The CD24 surface antigen in neural development and disease.

A cell's surface molecular signature enables its reciprocal interactions with the associated microenvironments in development, tissue homeostasis and pathological processes. The CD24 surface antigen (heat-stable antigen, nectadrin; small cell lung cancer antigen cluster-4) represents a prime example of a neural surface molecule that has long been known, but whose diverse molecular functions in intercellular communication we have only begun to unravel. Here, we briefly summarize the molecular fundamentals of CD24 structure and provide a comprehensive review of CD24 expression and functional studies in mammalian neural developmental systems and disease models (rodent, human). Striving for an integrated view of the intracellular signaling processes involved, we discuss the most pertinent routes of CD24-mediated signaling pathways and functional networks in neurobiology (neural migration, neurite extension, neurogenesis) and pathology (tumorigenesis, multiple sclerosis).

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 (125)I-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.

BACKGROUND: Breast 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. METHODS: We 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. RESULTS: We 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 L1CAM(high)AR(low) 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. CONCLUSIONS: These 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.

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, ß-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.

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.

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.

Role of miR-34a as a suppressor of L1CAM in endometrial carcinoma.

L1CAM promotes cell motility, invasion and metastasis formation in various human cancers and can be considered as a driver of tumor progression. Knowledge about genetic processes leading to the presence of L1CAM in cancers is of considerable importance. Experimentally, L1CAM expression can be achieved by various means. Over-expression of the transcription factor SLUG or treatment of cells with TGF-ß1 can induce or augment L1CAM levels in cancer cells. Likewise, hypomethylation of the L1CAM promoter on the X chromosome correlates with L1CAM expression. However, presently no mechanisms that might control transcriptional activity are known. Here we have identified miR-34a as a suppressor of L1CAM. We observed that L1CAM positive endometrial carcinoma (EC) cell lines HEC1B and SPAC1L lost L1CAM protein and mRNA by treatment with demethylating agents or knock-down of the DNA-methyltransferase-1 (DNMT1). Concomitantly, several miRNAs were up-regulated. Using miRNA profiling, luciferase reporter assays and mutagenesis, we identified miR-34a as a putative binder to the L1CAM-3'UTR. Over-expression of miR-34a in HEC1B cells blocked L1CAM expression and inhibited cell migration. In ECC1 cells (wildtype p53) the activation of p53 caused miR-34a up-regulation and loss of L1CAM expression that was miR-34a dependent. We observed an inverse correlation between L1CAM and miR-34a levels in EC cell lines. In primary tumor sections areas expressing high amounts of L1CAM had less miR-34a expression than those with low L1CAM levels. Our data suggest that miR-34a can regulate L1CAM expression by targeting L1CAM mRNA for degradation. These findings shed new light on the complex regulation of L1CAM in human tumors.

Body fluid exosomes promote secretion of inflammatory cytokines in monocytic cells via Toll-like receptor signaling.

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-1ß, 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.

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.

CD24 promotes tumor cell invasion by suppressing tissue factor pathway inhibitor-2 (TFPI-2) in a c-Src-dependent fashion.

CD24 is a glycosyl-phosphatidylinositol-anchored protein with mucin-type structure that resides exclusively in membrane microdomains. CD24 is often highly expressed in carcinomas and correlates with poor prognosis. Experimentally, the over-expression or depletion of CD24 alters cell proliferation, adhesion, and invasion in vitro and tumor growth in vivo. However, little is known about the mechanisms by which CD24 mediates these cellular effects. Here we have studied the mechanism of CD24-dependent cell invasion using transient CD24 knock-down or over-expression in human cancer cell lines. We show that CD24 depletion reduced tumor cell invasion and up-regulated expression of Tissue Factor Pathway Inhibitor 2 (TFPI-2), a potent inhibitor of extracellular matrix degradation that can block metastases formation and tumor cell invasion. Over-expression of CD24 in A125 cells resulted in reduced TFPI-2 expression and enhanced invasion. We provide evidence that the activity of c-Src is reduced upon CD24 knock-down. The silencing of c-Src, similar to CD24, was able to enhance TFPI-2 expression and reduce tumor cell invasion. An inverse expression of CD24 and TFPI-2 was observed by immunohistochemical analysis of primary breast cancers (N = 1,174). TFPI-2 expression was highest in CD24 negative samples and lowered with increasing CD24 expression. Patients with a CD24 low/TFPI-2 high phenotype showed significantly better survival compared to CD24 high/TFPI-2 low patients. Our results provide evidence that CD24 can regulate cell invasion via TFPI-2 and suggests a role of c-Src in this process.

CD24 Ala57Val polymorphism predicts pathologic complete response to sequential anthracycline- and taxane-based neoadjuvant chemotherapy for primary breast cancer.

Overexpression of CD24 is an independent prognostic factor for breast cancer. Recently, two polymorphisms in the CD24 gene were linked to disease risk and progression in autoimmune diseases. Here, we evaluated the clinical relevance of these polymorphisms with respect to their potential to predict a pathologic complete response (pCR) to neoadjuvant chemotherapy (NCT) for primary breast cancer (PBC), one of the strongest prognostic factors in this setting. A total of 257 patients were randomized to either doxorubicin/cyclophosphamide (AC) or doxorubicin/pemetrexed (AP), both followed by docetaxel (Doc) as NCT for T2-4 N0-2 M0 PBC as part of an international, multicenter, randomized phase II trial. CD24 polymorphisms were analyzed on germ line DNA and correlated with clinicopathologic variables and pCR. No significant associations were found between either of the polymorphisms and any of the clinicopathologic variables. In a multivariate analysis, CD24 Val/Val genotype was the only significant predictor of pCR (OR: 4.97; P = 0.003). The predictive potential was significant in both treatment arms and in the hormone receptor-positive subgroup. There was no correlation between CD24 3'UTR (TG/Del) genotype and pCR. We did not observe any association between CD24 genotype and CD24 protein expression or in vitro chemosensitivity, but there was a significant correlation between CD24 Val/Val and intratumoral lymphocyte aggregates. In conclusion, CD24 Ala/Val SNP is a strong and independent predictor of pCR after NCT for PBC and may affect immune functions rather than tumor characteristics. Further evaluation of the CD24 function and validation of its predictive potential are clearly warranted.

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 ß1-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 ß1 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 CD24(high) and CD24(low) transfectants was determined in three-dimensional ECMs. The percentage of invasive cells and their invasion depth was increased in CD24(high) cells compared with CD24(low) cells. Knockdown of CD24 and of the ß1-integrin subunit in CD24(high) cells decreased their invasiveness, indicating that the increased invasiveness is CD24- and ß1-integrin subunit-dependent. Fourier transform traction microscopy revealed that the CD24(high) cells generated 5-fold higher contractile forces compared with CD24(low) 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 CD24(high) cells but not in CD24(neg) cells. Moreover, after addition of lysophosphatidic acid or calyculin A, an increase in pre-stress in CD24(neg) cells was observed, which enhanced cellular invasiveness. In addition, inhibition of the Src kinase or STAT3 strongly reduced the invasiveness of CD24(high) cells, slightly reduced that of CD24(low) cells, and did not alter the invasiveness of CD24(neg) 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.

Oligodendrocytes secrete exosomes containing major myelin and stress-protective proteins: Trophic support for axons?

Oligodendrocytes synthesize the CNS myelin sheath by enwrapping axonal segments with elongations of their plasma membrane. Spatial and temporal control of membrane traffic is a prerequisite for proper myelin formation. The major myelin proteolipid protein (PLP) accumulates in late endosomal storage compartments and multivesicular bodies (MVBs). Fusion of MVBs with the plasma membrane results in the release of the intralumenal vesicles, termed exosomes, into the extracellular space. Here, we show that cultured oligodendrocytes secrete exosomes carrying major amounts of PLP and 2'3'-cyclic-nucleotide-phosphodiesterase (CNP). These exosomes migrated at the characteristic density of 1.10-1.14 g/mL in sucrose density gradients. Treatment of primary oligodendrocytes with the calcium-ionophore ionomycin markedly increased the release of PLP-containing exosomes, indicating that oligodendroglial exosome secretion is regulated by cytosolic calcium levels. A proteomic analysis of the exosomal fraction isolated by sucrose density centrifugation revealed in addition to PLP and CNP, myelin basic protein (MBP) and myelin oligodendrocyte glycoprotein (MOG) as constituents of oligodendroglial exosomes, together with a striking group of proteins with proposed functions in the relief of cell stress. Oligodendroglial exosome secretion may contribute to balanced production of myelin proteins and lipids, but in addition exosomes may embody a signaling moiety involved in glia-mediated trophic support to axons.