mTOR/MYC Axis Regulates O-GlcNAc Transferase Expression and O-GlcNAcylation in Breast Cancer

Abstract

Cancers exhibit altered metabolism characterized by increased glucose and glutamine uptake. The hexosamine biosynthetic pathway (HBP) utilizes glucose and glutamine, and directly contributes to O-linked-β-N-acetylglucosamine (O-GlcNAc) modifications on intracellular proteins. Multiple tumor types 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, it is shown that the PI3K/mTOR/MYC signaling pathway is required for elevation of OGT and O-GlcNAcylation in breast cancer cells. Treatment with PI3K and mTOR inhibitors reduced OGT protein expression and decreased levels of overall O-GlcNAcylation. In addition, both AKT and mTOR activation is sufficient to elevate OGT/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 mechanistically requires the expression of c-MYC transcriptional target HSP90A. Lastly, mammary tumor epithelial cells derived from MMTV-c-myc transgenic mice contain elevated OGT and O-GlcNAcylation and OGT inhibition in this model induces apoptosis. Thus, OGT and O-GlcNAcylation levels are elevated via activation of an mTOR/MYC cascade.

The Biochemistry of O-GlcNAc Transferase: Which Functions Make It Essential in Mammalian Cells?

“…Taken together, these studies suggest that nutrient responsiveness at the extremes of O-GlcNAc levels is tempered by feedback mechanisms. Meanwhile, there is evidence of crosstalk with the nutrient-sensitive mammalian target of rapamycin complex 1 (mTORC1) (98,99) and adenosine monophosphateactivated protein kinase (AMPK) (89,91) signaling pathways, which appear to regulate OGT's stability and subcellular localization, respectively.…”

Erratum: From Krebs to clinic: glutamine metabolism to cancer therapy

“…Aberrant expression and activity of O-Linked β-Nacetylglucosamine transferase (OGT), which links UDP-GlcNAc to proteins, was shown to be critical for the survival and progression of breast cancer, prostate cancer, and chronic lymphocytic leukemia [103][104][105] . Thus, glutamine input directly maintains translation, protein trafficking, and survival through suppression of the ISR and ER stress pathways 106,107 .…”

A Chemoenzymatic Histology Method for O‐GlcNAc Detection

“…Multiple studies have provided evidence that aberrant levels of O-GlcNAc exist in breast, [37,38] colon, [39] and prostate cancer. [40] As a second application, we applied chemoenzymatic histology analysis of O-GlcNAc to a commercial tissue array containing normal and cancerous specimens from 12 different organs to examine the differences in O-GlcNAc expression.…”

“…[31] Significantly, the efficacy and utility of chemoenzymatic histology for such applications has been validated by mass-spectrometry-based glycomics analysis. 38 …”