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Am J Pathol. Jan 2001; 158(1): 25–32.
PMCID: PMC1850253

Increased Expression of the Interleukin-11 Receptor and Evidence of STAT3 Activation in Prostate Carcinoma

Abstract

Previous investigations have shown that interleukin-6, a member of the JAK-STAT activating family of cytokines, plays an important role in prostate carcinoma. Here we demonstrate the co-expression of another member of this cytokine family, interleukin-11 (IL-11), and components of its receptor (interleukin-11 receptor; IL-11R), ie, IL-11Rα (involved in ligand recognition), and gp130 (involved in signal transduction) in cultured normal and malignant prostate-derived epithelial cell lines. In the DU-145 prostate carcinoma cell line, rhIL-11 stimulates a transient and dose-dependent increase in the tyrosine 705-phosphorylated, active form of STAT3 (STAT3 P-Tyr705), involved in the downstream signaling of IL-11R and other members of the gp130-dependent receptors. The ability of IL-11 to activate STAT3 in prostate-derived cells may be mechanistically important, given recent data suggesting that constitutively activated STAT3 may be associated with the malignant phenotype. In 51 human primary tissues derived from normal prostate, benign prostatic hyperplasia, and prostate carcinomas, IL-11Rα and gp130 were commonly expressed, with a statistically significant elevation in the expression of IL-11Rα in prostate carcinoma. Also, the tyrosine-phosphorylated, activated form of STAT3 was observed more prominently in the nuclei of cells residing in malignant glands compared to those in nonmalignant samples. Thus, the IL-11 receptor system is up-regulated in prostate carcinoma, and may be one part of a cytokine network that maintains STAT3 in its activated form in these tissues.

Prostate carcinoma is one of the leading causes of cancer deaths among men in the United States. 1 As part of an effort to better understand the biology of normal prostate tissue and its neoplastic counterpart, we have been investigating the presence of cytokine and growth factor networks in these tissues, because these cell signaling systems play a key role in the cellular events that are important in malignancy, such as growth, cell survival, and motility. Here we report that the receptor for interleukin-11 (IL-11) is present in prostate-derived tissue, and is expressed to a greater degree in prostate carcinoma compared to nonmalignant prostate tissue.

The IL-11 receptor (IL-11R) mediates the action of IL-11, a 19.1-kd pleiotropic cytokine that was initially cloned from a bone-marrow stromal cell line. 2 Whereas the hematopoietic effects of IL-11, which include stimulation of megakaryocyte maturation and platelet production, 3 and growth stimulation of CD34+ hematopoietic progenitor cells, 4 have been well studied, this cytokine has also been shown to mediate inhibition of adipogenesis, 4 stimulation of osteoclasts, 5 and cytoprotection of gut mucosa. 6-9 The IL-11 receptor is a member of a family of cytokine receptors, sometimes referred to as the gp130-dependent family of receptors, which includes the receptors for IL-6, leukemia inhibitory factor, ciliary neurotrophic factor, oncostatin M, and cardiotrophin. 10 The α subunit of the IL-11R, IL-11Rα, is required for high affinity binding of the ligand; on ligand binding, gp130, the subunit responsible for signal transduction, is recruited to the receptor complex. 11 It is not known whether IL-11R-associated gp130 undergoes homodimerization, as it does in the case of the IL-6 receptor, or if an additional, as yet unidentified, subunit is involved. 12 Among the signaling systems activated by IL-11R and other members of this receptor family is the JAK-STAT pathway. On ligand binding, these receptors activate members of the JAK kinase family; these kinases phosphorylate tyrosine residues in the cytoplasmic domains of the gp130 subunit, which in turn serve as docking sites for members of a family of latent transcription factors, the so-called signal transducers and activators of transcription proteins, ie, STAT proteins. 13,14 These recruited STAT proteins undergo tyrosine phosphorylation, which permits their dimerization and translocation to the nucleus where they alter gene expression. 15 Indeed, IL-11 has been shown to activate the JAK1 and JAK2 receptor-associated kinases, 16,17 triggering the activation of STAT1 and STAT3. 12,18 The latter action may be especially important, as constitutive or aberrant activation of STAT3 has recently been associated with the malignant phenotype. Evidence for this includes: 1) constitutively activated STAT3 has been reported in a variety of carcinomas and hematological malignancies, 19-24 2) cell transformation by src seems to be associated with STAT3 activation, 25,26 3) a mutant form of STAT3 that spontaneously dimerizes and self-activates can act as an oncogene, 27 and 4) transfection of cell lines with dominant-negative forms of STAT3 results in growth suppression and/or induction of apoptosis. 23,24,28 Other signaling systems that may be activated by the IL-11R include MAP kinase, the ribosomal S6 protein kinase, pp90rsk, 29 src-family tyrosine kinases, eg, p60src and p62yes, and phosphatidylinositol-3 kinase. 30,31

Two developments prompted this study. First, there is evidence that one member of the cytokine family that activates the JAK-STAT pathway, IL-6, may be important in prostate biology. IL-6 is present in seminal fluid and elevated plasma levels of this cytokine have been associated with increased morbidity in prostate cancer patients. 32 In addition, IL-6 acts as an autocrine growth factor in several prostate cancer cell lines, and this growth stimulation is accompanied by activation of STAT3. 33 These data suggest that other members of this cytokine family might also be important in both normal and malignant prostate epithelial cells. Second, using a reverse transcriptase-polymerase chain reaction (RT-PCR)-based screen for the expression of a number of prototypical cytokines and their receptors, we have discovered that one of the potential autocrine loops expressed by a normal human prostate epithelial cell line involves IL-11 and its receptor. As a result, we investigated the expression of components of the IL-11 system in prostate-derived cells and tissues.

Materials and Methods

Cell Culture

Normal human prostate epithelial cells were obtained from Clonetics (San Diego, CA); these cells were cultured for 2 to 4 passages in a defined medium as previously described. 34 Culturing of the DU-145, PC-3, and LNCaP prostate carcinoma cell lines (all obtained from the ATCC, Rockville, MD) were performed as outlined previously. 35 Recombinant human IL-11 (rhIL-11) was obtained from R&D Systems (Minneapolis, MN).

Detection of IL-11 and IL-11Rα mRNA by RT-PCR

The basic RT-PCR methodology used here has been described. 34 For the detection of IL-11 mRNA, the following primers were used: sense: 5′-ACT GCT GCT GCT GAA GAC TCG GCT GTG A-3′, antisense: 5′-ATG GGG AAG AGC CAG GGC AGA AGT CTG T-3′; for IL-11Rα, sense: 5′-GCC AAG CAG CCG ACT ATG AGA A-3′, antisense: 5′-AGT AGC CGA GGG TGT GGT TGG A-3′. Amplification was performed in a Stratagene Robocycler Gradient 40 thermal cycler (Stratagene, La Jolla, CA) under the following conditions: denaturation, 94°C, 45 seconds; annealing, 63°C for IL-11, 64°C for IL-11Rα, 45 seconds; extension 72°C, 2 minutes; 35 cycles followed by a 10-minute polishing step at 72°C. PCR products (expected sizes: 322 bp for IL-11, 712 bp for IL-11Rα) were resolved by electrophoresis using 1.5% agarose gels containing ethidium bromide. To confirm the identity of the PCR products, a Southern blotting technique was used. Gels were denatured in 0.2 N NaOH, 0.6 mol/L NaCl for 30 minutes, then washed in 25 mmol/L sodium phosphate buffer, pH 6.5; DNA fragments were transferred onto a GeneScreen nylon membrane (New England Nuclear-Dupont, Boston, MA) in the latter buffer overnight. The following antisense oligonucleotides to internal sequences within the expected PCR products, IL-11, 5′-GAT CTG GCT TTG GAA GGA CGG TGG TGG CT-3′, and IL-11Rα, 5′-GGA CGG TAC TGC AAA CGG AAC TTG AGC A-3′, were 32P-end-labeled with T4 polynucleotide kinase and used to probe the membrane in a hybridization buffer consisting of 2× standard saline citrate (SSC), 50% deionized formamide, 10% dextran sulfate, 0.02% bovine serum albumin, 0.02% polyvinyl-pyrrolidone, 0.02% Ficoll, 1% sodium dodecyl sulfate (SDS), and 500 mg/ml tRNA overnight at 42°C on an orbital shaker. The membrane was then washed in 2× SSC, 1% SDS at 55°C (20 minutes), two washes (15 minutes) in 0.1× SSC, 0.1% SDS, 55°C, and one 5-minute wash in 0.1% SSC at room temperature, then sealed and exposed to Kodak BioMax MS film (Eastman-Kodak, Rochester, NY) for 2 hours.

Secretion of IL-11 by Cell Lines as Determined by Enzyme-Linked Immunosorbent Assay

Normal and malignant cell lines (see above) were plated in their respective growth media at 1 × 10 5 cells/well in six-well dishes and incubated in a humidified 37°C incubator in a 95% air-5% CO2 atmosphere for 96 hours. Culture media incubated without cells served as controls. Conditioned media was then harvested and stored at −70°C, and cells were trypsinized and counted using a hemocytometer. The conditioned media was thawed and centrifuged briefly before assay for soluble IL-11 protein using a commercially available enzyme-linked immunosorbent assay kit (R&D Systems).

Immunoblotting of STAT3 Phosphorylated at Tyr705

DU-145 prostate carcinoma cells (0.5 × 106) were plated in minimal essential medium containing 0.5% fetal bovine serum and incubated at 37oC for 48 hours to reduce basal levels of activated STAT3. Cells were then incubated in serum-free minimal essential medium for 4 hours, then treated with serum-free medium containing various concentrations of rhIL-11 for periods of up to 60 minutes. Cells were lysed in 62.5 mmol/L Tris-HCl, pH 6.8, 2% SDS, 10% glycerol, 50 mmol/L dithiothreitol, 0.1% bromphenol blue, heated to 95oC for 5 minutes, placed on ice, and loaded onto 10 to 15% gradient SDS-polyacrylamide gel electrophoresis gels (Amersham Pharmacia, Piscataway, NJ). After electrophoresis in a PhastSystem apparatus (Amersham Pharmacia), gels were electrotransferred onto nitrocellulose membranes; these membranes were analyzed for the presence of Tyr705-phosphorylated STAT3 using an antibody from New England Biolabs (Beverley, MA), according to the manufacturer’s procedures.

Immunohistochemical Analysis of Prostatic Tissues and Cell Lines

Formalin-fixed, paraffin sections (5 μm) of normal prostate, benign prostatic hyperplasia, and prostate carcinoma were obtained either from the Cooperative Human Tissue Network (Columbus, OH) or the Department of Pathology at the University of Massachusetts/Memorial Health Care System (Worcester, MA) under a human subjects protocol approved by the University of Massachusetts Medical School human subjects institutional review board. These sections were deparaffinized by heating the slides to 60°C for 15 minutes and then subjecting them to two 5-minute changes in 100% xylene; the sections were then rehydrated by serial incubations in 100%, 90%, and 80% ethanol, followed by phosphate-buffered saline (PBS). The sections were then subjected to an antigen retrieval procedure: slides were placed in 10 mmol/L sodium citrate, pH 6.0, autoclaved at 121°C for 10 minutes, then placed in PBS for 30 minutes. In the case of cell lines, cultured cells were harvested by trypsinization and centrifuged onto poly-l-lysine-treated glass slides, or the cells were plated in their appropriate growth media on glass slides and grown for 24 hours; the cells were then fixed for 5 minutes in methanol at −20°C. After this, specimens were treated with methanol containing 0.3% H2O2 for 5 minutes to inactivate endogenous peroxidase activity. The samples were washed with PBS and nonspecific binding sites blocked by incubating with 5% goat serum in PBS for 90 minutes in a humidified chamber at room temperature. Samples were then incubated for 60 minutes at room temperature with rabbit anti-human IgG polyclonal antibodies to IL-11Rα or gp130 (Santa Cruz Biotechnology, Santa Cruz, CA) or rabbit polyclonal IgG controls (Santa Cruz Biotechnology) at concentrations of 3.5 μg/ml (IL-11Rα) or 1.0 μg/ml (gp130) in PBS containing 5% goat serum. Specimens were then rinsed in wash buffer (PBS containing 0.5% bovine serum albumin, 0.1% Tween-20) and incubated for 30 minutes with biotinylated goat anti-rabbit IgG (rabbit ABC staining kit, Santa Cruz Biotechnology) diluted according to the manufacturer’s protocol. Next, a solution of avidin-conjugated horseradish peroxidase (ABC staining kit) was applied for 30 minutes, according to the manufacturer’s recommendations. Slides were then rinsed twice in wash buffer and treated at room temperature with a H2O2/3-amino-9-ethylcarbazole substrate solution prepared as recommended by the manufacturer (Sigma Chemical Co., St. Louis, MO): color development time was in the range of 4 to 8 minutes. To confirm the specificity of the anti-IL-11Rα staining, the anti-IL-11Rα antibody was preincubated for 2 hours at room temperature with a fivefold excess by weight of the specific IL-11Rα peptide that this antibody was raised against (Santa Cruz Biotechnology) before using this antibody in the above-described immunostaining method. This preabsorption procedure was found to block immunostaining in prostate tissues, indicating that the staining observed was specific for IL-11Rα. Similarly, preincubation of the anti-gp130 antibody with a 10-fold excess of the specific gp130 peptide that this antibody was raised against (Santa Cruz Biotechnology) also blocked tissue staining. None of the tissues treated with control, nonspecific antibodies demonstrated staining.

A subset of paraffin sections was also screened for the presence of STAT3 phosphorylated at Tyr705 (STAT3 P-Tyr705), the activated form of STAT3 that is generated downstream of IL-11R/gp130 activation. Sections were processed and stained using a 1:100 dilution of a rabbit polyclonal IgG anti-human STAT3 P-Tyr705 antibody (Cell Signaling Technology, Beverly, MA), or rabbit polyclonal IgG controls using the same immunohistochemical methodology described above, except that the incubation with the primary antibody was performed for 24 hours at 4°C and the secondary antibody incubation time was increased to 1 hour. No staining is observed if nonspecific rabbit polyclonal IgG is used. To further determine the specificity of the staining, the anti-STAT3 P-Tyr705 antibody was preabsorbed for 2 hours at room temperature with a 10-fold excess by weight of the phosphorylated peptide which the antibody was raised against (SAAPY*LKTK, where Y* is a phosphotyrosyl residue; a gift from Dr. Nicole Stark, Cell Signaling Technology). This absorption procedure completely eliminated the nuclear staining of the prostate tissue samples

Results

Expression of IL-11 and Its Receptor in Normal and Malignant Cell Lines

As part of our laboratory’s effort to uncover novel autocrine and paracrine axes that may play important functional roles in the prostate, we examined a commercially obtained normal prostate epithelial cell line (referred to as PrECs) for the expression of a variety of cytokines, growth factors, and their cognate receptors using an RT-PCR-based screen. Among the factors tested were interleukins 1 (α and β) through 15, and their receptors, excepting the receptors for IL-12, 13, and 14. One of the salient findings of this study was the co-expression of IL-11 and its receptor in the PrECs (Figure 1A [triangle] , lane 2). To determine whether co-expression of IL-11 and IL-11Rα occurred in malignant prostate epithelial cells as well, we also tested several frequently studied prostate carcinoma cell lines, DU-145, PC-3, and LNCaP cells, for the presence of these mRNAs using RT-PCR. As shown in Figure 1A [triangle] , each of these cell lines also expressed both IL-11 and IL-11Rα. The identities of these mRNAs were confirmed by Southern blotting using internal probes specific to the respective sequences. We then determined if these mRNAs were expressed as proteins in these cells. Cell lines were plated in their respective growth media and incubated for 96 hours; the conditioned media was then harvested and assayed for human IL-11 using an enzyme-linked immunosorbent assay. Each of the three cell lines tested (PC-3 cells were not studied) secreted IL-11 (Figure 1B) [triangle] ; DU-145 cells expressed this cytokine at the highest level (97.4 ± 1.6 pg/10 5 cells), followed by PrECs (57.4 ± 5.1 pg/10 5 cells), then LNCaP cells (33.1 ± 13.2 pg/10 5 cells). Control media, ie, media that was incubated in the absence of cells, contained less than detectable levels of human IL-11 (<8 pg/ml). By immunohistochemistry, the specific expression of IL-11Rα was demonstrated in all four of these cell lines (for examples, see Figure 1C [triangle] ). All these cell lines stained positively for gp130, the other IL-11 receptor subunit (data not shown). That this IL-11 receptor is functional was demonstrated in studies with serum-starved DU-145 cells. rhIL-11 was able to mediate a transient activation of the phosphorylation of STAT3 on Tyr705 (STAT3 P-Tyr705) in these cells, with a peak at 10 minutes and a return to basal levels by 60 minutes; this activation was also dose-dependent (Figure 1D) [triangle] . Thus co-expression of IL-11 and its receptor, ie, a potential autocrine loop, is commonly observed in human cultured prostate cells.

Figure 1.
Evidence for the expression of IL-11 and IL-11Rα in normal and malignant prostate epithelial cell lines. A: Expression of the mRNA for IL-11 (left) and IL-11Rα (right) in the normal prostate epithelial cell line, PrEC (lane 2) and the ...

Expression of IL-11R Components in Normal Human Prostate, Benign Prostatic Hyperplasia (BPH), and Prostate Carcinoma

We then asked if components of the IL-11 system were present in primary prostate tissues. Twenty-three primary prostate carcinomas (with combined Gleason scores ranging from 6 to 9) and 28 nonmalignant specimens (normal prostate or BPH) were analyzed for the expression of IL-11Rα by immunohistochemistry. Several examples of these immunostains are presented in Figure 2 [triangle] . It was noted that the epithelial cells of the nonmalignant specimens displayed weak staining for IL-11Rα (Figure 2A) [triangle] , whereas more frequent and prominent staining occurred within the high-grade prostatic intraepithelial neoplasia (PIN) and invasive carcinoma (Figure 2, B–D) [triangle] . Staining was observed in the stroma of all prostate-derived specimens. To quantify these observations, epithelial cells within the glands of the prostate samples were scored for positive staining using the following scale: if no cells were positive, a score of 0 was given; if >0 but ≤25% of the cells stained positively, a score of 1+ was given; if >25 but ≤50% of the cells were positive, a score of 2+ was assigned; and if the number of positive cells was >50%, a score of 3+ was given. Fourteen of the 23 carcinomas tested had a score of 3+ (61%) for IL-11Rα staining, representing frequent expression of this receptor, whereas only nine (39%) had scores of 2+ or 1+, representing moderate to low frequency of IL-11Rα expression; none of the carcinoma samples stained negatively (Table 1) [triangle] . In contrast, among the 28 nonmalignant samples, six (21%) displayed no staining for IL-11Rα, 22 (79%) had scores of 2+ or 1+, and none had a 3+ score. Statistically, the scoring difference between the malignant versus nonmalignant samples was highly significant (P < 0.001) using a two-sided Fisher’s exact test. There was no significant association, however, between IL-11Rα staining and Gleason score, nor was there was any significant difference in extent of IL-11Rα staining between BPH and normal prostate tissues. These results indicate that IL-11Rα may become up-regulated during the malignant transformation of prostate epithelial cells, and/or that there is an expansion of an IL-11Rα-expressing cell type that occurs during this process. We also performed immunohistochemical staining for gp130, the signal transducing component of the IL-11 receptor; gp130 was universally expressed in the epithelial and stromal portions of these samples (data not shown), as has been observed in many other tissues and cell types. 10 We were unable to obtain an antibody to human IL-11 that was satisfactory for the immunostaining of formalin-fixed paraffin sections; as such, we were unable to determine whether potential IL-11 autocrine or paracrine axes are found in these prostate specimens.

Figure 2.
Immunohistochemical staining of normal human prostate and primary prostate carcinoma for IL-11Rα. Experimental procedures are described in Materials and Methods. A: Normal prostate tissue section showing weak epithelial cell staining, but high ...
Table 1.
Expression of IL-11Rα in Primary Prostate Carcinomas Versus Normal Prostate and Benign Prostatic Hyperplasia (BPH) as Determined by Immunohistochemistry

Expression of Tyrosine-Phosphorylated STAT3 in Normal Human Prostate, BPH, and Primary Prostate Carcinoma

We then asked if the activated form of STAT3, ie, STAT3 phosphorylated at Tyr705 (STAT3 P-Tyr705), which would be produced as a consequence of activation of the IL-11R, other members of the gp130-dependent family of receptors, or certain growth factor receptors such as erbB1, 36 were present in these prostate-derived tissues. An immunohistochemical assay was developed that used antibodies that specifically recognize only this phosphotyrosine form of STAT3. Because STAT3 P-Tyr705 is the species that dimerizes, enters the nucleus, and acts as a transcription factor, 15 it would be predicted that one would observe primarily nuclear staining in tissues where STAT3 is activated. A total of 21 prostate carcinomas and 28 specimens of normal or BPH were immunostained for STAT3 P-Tyr705. As shown in Figure 3 [triangle] , nuclear staining was obtained in the glandular epithelial cells of both the malignant (Figure 3A) [triangle] and nonmalignant samples (Figure 3B) [triangle] . The immunostains were quantitated using the scoring system described above for IL-11Rα staining. As in the case of IL-11Rα, it was noted that the epithelial cells in many of the nonmalignant specimens stained relatively weakly for STAT3 P-Tyr705 as compared to those in the malignant samples. For example, 15 of the 21 carcinomas (71%) displayed frequent staining for STAT3 P-Tyr705 (>50% of the cells staining positively), four (19%) displayed low to moderate staining (1+ or 2+), and only two samples were negative for this form of the protein (Table 2) [triangle] . In contrast, seven of the 28 normal/BPH samples (25%) did not stain for STAT3 P-Tyr705,19 (66%) exhibited low to moderate staining (1+ or 2+), and only two displayed a high percentage of positively staining cells (3+). The scores for the malignant samples were significantly different from the nonmalignant (P = < 0.0001) using a two-sided Fisher’s exact test. A comparison of the scores for IL-11Rα expression and STAT3 P-Tyr705 among the benign samples reveals that there is some discordance between the two. In at least five cases, moderate expression of activated STAT3 was observed in samples where IL-11Rα was absent from the epithelial cells. This suggests that cytokines or growth factors other than those that activate the IL-11 receptor, or some other mechanism, are contributing to the activation of STAT3 in these cases.

Figure 3.
Immunohistochemical staining of prostate carcinoma (A) and normal prostate tissue (B) for the tyrosine-phosphorylated, activated form of STAT3 (STAT3 P-Tyr705). Note the strong nuclear staining in prostate carcinoma (A) as compared to the weak, more sporadic ...
Table 2.
Presence of STAT3 P-Tyr705 in Primary Prostate Carcinomas Versus Normal Prostate and BPH as Determined by Immunohistochemistry

Discussion

The major finding of this investigation is that components of the IL-11 receptor system are commonly expressed in normal prostate, BPH, prostate carcinomas, and cell lines, and that the expression of the IL-11Rα subunit is elevated in prostate carcinoma relative to their BPH or normal prostate counterparts. Furthermore, the tyrosine-phosphorylated, activated form of STAT3, a signal transducing protein downstream of the IL-11 and other cytokine receptors, appears to be more prominent in primary prostate carcinoma cells compared to the epithelial cells of BPH or normal prostate. The presence of the IL-11 system in prostate tissue is not without precedent. Douglas and colleagues 37 found the mRNA for IL-11Rα in 15 of 15 primary breast carcinomas; IL-11 has also been reported in some breast cancer cell lines. 38,39 A recent immunohistochemical-based study from our laboratory revealed that the malignant epithelial cells of 41 of 44 primary human ovarian tumors co-express IL-11Rα and gp130, and that co-expression of IL-11 and its receptors are frequently observed in cell lines derived from both a variety of solid tumors and hematological malignancies. 40 Thus the IL-11 system may be more commonly expressed in malignant tissues and some of their normal counterparts than originally appreciated.

The exact function of IL-11 and other JAK-STAT activating cytokines in the prostate remains unclear. IL-6, which shares many overlapping actions with IL-11 in a number of cell systems, has been shown to activate androgen receptor-mediated gene expression in a STAT3-dependent manner in LNCaP prostate cancer cells. 41 Importantly, IL-6 was found to mediate androgen receptor activation synergistically with androgens, but was capable of triggering androgen receptor activation in the absence of androgen. 41 Thus in the normal prostate IL-6 (and IL-11) might act to sensitize androgen receptor-expressing prostate epithelial cells to the actions of androgens, whereas in the case of malignancy, up-regulation of these STAT3 activating cytokine systems might drive androgen receptor activation in the absence of androgens, thereby playing a role in the development of androgen independence. Another potentially relevant finding is that IL-11-expressing breast cancer cells can up-regulate the expression of aromatase in breast adipose stromal cells; by this paracrine interaction, the carcinoma cells might increase the level of stromal cell-generated estrogen. 38 Based on these observations, the relationship between expression of JAK-STAT activating cytokines and sex steroid metabolism in steroid-response tissues such as prostate and breast deserves further study. One may also consider the potential role of the IL-11 system in aspects of the biological behavior of prostate carcinoma, such as the predilection of this malignancy to metastasize to bone. Because IL-11 is produced by bone-marrow stromal cells, 3 IL-11R-bearing prostate carcinoma cells might be chemotactically attracted to this site; alternatively, if IL-11 serves as a growth or survival factor for these cells, the IL-11-rich bone marrow might be a hospitable environment for metastasizing cells.

The presence of the activated form of STAT3 and its up-regulation in prostate carcinoma, has important implications in terms of prostate cancer biology. As previously discussed, there is a growing body of evidence associating constitutive or aberrant activation of STAT3 with oncogenic transformation in human cancers. 19-28 Constitutively activated STAT3 has recently been reported in prostate carcinoma cell lines; 33 furthermore, transfection of these cells with dominant-negative forms of STAT3 triggers a suppression of their proliferation. 42 The data presented here provides evidence that activated forms of STAT3 occur in primary prostate carcinomas as well. Based on the findings presented here that the IL-11 receptor is commonly and prominently expressed in prostate cancers, that IL-11 can trigger STAT3 activation in prostate-derived cells, and the recent data that IL-6 may be an important inducer of STAT3 activation and prostate epithelial cell growth, 33,42 one may postulate that there are multiple cytokine networks involving the gp130-dependent receptors and other systems operating in prostate carcinoma whose function is to maintain STAT3 in an activated state, thus helping to drive the malignant phenotype.

Footnotes

Address reprint requests to Todd M. Savarese, Ph.D., UMass Cancer Center, Rm HB-774, University of Massachusetts Medical School, 55 Lake Ave. North, Worcester, MA 01655. E-mail: .ude.demssamu@eseravas.ddot

Supported by an American Cancer Society institutional grant, the University of Massachusetts Cancer Center, and the Department of Pathology, University of Massachusetts/Memorial Heath Care.

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