Molecules promoting circulating clusters of cancer cells suggest novel therapeutic targets for treatment of metastatic cancers

Abstract

Treatment of metastatic disease remains among the most challenging tasks in oncology. One of the early events that predicts a poor prognosis and precedes the development of metastasis is the occurrence of clusters of cancer cells in the blood flow. Moreover, the presence of heterogeneous clusters of cancerous and noncancerous cells in the circulation is even more dangerous. Review of pathological mechanisms and biological molecules directly involved in the formation and pathogenesis of the heterotypic circulating tumor cell (CTC) clusters revealed their common properties, which include increased adhesiveness, combined epithelial-mesenchymal phenotype, CTC-white blood cell interaction, and polyploidy. Several molecules involved in the heterotypic CTC interactions and their metastatic properties, including IL6R, CXCR4 and EPCAM, are targets of approved or experimental anticancer drugs. Accordingly, analysis of patient survival data from the published literature and public datasets revealed that the expression of several molecules affecting the formation of CTC clusters predicts patient survival in multiple cancer types. Thus, targeting of molecules involved in CTC heterotypic interactions might be a valuable strategy for the treatment of metastatic cancers.

Keywords: cancer; circulating cancer cell; heterotypic cell interactions; metastasis; tumor microenvironment.

Figure 1

Mechanisms of circulating tumor cells (CTC) cluster formation and their properties. Highlighted in green, the process of CTC clusters formation; Tumor cells can separate from adjacent cells with more mobile mesenchymal cells at the leading edge of the invasion and cells with more epithelial properties behind (4, 5). Accordingly, mixed epithelial mesenchymal phenotype and polyploidy are frequently observed in the CTC clusters (21, 22). CTCs and CTC clusters are able to withstand the shear stress in the blood circulation and escape natural killer (NK). CTCs can form homotypic clusters or interact with CAFs, neutrophils, PMN-MDSCs, Tumor associated macrophages (TAM) or platelets forming heterotypic clusters (14, 16, 17). CAFs circulate in the bloodstream in heterotypic CTC clusters and promote cancer cell clustering by secreting CXCL12 and TGF-β (4, 13, 20). During transit and metastasis, platelet-rich thrombus form around CTCs providing protection from shear stress and against lysis by NK cells (–58). CTC-associated neutrophils express TNF-α, OSM, IL-1β and IL-6 cytokines and their receptors are expressed correspondingly in CTCs. The interaction between CTCs and neutrophils is mediated by VCAM1, whereas TNF-α, OSM, IL-1β, and IL-6 promote proliferation of CTCs (16). In addition, neutrophils promote metastasis by releasing their DNA forming neutrophil extracellular traps (NET) (39). In turn, interaction with polymorphonuclear myeloid derived suppressor cells promote survival of the CTC clusters (17).

Figure 2

Overview of signaling pathways involved in the heterotypic cancer cell interactions pivotal for circulating tumor cells complex formation and metastasis. Specifically, PMN-MDSC-cancer cell interaction promotes ROS-induces Jagged1/Notch1/Nodal signaling that induces CTC cluster formation and metastasis (17), In turn, neutrophil interact with cancer cells via ITGAM/ICAM-1 adhesion facilitating interaction with liver sinusoids and metastasis (30). In addition, VCAM1 is required for the neutrophil-CTC cluster formation (29). Neutrophils produce IL1 and IL-6 that promote growth of neutrophils-CTC clusters via IL6ST and IL1R1 receptors (16), The Il-6 is also produced by the tumor associated macrophages which induce JAK2/STAT3/miR-506-3p/FoxQ1 signaling in cancer cells promoting epithelial mesenchymal transition (EMT), metastasis and further attraction of macrophages by the CCl2 secretion (31). A similar positive feedback loop is organized by the cancer-associated fibroblasts (CAFs) and cancer cells interactions. The CAFs produce TGF-β and CXCL12 that interact with TGFBR2 and CXCR4 receptors, inducing cancer cell EMT, CTC clusters and metastasis (4). In turn, cancer cells produce TGF-β and induce CAFs myofibroblast differentiation (83).

Figure 3

Set of selected CTC-associated genes differentially predict survival in several human cancer types. (A) An example of Kaplan-Meier curve for breast cancer (BRCA) patients stratified by high or low IL1R1 expression in tumors. Shaded areas represent 95% confidence intervals. Time is shown in days. (B) Hazard ratios (HR) and significance of the differences between Kaplan-Meier curves for patients stratified by the expression of indicated genes for the panel of solid tumors in TCGA database. Only genes with p<0.05 are shown. Note that log₂(HR)<0 for good prognosis corresponding to high gene expression and log₂(HR)>0 for bad prognosis. (C) Hazard ratio clustering for gene - cancer combinations revealed two major gene clusters. Top-right triangles depict approximations of the HR collected from the literature. Blue colors represent genes or gene products whose expression is associated with worse prognosis, pink colors represent genes or gene products whose expression is associated with good prognosis while white colors represent cases where the data is controversial.