Background: Prostate cancer is the most common cancer in American men and the second leading cause of cancer death. All clinical observations correlate poorly differentiated high-grade prostate cancer with disease-specific mortality. The HER2 cell membrane tyrosine kinase, a member of the epidermal growth factor receptor family that is the transcription product of the erbB2neu oncogene, and HER3, a receptor protein of the same family, are overexpressed in prostate cancer and prostatic intraepithelial neoplasia. The ligand for these receptors and another related family member, HER4, has recently been identified by independent investigator groups and called neu differentiation factor (NDF) or heregulin. In vitro treatment of HER2- and HER3- or HER2- and HER4-expressing breast cancer cells stimulates tyrosine phosphorylation of HER2 and produces changes in the rate of proliferation, degree of cellular differentiation, and synthesis of physiologic secretion products. There are no published reports on the expression of NDF and HER4 in prostate cancer or the in vitro effects of NDF in prostate cancer cells.
Methods: Expression of NDF, HER2, HER3, and HER4 was studied in 24 frozen prostatectomy specimens by immunohistochemistry. The biologic effect of human recombinant NDF was studied in vitro, using the LNCaP, PC3, and DU145 human prostate cancer cell lines. HER and NDF protein expression was studied by immunohistochemistry. NDF mRNA was analyzed using reverse transcriptase polymerase chain reaction from whole RNA. The biologic effects of NDF on prostate cancer cells in vitro included cell proliferation, thymidine synthesis, induction of prostate-specific antigen mRNA, anchorage-dependent and anchorage-independent cell growth, and ploidy analysis. Data analysis was performed using Student's t test.
Results: Observations in clinical prostatectomy specimens: Immunohistochemistry studies in clinical prostatectomy specimens demonstrate absence of significant NDF expression in prostate cancer, whereas it is expressed in 100% of the stroma, 100% of basal epithelial cells, and 58% of luminal cells in normal and benign hyperplastic prostatic tissue. The HER4 receptor protein is strongly expressed by normal prostate luminal cells, but not prostate cancer. Benign prostate tissue exhibits strong expression of HER2, HER3, and HER4 by basal cells, but only luminal cells significantly express HER4. Only 23% of prostate cancer specimens express HER4, while 95% express HER3 and 82% HER2. Prostatic intraepithelial neoplasia stained similarly to cancer for all proteins studied. Observations in prostate cancer cell lines: In vitro treatment with NDF significantly reduces aneuploidy and proliferation and growth of androgen-sensitive prostate cancer cells. Incubation with NDF also induces prostate-specific antigen mRNA in prostate cancer cells. In spite of displaying NDF mRNA, prostate cancer cells do not produce detectable NDF protein, but express HER2 and HER3 proteins.
Discussion: These data suggest that NDF may be a paracrine differentiation factor involved in normal adult prostate physiology and that functional loss of the NDF/HER ligand/ receptor loop may be an early event associated with prostate tumorigenesis.