Inactivation of Rheb by PRAK-mediated phosphorylation is essential for energy-depletion-induced suppression of mTORC1

Abstract

Cell growth can be suppressed by stressful environments, but the role of stress pathways in this process is largely unknown. Here we show that a cascade of p38β mitogen activated protein kinase and p38 regulated/activated kinase (PRAK) plays a role in energy starvation-induced suppression of mammalian target of rapamycin (mTOR), that energy starvation activates the p38β-PRAK cascade, and that p38β- or PRAK-deletion diminishes energy depletion-induced suppression of mTORC1 and reduction of cell size. We show that p38β-PRAK operates independent from the known mTORC1 inactivation pathways – phosphorylation of tuberous sclerosis protein 2 (TSC2) and raptor by AMP activated protein kinase (AMPK), and surprisingly, PRAK directly regulates Ras homolog enriched in brain (Rheb), a key component of the mTORC1 pathway by phosphorylation. Phosphorylation of Rheb at serine 130 by PRAK impairs Rheb’s nucleotide-binding ability and inhibits Rheb-mediated mTORC1 activation. The direct regulation of Rheb by PRAK integrates a stress pathway with the mTORC1 pathway in response to energy depletion.

A matter of energy stress: p38β meets mTORC1

“…Finally, it was reported that mTORC1 inhibition by either glycolytic or mitochondria respiratory inhibitors is transient and that the activity of mTORC1 recovers over time npg due to compensation of the alternative energy source [2]. It is surprising that in this new study such a transient effect on mTORC1 signaling upon 2-DG treatment was not observed [8].…”

“…This represents a novel mode of regulation of Rheb function, since only changes in GTP hydrolysis [13] or its sequestration by other proteins have been documented [7]. Since the nucleotide binding capacity of the Rheb mutants was distinct, it was not possible to accurately evaluate the effect of phosphorylation on GTP hydrolysis [8]. An intriguing observation made by the authors is the dependency of glycolytic inhibitors, but not mitochondrial respiratory inhibitors, on PRAK.…”

“…For instance, Inoki et al demonstrated earlier that high concentrations of 2-DG caused osmotic shock, which led to the down-regulation of mTORC1 signaling in a TSC2-independent manner [4]. Zheng et al reported now that high concentrations of sorbitol did not require PRAK protein to down-regulate mTORC1 signaling [8]; arguing that either pathway may substitute for the other or that osmotic shock alters mTORC1 signaling through yet another pathway whose identity is unknown. Likewise, Guan and colleagues reported earlier that p38-MK2, unlike p38β-PRAK cassette [8], positively regulates mTORC1 signaling by phosphorylating TSC2 [10].…”

“…Zheng et al reported now that high concentrations of sorbitol did not require PRAK protein to down-regulate mTORC1 signaling [8]; arguing that either pathway may substitute for the other or that osmotic shock alters mTORC1 signaling through yet another pathway whose identity is unknown. Likewise, Guan and colleagues reported earlier that p38-MK2, unlike p38β-PRAK cassette [8], positively regulates mTORC1 signaling by phosphorylating TSC2 [10]. It will be of interest to elucidate upstream events by which these distinct and opposing roles of p38 are involved in regulating mTORC1 function.…”

“…In a recent study, Zheng and colleagues have brought a new player into the field of energy balance, the p38 pathway [8]. The p38 pathway belongs to a MAPK signaling module, a highly conserved pathway known for its role in stress responses, including those to pro-inflammatory cytokines and UV irradiation as well as heat and osmotic shock [9].…”