Once within a lymph or blood vessel, tumor cells become circulating tumor cells (CTCs) that are targeted by different immune cells to varying extents
[79]. Individual CTCs resist immune system attacks through a myriad of mechanisms, such as the degradation of apoptotic receptors such as TRAIL, and the expression of attack-arresting surface markers such as CD47 and PD-L1
[80][81][82]. However, individual CTCs may undergo apoptosis under the influence of immune cytokines and fluid shear forces, and undergo anoikis upon loss of attachment to the extracellular matrix (ECM) and neighboring cells due to a lack of fibronectin mediation
[83][84]. CTCs that aggregate through CD-44 cohesion have been shown to be more resilient, particularly when the aggregates include platelets and neutrophils which disguise them
[85][86]. Heterogenous cell aggregates also lead to CTC proliferation through IL-1β and IL-6 crosstalk
[87]. CTCs spread with circulation to remote sites and, subsequently, they can extravasate from the vascular system through blood vessel walls into the surrounding tissue forming tumors at host organs, as shown in
Figure 4. The site of CTC arrest is known as the secondary site and is found to occur primarily in selective organs. This targeted arrest is known as organotropism and is described in
Figure 5, with breast cancer secondary sites appearing mainly in the bone, brain, liver, and lungs, in addition to the axillary lymph nodes, which are diagnostic for metastatic disease in ~97% of patients
[13][14][88]. Organotropism is presumed to originate from the primary tumor secreting context-dependent signals to distal sites that sensitize them to receive CTCs which, in cooperation with neutrophils, seek the pre-conditioned sites
[89][90][91]. Certain signals have been implicated with specific organotrophic targets, such as TGFβ and COX2 for the lungs, SRC-dependent pathways for bone, and IGF1 for the brain
[92][93][94]. mi-RNAs have also been identified as significant in targeted organotropism, with miR-16/148a for the lungs and miR-127/197/222/223 for bone
[89][90][91]. Tumor exosomes may be involved in organotropism; tumor-derived exosome uptake by organ-specific cells was observed to prepare the pre-metastatic niche, and treatment with exosomes from lung-tropic models redirected the metastasis of bone-tropic tumor cells. Moreover, exosome proteomics revealed distinct integrin expression patterns, in which the exosomal integrins α6β4 and α6β1 were associated with lung metastasis, whereas exosomal integrin αvβ5 was linked to liver metastasis. Thus, targeting integrins α6β4 and αvβ5 decreased exosome uptake as well as lung and liver metastasis, respectively. Exosome integrin uptake by resident cells was found to activate Src phosphorylation and pro-inflammatory
S100 gene expression
[95]. Outside of chaperoned extravasation, CTCs and CTC clusters may arrest along the vasculature through weak CD-44 and integrin αvβ3 binding, and are then reinforced with stronger fibronectin and integrin α5β1 bonds over time
[96]. Once the CTCs affix to their target organ, a similar process to intravasation then occurs, where CTCs excrete signals with neutrophils to permeabilize endothelial cells for infiltration
[97]. Trans-endothelial migration enables cancer cells to embed between endothelial cells, mesenchymal stem cells (MSCs), and vascular pericytes
[98]. This perivascular niche produces supportive signals such as PI3K, and is a main focus of anti-cancer research
[99][100]. Pericytes also play a significant role in breast cancer extravasation, and have been shown to respond to tumor-derived signals by performing an embryogenesis-derived program of angiotrophic migration and fibroblast differentiation, enhancing tumor extravasation and promoting pericyte mimicry (PM) in cancer cells
[101][102][103]. Pericyte- and MSC-expressed MCAM/CD146 were reported to alter the expression of the estrogen receptors ErbB3 and ErbB4, resulting in an increase in chemoresistance, EMT, and a worse prognosis, in contrast to previous findings
[104][105][106][107]. Confinement within the perivascular niche has been found to induce cancer cells to enter a motile amoeboid morphology that initiates the invasion of surrounding tissue, although invasion may also occur through slower collective and mesenchymal cell migration modes
[38][39]. Signals such as EGF, BMP2/7, AKT, and WNT may then reverse EMT by producing a mesenchymal-to-epithelial transition (MET) or amoeboid-to-epithelial transition (AET), indicated by a FGFR2b/FGFR2 switch that leads to migratory arrest and tumorigenesis
[108][109].
Microfluidic devices could provide a powerful tool for improving the scientific understanding of CTC behavior, organotropism, and extravasation. A microfluidic method was introduced for the integrated capture, isolation, and analysis of membrane markers, as well as the quantification of proteins secreted by single CTCs and CTC clusters. The proposed platform was tested with multiple breast cancer cell lines spiked into human blood and mouse-model-derived CTCs. The quantified secretion level of granulocyte growth-stimulating factor (G-CSF), which is involved in neutrophil recruitment, was found to be highly expressed across cancer cell lines. Incorporating barcoded magnetic beads, this platform can be adapted for multiplexed analysis enabling comprehensive functional CTC profiling
[110]. A thorough investigation of CTC–neutrophil adhesion was conducted using droplet formation techniques, which showed that CTC–neutrophil aggregates upregulated the expression of VCAM-1, E-cadherin, and macrophage recruitment cytokines such as CCL4/24/22 and PPBP
[111]. Utilizing a commercially available hepatic microphysiologic system (LiverChip, CN Bio Innovations Limited, Oxford, UK), the liver system was established as a relevant microfluidic model for the study of breast cancer metastasis
[112]. A microfluidic system was developed and characterized for the in vitro systematic studies of organ-specific extravasation of CTCs. The system recapitulated the two major aspects of the in vivo extravasation microenvironment: local signaling chemokine gradients in a vessel lined up with an endothelial monolayer. The system was utilized to demonstrate the extravasation of CXCR4-expressing MDA-MB-231 cancer cells, across a confluent HUVEC monolayer, in the presence of CXCL12 chemokine gradients. Consistent with the hypothesis of organ-specific extravasation, control experiments were presented to substantiate the observation that the MDA-MB-231 cell migration was due to controlled chemotaxis rather than a random process
[113]. A multi-site metastasis-on-a-chip microphysiological system was described for assessing the metastatic preference of cancer cells. The device housed multiple bioengineered 3D organoids established by a 3D photopatterning technique employing extracellular-matrix-derived hydrogel biomaterials. Under recirculating fluid flow, tumor cells grew in the primary site, entered circulation, and preferentially homed to specific organ constructs. The platform can be implemented to better understand the mechanisms underlying metastasis and, perhaps, leading to the identification of targets for intervention
[114]. The impact of hypoxia, a common feature of the tumor microenvironment, on the extravasation potential of breast cell lines, was studied in a 3D microvascular network model. Using HIF-1α knock-down cell lines, the importance of HIF-1α in the transmigration ability of human breast cell lines was validated. Under hypoxic conditions, the HIF-1α protein level was increased, and coincided with changes in cell morphology, viability, and an elevated metastatic potential; these changes were accompanied with an increase in the rate of extravasation compared with normoxia (21% O
2). Such a microfluidic model can be a reliable in vitro tool for systematically interrogating individual factors and their accompanying downstream effects, which may otherwise be difficult to study in complex tumor tissues
[115]. A physiologically relevant vascularized bone matrix to model CTC extravasation into the perivascular niche was created in a similar device, which was then used to demonstrate the anti-metastatic role of interstitial flow
[116]. Other projects on the perivascular niche added to the investigation of extravasation by providing alternative designs for imaging and quantifying CTC extravasation
[116][117][118][119]. A dynamic in vivo-like 3D microfluidic system replicating key structural, functional, and mechanical properties of the blood–brain barrier (BBB) in vivo was constructed to probe metastatic brain tumors. Multiple factors in this organotypic BBB model work synergistically to accentuate BBB-specific attributes with the complex microenvironment reproduced via physical cell–cell interaction and vascular mechanical cues. The interactions between cancer cells and astrocytes in the BBB microenvironment seemed to affect the ability of malignant brain tumors to traverse between brain and vascular compartments. The model offers the capability of examining brain metastasis of human breast cancer cells and their therapeutic responses to chemotherapy. Furthermore, the quantification of spatially resolved barrier functions exists within a single assay, providing a versatile platform for pharmaceutical development, drug testing, and neuroscientific research
[120].