Niko P. Bretz

Body Fluid Exosomes Promote Secretion of Inflammatory Cytokines in Monocytic Cells via Toll-like Receptor Signaling

Background: Exosomes, secreted from cells, have immunomodulatory capacities. Results: NFκB- and STAT3-mediated cytokine release is triggered by various types of ex vivo exosomes in a TLR-dependent fashion. Conclusion: Exosomes have inherent signaling capacities important for global inflammatory responses. Significance: Detailed knowledge about intercellular communication in cancer and inflammatory diseases is crucial for development of new therapeutic approaches. Tumor-derived exosomes have been shown to induce various immunomodulatory effects. However, the underlying signaling pathways are poorly understood. Here, we analyzed the effects of ex vivo-derived exosomes on monocytic cell differentiation/activation using THP-1 cells as model. We isolated exosomes from various body fluids such as amniotic fluid, liver cirrhosis ascites, and malignant ascites of ovarian cancer patients. We observed that exosomes were internalized by THP-1 cells and induced the production of IL-1β, TNF-α, and IL-6. Analysis of the signaling pathways revealed a fast triggering of NFκB and a delayed activation of STAT3. Pharmacologic and antibody-blocking experiments showed that the initial production of IL-6 was instrumental for subsequent activation of STAT3. Importantly, triggering of cell signaling was not a unique property of tumor exosomes but was also observed with exosomes of noncancerous origin. Exosomal signaling was TLR-dependent as the knockdown of Toll-like receptor 2 (TLR2) and TLR4 blocked NFκB and STAT3 activation. Similar results were obtained with TLR-neutralizing antibodies. Exosomes also triggered the release of cytokines from mouse bone marrow-derived dendritic cells or macrophages. This process was MyD88-dependent, further supporting a role of TLR signaling. Our results suggest that exosomes trigger TLR-dependent signaling pathways in monocytic precursor cells but possibly also in other immune cells. This process could be important for the induction of immunosuppressive mechanisms during cancer progression and inflammatory diseases.

Contractile Forces Contribute to Increased Glycosylphosphatidylinositol-anchored Receptor CD24-facilitated Cancer Cell Invasion

The malignancy of a tumor depends on the capability of cancer cells to metastasize. The process of metastasis involves cell invasion through connective tissue and transmigration through endothelial monolayers. The expression of the glycosylphosphatidylinositol-anchored receptor CD24 is increased in several tumor types and is consistently associated with increased metastasis formation in patients. Furthermore, the localization of β1-integrins in lipid rafts depends on CD24. Cell invasion is a fundamental biomechanical process and usually requires cell adhesion to the extracellular matrix (ECM) mainly through β1 heterodimeric integrin receptors. Here, we studied the invasion of A125 human lung cancer cells with different CD24 expression levels in three-dimensional ECMs. We hypothesized that CD24 expression increases cancer cell invasion through increased contractile forces. To analyze this, A125 cells (CD24 negative) were stably transfected with CD24 and sorted for high and low CD24 expression. The invasiveness of the CD24high and CD24low transfectants was determined in three-dimensional ECMs. The percentage of invasive cells and their invasion depth was increased in CD24high cells compared with CD24low cells. Knockdown of CD24 and of the β1-integrin subunit in CD24high cells decreased their invasiveness, indicating that the increased invasiveness is CD24- and β1-integrin subunit-dependent. Fourier transform traction microscopy revealed that the CD24high cells generated 5-fold higher contractile forces compared with CD24low cells. To analyze whether contractile forces are essential for CD24-facilitated cell invasion, we performed invasion assays in the presence of myosin light chain kinase inhibitor ML-7 as well as Rho kinase inhibitor Y27632. Cell invasiveness was reduced after addition of ML-7 and Y27632 in CD24high cells but not in CD24neg cells. Moreover, after addition of lysophosphatidic acid or calyculin A, an increase in pre-stress in CD24neg cells was observed, which enhanced cellular invasiveness. In addition, inhibition of the Src kinase or STAT3 strongly reduced the invasiveness of CD24high cells, slightly reduced that of CD24low cells, and did not alter the invasiveness of CD24neg cells. Taken together, these results suggest that CD24 enhances cell invasion through increased generation or transmission of contractile forces.

Molecular and clinical dissection of CD24 antibody specificity by a comprehensive comparative analysis.

CD24 is a small, highly glycosylated cell surface protein that is linked to the membrane through a glycosyl-phosphatidylinositol anchor. It is overexpressed in many human carcinomas and its expression is linked to bad prognosis. Lately, lack or low expression of CD24 was used to identify tumor stem cells resulting in conflicting data on the usefulness of this marker. In many immunohistochemical studies, the mAb SN3b was used but the epitope and specificity of this antibody have never been thoroughly investigated. In other studies based mainly on cytofluorographic analysis, the mAb ML-5 was applied. In this study, we compared the epitope of mAb SN3b to the CD24 mAbs SWA-11 and ML-5 that both bind to the core protein of CD24. Using tissue microarrays and affinity-purified CD24 glycoforms, we observed only a partial overlap of SN3b and SWA11 reactivity. The mAb SN3b recognizes sialic acid most likely on O-linked glycans that can occur independently of the CD24 protein backbone. The SN3b epitope was not related to common sialylated cancer-associated glycan structures. Both SN3b epitope positive or negative CD24 glycoforms supported the binding of P-selectin and Siglec-5. In breast cancer, the SN3b reactivity was associated with bad prognosis, whereas SWA11 was not. In renal cell cancer, the SN3b epitope was completely absent but SWA11 reactivity was a prognostic factor. Our results shed new light on the tumorbiological role of CD24 and resolve discrepancies in the literature related to the use of different CD24 mAbs.

Novel insights into exosome-induced, tumor-associated inflammation and immunomodulation

The immune system of cancer patients is often suppressed. Accumulating evidence suggests that exosomes released from tumor cells may play an essential role in this process but the mechanisms are not fully understood. Here we review recent papers showing that exosomes trigger the release of cytokines/chemokines from immune cells. We suggest that this process will either result in the stimulation of anti-tumor immune reactions or in a systemic immunosuppression. The direction appears to be largely dependent on the duration of interactions between immune cells and exosomes leading to the accumulation of inflammatory factors, i.e. on the length of the exposure to these factors. We propose that a long-term interaction of the immune system with elevated levels of tumor exosomes contributes to the development of immunosuppression in cancer patients.

CD24 controls Src/STAT3 activity in human tumors

CD24 is a glycosyl-phosphatidylinositol-anchored membrane protein that is frequently over-expressed in a variety of human carcinomas and is correlated with poor prognosis. In cancer cell lines, changes of CD24 expression can alter several cellular properties in vitro and tumor growth in vivo. However, little is known about how CD24 mediates these effects. Here we have analyzed the functional consequences of CD24 knock-down or over-expression in human cancer cell lines. Depletion of CD24 reduced cell proliferation and adhesion, enhanced apoptosis, and regulated the expression of various genes some of which were identified as STAT3 target genes. Loss of CD24 reduced STAT3 and FAK phosphorylation. Diminished STAT3 activity was confirmed by specific reporter assays. We found that reduced STAT3 activity after CD24 knock-down was accompanied by altered Src phosphorylation. Silencing of Src, similar to CD24, targeted the expression of prototype STAT3-regulated genes. Likewise, the over-expression of CD24 augmented Src-Y416 phosphorylation, the recruitment of Src into lipid rafts and the expression of STAT3-dependent target genes. An antibody to CD24 was effective in reducing tumor growth of A549 lung cancer and BxPC3 pancreatic cancer xenografts in mice. Antibody treatment affected the level of Src-phosphorylation in the tumor and altered the expression of STAT3 target genes. Our results provide evidence that CD24 regulates STAT3 and FAK activity and suggest an important role of Src in this process. Finally, the targeting of CD24 by antibodies could represent a novel route for tumor therapy.

miR-21-3p is a positive regulator of L1CAM in several human carcinomas

Expression of L1 cell adhesion molecule (L1CAM) occurs frequently in human cancers and is associated with poor prognosis in cancers such as ovarian, endometrial, breast, renal cell carcinoma and pancreatic ductal adenocarcinoma. L1CAM promotes cell motility, invasion, chemoresistance and metastasis formation. Elucidating genetic processes involved in the expression of L1CAM in cancers is of considerable importance. Transcription factors such as SLUG, β-catenin/TCF-LEF, PAX8 and VHL have been implicated in the re-activation of L1CAM in various types of cancers. There is increasing evidence that micro-RNAs can also have strong effects on gene expression. Here we have identified miR-21-3p as a positive regulator of L1CAM expression. Over-expression of miR-21-3p (miR-21*) but not the complementary sequence miR-21-5p (miR-21) could strongly augment L1CAM expression in renal, endometrial and ovarian carcinoma derived cell lines by an unknown mechanism involving transcriptional activation of the L1CAM gene. In patient cohorts from renal, endometrial and ovarian cancers we observed a strong positive correlation of L1CAM and miR-21-3p expressions. Although L1CAM alone was a reliable marker for overall and disease free survival, the combination of L1CAM and miR-21-3p expressions strongly enhanced the predictive power. Our findings shed new light on the complex regulation of L1CAM in cancers and advocate the use of L1CAM/miR-21-3p for diagnostic application.

Antibody therapy to human L1CAM in a transgenic mouse model blocks local tumor growth but induces EMT

L1 cell adhesion molecule (L1CAM) is overexpressed in many human cancers, confers bad prognosis and augments cell motility, invasion and metastasis. Results from xenograft mouse models suggested that L1CAM antibodies might be promising tools for cancer therapy. Here, we generated human L1CAM‐transgenic mice to study therapeutic efficacy and putative side effects in a model system. We established three transgenic lines (M2, M3 and F4) expressing the human L1CAM transgene in brain, kidney and colon with decreasing intensity (M2, M3 > F4). The expression pattern was similar to that of L1CAM in humans. No interference of the transgene with the expression of endogenous L1CAM was observed. Immunohistochemical analysis revealed correct expression of the transgene in mouse cortex and collective duct of the kidney. Injection of 125I‐labeled L1CAM antibodies resulted in specific enrichment in the kidney but not in the brain. The injection of the therapeutic anti‐human L1CAM mAb L1‐9.3/2a into transgenic mice even at high doses did not cause behavioral changes or other side effects. Similar results were obtained using a mouse specific L1CAM mAb in normal mice. Tumor therapy experiments were performed using syngeneic mouse tumor cells (RET melanoma and Panc02 pancreatic adenocarcinoma) transduced with human L1CAM. MAb L1‐9.3/2a efficiently and specifically attenuated local tumor growth in both model systems without apparent side effects. The therapeutic effect was dependent on immune effector mechanisms. Analysis of Panc02‐huL1CAM tumors after therapy showed elevated levels of EGF and evidence of immune‐induced epithelial‐mesenchymal transition. The results suggest that our transgenic mice are valuable tools to study L1CAM‐based antibody therapy.

L1CAM is expressed in triple-negative breast cancers and is inversely correlated with androgen receptor.

BackgroundBreast cancer is a heterogeneous disease displaying distinct molecular features and clinical outcome. The molecular profile of triple-negative breast cancers (TNBCs) overlaps with that of basal-like breast cancers that in turn show similarities with high-grade serous ovarian and endometrial carcinoma. L1CAM is an established biomarker for the latter cancers and we showed before that approximately 18% of primary breast cancers are positive for L1CAM and have a bad prognosis. Here we analysed the expression of L1CAM breast cancer subtypes.MethodsWe analyzed mRNA and protein expression data from different breast cancer cohorts for L1CAM, estrogen receptor, progesterone receptor, Her-2 and Androgen receptor (AR) and correlated the data. We performed Western blot analysis on tumor cell lysates and carried out chromatin-immuno-precipitation (CHIP) after AR overexpression.ResultsWe find that L1CAM is expressed preferentially though not exclusively in TNBCs. Using the human cancer genome atlas database and two independent breast cancer cohorts we find that L1CAM is inversely correlated with androgen receptor (AR) expression. We found that L1CAMhighARlow primary breast tumors have the worst clinical outcome. Overexpression of AR in MDA-MB436 breast cancer cells decreased L1CAM expression at the protein and mRNA level and CHIP-analysis revealed binding of AR to the L1CAM promoter region.ConclusionsThese results suggest that L1CAM in breast cancer is under AR control. The data also strongly advocate the use of L1CAM assessment in breast cancer diagnosis. We suggest that L1CAM expression could be causally related to the bad prognosis of TNBCs.

Lack of CD24 expression in mice reduces the number of leukocytes in the colon

CD24 is an extensively glycosylated membrane protein that is linked to the membrane via a glycosyl-phosphatidylinositol (GPI)-anchor. In mice, CD24 is expressed by hematopoietic and non-hematopoietic cells. CD24-/- mice do not have gross immunological defects, but detailed analysis revealed strongly reduced responses in an experimental autoimmune encephalomyelitis (EAE) model and a massive proliferation of T cells under lymphopenic conditions. It was also demonstrated that preB cells from CD24-/- mice are impaired in α4-integrin-mediated cell binding. Here we report that CD24-/- mice have strongly reduced numbers of leukocytes in the colon compared to wildtype mice. The reduction comprized all subpopulations. Leukocyte counts in spleen, mesenteric lymph nodes or small intestine were not significantly different. We find that beside leukocytes, CD24 is widely expressed in EpCAM+ epithelial and CD31+ endothelial cells of colon and small intestine. However, in CD24-/- mice the number of CD31+ endothelial cells in colons was strongly reduced and the number of epithelial cells was augmented. Leukocyte transfer experiments provided evidence that the CD24 status of recipient mice, rather than of the transferred cells, is crucial for leukocyte recruitment to the colon. We hypothesize that CD24 on colonic epithelial and endothelial cells is required for the retention and positioning of leukocytes most likely by affecting integrin function.

Abstract B3: CD24 affects tumor cell properties and cytokine secretion in a STAT3-dependent fashion

CD24 is a glycosyl-phosphatidylinositol (GPI)-anchored membrane protein frequently over-expressed in human carcinomas and associated with metastasis and poor prognosis. Previous work has shown that CD24 expression affects various cellular properties such as invasion, adhesion, proliferation and apoptosis in vitro as well as tumor growth in vivo. By analyzing underlying mechanisms of CD24-dependent tumor cell behaviour we discovered the influence of CD24 on STAT3 signaling and gene regulation via the tyrosine kinase c-Src. Altered activity of c-Src mediated by CD24 knock-down was responsible for reduced levels of phosphorylated STAT3 (Y705). Diminished activity of STAT3 was confirmed by luciferase-reporter assays. Silencing of c-Src, similar to CD24, reduced the expression of prototype STAT3-regulated genes. Likewise, the over-expression of CD24 augmented c-Src activity, the recruitment of c-Src into lipid rafts, levels of p-STAT3 and the expression of STAT3-dependent target genes. Functionally, depletion of CD24 reduced cell proliferation and increased apoptosis, suggesting a pivotal role of regulated target genes for these functional effects. Even more profoundly, CD24 expression was accompanied with the secretion of the STAT3 target IL-6. We could show a strong correlation between IL-6 secretion and CD24 expression in tumor cell lines. Furthermore, treatment with conditioned cell culture medium from CD24 over-expressing- or CD24 knock-down tumor cells initiated paracrine STAT3 signaling and gene regulation in the monocytic precursor cell line THP-1. This suggested a regulative role of CD24 on the tumor microenvironment. We propose that CD24 is capable of influencing not only tumor cellular properties but also has impact on the cytokine milieu of the tumor microenvironment. Citation Format: Niko P. Bretz, Alexei V. Salnikov, Gerhard Moldenhauer, Peter Altevogt. CD24 affects tumor cell properties and cytokine secretion in a STAT3-dependent fashion. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr B3.