Beyond tumor‑associated macrophages involved in spheroid formation and dissemination: Novel insights for ovarian cancer therapy (Review)

Abstract

Ovarian cancer (OC) is the most common and deadly malignant tumor of the female reproductive system. When OC cells detach from the primary tumor and enter the ascitic microenvironment, they are present as individual cells or multicellular spheroids in ascites. These spheroids, composed of cancer and non‑malignant cells, are metastatic units and play a crucial role in the progression of OC. However, little is known about the mechanism of spheroid formation and dissemination. Tumor‑associated macrophages (TAMs) in the center of spheroids are key in spheroid formation and metastasis and provide a potential target for OC therapy. The present review summarizes the key biological features of spheroids, focusing on the role of TAMs in spheroid formation, survival and peritoneal metastasis, and the strategies targeting TAMs to provide new insights in treating OC.

Keywords: ovarian cancer; peritoneal metastasis; spheroid; tumor‑associated macrophage.

Figure 1

Role of spheroids in the progression of ovarian cancer. Spheroids floating in ascites include homogenous spheroids and heterogenous spheroids. Homogenous spheroids are composed of cancer cells only, while heterogenous spheroids are composed of cancer and other stromal cells, such as TAMs and CAFs. These spheroids primarily detach from the primary tumor; few are formed by aggregation of individual stromal cells and cancer cells. Compared with individual cells, multicellular spheroids are more likely to survival in ascites as they exhibit anoikis and chemotherapeutic drug resistance and immune escape. Spheroids adhere to the peritoneum covering by a single layer of mesothelium cells with cell-cell interactions or cell-ECM interactions. These mesothelial cells are induced to CAMs by cancer cells. Once adhered, spheroids disaggregate on ECM and invade the ECM by activating MMP2/MMP9 and MT1-MMP. TAM, tumor-associated macrophages; CAF, cancer-associated fibroblast; ECM, extracellular matrix; CAM, cancer-associated mesothelial cell; CSC, cancer stem cell; MT1, melatonin receptor type 1.

Figure 2

Survival mechanisms of ovarian cancer spheroids in ascites. Several signaling pathway are involved in anoikis resistance, such as AKT pathway, Bcl-2 pathway, and Wnt/β-catenin pathway. Resistance to chemotherapeutic drugs may be due to spherical structure, drug resistance-related proteins, CSCs and other non-tumor cells present in spheroids. Other mechanisms such as immune escape may also contribute to survival of spheroids. FZD7, Frizzled family receptor 7; PIK3R3, phosphoinositide-3-kinase regulatory subunit 3; COL4A2, type IV α2 collagen; TrkB, tropomysin related kinase B; HGF, hepatocyte growth factor; ERK, extracellular signal-regulated kinase; FAK, focal adhesion kinase; CSC, cancer stem cell.

Figure 3

Role of TAMs in spheroid formation, survival and metastasis. TAM-secreted VEGF binds to its receptor KDR/Flk-1 on endothelial cells, then decrease expression of junctional protein claudin-5 on endothelial cells, increase vascular permeability and contributes to ascites accumulation. Macrophages are recruited to ascites by soluble factors released by cancer cells including UBR5, M-CSF and POSTN, and chemokines released by TAMs, such as CCL18. TAM spheroids in ascites primarily detach from the primary tumor. TAM-derived EGF activates EGFR on cancer cells and increases expression of VEGF-C and VEGFR3 in cancer cells. EGF also increases the expression of αMβ2 integrin in TAMs and ICAM-1 in cancer cells to facilitate adhesion between TAMs and cancer cells. TAM-released chemokine CCL18 interacts with CCR8 to promote EMT of cancer cells. TAMs promote anoikis and drug resistance via the activation of STAT3 and Sonic hedgehog pathways and upregulation of P-gp and MRP1 in cancer cells and promote immune escape via suppressed phagocytosis of TAMs via SIRPα, inhibit T cell proliferation via B7-H4 and PD-L1 and increase Treg recruitment via CCL12. MIP-1β secreted by TAMs binds to CCR5, resulting in overexpression of P-selectin on the mesothelial cell surface. Cancer cells attached to P-selectin via CD24. TAM-derived soluble factors (such as FLT3L, leptin and HB-EGF) increase expression of MMP-9 which mediates spheroid spreading and invasion. TAM, tumor-associated macrophage; VEGF, vascular endothelial growth factor; KDR, kinase insert domain receptor; Flk, fetal liver kinase; UBR, ubiquitin protein ligase E3 component N-recognin; M-CSF, macrophage colony-stimulating factor; POSTN, periostin; CCL, chemokine (C-C motif) ligand; EGFR, epidermal growth factor receptor; CCR, C chemokine receptor; P-gp, P-glycoprotein; MRP, multidrug resistance-associated protein; SIRP, signal regulatory proteins; ASK, apoptosis signal-regulating kinase; ZEB, Zinc finger E-box binding homeobox; EMT, epithelial-mesenchymal transition; ICAM, intercellular adhesion molecule; VE-CAD, vascular endothelial-cadherin; B7-H4, B7 homolog 4; PD-L1, programmed cell death-ligand 1; MIP, macrophage Inflammatory Protein-1; Treg, regulatory T cells; FLT3L, FMS-like tyrosine kinase 3 ligand; HB, heparin-binding epidermal growth factor-like growth factor.