Tumor cells can spread throughout the body and cause metastases – tumors in organs remote from the primary focus. Metastases are the leading cause of death from cancer.
What contributes to the metastases formation
To gain a foothold in healthy organs, tumor cells trigger the formation of specialized sites in them – pre-metastatic niches (PMN). PMNs are formed before the arrival of tumor cells and are characterized by vascular permeability, fibronectin deposition, and involvement of bone marrow cells. PMNs have been proven to contribute to cancer metastasis as melanoma, pancreatic, and breast carcinoma.
The tumor forms PMN by secreting vesicles – extracellular vesicles (EV). Cells release EV due to the outgrowth of the plasma membrane (microvesicles) or the secretion of small vesicles of endocytic origin (exosomes). EVs act as intracellular messengers that carry bioactive molecules: nucleic acids, proteins, and lipids. EVs change the functions of the cells that absorb them. For example, the absorption of tumor EV by bone marrow cells contributes to the formation of pre-metastatic niches. However, the mechanisms regulating the uptake of tumor EV (TEV) remain poorly defined.
Tumor extracellular vesicles suppress the expression of the interferon receptor
To investigate the mechanism of TEV uptake, the scientists tested how TEV affects the expression of cell surface receptors in a human melanoma cell line model. Scientists found that TEV suppresses the IFNAR1 subunit of the interferon type I receptor (IFN):
- EV from the plasma of patients with melanoma (MPEV) reduced the expression of IFNAR1 in monocytes in vitro.
- Peripheral blood cells of patients with melanoma reduced IFNAR1 levels.
What do tumor vesicles and viruses have in common
Transmission of type I IFN signals is the first line of antiviral protection. In a viral infection, cells secrete IFN I, triggering the expression of IFN-stimulated genes (ISG), and ISG prevents viral infection.
To undermine antiviral protection, viruses can suppress IFNAR1. In this, the mechanisms of viruses and TEV are similar. IFNAR1 suppression is essential for TEV uptake:
- TEV-induced degradation of IFNAR1 disrupts ISG expression.
- Cells can express the degradation-resistant mutant SA IFNAR1. Despite the impact of TEV, it provides stable transmission of IFN signals, reducing further absorption of TEV.
Thus, as during antiviral responses, cells activate IFN I pathways after exposure to TEV. TEV weakens this protection by suppressing IFNAR1 and ensuring further absorption of TEV. But the constant activation of IFN I stops the absorption of TEV.
The scientists demonstrated that normal EV fibroblasts (FEV) triggered a powerful type I IFN signaling response, which correlated with their inability to suppress IFNAR1 and be continuously absorbed. These data show that cells use a mechanism to prevent unwanted absorption of EV, and TEV carries a unique load that allows them to suppress this protective mechanism.
Suppression of the interferon response causes metastases
Using mouse melanoma B16F10, the scientists showed that inactivation of IFNAR1 is necessary for the formation of PMN in the lungs and metastasis. Compared to wild-type mice, mice expressing SA IFNAR1 did not form PMNs under the influence of TEV B16F10. In addition, these mice were less likely to form spontaneous metastases. It suggests that TEV-dependent suppression of IFN is necessary for the formation of metastases. These results complement the study of Australian scientists who showed that interferon has antimetastatic activity, suppresses TEV absorption, and suggested using IFN to treat metastases.
The CH25H enzyme prevents the absorption of tumor vesicles
Interferon-stimulated genes encode many factors. One of them is cholesterol-25-hydroxylase – CH25H), an enzyme that produces 25-hydoroxysterol (25HC). The study showed that the expression of CH25H blocks the fusion of viruses with cell membranes.
Scientists were interested in whether this ISG can also block the absorption of EV. Since TEV inhibits the transmission of IFN signals, TEV also suppressed the expression of CH25H. However, the treatment of 25HC cells reduced the absorption of TEV. Consequently, as during the antiviral response, ISG CH25H stops EV capture, and TEV contribute to its own capture by suppressing IFN I-dependent synthesis of 25HC:
- CH25H inhibited the formation of pre-metastatic niches under the influence of TEV in vivo and suppressed metastasis in mice with B16F10 tumor.
- MPEV reduced the expression of CH25H in vitro. The expression of CH25H was lowest in the white blood cells of metastatic patients with melanoma than non-metastatic patients with melanoma or white blood cells of healthy donors.
These results show how TEV interferes with the body’s defense, contributing to their absorption and the formation of PMN. The use of pathways that prevent TEV capture may be a strategy for eliminating PMN and metastases.
Reserpine eliminates metastases
The scientists had a task: to find small molecules that could mimic the action of 25HC. Scientists have found that the antihypertensive drug reserpine reduces the absorption of TEV and, as a result, the formation of PMN in vivo. Mice with B16F10 tumors were injected with reserpine before and after surgical removal of tumors. Reserpine eliminated lung metastases and improved the overall survival of mice. Therefore, reserpine, already approved for clinical use, can also be used to treat metastatic melanoma.
Transmission of interferon signals in tumors can be harmful
The study showed that in breast cancer, stromal EV triggers an ISG response in tumor cells. ISG expression supported tumor progression and metastasis. Taken together with current research, these results highlight that EV evokes different responses related to IFN signaling. Since TEV can bypass the body’s immune defense, the results of exposure to the IFN signaling pathway may be different in terms of tumorigenesis and metastasis. Therefore, when treating cancer by exposure to IFN, it is necessary to consider the context.
Conclusions
Reserpine eliminates metastases in vivo when administered before/after the removal of tumors. That proves the effectiveness of the strategy of influencing pre-metastatic niches. But since patients may suffer from low blood pressure during chemotherapy, caution should be exercised when choosing adjuvant treatment with reserpine.
Source
Tumor Extracellular Vesicles Impede Interferon Alert Responses
