The USF transcription factors are functionally related to c-Myc and share with Myc a common DNA-binding specificity. Ectopic USF expression inhibits Myc-mediated cellular transformation in vitro, suggesting involvement of USF in tumor suppression. However, direct evidence for a role of USF in human cancer is lacking. Considering that several prostate cancer-associated oncogenes and tumor suppressor genes are regulated by USF, we examined the endogenous expression of USF2 in several prostate cell lines. We found that USF2 expression is much higher in normal prostate cells and in the androgen-dependent cell line LNCaP than that in the androgen-independent cell lines, PC3 and DU145. Then, we investigated whether introduction of USF2 could suppress the tumorigenic properties of PC3. By transfecting USF2 and its deletion mutants, we found that USF2 inhibited antibiotic-resistant colony formation, and the inhibitory efficiency of the USF mutants is correlated with USF2 transcriptional activity. As compared to control subclones and the parental cell line, stable PC3 subclones constitutively expressing USF2 displayed greatly decreased anchorage-independent growth in soft agar (90-98%) and decreased xenograft tumor development in mice (80-90%). No significant differences were found in cell growth rate, cell cycle distribution and apoptosis between USF2-overexpressing clones and control clones by MTT assay, BrdU incorporation, and annexin V assays. However, USF2-expressing clones arrested in the G1 phase when cultured in serum-free medium for 48 hours, while control clones failed to arrest. Cell cycle arrest was also evident by a decrease in PCNA expression following serum starvation in the USF2-expressing clones. We propose that USF2 inhibits tumorigenicity in PC3 cells by restoring a normal response to external environmental changes. Our results indicate that USF2, because of its potent tumor reversion function, is a promising novel target for prevention and treatment of prostate cancer.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]