Crosstalk of Hedgehog and mTORC1 Pathways

Abstract

Hedgehog (Hh) signaling and mTOR signaling, essential for embryonic development and cellular metabolism, are both coordinated by the primary cilium. Observations from cancer cells strongly indicate crosstalk between Hh and mTOR signaling. This hypothesis is supported by several studies: Evidence points to a TGFβ-mediated crosstalk; Increased PI3K/AKT/mTOR activity leads to increased Hh signaling through regulation of the GLI transcription factors; increased Hh signaling regulates mTORC1 activity positively by upregulating NKX2.2, leading to downregulation of negative mTOR regulators; GSK3 and AMPK are, as members of both signaling pathways, potentially important links between Hh and mTORC1 signaling; The kinase DYRK2 regulates Hh positively and mTORC1 signaling negatively. In contrast, both positive and negative regulation of Hh has been observed for DYRK1A and DYRK1B, which both regulate mTORC1 signaling positively. Based on crosstalk observed between cilia, Hh, and mTORC1, we suggest that the interaction between Hh and mTORC1 is more widespread than it appears from our current knowledge. Although many studies focusing on crosstalk have been carried out, contradictory observations appear and the interplay involving multiple partners is far from solved.

Keywords: 4E-BP1; GLI1; GLI2; GLI3; Mammalian target of rapamycin; S6K; TSC; autophagy; eIF4E; primary cilia.

Figure 1

This is a model of Hh signaling in the primary cilium. To the left, Hh in the Off state. To the right, Hh in the On state. In the Off state, GRP161 and PTCH1 are in the primary cilia. GRP161 inhibits Hh signaling by activation of PKA, leading to phosphorylation of GLI. GLI2/3 are subsequently phosphorylated by CK1 and GSK3B and either proteolytically degraded to GLI2/3Rep or completely degraded. AMPK can also perform the priming phosphorylation of GLI. SUFU inhibits GLI. In the On state, Hh activation leads to removal of PTCH from the cilium, and entrance of SMO. The presence of active SMO in the cilium permits the export of GRP161. SMO prevents GLI degradation by downregulation of PKA. The full-length GLI1/2/3FL are accumulated and translocated to the nucleus in the active GLI1/2/3A form, leading to transcription of the Hh target genes (e.g., GLI1, PTCH1, AKT, NKX2.2) The IFT-B complex is involved in anterograde transport and the IFT-A in retrograde transport. See text for further details.

Figure 2

The mTOR pathway. mTOR is a member of the mTORC1 and the mTORC2 complexes. mTORC1 is composed of mTOR, Deptor, mLST8, RAPTOR, and PRAS40. mTORC2 is composed of mTOR, Deptor, mLST8, RICTOR, mSIN1, and PROTOR1. The tumor-suppressor TSC1/2 complex negatively regulates mTORC1 by inhibiting the protein RHEB. P13K, the RAS–RAF–MEK–ERK (MAPK) pathway, PTEN, LKB1, and NF1 are all members of the mTORC1 signaling pathway. Activation of mTORC1 begins when, e.g., growth factors activate PI3K. PI3K activates AKT1, which inactivates the TSC1/2 complex. RHEB activates mTORC1, which regulates the activity of S6K and 4E-BP1. The lysosomal localization of active mTORC1 is dependent on Rag GTPases. mTORC1 regulates nucleotide synthesis, glucose metabolism, lipid synthesis, and protein synthesis positively and regulates autophagy negatively. AMPK regulates mTORC1 negatively. Apoptosis is negatively regulated by AKT. See text for further details.

Figure 3

Hh and mTOR crosstalk via TGFβ signaling. The TGF-β-activated TGF-β receptor complex (TGF-βI/II) activates, by phosphorylation, the GLI2 transcription factor SMAD2/3. For this activation, TSC1 is required. See text for further details.

Figure 4

Connection between the PI3K/AKT/mTOR pathway and GLI activity. Increased Akt activity leads to increased GLI2 activity due to inhibition of degradation via PKA/CK1/GSK3β. IGF-induced activation of PI3K/AKT leads to increased expression of GLI1/2. Active RAS/MEK/AKT leads to increased nuclear localization of GLI1. Rapamycin inhibition of mTORC1 leads to increased PP2A activity, followed by decreased nuclear localization and transcriptional activity of GLI3 (but not GLI1 or GLI2). Glaucocalyxin A inhibits AKT. S6K1 phosphorylates GLI1 at p.Ser84, leading to increased nuclear location of GLI1. This effect could however not be confirmed by KD of S6K1. mTORC1-mediated inhibition of 4E-BP1 has been demonstrated to be required for canonical Hh signaling.

Figure 5

GSK3 in Hh and mTOR crosstalk. GSK3 inhibits Hh signaling by phosphorylation of GLI transcription factors, leading to their degradation. Upon an AMPK-mediated priming phosphorylation of TSC2, GSK3β could phosphorylate and activate TSC2, leading to inhibition of mTORC1. Inhibition of mTORC1 was demonstrated to change the cellular distribution of GSK3β from the cytosol to the nucleus. In contrast, GSK3β activates mTORC1, through direct phosphorylation and stabilization of Raptor. GSK3β also positively regulates S6K1 and negatively regulates 4E-BP1, by phosphorylation, in both cases leading to increased protein synthesis. S6K2 and S6K1 could both inhibit GSK3β through p.Ser9 phosphorylation.