Based on our literature search performed in PubMed, the first reports on brain metastases of lung cancer date back to 1981. Since 2014, 20–30 studies have been published every year. It is important to conduct in-depth research in this largely unexplored field. Yousefi et al. [9] claimed that targeted therapy was most necessary, unfortunately, however, the lack of understanding of the molecular mechanisms underlying brain metastases has hampered the development of this area of research. It is well recognized that lung cancer preferentially metastasizes to the brain, where it interacts with other molecules present within the tissue microenvironment to form a complex biological network that supports the survival and proliferation of cancer cells [10]. CXCL12 and its receptor CXCR4 have long been the target of research of brain metastases of lung cancer. Paratore et al. [11] reported that immune response of cavallo, CXCR4 and CXCL12 can help distinguish primary non-small cell lung cancer with or without brain metastases. Along the same lines, Chen et al.[12] suggested that the high level of CXCR4 expression was related to brain metastases of non-small cell lung cancer. However, the role of TTF1, which was previously considered to be a specific marker of brain metastases of lung cancer, remains controversial. Unal et al.[13] claimed that although TTF1 was highly expressed in of brain metastases of lung cancer, it is also expressed in primary gliomas. Therefore, its specificity brain metastases of lung cancer are questionable. In our study, except for Case No. 5, TTF1 was highly expressed in both the primary tumor and metastases.
So far, EGFR-related research [7] has mainly discussed the issue of tumor immunity, and has not yet explored the relationship between cancer and HCMV infection. In this study, we aimed to further our understanding of the links between brain metastases and HCMV infection. Although preliminary, our results suggest a potentially important effect resulting from the combination of IE and GBP4 protein expressions.
To the best of our knowledge, no relevant reports on the role of GBP4 in brain metastases of lung cancer have been reported. However, it is known that GBP4 is a GTPase protein induced by IFN-g and is essential for the activation of inflammasomes. It is also known that CXCL8 and LTB4 of the inflammasome play a role in the recruitment of neutrophils to the site of infection, where pathogens are then eliminated by prostaglandin D2 [14]. The expression of tumor suppressing genes is affected in brain metastases, nevertheless, in seven samples of the five cases of brain metastases of lung cancer reported in this study we found high expression of the GBP4 protein. How to explain these findings is an important endeavor. Xu et al. [15] reported that the high expression of GBP3 protein observed in glioma is involved in the regulation of the malignant progression of glioma cells by activating p62. For this reason, it is worth speculating whether, in the cases we have analyzed, brain metastases were facilitated by the action of GBP4 through CXCR4.
With regards to the IE protein, it expression is commonly found in cancer patients infected with HCMV. Halwachs et al. [16] reported that gene expression [author: gene expression of what? Please clarify which genes/pathways] after HCMV infection occurs in a transient cascade, and includes three phases: major immediate-early (MIE), early (E) and late (L) phases. Briefly, after the virus envelope fuses with the plasma membrane of the host cell, the enveloped virus particles are released into the cytoplasm, enter the nucleus through the nuclear pore within a few minutes and release viral DNA triggering the different phases of gene expression. Dooley et al. [17] reported that after HCMV infection, through the combined action of chromatin remodeling and transcription factors, the genes expressed in the MIE phase play an important regulatory role in the balance between latent infection and lytic infection. This molecular switch involves the action of a large number of host and viral proteins. In addition to the host-encoded MIE site regulatory factors, HCMV also encodes proteins such as UL138 [18] and US28 [19] during the incubation period, in order to inhibit immune responses that results in MIE transcription. Currently, only a number of the molecules involved in these processes are known, and our knowledge of this topic is still very limited. The results of this study show that, overall, the levels of IE expression are low in all tested samples, although there were also a few cells where the immune complexes were dark brown and granular, which is in line with the high expression of viral DNA characteristics, as shown in Fig. 6. Hence, IE positive expression in these few cells may be caused by HCMV infection in the MIE stage. The fact that the cases we analyzed are in the late stage of brain metastases of lung cancer, and therefore most of the cells infected by HCMV should be in the late stage of E and L, may be the explanation why only a few positive IE cells were observed. As mentioned above, GBP4 protein may be a member of a large number of host and viral proteins involved in these mechanisms. In the E and L phases after HCMV infection, a large amount of HCMV is required to participate in DNA replication. Therefore, among the five proteins, the expression of GBP4 is the strongest and the most extensive.
Targeted therapy of brain cancer of lung metastases, including targeted therapy that inhibits EGFR, can increase the median survival time to 12 months [20]. Using TKIs (EGFR-specific tyrosine kinase inhibitors) to treat brain cancer of lung metastases with EGFR mutations can prolong the median survival time to 15–20 months, with a remission rate of 60%-100% [21]. Inhibitors include Erlotinib[22], Erbitux [23], Gefitinib [24] and mouse monoclonal (mab) 225 and 528 [25]. Unfortunately, drug resistance cases following the above-mentioned target molecule treatment is a common occurrence. As suggested in Table 1, Fig. 4 and Fig. 5, if patients with brain cancer of lung metastases are infected by HCMV, GBP4 inhibitors may be more effective given that GBP4 protein expression appear broader than EGFR expression.
Research of the mechanisms underlying brain cancer of lung metastases is a hot topic of research that remains largely unexplored. This study contributes to the existence knowledge in this area. The limitation of this study is that the number of cases is too small and the detection indicators are single. Our findings should be confirmed in larger studies.