Abstract 2067: Putative role of Nrf2 as a tumor suppressor through inhibition of angiogenesis

Abstract

Abstract An increase in reactive oxygen species (ROS) is a common biochemical change in cancer cells. To investigate the mechanisms of ROS accumulation during the process of tumorigenesis we employed a well-characterized model of stepwise human mesenchymal stem cell (MSC) transformation. Transformed MSC (tMSC) became dependent upon oxidative stress, as treatment with antioxidants impaired their tumorigenic potential. The increased ROS in tMSC was a consequence of a striking global down-regulation of the cellular antioxidant machinery correlating with reduced levels of the transcription factor Nrf2, a key regulator of the cellular antioxidant response. Expression of constitutive active versions of oncogenic H-Ras, Raf and PI3K/AKT led to decreased levels of Nrf2 in non-transformed MSC. Down-regulation of Nrf2 plays a central role in the defective ROS-scavenging capacity and oncogenic potential of tMSC, since its over-expression induces antioxidants, reduces the levels of ROS and impairs in vivo tumor growth. Mechanistically we show that restoration of Nrf2 expression sensitizes transformed cells to apoptosis and reduces an angiogenic response. Elevated Nrf2, in turn, led to destabilization of HIF-1α and decreased expression of angiogenic factors such as adrenomedullin and VEGF during hypoxic conditions. Moreover, we found a significant reduction in both proliferation and sprouting of human umbilical vein endothelial cells after being cultured in hypoxic/conditioned media from tMSC over-expressing Nrf2 compared with control. Down-regulation of Nrf2 during human cell transformation is not restricted to MSC as Nrf2 expression was also down-regulated when human mammary epithelial cells (HMEC) were genetically modified and transformed with the same oncogenic elements used for MSC. We performed microarray comparison studies and observed that expression of Nrf2 was down-regulated in a panel of human tumors. Furthermore, expression of Nrf2 and its downstream gene NQO1 was enhanced in breast cancer cell lines such as MCF-7 and MDA-MB-231 when the cells were treated with U0126, an inhibitor of ERK1/2 pathway. These results suggest that down-regulation of Nrf2 expression through activation of Ras/Raf/ERK pathway is not unique to in vitro transformed MSC or HMEC, but also occurs in established human breast cancer cell lines. Overall, our results indicate that Nrf2 expression is reduced in certain human tumor cells by mechanisms that involve activation of Ras signaling, and reveal a new role for Nrf2 as a tumor suppressor through inhibition of angiogenesis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2067. doi:1538-7445.AM2012-2067