The mammalian target of rapamycin (mTOR) signaling pathway is upregulated in the pathogenesis of many cancers. Arachidonic acid (AA) and its metabolites play critical role in the development of breast cancer, but the mechanisms through which AA promotes mammary tumorigenesis and progression are poorly understood. We found that the levels of AA and cytosolic phospholipase A2 (cPLA2) strongly correlated with the signaling activity of mTORC1 and mTORC2 as well as the expression levels of vascular epithelial growth factor (VEGF) in human breast tumor tissues. In cultured breast cancer cells, AA effectively activated both mTOR complex 1 (mTORC1) and mTORC2. Interestingly, AA-stimulated mTORC1 activation was independent of amino acids, phosphatidylinositol 3-kinase (PI3-K) and tuberous sclerosis complex 2 (TSC2), which suggests a novel mechanism for mTORC1 activation. Further studies revealed that AA stimulated mTORC1 activity through destabilization of mTOR-raptor association in ras homolog enriched in brain (Rheb)-dependent mechanism. Moreover, we showed that AA-stimulated cell proliferation and angiogenesis required mTOR activity and that the effect of AA was mediated by lipoxygenase (LOX) but not cyclooxygenase-2 (COX-2). In animal models, AA-enhanced incidences of rat mammary tumorigenesis, tumor weights and angiogenesis were inhibited by rapamycin. Our findings suggest that AA is an effective intracellular stimulus of mTOR and that AA-activated mTOR plays critical roles in angiogenesis and tumorigenesis of breast cancer.

The IjB kinase (IKK)/NF-jB pathway has been shown to be a major regulator in cell survival. However, the mechanisms through which IKK mediates cell death are not clear. In this study, we showed that IKK-b contributed to hydrogen peroxide (H 2 O 2 )-induced cell death independent of the NF-jB pathway. Our results demonstrated that the pro-death function of IKK-b under oxidative stress was mediated by p85 S6K1 (S6 kinase 1), but not p70 S6K1 through a rapamycin-insensitive and mammalian target of rapamycin complex 1 kinase-independent mechanism. We found that IKK-b associated with p85, but not p70 S6K1, which was required for H 2 O 2 -induced activation of p85 S6K1. IKK-b and p85 S6K1 contributed to H 2 O 2 -induced phosphorylation of Mdm2 (S166) and p53 accumulation. p85 S6K1 is critical for IKK-b-mediated cell death. Thus, these findings established a novel oxidative stress-responsive pathway that involves IKK-b, p85 S6K1 and Mdm2, which is response for H 2 O 2 -induced cell death. Our results have important implications for IKK-b and p85 S6K1 as potential targets for the prevention of diseases involved in oxidative stress-induced aberrant cell death. Reactive oxygen species (ROS), such as hydrogen peroxide (H 2 O 2 ), are generated in cells as a consequence of oxidative metabolism. Although low levels of ROS are usually detoxified quickly by antioxidant enzymes, an excessive accumulation of ROS may result in oxidative stress. High and/or persistent levels of oxidative stress represent a major cause of cellular damage and aberrant death in a plethora of pathological conditions during the initiation and progression of a plethora of pathological conditions, including neurodegenerative diseases, cancer, autoimmune and the ageing process.1 While ROS may trigger cell death through multiple mechanisms, 2 recent studies have established the nuclear factor-kB (NF-kB) pathway as a critical signaling pathway in mediating the action of ROS in cell survival/cell death.