Regulation of mast cell survival and function by tuberous sclerosis complex 1

Abstract

Mast cells play critical roles in allergic disorders and asthma. The importance of tuberous sclerosis complex 1/2-mammalian target of rapamycin (TSC1/2-mTOR) signaling in mast cells is unknown. Here, we report that TSC1 is a critical regulator for mTOR signaling in mast cells downstream of FcRI and c-Kit, and differentially controls mast cell degranulation and cytokine production. TSC1-deficiency results in impaired mast cell degranulation, but enhanced cytokine production in vitro and in vivo after FcRI engagement. Furthermore, TSC1 is critical for mast cell survival through multiple pathways of apoptosis including the downregulation of p53, miR-34a, reactive oxygen species, and the up-regulation of Bcl-2. Together, these findings reveal that TSC1 is a critical regulator of mast cell activation and survival, suggesting the manipulation of the TSC1/2-mTOR pathway as a therapeutic strategy for mast cell-mediated diseases. IntroductionMast cells play pivotal roles in chronic allergic inflammation and acute anaphylaxis, which are largely mediated by the high-affinity immunoglobulin E (IgE) receptor (FcεRI) on their surface. Crosslinking of IgE-bound FcεRI by cognate antigen (Ag) initiates multiple signal transduction pathways that trigger the release of proinflammatory mediators, such as histamine from granules and de novo synthesis and secretion of cytokines. [1][2][3] FcεRI engagement activates the Src-family protein tyrosine kinases (PTK) Lyn, which phosphorylates immunoreceptor tyrosine-based activation motifs (ITAMs) of the ␤ and ␥-subunits of FcεRI and subsequently activates Syk. 4 These events are followed by recruiting and activating downstream effector and adaptor molecules, such as linker for activated T cells (LAT), 5 SH2 domain-containing leukocyte phosphoprotein of 76 kDa, 6 Rac GTPase guanine nucleotide exchange factor Vav1, 7 Tec family kinase Bruton tyrosine kinase, 8 and phospholipase C␥ (PLC␥). 9 In addition to Lyn, another Src PTK, Fyn, induces activation of Grb2-associated binder 2 without requirement of Lyn and LAT to promote phosphatidylinositol 3-kinase (PI3K) activation. [10][11][12][13] In turn, these signals are transmitted to downstream signaling molecules including PKCs and MAPKs that are important for mast cell activation. [14][15][16][17] The mammalian target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase linked with the PI3K pathway via Akt. 18 mTOR senses both environmental and intracellular stimuli such as growth factors, nutrients, energy, and stress, and is capable of integrating diverse biologic processes including cell metabolism, growth, autophagy, and survival. 18,19 mTOR forms 2 functionally and structurally distinct complexes: rapamycinsensitive mTOR complex 1 (mTORC1) and rapamycin-insensitive mTORC2. 19 mTORC1 phosphorylates ribosomal S6 kinases (S6Ks) and eIF4E-binding proteins (4E-BPs) to promote ribosomogenesis and cap-dependent translation. 18 mTORC2 directly phosphorylates Akt at Ser473 to promote Akt activation, and is also necessary to p...

Mast cell activation: A complex interplay of positive and negative signaling pathways

“…The roles of newly characterized Fyn-dependent downstream molecules have been recently investigated [24]. Shin et al described a previously unknown mechanism of mammalian target of rapamycin (mTOR) regulation also in mast cell biology [24][25][26].…”

“…Shin et al described a previously unknown mechanism of mammalian target of rapamycin (mTOR) regulation also in mast cell biology [24][25][26]. mTOR is an evolutionarily conserved serine/threonine kinase activated by PI3K/Akt signaling.…”

“…The authors showed fine modulation of mTOR and regulation of the TSC1. TSC1 deficiency resulted in increased cytokine production and impaired degranulation (via PKC-δ activation) [24].…”

“…The authors showed fine modulation of mTOR and regulation of the TSC1. TSC1 deficiency resulted in increased cytokine production and impaired degranulation (via PKC-δ activation) [24].The entire apparatus necessary for mast cell degranulation, as well as granule trafficking, involves the rearrangement of cytoskeletal actin and myosin microfilaments, in a way that largely requires the activation of Fyn [5]. In this regard, p21-activated kinases (Pak) have been recently characterized to transduce the Fyn-dependent signaling to the cytoskeleton.…”

“…The association of 3BP2 itself (3BP2 is active in the Lyn pathway as well) to SHP-1 regulates SHP-1-mediated production of TNF-α without effects on mast cell degranulation [22]. Moreover, SHP-2 enhances degranulation, calcium mobilization, and the synthesis of IL-1β, IL-10, and monocyte chemoattractant protein 1 (MCP-1) transcripts [23].The roles of newly characterized Fyn-dependent downstream molecules have been recently investigated [24]. Shin et al described a previously unknown mechanism of mammalian target of rapamycin (mTOR) regulation also in mast cell biology [24][25][26].…”