The STAT3 inhibitor GPB730 enhances the sensitivity to enzalutamide in prostate cancer cells.

Enzalutamide is a second-generation anti-androgen which has shown increased survival in patients with metastatic prostate cancer. However, some patients do not respond to this therapy or will develop resistance to treatment over time. Signal Transducer and Activator of Transcription 3 (STAT3) is known to be involved in castration-resistant prostate cancer and to interact with androgen receptor (AR)-signaling. This study aims to investigate the combination enzalutamide and the small molecule STAT3 inhibitor GPB730 for enhanced therapeutic effect in advanced prostate cancer in vitro. The prostate cancer cell lines LNCaP (androgen dependent) and C4-2 (androgen insensitive) were used. The effect of enzalutamide and GPB730, alone and in combination, was investigated on viability and IC50 values calculated. Enzalutamide and GPB730 treated LNCaP and C4-2 cells were subjected to western blot and QPCR analyses in order to investigate the expression of AR, STAT3 and down-stream targets. C4-2 were less sensitive to growth inhibition by enzalutamide than LNCaP cells. GPB730 enhanced the growth inhibitory effect of enzalutamide in LNCaP and C4-2 cells. The addition of GPB730 to enzalutamide decreased the IC50 values for enzalutamide by 3.3-fold for LNCaP and by 12-fold for C4-2. In C4-2 cells, GPB730 alone decreased PSA expression and enhanced the enzalutamide induced decrease in NKX3.1 expression. GPB730 and enzalutamide in combination enhanced inhibition of c-myc and survivin expression. This study suggests that enzalutamide may be combined with the STAT3 inhibitor GPB730 in order to enhance the efficacy of enzalutamide, offering a new therapeutic approach in advanced prostate cancer.

Nuclear expression of pSTAT3Tyr705 and pSTAT3Ser727 in the stromal compartment of localized hormone-naïve prostate cancer.

BACKGROUND: The signal transducer and activator of transcription 3 (STAT3) is involved in the progression of different tumors including prostate cancer (PCa). The expression of STAT3 in benign and malignant epithelium has been described previously but it has not been described in the stromal compartment. The aim of the present study was to evaluate the nuclear expression and prognostic value of different forms of phosphorylated STAT3 in the stromal compartment of non-cancer and cancer areas of prostatic tissue. MATERIAL AND METHODS: Tissue microarray cores from radical prostatectomy of 225 patients with hormone-naïve localized PCa were immunostained for two phosphorylated forms of STAT3, pSTAT3Tyr705 and pSTAT3Ser727. The prognostic value of the expression levels was studied by Cox regression analysis and biochemical recurrence (BCR)-free survival illustrated by Kaplan-Meier curves. RESULTS: Expression of pSTAT3Tyr705 and pSTAT3Ser727 in the stromal compartment of cancer tissue was lower compared with non-cancer areas. In univariable and multivariable Cox regression analysis, expression levels of pSTAT3Tyr705 and STAT3Ser727 showed similar prognostic value as pathological T-stage, Gleason score and surgical margin status. Kaplan-Meier survival analysis showed that low nuclear expression levels of pSTAT3Tyr705 and pSTAT3Ser727 in stromal cells in cancer compartment and in non-cancer areas were related to BCR-free survival. CONCLUSIONS: Nuclear expression of pSTAT3Tyr705 and pSTAT3Ser727 in the stromal cells mirrors previous findings in the epithelial component in that it displays prognostic value in men undergoing radical prostatectomy for localized hormone-naïve PCa.

Inhibition of STAT3 augments antitumor efficacy of anti-CTLA-4 treatment against prostate cancer.

There is an urgent need for new treatment options in metastatic drug-resistant prostate cancer. Combining immunotherapy with other targeted therapies may be an effective strategy for advanced prostate cancer. In the present study, we sought to investigate to enhance the efficacy of anti-CTLA-4 therapy against prostate cancer by the combination with STAT3 inhibition.Male C57BL6 mice were subcutaneously inoculated with the murine prostate cancer cell line RM-1. Tumor progression was monitored following treatment with vehicle, the small molecule STAT3 inhibitor GPB730, anti-CTLA-4 or GPB730 + anti-CTLA-4. Treatment with anti-CTLA-4 or anti-CTLA-4 + GPB730 significantly inhibited tumor growth and enhanced survival compared to vehicle. Combining anti-CTLA-4 treatment with GPB730 resulted in a significantly prolonged survival compared to anti-CTLA-4 alone. GPB730 significantly increased infiltration of CD45 + cells in tumors of anti-CTLA-4-treated mice compared to anti-CTLA-4 alone. The levels of tumor-infiltrating Tregs were significantly decreased and the CD8:Treg ratio significantly increased by GPB730 treatment in combination with anti-CTLA-4 compared to anti-CTLA-4 alone. Immunohistochemical analysis showed a significant increase in CD45-positive cells in anti-CTLA-4 and anti-CTLA-4 + GPB730-treated tumors compared to vehicle or GPB730 monotherapy. Plasma levels of IL10 were significantly increased by anti-CTLA-4 compared to vehicle but no increase was observed when combining anti-CTLA-4 with GPB730.In conclusion, STAT3 inhibition by GPB730 enhances the antitumoral activity of anti-CTLA-4 and decreases the intratumoral Treg frequency in a prostate cancer mouse model. These results support the combination of STAT3 inhibition with anti-CTLA-4 therapy to increase clinical responses in patients with prostate cancer.

STAT3 inhibition with galiellalactone effectively targets the prostate cancer stem-like cell population.

Cancer stem cells (CSCs) are a small subpopulation of quiescent cells with the potential to differentiate into tumor cells. CSCs are involved in tumor initiation and progression and contribute to treatment failure through their intrinsic resistance to chemo- or radiotherapy, thus representing a substantial concern for cancer treatment. Prostate CSCs' activity has been shown to be regulated by the transcription factor Signal Transducer and Activator of Transcription 3 (STAT3). Here we investigated the effect of galiellalactone (GL), a direct STAT3 inhibitor, on CSCs derived from prostate cancer patients, on docetaxel-resistant spheres with stem cell characteristics, on CSCs obtained from the DU145 cell line in vitro and on DU145 tumors in vivo. We found that GL significantly reduced the viability of docetaxel-resistant and patient-derived spheres. Moreover, CSCs isolated from DU145 cells were sensitive to low concentrations of GL, and the treatment with GL suppressed their viability and their ability to form colonies and spheres. STAT3 inhibition down regulated transcriptional targets of STAT3 in these cells, indicating STAT3 activity in CSCs. Our results indicate that GL can target the prostate stem cell niche in patient-derived cells, in docetaxel-resistant spheres and in an in vitro model. We conclude that GL represents a promising therapeutic approach for prostate cancer patients, as it reduces the viability of prostate cancer-therapy-resistant cells in both CSCs and non-CSC populations.

The STAT3 inhibitor galiellalactone inhibits the generation of MDSC-like monocytes by prostate cancer cells and decreases immunosuppressive and tumorigenic factors.

BACKGROUND: The transcription factor signal transducer and activator of transcription 3 (STAT3) is implicated in cancer drug resistance, metastasis, and immunosuppression and has been identified as a promising therapeutic target for new anticancer drugs. Myeloid-derived suppressor cells (MDSCs) play a major role in the suppression of antitumor immunity and STAT3 is involved in the accumulation, generation, and function of MDSCs. Thus, targeting STAT3 holds the potential of reversing immunosuppression in cancer. This study aims to investigate the effect of the small molecule STAT3 inhibitor galiellalactone on prostate cancer cell- induced generation of MDSCs from monocytes and the effect on immunosuppressive factors and inflammatory cytokines. METHODS: Primary human monocytes were cocultured with prostate cancer cells (DU145, PC3, and LNCaP-IL6) or with conditioned medium (CM) from prostate cancer cells in the presence or absence of the STAT3 inhibitor galiellalactone. Monocytes were analyzed by flow cytometry for an MDSC-like phenotype (CD14+ HLA-DR-/lo ). The secretion and gene expression of immunosuppressive factors and inflammatory cytokines from prostate cancer cells and monocytes were investigated. RESULTS: Galiellalactone blocked the prostate cancer cell-induced generation of MDSC-like monocytes with an immunosuppressive phenotype ex vivo. Monocytes cultured with CM from prostate cancer cells showed increased expression of phosphorylated STAT3. Prostate cancer cells increased the expression of interleukin1ß (IL1ß), IL10, and IL6 in monocytes which was inhibited by galiellalactone. In addition, galiellalactone decreased indoleamine 2,3-dioxygenase gene expression in monocytes. Galiellalactone reduced the levels of IL8 and granulocyte macrophage-colony stimulating factor in prostate cancer cells per se. CONCLUSION: The STAT3 inhibitor galiellalactone may prevent the prostate cancer cell-induced generation of MDSCs and reverse the immunosuppressive mechanisms caused by the interplay between prostate cancer cells and MDSCs. This is a potential new immunotherapeutic approach for the treatment of prostate cancer.

Expression of tSTAT3, pSTAT3727 , and pSTAT3 705 in the epithelial cells of hormone-naïve prostate cancer.

BACKGROUND: The signal transducer and activator of transcription 3 (STAT3) pathway is observed to be constitutively activated in several malignancies including prostate cancer (PCa). In the present study, we investigated the expression of total STAT3 (tSTAT3) and two forms of activated phosphorylated STAT3 (pSTAT3727 and pSTAT3705 ) in tissue microarrays (TMA) of two cohorts of localized hormone-naïve PCa patients and analyzed associations between the expression and disease outcome. METHODS: The expression of tSTAT3, pSTAT3727 , and pSTAT3705 was scored in the nuclei and cytoplasm of prostatic gland epithelial cells in two TMAs of paraffin-embedded prostatic tissue. The TMAs consisted of tissue originated from hormone-naïve radical prostatectomy patients from two different sites: Malmö, Sweden (n = 300) and Dublin, Ireland (n = 99). RESULTS: The nuclear expression levels of tSTAT3, pSTAT3727 , and pSTAT3705 in the epithelial cells of benign glands were significantly higher than in the cancerous glands. Cytoplasmic tSTAT3 levels were also higher in benign glands. Patients with low pSTAT3727 and pSTAT3705 levels in the cancerous glands showed reduced times to biochemical recurrence, compared with those with higher levels. No significant trends in nuclear nor in cytoplasmic tSTAT3 were observed in relation to biochemical recurrence in the Malmö cohort. Higher cytoplasmic tSTAT3 was associated with reduced time to biochemical recurrence in the Dublin cohort. Adding the tSTAT3 and pSTAT3 expression data to Gleason score or pathological T stage did not improve their prognostic values. CONCLUSIONS: Low pSTAT3727 and pSTAT3705 expression in epithelial cells of cancerous prostatic glands in hormone-naïve PCa was associated with faster disease progression. However, pSTAT3 and tSTAT3 expression did not improve the prognostic value of Gleason score or pathological T stage and may not be a good biomarker in the early hormone naïve stages of PCa.

Treatment with the WNT5A-mimicking peptide Foxy-5 effectively reduces the metastatic spread of WNT5A-low prostate cancer cells in an orthotopic mouse model.

Prostate cancer patients with high WNT5A expression in their tumors have been shown to have more favorable prognosis than those with low WNT5A expression. This suggests that reconstitution of Wnt5a in low WNT5A-expressing tumors might be an attractive therapeutic approach. To explore this idea, we have in the present study used Foxy-5, a WNT5A mimicking peptide, to investigate its impact on primary tumor and metastasis in vivo and on prostate cancer cell viability, apoptosis and invasion in vitro. We used an in vivo orthotopic xenograft mouse model with metastatic luciferase-labeled WNT5A-low DU145 cells and metastatic luciferase-labeled WNT5A-high PC3prostate cancer cells. We provide here the first evidence that Foxy-5 significantly inhibits the initial metastatic dissemination of tumor cells to regional and distal lymph nodes by 90% and 75%, respectively. Importantly, this effect was seen only with the WNT5A-low DU145 cells and not with the WNT5A-high PC3 cells. The inhibiting effect in the DU145-based model occurred despite the fact that no effects were observed on primary tumor growth, apoptosis or proliferation. These findings are consistent with and supported by the in vitro data, where Foxy-5 specifically targets invasion without affecting apoptosis or viability of WNT5A-low prostate cancer cells. To conclude, our data indicate that the WNT5A-mimicking peptide Foxy-5, which has been recently used in a phase 1 clinical trial, is an attractive candidate for complimentary anti-metastatic treatment of prostate cancer patients with tumors exhibiting absent or low WNT5A expression.

Expression of STAT3 in Prostate Cancer Metastases.

STAT3 and its upstream activator IL6R have been implicated in the progression of prostate cancer and are possible future therapeutic targets. We analyzed 223 metastatic samples from rapid autopsies of 71 patients who had died of castration-resistant prostate cancer (CRPC) to study protein and gene expression of pSTAT3 and IL6R. Immunohistochemical analysis revealed that 95% of metastases were positive for pSTAT3 and IL6R, with varying expression levels. Bone metastases showed significantly higher expression of both pSTAT3 and IL6R in comparison to lymph node and visceral metastases. STAT3 mRNA levels were significantly higher in bone than in lymph node and visceral metastases, whereas no significant difference in IL6R mRNA expression was observed. Our study strongly supports the suggested view of targeting STAT3 as a therapeutic option in patients with metastatic CRPC. PATIENT SUMMARY: We studied the levels of two proteins (pSTAT3 and IL6R) in metastases from patients who died from castration-resistant prostate cancer. We found high levels of pSTAT3and IL6R in bone metastases, suggesting that these proteins could be used as targets for new anticancer drugs.

Preclinical Characterization of 3ß-(N-Acetyl l-cysteine methyl ester)-2aß,3-dihydrogaliellalactone (GPA512), a Prodrug of a Direct STAT3 Inhibitor for the Treatment of Prostate Cancer.

The transcription factor STAT3 is a potential target for the treatment of castration-resistant prostate cancer. Galiellalactone (1), a direct inhibitor of STAT3, prevents the transcription of STAT3 regulated genes. In this study we characterized 6 (GPA512, Johansson , M. ; Sterner , O. Patent WO 2015/132396 A1, 2015 ), a prodrug of 1. In vitro studies showed that 6 is rapidly converted to 1 in plasma and is stable in a buffer solution. The pharmacokinetics of 6 following a single oral dose indicated that the prodrug was rapidly absorbed and converted to 1 with a tmax of 15 min. Oral administration of 6 in mice increased the plasma exposure of the active parent compound 20-fold compared to when 1 was dosed orally. 6 treated mice bearing DU145 xenograft tumors had significantly reduced tumor growth compared to untreated mice. The favorable druglike properties and safety profile of 6 warrant further studies of 6 for the treatment of castration-resistant prostate cancer.

The STAT3 Inhibitor Galiellalactone Effectively Reduces Tumor Growth and Metastatic Spread in an Orthotopic Xenograft Mouse Model of Prostate Cancer.

UNLABELLED: Signal transducer and activator of transcription 3 (STAT3) is known to be involved in the progression of prostate cancer (PCa) and is a key factor in drug resistance and tumor immunoescape. As a result, it represents a promising target for PCa therapy. We studied the effects of the STAT3 inhibitor galiellalactone (GL) on tumor growth and metastatic spread in vitro and in vivo. The effect of GL on cell viability, apoptosis, and invasion was studied in vitro using androgen-independent DU145 and DU145-Luc cell lines. For in vivo studies, mice were injected orthotopically with DU145-Luc cells and treated with daily intraperitoneal injections of GL for 6 wk. GL significantly reduced the growth of the primary tumor and the metastatic spread of PCa cells to regional and distal lymph nodes in vivo. Treatment with GL also resulted in decreased cell proliferation and increased apoptosis compared with controls. In vitro, GL reduces the viability and invasive abilities of DU145-Luc cells and induces apoptosis. Our results showed that tumor growth and early metastatic dissemination of PCa can be significantly reduced by GL, indicating its potential use as a therapeutic compound in advanced metastatic PCa. PATIENT SUMMARY: In this study, we tested the STAT3 inhibitor galiellalactone (GL) in an animal model of PCa. We found that mice treated with GL had smaller primary tumors and decreased lymph node metastases compared with mice treated with vehicle. GL has potential for treating advanced metastatic PCa.

Therapeutic Targeting of Nuclear γ-Tubulin in RB1-Negative Tumors.

UNLABELLED: In addition to its cytosolic function, γ-tubulin is a chromatin-associated protein. Reduced levels of nuclear γ-tubulin increase the activity of E2 promoter-binding factors (E2F) and raise the levels of retinoblastoma (RB1) tumor suppressor protein. In tumor cells lacking RB1 expression, decreased γ-tubulin levels induce cell death. Consequently, impairment of the nuclear activity of γ-tubulin has been suggested as a strategy for targeted chemotherapy of RB1-deficient tumors; thus, tubulin inhibitors were tested to identify compounds that interfere with γ-tubulin. Interestingly, citral increased E2F activity but impaired microtubule dynamics while citral analogues, such citral dimethyl acetal (CDA), increased E2F activity without affecting microtubules. The cytotoxic effect of CDA on tumor cells was attenuated by increased expression of either RB1 or γ-tubulin, and increased by reduced levels of either RB1 or γ-tubulin. Mechanistic study, in silico and in vitro, demonstrated that CDA prevents GTP binding to γ-tubulin and suggested that the FDA-approved drug dimethyl fumarate is also a γ-tubulin inhibitor. Finally, in vivo growth of xenograft tumors carrying defects in the RB1 signaling pathway were inhibited by CDA treatment. These results demonstrate that inhibition of γ-tubulin has the potential to specifically target tumor cells and may aid in the design of safer and more efficient chemotherapeutic regimes. IMPLICATIONS: The in vivo antitumorigenic activity of γ-tubulin inhibitors paves the way for the development of a novel broad range targeted anticancer therapy that causes fewer side effects.

Galiellalactone is a direct inhibitor of the transcription factor STAT3 in prostate cancer cells.

The transcription factor STAT3 is constitutively active in several malignancies including castration-resistant prostate cancer and has been identified as a promising therapeutic target. The fungal metabolite galiellalactone, a STAT3 signaling inhibitor, inhibits the growth, both in vitro and in vivo, of prostate cancer cells expressing active STAT3 and induces apoptosis of prostate cancer stem cell-like cells expressing phosphorylated STAT3 (pSTAT3). However, the molecular mechanism of this STAT3-inhibiting effect by galiellalactone has not been clarified. A biotinylated analogue of galiellalactone (GL-biot) was synthesized to be used for identification of galiellalactone target proteins. By adding streptavidin-Sepharose beads to GL-biot-treated DU145 cell lysates, STAT3 was isolated and identified as a target protein. Confocal microscopy revealed GL-biot in both the cytoplasm and the nucleus of DU145 cells treated with GL-biot, appearing to co-localize with STAT3 in the nucleus. Galiellalactone inhibited STAT3 binding to DNA in DU145 cell lysates without affecting phosphorylation status of STAT3. Mass spectrometry analysis of recombinant STAT3 protein pretreated with galiellalactone revealed three modified cysteines (Cys-367, Cys-468, and Cys-542). Here we demonstrate with chemical and molecular pharmacological methods that galiellalactone is a cysteine reactive inhibitor that covalently binds to one or more cysteines in STAT3 and that this leads to inhibition of STAT3 binding to DNA and thus blocks STAT3 signaling without affecting phosphorylation. This further validates galiellalactone as a promising direct STAT3 inhibitor for treatment of castration-resistant prostate cancer.

The fungal metabolite galiellalactone interferes with the nuclear import of NF-κB and inhibits HIV-1 replication.

Galiellalactone (GL) is a metabolite produced by the fungus Galiella rufa that presents antitumor and immunomodulatory activities. GL interferes with the binding to DNA of signal transducer and activator of transcription (STAT)-3 and also inhibits other signal pathways such as NF-κB, but the mechanism of action in this pathway remains unknown. In this study we report that GL inhibits vesicular stomatitis virus-recombinant HIV-1 infection and the NF-κB-dependent transcriptional activity of the HIV-LTR promoter. We found that GL prevents the binding of NF-κB to DNA but neither affects the phosphorylation and degradation of NF-κB inhibitory protein, IκBα, nor the phosphorylation and acetylation of the NF-κB p65 subunit. However, GL prevents the association of p65 with the importin α3 impairing the nuclear translocation of this transcription factor. Using a biotinylated probe we found that GL binds to p65 but not to importin α3. Therefore, GL is a dual NF-κB/STAT3 inhibitor that could serve as a lead compound for the development of novel drugs against HIV-1, cancer and inflammatory diseases.

Galiellalactone inhibits stem cell-like ALDH-positive prostate cancer cells.

Galiellalactone is a potent and specific inhibitor of STAT3 signaling which has been shown to possess growth inhibitory effects on prostate cancer cells expressing active STAT3. In this study we aimed to investigate the effect of galiellalactone on prostate cancer stem cell-like cells. We explored the expression of aldehyde dehydrogenase (ALDH) as a marker for cancer stem cell-like cells in different human prostate cancer cell lines and the effects of galiellalactone on ALDH expressing (ALDH+) prostate cancer cells. ALDH+ subpopulations were detected and isolated from the human prostate cancer cell lines DU145 and long-term IL-6 stimulated LNCaP cells using ALDEFLUOR® assay and flow cytometry. In contrast to ALDH- cells, ALDH+ prostate cancer cells showed cancer stem cell-like characteristics such as increased self-renewing and colony forming capacity and tumorigenicity. In addition, ALDH+ cells showed an increased expression of putative prostate cancer stem cell markers (CD44 and integrin α2ß1). Furthermore, ALDH+ cells expressed phosphorylated STAT3. Galiellalactone treatment decreased the proportion of ALDH+ prostate cancer cells and induced apoptosis of ALDH+ cells. The gene expression of ALDH1A1 was downregulated in vivo in galiellalactone treated DU145 xenografts. These findings emphasize that targeting the STAT3 pathway in prostate cancer cells, including prostate cancer stem cell-like cells, is a promising therapeutic approach and that galiellalactone is an interesting compound for the development of future prostate cancer drugs.

Galiellalactone is a novel therapeutic candidate against hormone-refractory prostate cancer expressing activated Stat3.

BACKGROUND: Signal transducer and activator of transcription 3 (Stat3) is constitutively active (phosphorylated) in several forms of cancer, including prostate cancer (PCa). Stat3 signaling may be an interesting target for cancer therapy since inhibition of this pathway mediates growth inhibition and apoptosis of these cells. In this study we investigated the in vitro and in vivo effects of the fungal metabolite galiellalactone, a direct inhibitor of Stat3, on PCa cells. METHODS: The human PCa cell lines DU145, PC-3, and LNCaP were used. Nude mice with subcutaneous PCa cell xenografts were subjected to daily intraperitoneal injections of galiellalactone for 3 weeks. The effect of galiellalactone on the induction of apoptosis of cultured PCa cells was investigated by Western blot analysis, immunocytochemistry, and annexin V staining. Effects of galiellalactone on Stat3 signaling were investigated by a luciferase reporter gene assay. Expression of Stat3 associated proteins and mRNA was investigated by Western blot and real-time quantitative PCR analysis. RESULTS: Galiellalactone induced apoptosis of p-Stat3 positive PCa cells (androgen-insensitive DU145 and PC-3) but not in cells lacking p-Stat3 (androgen-sensitive LNCaP). Galiellalactone inhibited Stat3-mediated luciferase activity (IC(50) approximately 5 microM) and reduced the expression of Bcl-2, Bcl-x(L), c-myc, and cyclin D1. Furthermore, galiellalactone significantly suppressed DU145 xenograft growth in vivo (42% growth reduction; P<0.002) and reduced the relative mRNA expression of Bcl-x(L) and Mcl-1. CONCLUSIONS: Galiellalactone induced growth inhibition and apoptosis in androgen-insensitive PCa cells expressing p-Stat3. We suggest that galiellalactone is a potential anti-tumor lead against hormone-refractory PCa with constitutively active Stat3.

A highly conserved protein secreted by the prostate cancer cell line PC-3 is expressed in benign and malignant prostate tissue.

In this study we characterize a novel gene on human chromosome 9 and its translation product, PC3-secreted microprotein (PSMP). The gene contains three exons that encode a protein of 139 amino acid residues, including a predicted signal peptide of 36 residues. The molecule is homologous to beta-microseminoprotein (MSP), a protein of unknown function, secreted at high concentration by the prostate gland. These two proteins have only 23% sequence identity, but their common origin is revealed by a preserved pattern of Cys residues. In contrast to MSP, which shows poor conservation between species, PSMP is very conserved. High transcript levels were detected in the prostate cancer cell line PC-3. Antiserum raised against PSMP detected a protein with an apparent molecular mass of 18 kDa in culture medium conditioned by PC-3 cells, but in cell lysates the antiserum also recognized a molecular species of 16 kDa, suggesting that PSMP undergoes post-translational modification. Xenografted PC-3 cell tumors in athymic nude mice showed strong staining for both PSMP protein and mRNA. Studies on human prostate cancer specimens showed immunohistochemical staining of both tumor and benign glandular cells. Our results suggest that PSMP is an important protein with significance in prostate cancer.