Engagement of variant CD44 confers resistance to anti-integrin antibody-mediated apoptosis in a colon carcinoma cell line

Ping-Pong-Tumor and Host in Pancreatic Cancer Progression

Metastasis is the main cause of high pancreatic cancer (PaCa) mortality and trials dampening PaCa mortality rates are not satisfying. Tumor progression is driven by the crosstalk between tumor cells, predominantly cancer-initiating cells (CIC), and surrounding cells and tissues as well as distant organs, where tumor-derived extracellular vesicles (TEX) are of major importance. A strong stroma reaction, recruitment of immunosuppressive leukocytes, perineural invasion, and early spread toward the peritoneal cavity, liver, and lung are shared with several epithelial cell-derived cancer, but are most prominent in PaCa. Here, we report on the state of knowledge on the PaCIC markers Tspan8, alpha6beta4, CD44v6, CXCR4, LRP5/6, LRG5, claudin7, EpCAM, and CD133, which all, but at different steps, are engaged in the metastatic cascade, frequently via PaCIC-TEX. This includes the contribution of PaCIC markers to TEX biogenesis, targeting, and uptake. We then discuss PaCa-selective features, where feedback loops between stromal elements and tumor cells, including distorted transcription, signal transduction, and metabolic shifts, establish vicious circles. For the latter particularly pancreatic stellate cells (PSC) are responsible, furnishing PaCa to cope with poor angiogenesis-promoted hypoxia by metabolic shifts and direct nutrient transfer via vesicles. Furthermore, nerves including Schwann cells deliver a large range of tumor cell attracting factors and Schwann cells additionally support PaCa cell survival by signaling receptor binding. PSC, tumor-associated macrophages, and components of the dysplastic stroma contribute to perineural invasion with signaling pathway activation including the cholinergic system. Last, PaCa aggressiveness is strongly assisted by the immune system. Although rich in immune cells, only immunosuppressive cells and factors are recovered in proximity to tumor cells and hamper effector immune cells entering the tumor stroma. Besides a paucity of immunostimulatory factors and receptors, immunosuppressive cytokines, myeloid-derived suppressor cells, regulatory T-cells, and M2 macrophages as well as PSC actively inhibit effector cell activation. This accounts for NK cells of the non-adaptive and cytotoxic T-cells of the adaptive immune system. We anticipate further deciphering the molecular background of these recently unraveled intermingled phenomena may turn most lethal PaCa into a curatively treatable disease.

Bioinformatics functional analysis of let-7a, miR-34a, and miR-199a/b reveals novel insights into immune system pathways and cancer hallmarks for hepatocellular carcinoma

Let-7a, miR-34a, and miR-199 a/b have gained a great attention as master regulators for cellular processes. In particular, these three micro-RNAs act as potential onco-suppressors for hepatocellular carcinoma. Bioinformatics can reveal the functionality of these micro-RNAs through target prediction and functional annotation analysis. In the current study, in silico analysis using innovative servers (miRror Suite, DAVID, miRGator V3.0, GeneTrail) has demonstrated the combinatorial and the individual target genes of these micro-RNAs and further explored their roles in hepatocellular carcinoma progression. There were 87 common target messenger RNAs (p ≤ 0.05) that were predicted to be regulated by the three micro-RNAs using miRror 2.0 target prediction tool. In addition, the functional enrichment analysis of these targets that was performed by DAVID functional annotation and REACTOME tools revealed two major immune-related pathways, eight hepatocellular carcinoma hallmarks-linked pathways, and two pathways that mediate interconnected processes between immune system and hepatocellular carcinoma hallmarks. Moreover, protein-protein interaction network for the predicted common targets was obtained by using STRING database. The individual analysis of target genes and pathways for the three micro-RNAs of interest using miRGator V3.0 and GeneTrail servers revealed some novel predicted target oncogenes such as SOX4, which we validated experimentally, in addition to some regulated pathways of immune system and hepatocarcinogenesis such as insulin signaling pathway and adipocytokine signaling pathway. In general, our results demonstrate that let-7a, miR-34a, and miR-199 a/b have novel interactions in different immune system pathways and major hepatocellular carcinoma hallmarks. Thus, our findings shed more light on the roles of these miRNAs as cancer silencers.

CD44/CD44v6 a Reliable Companion in Cancer-Initiating Cell Maintenance and Tumor Progression

Metastasis is the leading cause of cancer death, tumor progression proceeding through emigration from the primary tumor, gaining access to the circulation, leaving the circulation, settling in distant organs and growing in the foreign environment. The capacity of a tumor to metastasize relies on a small subpopulation of cells in the primary tumor, so called cancer-initiating cells (CIC). CIC are characterized by sets of markers, mostly membrane anchored adhesion molecules, CD44v6 being the most frequently recovered marker. Knockdown and knockout models accompanied by loss of tumor progression despite unaltered primary tumor growth unraveled that these markers are indispensable for CIC. The unexpected contribution of marker molecules to CIC-related activities prompted research on underlying molecular mechanisms. This review outlines the contribution of CD44, particularly CD44v6 to CIC activities. A first focus is given to the impact of CD44/CD44v6 to inherent CIC features, including the crosstalk with the niche, apoptosis-resistance, and epithelial mesenchymal transition. Following the steps of the metastatic cascade, we report on supporting activities of CD44/CD44v6 in migration and invasion. These CD44/CD44v6 activities rely on the association with membrane-integrated and cytosolic signaling molecules and proteases and transcriptional regulation. They are not restricted to, but most pronounced in CIC and are tightly regulated by feedback loops. Finally, we discuss on the engagement of CD44/CD44v6 in exosome biogenesis, loading and delivery. exosomes being the main acteurs in the long-distance crosstalk of CIC with the host. In brief, by supporting the communication with the niche and promoting apoptosis resistance CD44/CD44v6 plays an important role in CIC maintenance. The multifaceted interplay between CD44/CD44v6, signal transducing molecules and proteases facilitates the metastasizing tumor cell journey through the body. By its engagement in exosome biogenesis CD44/CD44v6 contributes to disseminated tumor cell settlement and growth in distant organs. Thus, CD44/CD44v6 likely is the most central CIC biomarker.

Hyaluronan Controls the Deposition of Fibronectin and Collagen and Modulates TGF-β1 Induction of Lung Myofibroblasts

The contribution of hyaluronan-dependent pericellular matrix to TGF-β1-driven induction and maintenance of myofibroblasts is not understood. Hyaluronan is an extracellular matrix (ECM) glycosaminoglycan important in cell adhesion, proliferation and migration, and is implicated in myofibroblast formation and maintenance. Reduced turnover of hyaluronan has been linked to differentiation of myofibroblasts and potentiation of lung fibrosis. Fibronectin is a fibril forming adhesive glycoprotein that is also upregulated following induction with TGF-β1. Although they are known to bind each other, the interplay between hyaluronan and fibronectin in the pericellular matrix during myofibroblast induction and matrix assembly is not clear. This study addresses the role of hyaluronan and its interaction with fibrillar matrix components during myofibroblast formation. Hyaluronan and fibronectin were increased and co-localized in the ECM following myofibroblast induction by TGF-β1. Inhibition of hyaluronan synthesis in TGF-β1-induced lung myofibroblasts over a 4day period with 4-methyl umbelliferone (4-MU) further enhanced myofibroblast morphology, caused increased deposition of fibronectin and type I collagen in the ECM, and increased expression of alpha-smooth muscle actin and hyaluronan synthase 2 (HAS2) mRNA. Hyaluronan oligosaccharides or hyaluronidase treatment, which more effectively disrupted the pericellular matrix, had similar effects. CD44 and β1 integrins co-localized in the cell membrane and along some stress fibers. However, CD44 and hyaluronan were specifically excluded from focal adhesions, and associated primarily with cortical actin. Time-lapse imaging of the immediate effects of hyaluronidase digestion showed that hyaluronan matrix primarily mediates attachment of membrane and cortical actin between focal contacts, suggesting that surface adhesion through hyaluronan and CD44 is distinct from focal adhesion through β1 integrins and fibronectin. Fluorescein-labeled hyaluronan bound regularly along fibronectin fibers and co-localized more with β1 integrin and less with CD44. Therefore, the hyaluronan matrix can interfere with the assembly of fibrillar ECM components, and this interplay regulates the degree of myofibroblast formation. These data also suggest that adhesion through hyaluronan matrix impacts cytoskeletal organization, and is potentially part of a clutch mechanism that regulates stick and slip of myofibroblasts by affecting the adhesion to and organization of fibronectin and collagen.

Curcumin induced nanoscale CD44 molecular redistribution and antigen-antibody interaction on HepG2 cell surface

The cell surface glycoprotein CD44 was implicated in the progression, metastasis and apoptosis of certain human tumors. In this study, we used atomic force microscope (AFM) to monitor the effect of curcumin on human hepatocellular carcinoma (HepG2) cell surface nanoscale structure. High-resolution imaging revealed that cell morphology and ultrastructure changed a lot after being treated with curcumin. The membrane average roughness increased (10.88 ± 4.62 nm to 129.70 ± 43.72 nm) and the expression of CD44 decreased (99.79 ± 0.16% to 75.14 ± 8.37%). Laser scanning confocal microscope (LSCM) imaging showed that CD44 molecules were located on the cell membrane. The florescence intensity in control group was weaker than that in curcumin treated cells. Most of the binding forces between CD44 antibodies and untreated HepG2 cell membrane were around 120-220 pN. After being incubated with curcumin, the major forces focused on 70-150 pN (10 μM curcumin-treated) and 50-120 pN (20 μM curcumin-treated). These results suggested that, as result of nanoscale molecular redistribution, changes of the cell surface were in response to external treatment of curcumin. The combination of AFM and LSCM could be a powerful method to detect the distribution of cell surface molecules and interactions between molecules and their ligands.

CD44: can a cancer-initiating cell profit from an abundantly expressed molecule?

Can an abundantly expressed molecule be a reliable marker for the cancer-initiating cells (CICs; also known as cancer stem cells), which constitute the minority of cells within the mass of a tumour? CD44 has been implicated as a CIC marker in several malignancies of haematopoietic and epithelial origin. Is this a fortuitous coincidence owing to the widespread expression of the molecule or is CD44 expression advantageous as it fulfils some of the special properties that are displayed by CICs, such as self-renewal, niche preparation, epithelial-mesenchymal transition and resistance to apoptosis?

Clinical significance of pAKT and CD44v6 overexpression with breast cancer

PURPOSE

CD44v6 contributes to apoptosis resistance and metastasis via upregulated activity of the PI3K/AKT pathway. The purpose of this study was to investigate the expression patterns and predictive value of phosphorylated AKT (pAKT) and CD44v6 in breast cancer tissues.

METHODS

Expression of pAkt and CD44v6 protein was detected using immunohistochemistry in breast cancer tissues and lymph node tissues in 98 patients. The correlation between the expression of pAkt, CD44v6 and other disease-related characteristics was investigated. The prognostic value of pAKT and CD44v6 overexpression for overall survival (OS) and disease free survival (DFS) was determined.

RESULTS

37 cases (37.8%) were observed as positive for pAkt expression, and 38 cases (38.8%) for CD44V6 overexpression. Staining was positive for pAKT in 20 of 38 (52.6%) CD44v6(+) and 17 of 60 (28.3%) CD44v6(-) (P = 0.016). A progressively increased trend of CD44v6 overexpression was observed from node(-) primary breast cancer tissues to node(+) primary cancer tissues to node(+) lymph tissues (P = 0.000). In univariate analysis lymph node status, pAKT and CD44V6 were significantly associated with a decreased OS and DFS. Multivariate analysis revealed that CD44v6 status was associated with a reduced OS (P = 0.003), and pAKT and CD44v6 with a shorter DFS (P = 0.012 and 0.016, respectively).

CONCLUSIONS

Our data suggested that there had an intensive relationship between pAKT and CD44v6 expression in breast cancer tissues. The overexpression of CD44v6 was an independent prognostic marker for predicting OS and DFS of breast cancer patients.

Expression of extra trinucleotide in CD44 variant of rheumatoid arthritis patients allows generation of disease-specific monoclonal antibody

Selective targeting of cells engaged in pathological activities is a major challenge for medical research. We generated monoclonal antibodies (mAbs) that exclusively bind, at concentrations ranging from 2 to 100 microg/ml, to a modified CD44 variant (designated CD44vRA) expressed on synovial fluid cells from joints of rheumatoid arthritis (RA) patients. These mAbs cross-reacted with keratinocytes expressing wild type CD44vRA (CD44v3-v10) only at a relatively high concentration (200 microg/ml). Sequence analysis of CD44vRA cDNA revealed, in 33 out of 43 RA and psoriatic arthritis patients, an extra intron-derived trinucleotide, CAG, which allows translation of an extra alanine. This insertion imposes a configurational change on the cell surface CD44 of RA synovial fluid cells, creating an immunogenic epitope and potentiating the ability to produce disease-specific antibodies. Indeed, the anti-CD44vRA mAbs (designated F8:33) were able to induce apoptosis in synovial fluid cells from RA patients, but not in peripheral blood leukocytes from the same patients, in keratinocytes from normal donors or in synovial fluid cells from osteoarthritis patients. Furthermore, injection of anti-CD44vRA mAbs reduced joint inflammation in DBA/1 mice with collagen-induced arthritis. These findings show that anti-CD44vRA mAbs are both bioactive and RA-specific.

A novel region of amplification at 11p12-13 in gastric cancer, revealed by representational difference analysis, is associated with overexpression of CD44v6, especially in early-onset gastric carcinomas

Diffuse-type gastric carcinomas (GCs) are often difficult to characterize because of contamination of tumor samples by surrounding normal tissue. As such, information regarding chromosomal aberrations in this subtype of GCs is limited. In this study, we used representational difference analysis to pinpoint genomic amplifications occurring in diffuse-type GCs. We found nine differential products from two novel regions of amplification in two tumors: one product mapped to 19p13.1 and eight mapped to a 1.8-Mb region in chromosomal segment 11p12-13. These amplifications were confirmed using Southern blot analysis and occurred in 3/16 and 6/15 diffuse-type GCs, respectively. CD44, a well characterized cellular adhesion molecule involved in several human malignancies, is encoded by a gene located within 200 kb of the 11p12-13 amplification fragments. We confirmed that overexpression of isoform CD44v6 was correlated with amplification at 11p12-13 in 11/12 diffuse-type GCs. Since diffuse-type GCs occur more frequently in early-onset gastric carcinomas (EOGCs, presented at 45 years of age or younger) than in "conventional" GCs, and the tumors carrying the original amplifications were EOGCs, we investigated overexpression of CD44v6 in 107 EOGCs and 88 conventional GCs using tissue microarrays. We found frequent CD44v6 overexpression in both tumor groups (76% and 57% respectively) and, interestingly, significantly more cases with overexpression of CD44v6 in EOGCs than in conventional GCs (P = 0.005), irrespective of histology. These findings provide further evidence for both the relevance of CD44 in GC and for distinct molecular characteristics of EOGCs when compared with those of GCs occurring at a later age.

CD44 promotes resistance to apoptosis in murine colonic epithelium

Dysregulated expression of CD44 isoforms occurs consistently in colon carcinogenesis, and this change occurs also in most other types of cancer. One of the basic features of malignant transformation is the acquisition of resistance to apoptosis. We previously found that the colonic epithelium of mice, deficient in CD44 is predisposed to apoptosis. In this study, we asked whether the expression of CD44 alters the response of the colon to an apoptotic stimulus, and what are the mechanisms involved. For this, we assessed the susceptibility of the murine colon to apoptosis by total body irradiation to induce apoptosis. Apoptotic and concomitant changes relevant to the mechanisms of apoptosis were monitored by molecular markers of apoptosis. We found enhanced susceptibility to apoptosis in CD44 deficient colonic epithelium based on an increase in the number of apoptotic bodies, and activation of caspase 3. This was not associated with alterations in proliferations as shown by comparable Ki-67 expression and BrdU labeling. Furthermore, upregulated active caspase 3 in CD44 deficient colon was accompanied by concomitant molecular alterations in caspase 9 and not caspase 8, and this indicated the involvement of the mitochondrial pathway in apoptosis execution. Overall, this is the first report demonstrating CD44 mediated resistance to apoptosis in the colonic epithelium in vivo. This implicates CD44 in promoting cell transformation into a malignant phenotype, in conjunction with other anti-apoptotic factors.

Functional proteomic screen identifies a modulating role for CD44 in death receptor-mediated apoptosis

Apoptotic evasion is a hallmark of cancer and its resistance to chemotherapeutic drugs. Identification of cellular proteins that mediate apoptotic programs is a critical step toward the development of therapeutics aimed at overcoming apoptosis resistance. We developed an innovative high-throughput screen to identify proteins that modulate Fas ligand-mediated apoptosis using fluorophore-assisted light inactivation (HTS-FALIpop). The FALI protein knockdown strategy was coupled to a caspase activity assay with the ability to detect both proapoptotic and antiapoptotic surface molecules expressed by HT-1080 human fibrosarcoma cells. FALI of the Fas receptor (Fas/CD95) using a fluorescein-conjugated anti-Fas antibody abrogated Fas ligand-mediated caspase activation. Ninety-six single-chain variable fragment antibodies (scFv), selected for binding to the surface of HT-1080 cells, were screened by HTS-FALIpop. Three of the scFvs caused decreases in caspase induction after FALI of their protein targets. One of the targets of these positive scFvs was identified as CD44 and was validated by performing FALI using a CD44-specific monoclonal antibody, which resulted in similar protection from Fas apoptosis. CD44-targeted FALI was antiapoptotic in multiple human cancer cell lines, including both Fas signaling type I and II cells, and was also protective against other ligands of the tumor necrosis factor death receptor family. FALI of CD44 inhibited formation and activation of the death-inducing signaling complex, suggesting that CD44 regulates Fas at the cell surface. This mechanism of death receptor regulation represents a novel means of apoptosis modulation that could be exploited by pharmacologic agents.

CD44 in cancer progression: adhesion, migration and growth regulation

It is well established that the large array of functions that a tumour cell has to fulfil to settle as a metastasis in a distant organ requires cooperative activities between the tumour and the surrounding tissue and that several classes of molecules are involved, such as cell-cell and cell-matrix adhesion molecules and matrix degrading enzymes, to name only a few. Furthermore, metastasis formation requires concerted activities between tumour cells and surrounding cells as well as matrix elements and possibly concerted activities between individual molecules of the tumour cell itself. Adhesion molecules have originally been thought to be essential for the formation of multicellular organisms and to tether cells to the extracellular matrix or to neighbouring cells. CD44 transmembrane glycoproteins belong to the families of adhesion molecules and have originally been described to mediate lymphocyte homing to peripheral lymphoid tissues. It was soon recognized that the molecules, under selective conditions, may suffice to initiate metastatic spread of tumour cells. The question remained as to how a single adhesion molecule can fulfil that task. This review outlines that adhesion is by no means a passive task. Rather, ligand binding, as exemplified for CD44 and other similar adhesion molecules, initiates a cascade of events that can be started by adherence to the extracellular matrix. This leads to activation of the molecule itself, binding to additional ligands, such as growth factors and matrix degrading enzymes, complex formation with additional transmembrane molecules and association with cytoskeletal elements and signal transducing molecules. Thus, through the interplay of CD44 with its ligands and associating molecules CD44 modulates adhesiveness, motility, matrix degradation, proliferation and cell survival, features that together may well allow a tumour cell to proceed through all steps of the metastatic cascade.

CD44v6 promotes proliferation by persisting activation of MAP kinases

CD44v6 is transiently expressed during T cell activation, and constitutively CD44v4-v7 expressing transgenic T cells show accelerated responses towards nominal antigens. The underlying mechanism is unknown. The mouse thymoma EL4 was transfected with CD44 standard isoform (CD44s) or CD44v6 cDNA (EL4-s, EL4-v6). Only EL4-v6 cells proliferated at an over 10-fold higher rate than untransfected cells, displayed up-regulated expression of CD69, CD25, and IL-2, and were protected from apoptosis by CD44v6 cross-linking. In the absence of any stimulus, ERK1/2 was partly phosphorylated, and phosphorylation was significantly increased by CD44v6 cross-linking. The same accounted for JNK, c-jun, and IkappaBalpha. Moreover, NF-kappaB was partly translocated into the nucleus. Instead, CD44s cross-linking induced ERK1/2, JNK, c-jun, and IkappaBalpha phosphorylation only in the context of TCR engagement. No selectively CD44v6 associated transmembrane proteins were uncovered in EL4 cells. However, CD44v6, as opposed to CD44s, did not colocalise with the TCR/CD3 complex after CD3 cross-linking. Furthermore, a CD44-associated 85-kDa protein became hypophosphorylated only after CD44v6 cross-linking. Threonine hypophosphorylation of this protein coincided with the activation of MAP and SAP kinases, which was prohibited in the presence of a phosphatase inhibitor. Thus, CD44v6, distinct to CD44s, stimulates autonomously growth and IL-2 secretion of a thymoma line and rescues cells from apoptosis.

CD44 negatively regulates apoptosis in murine colonic epithelium via the mitochondrial pathway

Regulation of epithelial cell proliferation and apoptosis are important determinants of colonic crypt homeostasis, and their dysregulations are key features of colon cancer. In this study, we investigated whether CD44, an adhesion protein overexpressed in colon cancer, plays a role in colonocyte proliferation and apoptosis, and the molecular mechanisms involved in these processes. Using a CD44 knockout mouse model devoid of a gross phenotype, we found that CD44 null colonocytes have alterations at the ultrastructural and molecular levels. Mitochondria in CD44 null colonocytes at the top of the crypt have disrupted cristae. The ratio of anti-apoptotic Bcl-xl to pro-apoptotic Bak was shifted toward apoptosis in CD44 null colon due to decreased Bcl-xl expression. Caspase 9 was upregulated and active in CD44 null colon. Its expression shifted from a location restricted to the top of the control crypts to the whole crypt axis in CD44 null colon. Caspase 3 was also activated in CD44 null colon suggesting that CD44 null colonocytes are apoptotic via the intrinsic pathway. Cell cycle regulators, cyclin A, p21, and pRb protein were abrogated in CD44 null mice. Overall, CD44 negatively regulates apoptosis via the mitochondrial pathway in the colonic epithelium through the regulators/effectors of cell cycle and apoptosis.

CD44 in cancer

CD44 is a multistructural and multifunctional cell surface molecule involved in cell proliferation, cell differentiation, cell migration, angiogenesis, presentation of cytokines, chemokines, and growth factors to the corresponding receptors, and docking of proteases at the cell membrane, as well as in signaling for cell survival. All these biological properties are essential to the physiological activities of normal cells, but they are also associated with the pathologic activities of cancer cells. Experiments in animals have shown that targeting of CD44 by antibodies, antisense,and CD44-soluble proteins markedly reduces the malignant activities of various neoplasms, stressing the therapeutic potential of anti-CD44 agents. Furthermore, because alternative splicing and posttranslational modifications generate many different CD44 sequences, including, perhaps, tumor-specific sequences, the production of anti-CD44 tumor-specific agents may be a realistic therapeutic approach. However, in many cancers (renal cancer and non-Hodgkin's lymphomas are exceptions), a high level of CD44 expression is not always associated with an unfavorable outcome. On the contrary, in some neoplams CD44 upregulation is associated with a favorable outcome. Even worse, in many cases different research grows analyzing the same neoplastic disease reached contradictory conclusions regarding the correlation between CD44 expression and disease prognosis, possibly due to differences in methodology. These problems must be resolved before applying anti-CD44 therapy to human cancers.

Hyaluronidase can modulate expression of CD44

CD44 is a family of transmembrane glycoproteins with multiple isoforms generated by alternative exon splicing of a single gene. CD44 and its variants are expressed on a wide variety of cells including cancer cells. The mechanisms by which splice variant exons are selected are unknown. The presence of hyaluronan in the environment of the cell appears to influence that selection process. The expression of particular splice variants of CD44 as well as the simultaneous presence of hyaluronan is important for motility, invasion, and the metastatic spread of some tumors. The influence of hyaluronidase digestion on the expression of CD44 in human cancer cell lines was examined. CD44 isoforms containing alternatively spliced exons were sensitive to hyaluronidase digestion in all lines examined, but differences between cell lines were observed. Expression of CD44s, the standard form, was resistant to digestion in two of three cell lines. A tentative model was formulated proposing that CD44 isoforms containing splice variants are unstable, requiring the continuous presence of ligand for expression. CD44s is relatively more stable, not requiring the continuous presence of hyaluronan. Additionally, a number of new CD44 variant isoforms, not previously observed, were identified.

Apoptosis, cell adhesion and the extracellular matrix in the three-dimensional growth of multicellular tumor spheroids

In the last few years, it has become increasingly apparent that cell survival and death, especially apoptosis, strongly depend on cell adhesion and the extracellular matrix. In addition, it has also become clear that the use of three-dimensional multicellular tumor spheroids, which mimick more closely solid tumors in vivo, are a realistic experimental model to investigate many aspects of tumor biology. In the present review, after a general overview of the current knowledge regarding apoptosis, cell adhesion and the extracellular matrix, the results obtained utilizing multicellular tumor spheroids in these types of studies are discussed. The main conclusion that may be drawn from a synthesis of the literature on these topics is that investigations with multicellular tumor spheroids yield much useful information that is sometimes in contradiction to that obtained with monolayer cultures, but is closer to that derived from in vivo studies. Consequently, the authors encourage that these three-dimensional systems be used in many studies in which cell death and adhesion are being examined.

Up-regulation of the tumour-associated marker CD44V6 in experimental kidney disease

CD44 is an adhesion molecule involved in a wide range of cell-cell and cell-matrix interactions. The standard form of CD44 (CD44S) is a 85-90-kD glycoprotein, but alternative splicing of RNA encoding 10 variable exons (V1-V10) can give rise to many different CD44 variant protein isoforms of higher molecular weight. CD44 isoforms containing the V6 exon play a crucial role in tumour metastasis and lymphocyte activation. However, the role of CD44V6 in the kidney is unknown. The aim of this study was to examined renal CD44V6 expression in health, disease and in vitro. Immunohistochemistry staining with the V6-specific 1.1ASML antibody identified constitutive CD44V6 expression by occasional cortical tubular epithelial cells and medullary tubules in normal rat kidney. In immune-induced kidney disease (rat anti-glomerular basement membrane glomerulonephritis), there was a marked increase in CD44V6 expression by cortical tubules, particularly in areas of tubulointerstitial damage, which was associated with focal macrophage infiltration. There was also a marked increase in CD44V6 expression by damaged tubules in a model of non-immune kidney disease (unilateral ureteric obstruction). Reverse transcription-polymerase chain reaction revealed a complex pattern of CD44V6-containing mRNA isoforms in normal rat kidney. This pattern of CD44V6 splicing was essentially unaltered in disease. The NRK52E normal rat kidney tubular epithelial cell line expresses both CD44S and CD44V6. Stimulation of NRK52E cells with IL-1 or transforming growth factor-beta 1 induced a two-to-five-fold increase in the expression of both CD44S and CD44V6. Furthermore, triggering of NRK52E cells by antibodies to CD44S or CD44V6, but not isotype control antibodies, induced secretion of monocyte chemoattractant protein-1. In conclusion, this study has identified expression of the tumour-associated marker CD44V6 in tubular epithelial cells in normal and diseased rat kidney, and suggests that signalling through the CD44V6 molecule may participate in the pathogenesis of experimental kidney disease.

CD44 cell adhesion molecules

The CD44 proteins form a ubiquitously expressed family of cell surface adhesion molecules involved in cell-cell and cell-matrix interactions. The multiple protein isoforms are encoded by a single gene by alternative splicing and are further modified by a range of post-translational modifications. CD44 proteins are single chain molecules comprising an N-terminal extracellular domain, a membrane proximal region, a transmembrane domain, and a cytoplasmic tail. The CD44 gene has only been detected in higher organisms and the amino acid sequence of most of the molecule is highly conserved between mammalian species. The principal ligand of CD44 is hyaluronic acid, an integral component of the extracellular matrix. Other CD44 ligands include osteopontin, serglycin, collagens, fibronectin, and laminin. The major physiological role of CD44 is to maintain organ and tissue structure via cell-cell and cell-matrix adhesion, but certain variant isoforms can also mediate lymphocyte activation and homing, and the presentation of chemical factors and hormones. Increased interest has been directed at the characterisation of this molecule since it was observed that expression of multiple CD44 isoforms is greatly upregulated in neoplasia. CD44, particularly its variants, may be useful as a diagnostic or prognostic marker of malignancy and, in at least some human cancers, it may be a potential target for cancer therapy. This review describes the structure of the CD44 gene and discusses some of its roles in physiological and pathological processes.