Melanoma macrometastases are red and cell nuclei are blue (DAPI), Magnification: 63. grown in 3D cultures (3D) express higher levels of CCR4 than cells grown in 2D cultures (2D). Addition of microglia-derived soluble factors to PFK-158 melanoma cells grown in 3D cultures (3D+MG) upregulated the expression of CCR4 even more. Graphs show the average % positive cells in three independent experiments, * 0.05. (CCD) Brain sections of normal mice (C) Hbegf and of mice inoculated via the intra-cardiac route with 1 106 mCherry-HBMMC (D). Brain sections were stained by PFK-158 immunofluorescence for CCR4 (green). Melanoma macrometastases are red and cell nuclei are blue (DAPI), Magnification: 63. Scale bar = 75 m (C), Scale bar = 50 m (D). Arrows indicate CCR4 expressing stromal cells in the brain microenvironment. We next asked whether the difference in CCR4 expression between local and HBMMC is also manifested under three-dimensional (3D) growth conditions, which represent more closely the reality [14C16]. We found that CCR4 expression is significantly higher ( 0.05) on local melanoma variants propagating in 3D culture than on the same cells growing under 2D conditions (Figure ?(Figure1B),1B), suggesting that the extracellular matrix in 3D cultures has a regulatory effect on the expression of CCR4. As mentioned above, the expression of CCR4 is regulated by the brain microenvironment . In an effort to create an system mimicking the brain microenvironment, we added soluble factors derived from microglia cells, an important constituent of the brain microenvironment, to cutaneous and HBMMC grown in 3D culture. The results (Figure ?(Figure1B)1B) demonstrated that microglia-derived soluble factors upregulated the expression of CCR4 by melanoma cells. The next set of experiments was aimed to establish whether CCR4 is expressed by brain-metastasizing melanoma cells 0.05) higher expression of CCR4 than paired PRMs (Figure 2AC2B). Open in a separate window Figure 2 CCR4 expression during melanoma progression to brain metastasis(A) Representative IHC staining with anti-CCR4 antibody for PRM, LNM and MBM specimens. PFK-158 Black bars indicate 100 m. The insets show a magnification of the melanoma lesions. Black arrowheads indicate CCR4-positive melanoma cells. Yellow bars indicate 20 m. (B) Box plot comparing H score for PRM, LNM and MBM. * 0.05. CCR4 ligands are expressed and secreted by human brain stromal cells We previously demonstrated that the CCR4 ligands CCL17 and CCL22 are expressed in the brain . Based on these results and those described above, (Figure ?(Figure1),1), we hypothesized that the targeted migration of CCR4-expressing melanoma cells is mediated by an interaction between CCR4 expressed by melanoma cells and CCR4 ligands expressed in the brain. In order to identify the cellular source of the CCR4 ligands in the brain, we performed qRT-PCR assays using cultures of human astrocytes, microglia and brain endothelial cells and found that all 3 types of brain cells express CCL17and CCL22. It should be noted that these cells require stress conditions (e.g. starvation medium) or activation signals (e.g. exposure to melanoma-derived supernatants C see below) to express the CCR4 ligands. We next utilized a human chemokine array to evaluate secretion of the ligands from astrocytes, microglia and brain endothelial cells. These cells were incubated in starvation medium containing 0.5% FCS for 24 h. Conditioned medium collected from these cells was analyzed for the relative expression of the CCR4 ligands CCL17 and CCL22. We found that all 3 types of brain cells secreted CCL17 (Figure ?(Figure3A)3A) and CCL22 (Figure ?(Figure3B,3B, suggesting that these cells are a physiological source of the CCR4 ligands. Open in a separate window Figure 3 CCR4 ligands are expressed and secreted by human brain stromal cells(ACB) Chemokine secretion analysis by human chemokine array. CCL17 (A) and CCL22 (B) are secreted by human endothelial cells, astrocytes (HA) and microglia (MG). (CCD) Chemokine secretion analysis by human chemokine array. Melanoma cells alter the secretion of CCL17 (C) and CCL22 (D) by microglial cells: Microglial cells treated with local melanoma cell-conditioned media (MG+Local), treated with brain metastasizing melanoma cell-conditioned media (MG+HBMMC), microglial cells alone served as control. (ECF) The bearing of melanoma tumors leads to alterations in the expression of CCR4 ligands in the brain. (E) Nude mice were s.c inoculated with 1 106 human brain metastasizing melanoma cells (YDFR.CB3). Three, five or ten weeks following inoculation (3W, 5W and 10W) brains were removed and analyzed for Ccl17 (E) and Ccl22 (F) expression levels using qRT-PCR. W = weeks. * 0.05, ** 0.005, = 3. As shown earlier (Figure ?(Figure1B)1B) microglial cells interacted with melanoma cells and altered the expression of CCR4.