The transmembrane protein CD82/KAI1 suppresses the metastatic potential of various cancer cell types. Moreover, decrease or loss of CD82 expression is closely associated with malignancy and poor prognosis in many human cancers including prostate cancer. Despite intense scrutiny, the mechanisms underlying the metastasis-suppressing role of CD82 are still not fully understood. Here, we found that a fibronectin matrix induced mesenchymal phenotypes in human prostate cancer cells with no or low CD82 expression levels. However, high CD82 expression rendered prostate cancer cells to have intensified epithelial characteristics upon fibronectin engagement, along with decreased cell motility and invasiveness. The CD82 function of inhibiting fibronectin-induced epithelial-to-mesenchymal transition (EMT) was dependent not only on CD82 interactions with fibronectin-binding α3β1/α5β1 integrins but also on the integrin-mediated intracellular signaling events. Notably, CD82 attenuated the FAK-Src and ILK pathways downstream of the fibronectin-receptor integrins. Immunofluorescence staining of human prostate cancer tissue specimens illustrated a negative association of CD82 with EMT-related gene expression as well as prostate malignancy. Altogether, these results suggest that CD82 suppresses EMT in prostate cancer cells adhered to the fibronectin matrix by repressing adhesion signaling through lateral interactions with the associated α3β1 and α5β1 integrins, leading to reduced cell migration and invasive capacities.

Background The epithelial‐to‐mesenchymal transition (EMT) is closely associated with cancer invasion and metastasis. Since the transforming growth factor β (TGF‐β) and Wnt signals induce EMT in various epithelial cell types, we examined whether and how the CD82/KAI1 metastasis suppressor affects the TGF‐β and Wnt signal‐dependent EMT in human prostate cancer cells. Methods The invasiveness of cancer cells was evaluated by examining their ability to pass through the basement membrane matrigel. The subcellular localizations of Smad4 and β‐catenin proteins were respectively examined by confocal microscopy following immunofluorescence antibody staining and immunoblotting analysis following subcellular fractionation. The transcriptional activities of the TGF‐β1‐responsive TRE and Wnt‐responsive Tcf/Lef promoters were determined by a luciferase reporter assay following transfection of the recombinant reporter vector into the cell. Results TGF‐β1 and Wnt3a treatments of human prostate cancer cells without CD82 expression resulted in not only increased invasiveness but also EMT involving the development of motile structures, downregulation of E‐cadherin, and upregulation of the mesenchymal proteins. However, in the cells with high levels of CD82, the TGF‐β1 and Wnt3a stimulations neither elevated invasiveness nor induced EMT. Furthermore, the TGF‐β1 signaling events occurring in the CD82‐deficient cells, such as phosphorylation of Smad2, nuclear translocation of Smad4, and transactivation of the TRE promoter, did not take place in the high CD82‐expressing cells. Further, high CD82 expression interfered with the Wnt signal‐dependent alterations in the phosphorylation pattern of glycogen synthase kinase 3β (GSK‐3β) in prostate cancer cells, which allowed GSK‐3β to continue phosphorylating β‐catenin, thereby attenuating the Wnt signaling effects on the nuclear translocation of β‐catenin and subsequent transactivation of the Tcf/Lef promoter. Conclusions The results of the present study suggest that CD82/KAI1 functions in suppressing TGF‐β1‐ and Wnt‐induced EMT in prostate cancer cells by inhibiting the TGF‐β1/Smad and Wnt/β‐catenin pathways. Therefore, loss or decrease of CD82 expression is likely to render prostate cancer cells prone to respond to the TGF‐β1 and Wnt signals with EMT, resulting in the development of a motile and invasive mesenchymal phenotype related to the initiation of the metastatic cascade.

Macrophage inhibitory cytokine‐1 (MIC‐1) is a cytokine with pleotropic actions and its expression is markedly increased by inflammation and cardiac injury and in cancers. In particular, MIC‐1 production after cardiac ischemia injury is associated with enhanced cardiac angiogenesis as well as myocardial protection. However, it remains uncertain whether MIC‐1 itself has proangiogenic activity. In this study, we tried to determine the precise role of MIC‐1 in physiological and pathological angiogenesis. Human microvessel endothelial cells responded to MIC‐1 with enhanced angiogenic behaviors. Employing various angiogenesis assays, MIC‐1 was found to promote vessel formation and development with a potency similar to that of vascular endothelial growth factor (VEGF). MIC‐1 transgenic (Tg) mice also displayed enhanced neovascularization in both developing embryos and neonatal mouse retinas, compared with wild‐type mice. Furthermore, endothelial cells (ECs) isolated from MIC‐1 Tg mouse lung exhibited higher angiogenic potential than ECs from wild‐type lung. MIC‐1‐induced angiogenesis was also observed in the recovery or healing processes of injuries such as hindlimb ischemia and skin wounds in mice. However, unlike VEGF, MIC‐1 induced neither endothelial inflammation nor increased vascular permeability. In ECs, the MIC‐1 signal exerted proangiogenic actions via the MEK/extracellular signal‐regulated kinase‐ and phosphatidylinositol 3‐kinase/Akt‐dependent pathways. Notably, these MIC‐1 signaling events in ECs were abrogated by small interfering RNA‐mediated knockdown of GFRAL, suggesting that GFRAL is an EC receptor for MIC‐1. In summary, we here show a novel role of MIC‐1 as a potent EC activator, which promotes both normal and injury‐related angiogenesis.

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The present data suggest that the KAI1 gene might be repressed by epigenetic alterations through the promoter CpG-site methylation during prostate cancer progression. This epigenetic mechanism could provide a clue for understanding how the KAI1 gene was silenced in metastatic prostate cancers. Prostate 77: 350-360, 2017. © 2016 Wiley Periodicals, Inc.