To explore the early response of HER2 + breast cancer cells to the biological effects of the brain microenvironment, the SKBR3 cells were cultured using serum-deprived conditions in the presence (T-treated) and absence (C-control) of conditioned medium combined from serum-deprived astrocytes (NHA), microglia (HMC3) and brain endothelial (HBEC5i) cells (Fig. 1A). It was anticipated that the combined CM from all three brain cells would mimic the microenvironment of the metastasized tumor cells in the perivascular niche. All brain cells were cultured to high confluence to ensure the secretion of factors in sufficient concentration for the stimulation experiments. The study was conducted in the absence of fetal bovine serum for multiple reasons. First, we intended to explore the behavior of cancer cells under conditions that induce cell cycle arrest—a state which is believed to be a major reason for metastatic latency of brain metastasized breast cancer cells, relapse being enabled by re-entering of the cancer cells in the cell cycle25,26. Under such conditions, it has been shown that adherence to endothelial cells is critical for the survival of single cancer cells that extravasate in the perivascular niche and encounter a foreign environment deprived of typical nutrients present in the blood stream. It has been also shown that after therapy, metastatic growth evolves from preexisting dormant disseminated cancer cells. Second, we intended to avoid the interference of the broad range of components of undefined concentration present in the serum (e.g., growth factors, hormones, enzymes, proteins, carbohydrates, etc.), as well as the variable batch-to-batch composition that can affect the reproducibility of findings27. The SKBR3 cells were depleted of serum for 24 h prior to, not just during, treatment to ensure that the impact of the treatment on SKBR3 cells stems only from the neural cell CM and not prior FBS supplementation. The control (C) and treated (T) nuclear (N) and cytoplasmic (C) cellular subfractions were annotated as follows: CN1–CN3 nuclear fractions of control cells, CC1–CC3 cytoplasmic fractions of control cells, TN1–TN3 nuclear fractions of treated cells, and TC1–TC3 cytoplasmic fractions of treated cells.Morphological characteristics of CM-treated SKBR3 cellsUnder basal conditions, the SKBR3 cells displayed a grape-like morphology with a lower intercellular adherence (Fig. 1B), as it has been confirmed by previous studies28. The cells stained strongly for the ERBB2 receptor which is known to be present in high abundance on the surface of the cells (Fig. 1C). Upon treatment with the CM, we observed the development of protruding structures resembling lamellipodia and filopodia emanating from the surface of cells, which could be also visualized by ERBB2 staining (Fig. 1D,E). DNA content analysis by flow cytometry revealed small but consistent differences, the treated SKBR3 cells displaying a larger proportion of cells in the G1 stage of the cell cycle in comparison to the controls [83% (CV 13%) treatment vs. 74% (CV 9%) control], suggesting an inhibition of growth upon treatment with the CM. Nonetheless, the proteomic data (see further) revealed a more complex response to the CM treatment that branched out into multiple biological processes beyond cell cycle arrest such as metabolism, adhesion, and immune response.Analysis of conditioned media using cytokine arraysMembrane-based cytokine arrays were used to measure the relative concentration of 105 different components in the CM of serum-deprived brain and cancer cells and assess the factors relevant to intercellular communication that may affect the behavior of cells (Fig. 2). Detailed results of the replicate cytokine measurements including images and raw and normalized pixel intensities are provided in Supplemental file 1. The functional impact of components that displayed noticeable pixel intensities (> ~ 2000) was evaluated. A set of ~ 50 proteins of interest emerged from the analysis: chemokines and interleukins, growth factors, and various other factors with roles in growth/development, angiogenesis, adhesion, migration, and immune response. The group of proteins with increased abundance in at least one of the brain cell secretomes included pro-inflammatory chemokines (CCL2, CCL5, CCL7/MCP3, CXCL1, CXCL5, CXCL8/IL8, CXCL10, CXCL11), homeostatic chemokines (CXCL12/SDF1), pro- and inflammatory cytokines and factors (IL6, IL15, TNFSF13B, FLT3LG, RBP4, CD14, CSF1/2), pro-inflammatory inhibitors and homeostatic cytokines (IL18BP, PTX3), growth and cell activator factors (BDNF, IGFBP2/3, PDGFA, GDF15, TNFSF13B, ANGPT1/ANGPT2, CSF1/2, CHI3L1, SPP1, DKK1), and cell adhesion and extracellular matrix (ECM) remodeling molecules (ICAM1, VCAM1, MMP9, THBS1, SERPINE1, PLAUR) (Fig. 2A–C,F).Figure 2Results from the cytokine microarray profiling of conditioned medium collected from serum-deprived cell cultures of (A) HBEC5i, (B) HMC3, (C) NHA, (D) SKBR3-control, and (E) SKBR3-stimulated cells. (F) Bar chart showing the average pixel intensities of the set of cytokines identified in the CM collected from the serum-starved brain and SKBR3-control cells (pixel intensities > 2000). Color code: blue-HBEC5i, green-HMC3, yellow-NHA, orange-SKBR3.The group of proteins with increased abundance in the SKBR3 secretome (not necessarily unique to SKBR3) included cytokines (pro-inflammatory MIF, CCL20, IL17A, and anti-inflammatory TFF3) and several groups of factors and receptors with relevance to growth and development (PDGFB, FGF19, TFRC), ECM remodeling, and invasiveness (LCN2, BSG) (Fig. 2D, F). Angiogenesis factors such as VEGFA, ANG and ENG were secreted by all cells, with ENG being more abundant in the HBEC5i and SKBR3 secretomes. A schematic representation of a subgroup of cytokines that provided visually discernable spots (pixel intensity > 7000), and of which some displayed discrepancies in abundance in the cell secretomes, is provided in a node/edge networked configuration in Fig. 3A with edges indicating detectability in either of the four cell lines. A cutoff of ~ threefold change (FC) in normalized pixel intensities in the group of HBEC5i/HMC3/NHA brain cells and SKBR3 was used to indicate unique association with a particular cell line.Figure 3Overview of cytokines that displayed discrepancies in abundance between the brain and the SKBR3-control cell secretomes. (A) Diagram representing the detectability of cytokines in the serum-starved brain and SKBR3 cells (nodes represent gene names; edges connect the nodes to the cell lines in which the cytokines were detected; nodes and edges are shown only for nodes with a normalized pixel intensity greater than ~ 7000). Symbol-code [size is proportional to log2(protein spot abundance)]: Growth/proliferation/survival/development/stress response, ⃟—Stimulation of immune responses and inflammatory conditions, Ο—Angiogenesis, Δ—ECM remodeling/adhesion/ migration/invasiveness. Color-code [indicates the cell line in which the microarray cytokine spot was most intense]: Red-SKBR3, Green-HBEC5i, Orange-HMC3, Blue-NHA, Yellow-cytokine secreted by multiple cell lines. (B) Bar chart of representative GO biological processes supported by all secreted proteins with pixel intensity > 2000 (FDR < 5%). The labels associated with each biological process indicate fold enrichment.Inflammatory cytokinesWith few exceptions, all pro-inflammatory cytokines with elevated abundance in the brain cell secretomes were secreted at the basal level by the HBEC5i cells, with only a few being secreted in similar abundance by the HMC3 (CCL2, CCL5, CXCL8/IL-8, PTX3, RBP4, FLT3LG, CD14) and NHA (CCL7, CXCL8) cells. Owing to their prime location in the bloodstream, the endothelial cells are the first to come in direct contact with any infectious entity in the body. Therefore, these cells advanced mechanisms for the recognition of damage-associated molecular patterns (DAMPs) for inciting an immune response and for recruiting immune cells to the site of pathogenesis29. Many of these functions are facilitated by the inflammatory cytokines and chemokines that are released by the endothelial cells in their microenvironment. Elevated levels of TNFSF13B, a cytokine involved in the regulation of immune responses and stimulation of B-/T-cells, were observed only in the HBEC5i cell secretome. IL6 was also preponderantly expressed in the HBEC5i secretome. Along with its well-known inflammatory functions, this cytokine has been implicated in regulating the proliferation, angiogenesis, invasion, and metabolism of cancer cells. IL6 mediates these outcomes primarily via the STAT3 or NFKB signaling axis, and induces the expression of factors which promote enhanced malignancy (GM-CSF, CCL2, MMP, VEGF)30. Previous studies have confirmed a basal level expression of CCL2 and CXCL8/IL8 by the human brain endothelial cells, however, CXCL10 and CCL5 were shown to be expressed only upon pro-inflammatory cytokine stimulation31. The levels of CCL2 were relatively constant in all cell secretomes, but there is accumulating evidence that CCL2 induces angiogenesis via increased VEGFA expression, while CXCL5, CXCL8/IL8 and CXCL1 directly affect tumor growth and metastasis by enhancing blood vessel supply through neovascularization32. On the other hand, CXCL10 and CXCL11 exert angiostatic effects and initiate T-cell or NK-cell mediated immune responses32. Previous studies suggested that CCL7 could be involved in promoting tumor invasion and metastasis, but tumor suppressor effects of this cytokine have been also identified33. The detection of CCL7 in this work was, however, just above the intensity threshold setting in HBEC5i and NHA cells. IL15 was also observable in low abundance, and only in the secretome of HBEC5i cells. By stimulating the proliferation and activation of immune cells (NK-, B- and T-cells), IL15 has many protective and anti-tumor roles. It is explored as a potential therapeutic agent by itself or as a target for the development of IL15 agonists and a number of immunotherapies34. IL18BPa (IL18 binding protein) which is an inhibitor of the proinflammatory IL18 cytokine was upregulated in HBEC5i, while the homeostatic/tissue remodeling PTX3 (Pentraxin-3) in the HMC3 and HBEC5i secretomes. IL18BPa is believed to play a buffering role for IL1835, and PTX3 to exert multiple tumor suppressive and supportive roles via complex involvement in mediating immune responses, ECM remodeling and angiogenic programs36,37. CXCL12 (SDF1α) was elevated in the HBEC5i secretome relative to SKBR3, and while this is a homeostatic chemokine, it is known to exert various roles in pathogenic conditions38,39. Different splice variants have specific activities, which are also controlled by PTMs38, and hypoxia and growth arrest in different cell types can elevate the expression level of this cytokine38. Increased CXCL12 levels produced under hypoxia by ovarian cancers have been shown to promote the expansion of endothelial cells and angiogenesis40. Altogether, aberrant expression of CXCL12 in the tumor microenvironment was correlated with tumor growth, proliferation, inhibition of apoptosis, and driving cancer cell migration to distant sites39. The cytokine signals along the MAPK, PI3K/AKT, Wnt and NFKB pathways to induce the expression of VEGF, FGF, cyclooxygenase-2 and IL6, all of which are critical mediators of angiogenesis39. CXCL12 was also observed in the NHA medium. Additional pro-inflammatory proteins such as the Fms-Related Tyrosine Kinase 3 Ligand and growth factor (FLT3LG), retinol binding protein 4 (RBP4), and receptor CD14, albeit in lower abundance than the other cytokines, complemented the ability of HBEC5i cells to stimulate the proliferation or activity of immune cells41. In addition, the cell adhesion glycoproteins VCAM1 and ICAM1 with roles in mediating cell adhesion, motility and maintaining tissue architecture, were also predominantly expressed in the HBEC5i secretomes, especially VCAM1 (ICAM1 was observable in low abundance). VCAM1 is overexpressed on endothelial cells under inflammatory conditions to allow for the adhesion and rolling of immune cells on the endothelium surface42. It has particular relevance during cancer metastasis, as it is one of the molecules that tumor cells bind for transmigrating across the endothelial barrier42.In microglia, the presence of IL6, CXCL8/IL8, CCL5, PTX3 was consistent with the role of these cytokines in the regulation of leukocyte activation and chemotaxis that is invoked in the brain in the case of neuropathological and inflammatory conditions, and the detection of PTX3 and FLT3LG with the involvement of microglia in mediating inflammation and phagocytosis, respectively43. Likewise, the secretion of CCL7 by NHA cells was associated with promoting microglia-mediated inflammation after brain injury44.In the SKBR3 secretome, pro-inflammatory cytokines were present, but generally in low abundance (excepting IL17A and MIF which were also present in the brain cell secretomes) or had mostly tumor-promoting functions (CCL20). Cytokines such as IL11, IL22, RETN and TNF which are associated with inflammatory responses were observable at very low intensities, but their emerging role in promoting the progression of cancer has been recognized45,46,47,48. The other cytokines have been shown to promote cell renewal, tissue regeneration, recruitment of immune cells and activation of pro-survival and mitogenic signaling (IL17A49); to induce cell invasiveness, angiogenesis and cell survival pathways (macrophage migration inhibitory factor (MIF50); or, to stimulate invasiveness via MMP secretion and impart chemotherapeutic resistance (chemokine CCL2051).Growth factorsWe further assessed the differences in the concentration of growth and stimulating factors between the SKBR3-control and the brain cells (Fig. 2A–D)41,52,53,54,55. The analysis revealed that the level of many pro-tumor factors, especially of those that are implicated in growth/proliferation, angiogenesis, migration, and metastatic progression was either similar or higher in the SKBR3 than in the brain cell secretomes (PDGFB, FGF19, GDF15, MIF, LCN2, TFF3, VEGFA, ANG, ENG, ANGPT2, TFRC, SERPINE1, PLAUR, BSG) (Figs. 2F, 3A). Specific biological processes sustained by these proteins included regulation of ERK1/ERK2 and MAPK signaling cascades, cell adhesion, cell migration and invasion, blood vessel development and angiogenesis, and response to stress/defense and inflammation. Selected GO biological processes represented by minimum 5 proteins from either of the cell secretomes, with a fold-enrichment of > 4, are provided in Fig. 3B. The multifunctional platelet derived growth factor subunit B (PDGFB) with roles in cell proliferation/survival and migration, in particular, was highly abundant in SKBR3 and essentially missing from all brain cells. GDF15 was expressed by all cells, and increased levels have been shown to be associated with advanced cancer and poor patient outcomes, as this factor promotes mechanisms of immune evasion in cancer cells and enhances cell viability and metastasis52. Others, however, have suggested a conflicting response to GDF15, when it was observed to induce apoptosis in some cancer cells52. Further evidence for the secretion of cancer progression supportive factors in SKBR3 was presented by the higher expression of lipocalin-2 (LCN2) which has important ramifications in EMT-related processes, invasion, and cell migration53 and of the anti-inflammatory trefoil factor 3 (TFF3) which supports invasion and metastasis in several carcinomas54.All three brain cell types secreted some if not all of the above cancer-supportive factors. In a broader context, these proteins are involved in developmental processes, and regulate cell proliferation, mitogenesis, survival, tissue repair, and chemotaxis (Fig. 3B). A few factors that were identifiable in SKBR3 were present in higher abundance in the brain cell secretomes (IGFBP2/3, PDGFA, SPP1, CSF1/2), some being particularly elevated in NHA (IGFBP2, ANGPT1, CHI3L1, SPP1) or HMC3 (PDGFA, BDNF) (Fig. 3A). NHA exclusively expressed the angiopoietin (ANGPT1) and chitinase 3 like 1 (CHI3L1) proteins. Together with VEGFA, angiopoietins secreted by astrocytes are important for governing the angiogenic remodeling processes for the development of vasculature in the brain56. Increased levels of CHI3L1 have been associated with neurodegenerative and inflammatory brain diseases, but aberrant expression has been also observed in brain tumors and metastasis. It has been suggested that CHI3L1 expression by activated astrocytes may play a role in tumor progression, angiogenesis, immune escape, and resistance to therapeutic drugs57. It is also worth highlighting osteopontin (SPP1) which plays a crucial role in driving metastasis, invasion, angiogenesis, chemotaxis, and suppression of anti-tumor immune responses58. SPP1 is known to be secreted by both cancer cells and the cells of the TME, and to have an overall positive impact on the metastatic progression of cancer58. HMC3 revealed the expression of the BDNF involved in the development and survival of CNS cells. Interestingly, BDNF activates TrkB (Tropomyosin-Related Kinase B or Neurotrophic Receptor Tyrosine Kinase 2) and HER2 receptors and enhances the proliferation and survival of brain metastatic HER2 + breast cancer cells59. On the other hand, while the exact function of PDGFA in microglia is not fully understood, this growth factor is recognized for its important contributions to supporting cell proliferation, angiogenesis, migration, as well as chemotaxis41. The matrix remodeling proteins and enzymes (SERPINE1, PLAUR, PTX3, LCN2, BSG, THBS1, MMP9), together with the CAMs (ICAM, VCAM1) that were overexpressed in the HBEC5i secretome, formed a group that plays key roles in mediating cell–cell/cell–matrix interactions and ECM remodeling processes that ultimately affect, again, the adhesion, migration, and the invasion capabilities of cancer cells.The culture medium collected from the SKBR3 cells grown in brain cell-conditioned media generated a signal for almost all factors that produced intense spots in the brain and SKBR3-control secretions (Fig. 2E). It was therefore difficult to conclusively determine which cytokines and biological processes were upregulated in the SKBR3 secretions after treatment with conditioned media from the brain cells.Proteomic analysis and quantification of the CM-treated versus non-treated SKBR3 cellsThe impact of treating the SKBR3 cells with the brain cell-conditioned media was next evaluated quantitatively at the proteome level. The expected outcome was the identification of novel proteins that mediate an early response in SKBR3 cells to the factors that they encounter in a foreign microenvironment (i.e., the brain TME). Three biological replicates of SKBR3-treated (T) cells were compared to three biological replicates of SKBR3-control (C) cells. Two complementary approaches were used for assessing protein differential expression, i.e., based on LC peak area and PSM count measurements. The two approaches produced rather complementary results due to: (a) the actual use of two distinct methods for assessing protein abundance; (b) the use of two distinct approaches to validate the PSMs, with the area-based measurements being enabled by a workflow that included the Percolator node that uses a semi-supervised machine learning approach to distinguish the correct from the incorrect PSMs, in contrast to the PSM count measurements that relied on using the results from the Target and Decoy database searches; and (c) the possible software-related misrepresentation of area measurements because some chromatographic peaks did not have actual PSMs associated with them in all treatment and control samples. The complementarity of such results is not surprising when only few proteins change abundance or the changes in abundance are small, and it was noted in previous studies, as well60. Longer exposure of the SKBR3 cells (> 24 h) to the brain CM could have amplified the changes in the proteome profiles, however, the prolonged lack of serum from the culture medium would have biased the results by inducing an excessive stress response.On the average, ~ 3200/4700 proteins per cell fraction were identified in the SKBR3 cells when using the Percolator and Target/Decoy validator nodes, respectively, with over 70% of proteins being identified with two or more unique peptides (Supplemental file 2). The reproducibility of protein identifications in biological replicates for the workflow that used the Percolator node is illustrated in Fig. 4 (# protein IDs, overlaps, PSM correlations), and the results of the quantitative comparisons in Fig. 5A–E,G [box and whisker plots of raw and normalized Log10(Protein Abundance) and Volcano plots] and Supplemental file 3. There were no major overlaps between the proteins that displayed differences in counts in the nuclear and cytoplasmic fractions, demonstrating the complementarity of the data (Fig. 5F and H). Taken altogether, the area and PSM-based upregulated datasets comprised 96/91 proteins, respectively, and the downregulated ones 256/46 proteins, combined from the analysis of both nuclear and cytoplasmic samples. A few representative proteins are illustrated in Fig. 6A. Selected proteins were validated using PRM-MS analysis and a few others by Western blotting (Supplemental file 4).Figure 4Evaluation of reproducibility in protein identifications. (A) Stacked bar chart showing the percentage of proteins identified by 2 or more unique peptides in a cell state and fraction (~ 70%, bars shaded in gray). The white boxes in the center display the total number of identified proteins in each cell state and fraction. (B–E) Venn diagrams displaying the overlap between the proteins identified with high/medium FDR and ≥ 2 unique peptides in three biological replicates of various SKBR3 cell fractions: (B) CN, (C) CC, (D) TN, and (E) TC. (F–I) Scatterplots displaying the degree of correlation between the PSMs of two biological replicates for (F) CN, (G) CC, (H) TN, and (I) TC datasets. Heatmaps on the right show the Pearson correlation coefficients for every comparison.Figure 5Overview of the proteomic label-free differential expression analysis approach. (A–D) Boxplots of area-based protein abundances for three biological and three technical replicates of SKBR3 cell fractions: (A) before and (B) after normalization for the quantitative analysis of TC vs CC fractions; (C) before and (D) after normalization for the quantitative analysis of TN vs CN fractions. (E, G) Volcano plots illustrating the results of differential expression analysis for proteins that displayed minimum 2-FC up- (red) or downregulation (blue) with a p-value ≤ 0.05 in TN vs CN and TC vs CC: (E) area-based measurements, and (G) PSM count-based measurements. (F, H) Venn diagrams showing the overlap between the proteins in the N and C fractions that were either up- or down-regulated in the area-based and PSM count-based measurements.Figure 6GO Biological processes represented by the proteins that changed abundance in the proteomic profiling of SKBR3 cells treated with CM medium from brain cells. (A) Box-whisker plots displaying the relative spectral counts for the selected proteins identified in the control and treated proteomic datasets. (B) Enriched biological processes represented by the combined nuclear/cytoplasmic proteins detected by the area and PSM-count measurements with increased or decreased abundance, respectively, in CM-treated vs non-treated SKBR3 cells. (C) Protein counts that emerged from the area and PSM measurements in the CM-treated vs non-treated SKBR3 cells mapped to GO biological processes of relevance to cancer (note: the proteins were mapped to biological processes by using GO controlled vocabulary terms).Quantification was performed separately for the nuclear and cytoplasmic fractions, however, the results were combined for a thorough understanding of the cellular biology. The short lists of differentially expressed proteins yielded only very few enriched biological functional categories. When combined, however, the nuclear and cytoplasmic results, as assessed either via area or PSM measurements, revealed the main processes that were affected by treating the cells with brain cell conditioned medium. The processes that were sustained by proteins with increased counts were dominated by altered gene expression, chromosome organization, trafficking, protein localization, and interferon signaling. The processes that were supported by proteins with decreased counts (represented mostly by the cytoplasmic cluster generated by area measurements) were broadly associated with various metabolic and transport processes, and included an over-representation of cytoplasmic translation and electron transport/mitochondrial respiratory processes (Fig. 6B and Supplemental file 5). Ribosome biogenesis was represented by proteins with both increased and decreased counts. Many of the above processes are mediated via posttranslational activation/deactivation or protein shuttling between various cellular organelles. Therefore, increased or decreased protein abundance measurements may not be reflective of actual protein up- or down-regulation, but rather of changes in PTMs or cellular location. Altogether, however, the results suggest that the treatment of cells with conditioned medium altered the cellular transcriptional/translational machinery with a net outcome of slowed metabolic processes. Ribosome biogenesis, for example, is an energy-intensive process, and a disruption in the availability of nutrients or energy in the cell that can affect any step of the biogenesis process leading to either up- or down-regulated ribosome components, has been shown to result in altered cell cycle progression or even cell death61.It is important to reiterate that upon treatment with the brain cell-conditioned media the SKBR3 cells were exposed to many inflammatory molecules that were either absent or present in low abundance in the secretome of the SKBR3-control cells. In the same time, the SKBR3-control cells secreted larger amounts of several important growth factors that may or may not have been at optimum levels in the brain cell-conditioned media to elicit a signaling response in the SKBR3 cells. Therefore, to derive a better understanding of the processes that were dysregulated in the SKBR3 treated cells, the proteins were further mapped directly to specific GO terms reflective of cancer-supportive processes and metastatic progression (Fig. 6C). The complex interplay between these proteins in affecting the cell fate is exemplified in a Sankey diagram that presents a selected list of SKBR3 proteins with changed abundance, matched by at least three unique peptides, mapped to such processes (Fig. 7 and Supplemental file 6).Figure 7Sankey diagram illustrating the mapping of dysregulated nuclear/cytoplasmic proteins with increased abundance to key biological processes of relevance to metastatic progression of cancer (note: only proteins identified by at least three unique peptides are shown).Based on our previous work on proteomic profiling of SKBR3 cells that has identified a broad range of receptors in the cell membrane (EGFR, FGFR, CSFR, IL, CD44/Cd47, IL, PDGFRA, TNFR, VEGFA receptor FLT1, ephrin, integrin, neuropilin NRP1, activin, chemokine ACKR2, adiponectin ADIPOR1, sortilin SORT1, low density lipoprotein LRP, syndecan SDC4)62, a depiction of the possible interactions between the SKBR3 breast cancer and the brain cells is provided in Fig. 8. Relevant SKBR3 receptors, along with the most abundant cytokines and growth factors secreted by the brain cells, are listed in the figure. The observed changes in the SKBR3 proteome were further interpreted based on the consideration that the SKBR3 cells that were exposed to the CM from brain cells experienced an environment deprived of optimal levels of growth factors needed for proliferation, but more abundant in inflammatory molecules. Proteins with similar trends in abundance changes, as emerged from the two quantification approaches, are discussed below.Figure 8Illustration of receptors and secreted factors that can enable interactions between the SKBR3 breast cancer and brain cells. Relevant SKBR3 receptors were detected by proteomic profiling58. Abundant cytokines and growth factors secreted by brain cells, but missing or detected in low abundance in the SKBR3 secretome, are depicted near each cell line. Abundant cytokines and growth factors secreted in common by all brain and/or cancer cells are shown inside the sidebars.
