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The Journal of Clinical Endocrinology & Metabolism Vol. 82, No. 11 3851-3858
Copyright © 1997 by The Endocrine Society


Original Studies

Expression and Localization of Activin Subunits and Follistatins in Tissues from Men with High Grade Prostate Cancer

T. Z. Thomas, H. Wang, P. Niclasen, M. K. O’Bryan, L. W. Evans, N. P. Groome, J. Pedersen and G. P. Risbridger

Institute of Reproduction and Development, Monash University (T.Z.T., H.W., P.N., M.K.O., G.P.R.), Clayton; and Melbourne Pathology (J.P.), Collingwood, Australia; and Oxford Brookes University (L.W.E., N.P.G.), Headington, United Kingdom

Address all correspondence and requests for reprints to: Dr. G. P. Risbridger, Institute of Reproduction and Development, Monash University, Level 3, Block E, Monash Medical Center, 246 Clayton Road, Clayton 3168, Australia. E-mail: gail.risbridger{at}med.monash.edu.au


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Activins are growth and differentiation factors that have growth inhibitory effects on LNCaP and DU145, but not PC3, human prostate tumor cell lines. Activin-binding proteins, follistatins, block the inhibitory actions of exogenously added activins on LNCaP and DU145 tumor cell lines. Based on these in vitro observations using human prostate tumor cell lines, the aims of this study were to determine whether activins and follistatins are expressed in the human prostate in tissues from men with high grade prostate cancer. The expression and cellular localization of these proteins in malignant and nonmalignant regions of these tissues were compared to determine whether any changes occur with progression to malignancy. The results demonstrate that activins and follistatins are synthesized in tissues from men with high grade prostate cancer, and that messenger ribonucleic acid (mRNA) and protein for the activin ßA- and ßB-subunits and follistatin is expressed and localized to poorly differentiated tumor cells. In the nonmalignant regions, activin ßA and ßB subunit mRNA and proteins are predominantly localized to the epithelium. Follistatin mRNA was expressed in the basal epithelial cells and in the fibroblastic stroma; however, the localization of follistatin proteins using two specific antisera demonstrated a difference between the follistatin isoforms expressed in basal cells and the stroma. In the progression to malignancy, the colocalization of follistatin and activins to the tumor cells in vivo implies that resistance to the growth inhibitory effects of activin may be conferred by follistatins.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
A NUMBER of growth factors are involved in the progression to malignancy, including members of the transforming growth factor-ß (TGFß) superfamily. Based on the significant homology between the ß-subunits, activins and inhibins are members of the TGFß superfamily and were first identified by their ability to regulate pituitary FSH. Inhibins inhibit FSH synthesis, whereas activins stimulate FSH synthesis (1, 2, 3). Inhibins are glycoprotein hormones that are formed by the dimerization of two subunits, i.e. an {alpha} with either ßA (inhibin A) or ßB (inhibin B). The inhibin ßA- and ßB-subunits can also form dimers called activins; three forms are known to exist: activin A (ßAßA), activin B (ßBßB), and activin AB (ßAßB) (4). Activins have been shown to have either proliferative or antiproliferative actions in reproductive, neuroendocrine, and erythroid tissues (5, 6, 7). The effects of activins on cultured cells include induction of cell cycle arrest (8) and apoptosis (9). Follistatins are structurally unrelated proteins (which also stimulate FSH secretion) (10) and bind activins, resulting in the neutralization of activin bioactivity (9, 11). Follistatins are glycosylated monomeric proteins that arise from two alternatively spliced messenger ribonucleic acid (mRNA; FS315 and FS288), resulting in three protein cores; nine mol wt forms are postulated, and six have been identified and characterized (12).

Recent studies from this laboratory have shown that inhibins, activins, and follistatins are synthesized by the human prostate gland. Thomas and colleagues (13) have reported mRNA expression for {alpha}- and ß-subunits, the activin type II receptor, and both follistatin transcripts in tissues from men with benign prostatic hyperplasia. Proteins for activin A, activin ßA and ßB subunits, and follistatins were localized to the hyperplastic human prostate by immunohistochemical techniques (13). More recently, inhibin {alpha}-subunit gene expression and protein synthesis were reported in benign prostate hyperplasia (BPH) tissues; however, in malignancy, down-regulation of inhibin {alpha} gene expression occurred, and no mRNA or protein was observed in tissues from patient with high grade cancers (Mellor SL, Richards MG, Pedersen JS, Robertson DM, and Risbridger GP, submitted for publication). The loss of inhibin {alpha} expression in vivo correlated with the failure to detect inhibin {alpha} gene or protein in human prostate tumor cell lines (14, 15, 16). Together with the findings that functional deletion of the inhibin {alpha}-subunit results in the development of gonadal tumors in mice (17), it was proposed that the inhibin {alpha} gene is a tumor suppressor (16).

In the absence of inhibin {alpha} gene expression in high grade prostate cancer, it remains to be determined whether there is additional regulation of activin ß-subunits, which may involve follistatin expression. In vitro studies using the human prostate tumor cell lines LNCaP, DU145, and PC3 have reported activin ß-subunit and follistatin mRNAs and proteins (14, 15, 16, 18), suggesting endogenous production of these proteins by prostate tumor cells. However, the effect of exogenously added activins A and B was to inhibit the growth of LNCaP and DU145 cells; this response was not observed by PC3 cells, which appear resistant to the effects of activin A (18). Although activin inhibits tumor cell proliferation in vitro, follistatin completely blocks this action on DU145 and LNCaP cells (18). Therefore, the different in vitro observations may be due to the production of different levels of follistatins and/or protein forms by each of the tumor cell lines. The aim of this study was to determine whether poorly differentiated tumor cells from men with high grade prostate cancer express activin ßA- and ßB-subunits and follistatins. The patterns of localization and expression in malignant tissue were compared with those in nonmalignant regions of the same tissue specimens.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Patient samples

Needle biopsy specimens were obtained from a total of 25 patients undergoing transrectal ultrasound (TRUS)-guided biopsy. Carcinoma of the prostate was confirmed by histological examination (Melbourne Pathology, Collingwood, Australia), and the patients selected were those diagnosed with high grade prostate cancer and a Gleason score between 7–10. The specimens were obtained in accordance with the requirements and approval of the standing committee for human ethics and experimentation at Monash Medical Center and Monash University. Four-micron sections were cut from the specimens and used for immunohistochemistry and/or in situ hybridization (shown in Table 1Go) as described below. The expression or localization of inhibin ß-subunits or follistatin was compared in nonmalignant regions and malignant (Gleason grade 4 or 5) regions of the tissue sections.


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Table 1. Analysis of patient samples for ßA, ßB, and follistatin proteins and mRNA by immunohistochemistry and in situ hybridization using specific antibodies and riboprobes

 
Immunohistochemical studies

Antibodies. Four antibodies for the inhibin ßA- and ßB-subunit proteins were used for these studies. The ßA and ßB Salk polyclonal antibodies (gifts from Dr. W. Vale) were raised in rabbits against synthetic human ßA or ßB peptides (19). The ßA and ßB Groome monoclonal antibodies (gifts from Dr. N. P. Groome) were raised against synthetic human ßA or ßB peptides and have previously been used for the measurement of inhibin A and inhibin B (20, 21).

Follistatin immunoreactivity was determined using two antisera. AS 202 was raised in an intact adult male New Zealand rabbit to purified bovine 39-kDa follistatin and has been used for the specific measurement of follistatin in serum samples by RIA (22). On Western blot analysis, the cross-reaction of bovine follistatin with AS 202 was approximately 16 times greater than that with human recombinant FS288 per ng protein (de Kretser, D. M., unpublished observation). Preabsorption of AS 202 was achieved by incubating 1 µg bovine follistatin with 25 µL of a 1:500 dilution of AS 202 overnight at 4 C. The mixture was centrifuged at 12,000 x g, and the supernatant was collected and used accordingly. The OxB288 antibody was raised to human recombinant FS288 and was generously provided by Dr. L. Evans (23).

Immunohistochemistry. Sections were dewaxed, rehydrated, and placed in Target Retrieval Solution (Dako, Carpenteria, CA). Antigenic sites were exposed by microwaving the sections (2.25 watts/mL·min for 3 min, followed by 0.3 watts/mL·min for 3–5 min or 20 min at 2.25 watts/mL·min) before the removal of endogenous peroxidase activity with 3% H2O2 for 30 min. Sections were incubated with 0.2% Triton X-100 (Sigma Chemical Co., St. Louis MO) for 10 min before blocking in either 1% blocking reagent (Boehringer Mannheim, Mannheim, Germany) or CAS block (Zymed, San Francisco, CA) plus 10% normal serum. The sections were incubated with the primary antibody overnight at 4 C (ßA, ßB, and OxB288) or at room temperature for 2 h (AS 202), washed in PBS (10 mmol/L phosphate buffer and 15 mmol/L NaCl, pH 7.4), and incubated with either biotinylated anti-rabbit IgG (1:200; Salk antibodies and AS 202; Zymed) or biotinylated antimouse IgG (1:200; Groome antibodies; Vector Laboratories, Burlingame, CA) at room temperature for 1 h. The sections were washed in 0.01 mol/L PBS and incubated for 1 h with streptavidin (1:50; Vectastain Elite ABC Kit, Vector Laboratories). After additional washes with 0.01 mol/L PBS, peroxidase activity was detected using 3',3'-diaminobenzidine tetrahydrochlorate (Liquid DAB Substrate Kit, Zymed). The reaction was terminated in distilled water, and the sections were counterstained with Mayer’s hematoxylin (Sigma Diagnostics, St. Louis, MO), dehydrated, and permanently mounted with DPX (BDH, Poole, U.K.).

In situ hybridization studies

Probe synthesis. Digoxygenin (DIG)-labeled riboprobes were prepared using the Boehringer Mannheim riboprobe labeling kit. Sense and antisense complementary RNA (cRNA) probes were generated from rat inhibin ßA and ßB (24) and rat follistatin (25) partial clones (370, 390, and 267 bp, respectively).

Hybridization. Sections were dewaxed, rehydrated, treated with 0.2 mol/L HCl, and then washed in diethyl pyrocarbonate-treated water (twice, 5 min each time). Sections were digested with proteinase K (Boehringer Mannheim) for 30 min at 37 C. After incubation in 0.2% glycine for 10 min at 4 C, sections were equilibrated in 0.1 mol/L triethanolamine and acetylated in 0.25% acetic anhydride for 5 min. After rinsing in diethyl pyrocarbonate-treated water, sections were prehybridized for a minimum of 30 min. Prehybridization solution contains 3 x SSC (1 x SSC is 0.15 mol/L sodium chloride and 15 mmol/L sodium citrate, pH 7), 1 x Denhardt’s solution, 50% deionized formamide, 66 mmol/L phosphate buffer (pH 8), 1000 µg/mL herring sperm DNA, and 200 µg/mL transfer RNA.

Sections were hybridized overnight at 42 C in hybridization buffer (prehybridization solution plus 10% dextran sulfate) containing a predetermined concentration (200–1000 ng/mL) of riboprobe. Excess probe was removed by sequential 15-min washes in 2 x SSC at room temperature, 2 x SSC at 42 C, 1 x SSC at 42 C, and finally either 0.1 or 0.5 x SSC (depending on the probe employed) at 42 C.

Anti-DIG detection. The tissues were briefly washed in buffer 1 (0.1 mol/L maleic acid and 0.15 mol/L sodium chloride, pH 7.5) before anti-DIG antibody detection. Anti-DIG Fab fragments conjugated to alkaline phosphatase (Boehringer Mannheim) were diluted to 1:1000 in 1% blocking reagent in buffer 1, and the sections were incubated for 1 h at room temperature. Alkaline phosphatase activity was detected using NBT/BCIP substrate (NBT/BCIP 1-step, Pierce Chemical Co., Rockford, IL), the reaction was stopped by immersion in water, and the sections were permanently mounted with GVA Histomount (Zymed).


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
ßA expression and localization

The pattern of immunostaining in the nonmalignant and malignant regions of tumor tissue obtained from patients with high grade prostate cancer using two specific ßA-subunit antibodies is shown in Fig. 1Go. Using the ßA Groome antibody, positive immunoreactivity was observed in basal and secretory epithelial cells in the nonmalignant regions of tissue (Fig. 1Go, A and B). Immunoreactivity for ßA-subunit was also observed in tumor cells from adjacent regions of the tissue containing poorly differentiated tumor (Fig. 1Go, D and E). No immunostaining was observed in either the nonmalignant or malignant regions of control sections (Fig. 1Go, C and F). Using the ßA Salk antibody, a similar pattern of immunostaining was observed; the nonmalignant epithelium showed positive immunoreactivity that was localized to the basal and secretory cells (Fig. 1Go, G and H, indicated by the arrow), although the intensity of staining was highly variable. In addition, weaker stromal staining was observed in these nonmalignant regions of the tissue sections, as demonstrated by the arrows (Fig. 1Go, G and H). The malignant epithelial tumor cells showed consistent and positive staining of localization for ßA-subunit protein (Fig. 1Go, J and K). No positive immunoreactivity was observed with rabbit IgG (Fig. 1Go, I and L).



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Figure 1. Localization and expression of ßA-subunit mRNA and protein in nonmalignant and malignant regions of prostate tissues from men with high grade prostate cancer. A–F, Localization of ßA-subunit protein using the ßA Groome antibody in nonmalignant (A magnification; B) and malignant (D, E) regions of prostate tissue. Arrows indicate positive immunoreactivity localized to the epithelial cell layer in the nonmalignant regions (A and B). No positive immunoreactivity was observed when the sections were treated with rabbit IgG (C, F). G–L, Localization of ßA-subunit protein using the ßA Salk antibody in the nonmalignant regions (G, H). Arrows indicate patches of immunoreactivity in the stroma (G) or epithelium (H). Immunoreactivity for ßA-subunit protein in malignant tumor cells is shown in J and K; control sections showed no positive immunoreactivity (I, L). M–P, The expression of ßA-subunit mRNA in nonmalignant (M) and malignant (N) regions. Arrows indicate mRNA expression in the epithelium and stroma (M). Using a sense cRNA probe as a control, no mRNA was localized in either nonmalignant (O) or malignant (P). The bar in A represents 100 µm in A, C, D, F, G, I, J, L, M, O, and P. The bar in B represents 50 µmol/L in E, H, K, and N.

 
In situ hybridization confirmed that mRNA for ßA-subunit was present in the nonmalignant regions of the sections from men with high grade prostate cancer and was predominantly localized to secretory and/or basal cells in the epithelium (Fig. 1MGo). Isolated groups of cells in the stroma also expressed ßA mRNA (shown by the arrows in Fig. 1MGo). The expression of ßA-subunit mRNA was detected in the malignant cells of the adjacent regions of tumor tissue (Fig. 1NGo). Using the sense ßA probe, no mRNA expression was detected (Fig. 1Go, O and P).

ßB-subunit expression and localization

The pattern of expression and localization for ßB-subunit in nonmalignant and malignant regions of tissue obtained from patients with high grade prostate cancer is shown in Fig. 2Go. Using the ßB Groome antibody, positive immunoreactivity was primarily localized to the epithelial compartment of nonmalignant regions of tissue, with more intense staining of ßB-subunit observed in the basal cells (Fig. 2Go, A and B). In adjacent malignant regions, immunoreactivity for ßB-subunit was localized to the tumor cells (Fig. 2Go, D and E), no positive immunostaining was observed in the control sections (Fig. 2Go, C and F). Using the ßB Salk antibody, immunoreactivity was localized to the nonmalignant epithelial tissue and was predominantly located in the basal, rather than the secretory, epithelial cells (Fig. 2Go, G and H). Positive immunoreactivity for ßB-subunit was also present in the malignant tumor cells (Fig. 2Go, J and K). Positive immunoreactivity was not observed with rabbit IgG (Fig. 2Go, I and L).



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Figure 2. Localization and expression of ßB-subunit mRNA and protein in nonmalignant and malignant regions of prostate tissues from men with high grade prostate cancer. A–F, Positive immunoreactivity using the Groome ßB antibody was observed in nonmalignant (A, B) and malignant (D, E) regions of tissue. Arrows indicate positive immunoreactivity in basal cells in B. No immunoreactivity was observed in control sections (C, F). G–L, ßB-Subunit protein was localized using the Salk antibody in nonmalignant (G, H) and malignant regions (J, K). The arrow indicates basal cell immunoreactivity (H). No positive immunoreactivity was observed in control sections (I magnification; L). M–P, ßB mRNA was localized in nonmalignant regions (M) and malignant regions (N). The arrow indicates mRNA expression in basal cells (M). Using a sense cRNA probe as a control, no mRNA was localized (O, P). The bar represents 100 µm in A, C, D, F, G, I, J, L, M, O, and P and 50 µm in B, E, H, K, and N.

 
mRNA for the ßB-subunit was expressed in the malignant and nonmalignant epithelium. In the latter regions, the level of expression of ßB-subunit mRNA was greater in the basal cells than in the secretory epithelial cells and was consistent with the immunoreactivity in the basal cells using both of the ßB antibodies (Fig. 2MGo). mRNA for ßB-subunit was also expressed in the tumor cells (Fig. 2NGo). No mRNA expression was detected using the ßB sense probe in either malignant or nonmalignant regions of the biopsy (Fig. 2Go, O and P).

Localization and expression of follistatin

Using the polyclonal antibody AS 202, follistatin immunoreactivity was localized to patchy regions of stromal tissue in the nonmalignant regions of needle biopsies from men with high grade prostate cancer (Fig. 3Go, A and B). No immunoreactivity was observed in the epithelium or in the control sections incubated with AS 202 preabsorbed with bovine follistatins (Fig. 3CGo). In contrast, using the OxB288 antibody, positive immunoreactivity was localized to the basal cells of the epithelium and was not recorded in surrounding stromal tissues (Fig. 3Go, G and H). In the regions of poorly differentiated tumor, intense positive immunoreactivity was localized to the malignant epithelial cells using both follistatin antibodies (Fig. 3Go, D and E, and J and K). No positive localization was observed with rabbit IgG (Fig. 3Go, F and L).



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Figure 3. Localization and expression of follistatin subunit mRNA and protein in nonmalignant and malignant regions of prostate tissues from men with high grade prostate cancer. A–F, Using AS 202, follistatin immunoreactivity was localized to nonmalignant (A, B) and malignant (D, E) regions. Arrows indicate positive immunoreactivity in the stroma (A and B). No positive immunoreactivity was observed in the controls (C, F). G–L, FS288 immunoreactivity was observed in nonmalignant regions (G, H) and malignant regions (J, K). Arrows indicate positive immunoreactivity in the basal cells (G and H). Positive immunoreactivity was not observed in the control sections (I, L). M–P, mRNA for follistatin was expressed in the nonmalignant (M) and malignant (N) regions of tissue. Arrows indicate basal and stromal cell expression in (M). Using a sense cRNA probe, no mRNA was detected in nonmalignant (O) or malignant (P) tissues. The bar represents 100 µm in A, C, D, F, G, I, J, L, M, O, and P and 50 µm in B, E, H, K, and N.

 
In the nonmalignant tissues, follistatin mRNA was expressed in patches of cells within the stromal tissue (Fig. 3MGo), consistent with the immunoreactivity observed with AS 202 (Fig. 3Go, A and B). Follistatin mRNA was also expressed in the nonmalignant epithelium, specifically in the basal cells (Fig. 3MGo); this site of expression of follistatin mRNA correlated with the detection of immunoreactivity for follistatin protein using the OxB288 antibody (Fig. 3Go, G and H). In the malignant tumor regions, follistatin mRNA was expressed in the tumor cells (Fig. 3NGo), which is consistent with the positive immunoreactivity that was recorded using both follistatin antibodies (Fig. 3Go, D and E, and J and K). mRNA expression was not detected using the follistatin sense riboprobe (Fig. 3Go, O and P).


    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
This study describes the pattern of expression and localization of activin ß-subunits and activin-binding proteins, follistatins; the results demonstrate that there is a difference between the malignant and nonmalignant regions of tissue from men with high grade prostate cancer.

In the nonmalignant regions of the tissue sections from men with high grade prostate cancer, activin subunits ßA and ßB were detected in the epithelium, consistent with our previous observations using tissues from men with BPH (13). Expression of mRNAs for the activin subunits in the epithelium was observed and, therefore, confirmed that the prostatic epithelium is a site of synthesis and expression of activin ligands. Although the expression of activin ßA-subunit mRNA was detected in all basal and secretory cells of the nonmalignant epithelia, the protein was not detected in a uniform pattern using the Salk ßA antibody. This pattern of variable staining, where some glands showed intense staining and others showed none at all, was previously observed using an antibody to dimeric activin A to detect this ligand in tissue sections from men with BPH (13). However, the Groome monoclonal antibody consistently detected ßA-subunit protein in basal and secretory cells of the nonmalignant epithelia, which was identical to the uniform detection of mRNA. The reason why there are differences using the two different ßA antibodies is not known.

In contrast to those of ßA, the expression and synthesis of ßB were consistently observed to be predominantly localized to the basal, rather than the secretory, cells. This pattern of subunit localization was identical using both antibodies and was consistent with the expression of ßB mRNA. The predominant synthesis and production of the ßA- and ßB-subunits in the prostatic epithelial cells provides these cell types with the capacity to form homo- or heterodimers of the ß-subunits and, thus, activin A, B, or AB ligands. This laboratory has previously demonstrated that the nonmalignant epithelium expresses and produces the inhibin {alpha}-subunit (13), and therefore, the capacity to synthesize inhibin A or B must also reside within these cells.

The actions of inhibin and activin ligands on nonmalignant prostatic epithelium have yet to be documented. In normal and hyperplastic acini of human prostate, it has been reported that 70% of proliferating cells are localized within the basal cell layer, which has led to the speculation that basal cells have an important role in the proliferative activity of the epithelium (26). It is noteworthy that the basal cells are the predominant site of synthesis of the activin and inhibin subunits, suggesting that these ligands have the potential to influence renewal of the prostatic epithelium. In the human prostate tumor cell lines LNCaP and DU145, a growth inhibitory response to activin A and B has been reported (18, 27, 28), and it is possible that these ligands have a similar action on the nonmalignant epithelium. Indeed, this might be predicted, as activins are known to have antiproliferative actions in other cells and tissues (5, 6, 7) and to induce apoptosis in certain cell lines (9).

In many instances, the inhibins have effects that oppose those of activins in some cells and tissues (5, 29). The demonstration that inhibin {alpha}-subunits are synthesized by the nonmalignant prostate suggests that they may regulate the actions of activins on the nonmalignant epithelium. In contrast, the {alpha}-subunit is down-regulated in tissues from men with high grade prostate cancer (Mellor SL, Richards MG, Pedersen JS, Robertson DM, and Risbridger GP, submitted for publication), and human prostate tumor cells do not express inhibin {alpha}-subunit in vitro (14, 15, 16). The loss of {alpha}-subunit with the continued expression of the ß-subunits results in the synthesis of activins, but not inhibins, in the malignant epithelium. However, the growth inhibitory actions of activin A and B on some prostatic tumor cell lines are not consistent with the malignant characteristics of these cells (18), and a mechanism for conferring resistance to activin ligands must be considered. By analogy with TGFß, it is possible that there are inactivating mutations of the activin receptors (30), but an alternate means of regulating activin action is via follistatins, i.e. binding proteins with a specific and high affinity for activins.

Follistatin neutralizes the diverse actions of activins, including apoptosis (9), in various cells and tissues by forming an inactive complex with activin. The results described herein demonstrate the expression and localization of follistatin in tumor cells from men with high grade prostate cancer. The localization of the activin subunits and follistatin to the malignant epithelial cells implies that follistatin neutralizes the bioactivity of the activin ligands in poorly differentiated prostate tumors. This is consistent with the in vitro observation that follistatin neutralizes the growth inhibitory effects of exogenously added activin A to the human prostate tumor cell lines LNCaP and DU145 (18); in contrast, PC3 cells are resistant to exogenously added activin A (18). The only difference between the three prostate tumor cell lines that we have observed is the expression of FS288 mRNA by the activin-resistant PC3 cells, but not by LNCaP or DU145 cells in which the predominant mRNA is FS315 (18). The relative levels and types of follistatin produced by the three prostate tumor cell lines are not known. It has been shown that FS288 has a higher neutralizing activity than FS315 (12), and therefore, it is possible that the presence of FS288 is important in conferring activin resistance, as demonstrated in the PC3 cells. In poorly differentiated tumor tissue, follistatin mRNA and proteins, detected by two different antibodies, localize with activin subunit immunoreactivity, which implies that the tumor cells may be resistant to activin in vivo due to the presence of follistatin.

In the nonmalignant tissues adjacent to the high grade tumors, mRNA expression for follistatin was detected in epithelial basal cells and stroma; using OxB288, FS288 protein was localized predominantly to the basal cells. This suggests that the interplay between activin and follistatin is important in regulating activin’s action on basal cells, but the effect of activins on basal cells is not known. In the nonmalignant stromal tissue, follistatin mRNA is expressed, and protein was localized using AS 202, but not the OxB288 antibody. These results suggest that the follistatin proteins produced in basal and stromal cells are different. Furthermore, in malignant and nonmalignant tissues, these data suggest that the expression of follistatin mRNA results in the production of different follistatin isoforms in specific cell types. The relative levels of follistatin proteins and whether they are bound or unbound to activin may also change in the progression to malignancy. The ability to resolve these issues requires the further development of methods that can discriminate among the different forms of follistatin in bound and unbound forms in situ.

Received May 27, 1997.

Revised July 23, 1997.

Accepted August 1, 1997.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Robertson DM, Foulds LM, Leversha L, et al. 1985 Isolation of inhibin from bovine follicular fluid. Biochem Biophys Res Commun. 126:226–226.
  2. Rivier C, Rivier J, Vale WW. 1986 Inhibin-mediated feedback control of follicle stimulating hormone secretion in the female rat. Science. 234:205–208.[Abstract/Free Full Text]
  3. Ling N, Ying S-Y, Ueno N, et al. 1986 Pituitary FSH is released by a heterodimer of the ß-subunits from the two forms of inhibin. Nature. 321:779–782.[CrossRef][Medline]
  4. Burger HG. 1988 Inhibin: definition and nomenclature including related substances. J Endocrinol. 117:159–160.[Abstract/Free Full Text]
  5. Hedger MP, Drummond AE, Robertson DM, Risbridger GP, de Kretser DM. 1989 Divergent actions of inhibin, activin and transforming growth factor ß on [3H]thymidine incorporation by rat thymocytes and 3T3 fibroblasts in vitro. Mol Cell Endocrinol. 61:133–138.[CrossRef][Medline]
  6. Mather JP, Attie KM, Woodruff T, Rice GC, Phillips DM. 1990 Activin stimulates spermatogonial proliferation in Germ-Sertoli cell co-cultures from immature rat. Endocrinology. 127:3206–3214.[Abstract/Free Full Text]
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  10. Robertson DM, Klein R, de Vos FL, et al. 1987 The isolation of polypeptides with FSH suppressing activity from bovine follicular fluid which are structurally different to inhibin. Biochem Biophys Res Commun. 149:744–749.[CrossRef][Medline]
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