mTORC1 and mTORC2 differentially regulate homeostasis of neoplastic and non-neoplastic human mast cells

Abstract

Increased mast cell burden is observed in the inflamed tissues and affected organs and tissues of patients with mast cell proliferative disorders. However, normal mast cells participate in host defense, so approaches to preferentially target clonally expanding mast cells are needed. We found that mammalian target of rapamycin complex 1 (mTORC1) and 2 (mTORC2) are up-regulated in neoplastic and developing immature mast cells compared with their terminally differentiated counterparts. Elevated mTOR mRNA was also observed in bone marrow mononuclear cells of patients exhibiting mast-cell hyperplasia. Selective inhibition of mTORC1 and mTORC2 through genetic and pharmacologic manipulation revealed that, whereas mTORC1 may contribute to mast-cell survival, mTORC2 was only critical for homeostasis of neoplastic and dividing immature mast cells. The cytostatic effect of mTORC2 down-regulation in proliferating mast cells was determined to be via inhibition of cell-cycle progression. Because mTORC2 was observed to play little role in the homeostasis of differentiated, nonproliferating, mature mast cells, these data provide a rationale for adopting a targeted approaching selectively inhibiting mTORC2 to effectively reduce the proliferation of mast cells associated with inflammation and disorders of mast cell proliferation while leaving normal differentiated mast cells largely unaffected. IntroductionMast cells (MCs) are considered to be critical components of both the innate and acquired immune defense systems. 1 Central to these functions is the ability of MCs to release a plethora of inflammatory mediators after activation through cell-surface receptors, primarily the high-affinity receptors for IgE (Fc⑀RI). 2 Although these reactions are considered to have evolved to protect host organisms against invading parasites and other microorganisms, 3 inappropriate or exaggerated activation of MCs manifests the reactions associated with allergic diseases.MCs develop from bone marrow (BM) CD13 ϩ /CD34 ϩ /CD117 (KIT) ϩ progenitor cells that enter into circulation and mature during migration to and residency in their target tissues. 4 MC numbers within tissues appear to be tightly regulated, with a several-fold increase in numbers occurring in inflammatory conditions and even higher numbers in association with parasitic inflammation. Clonal MC disorders may result in 10-fold or greater numbers of MCs in tissues such as the BM, liver, and spleen. [5][6] Similarly, a dysregulated increase in MC numbers is also observed in certain types of cancer, and in that context may contribute to cancer progression. [7][8][9] We observed previously that the mammalian target of rapamycin (mTOR) is overexpressed and constitutively phosphorylated in neoplastic MCs regardless of whether activating mutations in the MC growth factor receptor KIT are present. 10 MTOR is a serine/ threonine kinase that regulates divergent signaling pathways depending on its interactions with 2 regulatory proteins: raptor, a major component of mTOR complex 1 (mTORC1), a...

Advanced systemic mastocytosis: from molecular and genetic progress to clinical practice

“…Smrz et al showed that the expression and activation of mTORC1 and mTORC2 was increased in neoplastic human MC lines and in immature normal MCs, as compared with mature normal MCs. 92 Interestingly, the authors demonstrated that mTORC1 might contribute to MC survival, while mTORC2 might only fulfill critical functions in the context of proliferating (dividing) neoplastic and immature MCs. 92 Rapamycin, a specific inhibitor of mTORC1, has been shown to block FcεRI-and KITinduced mTORC1-dependent p70S6K phosphorylation in normal MCs.…”

“…92 Interestingly, the authors demonstrated that mTORC1 might contribute to MC survival, while mTORC2 might only fulfill critical functions in the context of proliferating (dividing) neoplastic and immature MCs. 92 Rapamycin, a specific inhibitor of mTORC1, has been shown to block FcεRI-and KITinduced mTORC1-dependent p70S6K phosphorylation in normal MCs. 88 Furthermore, BEZ235, a dual PI3-K/mTOR blocker, exerted strong growth-inhibitory effects on neoplastic MCs in vitro.…”

Mammalian Target of Rapamycin (mTOR) Inhibition in Advanced Bronchial Carcinoids

“…S6K1 from mTORC1 inhibits the PI3K/AKT pathway via suppression of insulin receptor substrate-1 (IRS-1); thus, the mTORC1/mTORC2 complex is both an upstream and downstream regulator of cellular function, and regulates cell survival, cytoskeletal remodelling and cell migration (see Figure 1). 22,23 Binding of insulin or insulin-like growth factors (IGFs) to their receptors leads recruitment of PI3K to the cell membrane, activating AKT (protein kinase B). Phosphatase and tensin homologue (PTEN) is an inhibitory regulator of the PI3K/AKT/mTOR pathway that antagonises the action of PI3K.…”

Providing the TORC for cell cycle progression in neoplastic mast cells

“…7 In addition, expression of mTOR mRNA was observed to be elevated in bone marrow mononuclear cells of patients with mastocytosis. 8 MTOR is a serine/threonine kinase that forms two distinct complexes by binding to associating proteins, including raptor, forming mTOR complex 1 (mTORC1), or rictor, forming mTOR complex 2 (mTORC2). 9 The mTORC1 complex primarily controls the activation of the downstream transcriptional regulators p70 S6 kinase and 4E-BP1, whereas mTORC2 primarily provides positive feedback regulation of AKT activation.…”

“…10 This allowed us to directly compare the relative roles of the mTOR complexes in developing terminally differentiated mature and neoplastic mast cells. 8 To this end, we adopted two approaches: (1) a pharmacological approach using inhibitors that selectively target mTORC1 (rapamycin) or both mTORC1 and mTORC2 (Torin1) and (2) a gene knockdown approach utilizing shRNA to downregulate the expression of raptor or rictor, respectively, blocking the function of mTORC1 and mTORC2. The combined results of these approaches revealed that mTORC1 and mTORC2 play different roles in the expansion and homeostasis of neoplastic and non-neoplastic mast cells.…”