The most central aspect of the biological behavior of colorectal cancer is invasion and metastasis, which is mediated by a series of ordered steps. Molecular imaging can visualize the metastatic process, and the living body shows changes in physiological functions at the cellular level. PET, as the main imaging device for molecular imaging, displays specific protein expression changes at the molecular level by means of specific targeting probes, which provides an effective method for visualization of tumor invasion and metastasis. In the early stage, our group established a correlation between GLUT-1 and HIF-1α by establishing molecular imaging features of liver metastasis nodules (18F-FMISO and 18F-FDG), in order to achieve early diagnosis of colorectal cancer liver metastasis . However, due to the initial stage of the experiment, the selected molecular imaging indicators and tumor-related biological characteristics indicators are limited, and it is impossible to detect the biological behaviors related to tumor metastasis in many aspects. Therefore, based on this study, the new tumor metastasis markers MACC1 and SPON2 were combined to detect the physiological function of liver metastasis of colorectal cancer. The results showed that 18F-FLT and 18F-FMISO uptake finally reflected the difference in expression of tumor biomarkers (MACC1, SPON2), predicted the metastatic potential of the two tumor cells, and provided early diagnosis and metastatic potential prediction for clinical colorectal cancer liver metastasis.
In recent years, many literatures have reported on the abnormally high expression of MACC1 and SPON2 in various cancers and their potential applications in the development, progression and prognosis of cancer. In hepatocellular carcinoma, MACC1 is considered to be a biomarker of survival prognosis. In lung adenocarcinoma, high levels of MACC1 expression have been shown to be associated with postoperative recurrence of tumors. SPON2 has been proposed as a diagnostic biomarker for ovarian cancer and prostate cancer [24,25]. In addition, the SPON2 gene is significantly upregulated in CRC compared to colorectal adenomas. These findings of MACC1 and SPON2 effects in different malignancies support the results of this study, suggesting that they may ubiquitously promote carcinogenesis.
First, we carried out in vitro cellular uptake experiments of LoVo and HCT8 on three tracers, which laid the foundation for the detection of different biological behaviors of PET in vitro. The results showed that LoVo had higher rates of uptake of the three tracers at 240 minutes than HCT8, with statistically significant differences (P = 0.001, 0.000, 0.020, respectively). Moreover, the uptake of 18F-FDG is always higher than the intake of the other two tracers, regardless of cell type. PET imaging results of small animals showed that the SUVmax and SUVmax ratio values of the three tracers in LoVo cells were higher than those of HCT8, which was consistent with the results of in vitro cell uptake experiments, and further confirmed the diagnostic value of different tracer PET imaging on tumor tissues/cells. Compared with the other two tracer PET images, the FDG SUVmax and FDG SUVmax ratio values of all liver metastases were not statistically different between the two cells, which was consistent with the 18F-FDG in vitro uptake experiment. We speculate that this difference may be related to the internal environment, indicating that the glucose consumption of these two liver metastases is more consistent, and 18F-FDG cannot distinguish the difference between the two cell metastases. Most cancer cells are metabolically active, with relatively little cell proliferation and hypoxia. The net absorption of FDG is higher than FLT and FMISO in tumors. Moreover, in addition to tumor cells, many inflammatory cells are usually present in malignant lesions, resulting in higher FDG uptake . The FLT SUVmax and SUVmax ratio values were both higher than FMISO, consistent with the results of the cellular uptake experiments. This suggests that FLT imaging is more sensitive to CRC liver metastasis than FMISO.
Tumor invasiveness is a key factor affecting the prognosis of colorectal cancer, including differentiation, growth rate, and metastatic potential. In this manuscript, the new biological characteristics of liver metastasis related to liver metastasis, MACC1 and SPON2, were combined to analyze the feasibility, specificity and effectiveness of multi-probe uptake on biological characteristics of metastasis from both in vivo and external aspects.
As a biological marker for the progression, metastasis and survival of many solid tumors, MACC1 has considerable basic and clinical research value [28,29]. However, it has not been reported in the pathological mechanism of liver metastasis of colorectal cancer and clinical marker research. Western blot results showed that there was a significant difference in the expression of MACC1 between LoVo and HCT8 liver metastases (P<0.0001), which was consistent with the results of immunochemical experiments. This indicates that the stronger the cell's ability to metastasize, the higher its MACC1 expression, and MACC1 can also reflect the metastatic ability of colorectal cancer cells. In this study, we found that MACC1 was significantly correlated with 18F-FLT SUVmax ratio and 18F-FMISO SUVmax ratio (r=0.737, P=0.0026; r=0.770, P=0.0013). It can be seen that MACC1, a novel tumor metastasis marker protein, can not only reflect the malignant degree of tumor, but also closely related to tumor proliferation and hypoxia metabolism. That is, the uptake characteristics of 18F-FLT and 18F-FMISO tracers can reflect the invasion and metastasis ability of colorectal cancer, and can be detected by MACC1 expression level, which has not been reported in previous studies.
SPON2 has been observed to increase SPON2 gene and protein expression in liver cancer, gastric cancer, ovarian cancer, pancreatic cancer, and prostate cancer[19, 25, 33, 34]. Studies have shown that the SPON2 gene is up-regulated in colorectal cancer compared to colorectal adenomas [26,35]. Western blot analysis showed that there was a significant difference in the expression of SPON2 between LoVo and HCT8 liver metastases (P=0.0003), which was consistent with the results of immunochemistry experiments. The stronger the cell's ability to metastasize, the higher its SPON2 expression, and SPON2 also reflects the metastatic ability of colorectal cancer cells. In this study, we found that SPON2 was significantly correlated with 18F-FLT SUVmax ratio (r=0.842, P=0.0002). It can be seen that the new tumor metastasis marker protein of SPON2 can not only reflect the malignant degree of tumor, but also closely related to tumor proliferation and metabolism. That is, the uptake characteristics of 18F-FLT can reflect the invasion and metastasis ability of colorectal cancer, and can be detected by SPON2 expression level. It can be seen that overexpression of SPON2 can enhance the proliferation of colon cancer cells. This is consistent with previous reports that SPON2 may be a biomarker for CRC diagnosis and prognosis .
This study shows that 18F-FLT and 18F-FMISO are more favorable for the detection of liver metastases from colorectal cancer than 18F-FDG. These three tracers have different metabolic pathways and reflect different physiological processes. The difference in imaging reflects differences in the levels of metabolic substrates (thymine, nitroimidazole and glucose), which may be due to microenvironment, blood. The effects of circulation, metabolism or related pathways. This study showed that the level of glucose metabolism was significantly higher than that of thymine and nitroimidazole at the cellular or tissue level, but there was no significant difference in the expression of 18F-FDG between cells with different metastatic potential and corresponding liver metastases. It is indicated that malignant tumors have high glucose metabolism, but glucose consumption is similar. 18F-FDG itself cannot distinguish the difference between different metastatic potential colorectal cancer liver metastases, which indirectly indicates that 18F-FDG is highly sensitive. Poor specificity. The 18F-FLT SUV values in both colorectal cancer liver metastasis models were higher than 18F-FMISO, which was consistent with the results of in vitro cell uptake experiments. It indicates that the proliferation and metabolism of tumors are higher than their hypoxia metabolism at the tissue or cell level, that is, the proliferation level can better reflect the invasion and metastasis ability of tumors. In addition, 18F-FMISO has lower sensitivity to 18F-FLT than liver metastases, but it has different degrees of correlation with tumor metastasis-related proteins. By comparing the characteristics of in vitro and in vivo nuclide uptake and the comprehensive analysis of MACC1 and SPON2 protein levels in colorectal cancer, we found that the higher the 18F-FLT uptake, the stronger the cell transfer ability, the higher the protein levels of MACC1 and SPON2, 18F-FLT The level of uptake was significantly correlated with the expression of MACC1 and SPON2. The higher the 18F-FMISO uptake, the stronger the cell transfer ability, and the higher the MACC1 protein level, the 18F-FMISO uptake level was significantly correlated with MACC1 expression. Individual differences between tumor phenotypes complicate the choice of tumor treatment options [37, 38]. As the results of this study show, 18F-FLT and 18F-FMISO PET can not only show the heterogeneity between tumors, but also have the ability to distinguish the biological characteristics of tumors. Therefore, the combination of 18F-FLT and 18F-FMISO PET imaging is also of great value for the choice of tumor treatment options.
This study has certain limitations. The experiment used ROI technology to semi-quantitatively analyze the lesions. Through visual analysis, the ROI region was drawn in the abnormal uptake area of the liver. Due to the small lesions of liver metastases in some nude mice, there was a certain error in the outline of the ROI region, and the liver metastasis model Less in quantity. In the future, it is necessary to expand the sample size and improve the experimental methods to provide greater clinical applicability for early detection, metastatic potential evaluation and prognosis evaluation of colorectal cancer liver metastasis.
In summary, although 18F-FDG has higher sensitivity than the other two tracers, it lacks specificity, and the inability to distinguish the diagnosis is still a necessary disadvantage. Both 18F-FLT and 18F-FMISO imaging can evaluate the invasion and metastasis ability of colorectal cancer, and can be detected by detecting the expression level of new tumor markers and multi-probe metabolic markers. It is a preferred PET tracer for the diagnosis of colorectal cancer liver metastasis.