Genetic alterations, including activation of proto-oncogenes and inactivation of tumor suppressor genes, are involved in transforming benign cells into malignant ones. In this study we wanted to investigate the role of five possible oncogenes, ERBB2, uPA, TRPS1, EIF3S3 and MYC in the development and progression of prostate cancer. ERBB2 gene copy number and expression was analyzed in both androgen- dependent primary and metastatic tumors, as well as recurrent hormone-refractory tumors. Using chromogenic in situ hybridization only one borderline amplification of ERBB2 (6-8 copies) was found in the prostate tumors studied. Immunohistochemical staining of ERBB2 protein was negative in all prostate samples, including the sample with the borderline amplification of ERBB2 gene. The expression level of ERBB2 mRNA in prostate tumors as well as in prostate cell lines was analyzed using real-time quantitative RT-PCR (Q-RT-PCR). No differences in the expression levels between tumor types were found, and the expression levels in prostate cancer corresponded to levels in breast carcinomas without ERBB2 amplification. The data suggest that the expression of ERBB2 is low in prostate cancer, regardless of the stage. Thus, it is unlikely that therapies based on overexpression of the ERBB2 gene will be effective in the treatment of prostate cancer. Hormone-refractory metastases and locally recurrent hormone-refractory tumors from patients who died of prostate cancer were analyzed for uPA gene copy number by using fluorescence in situ hybridization. Increased copy number but no high-level amplifications, of uPA was found in 21% of the locally recurrent hormone-refractory tumors. 31% of metastases showed increased copy number of uPA, and one case with a high-level amplification was also observed. The effect of uPA inhibitors on the invasion potential of prostate cancer cell lines was studied using Matrigel invasion assay. The data indicated that invasion of prostate cancer cells containing uPA amplification was inhibited with specific uPA inhibitors (p-aminobentzamidine and B428), whereas this was not the case in cells without amplification. To identify the target gene for the amplification on 8q, 3 candidate genes, TRPS1, EIF3S3, and MYC, were studied. All 3 genes were found co-amplified in about 30% of hormone-refractory prostate carcinoma tumors, analyzed by FISH. Copy number and mRNA expression (by Q-RT-PCR) of TRPS1, EIF3S3 and MYC genes were also analyzed in breast and prostate cancer cell lines. Only EIF3S3 mRNA was overexpressed in SK-Br-3 breast cancer cell line, which contained the highest copy number of all three genes. Next, the expression levels of TRPS1, EIF3S3 and MYC mRNAs were analyzed in benign prostate hyperplasia (BPH), untreated and hormone-refractory prostate tumors. EIF3S3 mRNA expression was higher in prostate carcinomas compared to BPH, but TRPS1 and MYC mRNA levels were similar in all prostate tumor types. The data suggest that expression of EIF3S3 mRNA is increased in prostate cancer, and gene amplification seems to be one mechanism for the overexpression. Finally, the effect of EIF3S3 overexpression on cell growth was studied in NIH 3T3 murine fibroblasts using pTet-Off expression system. Overexpression had a positive effect on growth rate and survival in soft agar. The effect of EIF3S3 inhibition was studied in prostate and breast cancer cell lines using the siRNA method. After four days, the reduction in cell growth was significant in all four cell lines. The results imply that EIF3S3 has a significant role in regulating cell growth, and its overexpression may give rise to improved cancer cell survival.