1 Departments of Cancer Research
2 Genomics and Gene Therapy
3 Immunology, Berlex Biosciences, Richmond 94804-0099
4 Department of Molecular Pharmacology, Stanford University Medical School, Stanford, California 94305-5174
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ABSTRACT |
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protein kinase B; cDNA array hybridization; estrogen inducible expression; oncogenesis; gene regulation; signal transduction networks
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INTRODUCTION |
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Overexpression, hyperactivation, or constitutive activation of AKT is oncogenic. Akt1 was first identified as the transforming gene, v-Akt, encoded by the AKT8 retrovirus. v-Akt differs from its cellular homolog, Akt1, in that it has a 5' fused viral gag sequence that causes constitutive activation of the AKT1 kinase. Constitutively active AKT causes cellular transformation in vitro, and cells transfected with constitutively active AKT1 form tumors in nude mice. In vivo, mice transgenic for constitutively activated AKT1 develop T cell lymphomas rapidly (33). Cotransfection with dominant-negative AKT blocks transformation, indicating that AKT activity is necessary and sufficient for oncogenic transformation. AKT activity is elevated in samples of ovarian, breast, prostate, lung, and glioblastoma tumors (4, 36).
The purpose of this study was to identify Akt1-regulated genes to aid in understanding the mechanism of AKT1-mediated oncogenesis. The use of an inducible form of the AKT1 protein, MERAKT, made it possible to study the pattern and kinetics of Akt1-regulated gene expression using a single cell line. Activity of the MERAKT kinase is rapidly stimulated in the presence of estrogen receptor (ER) ligands, and repressed in their absence. To create MERAkt, a mutant murine ER-hormone binding domain (ER-HBD) was fused to the 3' end of the kinase domain of Akt1, and a myristoylation sequence substituted for the pleckstrin homology (PH) domain at the 5' end of Akt1. Activation of the gene product, MERAKT, like AKT1, requires membrane localization and phosphorylation at two sites, Thr308 and Ser473 (23). The NH2-terminal myristoylation sequence localizes the MERAKT protein to the plasma membrane, but the ER-HBD blocks the phosphorylative activation sites until ER ligands are added to the culture (23). Mirza et al. (31) recently used MERAkt to show that induction of AKT1 activity is sufficient to transform Rat1 or (to a lesser extent) NIH3T3 fibroblasts, and to increase the apoptotic resistance and cell cycle progression of Rat1 fibroblasts. Increases in apoptotic resistance and cell cycle dysregulation contribute to oncogenic transformation (6).
Akt plays a role in the inhibition of apoptosis (9, 33, 36), increased cell-cycle progression (22), enhancement of tumor neovascularization (29), and stimulation of glycolysis (2), all of which contribute to tumor formation and maintenance. However, there are few reports of genes regulated by Akt to carry out these AKT-dependent functions. The few reported Akt-regulated genes are GLUT-1 and phosphoenolpyruvate carboxykinase (PEPCK), whose regulation increases glycolysis (3, 26); vascular endothelial growth factor (VEGF), which promotes angiogenesis (19); the anti-apoptotic factor Bcl-2 (34); and p27kip, a cell cycle inhibitor repressed by AKT (5, 11, 30, 39). In contrast, there are numerous reports of Akt-regulated transcription factors, including AP-1 (12), glucocorticoid receptor (35), E2F (7, 15), forkhead transcription factor FKHRL1 (11, 30, 32, 39), hepatocyte inducing factor 1 (HIF1
; 29), cAMP response element binding proteins (CREB), and nuclear factor
B (NF-
B) (20). Each of these transcription factors affects a number of characterized target genes, yet determination of which of these is Akt-regulated has not been reported. Investigation of Akt-regulated genes is complicated by the small magnitude of the changes in gene expression due to activation of AKT (2- to 3-fold) (34) and by the repeated observation that AKT-dependent signaling requires costimulation of other signaling pathways (12, 15, 20). Activation of other signaling pathways may also eliminate AKT-dependent signals through negative interference.
Previous studies using the inducible MERAkt gene have shown that nuclear signaling due to MERAKT activation is indistinguishable from signaling initiated by insulin-activated endogenous AKT1. Transcriptional regulation of PEPCK and GLUT-1 by 4-hydroxytamoxifen (Tam)-induced MERAKT mimics that of insulin-induced AKT1 (3, 26). We report here more than 20 AKT-regulated genes identified at 3 and 20 h after induction of AKT1 activity in MERAkt cells (retrovirally derived from NIH3T3 cells). Six chemically distinct ER ligands were used to activate MERAKT in replicate experiments. The distinct chemical structures and activation potencies of these ligands allowed AKT-dependent regulation to be distinguished from AKT-independent regulation. The known functions of a few of the genes contrast with the oncogenic phenotype induced by activation of AKT1. The regulation of many of the genes, however, is consistent with oncogenesis. Among these genes are anti-apoptotic factors; promoters of tumor neovascularization, invasion, and metastasis; and regulators of cell cycle progression. Our results identify for the first time genes regulated by activation of the oncogene Akt1 and also indicate that there is significant negative interference between AKT-dependent signals and signals from elements of the platelet-derived growth factor-ß (PDGFß) pathway upstream of AKT.
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MATERIALS AND METHODS |
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Both phenol red-free DMEM and charcoal-adsorbed FBS were tested in MERAkt cell proliferation assays and found to have no effect on generation time or cellular morphology.
To minimize any differences due to culture conditions, all treated and untreated MERAkt cultures were grown, serum-starved, treated, and handled in parallel in a given experiment. All experiments were repeated at least once on separate occasions, and untreated reference culture samples were prepared at each time point in triplicate. Samples were harvested at 3 and 20 h after drug treatment.
ER ligands and growth factors for activation of MERAKT.
17ß-Hydroxyestradiol (E2) and Tam (Sigma) were dissolved in ethanol at 1 mM and stored at -20°C. Raloxifene (Ral), ICI-182780 (ICI), ZK427, ZK819, and ZK955 were obtained from the Molecular Pharmacology Division of Berlex Biosciences, reconstituted to 10 mM in DMSO, and diluted to 1 mM in ethanol. The ER ligand concentration at which intensity of the phosphorylated MERAKT signal was half-maximal is defined as the EC50. All ER ligands were used at a final concentration of 0.1 µM. PDGFß (R&D Systems) was reconstituted according to the manufacturers instructions (fatty acid-free BSA, 0.1%, was used to avoid fatty acid stimulation of Akt activity) and used at 10 ng/ml.
Flow cytometric analysis.
The percentage of cells in each phase of the cell cycle under a given set of culture conditions was determined by flow cytometric analysis of propidium iodide-stained nuclei. MERAkt or NIH3T3 cells were harvested by trypsinization, washed in Dulbeccos phosphate-buffered saline with calcium and magnesium (GIBCO), and counted, and 3 million cells were resuspended in 0.25 ml 4°C phosphate-buffered glucose/EDTA solution on ice. Then, 0.75 ml of 95% nondenatured alcohol (-20°C) was added dropwise. Nuclei were pelleted, resuspended in 0.5 ml PBS containing 50 µg/ml RNase, and incubated at 37°C for 20 min; then 25 µg propidium iodide was added to each sample, and the samples incubated at ambient temperature for 30 min. Samples were analyzed on a Becton-Dickinson FACScan apparatus.
Protein analysis by Western blot.
Cell samples were lysed in anti-phosphatase buffer (1% Nonidet P-40, 15% glycerol, 137 mM NaCl, 20 mM Tris, pH 7.5, 2 µg/ml leupeptin, 2 µg/ml aprotinin, 2 mM benzamidine, 20 mM NaF, 10 mM NaPPi, 25 mM ß-glycerolphosphate, with fresh 1 mM Na3VO4 and 2 mM PMSF). Equivalent amounts of total protein (determined by bicinchoninic acid assay; BCA, Pierce) were analyzed by Western blot using Novex 10% acrylamide Tris-glycine gels (Novex) following the manufacturers instructions. Primary antibody was used at 1:1,000; secondary antibody (horseradish peroxidase-conjugated goat anti-rabbit antisera, New England Biolabs) was diluted 1:2,000, and developed with Luminol (New England Biolabs).
Antibodies for Western analysis.
Phosphorylated AKT and MERAKT were detected by anti-peptide sera purchased from New England Biolabs (NEB). NEB rabbit sera raised against an AKT1 decapeptide containing phosphoserine-473 or raised against a similar peptide containing AKT1 phosphothreonine-308 are referred to here as anti-phospho473AKT and anti-phospho308AKT sera, respectively. Phosphorylation of glycogen synthase kinase-3 (GSK3) was similarly detected using rabbit anti-phosphorylated-GSK3-peptide sera from NEB. Horseradish peroxidase-conjugated goat anti-rabbit sera, as well as the similar anti-mouse sera, were purchased from NEB. A mouse monoclonal antibody (P67920) from Transduction Laboratories was used to distinguish AKT1 from the other isoforms of AKT.
cDNA array hybridization studies.
The mouse arrays (Atlas 1.2, 7853-1; Clontech Laboratories, Palo Alto, CA) containing 1,176 genes representing all major cellular pathways and functions were employed in these studies. To prepare probe for hybridization, total RNA was purified from each sample using Qiagen RNeasy Midi kits according to manufacturers instructions. Probe was prepared from 4 µg of total RNA according to Clontechs November 1999 protocol, except that G-50 TE spin columns (Boehringer-Mannheim) were used to purify labeled complex probe away from unincorporated [-32P]dATP. Size distribution of the 32P-labeled complex probe was determined by electrophoresis on 6% acrylamide urea denaturing gels (Novex) followed by autoradiography.
Data quantification and background determination.
Fuji imaging screens were exposed to the hybridized gene array membranes for 72 h and read into a Storm PhosphorImager (Molecular Devices, Sunnyvale, CA) at 200 µm resolution. The hybridization signal intensities at each cDNA spot were quantified using Array Vision (Imaging Research, St. Catherines, Ontario, Canada). The grid definition protocol used an automated alignment algorithm to finely adjust the grid over the spots. The signal intensity was calculated as the mean pixel value minus a local regional background and reported for each spot in the array. The signal intensities were then normalized to the mean of all spots on the array.
Identification of differentially expressed genes by GEPView.
GEPView is a proprietary software, from Berlex Laboratories, for analysis of cDNA hybridization array data. Output of Array Vision (above) was entered into GEPView for determining changes in signal intensity and z score determination. GEPView calculates the fold change of the intensity of the hybridization signal for each gene (cDNA) in a treated sample to the signal of the same gene in the matched untreated reference sample. For fold change calculations, if the signal intensity value fell below a threshold set at background/2, then the intensity value for that gene was reset to a value of background/2. The criteria set for differential expression was that the fold change in expression comparing treated to untreated signals be greater than 2 or less than -2 and that the z score be at least 0.3. The z score reflects the difference between two intensity values being evaluated compared with the spread of the data for the entire array (mean/standard deviation). The algorithm used for z score determination was developed in Array Vision. Differentially expressed genes were then sorted using Microsoft Excel to identify those genes differentially expressed in response to multiple types of treatment.
Hybridization intensity normalization and statistical analyses.
Analyses were accomplished using in-house software implementing plotting techniques based on Students t-test calculations, as in Callow et al. (8). P values reported are uncorrected for multiple tests. A conservative (strict) correction for multiple tests was also applied, where the correct value was calculated as 1 - (1 - P)N, with P being the uncorrected value and N the number of expression levels tested. The null hypothesis for each genes level of expression was that treated and untreated sets of hybridization intensities were the same; the mean values for the treated samples and untreated samples were assumed to be equal. The t-test evaluates this, and the hypothesis may be rejected at the reported alpha level or "P value."
Quantile-quantile plots of statistics were used to investigate the distribution of statistics calculated. The quantile-quantile plot compares the t-statistics calculated for each gene, testing the difference between the mean values for treated and untreated samples, with the quantiles of the t-distribution derived from the number of samples tested and rank of the statistic in the dataset. Points on the plot which show much more extreme values in the calculated t statistic than in the quantile value may be outliers of note.
Prior to calculating t statistic values and plotting, each array of hybridization intensities was normalized to a vector consisting of the mean values for each gene using Locfit for the statistical package R (http://cm.bell-labs.com/cm/ms/departments/sia/project/locfit/index.html). Trends in values following mean intensities across arrays for expression levels were thus removed; if an array showed an idiosyncratically low set of values for highly expressed genes, for example, this trend was removed prior to statistical calculations.
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RESULTS |
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PDGFß-regulated genes.
MERAkt cells stimulated with 10 ng/ml PDGFß were analyzed by Western blot using anti-phospho473AKT and anti-phosphoGSK3 sera (Fig. 3, A and B). PDGFß stimulated transient activation of endogenous AKT1 with an 50-fold increase in phosphorylation by 15 min, which then decreased 10-fold by 3 h (lanes 3 and 4 of Fig. 3A). There was a coordinate increase in phosphorylation of GSK3, particularly of GSK3-
at 15 min, returning to basal levels by 3 h (lanes 3 and 4 of Fig. 3B). We note that handling cultures was sometimes sufficient to activate MERAKT, as evident in lane 2 of Fig. 3A, but this activation was 20- to 50-fold less than the level of phosphorylative activation induced by treatment with any of the potent MERAKT activators such as Tam (Fig. 2C, lane 3; Fig. 4, lane 1). This relatively low-level activation of MERAKT did not result in phosphorylation of GSK3 (Fig. 3B, lane 2), indicating that signal transduction from MERAKT was undetectable under these conditions. It follows that the low level of MERAKT phosphorylation seen in lane 3 of Fig. 3A results from handling of the culture during PDGFß treatment but does not contribute to the increase in GSK3 phosphorylation observed in lane 3 of Fig. 3B.
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Data were analyzed by two methods: GEPView, which reports the fold change in the intensity of the signals in matched treated vs. untreated samples for each gene and each treatment, and by a statistical analysis based on a modification of Students t-test. GEPView analysis facilitates comparison of the gene regulation effects of the individual treatments used to deduce the mechanism of gene regulation. Lists of genes identified as differentially expressed, using a twofold change cutoff, in response to each treatment were compiled and sorted to identify genes differentially regulated by the greatest number of treatments. The results of this sorting are presented in Tables 3 and 4. No genes were differentially regulated by all six ER ligand treatments. Those genes most frequently regulated by the treatments were commonly regulated by the four potent MERAKT activators and infrequently regulated by the weak MERAKT activators, E2 and ZK427. No genes were differentially regulated by both E2 and ZK427. Fold changes in gene signal intensity between zero and twofold are indicated by a "". Genes are listed in order of descending confidence that they are AKT regulated. Confidence that a given gene is AKT regulated is proportional to the number of experiments in which the genes expression was affected by any of the four potent MERAKT activators and is inversely proportional to the number of times the gene was affected by either of the weak MERAKT activators.
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To determine whether any of the genes identified in Tables 3 or 4 could be ER ligand regulated by an AKT-independent mechanism, we analyzed data from MERAkt cultures treated with E2 or ZK427. E2 regulates ER- and ERß-dependent genes on average one log more strongly than any of the four potent MERAkt activators, and ZK427 regulates ER
-dependent genes (41). As shown in Tables 3 and 4, none of the genes commonly regulated by the four potent MERAKT activators was consistently or strongly regulated by either E2 or ZK427, indicating that the mechanism of their regulation in unlikely to be ER dependent.
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DISCUSSION |
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This study depended on the use of ER ligands to induce AKT1 activity via binding to the mutant ER-HBD of MERAKT. Therefore, we investigated whether regulation of any of the genes commonly regulated by the four potent MERAKT activators could be ER dependent and AKT1 independent. We did not expect to observe ER-dependent regulation by any of the four potent MERAKT activators because 1) all four compounds are very weak inducers of ER-dependent gene expression, at least one log less potent than E2 (41), and 2) the NIH3T3 cell line used is reported to express only low levels of ER (21). We showed that ER-dependent genes (IGFBP-1 and prolactin receptor) were not differentially expressed in our system in response to E2 treatment, indicating that the functional level of ER in the cells used for this study is below that required for ER-dependent gene regulation. Additionally, we showed that the genes commonly regulated by the four potent MERAKT activators are not regulated by treatment with E2 or ZK427. E2 and ZK427 are strong and moderate inducers of ER-dependent gene expression respectively, but both are relatively weak MERAKT activators. If the genes commonly regulated by the four potent MERAKT activators were regulated by an ER-dependent mechanism, then they would have been more strongly regulated by E2 or ZK427 than by the four potent MERAKT activators. The opposite was observed. Last, ER-independent regulation common to the four potent MERAKT activators is unlikely given the dissimilarity of their chemical structures. Ral is a benzothiophene, ICI is a 7-substituted E2, and Tam and ZK955 are triphenylethylenes (41). These compounds are unlikely to have similar effects on gene expression independent of their common ability to bind to the mutant ER-HBD of MERAKT. We therefore conclude that genes commonly regulated by the four potent MERAKT activators are regulated by AKT1 activity.
This is the first time that global expression analysis has been used to identify Akt-regulated genes. The small-magnitude changes are consistent with those reported for Akt regulation of Bcl-2 (34), one of the only Akt-regulated genes for which quantitative data has been reported. Similarly small magnitude changes were recently reported for PTEN-regulated genes identified by global expression analysis. PTEN ("Phosphatase with tensin homology") is a negative regulator of AKT activation, acting immediately upstream of AKT in the PI3-kinase pathway. Forty-two of the 43 genes reported changed two- to threefold. Only one gene was greater than threefold affected by PTEN (18). Small magnitude changes in gene expression may be common for these pathways.
Akt regulation and oncogenesis.
The function of many of the Akt-regulated genes is consistent with tumor initiation or maintenance through promotion of cell cycle progression, inhibition of apoptosis, and enhancement of angiogenesis. The function of other Akt-regulated genes contrasts with Akts oncogenic phenotype, and the explanation for their regulation by Akt remains unclear.
Akt-regulated induction of 78-kDa glucose-regulated protein (GRP) and BAG-1 are consistent with AKTs anti-apoptotic function (40). Induction of MECP-2 (represses tumor suppressor gene expression), matrix metalloproteinase 2, extracellular matrix metalloproteinase inducer, 58-kDa inhibitor of RNA-activated protein kinase (an oncogene) (1), and repression of N-cadherin are all consistent with tumor formation and invasion or metastasis. Most classes of cathepsins (L, B1, D, H, W), proteases whose expression correlates with aggressive tumor growth (38), are induced by AKT at both 3 and 20 h.
C-myc, N-myc, and L-myc are all repressed by AKT1 activity. This finding contrasts with frequent reports of c-myc upregulation in cancers, most clearly illustrated in colorectal cancer (16). There are several ways in which these data may be reconciled. AKT increases translation of 5'-terminal oligo pyrimidine track (5'-TOP) mRNAs, including that of c-myc (22). The increased C-MYC levels may negatively regulate c-myc transcription, a testable hypothesis that may reconcile our results with those of He et al. (16). However, the kinetics of c-myc mRNA repression upon activation of AKT1 argues against this explanation. A second hypothesis is that the mechanism of AKT oncogenesis relies on inhibition of apoptosis rather than induction of mitogenesis (e.g., by C-MYC). Upregulation of c-myc can induce apoptosis (10); repression of c-myc may therefore be part of Akts anti-apoptotic mechanism. Finally, our observation that c-myc is induced by PDGFß stimulation (which activates AKT1) indicates that AKT1 repression of c-myc expression may be physiologically irrelevant. AKT was activated in our study in isolation of upstream signaling. AKT is unlikely to be activated in isolation under physiological conditions. Our results comparing PDGFß and AKT1 gene regulation indicate that signaling from upstream pathway elements may negatively interfere with c-myc signaling initiated by AKT1, discussed below.
Tumor neovascularization may be promoted by AKT-regulated coordinate repression of EGR1, thrombospondin-1 (THBS1), and several transforming growth factor ß (TGF-ß) family ligands and receptors. EGR1 suppresses tumor formation by stimulating the TGF-ß1 and fibronectin promoters, leading to enhanced cell attachment and regulated growth (28). Repression of EGR1 would allow anchorage-independent growth, loss of regulation, and reduced TGF-ß1 levels. TGF-ß1 normally cooperates with THBS1 in suppressing endothelial cell proliferation (37). Reductions in TGF-ß and TGF-ß receptor levels combined with reductions in THBS1 levels are therefore expected to promote angiogenesis and tumor neovascularization. Tumor neovascularization would be further aided by AKT-regulated induction of VEGF mRNA (observed at 20 h) and other HIF1-regulated genes, and phosphorylative activation of endothelial constitutive nitric oxide synthase (ecNOS) (14).
There are a number of genes whose regulation by AKT1 is inconsistent with AKTs anti-apoptotic or oncogenic functions. Apoptotic factors include AKT1-regulated induction of BAX, T-cell death-associated gene 51 (TDAG51), BAK1, and repression of SMN ("survival of motor neurons"). Oncogenic factors include repression of oncogenes pim1, Erb-B2, N-ras, and Met, and induction of TIMP3 ("tissue inhibitor of metalloproteinases 3"). We have no explanation for these apparent inconsistencies but note similar findings in a recent study of Raf-1-regulated genes. Many of the Raf-1-regulated genes functioned to inhibit, rather than promote, oncogenesis (17).
Akt regulation is cell-type specific.
Observations of gene regulation depend on the cell type studied. The following are examples of genes whose AKT regulation has been reported in other systems but which were not identified as AKT-regulated genes in our study. It has been reported that AKT regulates cell survival and induces cell cycle progression by phosphorylative inhibition of the transcription factor FKHRL1, consequently reducing p27kip1 expression (24, 25). MERAkt cells express low levels of FKHRL1 (unpublished results), possibly explaining why significant changes in p27kip1 mRNA levels were not observed. Similarly, AKT regulates VEGF expression under hypoxic conditions, by regulating the transcription factor HIF1 (29). Under the normoxic conditions here, we did not observe AKT regulation of VEGF expression at 3 h and observed only a minimal increase at 20 h. Additionally, very low steady-state levels of HIF1
mRNA were measured in these samples. GLUT-1 mRNA levels are affected by AKT1 activity (3), but GLUT-1 regulation was not observed in MERAkt cells, presumably because components of required signaling pathways are not expressed in these cells. The PEPCK gene was not represented on the arrays used in this study.
This is the first time that signaling from a pathway element (AKT1) has been compared with signaling initiated at the cell surface (PDGFß stimulation), allowing detection of interference of signals originating in a single pathway. Comparison of AKT1-regulated and PDGFß-regulated genes reported here shows that AKT1-regulated genes are not a subset of PDGFß-regulated genes. The only two genes that were similarly regulated by AKT1 and PDGFß were TDAG51 and 78-kDa GRP. Genes that are regulated by AKT1 but not by PDGFß are evidence of signals originating upstream of AKT that block, or negatively interfere with, AKT signaling regulating expression of that gene. More striking, there are a number of genes, including c-Myc, EGR1, and THBS1 that are oppositely regulated by AKT1 and PDGFß, indicating that signals from PDGFß elements upstream of AKT are dominant over those originating from AKT.
In conclusion, a number of genes were identified as AKT1-regulated under the conditions tested. Many of these genes may contribute to AKT1-mediated oncogenic transformation, tumor growth, or maintenance. Further work is needed to determine whether protein levels or activities increase or decrease in concert with the changes in steady-state mRNA levels reported here. To determine which of these changes in mRNA levels contribute to AKT-mediated oncogenesis, overexpression or antisense methods could be employed.
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ACKNOWLEDGMENTS |
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Present address of P. H. Johnson: EpiGenx Pharmaceuticals, 5385 Hollister Ave., Santa Barbara, CA 93111.
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FOOTNOTES |
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Address for reprint requests and other correspondence: I. Kuhn, Berlex Biosciences, 15049 San Pablo Ave., Richmond, CA 94804-0099 (E-mail: Irene_Kuhn{at}Berlex.com).
1 By convention, in this report, gene names are indicated by lower case abbreviations, and protein product names are indicated by upper case abbreviations.
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REFERENCES |
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