Reversion of human prostate tumorigenic growth by azatyrosine

Abstract

Objectives Azatyrosine, an antibiotic isolated from a Streptomyces species, has been previously shown to have antitumor activity against ras- and neu-transformed fibroblasts and human epithelial cells. In this study, we investigated the effect of azatyrosine on human prostate cancer cell growth and the reversion potential of this antibiotic on prostate tumorigenic cell lines. Methods Three androgen-independent human prostate cancer cell lines (TSU-Prl, DU-145, and PC-3) were cultured in the presence of azatyrosine and their growth rates were determined over a 7-day period. Following exhaustive treatment with azatyrosine for 5 weeks, three azatyrosine-resistant colonies were cloned from the PC-3 cell line and were subsequently established as stable cell lines. The growth characteristics of these azatyrosine-resistant clones were examined both in vitro and in vivo to establish their “potentially revertant” profiles. Results Incubation with azatyrosine (for 7 days) resulted in greater than 95% in vitro growth inhibition of the three parental prostate cancer cell lines. Analysis of the biologic properties of these azatyrosine-resistant cell lines revealed: (1) a significant reduction in in vitro growth rates; (2) a decreased rate of DNA synthesis as measured by thymidine uptake; and (3) a decreased ability for colony formation in soft agar. Moreover all three azatyrosine-resistant clones exhibited suppressed tumorigenicity in severe combined immunodeficient (SCID) mice when compared with the parental cell line. An important observation was that one revertant clone demonstrated complete loss of tumorigenicity. On the basis of this biologic behavior, these cell lines were characterized as revertants. Cytogenetic analysis revealed gross chromosomal differences between the revertant clones and the parental cell line. Northern hybridization analysis demonstrated elevated expression of the K- rev-1 and bcl-2 but not the rrg mRNA transcripts in the revertant cell lines. Conclusions These results suggest that azatyrosine inhibits prostate tumorigenic growth; it has a high reversion efficiency on human prostate cancer cells; and the K- rev-1 suppressor gene and the bcl-2 proto-oncogene could be potentially involved in the reversion mechanism mediated by azatyrosine. This reversion of prostate cancer cells to an apparently nontumorigenic phenotype points to a potentially significant therapeutic role for azatyrosine in the treatment of advanced prostate cancer.