Phosphatidylinositol 3 kinase (PI3K) modulates manganese homeostasis and manganese-induced cell signaling in a murine striatal cell line

• Published in: Neurotoxicology (Park Forest South) 2018-01, Vol.64, p.185-194
• Main Authors: Bryan, Miles R., Uhouse, Michael A., Nordham, Kristen D., Joshi, Piyush, Rose, Daniel I.R., O’Brien, Michael T., Aschner, Michael, Bowman, Aaron B.
• Format: Article
• Language: English
• Subjects: 1-Phosphatidylinositol 3-kinase; Activation; AKT protein; Animals; Ataxia; Ataxia telangiectasia mutated protein; Blocking; Cell Line; Cell lines; Cells; Chemical elements; Chromones - pharmacology; Corpus Striatum - drug effects; Corpus Striatum - metabolism; Exposure; Fura-2; HEK293 Cells; Homeostasis; Humans; Induced Pluripotent Stem Cells - drug effects; Induced Pluripotent Stem Cells - metabolism; Inhibition; Inhibitory Concentration 50; Intracellular; Kinases; KU55933; KU60019; LY294002; Manganese; Manganese - metabolism; Manganese transport; Mice; Morpholines - pharmacology; Neostriatum; Neural Stem Cells - drug effects; Neural Stem Cells - metabolism; Neurotoxicity; NU7441; p53; p53 Protein; Phosphatidylinositol 3-Kinase - metabolism; Phosphorylation; PI3K; Pluripotency; Progenitor cells; Protein kinase; Rodents; Serine; Signal Transduction; Signaling; STHdh; TOR protein; Tumor Suppressor Protein p53 - metabolism
• ISSN: 0161-813X; 1872-9711
• DOI: 10.1016/j.neuro.2017.07.026

•Pharmacological PI3K inhibition reduces Mn-induced p-AKT and p-S6 in multiple cell lines.•PI3K inhibition reduces Mn-induced p-p53 by reducing Mn uptake in the STHdh cell.•The mechanism for PI3K inhibition on net Mn uptake is not sufficiently explained by AKT or S6 signaling. In a recent study, we found that blocking the protein kinase ataxia telangiectasia mutated (ATM) with the small molecule inhibitor (SMI) KU-55933 can completely abrogate Mn-induced phosphorylation of p53 at serine 15 (p-p53) in human induced pluripotent stem cell (hiPSC)-differentiated striatal neuroprogenitors. However, in the immortalized mouse striatal progenitor cell line STHdhQ7/Q7, a concentration of KU55933 far exceeding its IC50 for ATM was required to inhibit Mn-induced p-p53. This suggested an alternative signaling system redundant with ATM kinase for activating p53 in this cell line- one that was altered by KU55933 at these higher concentrations (i.e. mTORC1, DNApk, PI3K). To test the hypothesis that one or more of these signaling pathways contributed to Mn-induced p-p53, we utilized a set of SMIs (e.g. NU7441 and LY294002) known to block DNApk, PI3K, and mTORC1 at distinct concentrations. We found that the SMIs inhibit Mn-induced p-p53 expression near the expected IC50s for PI3K, versus other known targets. We hypothesized that inhibiting PI3K reduces intracellular Mn and thereby decreases activation of p53 by Mn. Using the cellular fura-2 manganese extraction assay (CFMEA), we determined that KU55933/60019, NU7441, and LY294002 (at concentrations near their IC50s for PI3K) all decrease intracellular Mn (∼50%) after a dual, 24-h Mn and SMI exposure. Many pathways are activated by Mn aside from p-p53, including AKT and mTOR pathways. Thus, we explored the activation of these pathways by Mn in STHdh cells as well as the effects of other pathway inhibitors. p-AKT and p-S6 activation by Mn is almost completely blocked upon addition of NU7441(5μM) or LY294002(7μM), supporting PI3K’s upstream role in the AKT/mTOR pathway. We also investigated whether PI3K inhibition blocks Mn uptake in other cell lines. LY294002 exposure did not reduce Mn uptake in ST14A, Neuro2A, HEK293, MEF, or hiPSC-derived neuroprogenitors. Next, we sought to determine whether inhibition of PI3K blocked p53 phosphorylation by directly blocking an unknown PI3K/p53 interaction or indirectly reducing intracellular Mn, decreasing p-p53 expression. In-Cell Western and CFMEA experiments using multiple concentrati