TSC2 Integrates Wnt and Energy Signals via a Coordinated Phosphorylation by AMPK and GSK3 to Regulate Cell Growth

Abstract

Mutation in the TSC2 tumor suppressor causes tuberous sclerosis complex, a disease characterized by hamartoma formation in multiple tissues. TSC2 inhibits cell growth by acting as a GTPase-activating protein toward Rheb, thereby inhibiting mTOR, a central controller of cell growth. Here, we show that Wnt activates mTOR via inhibiting GSK3 without involving beta-catenin-dependent transcription. GSK3 inhibits the mTOR pathway by phosphorylating TSC2 in a manner dependent on AMPK-priming phosphorylation. Inhibition of mTOR by rapamycin blocks Wnt-induced cell growth and tumor development, suggesting a potential therapeutic value of rapamycin for cancers with activated Wnt signaling. Our results show that, in addition to transcriptional activation, Wnt stimulates translation and cell growth by activating the TSC-mTOR pathway. Furthermore, the sequential phosphorylation of TSC2 by AMPK and GSK3 reveals a molecular mechanism of signal integration in cell growth regulation.

P-21-activated protein kinase-1 functions as a linker between insulin and Wnt signaling pathways in the intestine

“…A report by Inoki et al (2006) supports the concept that the Wnt signaling activation also stimulates gene expression at both the transcriptional and translation levels. The latter was also associated with the activation of mTOR and S6K1, mediated by the inactivation of GSK-3, but not requiring b-cat.…”

“…The latter was also associated with the activation of mTOR and S6K1, mediated by the inactivation of GSK-3, but not requiring b-cat. In the absence of Wnt activation, GSK-3, in concert with AMP (adenosine monophosphate)-dependent protein kinase, catalyzed the pre-phosphorylation of Ser1341, phosphorylation of Ser1337 of TSC-2 and the increase in the GAP (GTPaseactivating protein) activity of the TSC1/2 (tuberous sclerosis complex) to inhibit mTOR (Inoki et al, 2006). Therefore, both Wnt and insulin/IGF1 signaling resulted in mTOR activation and stimulation of cat/TCF activity to augment both the translation and the transcription of their common targets.…”

Gli1 acts through Snail and E-cadherin to promote nuclear signaling by β-catenin

“…Wnt-mediated inhibition of GSK3b activates the mammalian target of rapamycin (mTOR) kinase, identifying rapamycin and related inhibitors of mTOR as antagonists of malignant cells with active Wnt signaling (Inoki et al, 2006) (Figure 7c). This signaling may explain how Gli1 activates mTOR and how mTOR inhibitors block transformation by Gli1, as we showed previously .…”

Rapamycin and mTOR kinase inhibitors

“…Under conditions in which intracellular ATP is depleted and the level of AMP is increased, AMP binds to AMPK and allows LKB1 to phosphorylate Thr172 on the catalytic α subunit to activate AMPK [19]. AMPK then phosphorylates TSC2 on Ser1345, which primes TSC2 for subsequent phosphorylation of Ser1341 and Ser1337 by glycogen synthase kinase 3 [20]. Together, these modifications are thought to enhance the GAP activity of TSC2, inactivate Rheb, and turn off mTORC1 signaling (Fig.…”