New insights into the correlations between circulating tumor cells and target organ metastasis

Abstract

Organ-specific metastasis is the primary cause of cancer patient death. The distant metastasis of tumor cells to specific organs depends on both the intrinsic characteristics of the tumor cells and extrinsic factors in their microenvironment. During an intermediate stage of metastasis, circulating tumor cells (CTCs) are released into the bloodstream from primary and metastatic tumors. CTCs harboring aggressive or metastatic features can extravasate to remote sites for continuous colonizing growth, leading to further lesions. In the past decade, numerous studies demonstrated that CTCs exhibited huge clinical value including predicting distant metastasis, assessing prognosis and monitoring treatment response et al. Furthermore, increasingly numerous experiments are dedicated to identifying the key molecules on or inside CTCs and exploring how they mediate CTC-related organ-specific metastasis. Based on the above molecules, more and more inhibitors are being developed to target CTCs and being utilized to completely clean CTCs, which should provide promising prospects to administer advanced tumor. Recently, the application of various nanomaterials and microfluidic technologies in CTCs enrichment technology has assisted to improve our deep insights into the phenotypic characteristics and biological functions of CTCs as a potential therapy target, which may pave the way for us to make practical clinical strategies. In the present review, we mainly focus on the role of CTCs being involved in targeted organ metastasis, especially the latest molecular mechanism research and clinical intervention strategies related to CTCs.

Fig. 1

The timeline of progress in research related to the CTCs. In 1869, Thomas Ashworth first reported the tumor cells escaping from solid tumors and entering the blood. And in 1976 that Nowell formally confirmed the definition of CTCs. The first- and second-generation CTC isolation technologies are based on the physical properties and surface antigens of CTCs, respectively. A prospective study published in 2004 used Cell Search, an immunomagnetic bead method for positive binding of CTCs to the CTC surface antigen EpCAM, to isolate CTCs, which has become the first FDA-approved method for CTC isolation. With the progress of various experimental techniques, the isolation of CTCs has been developed to the third generation of chip technology. Nanomaterials have also been integrated to enhance the purity and efficiency of CTC isolation. High-throughput sequencing technology and single-cell sequencing technology are also used in the study of CTCs. In recent years, research on CTCs has indicated that they can serve as an indicator of tumor metastasis and may have potential for early tumor diagnosis. Furthermore, CTCs have shown promise as a positive prognostic factor for organ-specific metastasis. Notably, CTC was accepted as a tumor marker by the American Society of Clinical Oncology (ASCO) in 2007, and included in the 8th edition of AJCC breast cancer TNM staging system in 2017

Fig. 2

The process by which CTCs detach from the primary tumor and survive in the circulation. a Tumor cells detach from the primary tumor. In the primary tumor, tumor cells will undergo epithelial-mesenchymal transition (EMT) triggered by some factors such as TGF-β, making them transition from the epithelial state to the mesenchymal state and acquire stronger invasion ability and higher metastatic potential. b Survival in the circulation. Tumor cells that escape from the primary tumor can enter the bloodstream as individual circulating tumor cells (CTCs) or as multicellular CTC clusters while the individual CTCs are likely to be exposed to physical stress or rapid natural killer (NK) cell clearance. Moreover, CTC clusters could bind to platelets or neutrophils. CTCs can activate platelets to secrete high levels of TGF-β, and activated platelets assist CTCs in evading immune surveillance and tumor-endothelial interactions. Tumor-derived inflammatory factors can stimulate neutrophil formation of “neutrophil extracellular traps” (NETs) that facilitate the attachment of CTCs to endothelial cells and promote metastatic dissemination. This figure was created with biorender.com

Fig. 3

Overall view of CTCs in organ-specific metastasis. The heterogeneity of the circulating tumor cell (CTC) repertoire in the blood reflects the diversity of molecular mechanisms leading to tumor dissemination. Different CTCs differ in their ability to extravasate to distant organs such as bone, lung, brain, liver, or lymph nodes and in their ability to colonize by forming overt metastases. The occurrence of distant metastasis in different organs is related to the characteristics of CTCs, the specific molecular mechanism, and the microenvironment of distant organs. The cells in this figure were created with biorender.com

Fig. 4

The molecular features and mechanisms of CTCs mediating brain, lung, liver and bone metastasis. a For brain metastasis, cancer stem cell-like CTCs or CTCs with specific phenotypes (e.g., HER2, EGFR, HPSE and Notch1) might be more adapted to the brain microenvironment. CTCs can cross the BBB with the help of mediators (e.g., COX2 and ST6GALNAC5) and tumor-derived EVs. CTCs express high levels of anti-PA serpins, which can inhibit the pro-apoptotic effects of reactive astrocytes. In addition, CTCs can induce astrocytes to secrete cytokines suitable for survival. b For lung metastasis, CTCs expressed FADS3 and ICAM can extravasate with the help of miRNAs and integrins. And miR-122 ensures sufficient glucose uptake by CTCs. The periostin (POSTN) and tenascin C (TNC) derived from CTCs and myofibroblasts stimulate CTC survival by amplifying Wnt and Notch signaling. Integrins α6β4 and α6β1 are preferentially taken up by lung-resident cells, creating a microenvironment suitable for CTCs. CTCs can express high levels of B7x to induce immune escape and avoid elimination by immune cells. c As for liver metastasis, CTCs can release exosomes and secrete TGF-β to activate stellate cells, thereby recruiting bone marrow-derived cells (BMDCs). CTCs also activate fibroblasts to produce IL-11 through the secretion of TGF-β, which support CTC survival in the liver. In the circulation, TPO binds to CTCs to regulate metabolism and activate Wnt signaling. High Src signaling protects CTCs from TRAIL-mediated apoptosis. d For bone metastasis, the activation of osteoclasts is facilitated by CTC-derived mediators, including PTHrP, IL-1. CTCs induce IL-6 secretion from osteoblasts, which in turn induces osteoclast differentiation. Activated osteoclasts undergo bone resorption and release other growth factors, such as TGF-β, which further stimulates the expression of osteolytic factors in CTCs, resulting in a vicious cycle. Some parts of the figure were created with biorender.com

Fig. 5

The molecular features and mechanisms of CTCs mediating lymph node metastasis. For lymph node metastasis, CTCs expressing VIM, uPAR and CXCR4 tended to develop lymph node metastasis. Various factors secreted by CTCs can induce the formation of a pre-metastatic niche (PMN) and an immunosuppressive environment in lymph nodes suitable for tumor colonization and growth. CXCL1 and CXCL8 secreted by CTCs can recruit neutrophils, promote the activation of the ERK and JNK signaling pathways and the expression of VEGF-A and MMP9 in neutrophils, induce tumor lymphangiogenesis, and promote lymph node metastasis. Tumor-derived PGE2 and TGF-β induce immune suppression of dendritic cells (DCs) and T cells. Some parts of the figure were created with biorender.com

Fig. 6

Strategies targeting CTCs. a EMT process. TGF-β promotes EMT in concert with other signaling pathways through a variety of pathway, which in turn activates the expression of transcription factors and regulates apoptosis and cell adhesion gene and avoid anoikis. The development of inhibitors that target pathways in the EMT process can inhibit EMT, allowing CTCs to upregulate epithelial markers and downregulate mesenchymal makers, thereby reducing the invasiveness of CTCs. b Tumor microenvironment. Cancer-associated fibroblasts (CAFs) and bone marrow-derived dendritic cells (BMDCs) are important supportive cells during metastasis, helping to maintain the invasive tumor microenvironment and the metastatic phenotype of CTCs. Targeting CAFs can reduce CTC leakage, and targeting BMDCs can eliminate premetastatic niche (PMN) formation so that the target organ cannot provide a suitable microenvironment for CTCs to colonize. c CTC clusters. CTC clusters can be decomposed in three ways. First, it can specifically capture CTCs and induce apoptosis. Second, antibodies or DNase can be used to destroy the binding between CTCs and neutrophils. Finally, antithrombotic drugs can be used to degrade platelets in CTC clusters to decompose CTC clusters. d Pan-CTC. Rg3-LP/DTX can accurately capture CTCs via the Glut1-Rg3 interaction. In addition, drugs such as mifepristone can be used to directly interact with Bcl-2, a member of the antiapoptotic protein family, and activate the p38 MAPK pathway to induce apoptosis of CTCs. Moreover, some CTCs have cell markers of CSCs; therefore, it is possible to target stem cell markers to achieve the goal of eliminating CTCs. This figure was created with biorender.com