Molecular and Cellular Biochemistry 208: 11–18, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
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Expression of receptor protein tyrosine phosphatase α mRNA in human prostate cancer cell lines Stanislav Zelivianski,1 Jeanenne Dean,1 Deepak Madhavan,1 Fen-Fen Lin1 and Ming-Fong Lin1,2,3,4 1
Department of Biochemistry/Molecular Biology; 2Section of Urologic Surgery, College of Medicine; 3UNMC Eppley Cancer Center, University of Nebraska Medical Center; 4Omaha VA Hospital, Omaha, NE, USA Received 8 July 1999; accepted 4 November 1999
Abstract Receptor protein tyrosine phosphatase α (RPTPα) is a transmembrane protein phosphatase, and has been proposed to be involved in the differentiation of the neuronal system. In the present study, we demonstrated the expression of RPTPα mRNA in several normal human tissues. We further investigated the regulation of expression of RPTPα mRNA in epithelial cells utilizing three commercially available human prostate cancer cell lines LNCaP, PC-3 and DU145. This is because these cells exhibit different levels of differentiation, defined by the expression of a tissue-specific differentiation antigen, prostatic acid phosphatase (PAcP), and their androgen sensitivity. LNCaP cells express PAcP and are androgen-sensitive cells, while PC-3 and DU145 cells do not express PAcP and are androgen-insensitive cells. Northern blot analyses revealed that, in LNCaP cells, fetal bovine serum (FBS) and 5α-dihydrotestosterone (DHT) down-regulates RPTPα mRNA expression, similar to the effect on PAcP. Contrarily, FBS up-regulated the RPTPα mRNA level in PC-3 and DU145 cells. In LNCaP cells, sodium butyrate inhibited cell growth and up-regulated RPTPα as well as PAcP mRNA expression. Although, sodium butyrate also inhibited the growth of PC-3 and DU145 cells, the level of RPTPα mRNA was decreased in PC-3, while increased in DU145 cells. Thus, data taken together indicate that the expression of RPTPα is apparently regulated by a similar mechanism to that of PAcP in LNCaP cells. (Mol Cell Biochem 208: 11–18, 2000) Key words: receptor protein tyrosine phosphatase α, cell differentiation, prostate cancer cell, prostatic acid phosphatase
Introduction It has been well established that protein tyrosine phosphorylation plays a critical role in regulating various cellular events, including cell differentiation and proliferation [1, 2]. Protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) function dynamically to maintain the cellular homeostasis [3]. Abnormal tyrosine phosphorylation may result in neoplastic diseases [4]. Extensive studies in the past decades have provided insight in the regulation of PTK expression, enzymatic activity, and roles of PTKs in
signal transduction [1, 5]. However, relatively little is known about the functional role of PTPs in epithelial cells. PTPs can be divided into two classes: the receptor-like, membrane-spanning PTPases which may act as receptors, and the cytosolic PTPases identified as intracellular low molecular weight enzymes [6]. Receptor-like PTPs (RPTPs) exhibit high homology in their PTP domains but differ considerably in their extracellular segments. Most of RPTPs contain two catalytic domains and are presumed to be controlled by binding of an extracellular ligand [1, 6]. Conversely, RPTPases themselves may serve as ligands for signaling [7,
Address for offprints: Ming F. Lin, Dept. of Biochemistry and Molecular Biology, University of Nebraska Medical Center, 984525 Nebraska Medical Center, Omaha, NE 68198-4525, USA
12 8]. The cytosolic PTPases have only one single PTPase domain and variable N- and C-terminal extensions. It has been postulated that these sequences target cytosolic PTPases to the specific intracellular location [9]. RPTPα, also known as leukocyte common antigen related phosphatase (LRP) [10], is a transmembrane member of the PTP family with a relatively short (123 amino acid) extracellular domain [11] that is highly glycosylated [12]. Like most transmembrane PTPs, RPTPα has two homologous cytoplasmic catalytic domains. The domain proximal to the cell membrane exhibits the majority of protein-tyrosine phosphatase activity, while the second domain has low but detectable phosphatase activity [13, 14]. It has been demonstrated that RPTPα is involved in GRB2mediated signaling because RPTPα can be tyrosine phosphorylated and subsequently recruited the adapter protein GRB2 [15, 16]. RPTPα may also participate in the activation of mitogen-activated protein kinase and c-Jun transcription factor [17]. Additionally, it has been shown that RPTPα dephosphorylates the negative regulatory site of Src PTK [18]. An overexpression of RPTPα leads to the dephosphorylation and activation of cytoplasmic PTK c-Src [19]. Interestingly, differentiation of neuroblastoma cells and embryonal carcinoma cells (EC) is accompanied by an increased Src activity [20, 21] and a up-regulated expression of RPTPα [22]. The functional role of RPTPα in neuronal differentiation is further demonstrated by the observation that its overexpression enhances the development of neurotransmitter response during the differentiation process [23]. However, the regulation of its expression and its biological role in epithelial cells remain mostly unknown. In this study, we demonstrated the expression of RPTPα at the mRNA level in several normal human tissues. To obtain insight into the regulation of expression of RPTPα in epithelial cells and their further differentiation, we investigated its expression in all three commercially available human prostate cancer cells LNCaP, PC-3 and DU145. These cells exhibit distinct levels of cell differentiation, as defined by the expression of a prostate-specific differentiation antigen, prostatic acid phosphatase (PAcP), and the androgen sensitivity [24]. PAcP is known to be an important marker for the differentiation of prostatic epithelium [24, 25]. Recent studies clearly show that the cellular form of PAcP functions as a neutral PTP, and regulates the growth of prostate epithelia [25]. Furthermore, LNCaP cells can be induced for further differentiation into neuroendocrine-like cells [26, 27], a cell type that is associated with the advanced stage of prostate cancer [28, 29]. While, undifferentiated cancer cell including PC-3 and DU145 do not express PAcP. Additionally, LNCaP cells are androgen-responsive, but not DU145 or PC-3 cells [24, 25]. Thus, PC-3 and DU145 cells can serve as controls for investigating the regulation of RPTPα expression in LNCaP cells. Together, LNCaP, PC-3 and DU145 cells represent
an interesting model system to study the regulation of expression of RPTPα in epithelial cells with different stages of differentiation. The study of RPTPα expression in this system will provide us with new information in understanding its putative role in prostate epithelium differentiation, in additional to its functional role in neuronal cells. Our results clearly show that the mRNA level of RPTPα is regulated coordinately with PAcP in LNCaP cells.
Materials and methods Materials Cell culture medium RPMI 1640, fetal bovine serum (FBS), gentamicin and glutamine were obtained from Life Technologies, Inc. [α-32P]dCTP was from Amersham. All other reagents were obtained as described in previous publications [24, 30].
Methods Cell culture Three commercially available human prostate carcinoma cell lines, LNCaP, PC-3, and DU145, were originally obtained from the American Type Culture Collection, and routinely maintained in RPMI-1640 medium supplemented with 7% FBS, 1% glutamine, and 0.5% gentamicin, as described in previous publications [24, 30]. LNCaP cells are the only commercially available human prostate cancer cells that are androgen-responsive and express PAcP, while PC-3 and DU145 cells are androgen-insensitive and lack the expression of PAcP [25, 31]. For experiments, LNCaP cells that had passage numbers less than 33 were designated as clone 33, passage numbers between 80 and 120 as clone 81, as described in our previous publication [25]. The growth rate of clone 81 cells is more rapidly than clone 33 cells with a doubling time half of clone 33 cells [25]. To quantify cell growth, attached cells were trypsinized and neutralized with medium-FBS. The total cell number was counted using the trypan blue exclusion method under a microscope and/or a Coulter Counter Z1 model [24, 25]. RNA isolation and Northern blot analysis To examine the expression of RPTPα in different normal cells, a multiple tissue membrane blot containing mRNAs from various normal human tissue specimen was purchased from Clontech Laboratories, Inc (Multiple Tissue Northern Blots Human II). The membrane was hybridized with cDNA probes according to the manufacturer’s instructions. To study the regulation of RPTPα gene expression in prostate cells, total RNAs were extracted from those cells using TRI Reagent
13 (Molecular Research Center Inc.) [25, 32]. Ten to twenty µg total RNA of each sample were electrophoresed on an agarose gel containing formamide, transferred to nylon membrane Zeta-Probe GT (Bio-Rad), and baked at 80°C for 1h. Hybridization was performed as described previously [25, 33]. RPTPα cDNA (1.1 kb) was originally obtained from the American Type Culture Collection. A specific cDNA probe containing 400 bp of 5′-end of RPTPα for hybridization was synthesized by polymerase chain reaction (PCR), and followed by sequencing to confirm the accuracy of the probe. RPTPα cDNA fragments were labeled with [α-32P]dCTP using the Prime-It Random Primer Labeling Kit purchased from Stratagene. PAcP and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) cDNA probes were labeled utilizing the Random Primers DNA Labeling System from Life Technologies [25]. Filters were exposed to X-AR film (Kodak) at –70°C with an intensifying screen for different exposure time periods. The intensity of hybridization band was semiquantified densitometrically. The ratio of RPTPα/GAPDH was calculated from the signal intensity that was obtained in the linear range of curves for the signal intensity. Androgen effect LNCaP cells were plated in T-175 culture flasks at a density of 2.5 ×106 cells/flask in RPMI-1640 medium supplemented with 7% FBS, 1% glutamine and 0.5% gentamicin, and maintained for 3 days. To investigate androgen effect on the expression of RPTPα mRNA, LNCaP cells were grown in a steroid-reduced medium containing 2% heat-inactivated steroid-reduced, dialyzed FBS (SR-FBS) for 2 days, and then grown in fresh SR-FBS in the presence or absence of 10 nM 5α-dihydrotestosterone (DHT, Sigma) for an additional 3 days, as described in previous publications [25, 30, 34]. FBS effect Cells were plated in T-175 culture flasks with 2.5 × 106 cells/ flask in RPMI-1640 medium supplemented with 7% FBS and maintained for 3 days. All cells were then grown in 2% SRFBS for 24 h, which was replaced with fresh medium supplemented 0.5% SR-FBS for an additional 48 h. Cells were then grown in fresh 0.5% SR-FBS or 10% SR-FBS for 3 days, and harvested for the preparation of total RNA. Sodium butyrate effect Cells were seeded in T-175 culture flasks with 2.5 × 106 cells/ flask for PC-3 and DU145 cells, or 3.5 × 106 cells/flask for LNCaP cells in RPMI-1640 medium supplemented with 7% FBS for 3 days. Cells were then fed with the fresh medium containing 7% SR-FBS with or without sodium butyrate (Sigma) at different concentrations. A phenol red-free medium was used for LNCaP cells. The maximal concentration of sodium butyrate was 1.5 mM for LNCaP cells and 2.0 mM for PC-3 and DU145 cells because 2 mM of sodium butyrate
has a toxic effect on LNCaP cells. After 72 h cells were harvested. An aliquot of cells was used for cell counting, while remaining cells were utilized for total RNA preparation. Phorbol ester effect LNCaP cells were plated in RPMI-1640 medium supplemented with 7% FBS for 3 days. Cells were then fed with the fresh medium containing 2% SR-FBS in a phenol red-free medium for 48h, and 10nM 12-O-tetradecanoylphorbol-13-acetate (TPA, Gibco) or DHT was added to the same fresh culture medium. Total RNA from cultured cells was isolated at different time points for Nothern blot hybridization. The level of mRNA was normalized to rRNA. Data analysis The intensities of hybridization bands were semiquantified by densitometric analyses of autoradiograms utilizing the Molecular Dynamics Computing Densitometer and its software program.
Results Expression of RPTPα gene in various human tissue specimens. Existence of two RPTPα isoforms has been demonstrated for COS-1 cells [12]. We first investigated the expression of RPTP at the mRNA level in various normal tissues by Northern blot analyses. The level of RPTPα mRNA expression differed by 2–5 fold among different tissue specimens. In most tissues (Fig. 1, upper panel), the probe of RPTPα cDNA detected two isoforms of RPTPα mRNA, while only one major species, the short isoform, was observed in prostate, testis and ovary (Fig. 1, lanes c-e). Prostate tissue (Fig. 1, lane c) expressed a relatively high level of RPTPα mRNA. In spleen and thymus (Fig. 1, lanes a and b), both isoforms were expressed at an apparently equal level. The short isoform was expressed 2 fold more in small intestine and colon (Fig. 1, lanes f, g), while in peripheral blood leukocytes the long isoform was dominantly expressed (lane h). Contrarily, the expression of PAcP mRNA was only detected in prostate tissue, indicating the tissue-specific expression (Fig. 1, middle panel, lane c). Thus, RPTPα is expressed in all normal tissues examined in additional to neuronal cells [19, 22, 23, 35]. DHT, FBS, and butyrate effects on RPTPα mRNA expression in LNCaP cells We investigated the regulation of expression of RPTPα mRNA in epithelial cells utilizing prostate cancer cell lines. To investigate the androgen effect on RPTPα mRNA expression,
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Fig.1. Expression of RPTPα mRNA in normal human tissues. The multiple tissue membrane contains 2 µg poly (A+) mRNA per lane; aspleen; b-thymus; c-prostate; d-testis; e-ovary; f-small intestine; gcolon; h-peripheral blood leukocyte. Hybridizations were performed using [32P]-labeled cDNAs of RPTPα and PAcP. GAPDH cDNA was used as a control. Positions of RPTPα isoform were marked by arrows.
To elucidate the biological significance of RPTPα expression in prostate cells, we compared its expression with that of PAcP. These two subcloned cells expressed different levels of PAcP mRNA (Fig. 2). The clone 81 cells exhibited a rapid growth rate [25], and expressed a low level of PAcP (Fig.2). Stimulation of LNCaP cells by DHT and FBS led to a decreased level of PAcP mRNA despite different basal levels of expression (Fig. 2). Thus, in LNCaP cells, a stimulation of cell growth correlates with a decrease of the RPTPα as well as PAcP mRNA level despite different basal levels of expression. To further investigate the inverse correlation of RPTPα gene expression with the proliferation of LNCaP cells, sodium butyrate, a cell growth inhibitor, was utilized to treat clone33 cells that have a high basal level of PAcP expression. As shown in Fig.3A, sodium butyrate inhibited the growth of
two subclones of LNCaP cells were grown in the presence or absence of DHT. As shown in Fig. 2, these two subcloned cells expresed only one form of RPTPα mRNA as that in normal prostate cells (Fig. 1). Although these two cells exhibit different growth rates [25], the RPTPα mRNA level was very similar (Fig. 2). DHT treatment resulted in a decreased level of RPTPα mRNAs in both subclones of LNCaP cells, although in slow-growing clone-33 cells the decrease of RPTPα mRNA was only about 20%. We further analyzed the regulation of the RPTPα gene expression at the mRNA level by another growth stimulating agent, FBS. Similarly, FBS suppressed the mRNA level of RPTPα in these two subcloned cells and the effect on the rapidly growing clone-81 cells was more pronounced.
Fig. 2. Northern blot analyses of RPTPα and PAcP gene expression in LNCaP cells. LNCaP cells were cultured at growth conditions with or without 10 nM DHT, and 0.5 or 10% SR-FBS as indicated in the figure. Twenty µg of total RNA per lane from LNCaP cells were blotted on a nylon membrane. Hybridizations were performed using [ 32 P]-labeled cDNA fragments of RPTPα, PAcP and GAPDH, subsequently. Similar results were obtained from three sets of independent experiments.
Fig. 3. Butyrate effect on the expression of RPTPα in LNCaP cells. (A) Butyrate effect on the growth of LNCaP cells. After seeded in 7% FBS for 3 days and then incubated in a 7% SR-FBS for 48h, clone-33 LNCaP cells in triplicate were exposed to different concentrations of sodium butyrate as indicated in the figure. Total cell number was counted at day 3. The bar represented standard deviation. Similar results were obtained from two sets of independent experiments; (B), Northern blot analyses of RPTPα and PAcP expression in butyratetreated LNCaP cells. Total RNA were prepared from cells that were harvested at the denoted time point. The membrane was probed with a cDNA for RPTPα and then for PAcP. GAPDH was used a control for normalizing the expression level and for the quality of RNA preparations.
15 clone 33 LNCaP cells. Concurrently, the level of RPTPα and PAcP mRNA was respectively increased upon treatment (Fig. 3B). Data taken together show that the expression of RPTPα mRNA inversely correlates with the proliferation of LNCaP cells.
Differential effects by FBS and sodium butyrate on the expression of RPTPα mRNA in PC-3 and DU145 cells To further examine the regulation of RPTPα mRNA expression, we utilized two other human prostate cancer cell lines, PC-3 and DU145. In contrast to LNCaP cells, PC-3 and DU145 cells are undifferentiated prostate cancer cells since they do not express PAcP and are androgen-insensitive [24, 25, 30]. Both PC-3 and DU145 cells expressed one species of RPTPα mRNA (Fig. 4) as LNCaP cells (Fig. 2 and 3). While, DU145 cells had a much higher basal level of RPTPα mRNA than PC-3 cells (Fig. 4). Interestingly, FBS stimulated cell growth (data not shown) and induced the expression of RPTPα mRNA in both PC-3 and DU145 cells (Fig. 4). PC-3 and DU145 cells were treated with sodium butyrate, a reagent that can suppress cell proliferation and also induce partial differentiation of several cancer cells [36]. Sodium butyrate inhibited the growth of both PC-3 and DU145 cells (Fig. 5A) as on LNCaP cells (Fig. 3A). Nevertheless, sodium butyrate treatment did not result in an induction of PAcP expression (data not shown). Surprisingly, sodium butyrate inhibited the RPTPα mRNA level in PC-3 cells, but stimulated its expression level in DU145 cells. Thus, sodium butyrate has differential effects on the mRNA level of RPTPα in undifferentiated PC-3 and DU145 cells.
Fig. 5. Butyrate effect on the expression of RPTPα in PC-3 and DU145 cells. (A) Sodium butyrate effect on the growth of PC-3 and DU145 cells. Cells were treated with different concentrations of sodium butyrate as indicated in the figure. Total cell number was counted at day 3. The data shown were the average of triplicate from one set of experiment. The bar represented the standard deviation. Similar results were obtained from two sets of independent experiments; (B) Northern blot analysis of RPTPα expression in butyrate-treated PC-3 and DU145 cells. Total RNA were harvested at the denoted time point, and the blot was probed with a cDNA for RPTPα. GAPDH was used as a control for the quality of RNA preparation and normalization of expression; while the ribosomal RNA (rRNA) was included as an additional control for loading errors.
Effect of phorbol ester on the RPTPα mRNA expression in LNCaP cells.
Fig. 4. FBS effect on RPTPα expression in PC-3 and DU145 cells. Total RNA was prepared as described above. After electrophoresis and transfer, the blot was hybridized with a RPTPα cDNA probe followed by GAPDH cDNA. Similar results were obtained from two sets of independent experiments.
To initially look at the regulatory mechanism of RPTPα mRNA expression in LNCaP cells, we treated those cells with TPA, comparing with DHT. After 24h treatment, only approximately 10% RPTPα mRNA level was inhibited by TPA and DHT each. With an additional 48 h exposure, DHT and TPA inhibited RPTPα mRNA expression by up to 40% and 50%, respectively. Thus, TPA as well as DHT can inhibit RPTPα expression, although TPA is slightly more effective than DHT.
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Fig. 6. TPA effect on the expression of RPTPα mRNA. LNCaP cells were treated with 10nM DHT or TPA, respectively. After incubation, cells were harvested and total RNA was isolated for analyzing RPTPα mRNA expression. C, control; D, DHT; T, TPA.
Discussion PTPs have been classified as a separate family of phosphatases, distinct from protein-serine/threonine phosphatases, with a high specificity for tyrosine residues [6]. Relatively little is known about the regulation of PTP expression, activity, and the role of PTPs in signal transduction. Many receptor-linked tyrosine phosphatases exist in multiple forms that differ in their extracellular domains, due to the alternative splicing [37] or polyadenylation [23]. Our data clearly show the expression of RPTPα isoforms in numerous normal human tissues (Fig. 1). The data allow us to propose, that RPTPα may be involved in the signal transduction cascade in many cells. Regulation of its expression and functional role in nonneuronal cells deserves further investigation. An induction of PTP activity has been demonstrated to be associated with differentiation and growth inhibition of many cell systems [24, 38–41]. In our studies, RPTPα is expressed in all investigated human prostate cells. Normal prostate tissue (Fig. 1, upper panel, lane c) and cancer cells (Figs. 2–5) express only one form of RPTPα mRNA with a relatively high basal level. Thus, prostate cancer cells serve as an interesting model system to elucidate the regulation of expression of this enzyme in epithelial cells. LNCaP cells are partially differentiated androgensensitive prostate cancer cells and express prostate-specific differential markers, PAcP and prostate specific antigen (PSA) [25, 30]. Thus, LNCaP cells represent an interesting cell model in studying the functional role of RPTPα in the differention process of non-neuronal cells. It has been demonstrated that stimulation of cell growth by DHT and FBS correlates with a decline of PAcP activity in those cells [24,25]. In this report (Fig. 2), the down-regulation of RPTPα by DHT, FBS and TPA in LNCaP cells concurs with the decline in PAcP expression. Conversely, suppression of cell growth by butyrate correlates with an increase of RPTPα as well as PAcP (Fig. 3). These results taken together indicate that RPTPα may have a specific functional role in LNCaP cells, as that proposed for cellular PAcP [24, 25]. The regulation of RPTPα expression however follows a different mechanism from that of PAcP. The basal level of
PAcP expression is decreased in clone-81 LNCaP cells with a rapid growth rate (Fig. 2 and [25]), while the expression of RPTPα is not noticebly altered (Fig 2). Sodium butyrate can induce a partial differentiation phenotype of some cancer cells, and thus has been used to modify the growth and the phenotype of prostate cancer cells [36]. Sodium butyrate suppresses the proliferation of LNCaP cells and increases the RPTPα as well as PAcP mRNA level. Interestingly, the elevation of RPTPα mRNA follows a dose-dependent fashion, while up-regulation of PAcP mRNA follows a bell-shape dependence (Fig. 3B). Although the mechanism of decreased expression of PAcP mRNA in 1.5 mM butyrate remains unknown, these observations clearly show that the regulatory mechanism of RPTPα and PAcP expression is similar, but not identical. PC-3 and DU145 cells are undifferentiated cell lines and do not express differentiation markers including PSA and PAcP [30]. In contrast to LNCaP cells, FBS induces the expression of RPTPα mRNA in PC-3 and DU145 cells. These facts allow us to propose that the regulatory mechanism of certain gene expressions in PC-3 and DU145 cells is different from those in LNCaP cells. This differential response of RPTPα gene expression to FBS in different prostate cells is similar to the response of c-myc gene expression to protein kinase C (PKC) action on those cells [42]. PKC elicits a plethora of cellular responses on cell growth and differentiation and triggers the phosphorylation of RPTPα [43]. It has been shown that the PKC signal transduction pathway functions differently in androgen-sensitive LNCaP and androgeninsensitive PC-3 and DU145 prostate cancer cells [42]. For example, TPA down regulates 50% of c-myc mRNA in LNCaP, while it up-regulates two folds of c-myc expression in DU145 and PC-3 cells after a 3h treatment [42]. We also investigate the mechanism of regulation of RPTPα expression in LNCaP cells. TPA as well as DHT can inhibit RPTPα expression up to 50% after 72h treatment. Thus, the prolonged treatment with TPA leads to the down-regulation of PKC pathway (data not shown and ref. [38]), concurrently the expression of RPTPα is down-regulated. Thus PKC is possibly involved in the regulation of RPTPα expression. Nevertheless, the slow effect by TPA indicates that the TPA inhibition may not be a direct response. In this study, we describe the expression of RPTPα in several normal human tissues and its regulation in prostate cancer cells. In partially ‘differentiated’ androgen-sensitive LNCaP cells, there is a strong correlation in the regulation of expression of RPTPα with PAcP – a prostate-specific differentiation marker. However, the mechanism of regulation of RPTPα expression is different in androgen-insensitive undifferentiated PC-3 and DU145 cells. Additionally, there is an unexpected up-regulation of RPTPα by butyrate in DU145 cells than in PC-3 cells (Fig. 5). This may be due to DU145 cells which are much less differentiated than PC-3
17 cells. In terms of morphonuclear characteristics and population dynamics, the PC-3 cell line is more differentiated, hormonesensitive than the DU145 one [44]. Furthermore, PC-3 cells express a low level of androgen receptor but not DU145 cells [25]. Thus PC-3 cells maintain a higher degree of differentiation than DU145 cells. This hypothesis is further supported by the analysis of signaling pathways [45]. The data taken together support the idea that DU145 cells have more deregulated signal transduction pathways than PC-3 cells, and, consequently, the regulation of RPTPα is different. In summary, results indicate that, in differentiated prostate epithelial cells, RPTPα may serve as an adaptor molecule by linking other regulatory components as it in neuronal cells [19]. Future studies are required to determine the role of RPTPα in the ‘differentiated’ prostate epithelium cells.
Acknowledgements We thank Dr. Tzu-Ching Meng and Juliette E. Petersen for critical comments. This work was supported in part by a grant (CA 72274) from the National Cancer Institute, NIH and LB 506 (#2000–19) from the Nebraska Department of Health.
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