Pancreaticobiliary maljunction(PBM) is closely related to the occurrence and development of gallbladder stones and bile duct cancer(Funabiki, Matsubara, Miyakawa, Ishihara 2009; Tashiro, Imaizumi, Ohkawa, Okada, Katoh, Kawaharada, Shimada et al. 2003). PBM and long common passage of pancreatic juice reflux into the biliary tract are related to proliferation changes, metaplasia and cancer of bile duct and gallbladder epithelium(Funabiki, Matsubara, Miyakawa, Ishihara 2009; Ichikawa, Kamiyama, Sekido, Ishikawa, Miura, Kamiya, Morita et al. 2004). Some scholars have studied the causes of biliary tract cancer and found that the cancer development rate of patients with PBM is 200 times higher than that of normal pancreaticobiliary junction in Japan(Matsuda, Marugame, Kamo, Katanoda, Ajiki, Sobue, Japan Cancer Surveillance Research 2009; Morine, Shimada, Takamatsu, Araida, Endo, Kubota, Toki et al. 2013). Recently, it has been recognized that patients with normal pancreaticobiliary junction may have OPBR, just like pancreaticobiliary reflux associated with PBM .OPBR could lead to informative changes of the biliary epithelium and progress towards the development of precancerous biliary changes and gallader cancer(McKee 2007; Mizuno, Kato, Koyama 1996). However, the underlying molecular mechanism of gallstone formation and cancer development in OPBR is still unclear.
Metabolomics is a newly developed subject after genomics and proteomics, and it is a promising technology. Metabolomics studies usually use liquid chromatography-mass spectrometry (LC/MS) for organic metabolites and the screening of biomarkers among them(Dunn, Bailey, Johnson 2005; Lu, Zhao, Bai, Zhao, Lu, Xu 2008). Some scholars reported that metabolomics analysis can be used as a new method for the diagnosis of breast cancer, biliary tract cancer and PBM(Claudino, Quattrone, Biganzoli, Pestrin, Bertini, Di Leo 2007; Mori, Morine, Mawatari, Chiba, Yamada, Saito, Ishibashi et al. 2021; Xu, Cheng, Ding, Lv, Chen, Wu, Zheng 2015). In the past, metabolomics methods were only applied in biliary tract cancer and PBM, and few studies were applied, including small sample sizes, different results and different methods(Mori, Morine, Mawatari, Chiba, Yamada, Saito, Ishibashi et al. 2021; Sharif, Williams, Lampejo, Khan, Bansi, Westaby, Thillainayagam et al. 2010). In this study, we applied LC/MS method to metabolomic analysis of bile samples from large study samples of 60 patients. The results of metabolomics analysis showed that there was a certain separation trend between PBR group and Normal group in the metabolomic 2D and 3D score plots.. These results reflect the reliability of the present study and the applied model, which was further confirmed using the OPLS-DA model established from the positive and negative ion mode data of this experiment was good-fit and is statistically vaild.
A strong association has been found between gallstone formation and gallbladder cancer and higher levels of amylase in the gallbladder bile of patients with PBR and NPBJ(Beltran, Vracko, Cumsille, Cruces, Almonacid, Danilova 2007; Horaguchi, Fujita, Noda, Kobayashi, Ito, Takasawa, Obana et al. 2008; Sai, Suyama, Nobukawa, Kubokawa, Sato 2005). Biliary tract cancer associated with PBR usually results from the reflux of pancreatic juice into the biliary tract leading to metabolic disturbances of compounds in the bile causing chronic inflammation and damage to the biliary mucosa, especially amino acid and lipid metabolism disturbances(Funabiki, Matsubara, Miyakawa, Ishihara 2009; Mizuno, Kato, Koyama 1996; Mori, Morine, Mawatari, Chiba, Yamada, Saito, Ishibashi et al. 2021). LC/MS metabolomics analysis was used to determine the characteristics of bile metabolism in the OPBR. PCA and OPLS-DA models showed sufficient sensitivity and specificity to distinguish the OPBR group from the Normal group. Compared with the normal group, the levels of 106 metabolites related to energy homeostasis, amino acid metabolism, Ba metabolism and lipid synthesis in the bile metabolomics were changed significantly in OPBR group. In addition, potential biomarkers with the greatest contribution to this discrimination were then selected according to the VIP values and were identified. The results showed that the levels of phosphatidylcholine and PC (20:3 (8Z, 11z, 14z) / 14:0) were significantly lower in opbr group, while the levels of lysoPC (16:1 (9z) / 0:0), lysoPC (15:0), lysoPC (16:0), palmitic acid, leucine, methionine, L-tyrosine and phenylalanine were significantly higher in OPBR group.
Phenylalanine is an essential amino acid, synthesizes important neurotransmitters and hormones together with tyrosine, and participate in the body's glucose metabolism and fat metabolism. Tyrosine is oxidized from phenylalanine by phenylalanine hydroxylase in vivo and it is an important precursor of melanin, catecholamine neurotransmitter and thyroid hormone. Some studys reported that the abnormal proliferation of malignant tumors in the early and middle stages of lung cancer caused normal cell stress reactions, resulting in abnormal amino acid metabolism, such as a significant increase in tyrosine concentration(An, Chen, Zhang, Song, Sun, He, Bai et al. 2010). Furthermore, phenylalanine and tyrosine biosynthesis were shown to be significantly increased at the stage of severe dysplasia of the stomach and gastric cancer(Lario, Ramirez-Lazaro, Sanjuan-Herraez, Brunet-Vega, Pericay, Gombau, Junquera et al. 2017), anti androgen prostate cancer cells present high levels of phenylalanine and tyrosine(Corsetti, Rabl, McGloin, Nabi 2018). Other studies have confirmed that a diet restricted with phenylalanine inhibits the growth and metastasis of several malignancies(Elstad, Meadows 1990) (Elstad, Meadows, Abdallah 1990). The results of this study suggest that phenylalanine and tyrosine may be related to biliary tract lesions in OPBR, which is similar to the results of a metabolomic study of biliary tract cancer(Mori, Morine, Mawatari, Chiba, Yamada, Saito, Ishibashi et al. 2021).
The analysis results of this study revealed that the circulating leucine levels in OPBR were significantly higher than those in Normal (Table I). Branched chain amino acids (BCAA) have been reported to be associated with a variety of cancers. These BCCA include valine, leucine and isoleucine. An adequate intake of BCAAs is required for protein synthesis and several metabolic and signalling functions, including insulin metabolism(Wiggins, Kumar, Markar, Antonowicz, Hanna 2015). The catabolism of BCAA is an important process in amino acid synthesis(Holecek 2021). In order to support sustained biomass accumulation, cancer cells need to increase the level of nutrients such as amino acids. Indeed, cancer cells have increased BCAAs uptake(Hattori, Tsunoda, Konuma, Kobayashi, Nagy, Glushka, Tayyari et al. 2017), and BCAA aminotransferases (BCATs) that catalyze the first reaction in the catabolism of BCAAs are overexpressed in cancer cells(Fickert, Fuchsbichler, Wagner, Zollner, Kaser, Tilg, Krause et al. 2004; Tonjes, Barbus, Park, Wang, Schlotter, Lindroth, Pleier et al. 2013). Therefore, the increase of leucine in OPBR patients may be because both the host and the tumor need energy and proliferation. Similarly, other studies have reported that there is a positive correlation between higher intake of BCCA in diet and all-cause mortality of colorectal cancer patients(Long, Yang, Liu, Tobias, Katagiri, Wu, Jin et al. 2020). In PDAC cells, bcat2 protein levels are significantly increased, and downregulation of bcat2 can inhibit the proliferation of these cells(Dey, Baddour, Muller, Wu, Wang, Liao, Lan et al. 2017). Serum leucine and isoleucine levels of lung cancer patients are increased(Zhang, Zhu, Wang, Zhang, Cai 2016), and BCAA play an important role in cancer metabolism by regulating the bioenergetics and biosynthesis of cells through the tricarboxylic acid cycle(Reyes-Castellanos, Masoud, Carrier 2020).
Many studies have demonstrated that the disorders of lipid profles played a pivotal pathogenetic role in the initiation and progression of gallstons and GBC(Lee, Lee, Kweon, Kim 2019; Mori, Morine, Mawatari, Chiba, Yamada, Saito, Ishibashi et al. 2021). Lyso-PC is one of the major lysophospholipids and is mainly generated by hydrolysis of phosphatidylcholine. Phosphatidylcholine is the main phospholipid in mammalian cells and plays an important role in cell membrane structure, signal transduction and lipoprotein metabolism(Billah, Anthes 1990). It is synthesized in hepatocytes and subsequently transported to bile ducts by flippase multidrug resistance protein 3. Its main function in bile is to form mixed micelles with bile acids and cholesterol, which is necessary for fat emulsification. Phosphatidylcholine has also been confirmed to have cytoprotective effects and reduce the damaging effect of bile acids on biliary epithelium(Barrios, Lichtenberger 2000; Komichi, Tazuma, Nishioka, Hyogo, Une, Chayama 2003). Decreased phosphatidylcholine in bile will lead to the formation of bile acid monomers, which are more damaging to biliary cell membranes and predispose to biliary malignancies(Komichi, Tazuma, Nishioka, Hyogo, Chayama 2005). Phosphatidylcholine reduction has been found in the bile of biliary cancer(Albiin, Smith, Arnelo, Lindberg, Bergquist, Dolenko, Bryksina et al. 2008; Hashim Abdalla, Taylor-Robinson, Sharif, Williams, Crossey, Badra, Thillainayagam et al. 2011). Lyso-PC can damage vascular endothelial cells, promote the proliferation and migration of tumor cells(Alpini, Invernizzi, Gaudio, Venter, Kopriva, Bernuzzi, Onori et al. 2008; Dial, Dawson, Lichtenberger 2015), exhibit cytotoxicity by inducing biliary epithelial cell damage, and also cause cholangiocyte senescence, which may ultimately lead to biliary tumors(Fujita, Sugiyama, Otoshi, Taogoshi, Kimura, Kishikawa, Kodama et al. 2014). In present study, lyso-PC in bile of OPBR patients was significantly higher than that of Normal.
As for the mechanism of gallstone formation, it is believed that the changes of bile composition caused by various reasons is one of the main reasons of gallstone formation, including cholesterol, phospholipids and free fatty acids(FFA)(Carey, Lamont 1992; Portincasa, Moschetta, Palasciano 2006). Bile FFA play an important role in maintaining the stable dissolution of cholesterol, and the main component is palmitic acid(Fujino, Tamura, Uraya, Yonaga 1987). Some studies have revealed that the total FFA content in bile of patients with gallstones is significantly higher than that of normal groups. With the increase of bile FFA concentration, it will promote cholesterol crystallization and ultimately stone formation. In addition, a large number of unsaturated FFA can cause damage to the gallbladder mucosa, damage the gallbladder contraction function, and promote the high secretion of gallbladder mucin. These factors are all considered to play a positive role in the formation of gallstones(Schone, Jungst, Meyer, Hernandez-Richter, Fischer 2000). Similarly, we found that palmitic acid was significantly elevated in OPBR patients, which may be one of the reasons for gallstone formation. Meanwhile, arachidonic acid in bile of OPBR patients was higher. Phospholipase A2 plays a crucial role in arachidonic acid metabolism and secretion and is upregulated in patients with multiple cholesterol stones(Shoda, Ueda, Ikegami, Matsuzaki, Satoh, Kano, Matsuura et al. 1997). Arachidonic acid metabolism may play an active role in the formation of gallstones OPBR patients.
In this study, we performed a bile metabolomic analysis to compare the metabolites of bile obtained from patients with OPBR and benign diseases. Studys using metabolomics for the pathogenesis of OPBR has not been reported thus far. This is the first study on the metabolomic analysis of OPBR bile, and also a highly original study with clinical application value. In addition, we found that there were significant differences in bile lithogenic and carcinogenic metabolites between OPBR and benign diseases. This study can actually identify carcinogenic related metabolites, which is helpful to identify high-risk groups of clinical biliary tract tumors. All of the above are the advantages of this study and have strong clinical applicability. However, the current study has some limitations. Firstly, one limitation was that the sample size was small. Therefore, it is necessary to carry out validation studies. We are collecting more bile samples from OPBR patients and OPBR patients with biliary tract tumors. Another limitation is that there is no detection of markers of precancerous lesions in the gallbladder resected from OPBR patients. Although this is a preliminary study, our results provide a basis for further study of the biliary carcinogenesis mechanism of OPBR.Because bile metabolites involve complex interaction networks in the process of cancer development, further targeted metabolomics research is needed to analyze the association between changes in amino acid, bile acid and lipid metabolism and the occurrence and progression of cancer.
In conclusion, the results of this study demonstrate that LC/MS-based metabolomic approaches are effective and promising methods to differentiate OPBR from benign biliary diseases and to identify specific metabolites. The results of this study may help to elucidate the oncogenic mechanism of the disease in the future. Bile metabolites were significantly altered in OPBR patients compared with healthy controls. Therefore, OPBR patients can be considered as a high-risk group for biliary tract tumors.