomarker panel. The abundance of bile acid metabolites varied within the non-obstructive CCA group, implicating those factors other than jaundice status affected bile acid homeostasis in a subgroup of patients. Accumulation of bile acids in urine has been associated with an inflammatory signature in various forms of hepatobiliary diseases (33). Furthermore, dysregulation of bile acid metabolism was found to be implicated in biliary tract carcinogenesis by activating pathways that promote cholangiocellular proliferation and increases CCA invasiveness, such as the G protein-coupled bile acid receptor TGR5 (34).
In conclusion, the study highlights several metabolic alterations in biliary carcinoma and builds on the initial pilot study, where we studied 48 Thai subjects at high risk of infection, 41 with active O. viverrini infection, 34 with periductal fibrosis and owing to the difficulty of sample collection, only 14 with CCA, all of whom were non-jaundiced (4). In the current study, pronounced bile acid urinary excretion was mostly associated with clinically icteric CCA patients. Similar to our initial study, patients without jaundice, acylcarnitine and steroid species were the most discriminant, compared to the disease-free cohort. Targeted acylcarnitine and steroid assay could potentially be informative; it can provide a greater understanding with the benefits of higher specificity, lower potential for false positive identifications and better control for environmental confounders (such as diet and drugs). Gut microbial co-metabolites (such as hippurate and indoxyl sulfate) and bile acids have been shown to exhibit a different pattern in cancer patients. Emerging evidence has identified a distinct colonic microbial population “on” and “off” the tumour microenvironment, implicating an interplay between intestinal microbial ecology and carcinogenesis (35). Better understanding of the gut microbial-mammalian co-metabolism and the role of the oncogenic microbiome in disease initiation and progression may provide novel diagnostic and therapeutic tools.
The rare occurrence of CCA in Western countries limits biomarkers validation in large human cohorts. Nevertheless, CCA is one of the most common malignancies in Thailand and the framework for a large-scale surveillance programme is already available. Preclinical validation of a urinary prognostic and diagnostic metabolic panel could be implemented in parallel to the exciting ultrasound screening programme known as CASCAP (36).