Gamma-glutamyl transpeptidase and indirect bilirubin may participate in systemic inflammation of patients with psoriatic arthritis
Advances in Rheumatology volume 63, Article number: 53 (2023)
Previous studies have suggested that systemic metabolic abnormalities are closely related to psoriatic arthritis (PsA). Gamma-glutamyl transpeptidase (GGT) and indirect bilirubin (IBIL), two essential active substances in hepatic metabolism that have been demonstrated as an oxidative and anti-oxidative factor respectively, have been proved to be involved in oxidative stress damage and inflammation in several human diseases. However, their role in PsA remains unclear.
In this retrospective comparative cohort study, a case group of 68 PsA patients and a control group of 73 healthy volunteers from the Third Hospital of Hebei Medical University were enrolled. Serum GGT, IBIL, GGT/IBIL ratio and C-reactive protein (CRP), a well applied bio-marker of systemic inflammatory in PsA, were compared between the two groups. Furthermore, the relationship of GGT, IBIL and GGT/IBIL with CRP were explored in PsA patients. Finally, the patients were divided into high inflammation group and low inflammation group according to the median value of CRP. Multivariate logistic regression analyses were used for the association of systemic inflammation level with GGT, IBIL and GGT/IBIL.
Compared with healthy controls, PsA patients exhibited significantly higher serum GGT, GGT/IBIL, and CRP levels and lower IBIL levels. Serum GGT and GGT/IBIL were positively correlated with CRP, whereas IBIL were negatively correlated with CRP. Binary logistic regression analysis revealed that serum GGT was a risk factor for high CRP in PsA, whereas IBIL was a protective factor. Furthermore, GGT/IBIL was a better indicator of high CRP condition in PsA patients than either GGT or IBIL alone, as determined by the receiver operating characteristic curves.
GGT and IBIL may participate in the pathogenesis of PsA. Additionally, GGT, IBIL and the balance of the two may reflect systemic inflammation mediated by oxidative stress events related to metabolic abnormalities to a certain extent.
Psoriatic arthritis (PsA) is a chronic inflammatory joint disease that occurs in approximately 30% of patients with psoriasis [1, 2]. Currently, the pathogenesis of PsA is unclear. However, oxidative stress is considered an important factor associated with the pathogenesis of both psoriasis and PsA .
Gamma-glutamyl transpeptidase (GGT) is a crucial enzyme involved in the catabolism of glutathione (GSH), catalyzing the transfer of glutamyl to amino acids . This process produces large amounts of reactive oxygen species (ROS), leading to the development of oxidative stress in the body . The role of GGT in oxidative stress has received increasing attention in recent years, and previous studies have shown that serum GGT can predict the occurrence of metabolic syndrome, cardiovascular malfunctions and other diseases [6, 7]. However, only a few studies have analyzed the GGT expression in patients with PsA. Indirect bilirubin (IBIL) is the final product of heme catabolism. As early as 1987, Stocker et al.  suggested that bilirubin may be a natural antioxidant that can effectively eliminate free radicals and participate in the reduction of oxidative stress. It is known to have anti-inflammatory and immunosuppressive properties and plays a protective role in rheumatoid arthritis and polymyositis [9, 10]. Interestingly, previous reports have proposed contrasting roles for serum GGT and IBIL in metabolic syndrome and systemic lupus erythematosus [11, 12]. However, the roles of GGT and IBIL in PsA are currently unknown.
C-reactive protein (CRP) is a sensitive marker that reflects the inflammatory status of the body . Previous studies have concluded that CRP is the best serum indicator of PsA activity . Therefore, in this study, we used CRP as an indicator of the systemic inflammatory response to PsA and investigated the relationships between serum GGT, IBIL, and CRP levels in patients with PsA.
Materials and methods
Patients and healthy controls
We recruited patients with PsA who visited the Third Hospital of Hebei Medical University from January 2016 to December 2021. All patients met the 2006 Classification Criteria for Psoriatic Arthritis (CASPAR) . Exclusion criteria included the following: pregnancy/breastfeeding, alcohol abuse, acute/chronic inflammatory disease, infection or trauma, hepatobiliary disease, renal insufficiency, coronary artery disease, cancer, hemolytic and autoimmune diseases, and treatment with antioxidants, non-steroidal anti-inflammatory drugs, steroidal drugs, immuno- suppressants, or biologics in the month prior to evaluation. Based on these criteria, 68 patients were enrolled in the study group. An additional 73 healthy volunteers were recruited from the physical examination center of our hospital as controls during the same period. Our trials were conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committee of the Third Hospital of Hebei Medical University (2022-023-1).
We retrospectively collected the personal and clinical data of all participants, including age, sex, and serum levels of GGT, IBIL, CRP, alanine aminotransferase (ALT), and aspartate aminotransferase (AST). Because GGT and IBIL are known to have oxidant and antioxidant properties, respectively, the GGT/IBIL ratio was included as an additional variable in the analysis.
All data were analyzed using SPSS 25.0 (SPSS Inc., Chicago, IL, USA). Normally distributed data were presented as mean ± standard deviation, and differences between groups were assessed using Student's t-tests. Nonparametric data were described by median values (interquartile range) and subjected to the Mann–Whitney U test. The chi-square test was used to compare the distribution of categorical variables between groups. Relationships between GGT, IBIL, GGT/IBIL, and CRP were analyzed by Spearman’s correlation analysis. The effect of GGT and IBIL on the severity of inflammation was evaluated through binary logistic regression. Finally, the receiver operating characteristic (ROC) curves were constructed based on the results of the logistic regression analysis to evaluate the predictive value of GGT, IBIL, and GGT/IBIL for the degree of inflammation in patients with PsA. All tests were two-tailed with a statistical significance threshold of P < 0.05.
The characteristics of 68 patients with PsA and 73 healthy controls are presented in Table 1. No significant differences were observed in the age and sex distributions between patients and controls (all P＞0.05). The levels of serum GGT, GGT/IBIL, and CRP were significantly higher (P = 0.036; P < 0.001; P < 0.001) in the patient group than in the control group, whereas IBIL concentrations were significantly lower (P < 0.001). ALT and AST levels showed no significant differences between the two groups (P = 0.068; P = 0.111).
Correlations of serum GGT, IBIL, and GGT/IBIL with CRP
Spearman correlation analysis was used to evaluate the associations of serum GGT, IBIL, and GGT/IBIL with CRP. We found that serum GGT levels were positively correlated with serum CRP levels (r = 0.4427, P < 0.001), while serum IBIL levels were negatively correlated with serum CRP levels (r = − 0.4187, P < 0.001). Furthermore, the ratio of GGT/IBIL was positively correlated with CRP levels (r = 0.6292, P < 0.001) (Fig. 1a–c).
Relationship between GGT, IBIL, and the degree of inflammatory response in patients with PsA
To assess the relationship between GGT, IBIL levels and inflammation in patients with PsA, we used CRP levels as a measure of the inflammatory response and divided patients into two groups based on their median serum CRP concentration. No significant differences in age, gender, ALT level, or AST level between the CRP-low (PsA1) and CRP-high (PsA2) groups were detected (all P＞0.05). Serum GGT levels and GGT/IBIL were significantly higher in the PsA2 group than in the PsA1 group (P = 0.002; P < 0.001). Conversely, IBIL levels were significantly lower in the PsA2 group than in the PsA1 group (P = 0.003; Table 2). Multivariate logistic regression analysis adjusted for age, sex, and ALT, AST, GGT, and IBIL levels revealed that a high GGT level was a risk factor for high systemic inflammation in patients with PsA (OR = 1.073, 95% confidence interval [CI] 1.006–1.145, P = 0.032), whereas a high IBIL level was a protective factor (OR = 0.766, 95% CI 0.633–0.928, P = 0.006; Table 3). ROC curves showed that the area under the curve (AUC) value for GGT/IBIL (AUCGGT/IBIL = 0.814) was higher than that for GGT and IBIL (AUCGGT = 0.718; AUCIBIL = 0.707) (Fig. 2).
PsA is a chronic inflammatory joint disease with complex pathogenesis. An increasing number of studies have recently proved that oxidative stress is associated with the pathogenesis of PsA. Firuzi et al.  found that plasma peroxide levels were significantly elevated and sulfhydryl levels were decreased in patients with PsA, suggesting that an imbalance of oxidative and antioxidative processes may contribute to PsA pathogenesis. Oxidative stress-associated ROS production triggers the activation of the nuclear factor-kappaB (NF-κB) signaling pathway in dendritic cells, macrophages, and other immune cells, which results in a release of inflammatory cytokines, such as interleukin 1 (IL-1) and tumor necrosis factor-α (TNF-α). These cytokines stimulate osteoclasts to invade adjacent cartilage and mediate bone resorption, leading to joint deformity and loss of function [17,18,19].
GGT transports glutamyl residues on cell membranes and is responsible for the catabolism of the antioxidant GSH in vivo. Studies have shown that GGT undergoes redox reactions when it hydrolyzes GSH and reduces Fe3+, which can produce large amounts of superoxide anions , leading to many events related to oxidative stress in the body. A previous study on coronary artery risk development in young adults suggested that increased serum GGT levels may serve as an early marker of oxidative stress and predict the serum levels of inflammatory factors, such as CRP . Recent studies have demonstrated the involvement of GGT in the pathogenesis of cardiovascular and metabolic syndrome diseases through the oxidative stress reaction, concluding that GGT plays a pro-inflammatory role in these diseases [6, 7]. Furthermore, GGT levels were significantly increased in the inflamed synovial membranes of patients with rheumatoid arthritis. Moreover, injecting anti-GGT monoclonal antibodies into the intraperitoneal cavity of arthritic mice significantly reduced the number of osteoclasts and attenuated bone erosion . Interestingly, GGT is an endogenous activator of Toll-like receptor 4-mediated osteoclastogenesis . Our study builds upon these previous works, as we discovered a positive correlation between the levels of GGT and CRP in patients with PsA, indicating the GGT may serve as a risk factor for PsA-related inflammation.
IBIL is a powerful antioxidant, and the work by Zhao et al.  demonstrated that IBIL can directly scavenge superoxide anion radicals at the sites of inflammation. Furthermore, in a mouse model of autoimmune arthritis, exogenous IBIL injections significantly reduced oxidative DNA damage, neutrophil infiltration, and fibrin deposition in the joint cavity, leading to a decrease in inflammation and alleviating the symptoms of joint damage . Additionally, IBIL has been shown to promote the expansion of regulatory T cells , which may disrupt the activation and proliferation of autoreactive T cells in PsA, thereby inhibiting the immune response. Moreover, in a study by Balta et al. , patients with psoriasis vulgaris exhibited decreased serum IBIL levels and elevated CRP levels, which is consistent with our findings. Thus, we conclude that IBIL plays a protective role in inflammatory and autoimmune diseases, including PsA.
At present, the specific mechanisms underlying the roles of GGT and IBIL in the inflammatory response of patients with PsA are unclear. However, we can glean some insights by connecting the findings of previous reports. The stimulation of bone marrow-derived macrophages with recombinant human GGT1 protein was shown to increase the expression of TNF-α, IL-1β, IL-6, and MIP-1α . Both IL-1β and IL-6 trigger the differentiation of naïve T cells into Th17 cells. These cells secrete IL-17, as a key cytokine driving inflammation in PsA that functions in concert with other cytokines to increase synovial inflammation . Furthermore, TNF-α and IL-6 activate Th22 cells, which release IL-22. This cytokine activates fibroblast-like synoviocytes through the PI3K-mTOR pathway, thereby inducing osteoclastogenesis [28, 29]. On a related note, Taniguchi et al.  demonstrated that in bone marrow stromal cells, GGT stimulates the expression of receptor activator of NF-κB ligand (RANKL), which is a major player in the differentiation of osteoblasts into osteoclasts . In fact, RANKL expression in the synovial membrane was significantly increased in patients with PsA . As an inhibitor of cytoplasmic protein kinase, IBIL could prevent the translocation of NF-κB to the nucleus by blocking the phosphorylation of IκB kinase, which is a master regulator of NF-κB signaling [33, 34]. Besides, NF-κB is also considered to be an important mediator of the pathogenesis of psoriasis . When pro-inflammatory signals downstream of NF-κB are intercepted, the production of inflammatory cytokines such as TNF-α and IL-6 are impaired , while IL-6 in turn is the strongest stimulator of CRP formation . The above findings may explain the negative correlation between IBIL and CRP levels identified in our study. Furthermore, our finding that GGT/IBIL was significantly positively correlated with CRP levels suggests that the ratio of GGT to IBIL better reflects the overall inflammatory status of PsA and indicates that the imbalance in oxidation/antioxidation ultimately leads to inflammatory reactions in vivo.
It is important to note that our study is not without limitations. As this was a retrospective study, the prognostic effect of serum GGT and IBIL on PsA could not be evaluated due to the lack of follow-up. In addition, relatively few patients were included in this study, and additional larger-scale studies are required to confirm these findings.
In conclusion, GGT and IBIL may participate in the pathogenesis of PsA. Additionally, GGT, IBIL and the balance of the two may reflect systemic inflammation mediated by oxidative stress events related to metabolic abnormalities to a certain extent.
Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
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This study was approved by the Ethics Committee of the Third Hospital of Hebei Medical University (Ethics number: 2022-023-1) and conducted in accordance to the Declaration of Helsinki principles.
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Wang, X., Mao, Y., Ji, S. et al. Gamma-glutamyl transpeptidase and indirect bilirubin may participate in systemic inflammation of patients with psoriatic arthritis. Adv Rheumatol 63, 53 (2023). https://doi.org/10.1186/s42358-023-00334-y