Abstract 2886: Differential effects of soy isoflavones on Akt and vimentin expression in human prostate cancer cells and mouse xenograft tumors

Abstract

Abstract Studies have reported that soy isoflavones inhibit growth in a number of carcinoma cell lines. The overexpression of Akt, a prosurvival pathway, and vimentin, a mesenchymal marker, in urological carcinomas has been associated with tumor progression and poor prognosis. Our study investigated the effects of a soy isoflavone concentrate (ISF) on the expression of Akt and vimentin in a) cultured human prostate cancer cells and b) xenograft tumors in mice. METHODS: Human prostate cancer PC-3 cells, which are androgen-insensitive and do not express p53 or PTEN, were grown in culture. Hypoxia exposure was at 1% oxygen vs room air for 48 hours. Akt, phosphorylated-Akt (phosAkt), carbonic anhydrase IX (CAIX) and vimentin expression in cells was measured by Western immunoblot analysis. Male nude mice on a soy-free control diet were injected with PC-3 cells into the hind flank. On day 5, half the mice were placed on a diet containing 0.5% soy isoflavone concentrate (ISF). Tumor size was monitored biweekly. Resected tumors were fixed in formalin and paraffin-embedded. Immunohistochemical (IHC) analysis of tumors used antibodies against PCNA, TUNEL, CAIX, Akt, phosAkt, and vimentin. RESULTS: In culture, PC-3 cells constitutively express high levels of Akt and vimentin proteins, which is not altered by hypoxia; in contrast, phosAkt and CAIX proteins are highly expressed only under hypoxic conditions. ISF caused dose-dependent inhibition of DNA synthesis and cell viability without evidence of apoptotic cell death. In culture, ISF decreased the constitutive expression of vimentin, as well as the hypoxia-mediated induction of pAkt and CAIX. In mice, the soy ISF diet significantly inhibited xenograft tumor growth. At 39 days, median tumor size was 862 mm3 in soy-free controls, and 520 mm3 in mice on the ISF diet (p=0.02). IHC analysis of tumors shows that ISF decreased the expression of nuclear PCNA, while TUNEL positive apoptotic cells was increased 4-fold. The ISF diet markedly suppressed levels of total Akt and activated phosAkt. In contrast, staining for CAIX protein was not different in control vs ISF-tumors. Vimentin was clearly expressed in PC-3 tumors and was increased by exposure to irradiation (IR, 2Gy). Interestingly, ISF suppressed IR-induced vimentin expression. Thus, soy ISF treatment inhibited PC-3 cell proliferation both in vitro and in vivo, but different mechanisms are involved in the regulation of expression of the dominant Akt prosurvival signaling pathway, as well as a major mesenchymal marker vimentin. CONCLUSIONS: Together, data indicate that a soy ISF diet strongly inhibits the growth of prostate xenograft tumors which is mediated by differential effects on hypoxia-regulated genes in the tumor microenvironment. (Supported by NIH CA102386 and the Maren Foundation) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2886.