Phosphorylation of Raptor by p38β Participates in Arsenite-induced Mammalian Target of Rapamycin Complex 1 (mTORC1) Activation

Abstract

Cell growth is influenced by environmental stress. Mammalian target of rapamycin (mTOR), the central regulator of cell growth, can be positively or negatively regulated by various stresses through different mechanisms. The p38 MAP kinase pathway is essential in cellular stress responses. Activation of MK2, a downstream kinase of p38␣, enhances mTOR complex 1 (mTORC1) activity by preventing TSC2 from inhibiting mTOR activation. The p38␤-PRAK cascade targets Rheb to inhibit mTORC1 activity upon glucose depletion. Here we show the activation of p38␤ participates in activation of mTOR complex 1 (mTORC1) induced by arsenite but not insulin, nutrients, anisomycin, or H 2 O 2 . Arsenite treatment of cells activates p38␤ and induces interaction between p38␤ and Raptor, a regulatory component of mTORC1, resulting in phosphorylation of Raptor on Ser 863 and Ser 771 . The phosphorylation of Raptor on these sites enhances mTORC1 activity, and contributes largely to arsenite-induced mTORC1 activation. Our results shown here and in previous work demonstrate that the p38 pathway can regulate different components of the mTORC1 pathway, and that p38␤ can target different substrates to either positively or negatively regulate mTORC1 activation when a cell encounters different environmental stresses.The p38 mitogen-activated protein kinase (MAPK) signal pathway plays an important role in a variety of biological processes, including inflammation, cell differentiation, and cell death (1-3). The p38 group of MAPK has four members: p38␣, p38␤, p38␥, and p38␦ (4 -7). Although similarities in activation and function have been observed, each p38 isoform also has distinct functions (8). Although activation of p38 MAPKs by different stimuli is cell type-dependent, various stress stimuli appear to activate the p38 pathway in all types of cells, and thus the p38 pathway is considered to be a major stress-activated signaling pathway (9). The activation of transcription factors and subsequent gene expression is a major mechanism by which the p38 pathway mediates biological responses (10 -12). The activation of other types of cellular proteins is also essential for the p38 pathway to execute its function (13).The mammalian target of rapamycin (mTOR) 3 is a serine/ threonine kinase that acts as an environmental sensor to regulate a plethora of cellular biosynthetic processes (14). mTOR activation promotes cell growth and proliferation, whereas mTOR inhibition stops cell growth and initiates catabolic processes (15). The p70 S6 kinase (S6K) and eIF4E-binding protein 1 (4EBP1) are key regulators of mRNA translation, and are the most well characterized targets of mTOR (16,17). Phosphorylation of S6K and 4EBP1 by mTOR leads to increased levels of translation of specific mRNAs (18,19). mTOR exists in two distinct functional complexes, mTOR complex 1 (mTORC1) and mTORC2. mTORC1 is potently and specifically inhibited by rapamycin, and it regulates cell size, autophagy, ribosome biogenesis, protein translation, transcription, and cellular viability (15)...

Lewis lung carcinoma regulation of mechanical stretch-induced protein synthesis in cultured myotubes

“…Several studies provide support for ERK1/2's role in the mechanical induction of skeletal muscle mTOR signaling (38,71). Additionally, mechanical activation of p38 signaling has been associated with peroxisome proliferator-activated receptor-␣ coactivator-transcription (2), glucose uptake (12), and mTOR signaling activation (17,72). However, chronic inflammatory diseases also activate muscle MAPK signaling, resulting in increased protein degradation and muscle wasting (51,76).…”

“…Activation of mTOR signaling by mechanical stimulation can occur independent of Akt, and there is evidence that signaling involving phospholipase D (25,73) and extracellular signal-regulated kinases 1/2 (ERK1/2) (34, 38) is involved in the process. Mechanical stimuli can activate muscle ERK1/2 and p38 signaling (37,44), which can both induce mTORC1 (38,42,72). Chronic systemic inflammation has the potential to inhibit muscle protein synthesis activation.…”

NLK phosphorylates Raptor to mediate stress-induced mTORC1 inhibition

“…Raptor S863 has been reported to be a common phosphorylation site by multiple kinases (Wang et al 2009;Foster et al 2010;Carriere et al 2011;Wu et al 2011;Kwak et al 2012), including ERK1/2, JNK, p38β, and mTOR. Therefore, Raptor S863 phosphorylation can occur in response to a number of stimuli.…”

Investigation of Receptor interacting protein (RIP3)-dependent Protein Phosphorylation by Quantitative Phosphoproteomics

“…Immunocomplexes were subjected to an in vitro kinase assay in a kinase buffer (25mM Tris-HCl at pH 7.5, 10 mM MgCl 2 , 2 mM dithiothreitol, 5 mM ␤-glycerophosphate and 0.1 mM Na 3 VO 4 ) containing ␥-32 P ATP at 30°C for 30min. The autophosphorylation of RIP3 was detected as described (27).…”