Spheroid Formation and Peritoneal Metastasis in Ovarian Cancer: The Role of Stromal and Immune Components

Abstract

Ovarian cancer (OC) is one of the most common gynecological cancers, with the worst prognosis and the highest mortality rate. Peritoneal dissemination (or carcinomatosis) accompanied by ascites formation is the most unfavorable factor in the progression and recurrence of OC. Tumor cells in ascites are present as either separate cells or, more often, as cell aggregates, i.e., spheroids which promote implantation on the surface of nearby organs and, at later stages, metastases to distant organs. Malignant ascites comprises a unique tumor microenvironment; this fact may be of relevance in the search for new prognostic and predictive factors that would make it possible to personalize the treatment of patients with OC. However, the precise mechanisms of spheroid formation and carcinomatosis are still under investigation. Here, we summarize data on ascites composition as well as the activity of fibroblasts and macrophages, the key stromal and immune components, in OC ascites. We describe current knowledge about the role of fibroblasts and macrophages in tumor spheroid formation, and discuss the specific functions of fibroblasts, macrophages and T cells in tumor peritoneal dissemination and implantation.

Keywords: T-lymphocytes; cancer-associated fibroblasts; composition; malignant ascites; ovarian cancer; peritoneal metastasis; spheroid; tumor-associated macrophages.

Figure 1

Tumor cell survival and spheroid formation in malignant ascites. (A). Downregulation of E-cadherin and upregulation of N-cadherin leads to EMT and a loss of cell–cell contact between tumor cells, followed by their detachment from the primary site. In order to evade anoikis, tumor cells form clusters through α5β1 integrin and fibronectin interactions. Activation of AKT leads to inhibition of caspase-3, which also hinders apoptosis. (B). One of the possible mechanisms of spheroid formation is mediated by the EGF-EGFR axis between TAMs/CAFs and tumor cells. Fibroblasts secrete EGF that upregulates ITGA5 expression in tumor cells as well as TGFβ, leading to strengthened tumor–stromal interaction inside spheroids. In turn, CAFs activated by TGFβ- and ITGA5-overexpressing tumor cells secrete EGF and other TGFβ-associated factors. EGF+ TAMs induce EGFR+ tumor cell migration and spheroid formation. Free-floating TAMs abundantly express CD206 and CD163 in ascites. A major marker for CAFs is α-SMA. VEGF is a key factor in ascitic TME.

Figure 2

Mechanisms of implantation metastasis. α5β1 integrin on tumor cells binds to exposed fibronectin on mesothelial cells, facilitating the development of peritoneal metastasis. The morphology of implanted tumor cells resembles that of mesenchymal cells with increased expression of mesenchymal markers (ZEB1, SNAIL and TWIST) and decreased E-cadherin expression during invasion to the omentum/peritoneum. TAM-derived CCL18 promotes EMT in tumor cells. IL-10, IL-6, MMP9 and LIF produced by TAMs enable the implantation of tumor cells. TGFβ-activated fibroblasts induce the adhesion of tumor cells to a layer of mesothelial cells.