In this study, we utilized genetic variants to infer causal relationships, revealing potential causal associations among 11 CTSs, plasma proteins, and MLC through reciprocal MR analysis. Based on the European population, we draw the following conclusions: (1) CTSF, D, and V may reduce the risk of MLC; (2)42 plasma proteins may have a causal relationship with MLC; (3)13 plasma proteins potentially mediating the relationship between the 3 CTSs and MLC.
The mechanisms underlying the development and progression of MLC have garnered significant interest. The classic "seed and soil" hypothesis posits that the interplay of complex biological systems facilitates the metastatic process to the liver(49). Indeed, MLC is contingent upon microenvironment changes in the the primary tumor, metastasis pathways and the liver, involving diverse processes such as immunity and metabolism(50). Various proteins, including CTSs and plasma proteins, have been confirmed to play crucial role in these processes.
CTS, a key hydrolase in lysosomes, is the principal effector of protein catabolism and autophagy(51), and it plays a vital role in numerous physiological processes in the human body(5). The relationship between CTSs and tumors has attracted considerable attention. Studies have demonstrated that CTSs alters the tumor microenvironment through the transport and degradation of extracellular matrix (ECM)(52), as well as the activation, processing or degradation of various chemokines, growth factors and cytokines. CTSs can also release cell adhesion molecules involved in tissue invasion and metastasis(53), highlighting the close connection between CTSs and tumors and their significant potential in the field of oncology. On one hand, CTSs have been considered predictive and prognostic indicators for various tumors, with their levels changing in glioblastoma (54), breast cancer(55) lung cancer(56), colorectal cancer(57), liver cancer(58) and other malignancies. The increase of CTSs in most tumors indicates a state of progression, poor prognosis and low survival probability. On the other hand, the role of CTSs as tumor therapeutic targets has been continuously explored. For instance, ASPER-29, a novel inhibitor of CTSL and CTSS, has shown a marked ability to inhibit pancreatic cancer cell metastasis(14). Additionally, the pan-CTS inhibitor JPM-OEt significantly reduces tumor burden, angiogenesis, and invasion in RIP1-Tag2 mice(59), and its derivative E-64 has also been confirmed to prevent liver colonization of lung cancer cells in experiments(60). Recent studies have even suggested that CTS may affect the efficacy of tumor treatment(61, 62).
The possible inhibitory effect of three CTSs on MLC found in this study aligns with the conclusions of some previous studies. In brain tumors, CTSF levels were lower in ependymomas, glioblastomas and medulloblastomas than in normal brain(63). Ji C et al. (64)found that down-regulation of CTSF expression can effectively inhibit GC cell apoptosis and promote its proliferation, suggesting that the CTSF gene plays a tumor suppressor role in GC and may become a target for GC treatment. Zheng L et al. also discovered that LINC00982 promotes the expression of CTSF, thereby inhibiting gastric cancer progression(12).Further more, knockdown of CTSF in the absence of PUMA and p21 can induce leukemia development(65), and another study suggested that CTSF may play an anti-tumor role by regulating the immune response in NSCLC(66). Regarding the effect of CTSD on tumors, it is believed that CTSD can induce or inhibit cell apoptosis according to microenvironmental changes(11). Previous studies have shown that CTSD can promote tumor proliferation, invasion and prevent apoptosis(67, 68), as well as inhibit tumor growth(11, 69, 70). CTSV, a relatively newly identified CTS, has been suggested to play a pro-tumor role(71), but it has also been reported to be down-regulated in highly metastatic LM5 and LM7 cells(9), and is associated with good prognosis in thymoma(72, 73) and estrogen ER-negative breast cancer(74). Conversely, CTSB, CTSL, CTSS, CTSZ, etc, which have been more extensively studied, have been indicated to promote tumor progression in previous researches(5, 9, 54–57, 75), while IVW in this study suggested no causal relationship between them and MLC. Only MR-Egger results suggested that increased CTSB levels promotes MLC progress and BW suggested a negative relationship between CTSZ and MLC.
Similarly, plasma exert significant influence on tumor growth, migration, invasion and the construction of microenvironment. Numerous prior investigations have endeavored to harness plasma proteins as pivotal biomarkers and therapeutic targets for cancer, achieving measurable success and practical clinical application(15, 34). In an effort to delve into the interplay between plasma proteins and metastatic liver cancer (MLC), and to uncover potential mediators, this study employed a series of Mendelian randomization (MR) analyses. The findings revealed 43 plasma proteins with a causal link to MLC, among which 13 were identified as intermediary proteins, including Endostatin. The relationship between the majority of these proteins and MLC aligns with prior research: Endostatin(24–26), Ficolin-3(27–29), sRAGE(30), Prekallikrein(31), GIB(32) and ARGI1(33) were negatively correlated with MLC, implying that elevated levels of these proteins may confer a reduced risk of MLC. Conversely, increased levels of CNTN2(16), RBP(17) and Tropomyosin 2(18) may heighten MLC risk. However, divergences from previous findings was observed with CPNE1(19, 20), MAPK14(21) and CDK8NAcyclin C(23), that are, this study suggested an inverse causal relationship between the three proteins and MLC, which contrary to their previously proposed roles in promoting tumorigenesis. In conclusion, plasma proteins may play different roles in different tumor microenvironments and have different or even opposite effects on tumors.
While this study demonstrates some important findings, there are several limitations. Firstly and foremost, data constraints hindered access to a comprehensive pool of MLC-related GWAS data from diverse sources, precluding a more nuanced and detailed investigation. Secondly, the number of SNPs limited the application of a comprehensive suite of MR methods for sensitivity analyses, necessitating further validation of the robustness of certain results. Nonetheless, by combining multiple sensitivity analyses, our study provides strong evidence for most of the positive results. Additionally, the study's focus on the European population means that the findings may not be generalizable to other ethnicities, while it do hold referential value for analogous research in diverse populations. Lastly, the discordance between the effects of certain proteins observed in this study and previous research warrants further investigation to uncover potential unknown mechanisms.
In summary, this study identified increased levels of CTS variants CTSF, CTSD, and CSTV as potentially reducing the risk of MLC. It also identified plasma proteins that may influence MLC risk, explored mediating factors, and established a causal network encompassing three CTS variants, 14 plasma proteins, and MLC. These proteins could offer valuable insights into the mechanisms underlying MLC, serve as markers for its occurrence, progression, and prognosis, and guide early diagnosis and evaluation of MLC outcomes. They may also represent potential therapeutic targets for MLC.