Regulation of Antioxidant Metabolism by PI3K/Akt Signaling in Breast Cancer

Abstract

In cancer cells, oncogenic signaling frequently mediates metabolic reprogramming to divert nutrients toward anabolic processes to facilitate enhanced growth and proliferation. Understanding these metabolic changes will reveal cancer-associated metabolic vulnerabilities that can be exploited for cancer therapy. In breast cancer, the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway is frequently dysregulated. Here, we report mechanisms by which oncogenic PI3K/Akt signaling regulates antioxidant metabolism to support breast cancer progression. First, we find that in mammary epithelial cells, oncogenic PI3K/Akt stimulates glutathione (GSH) biosynthesis by stabilizing and activating the transcription factor NRF2 to up-regulate the GSH biosynthetic genes. Increased NRF2 stability is dependent on the Akt-mediated accumulation of p21Cip1/WAF1 and GSK-3β inhibition. Consistently, in human breast tumors, up-regulation of NRF2 targets is associated with PI3K pathway mutation status and oncogenic Akt activation. Elevated GSH biosynthesis is required for PI3K/Akt-driven resistance to oxidative stress, initiation of tumor spheroids, and anchorage-independent growth. Furthermore, inhibiting GSH biosynthesis with buthionine sulfoximine synergizes with cisplatin to selectively induce tumor regression in PI3K pathway mutant breast cancer cells. Our findings provide insight into GSH biosynthesis as a metabolic vulnerability associated with PI3K pathway mutant breast cancers. We also find that PI3K/Akt signaling regulates the utilization of homocysteine (Hcy) in either the methionine cycle to produce methionine (Met) or in the transsulfuration pathway to synthesize cysteine (Cys), which is involved in multiple cellular antioxidant systems. Unlike most non-tumorigenic cells, many cancer cells exhibit “methionine dependency”, where their proliferation is impaired in growth media in which Met is replaced by its precursor Hcy (Met⁻Hcy⁺ media). Here, we demonstrate that oncogenic PI3K/Akt signaling is sufficient to confer methionine dependency to mammary epithelial cells by decreasing cystine (Cys₂) uptake through the regulation of xCT, a Cys₂ transporter. Manipulation of xCT activity alters the proliferative capacity of breast cancer cells in Met⁻Hcy⁺ media, suggesting that it functionally mediates methionine dependency. We propose that by down-regulating Cys₂ uptake through xCT, oncogenic PI3K/Akt instead maintains cellular Cys pools by primarily converting Hcy to Cys through the transsulfuration pathway. Consequently, less Hcy is devoted to the methionine cycle to produce Met, leading to the methionine dependency phenotype. This increased dependence on the transsulfuration pathway may subsequently constitute a metabolic vulnerability that can be therapeutically exploited to target PI3K pathway mutant breast cancer cells.