Whole-brain radiation therapy (WBRT) has been one of the mainstays of treatment for brain metastases, especially in patients who do not have a chance to operate [1]. Unfortunately, current clinical evidence suggests that whole-brain radiotherapy does not significantly improve outcomes in patients with brain metastatic cancer [2].
Therefore, exploring the molecular mechanisms of radiation therapy resistance of brain metastases and finding molecular markers that predict radiotherapy sensitivity will help to find ways to address radiation therapy resistance and the formulation of individualized radiotherapy regimens for patients.
Recently, The team of Professor Manuel Valiente from the National Cancer Research Center in Spain discovered a new mechanism for brain metastatic cancer's resistance to whole brain radiotherapy.
They found that brain metastatic cancer cells can express and secrete the S100A9 protein at a high level, which mediates radiotherapy resistance by binding to the receptor RAGE on the surface of tumor cells to activate the intracellular NF-κB/JunB signaling pathway, which can be reversed by the AGE inhibitor FPS-ZM1.
In addition, they found that S100A9 can be used as a biomarker (either blood testing or tumor tissue testing) to predict the sensitivity of patients with brain metastases to whole-brain radiotherapy, thereby personalizing radiation therapy for patients.
The results of this study provide a theoretical basis for personalized radiation therapy for brain metastase patients, and at the same time, a radiosensitizer with potential clinical application value was found, which has very important clinical translational significance, and the relevant research results were published in the famous journal Nature Medicine [3].
Screenshot of the first page of the article
To investigate the mechanism by which brain metastatic carcinoma (BrM) is resistant to whole-brain radiotherapy, Professor Manuel Valiente's team first constructed an animal model of brain metastases of lung cancer (intracardiac inoculation of lung adenocarcinoma cell line H2030-BRM) and an animal model of breast cancer brain metastases (TNBC cell line E0771-BRM inoculation).
They found that none of the three different regimens of whole-brain radiotherapy extended mouse survival compared to the control group, and curiously, the two cell lines were sensitive to radiotherapy when cultured in vitro.
Animal models of brain metastatic cancer are resistant to whole-brain radiotherapy, but cell lines are sensitive to radiotherapy when cultured in vitro
This interesting phenomenon led the researchers to wonder if it was because tumor cells came into contact with brain tissue that they became resistant to radiation therapy.
So they placed H2030-BRM or E0771-BRM cells on thin slices of brain tissue cultured in vitro and subjected them to the same radiation treatment as before, and found that the cancer cells did indeed become radiotherapy resistant, indicating that the tumor-growing brain microenvironment had substances that altered the sensitivity of cell radiotherapy.
Previous studies have found that the interaction of brain metastatic cancer cells with glial cells plays an important role in the growth of brain metastatic cancer cells [4]. After co-culturing brain metastatic cancer cells and glial cells, the researchers found that resistance to radiation therapy only occurs when brain metastatic cancer cells come into direct contact with glial cells (separating the two types of cells from culture is ineffective), especially with astrocytes.
Brain metastatic cancer cells come into contact with astrocytes to resist radiation therapy
To study the molecular mechanisms, the researchers sequenced RNA from control cells and co-cultured brain metastatic cancer cells, and found that S100A9 was the most significant upregulation gene, and S100A9 was also highly expressed in animal models and human brain metastatic carcinoma specimens.
The high expression of S100A9 is induced by CXCL1 and TGG-α secreted by astrocytes, and the addition of the above two cytokines to cells cultured in vitro can induce high expression of S100A9 in brain metastatic cancer cells and become radiotherapy resistant.
The above study suggests that S100A9 may be involved in the mechanism of radiation therapy resistance of brain metastatic cancer cells.
To verify this, the researchers added recombinant S100A9 to a radiation-sensitive brain metastatic cancer cell line in vitro and found that tumor cells were 3 times more sensitive to radiation therapy. After silencing cell S100A9 and then cultured with thin slices of brain tissue, tumor cells remain sensitive to radiation therapy. This confirms that S100A9 is the "culprit" that leads to tumor radiotherapy resistance.
So how did S100A9 cause brain metastases that were originally sensitive to radiation therapy to become resistant to radiation?
Through transcriptomic analysis, the researchers found that in radiation-resistant brain metastases, the receptor RAGE of S100A9 was highly expressed after radiation stimulation, and the same was true in animal models and human brain metastatic carcinoma specimens, and the pathway enrichment analysis showed that the NF-κB signaling pathway downstream of AGE was significantly activated.
These data suggest that brain metastatic cancer cells can express and secrete S100A9 and bind to radiation-induced high-expression RAGE receptors, activating NF-κB-mediated radiotherapy resistance.
Brain metastatic cancer cells mediate radiotherapy resistance mechanisms through S100A9
Subsequently, the researchers studied the effect of S100A9 on radiotherapy sensitivity in vivo and constructed a lung cancer brain metastase model in which H2030-BrM cell S100A9 was silenced.
The results showed that tumor resistance to radiotherapy disappeared, and radiotherapy significantly prolonged the survival of mice compared with the control group (H2030-BrM cell wild-type lung cancer brain metastasis model). A similar phenomenon is seen in animal models of brain metastases of breast cancer.
The researchers then confirmed in vivo that S100A9 does mediate tumor radiation resistance by activating the NF-κB/JunB signaling pathway.
Since S100A9 can be expressed not only by tumor cells, S100A9 can also be secreted in neutrophils and monocytes. To verify whether radiation therapy resistance is mediated by tumor-specific secretion of S100A9, the researchers constructed a mouse model of S100A9 gene knockout, and then implanted wild-type E0771-BRM cells into the brain to construct a brain metastase model, showing that the tumor was still resistant to radiotherapy. This confirms that radiation therapy resistance is indeed mediated by S100A secreted by brain metastatic cancer cells.
S100A9 knockout mouse brain transfer model is radiation therapy resistant
Given the relevance of S100A9 to radiation sensitivity to brain metastases, the researchers studied the relationship between S100A9 and the prognosis of patients with brain metastases who received standard treatment, including radiation therapy, in a clinical population.
By analyzing the clinical specimens and follow-up data of patients with lung cancer (n=22), breast cancer (n=42), and melanoma (n=34) brain metastases, the researchers found that the expression level of S100A9 in lung brain metastatic carcinoma specimens was related to the tumor recurrence time after whole brain radiotherapy, and the expression level of S100A9 in breast cancer and melanoma brain metastatic carcinoma specimens was significantly correlated with survival rate. This strongly suggests the value of S100A9 as a predictor of radiotherapy sensitivity.
Brain tumor specimen S100A9 levels can be used as a predictor of radiotherapy sensitivity
Considering that some patients with brain metastases have lost the opportunity to operate and cannot obtain intracranial tumor specimens to evaluate the expression level of S100A9, the researchers also studied the relationship between S100A9 levels in blood and the sensitivity of patients to radiation therapy.
By analyzing blood samples and clinical data from 71 patients with brain metastatic cancer, they found that S100A9 levels in the blood could also be used to predict the prognosis of patients receiving standard treatments, including radiation therapy, although this needs to be verified by larger clinical studies.
In order for those patients with high S100A9 levels to enjoy the benefits of radiation therapy, researchers can't help but wonder if this radiation resistance can be reversed.
Very fortunately, the existing AGE inhibitor FPS-ZM1 has very good penetration of the blood-brain barrier [5]. After using FPS-ZM1 in lung cancer brain metastase mice and breast cancer brain metastase mice, the researchers found that tumors that were supposed to be resistant to radiation therapy became sensitive to treatment.
The RAGE inhibitor FPS-ZM1 reverses radiation resistance to tumors
Overall, Valiente's team found that S100A9 mediates radiation resistance to brain metastatic cancer and serves as a potential biomarker to predict patient sensitivity to radiation therapy for personalized treatment. It is understood that researchers have initiated a prospective multicenter study to assess the predictive value of S100A9 for radiotherapy sensitivity.
More importantly, this radiotherapy resistance can be reversed by AGE inhibitors, and the use of AGE inhibitors for the treatment of Alzheimer's disease has entered clinical trials [6]. In the near future, the drug may indeed be used as a radiotherapy sensitizer to improve the prognosis of patients with brain metastatic cancer.
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6.Burstein AH, Sabbagh M, Andrews R, Valcarce C, Dunn I, Altstiel L: Development of Azeliragon, an Oral Small Molecule Antagonist of the Receptor for Advanced Glycation Endproducts, for the Potential Slowing of Loss of Cognition in Mild Alzheimer's Disease. J Prev Alzheimers Dis 2018, 5(2):149-154.
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