| Author: | Hu, Bing |
| Title: | Mechanical regulation of breast tumor cell death in blood circulation and brain metastasis under natural killer cell surveillance |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Biomedical Engineering |
| Pages: | i, 165 pages : color illustrations |
| Language: | English |
| Abstract: | Metastasis is the primary cause of death among cancer patients. Malignant tumor cells preferentially metastasize to specific organs, termed organotropism. Compared with other organs, brain metastasis shows higher lethality with limited clinical strategies for treatment. The molecular mechanisms underlying brain metastasis, however, remain largely elusive. Tumor metastasis is tightly regulated by both intrinsic and extrinsic factors, such as the immune system, the surrounding mechanical microenvironment, and the mechanics of tumor cells. In particular, at the primary tumor site, infiltrated immune cells, like natural killer (NK) cells, selectively kill some tumor cells but spare others. Whether the mechanical properties of tumor cells play any role in this immunosurveillance process is yet to be addressed. Whether the remaining immune-resistant tumor cells contribute to organotropism remains unknown. Upon entry into blood circulation, circulating tumor cells (CTCs) experience fluid shear stress (FSS) and robust immune surveillance from NK cells. However, whether NK cells and FSS affect CTCs’ viability synergistically or independently during hematogenous dissemination is still poorly understood. To solve these fundamental questions, we first investigated the co-localization between different types of infiltrated immune cells and the organotropism pattern by analyzing spatial transcriptomics data from patients, followed by a series of in vitro and in vivo assays. Our results showed that NK cells, rather than other immune cells, correlated with brain metastasis in breast cancer. Importantly, co-culture with NK cells selected a subpopulation of breast cancer cells with enhanced brain metastatic ability. Mechanistically, NK cells secreted and injected granzyme B into tumor cells that directly cleave ezrin, leading to the reduction of cell membrane tension. The low membrane tension activated PI3K-Akt signaling in NK-selected cells to upregulate GLUT3 via CREB signaling, which potentiated glucose uptake and metabolism to promote brain colonization. The intervention of ezrin-PI3K/Akt-GLUT3 signaling inhibited brain metastatic potential of NK-selected cells. Single-cell transcriptomics analysis of patient-derived tumor biopsies revealed a clinical correlation between ezrin-PI3K/Akt-GLUT3 signaling and brain metastasis-associated genes. Further, lower membrane tension rendered tumor cells more resistant to NK-mediated cytotoxicity. These findings demonstrate that except the well-known cytotoxic effect, NK cells can promote breast cancer brain metastasis via the reduction of cell membrane tension, providing compelling evidence to unveil the pro-organotropism effect of NK cells. To investigate the effect of FSS on NK-mediated cytotoxicity against CTCs, we applied physiologically relevant magnitudes of FSS on CTCs with or without NK cells. FSS enhanced NK cell cytotoxicity by delivering more granzyme B into CTCs with negligible changes in the secretion of cytokines or the expression of Trail. Knocking down a mechano-sensitive receptor, NKG2D, erased the FSS-enhanced cytotoxicity. NKG2D mechano-sensing was further validated by FSS-dependent CD107a expression induced by direct NKG2D-ligand interaction. These results indicate that the cytotoxicity of NK cells is augmented by FSS for the effective elimination of metastatic CTCs during hematogenous dissemination. To summarize, these studies provide evidence to unveil that 1) NK cells' roles in cancer are not only anti-metastatic (killing malignant tumor cells) but also pro-metastatic (promoting brain metastasis); 2) tumor cell membrane tension is a mechanical checkpoint for NK cell surveillance and dictates both immune evasion and brain metastatic potential. FSS strengthens NK cell cytotoxicity through NKG2D-mediated mechano-sensing. Thus, targeting the membrane tension of breast tumor cells and the NKG2D-ligand axis could be new mechanotargeting strategies for the treatment of metastasis. |
| Rights: | All rights reserved |
| Access: | open access |
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