New Insights into the Regulation of mTOR Signaling via Ca2+-Binding Proteins

Abstract

Environmental factors are important regulators of cell growth and proliferation. Mechanistic target of rapamycin (mTOR) is a central kinase that maintains cellular homeostasis in response to a variety of extracellular and intracellular inputs. Dysregulation of mTOR signaling is associated with many diseases, including diabetes and cancer. Calcium ion (Ca²⁺) is important as a second messenger in various biological processes, and its intracellular concentration is tightly regulated. Although the involvement of Ca²⁺ mobilization in mTOR signaling has been reported, the detailed molecular mechanisms by which mTOR signaling is regulated are not fully understood. The link between Ca²⁺ homeostasis and mTOR activation in pathological hypertrophy has heightened the importance in understanding Ca²⁺-regulated mTOR signaling as a key mechanism of mTOR regulation. In this review, we introduce recent findings on the molecular mechanisms of regulation of mTOR signaling by Ca²⁺-binding proteins, particularly calmodulin (CaM).

Keywords: Ca2+ mobilization; TSC2; calmodulin; mTOR.

Figure 1

General functions of mTOR signaling. mTORC1 is primarily composed of mTOR, Raptor, and mLST8. mTORC2 is composed of mTOR, Rictor, mSin1, and mLST8. Multiple stimuli, including amino acids, growth factors, and energy stress regulate mTORC1 activity via the GATOR2–GATOR1–Rag GTPases axis and the TSC2–Rheb GTPase axis. mTORC1 mainly functions to activate anabolism, protein synthesis, or lipid synthesis and to inhibit catabolism, lysosome biogenesis, or autophagy by directly phosphorylating various substrates (closed red curves in oval shapes). In contrast, mTORC2 is thought to respond to growth factors or insulin. Activated mTORC2 regulates cell survival, metabolism, and cytoskeleton dynamics by directly phosphorylating the AGC kinase protein family of Akt, PKC, and SGK1. Major direct substrates of mTORC2 are shown by closed orange curves in oval shapes.

Figure 2

Ca²⁺/CaM-mediated regulation of mTORC1. (A) Amino acids have been reported to elevate intracellular Ca²⁺ concentration and enhance hVps34 kinase activity by promoting the binding between hVps34 and CaM in a Ca²⁺-dependent manner. hVps34 has also been suggested to be activated by amino acids by binding to hVps15. Activated hVps34 generates PI3P from PI in the lysosome, which can trigger PLD1 recruitment. PLD1 then produces PA, which leads to mTORC1 activation. (B) The lysosomal Ca²⁺ channel TRPML1 is negatively regulated by mTORC1. Prolonged nutrient starvation may decrease mTORC1 activity and relieve its inhibition. TRPML1-mediated Ca²⁺ release from the lysosome is sensed by CaM and might activate mTORC1 by the direct binding of Ca²⁺/CaM to mTOR. (C) Amino acids provoke extracellular Ca²⁺ influx. Ca²⁺/CaM activates mTORC1 through the TSC2–Rheb axis by binding to and inhibiting TSC2. Binding of Ca²⁺/CaM to TSC2 might affect TSC2 actions by affecting TSC2 binding to Rheb and/or TSC2 localization, although the detailed mechanism of how Ca²⁺/CaM suppresses TSC2–Rheb remains unclear. Ca²⁺ is indicated by yellow circles.

Figure 3

Potential mechanisms of Ca²⁺/CaM-mediated regulation of Akt and mTORC2. (A) Ca²⁺/CaM phosphorylated at Tyr99 interacts with the N-terminal domain (nSH2) and the C-terminal domain (cSH2) of the 85kDa regulatory subunits of PI3K (p85) and stimulates kinase activity of PI3K. The generated PIP₃ recruits Akt and PDK1 to the plasma membrane. Akt is fully activated by two kinases, PDK1 and mTORC2. (B) EGF induces an intracellular Ca²⁺ rise and enhances Akt activity by promoting CaM–Akt binding as follows: (1) EGF binding to EGFR activates PI3K to generate PIP₃ from PIP₂. (2) EGFR also activates PLCγ and induces Ca²⁺ release from the ER via an IP₃-dependent mechanism. (3) Binding of Ca²⁺/CaM to the PH domain of Akt is proposed to change the conformation of Akt and to facilitate its targeting to the plasma membrane. (4) Binding of Akt to PIP₃ promotes the dissociation from CaM. (5) Akt is phosphorylated by PDK1 and mTORC2 and is fully activated. (C) TMBIM6 Ca²⁺ channel-like protein interacts with mTORC2. Ca²⁺ released from the ER through TMBIM6 is necessary for mTORC2 assembly and association with ribosomes. Because CaM binds to Rictor in a Ca²⁺-dependent manner [33], CaM may mediate Ca²⁺ signaling to mTORC2 activation. Ca²⁺ is indicated by yellow circles.