Dexamethasone Represses Signaling through the Mammalian Target of Rapamycin in Muscle Cells by Enhancing Expression of REDD1

Abstract

The mammalian target of rapamycin (mTOR), a critical modulator of cell growth, acts to integrate signals from hormones, nutrients, and growth-promoting stimuli to downstream effector mechanisms involved in the regulation of protein synthesis. Dexamethasone, a synthetic glucocorticoid that represses protein synthesis, acts to inhibit mTOR signaling as assessed by reduced phosphorylation of the downstream targets S6K1 and 4E-BP1. Dexamethasone has also been shown in one study to up-regulate the expression of REDD1 (also referred to RTP801, a novel stress-induced gene linked to repression of mTOR signaling) in lymphoid, but not nonlymphoid, cells. In contrast to the findings of that study, here we demonstrate that REDD1, but not REDD2, mRNA expression is dramatically induced following acute dexamethasone treatment both in rat skeletal muscle in vivo and in L6 myoblasts in culture. In L6 myoblasts, the effect of the drug on mTOR signaling is efficiently blunted in the presence of REDD1 RNA interference oligonucleotides. Moreover, the dexamethasone-induced assembly of the mTOR regulatory complex Tuberin⅐Hamartin is disrupted in L6 myoblasts following small interfering RNA-mediated repression of REDD1 expression. Finally, overexpression of Rheb, a downstream target of Tuberin function and a positive upstream effector of mTOR, reverses the effect of dexamethasone on phosphorylation of mTOR substrates. Overall, the data support the conclusion that REDD1 functions upstream of Tuberin and Rheb to down-regulate mTOR signaling in response to dexamethasone.In contrast to the anabolic actions of growth-promoting hormones such as insulin and insulin-like growth factor 1, glucocorticoids act to repress protein synthesis in skeletal muscle of animals in vivo (1), in perfused hind limb preparations (2, 3), and in isolated muscle preparations (4, 5). In part, the repression of protein synthesis in response to glucocorticoids is a result of decreased signaling through a protein kinase referred to as the mammalian target of rapamycin (mTOR).2 mTOR phosphorylates at least two proteins involved in the regulation of mRNA translation, the eukaryotic initiation factor (eIF)-4E-binding protein 1 (4E-BP1) and the ribosomal protein S6 kinase 1 (S6K1) (6). 4E-BP1 acts to repress mRNA translation by sequestering the mRNA cap-binding protein eIF4E into an inactive complex (7,8). Phosphorylation of 4E-BP1 by mTOR initiates a series of phosphorylation events that ultimately result in the release of eIF4E from the inactive 4E-BP1⅐eIF4E complex, allowing it to bind to eIF4G to form the active eIF4F complex. Phosphorylation of S6K1 by mTOR generates a docking site for a second protein kinase, phosphoinositide-dependent protein kinase 1 (PDK1), allowing PDK1 to phosphorylate and activate S6K1 (9, 10). Subsequently, S6K1 phosphorylates ribosomal protein rpS6 (11), eIF4B (12, 13), and eukaryotic elongation factor (eEF2) kinase (14). Thus, repression of mTOR signaling results in a reduction in both the initiation and elongation phases of mRNA translati...

Cellular mechanisms regulating protein synthesis and skeletal muscle hypertrophy in animals

“…Cros et al (19) identified REDD2 (also named SMHS1) as a highly upregulated gene in soleus muscle undergoing atrophy following 14 days of hindlimb unloading (78). Biochemical studies of both REDD1 and REDD2 found that they inhibit mTOR kinase activity in non-muscle and muscle cells downstream of Akt/PKB but upstream of TSC2 (14,18,89,105). In skeletal muscle, REDD1 has been shown to inhibit mTOR signaling and protein synthesis in response to dexamethasone and alcohol intoxication (67,105).…”

“…Biochemical studies of both REDD1 and REDD2 found that they inhibit mTOR kinase activity in non-muscle and muscle cells downstream of Akt/PKB but upstream of TSC2 (14,18,89,105). In skeletal muscle, REDD1 has been shown to inhibit mTOR signaling and protein synthesis in response to dexamethasone and alcohol intoxication (67,105). In addition, Kimball et al (65) found that rapid degradation of REDD1 in muscle was associated with enhanced mTOR signaling.…”

Interleukin-6 influences stress-signalling by reducing the expression of the mTOR-inhibitor REDD1 in a STAT3-dependent manner

“…Several stressors such as energy depletion [6], ER stress [7,8], hypoxia [9,10], DNA damage by irradiation [11] and chemicals [12] induce the expression of REDD1. Additionally REDD1 expression is increased by pharmacological derivatives of cortisol such as dexamethasone [8,13] and by insulin [14].…”

Expression of growth-related genes in young and older human skeletal muscle following an acute stimulation of protein synthesis

“…On the other hand, REDD1 and REDD2 have been identified as negative regulators of mTOR (17). REDD1 is altered in response to various cellular stresses such as hypoxia, dexamethasone, starvation, and muscle contraction (11,17,24,48,55). REDD1 has been shown to interact with 14 -3-3 and modify the function of the tuberous sclerosis 1 (TSC1) and 2 (TSC2) complex (18) leading to inhibition of mTOR signaling.…”