Cellular growth and homeostasis via amino acid-responsive pathways are mediated by the mTOR signaling pathway. Rag GTPases and Map4K3 modify mTOR signaling as amino acid sensors. Altered mTOR signaling in relation to amino acid sensors might represent factors that modify proliferation and treatment responses in astrocytic tumors. To investigate this hypothesis, RagC and Map4K3 expression was studied in human gliomas, glioma cells (U87MG/U138MG), and nonglial cells (MCF-7, IOMM-Lee). RagC and Map4K3 knockout in glioma cells was generated using CRISPR-Cas and shRNA. High-grade astrocytomas had significantly reduced immunoreactivity for RagC and Map4K3 compared to low-grade astrocytomas. RagC- and Map4K3-deficient glioma cells had significantly increased proliferation and showed altered morphology and motility. Induced amino acid deficiency (leucine deprivation) reduced proliferation in Map4K3- but not in RagC-deficient cells. mTOR signaling in RagC- and Map4K3-deficient U87 cells was altered with increased phosphorylation of p70S6K and increased expression of RagD and transcription factor EB. In this context, uncoupled, exaggerated autophagy occurred in Map4K3-deficient U87 cells. In contrast, RagC-deficient U87 cells showed increased senescence but no autophagy induction. These data show that losses of RagC and Map4K3 in malignant gliomas have proliferation-inducing effects and differentially modulate key mTOR signaling-dependent cellular mechanisms.
Kahr et al. (Sun,) studied this question.