The pathogenesis of IPF involved a series of pathological process, such as early inflammation, oxidative stress, myofibroblasts proliferation, excessive wound healing and fibrosis. Deficient understanding of the mechanisms underlying IPF has hampered the development of efficient tools for diagnosis and interventions[17]. For the discovery of new biomarkers, the application of metabolomics is emerging and significantly contributes to a deeper understanding of the metabolic pathways in IPF [18]. Moreover, previous studies have showed that dysregulated metabolism may be used to discover new biomarkers and targets[19]. In this work, a simple, repeatable and sensitive method was validated for the simultaneous determination of metabolites for diagnosis of IPF disease. Herein, a non-targeted UPLC-Q-TOF-MS plasma metabolism method was applied for exploring the metabolic characteristics to screen efficient predictors of IPF. The presented results demonstrated the metabolic profiling of IPF patients differing from those of controls and COPD, which showed satisfying data quality. We compared plasma metabolic profiles of 30 subjects to identify its metabolic signatures. Previous studies on the metabolites of the three groups have been really rare.
Previous study reported that the advantage of plasma metabolic profiling for characterizing metabolite signature of IPF and changes in metabolic pathways that might be helpful for understanding the metabolic mechanism of pulmonary fibrosis[20]. In the current study, PCA did not show a clearly different distribution in the positive/negative ion model. Then, we applied OPLS-DA statistical approach to select differential metabolites. 47 significantly changed metabolites related to IPF were found to be differentialy altered among the three groups in positive/negative ion model. Furthermore, recent many studies have been proved that establishing a diagnostic model to predict the IPF patients could potentially improve the diagnosis[21]. ROC curves of classification models base on the 8 metabolites with high AUC above 0.7 were plotted to distinguish IPF and normal control. Therefore, a combination of more than one discriminatory metabolite will be necessary to increase the diagnostic performance of IPF. To research their metabolism mechanisms, the enriched pathways of the metabolites were also analyzed according to the KEGG database, which reflected the key pathway related with differential metabolites. The most enriched pathway terms were biosynthesis of unsaturated fatty acids, caffeine metabolism, Arginine biosynthesis, linoleic acid metabolism, central carbon metabolism in cancer, protein digestion and absorption, retrograte endocannabinoid signaling and ABC transporters.
Endocannabinoid, interacts with CB1 and CB2 receptors of endocannabinoid system, help coordinate and regulate everything which we feel, think and do. The endocannabinoid/cannabinoid receptor system may be a rational therapeutic target in IPF and promote inflammatory by CB1 activation in many chronic inflammatory diseases[20]. In addition to promoting inflammatory, activation of CB1 also promote fibrosis progression in many organs, such as liver, kidney, heart and skin[22–24]. Simultaneously, it is reported that the brain-penetrant CB1 antagonist attenuates liver fibrosis in mice model[25]. However, the potential role of CB2 has not yet been investigated in patients with IPF or pulmonary fibrosis animal model. As shown in Fig. 5D, CB2 is predominantly expressed in ATII and myofibroblast cells of lung. Rice W et al. reported that CB1 is always expressed in ATII cells, bronchial epithelial cells, and alveolar macrophages[26]. However, we found a higher expression of CB1 in alveolar macrophages than other cell populations following BLM treatment.
Although the FDA have approved pirfenidone and nintedanib for the cure of IPF patients, both compounds still have modest efficacy, but not lead to good overall survival.Therefore, the ideal application implies that simultaneously targeting alveolar inflammation and fibrotic process. Polypharmacology may offer a model for the way to drug discovery. As was demonstrated in Fig. 6, it was observed that the CB1 and CB2 were specifically activated in rat PF model. Furthermore, Resat Cinar etal. reported that elevated activity of the endocannabinoid/CB1 system parallels disease progression in the patients with IPF and in mice with BLM-induced lung fibrosis, thus making it a feasible candidate for the treatment of IPF. Next, cannabinoid receptors antagonist or CB2 deletion will be employed to explore the detail mechanism of antifibrosis and find the downstream target of endocannabinoid/cannabinoid receptor system in the lung with PF. In summary, the results of this study indicate that the endocannabinoid/cannabinoid receptor system and related signal transduction molecules are important targets for the diagnosis and treatment of PF, and that the treatment of ICA protect against pulmonary fibrosis induced by BLM in rats for the first time. Further research should be encouraged to clarify the mechanism of endocannabinoid/cannabinoid receptor system and evaluate the effectiveness of ICA in patients with IPF.