Colorectal cancer (CRC) was one of the most common malignancies of the gastrointestinal tract. CRC ranked the third in incidence and was the second leading cause of cancer-related deaths worldwide (1). CRC was also one of the most prevalent cancers in China, where the mortality rate of CRC was about 13.13/100, 000, accounting for 7.8% of the total number of deaths among patients with malignant tumors (2). Recurrence and distant metastasis were the two main factors affecting the survival of CRC (3). The most common target organ of distant hematogenous metastases of CRC was liver (4). Colonic venous blood converged into the hepatic portal vein through the superior and inferior mesenteric veins, respectively. It was the anatomical structure and portal circulation pathway that made the liver the preferred site for distant metastases. About 15-25% of patients suffered from synchronous liver metastasis (SLM), while another 15-25% developed metachronous liver metastases postoperatively (5). Ultimately, approximately 50% of patients developed liver metastases at some point throughout the course of their disease (6). Patients with untreated liver metastases had a median survival of only 6.9 months (7). Although complete surgical resection was provided, the median survival period was less than 35 months (7). Obviously, CRC liver metastasis was a thorny problem in clinical diagnosis and treatment.

According to WHO, 1.1 million people were newly infected with chronic hepatitis B virus (HBV) in 2017 (7). As of 2016, there were 267 million chronic hepatitis B (CHB) infections worldwide and 1.4 million deaths from viral hepatitis, 96% of which were caused by hepatitis B and C viruses (7, 8). China conducted the first national hepatitis seroepidemiological survey in 1992, according to which approximately 120 million people in China carried HBsAg(+), and nearly 300, 000 died from HBV infection each year (9). CRC patients with comorbid chronic HBV infection were also more common in clinical practice. However, whether CHB promoted or suppressed synchronous liver metastasis was controversial. Some concluded that the incidence of liver metastases was reduced in CRC patients with concomitant CHB infection (10). Obviously, CHB infection had a suppressive effect on liver metastases, but the sample size included was small. Although others thought that CRC with concomitant CHB infection promoted liver metastases (11, 12), the inclusion criteria were controversial.

Thus, in our study, a retrospective analysis of a large sample was conducted to explore the effect of HBV on SLM of CRC, aiming to clarify a clear connection between HBV and SLM in CRC, to provide a basis for further clinical and basic research on CRC liver metastasis.

Recurrence and metastasis were the leading causes of death in CRC patients (27). The liver was the most common metastatic organ of CRC (28). Resection of liver metastases was the preferred method for the treatment of CRC with liver metastases (29). However, approximately 75% of patients relapsed within 2 years (30). Due to a large number of HBV infective patients and CRC patients worldwide, so what was the relationship between HBV infection and CRLM? Before discussing the relationship between HBV and CRLM, we first defined the definition of synchronous CRLM. The Expert Group on the Treatment of Liver Metastases discussed this issue and reached a consensus (31) that SLM were referred to as simultaneously discovered liver metastases detected at the time of primary CRC tumor diagnosis. Although the classification of SLM had reached an international consensus, actually the standards in researches were not uniform. Some argued that SLM were liver metastases found at the time of CRC diagnosis or within 6 months after radical resection of the primary CRC (15). Nevertheless, if liver metastases happened within 6 months after surgery, it meant that metastases had already occurred before surgery. In the early stages of metastasis, minimal residual diseases were undetectable. Because CT only could distinguish lesions larger than 0.5 cm, while B-ultrasound only larger than 1 cm (32). In addition, the reports on the incidence of synchronous and metachronous liver metastases were controversial due to the limited sample size (21, 23). Here, we selected SLM according to the international consensus (31) that liver metastases found before or at the time of diagnosis of CRC, which could allow us to judge the occurrence of SLM more accurately.

The controversy was still ongoing regarding the impact of HBV infection on the risk of CRC liver metastases. Most studies thought that HBV infection inhibited the occurrence of CRC liver metastases (26, 33); at the same time, other few studies held an opposite view (11, 17). A retrospective study by Huo et al. (12) collected 4,033 CRC patients to conclude that concomitant chronic HBV infection significantly increased the risk of CRC liver metastases with a higher hazard risk (2.317), compared with CRC patients not infected with HBV. However, the mechanism of HBV infection promoting CRC liver metastasis was unclear. Chemokines in tumor microenvironment promoted malignant tumor metastasis through multiple mechanisms (34). CRC cells recruited specific subsets of myeloid cells to facilitate cancer cell growth in the liver through the chemokine CCL2 (35). They were combined with monocyte chemoattractant protein (MCP-1) to form MCP-1/CCR2, which promoted the growth of CRC in the animal. Once HBV infection occurred, the expression level of MCP-1 was up-regulated (36). This might hint that HBV infection facilitated liver metastasis of CRC. Also, the expressions of chemokines CCL20, CXCL6, and CXCL9/10/11 increased in HBV-infected patients, which were all related to the occurrence of CRC (36–38).

We found that HBV may inhibit SLM in CRC. We enrolled 6, 020 cases, of which 802 patients developed SLM. There were 44 cases with simultaneous liver metastasis in the HBV infective group. Compared with the HBsAg (–) group, the proportion of SLM in the HBsAg(+) group was lower (9.80% vs. 13.6%). Utsunomiya et al. (24) found that liver metastases were rare in HBV or HCV-infected CRC patients. Song et al. (22) reported that HBV-infected patients had fewer CRC liver metastases and more prolonged survival than non-HBV-infective patients. Another research showed that HBV infection and liver cirrhosis could reduce the incidence of liver metastases in CRC patients, but did not affect their survival rate. Wang et al. (25) and Qiu et al. (10) also came to a similar conclusion that HBV inhibited the SLM of CRC.

The mechanism by which HBV infection inhibited CRC liver metastasis was also still unclear. Some studies held that HBV enhanced the host’s cellular and humoral immune function after HBV entered the body. HBV replication not only enhanced the killing of cytotoxic T lymphocytes (CTL) and Kupffer cells to wipe out cancer cells, but activated cytokines such as TNF-α and INF-γ to boost the antitumor effects (10, 39). HBV infection promoted the production of cytokines such as INF-γ and IL-6 by activating Kupffer cells, CTLs, and monocytes, while INF-γ inhibited the formation of neovascularization in cancer metastases. IL-6 indirectly increased liver ECM, thereby inhibiting CRC cell metastasis or making it difficult for CRC cells to transfer to the liver for growth and proliferation (40). During the progression from CHB to cirrhosis, Kupffer cell activation led to tissue damage and even liver fibrosis, and inhibited CRC liver metastasis. Other studies reported that microRNAs silenced target genes through mRNA degradation or translation inhibition to inhibit the occurrence of liver metastasis, such as miRNA-145, Let-7, etc (41, 42). Also, tumor liver metastases were intrinsic to tumor cells and influenced by the local metastatic tumor microenvironment (43). The imbalance between matrix metalloproteinases (MMPs) and their inhibitors contributed to CRC progression and invasion (44). MMP inhibitors used to treat CRC in animal models suggested that increased expression of MMPs inhibited the colonization of chronic hepatitis-infected tumor cells and hindered colon cancer liver metastasis (45). Another possible explanation we thought was that CRC secreted CEA that could specifically bind to the CEA receptor on liver Kupffer cells so that Kupffer cells produced IL-α, IL-1β, IL-6, and TNF-α, inducing liver Sinusoidal endothelial cells to express intercellular adhesion molecules. Next, metastatic cancer cells adhered to the liver sinusoidal endothelial cells, so as not to enter the liver.

Liver cirrhosis was a common clinical chronic progressive liver disease, diffuse liver damage formed by long-term or repeated action of one or more causes (46–49). In China, most of them were post-hepatitis cirrhosis, while a few were alcoholic cirrhosis and schistosomiasis (12, 50, 51). What was the relationship between post-hepatitis cirrhosis and CRC liver metastasis? Huo et al. (12) used APRI as an evaluation index for the severity of liver cirrhosis and found that in CRC patients with positive HBsAg, patients with high APRI (>0.5) had a lower probability of developing SLM than patients with low APRI (≤0.5). Liver metastases from CRC were rarely shown in patients with liver cirrhosis, a retrospective study in the United States showed (52). However, a study in Taiwan put forward the opposite view, arguing that the risk of liver metastases in CRC patients with liver cirrhosis was underestimated, presenting that the risk of liver metastases in CRC patients with liver cirrhosis was higher (53). Nevertheless, in 449 cases of HBsAg(+) CRC patients in our study, we did not find that e-antigen and liver cirrhosis indicators (APRI, FIB-4) had any effect on the occurrence of SLM. This suggested that HBV-induced liver cirrhosis did not further affect the occurrence of SLM. There might be the following reasons we thought for the above results: 1. A better indicator of HBV replication was the level of DNA replication. 2. APRI and FIB-4 could not accurately reflect the actual degree of liver cirrhosis or liver fibrosis. 3. The information on whether patients took hepatoprotective or antiviral drugs or not was missing. But interestingly, we found that in HBsAg (–) CRC patients, the incidences of SLM in the high APRI and FIB-4 groups were 24.9% and 14.9%, respectively, which were significantly higher than those in the low APRI and FIB-4 groups (12.3% and 12.5%), suggesting that non-HBV factors in liver cirrhosis promoted the occurrence of SLM from CRC. The possible underlying mechanism was the effect of mechanical factors, such as mesenteric circulation and hepatic capillaries, which promoted liver metastasis (54). Patients with liver cirrhosis had intestinal epithelial barrier dysfunction compared with healthy subjects (55–57). In addition, vascular remodeling and tortuosity led to direct shunting of portal and arterial blood supply to the hepatic outflow tract, and eventual vessel tortuosity and slow blood flow further facilitated cancer cell seeding (57, 58). The new finding opened up new ideas for us to further study the pathogenetic mechanism of synchronous liver metastasis of CRC, but it still needed prospective data verification.

This study had the following deficiencies and limitations: 1. Status of HBV carriers. It would be more convincing to clarify the role of HBV-DNA status in tumor pathogenesis. 2. HBV treatment and outcome. Antiviral therapy duration, regimen, and outcomes also affected final clinical outcomes. Besides, many retrospective studies have not been able to investigate whether the tumor occurred or HBV infection first. 3. Different definitions of liver metastases and defects in detection methods. Different definitions of liver metastases in CRC would inevitably lead to bias in the analysis of results. Also, the resolutions and models of imaging equipment in different hospitals or different periods of the same hospital were quite different, resulting in diagnostic defects and final research bias.

In conclusion, despite the controversies shown in the review of literature, the retrospective analysis of a large number of cases in our study found that in patients with primary CRC, carrying positive HBsAg might inhibit the occurrence of SLM. As for early-onset CRC patients, it seemed that HBsAg status was not associated with the occurrence of SLM. The rate of concurrent liver metastases from colon cancer was higher than that in rectal cancer. However, HBsAg status seemed unrelated to the occurrence of SLM in colon cancer. Besides, HBV-induced liver cirrhosis appeared not to further affect the occurrence of SLM while liver cirrhosis caused by non-HBV factors promoted the occurrence of SLM. Meanwhile, it seemed that HBsAg status had no effects on the incidences of lung metastasis in CRC. These findings still required prospective data validation.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by the ethics committee of Shanghai Changzheng hospital. The patients/participants provided their written informed consent to participate in this study.

LZ, PG, and YL contributed equally to this work and share first authorship. All authors contributed to the article and approved the submitted version.