Our study deployed several MR methods to determine the causal link between inflammatory CKs and CC. To our knowledge, we are the first to demonstrate a causal relation between 132 inflammatory CKs and CC, revealing that low IFN-γ levels and high CysD, IL-8, LIF, and MCP-2 levels are associated with a higher CC risk.
IFN-γ, a CK of the type II interferon family, has the functions of activating macrophages, anti-tumor, anti-virus, and immune regulation[5, 34]. Nonetheless, epidemiological evidence is limited for the relation between IFN-γ and CC. For example, in an observational study that included 30 patients with CC and 30 controls, patients with CC had higher IFN-γ levels, unlike the control group (P<0.05)[35]. In addition, patients with CC experienced significantly decreased plasma IFN-γ levels after radical hysterectomy, which may be related to the 5-year recurrence rate and/or CC metastasis[36]. However, our MR analysis found that IFN-γ possesses a probable protective effect on CC risk, suggesting that the results of observational studies may necessitate additional investigation. Some studies have found that IFN-γ can overexpress indoleamine-2,3-dioxygenase-1 (IDO1) and induce its activity in CC cells, resulting in tryptophan conversion to canine uric acid, thus promoting autophagy induction in CC cells, further inducing macrophage activation and phagocytosis, and finally limiting tumor growth[37]. Although our MR analysis showed a causal correlation between heightened IFN-γ levels and decreased CC risk, reverse MR analysis demonstrated no causal connection between CC and IFN-γ. In view of these contradictory studies, further research is required, especially in the direction of potential mechanisms, which is essential to clarify the potential role of IFN-γ in CC development.
CysD, a member of the cathepsin inhibitor cysteine aminotransferase superfamily, is a small protein composed of 112 amino acids encoded by CST5[38]. CysD can inhibit cathepsin H, S, and L, thus playing the role of immunosuppression and anti-tumor[39]. So far, there are no studies on CysD in CC, but some studies have found that CYSD is an anti-tumor gene for hepatitis B virus-associated hepatocellular carcinoma (HCC) as well as colon, gastric, and prostate cancers[40–42]. The CST5 expression level in HCC tissues of patients with hepatitis B virus-associated HCC has been indicated to be much higher in comparison to normal liver tissues. Moreover, CST5 has been elucidated to be the downstream target of p53[40], and p53 is an important tumor suppressor gene in CC[43], so we can boldly speculate that CST5 may possess a certain role in CC occurrence and development. Our MR analysis investigated the causal connection between CysD and CC, and the results were consistent with our guess that elevated CysD levels were positively correlated with CC risk. Therefore, we can conclude that CysD may contribute to CC occurrence and development, but we still need to do a lot of clinical and basic research to verify this. Finally, our reverse MR analysis revealed the absence of a direct causal relation between CC and CysD.
IL-8, known as CXC chemokine ligand 8 (CXCL8), is a CK of the CXC chemokine family[44, 45]. One case-control report included 227CC patients and 419 healthy female controls. The serum IL-18 level in CC patients was significantly higher than that in healthy controls (110.5 ± 252.3 vs. 90.3 ± 215.4%, P < 0.05)[46]. However, the results of another case-control report manifested that the serum LI-8 levels in CC patients were lower than those in control groups (292.79 ± 6.24 vs. 340.52 ± 11.91%, P < 0.05)[11]. Although our MR analysis showcased a direct causal connection between heightened IL-8 and increased CC risk, the link between IL-8 and CC pathogenesis remains unclearly studied, but it has been reported that IL-8 can down-regulate protein Numb and up-regulate IL-8 receptor (IL-8R) A/B and signal-regulated protein kinase (ERKs)[47]. Thus, participate in CC occurrence and development. In view of the inconsistent results, additional studies should determine the precise mechanism of IL-8 in CC occurrence and development.
LIF is a multifunctional CK of the IL-6 superfamily, which consists of 180 amino acids[48, 49]. The LIF level in CC has been reported to be significantly overexpressed than that in normal cervical tissue, and high expression is related to the low survival rate of patients with CC[50]. However, up to now, no study has been found between serum LIF and CC, although serum LIF has been revealed to be significantly increased in patients with pancreatic, rectal, and prostate cancers, among others[51]. Our MR analysis found a causal correlation between LIF and CC, but there is a lack of research to further verify it. Some studies have found that LIF treatment of HPV-16 transformed CC cells can inhibit the virus length control region (LCR), resulting in a significant decrease in E6/E7mRNA abundance[52]. It has also been reported that LIF is the direct transcriptional target of tumor suppressor gene p53, and the combination of p53 and LIF can improve the expression and function of LIF in mouse uterus[53]. To sum up, LIF is involved in CC occurrence and development, but we need further clinical research and basic research to clarify the probable mechanism behind LIF in CC.
MCP-2, also known as C-C motif chemokine 8 (CCL8), is a CK of the CC chemokine family, which has the function of recruiting macrophages into the hypoxic tumor microenvironment[54]. The expression of MCP-2 is significantly increased in breast cancer, colorectal cancer, glioblastoma, lung cancer, and other patients[55]. However, there is no research on serum MCP-2 and CC. In a clinical control trial including 9 cases of p53 wild-type serous ovarian cancer and 62 cases of p53 mutant serous ovarian cancer, the expression level of MCP-2 in p53 mutant ovarian cancer was significantly increased[56], while the p53 mutant was closely related to CC[57]. In addition, our MR analysis elucidated a positive relation between the increase of MCP-2 and the increased risk of CC. We can speculate that MCP-2 is crucial in CC occurrence and development. Hypoxia can increase ZEB1 expression in CC cells, directly promote MCP-2 production, and attract macrophages via the CCR2-NF-κB pathway, leading to CC progression[58]. Therefore, we need to further verify the causal link between MCP-2 and CC.
Until now, although many studies have manifested the vital role of inflammatory CKs in cancer occurrence and development, no MR analysis has verified the causal link between inflammatory CKs and CC. To our knowledge, this systematic and comprehensive MR study is the first on the causal connection between inflammatory CKs and CC. We have found a potential causal association between IFN-γ, CysD, IL-8, LIF, MCP-2, and CC. Finally, a reverse MR analysis was also performed to further verify the causal relation between CC and them.
Although MR analysis is a data analysis method based on large-scale experimental samples, it also has some constraints. (1) While we conducted Mendelian Randomization (MR) analysis using GWAS data from European populations, the applicability of these findings to other populations requires further validation. We adopted a relaxed significance threshold of p < 5 × 10–6 for selecting SNPs, which might incorporate false positive variations, potentially leading to errors in our analysis. (3) The data of CC are unique to women, while inflammatory CKs come from men and women; due to the lack of specific information, we are unable to carry out subgroup analysis, which may lead to biased results. (4) After Bonferron correction, we revealed that inflammatory CKs and CC risk had no statistically significant correlation, so we need more comprehensive data for further research.