doi:10.1369/jhc.6A7001.2006
Volume 54 (12): 1413-1423, 2006 Copyright ©The Histochemical Society, Inc. Analysis of Protein Expression in Cell Microarrays: A Tool for Antibody-based Proteomics
Department of Genetics and Pathology, Rudbeck Laboratory, University Hospital, Uppsala, Sweden (A-CA,SS,HB,CK,KW,FP), and Department of Biotechnology, Royal Institute of Technology, Stockholm, Sweden (MU) Correspondence to: Fredrik Pontén, Dept. of Genetics & Pathology, Rudbeck Laboratory C5:3, SE-751 85 Uppsala, Sweden. E-mail: fredrik.ponten{at}genpat.uu.se
Tissue microarray (TMA) technology provides a possibility to explore protein expression patterns in a multitude of normal and disease tissues in a high-throughput setting. Although TMAs have been used for analysis of tissue samples, robust methods for studying in vitro cultured cell lines and cell aspirates in a TMA format have been lacking. We have adopted a technique to homogeneously distribute cells in an agarose gel matrix, creating an artificial tissue. This enables simultaneous profiling of protein expression in suspension- and adherent-grown cell samples assembled in a microarray. In addition, the present study provides an optimized strategy for the basic laboratory steps to efficiently produce TMAs. Presented modifications resulted in an improved quality of specimens and a higher section yield compared with standard TMA production protocols. Sections from the generated cell TMAs were tested for immunohistochemical staining properties using 20 well-characterized antibodies. Comparison of immunoreactivity in cultured dispersed cells and corresponding cells in tissue samples showed congruent results for all tested antibodies. We conclude that a modified TMA technique, including cell samples, provides a valuable tool for high-throughput analysis of protein expression, and that this technique can be used for global approaches to explore the human proteome. (J Histochem Cytochem 54:14131423, 2006)
Key Words: immunohistochemistry cell line tissue microarray affinity proteomics antibody-based proteomics
IMMUNOHISTOCHEMISTRY (IHC) is a widely accepted method to visualize patterns of protein distribution and localization in tissues and cells. The recent development of tissue microarrays (TMA) enables IHC to be used as a high-throughput method for simultaneous analysis of protein expression in a multitude of tissue samples (Warford et al. 2004
In vitro cultured cell lines have been used extensively in basic biomedical research, and much knowledge today concerning malignant transformation, differentiation, and other cellular processes originates from experiments performed in cell lines. Although cell lines in several aspects are poor representatives of corresponding malignant tumors, in vitro cultured cells provide well-defined model systems for analyzing molecular events in a cellular context. The possibility to culture indefinite amounts of clonal cells in addition to an option to perform controlled manipulations is also advantageous. In this study we used IHC to analyze protein expression in 46 unique cell lines and 12 patient cell samples, using 20 different antibodies. Frequently used cell lines were selected to represent the major forms of solid tumors and hematological malignancies. As the cell TMA was designed as a complement to the TMAs containing human normal and cancer tissues used in an antibody-based proteomics project (Uhlen et al. 2005
The gold standard for IHC evaluation is manual examination of immunoreactivity using light microscopy. To reduce subjectivity and increase reproducibility, computerized image-analysis systems have been developed, both for enzyme-based (Ranefall et al. 1998
Antibody-based proteomics has been developed to enable mapping of protein expression in human tissues on a proteome-wide scale (Uhlen and Ponten 2005 Here we present a technique for the assembly of cells in a TMA format, including a review on how to create an artificial tissue by suspension of cultured cells in agarose. Optimal cell concentration for three-dimensional spreading of cells, harvesting protocols, and sectioning strategies is presented. In conclusion, the described strategy can be used for high-throughput antibody-based proteomics.
Design of TMAs For comparison of the staining quality between cells and tissues, TMAs containing tissue samples were designed and generated as previously described (Kampf et al. 2004
Cell Culture and Handling of Cell Aspirates and PBMCs Cell lines were cultured in 37C in a humidified atmosphere with 5% CO2 and in accordance with recommendations from the respective distributor (Table 1). Cell lines were grown and harvested in proliferation phase, i.e., 1624 hr before harvest confluent cells were split 1:2. All cell lines were tested for mycoplasma infection by inoculation on indicator cells and Hoechst staining (SVA; Mycoplasma Laboratory, Uppsala, Sweden) (Chen 1977 To obtain the best cell morphology, two strategies for detachment of adherent cells were tested: 0.5% trypsin or a rubber policeman. Adherent cells were washed with 1X PBS, 2 mM EDTA prior to detachment. After detaching the cells from the plastic surface using either trypsin or a rubber policeman, cells were resuspended in 8 ml (Petri dish, 10 cm in diameter) of respective medium with the addition of 2% fetal bovine serum (Gibco, Invitrogen, Life Technologies; Paisley, Scotland). Cell suspension was then removed from the plate and transferred to a 250-ml centrifugation flask and centrifuged at 400 x g at 4C. Following removal of the supernatant, the cell pellet was resuspended in ice-cold 1X PBS and transferred to a 50-ml tube where the volume was readjusted to 50 ml with 1X PBS. Finally, cells were counted and the centrifugation step was repeated. Suspension-grown cell lines were started at a concentration of 0.2 x 106 cells/ml and harvested on ice by centrifugation as above in 250-ml centrifugation flasks at 400 x g at 4C, followed by the washing process in PBS as described for the adherent cell lines. Patient cells and blood donor cells were collected with permission from the local Ethics Committee, and informed consent was obtained, respectively. Patient leukemia cells stored frozen in fetal bovine serum and 10% DMSO were thawed and washed twice in 1X PBS. In the second wash, cells were counted and viability was tested with trypan blue in a Bürker chamber. PBMCs were isolated from buffy-coat preparations with Ficoll Paque Plus according to the manufacturer's recommendation (GE Healthcare, Amersham Biosciences; Uppsala, Sweden), with the addition of lysis of the red blood cells.
Preparation of the Agarose Cell Gel
TMA Production
IHC
Image Acquisition and Image Analysis For evaluation of the three-dimensional cell distribution in the cell gel, one section at every 100-µm depth was taken from theparaffin-embedded cell gels and stained with hematoxylin. The 756 x 572 pixel color images with 3x 256 gray levels were captured using a color video camera (DXC-151; Sony, Tokyo, Japan) attached to a standard microscope (BH-10; Olympus, Tokyo, Japan) using a x10 objective. This resulted in a final magnification of x25 and a pixel size of 1.6 µm for a wavelength of 550 nm. A semi-automatic color-based quantification method was used to count the cells (Ranefall et al. 1997
Cell Culture and Handling of Cell Aspirates and PBMCs Suspension-grown cell lines appear ideal for cell gel preparations, as no manipulation, e.g., trypsinization, is needed prior to fixation and histoprocessing. For adherent cell lines, there was no visible morphological difference if cells were harvested using trypsin or a rubber policeman. Both methods provided intact immunogenic properties of tested membrane proteins (Figure 1 ). Because the protocol using trypsin was demonstrated to be more efficient for detaching the cells, it was used for harvesting all adherent-grown cell lines to be included in the cell TMA. Both suspension- and adherent-grown cell lines embedded in agarose showed excellent cell morphology. For each cell line that tested positive for mycoplasma infection, a new negative batch was cultured, which together with its positive counterpart was included in the array.
Preparation of the Agarose Cell Gel Three-dimensional distribution of cells in the cell gel was evaluated by quantifying hematoxylin-stained 4-µm sections taken at 100-µm intervals. This was done using cell concentrations of 25 and 50 x 106 cells/ml from the U-937 cell gel. Results from image analysis showed that the mean number of cells/mm2 in 28 sections was 311 (95% CI 288336) and 884 (95% CI 7201048), respectively. As visually observed, cells were homogeneously distributed in both concentrations.
Based on visual examination of immunostained slides from all four cell concentrations (5, 25, 50, and 100), a cell concentration of 100 x 106 cells/ml agarose was selected for cell gel production. This resulted in a representation of
TMA Production
IHC The original protocol used for HIER on tissues was 10 min. This was adjusted to 7 min for cells to enable a strong and distinct hematoxylin staining in the cell nuclei. To evaluate the quality and morphology of the cells in the cell TMA, 20 well-characterized antibodies with known immunostaining patterns were selected (Table 2). As shown in Figure 2 , antibodies toward antigens in six specific subcellular compartments (nucleus, cytoplasm, plasma membrane, nuclear membrane, nucleolus, and mitochondria) successfully recognized their respective antigen in sections from the cell TMA. For further validation, immunostaining of the cell TMA was compared with staining in the corresponding tissue sections. In all cases where such a comparison was applicable, the staining pattern was in agreement with published data on the expected protein distribution. This is exemplified by the anti-LCA (CD45) antibody, which was positive in a vast majority of leukemia/lymphoma cell lines and also showed positivity in lymphoid cells in the tissue sections (Figures 3A and 3B). The anti-desmin antibody recognized myocytes in striated muscular tissue as well as in the corresponding rhabdomyosarcoma-derived cell line RH-30 (Figures 3C and 3D). Congruent staining was also evident using the HER-2 antibody, which showed strong membranous staining in breast carcinomas and the cell line SK-BR-3 with known high Her-2 expression (Figures 3E and 3F).
Statistical significance of TMA technology, i.e., using core biopsies as representatives for large tissue biopsies, has been evaluated in earlier studies, and it has been shown that duplicate or triplicate biopsies provide a high degree of concordance (Camp et al. 2000 It may be necessary to limit the number of cells in a study when large-scale efforts including several different experiments are to be performed. However, cells dispersed at relatively low cell densities in a cell gel reduce the amount of cells needed as compared with a cell pellet. This is advantageous because it also allows extensive studies on scarce patient samples from fine needle biopsies where only a limited number of cells are available. One requirement for a suitable cell gel is a viscosity at 65C that facilitates mixing of the cells and subsequent transferal into a plastic well. Furthermore, the cell-embedding medium must endure fixation, histoprocessing, and paraffin embedding with minimal shrinkage and without fragmentation. It must also preserve acceptable cell morphology and be easy to section in a microtome. This was true for the described cell gel preparation technique. The use of IHC is hampered by the lack of standardized protocols for fixation, antigen retrieval, and antibody-based targeting of proteins. IHC is, at best, semiquantitative; however, in a complex tissue context, data obtained from a single tissue including different cell populations can be of significant value. Such qualitative data are often impossible to assess using other methods. Although IHC using enzyme-based protocols results in a relatively narrow dynamic range of staining intensity, appraisal of immunoreactivity in tissues and cells allows for an outstanding method to perform a relative mapping of protein expression. The inborn mixture of different cells in a tissue enables the analysis of immunoreactivity in a given cell type as a relative measure of protein content, as compared with other cells in the same tissue. This is of particular importance when antibodies generated toward proteins of unknown tissue distribution and cellular localization are analyzed.
Selection of cells used in the described cell TMA includes cell lines from both hematopoietic cell lines and solid tumors, of which eight cell lines also are included in the NCI-60 cell line panel used for anticancer drug screening (Monks et al. 1991 In conclusion, we have provided a strategy for using cells in a TMA format that enables high-throughput IHC as a method to screen for protein expression levels and distribution in a large number of cell samples. This approach could well be integrated in existing antibody-based proteomic strategies utilizing tens of thousands of antibodies. A comprehensive cell atlas can be envisioned as a complement to the existing protein profiling in human tissues and cancer (www.proteinatlas.org).
This work was financed by a grant supporting the Swedish Human Proteome Resource project from the Knut and Alice Wallenberg Foundation, Stockholm, Sweden. We thank Prof. Kenneth Nilsson, Prof. Bengt Westermark, Prof. Gunnar Nilsson, Assoc. Prof. Nils-Erik Heldin, Prof. Lena Claesson-Welsh, Prof. Martin McMahon, and Prof. Norbert E. Fusenig for discussions and for permission to use specific cell lines cited in the manuscript.
Received for publication April 25, 2006; accepted August 2, 2006
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