Insulin Activation of Rheb, a Mediator of mTOR/S6K/4E-BP Signaling, Is Inhibited by TSC1 and 2

Abstract

Tumor suppressor genes evolved as negative effectors of mitogen and nutrient signaling pathways, such that mutations in these genes can lead to pathological states of growth. Tuberous sclerosis (TSC) is a potentially devastating disease associated with mutations in two tumor suppressor genes, TSC1 and 2, that function as a complex to suppress signaling in the mTOR/S6K/4E-BP pathway. However, the inhibitory target of TSC1/2 and the mechanism by which it acts are unknown. Here we provide evidence that TSC1/2 is a GAP for the small GTPase Rheb and that insulin-mediated Rheb activation is PI3K dependent. Moreover, Rheb overexpression induces S6K1 phosphorylation and inhibits PKB phosphorylation, as do loss-of-function mutations in TSC1/2, but contrary to earlier reports Rheb has no effect on MAPK phosphorylation. Finally, coexpression of a human TSC2 cDNA harboring a disease-associated point mutation in the GAP domain, failed to stimulate Rheb GTPase activity or block Rheb activation of S6K1.

Rheb activates AMPK and reduces p27Kip1 levels in Tsc2-null cells via mTORC1-independent mechanisms: implications for cell proliferation and tumorigenesis

“…As Rheb is constitutively activated in Tsc2-deficient cells (Garami et al, 2003;Finlay et al, 2007), we depleted Rheb using RNA interference to investigate its role in regulating AMPK activity. Rheb small interfering RNA (siRNA) decreased Rheb mRNA and protein levels ( Figure 1a).…”

“…Along with others, we showed that the small GTPase Rheb is the molecular link between TSC1/2 and mTORC1. Rheb activates mTORC1, and TSC2 inhibits Rheb by acting as a GTPase-activating protein (Castro et al, 2003;Garami et al, 2003;Tee et al, 2003;Inoki et al, 2003a;Zhang et al, 2003b).…”

“…As Rheb is constitutively activated in Tsc2-null cells (Garami et al, 2003;Finlay et al, 2007), we sought to determine whether it regulates AMPK activity. Here, we show that Rheb but not mTORC1 activates AMPK.…”

The Kaposi's sarcoma‐associated herpesvirus G protein‐coupled receptor: Lessons on dysregulated angiogenesis from a viral oncogene

“…Akt has been recognized as an essential link between the PI3K pathway and the mammalian target of rapamycin (mTOR) through the inactivation of the tuberous sclerosis complex (TSC) [Manning and Cantley, 2003; Richardson et al, 2004]. The TSC complex, formed by hamartin (TSC1) and tuberin (TSC2), functions as a GTPase-activating protein (GAP) for Rheb, a Ras-related small GTP binding protein that promotes the activation of mTOR [Garami et al, 2003; Inoki et al, 2003; Tee et al, 2003]. Phosphorylation of the tumor suppressor TSC2 by Akt results in its inactivation, thereby promoting the accumulation of (active) Rheb-GTP and the induction of mTOR activity.…”

Carboxyl‐Terminal Modulator Protein Ameliorates Pathological Cardiac Hypertrophy by Suppressing the Protein Kinase B Signaling Pathway

“…Moreover, the phosphorylation of serine 473 in the carboxyl terminal regulatory domain of AKT is necessary for the full activation of AKT 25. When activated, AKT phosphorylates its downstream molecules mTOR and GSK3β via the phosphorylation of tuberous sclerosis complex and the inactivation of Ras homolog enriched in brain; mTOR phosphorylates P70S6K 27, 28. Furthermore, the expression of mTOR is increased in the cardiac tissue of patients with heart failure compared with healthy controls 29.…”