Cellular acidosis triggers human MondoA transcriptional activity by driving mitochondrial ATP production

Abstract

Human MondoA requires glucose as well as other modulatory signals to function in transcription. One such signal is acidosis, which increases MondoA activity and also drives a protective gene signature in breast cancer. How low pH controls MondoA transcriptional activity is unknown. We found that low pH medium increases mitochondrial ATP (mtATP), which is subsequently exported from the mitochondrial matrix. Mitochondria-bound hexokinase transfers a phosphate from mtATP to cytoplasmic glucose to generate glucose-6-phosphate (G6P), which is an established MondoA activator. The outer mitochondrial membrane localization of MondoA suggests that it is positioned to coordinate the adaptive transcriptional response to a cell’s most abundant energy sources, cytoplasmic glucose and mtATP. In response to acidosis, MondoA shows preferential binding to just two targets, TXNIP and its paralog ARRDC4. Because these transcriptional targets are suppressors of glucose uptake, we propose that MondoA is critical for restoring metabolic homeostasis in response to high energy charge.

Protein synthesis inhibitors stimulate MondoA transcriptional activity by driving an accumulation of glucose 6-phosphate

“…Likewise, and consistent with a requirement for mtATP, blocking the activity of ATP synthase (complex V) with oligomycin also robustly inhibited TXNIP expression ( Figure 3A). To test the requirement of ATP synthesis further, we used siRNA to deplete ATP5I, which is an essential component of ATP synthase: our previous work established that ATP5I knockdown in HeLa cells blocks the production of mtATP (13). In this experimental context, ATP5I knockdown reduced background TXNIP expression and completely suppressed its induction by RocA ( Figure 3B).…”

“…Mechanistically, glucose-6-phosphate (G6P) drives translocation of MondoA from the outer mitochondrial membrane (OMM) to the nucleus where it binds the promoters of its target genes and recruits cofactors that initiate transcription (10,12,13). MondoA binds a double E-box Carbohydrate Response Element (ChoRE) in the TXNIP promoter to drive its expression in response to elevated glucose levels (11,14,15).…”

“…In addition to an absolute functional requirement on glucose (10,11), we recently showed that MondoA transcriptional activity is also highly dependent on mtATP (13). Our data suggest that MondoA functions as a coincidence detector, only being active when above threshold levels of glucose and mtATP are available to generate enough G6P to drive MondoA activity (13). Collectively, our data suggest that MondoA is sensor of high cellular energy charge exemplified by its two most prevalent nutrient sources and is critical for the adaptive transcriptional response to a hyper-nutrient state.…”

“…We next investigated how protein synthesis inhibitors increase MondoA transcriptional activity. We focused on a potential role for mitochondrial function and mtATP for three reasons: 1) protein translation is the most ATP-consuming biosynthetic reaction, 2) MondoA transcriptional activity depends on mtATP (13), 3) higher mtATP levels may sensitize MondoA transcriptional activity and TXNIP expression to lower levels of glucose by increasing levels of G6P (13). Consistent with a requirement for functional electron transport, inhibition of complex I with metformin completely abrogated TXNIP induction by CHX ( Figure 3A).…”

mTOR Senses Intracellular pH through Lysosome Dispersion from RHEB

“…Given that glycolysis is the main acid source in tumors, it makes teleological sense that a similar feedback loop might exist to downregulate glycolysis in response to decreasing pH i . To this end, acidification rapidly induces thioredoxin‐interacting protein (TXNIP) transcription, largely due to activation of the MondoA/Mlx heterodimeric transcription factor, a member of the extended MYC protein family . TXNIP, in turn, strongly suppresses glycolysis by suppressing the transcription of GLUT1 and promoting GLUT1 internalization .…”