Cancerous cells forming a lump in the pancreatic tissue. [Image credit: Wikimedia Commons]
In summary, this study systematically elucidates the molecular mechanism of co-adaptation between tumor cells and liver cells in the process of pancreatic cancer liver metastasis by explaining the important role and regulatory mechanism of the SLIT2-ROBO1 axis in the formation of PMNs and MMNs, which offers direction to new therapeutic targets and strategies.
Pancreatic cancer is one of the deadliest malignant tumors. The main treatment method is still radical resection combined with chemotherapy. However, nearly 80% of patients cannot be surgically removed due to complicated distant metastasis, especially liver metastasis . Therefore, revealing the molecular mechanism of pancreatic cancer liver metastasis is of great significance for improving the prognosis and prolonging survival of pancreatic cancer.
In the process of tumor metastasis, there is a co-adaptation phenomenon between the disseminated tumor cells (DTC) derived from the primary tumor and the microenvironment of the target organ, that is, the mutual adaptation of “seed and soil”[2,3]. Therefore, deciphering the key events of mutual adaptation between pancreatic cancer disseminated cells and normal cells in the liver microenvironment is crucial to elucidate the molecular mechanism of pancreatic cancer liver metastasis.
The formation of pre-metastatic niche (PMN) and post-metastatic niche (MMN) is a key event in tumor metastasis . Studies have shown that PMN formation is the first step in tumor metastasis, and PMN formation can induce normal cells in target organs to recruit cells needed to form a complex tumor microenvironment (TME) to better form “soil” [5,6 ]. Therefore, analyzing the mechanism of PMN and MMN formation has important guiding significance for solving the liver metastasis of pancreatic cancer.
Recently, a research team led by Zhang Zhigang, Li Jun and Sun Yongwei of Shanghai Jiaotong University published an important research result in the journal Nature Communications. This study found that the interaction between the dependence receptor ROBO1 expressed on the tumor surface and the ligand SLIT2 derived from liver cells promoted the mutual adaptation between DTC and liver cells, which led to liver metastasis of pancreatic cancer; and the targeting of ROBO1 And antibodies can block this co-adaptation effect and thus inhibit liver metastasis of pancreatic cancer .
In conclusion, this study deciphered the mechanism of mutual adaptation between tumor cells and liver cells in the process of pancreatic cancer liver metastasis and provided a new therapeutic strategy for pancreatic cancer liver metastasis.
In order to simulate the formation of PMN and MMN in pancreatic cancer liver metastasis, Zhang Zhigang’s team injected KPC1199 and Panc02 cell lines into the spleen to construct normal liver, PMN (7-11 days after KPC1199 injection, 5-8 days after Panc02 injection) and MMN Models (5-8 days after KPC1199 injection, 9 days after Panc02 injection), and use small animal CT, bioluminescence and tissue staining techniques to prove the reliability of the model.
To reveal the mechanism of co-adaptation during pancreatic cancer liver metastasis, they used transcriptomics to detect differentially expressed genes in normal liver, PMN and MMN liver cells. The results showed that compared with normal liver, hepatic cells in PMN and MMN stage had more obvious changes in axon guidance pathway, and three genes, SEMA3e, SLIT2 and EFNA4, were found to be significantly upregulated in PMM and MMN.
Further research results showed that SLIT2 began to appear during the formation of PMNs and continued to express until the completion of the MMN stage, and staining of samples from patients with liver metastases from pancreatic cancer revealed that SLIT2 originated from liver cells rather than tumor cells.
That is, SLIT2 is a key ligand in the “soil”. But to mediate the “soil-and-seed” co-adaptation effect, ligands in tumor cells are also required.
Members of the ring-boot family (ROBO) are the main receptors for SLIT2 . Here, Zhang Zhigang’s team found that only ROBO1 was expressed in metastatic pancreatic cancer cells, and there was a strong correlation between ROBO1 and SLIT2 in liver metastases, and there was a direct interaction.
In order to clarify the role of SILT2 and ROBO1 in pancreatic cancer liver metastasis, they constructed SLIT2 knockout mice, restored the expression of SLIT2 in liver cells by lentivirus injection, and then injected tumor cells expressing ROBO1 into the spleen.
The results showed that loss of SLIT2 significantly inhibited the formation of PMNs and MMNs in pancreatic cancer liver metastases, and this effect was reversed after the restoration of SLIT2.
They also found that the administration of ROBO1-neutralizing antibodies could significantly reduce the formation of PMM and MMN in mice with liver metastases from pancreatic cancer. In addition, ROBO1-expressing tumor cells and SLIT2-expressing hepatocytes can undergo cooperative adaptation, and the absence of SLIT2 in the tumor microenvironment or the blockade of ROBO1 can weaken the competitive advantage of ROBO1-positive tumor cells in the liver microenvironment.
Not only that, Zhang Zhigang’s team also constructed a wild-type and Ig domain deletion (ΔROBO1, deleting the fragment that interacts between ROBO1 and SLIT2) ROBO1 overexpressing tumor cell lines, and injected them into the spleen of mice.
The results showed that the overexpression of wild-type ROBO1 could significantly up-regulate the formation of PMM and MMN, but this phenomenon did not occur when overexpression of ΔROBO1. This shows that the effect of ROBO1 on pancreatic cancer liver metastasis depends on the interaction with SLIT2.
Since the mitogen-activated protein kinase (MAPK) pathway plays an important role in the process of tumor proliferation and metastasis, Zhang Zhigang’s team hypothesized that ROBO1 can promote liver metastasis of pancreatic cancer by activating the MAPK pathway.
And it did. They found that the binding of SLIT2 to ROBO1 significantly increased the phosphorylation of p38α MAPK; while the level of p38α MAPK phosphorylation was significantly decreased when using ROBO1 neutralizing antibody. Specifically, ROBO1 directly binds to p38α MAPK and MEK3/6, while the binding of SLIT2 further enhances the phosphorylation level of p38α MAPK.
In short, from a mechanistic point of view, the complex of SLIT2 and ROBO1 can increase the interaction between p38α MAPK and its kinase MEK3/6, thereby activating the downstream MAPK pathway and promoting liver metastasis of pancreatic cancer.
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