Search

Cell adhesion molecules have been suggested to function as tumor suppressor molecules. We have been studying one of the epithelial cell adhesion molecules (C-CAM), which belongs to the immunoglobulin gene superfamily. Transfection of a C-CAM cDNA expression vector into a highly tumorigenic human prostate cancer cell line (PC-3) suppresses tumor formation in nude mice. Alternatively, reducing C-CAM expression levels in the nontumorigenic rat prostate epithelial cell line NbE by the antisense expression vector markedly increases tumorigenicity of NbE cells in nude mice. These results suggest that C-CAM may be a tumor suppressor in prostate cancer. In this study, we examined the relationship between C-CAM expression during human prostate development and neoplastic progression by immunohistochemical staining of frozen sections. C-CAM predominantly localized on the plasma membrane of the basal cell layer in both the fetal and normal adult prostate gland. However, an overall decreased staining was seen in benign prostatic hyperplasia and high grade prostatic intraepithelial neoplasia. Furthermore, C-CAM was not detected in prostate carcinomas. Thus, a decrease in C-CAM expression may be an early event in hyperplastic/neoplastic transformation. These observations support the suggestion that C-CAM is a tumor suppressor in prostate cancer progression.

Epidermal growth factor (EGF) and transforming growth factor-alpha (TGF alpha) are two closely related peptides that interact with cell-surface epidermal growth factor receptors (EGFR) to induce receptor tyrosine phosphorylation and activation of intracellular signal-transduction pathways. EGF appears to be the predominant EGF-related growth factor in the normal prostate and in benign prostatic hyperplasia (BPH). Evidence indicates that EGF and TGF alpha are important for maintainence of the structural and functional integrity of the benign prostatic epithelium. The EGF-related peptides are primarily localized to the secretory epithelium of the benign prostate, and their production and secretion is augmented by the presence of circulating androgens. EGFR are located in the basal/neuroendocrine (NE) compartment of the benign prostate and exhibit relatively androgen-independent expression. The EGF-related peptides and EGFR are also present in neoplastic prostatic tissues. There is currently no direct evidence to implicate EGFR activation in the pathogenesis of BPH. However, the EGF-related peptides appear to play a functional role in the growth of prostatic carcinoma cells, with TGF alpha being the predominant growth factor. Numerous investigators have demonstrated the functional significance of a TGF alpha/EGFR-mediated autocrine growth pathway in cultured prostatic carcinoma cells. Studies of cultured prostate cancer cells, but not normal epithelial cells, demonstrate constitutive activation of EGFR. Androgen-independent cancer cells exhibit more EGFR expression and phosphorylation than do androgen-responsive prostate cancer cells. Most studies indicate that EGFR do not play a functional role in androgen-stimulated growth of prostate cancer cells. Several studies have correlated EGFR expression with increased nuclear size and tumor dedifferentiation. Future studies should focus on determining both the prognostic significance of EGFR expression and whether manipulation of EGFR-mediated growth can be exploited for therapeutic benefit in human prostate cancer.

The treatment of advanced metastatic prostate cancer by hormone manipulation or orchiectomy is frequently followed by the appearance of hormone-insensitive and highly chemoresistant tumor cells. In this study we have investigated the contribution of the P-glycoprotein-mediated drug efflux (multidrug-resistance; MDR) to the cellular resistance of prostate carcinoma-derived cell lines to diverse cytotoxic drugs by detection of P-glycoprotein (P-gp) measurement of P-gp-mediated drug transport and reversal of MDR by chemosensitizers. The in vitro chemosensitivity of three prostate cancer cell lines (PC-3, DU-145 and LNCaP) to doxorubicin was measured in a thymidine incorporation proliferation assay. Growth of the partially hormone-sensitive cell line LNCaP is inhibited by low doses of doxorubicin (IC50:27 ng./ml.), but PC-3 and DU-145 are highly resistant to the drug, with IC50 values of 10 micrograms./ml. and 7.5 micrograms./ml., respectively. The chemosensitivity of the PC-3 and DU-145 cells is increased in response to 1 microM. verapamil, 1 micrograms./ml. cyclosporine A and 2 microM. tamoxifen, which are known to partially reverse the MDR phenotype in other resistant tumors. A verapamil-sensitive drug efflux has been demonstrated for the PC-3 and Du-145, but not for the LNCaP, cell lines, using flow cytometric measurements of the P-gp substrate rhodamine 123 efflux from preloaded cells. In agreement with the functional measurements, the expression of the P-glycoprotein was detected in the PC-3 and Du-145 cell lines in Western blots using the monoclonal C 219 antibody. In conclusion, the chemoresistant and hormone-insensitive PC-3 and Du-145 cell lines express P-gp and exhibit verapamil-sensitive drug efflux, indicative of MDR. However, the low MDR-reversal rates observed in these cell lines in response to chemosensitizers in clinically achievable concentrations (approximately 2- to 3-fold reversal), point to non-MDR-associated cellular mechanisms as dominant factors of chemoresistance in prostate cancer.

We studied the effect of retinoic acid on the growth and secretory activity of the androgen-responsive prostatic carcinoma cell line LNCaP. Our data showed that retinoic acid at 0.01 microM. stimulated the proliferation of LNCaP cells but inhibited their growth at 0.1 microM. under androgen-free conditions. In the presence of 0.1 nM. dihydrotestosterone (DHT), LNCaP cell proliferation was inhibited by 10 microM. retinoic acid but not by lower concentrations of retinoic acid. Retinoic acid reduced LNCaP cell growth at concentrations of 0.1 microM. in the presence of 10 nM. DHT. Retinoic acid (10 microM.) also reduced the growth response of LNCaP cells to epidermal growth factor and transforming growth factor alpha and potentiated the inhibitory effect of transforming growth factor beta. In additional studies, retinoic acid induced a dose-dependent increase in prostate specific antigen (PSA) secretion at concentrations of 0.1 to 1 microM. Dihydrotestosterone (10 nM.) also enhanced the secretion of PSA by LNCaP cells, and this effect was potentiated in a dose-dependent fashion by the addition of retinoic acid at 0.1-10 microM. Competitive binding studies showed that retinoic acid did not bind to androgen receptors. Overall, retinoic acid had a biphasic effect on LNCaP proliferation and promoted the secretion of PSA. The biphasic effect of retinoic acid on LNCaP growth should be considered in designing in vivo studies to determine the impact of retinoic acid on solid prostatic tumor growth. In addition, the ability of retinoic acid to increase PSA secretion may complicate the interpretation of serum PSA levels used for diagnostic and prognostic purposes.

Results of recent studies indicate that cultured, androgen-independent prostatic carcinoma cells synthesize and secrete transforming growth factor alpha, which interacts with epidermal growth factor receptors (EGFRs) to promote autonomous growth. In the present study, we evaluated the expression and constitutive activation of EGFRs in normal prostatic epithelial cells and the androgen-independent prostatic carcinoma cell lines PC3 and DU145. Our studies showed that cultured normal epithelial cells and androgen-independent prostatic carcinoma cells actively synthesize and exhibit constitutive phosphorylation of the M(r) 170,000 EGFR. The addition of monoclonal anti-EGFR reduced receptor phosphorylation and significantly inhibited the proliferation of prostatic tumor cells. The observed reduction in EGFR phosphorylation could be partially attributed to an antibody-induced decrease in the expression of metabolically labeled EGFR. Results of further studies showed that anti-EGFR enhanced the sensitivity of PC3 cells to the cytotoxic and cytostatic effects of tumor necrosis factor alpha. These studies demonstrate that constitutive activation of EGFR in androgen-independent prostatic carcinoma plays a functional role in the regulation of cellular proliferation in vitro. In addition, the enhanced sensitivity of prostatic carcinoma cells to tumor necrosis factor alpha in the presence of anti-EGFR provides a rationale for the further investigation of combination therapy in the treatment of disseminated, androgen-independent disease.

Previous studies from our laboratory have shown that reconstituted basement membrane and stromal secretory products are important regulators of benign prostatic epithelial cell growth and differentiation. In the present study we evaluated the impact of extracellular matrix (ECM) and soluble stromal secretory products on the proliferation and secretory activity of the androgen-responsive prostatic carcinoma cell line LNCaP. In these studies, dihydrotestosterone (DHT) was a potent mitogen for LNCaP cells cultured on plastic or on type I collagen. The growth response to DHT was greatly attenuated when LNCaP cells were grown on prostatic stromal ECM. Cells grown on stromal ECM also exhibited clustered morphology compared to the monolayer growth observed on plastic and secreted elevated levels of prostate specific antigen (PSA) and prostatic acid phosphatase (PAP). These findings indicate that cultivation of LNCaP on stromal ECM will promote the expression of differentiated functions. In additional studies, stromal cell conditioned medium (SCM) significantly increased PSA/PAP secretion by LNCaP cells in the presence of 10 nM DHT. The enhancement of DHT-induced PSA/PAP secretion by SCM was most pronounced when LNCaP cells were grown on stromal ECM. SCM did not significantly alter LNCaP proliferation. These studies indicate that prostatic stromal ECM and soluble secretory products will promote differentiated function in cultured LNCaP cells. In addition, we show that DHT can act as either a growth or differentiation-promoting stimulus depending on the presence of stromal factors.

Suramin is a trypanocidal drug that has generated recent interest as an antineoplastic agent because of its ability to inhibit the binding of growth factors to their cell surface receptors. Our studies, and others, suggest that the androgen-independent human prostatic carcinoma cell lines PC3 and DU145 proliferate via autocrine growth mechanisms mediated by transforming growth factor alpha (TGFa) and its receptor, the epidermal growth factor (EGF) receptor. The present studies were designed to evaluate the ability of suramin to inhibit PC3 and DU145 proliferation by interfering with TGFa-mediated autocrine growth. Suramin induced a dose-dependent reduction of prostatic tumor cell proliferation which was reversed by removal of suramin from the culture medium. 3H-thymidine release studies showed that suramin had little direct cytotoxicity to either cell line. These findings suggest that the effects of suramin are mediated by cytostatic, rather than cytotoxic, mechanisms. Suramin also interfered with TGFa-mediated growth mechanisms. Specifically, suramin reduced the specific binding of TGFa to PC3 and DU145 cells. Additionally, the inhibitory effect of suramin on DU145 was reversed by cultivation of cells in the presence of excess TGFa. Further investigations revealed that suramin increased the percentage of cells in the S phase of the cell cycle for both cell lines. These studies show that the inhibitory effect of suramin on PC3 and DU145 cell growth is mediated, in part, by alteration of TGFa-mediated autocrine growth mechanisms and cell cycle kinetics.

The androgen-independent prostatic carcinoma cell line PC3 is known to exhibit autonomous growth in vitro and in vivo. The purpose of the present study was to investigate the role of transforming growth factor alpha (TGF-alpha) and its receptor, the epidermal growth factor (EGF) receptor, in the regulation of PC3 cell proliferation. Results showed that PC3 cells secrete factors into conditioned medium that are mitogenic for the less aggressive prostatic carcinoma lines DU145 and LNCaP. Gel filtration chromatography of PC3-conditioned medium revealed a major peak of mitogenic activity at a molecular weight of 5,000 to 10,000 which was inhibited by the addition of antibody to TGF-alpha. The synthesis and secretion of TGF-alpha by PC3 cells were further demonstrated by immunoblotting and radioimmunoassay. Radioreceptor analysis showed a single class (Kd 5.3 nM) of EGF receptors on PC3 cells. The presence of Mr 170,000 EGF receptors on PC3 cells was further demonstrated by immunoprecipitation of metabolically labeled proteins. TGF-alpha was effective in stimulating the growth of low-density, but not high-density, PC3 cultures. In addition, the proliferation of PC3 cells under serum-free defined conditions was inhibited by antibodies to TGF-alpha and/or the EGF receptor. These data indicate that TGF-alpha/EGF receptor interactions are partially responsible for autonomous growth of the PC3 cell line and may explain one mechanism of escape from androgen-dependent growth in human prostatic carcinoma.

Prostatic carcinoma represents the second leading cause of cancer mortality in men and is responsible for over 25,000 deaths each year. Currently, no curative therapy is available for metastatic carcinoma of the prostate. The present studies were undertaken to assess the efficacy of recombinant tumor necrosis factor alpha (TNF) in the therapy of experimental prostatic carcinoma. TNF was cytotoxic to the prostate cancer cell lines PC3, DU145, and LNCAP but not benign prostatic epithelial and stromal cells in vitro. The sensitivity of the prostatic carcinoma lines to TNF-mediated cytotoxicity was enhanced by the presence of actinomycin D. Intravenous administration of TNF (50-100 micrograms/kg) to nude mice bearing subcutaneous PC3 tumors resulted in significant inhibition of primary tumor growth compared to control. TNF was also effective in reducing the growth of intraabdominal PC3 tumors induced by intrasplenic injection of PC3. Furthermore, the incidence of microscopic PC3 foci in the spleen, liver, lung, and diaphragm was diminished in mice receiving TNF therapy. These studies demonstrate the potential of TNF in the therapy of human prostatic carcinoma.

Studies were undertaken to define the expression of cytokeratins in normal, hyperplastic and malignant epithelial cells from human prostate. Cytokeratin (CK) polypeptides, separated by two-dimensional electrophoresis, were identified by immunoblotting with CK-specific monoclonal antibodies. CK polypeptides 5, 7, 8, 15, 18 and 19 were identified in fresh normal and hyperplastic prostate. Expression of CK 15 has not been previously reported in human prostate. Analysis of central and peripheral zone tissues from human prostate did not reveal qualitative differences in CK expression between these areas. Epithelial cells harvested from fresh BPH tissue by percoll gradient centrifugation and propagated in vitro using selective culture techniques showed alterations in CK expression compared to intact human prostate. Specifically, CKs 6, 14, 16 and 17 were noted in cultured BPH epithelial cells but not fresh normal prostate or BPH tissue. Immunoblot analysis of the established prostate cancer cell lines PC3, DU145 and LNCAP showed expression of CKs 8 and 18 but not CKs 5, 7 and 15 which were observed in benign prostate. These studies further characterize CK expression in benign and malignant human prostate and provide insights which may be useful in differentiating normal, hyperplastic and malignant epithelial cells in the human prostate gland.