Frans C. S. Ramaekers

Annexin V‐Affinity assay: A review on an apoptosis detection system based on phosphatidylserine exposure

Apoptosis is a programmed, physiological mode of cell death that plays an important role in tissue homeostasis. Understanding of the basic mechanisms that underlie apoptosis will point to potentially new targets of therapeutic treatment of diseases that show an imbalance between cell proliferation and cell loss. In order to conduct such research, techniques and tools to reliably identify and enumerate death by apoptosis are essential. This review focuses on a novel technique to detect apoptosis by targeting for the loss of phospholipid asymmetry of the plasma membrane. It was recently shown that loss of plasma membrane asymmetry is an early event in apoptosis, independent of the cell type, resulting in the exposure of phosphatidylserine (PS) residues at the outer plasma membrane leaflet. Annexin V was shown to interact strongly and specifically with PS and can be used to detect apoptosis by targeting for the loss of plasma membrane asymmetry. Labeled annexin V can be applied both in flow cytometry and in light microscopy in both vital and fixed material by using appropriate protocols. The annexin V method is an extension to the current available methods. This review describes the basic mechanisms underlying the loss of membrane asymmetry during apoptosis and discusses the novel annexin V-binding assay.

Immunocytochemical detection and mapping of a cytokeratin 18 neo-epitope exposed during early apoptosis

A neo‐epitope in cytokeratin 18 (CK18) that becomes available at an early caspase cleavage event during apoptosis and is not detectable in vital epithelial cells is characterized. The monoclonal antibody M30, specific for this site, can be utilized specifically to recognize apoptotic cells, which show cytoplasmic cytokeratin filaments and aggregates after immunohistochemistry with M30, while viable and necrotic cells are negative. The number of cells recognized by the antibody increases after induction of apoptosis in exponentially growing epithelial cell lines and immunoreactivity is independent of the phosphorylation state of the cytokeratins. The generation of the M30 neo‐epitope occurs early in the apoptotic cascade, before annexin V reactivity or positive DNA nick end labelling. In a flow cytometric assay, the majority of the M30‐positive cells appear in the ‘apoptotic’ subG1 peak. Tests with synthetic peptides define positions 387–396 of CK18, with a liberated C‐terminus at the caspase cleavage site DALD‐S, as the ten‐residue epitope of M30. This epitope starts at the end of coil 2 of the predicted CK18 structure, at a probable hinge region, compatible with the sensitivity to proteolytic cleavage. The definition of a specific caspase cleavage site in CK18 as a neo‐epitope can be used for quantification of apoptotic epithelial cells with immunocytochemical techniques and is applicable to both fresh and formalin‐fixed material. Copyright

A novel assay to measure loss of plasma membrane asymmetry during apoptosis of adherent cells in culture.

Early during the process of apoptosis, cells lose their phospholipid membrane asymmetry and expose phosphatidylserine (PS) at the cell surface while maintaining their plasma membrane integrity intact. This process can be monitored for suspended cell types by using annexin V-FITC, which is a Ca(2+)-dependent, phospholipid-binding protein with high affinity for PS, and flow cytometry. If adherent cell types are to be studied for this apoptosis-associated phenomenon, then a problem is encountered, in that specific membrane damage occurs during harvesting. In this paper, a flow cytometric-based method is described that allows the measurement of loss of phospholipid asymmetry during apoptosis of adherent cells in culture. The method relies on the phospholipid binding property of biotinylated annexin V. Furthermore, the use of this conjugate allows tricolor flow cytometric analysis of apoptosis. Employing the method to MR65 cells, which were initiated by olomoucine to enter apoptosis, it is shown that PS exposure occurs early after the onset of apoptosis and, at the prevalent time-resolution, that PS exposure is accompanied by loss of both cytokeratin and DNA. The annexin V+ cells appear as a characteristic sub-G1 peak in the DNA histogram.

Contribution of mutations in the cytochrome P450 14α-demethylase (Erg11p, Cyp51p) to azole resistance in Candida albicans

The cytochrome P450 14alpha-demethylase, encoded by the ERG11 (CYP51) gene, is the primary target for the azole class of antifungals. Changes in the azole affinity of this enzyme caused by amino acid substitutions have been reported as a resistance mechanism. Nine Candida albicans strains were used in this study. The ERG11 base sequence of seven isolates, of which only two were azole-sensitive, were determined. The ERG11 base sequences of the other two strains have been published previously. In these seven isolates, 12 different amino acid substitutions were identified, of which six have not been described previously (A149V, D153E, E165Y, S279F, V452A and G4655). In addition, 16 silent mutations were found. Two different biochemical assays, subcellular sterol biosynthesis and CO binding to reduced microsomal fractions, were used to evaluate the sensitivity of the cytochromes for fluconazole and itraconazole. Enzyme preparations from four isolates showed reduced itraconazole susceptibility, whereas more pronounced resistance to fluconazole was observed in five isolates. A three-dimensional model of C. albicans Cyp51p was used to position all 29 reported substitutions, 98 in total identified in 53 sequences. These 29 substitutions were not randomly distributed over the sequence but clustered in three regions from amino acids 105 to 165, from 266 to 287 and from 405 to 488, suggesting the existence of hotspot regions. Of the mutations found in the two N-terminal regions only Y132H was demonstrated to be of importance for azole resistance. In the C-terminal region three mutations are associated with resistance, suggesting that the non-characterized substitutions found in this region should be prioritized for further analysis.

Marked differences in survival rate between smokers and nonsmokers with HPV 16-associated tonsillar carcinomas

Oncogenic human papillomavirus (HPV) is a causative agent in a subgroup of head and neck carcinomas, particularly tonsillar squamous cell carcinomas (TSCC). This study was undertaken because controversial data exist on the physical status of HPV‐DNA and the use of p16INK4A overexpression as surrogate HPV marker, and to examine the impact of HPV and tobacco consumption on the clinical course of TSCC. Tissue sections of 81 TSCC were analyzed by HPV 16‐specific fluorescence in situ hybridization (FISH) and p16INK4A‐specific immunohistochemistry. Results were correlated with clinical and demographic data. HPV 16 integration was detected by FISH as punctate signals in 33 out of 81 (41%) TSCC, 32 of which showed p16INK4A accumulation. Only 5 out of 48 HPV‐negative tumors showed p16INK4A immunostaining (p < 0.0001). The presence of HPV furthermore correlates significantly with low tobacco (p = 0.002) and alcohol intake (p = 0.011), poor differentiation grade (p = 0.019), small tumor size (p = 0.024), presence of a local metastasis (p = 0.001) and a decreased (loco)regional recurrence rate (p = 0.039). Statistical analysis revealed that smoking significantly increases the risk of cancer death from TSCC and that non‐smoking patients with HPV‐containing TSCC show a remarkably better disease‐specific survival rate. HPV 16 is integrated in 41% of TSCC and strongly correlates with p16INK4A overexpression, implicating the latter to be a reliable HPV biomarker. Patients with HPV‐positive tumors show a favorable prognosis as compared to those with HPV‐negative tumors, but tobacco use is the strongest prognostic indicator. These findings indicate that oncogenic processes in the tonsils of non‐smokers differ from those occurring in smokers, the former being related to HPV 16 infection.

A subset of head and neck squamous cell carcinomas exhibits integration of HPV 16/18 DNA and overexpression of p16INK4A and p53 in the absence of mutations in p53 exons 5-8.

Besides well‐known risk factors such as tobacco use and alcohol consumption, oncogenic human papillomavirus (HPV) infection also has recently been suggested to promote head and neck tumorigenesis. HPV is known to cause cancer by inactivation of cell cycle regulators p53 and pRb via expression of viral oncoproteins E6 and E7. This indicates that p53 mutations are not a prerequisite in HPV‐induced tumor development. However, discrepancy exists with respect to the frequency of head and neck squamous cell carcinomas (HNSCC) harboring DNA of oncogenic HPV and the fraction of these tumors showing p53 mutations. In our study, we examined the frequency of HNSCC demonstrating HPV 16/18 integration as identified by fluorescence in situ hybridization (FISH) and investigated their p53 (mutation) status by immunohistochemistry and single‐strand conformation polymorphism (SSCP) analysis of exons 5–8. Paraffin‐embedded, archival biopsy material from 27 premalignant mucosal lesions and 47 cases of HNSCC were analyzed. Ten of the 47 (21%) HNSCC unequivocally exhibited HPV 16 integration, including 8 of 12 (67%) tonsillar carcinomas. This is supported by the immunohistochemical detection of p16INK4A overexpression in all 10 HPV‐positive tumors. Although FISH is considered to be less sensitive than PCR‐based methods for HPV detection, our data clearly demonstrate clonal association of HPV with these tumors, as illustrated by the presence of integrated HPV 16 in both the primary tumor and their metastases in 2 patients. In contrast, HPV 16/18 DNA could not be detected in the premalignant lesions. In 30 of 47 (64%), HNSCC accumulation of p53 was observed, including 8 of the 10 HPV‐positive carcinomas. However, in none of the latter cases could mutations in exons 5–8 be identified, except for a polymorphism in codon 213 of exon 6 in one patient. Evaluation of clinical data revealed a significant inverse relation between tobacco use with or without alcohol consumption, and HPV positivity of the tumors.

The inner nuclear membrane protein Emerin regulates β‐catenin activity by restricting its accumulation in the nucleus

Emerin is a type II inner nuclear membrane (INM) protein of unknown function. Emerin function is likely to be important because, when it is mutated, emerin promotes both skeletal muscle and heart defects. Here we show that one function of Emerin is to regulate the flux of β‐catenin, an important transcription coactivator, into the nucleus. Emerin interacts with β‐catenin through a conserved adenomatous polyposis coli (APC)‐like domain. When GFP‐emerin is expressed in HEK293 cells, β‐catenin is restricted to the cytoplasm and β‐catenin activity is inhibited. In contrast, expression of an emerin mutant, lacking its APC‐like domain (GFP‐emerinΔ), dominantly stimulates β‐catenin activity and increases nuclear accumulation of β‐catenin. Human fibroblasts that are null for emerin have an autostimulatory growth phenotype. This unusual growth phenotype arises through enhanced nuclear accumulation and activity of β‐catenin and can be replicated in wild‐type fibroblasts by transfection with constitutively active β‐catenin. Our results support recent findings that suggest that INM proteins can influence signalling pathways by restricting access of transcription coactivators to the nucleus.

A- and B-type lamins are differentially expressed in normal human tissues

Abstract A selection of normal human tissues was investigated for the presence of lamins B1, B2, and A-type lamins, using a panel of antibodies specific for the individual lamin subtypes. By use of immunoprecipitation and two-dimensional immunoblotting techniques we demonstrated that these antibodies do not cross-react with other lamin subtypes and that a range of different phosphorylation isoforms is recognized by each antibody. The lamin B2 antibodies appeared to decorate the nuclear lamina in all tissues examined, except hepatocytes, in which very little lamin B2 expression was observed. In contrast to previous studies, which suggested the ubiquitous expression of lamin B1 in mammalian tissues, we show that lamin B1 is not as universally distributed throughout normal human tissues as was to be expected from previous studies. Muscle and connective tissues are negative, while in epithelial cells lamin B1 seemed to be preferentially detected in proliferating cells. These results correspond well with those obtained for lamin B1 in chicken tissues. The expression of A-type lamins is most prominent in well-differentiated epithelial cells. Relatively undifferentiated and proliferating cells in epithelia showed a clearly reduced expression of A-type lamins. Furthermore, most cells of neuroendocrine origin as well as most hematopoietic cells were negative for A-type lamin antibodies.

Keratins 7 and 20 as diagnostic markers of carcinomas metastatic to the ovary

The most common carcinomas metastatic to the ovary that mimic ovarian primaries are colonic adenocarcinomas and endometrial carcinomas. Conventional histochemical staining procedures, even in combination with additional immunohistochemical assays, are of limited value in distinguishing between these metastases and primary ovarian carcinomas. In this study we investigated whether the application of monoclonal antibodies against keratins 7, 8, and 20 could help in differentiating between these categories. The reactivity patterns of 40 carcinomas metastatic to the ovary were compared with those of their primary carcinomas on the one hand and with various primary ovarian carcinomas and mesotheliomas on the other. Colon cancer metastatic to the ovary was keratin 7 negative and keratin 20 positive in 94% of the cases; in contrast, all primary ovarian carcinomas were keratin 7 positive and keratin 20 negative, with the exception of two cases of mucinous cystadenocarcinoma. Ovarian metastases of gastric cancer usually contained keratins 7 and 20. Metastases of endometrial cancer to the ovary and primary ovarian carcinomas usually showed similar keratin expression. We propose that keratin 7 and 20 antibodies may be of help to distinguish between primary ovarian carcinomas and carcinoma metastases in the ovary.

24(S)-HYDROXYCHOLESTEROL PARTICIPATES IN A LIVER X RECEPTOR- CONTROLLED PATHWAY IN ASTROCYTES THAT REGULATES APOLIPOPROTEIN E-MEDIATED CHOLESTEROL EFFLUX

Both apolipoprotein E (apoE) and 24(S)-hydroxycholesterol are involved in the pathogenesis of Alzheimer disease (AD). It has been hypothesized that apoE affects AD development via isoform-specific effects on lipid trafficking between astrocytes and neurons. However, the regulation of the cholesterol supply of neurons via apoE-containing high density lipoproteins remains to be clarified. We show for the first time that the brain-specific metabolite of cholesterol produced by neurons, i.e. 24(S)-hydroxycholesterol, induces apoE transcription, protein synthesis, and secretion in a dose- and time-dependent manner in cells of astrocytic but not of neuronal origin. Moreover, 24(S)-hydroxycholesterol primes astrocytoma, but not neuroblastoma cells, to mediate cholesterol efflux to apoE. Similar results were obtained using the synthetic liver X receptor (LXR) agonist GW683965A, suggesting involvement of an LXR-controlled signaling pathway. A 10-20-fold higher basal LXRα and -β expression level in astrocytoma compared with neuroblastoma cells may underlie these differential effects. Furthermore, apoE-mediated cholesterol efflux from astrocytoma cells may be controlled by the ATP binding cassette transporters ABCA1 and ABCG1, since their expression was also up-regulated by both compounds. In contrast, ABCG4 seems not to be involved, because its expression was induced only in neuronal cells. The expression of sterol regulatory element-binding protein (SREBP-2), low density lipoprotein receptor, 3-hydroxy-3-methylglutaryl-CoA reductase, and SREBP-1c was transiently up-regulated by GW683965A in astrocytes but down-regulated by 24(S)-hydroxycholesterol, suggesting that cholesterol efflux and synthesis are regulated independently. In conclusion, evidence is provided that 24(S)-hydroxycholesterol induces apoE-mediated efflux of cholesterol in astrocytes via an LXR-controlled pathway, which may be relevant for chronic and acute neurological diseases.