Regulation of O-GlcNAc Transferase in breast cancer and its role in regulating lipid metabolism

Cancer cells exhibit altered metabolism characterized by increased glucose and glutamine uptake. Altered utilization of these substrates directly contributes to O-linked-[beta]-N-acetylglucosamine (O-GlcNAc) modifications on intracellular proteins. Multiple cancers contain elevated total O-GlcNAcylation, in part, by increasing O-GlcNAc transferase (OGT) levels, the enzyme that catalyzes this modification. Although cancer cells require OGT for oncogenesis, it is not clear how tumor cells regulate OGT expression and O-GlcNAcylation. Here, we show that the PI3K/mTOR/MYC pathway is required for elevation of OGT and O-GlcNAcylation in breast cancer cells as treatment with PI3K and mTOR inhibitors reduced OGT protein expression and decreased levels of overall O-GlcNAcylation. Downstream of mTOR, the oncogenic transcription factor c-MYC is required and sufficient for increased OGT protein expression in an RNA-independent manner and c-MYC regulation of OGT in cancer cells requires the expression of c-MYC target HSP90A. Mammary tumor epithelial cells derived from MMTV-c-myc transgenic mice contain elevated OGT and O-GlcNAcylation and OGT inhibition in these tumor cells induces apoptosis. In an attempt to further address consequences of elevated O-GlcNAcylation in breast cancer, we performed comprehensive metabolomics. Metabolomics analysis revealed that reducing OGT levels in breast cancer cells altered a number of metabolic pathways including amino acids and nucleotides, however, the largest change was seen in fatty acid metabolism. We show that O-GlcNAcylation regulates the transcription factor sterol regulatory element binding protein 1 (SREBP-1) and its transcriptional targets, critical enzymes involved in lipid synthesis. OGT regulates SREBP-1 protein expression and stability/degradation via AMP Activated protein kinase (AMPK). SREBP-1 is critical for OGT-mediated regulation of cell survival and of lipid synthesis, as overexpression of SREBP-1 rescues lipogenic defects associated with OGT suppression, and tumor growth in vitro and in vivo. We also find that OGT regulates these factors in lactating mammary glands, highly lipogenic normal tissue. These results unravel a previously unidentified link between O-GlcNAcylation, lipid metabolism and the regulation of SREBP-1 in cancer. These finding suggests OGT may be a novel therapeutic target for the treatment of lipid-dependent cancers.