Modulation of autophagy signaling with resistance exercise and protein ingestion following short-term energy deficit

Abstract

Autophagy contributes to remodeling of skeletal muscle and is sensitive to contractile activity and prevailing energy availability. We investigated changes in targeted genes and proteins with roles in autophagy following 5 days of energy balance (EB), energy deficit (ED), and resistance exercise (REX) after ED. Muscle biopsies from 15 subjects (8 males, 7 females) were taken at rest following 5 days of EB [45 kcal·kg fat free mass (FFM) Ϫ1 ·day Ϫ1 ] and 5 days of ED (30 kcal·kg FFM Ϫ1 ·day Ϫ1 ). After ED, subjects completed a bout of REX and consumed either placebo (PLA) or 30 g whey protein (PRO) immediately postexercise. Muscle biopsies were obtained at 1 and 4 h into recovery in each trial. Resting protein levels of autophagy-related gene protein 5 (Atg5) decreased after ED compared with EB (ϳ23%, P Ͻ 0.001) and remained below EB from 1 to 4 h postexercise in PLA (ϳ17%) and at 1 h in PRO (ϳ18%, P Ͻ 0.05). In addition, conjugated Atg5 (cAtg12) decreased below EB in PLA at 4 h (ϳ20, P Ͻ 0.05); however, its values were increased above this time point in PRO at 4 h alongside increases in FOXO1 above EB (ϳ22-26%, P Ͻ 0.05). Notably, these changes were subsequent to increases in unc-51-like kinase 1 Ser757 phosphorylation (ϳ60%) 1 h postexercise in PRO. No significant changes in gene expression of selected autophagy markers were found, but EGR-1 increased above ED and EB in PLA (ϳ417-864%) and PRO (ϳ1,417-2,731%) trials 1 h postexercise (P Ͻ 0.001). Postexercise protein availability, compared with placebo, can selectively promote autophagic responses to REX in ED. autophagy; skeletal muscle; resistance exercise; muscle protein breakdown MAINTENANCE AND REMODELING of skeletal muscle mass depends on the net balance between simultaneous processes of muscle protein synthesis (MPS) and muscle protein breakdown (MPB). Periods of energy deficit (ED) are associated with losses of lean body mass (35), and we (2) and others (44) have demonstrated that short-term ED through dietary restriction reduces postabsorptive rates of MPS compared with energy balance (EB). The protein degradation signaling pathways underlying this increased MPB response are not well characterized, although increased mRNA expression of ubiquitin proteasomal system (UPS) markers have been reported following short-term ED (9, 10). However, this UPS response is repressed following protein consumption (9), suggesting a possible muscle protein "sparing" effect of protein ingestion when in ED.The loss of muscle proteins resulting from ED (35) may, in part, be attributed to enhanced activity of the autophagylysosomal pathway (henceforth, termed autophagy). For example, chronic (6 mo) ED increases mRNA and protein markers of autophagy (53). Autophagy degrades, via lysosomal proteolysis, protein aggregates and damaged organelles that are then "recycled" to yield energy to maintain cellular metabolism (11). This process commences with the sequestration of a portion of cytoplasm by a double-membrane vacuole called the autophagosome, which subsequently releases its...

Pronounced energy restriction with elevated protein intake results in no change in proteolysis and reductions in skeletal muscle protein synthesis that are mitigated by resistance exercise

“…In agreement with the acute rates of MPB in the current study, we did not observe any changes in gene or protein expression of targets in the ubiquitin-proteasome and autophagic-lysosomal pathways, which is consistent with some short-term studies in humans (6,12,13). Other ER studies have observed only small increases in these proteolytic genes and proteins.…”

“…Whereas a significant amount of research exists that has characterized changes in the rates of MPS during ER, relatively little is known about the effect of ER on the rates of MPB. Studies of molecular markers of MPB, such as changes in the gene and protein expression of targets that are involved in the ubiquitin-proteasome and autophagic-lysosomal pathways, have yielded inconsistent results (11)(12)(13). Few direct measurements of the rates of MPB during ER have been made; however, a single report has shown that after a 10-d 20% energy deficit in healthy adults (consuming 1.5 g/kg/d of protein), there was a 60% increase in the rates of postabsorptive MPB (12).…”

Severe energy deficit at high altitude inhibits skeletal muscle mTORC1‐mediated anabolic signaling without increased ubiquitin proteasome activity

“…In vitro (19) and rat studies (20,21) additionally demonstrate that reductions in translation and phosphorylation of mTORC1 signaling proteins occur after underfeeding. Anabolic resistance that results from negative energy balance at SL does so without consistent and appreciable changes in muscle proteolysis (15,(22)(23)(24). These findings add to the theory that the modulation of protein synthesis exerts a much more pronounced regulatory effect on muscle mass in healthy muscle than alterations in proteolysis (23,25); however, the extent to which muscle anabolism and proteolysis are modulated in healthy individuals who are exposed to the combined environmental and physiologic stresses of hypoxia and negative energy balance has not been documented.…”

“…Likewise, several recent studies have shown little to no alteration in muscle proteolysis after short-term energy deficit (15,23,24). Our group previously reported that 21 d of consuming a 40% energy deficient diet at SL did not alter basal or postprandial ubiquitin proteasome subunit activities relative to weight maintenance values (24), and Hector et al (15) reported no changes in muscle protein breakdown in response to consuming a 40% energy deficient diet for 10 d. Furthermore, Smiles et al (23) reported no difference in the intramuscular markers of proteolysis after 5 d of ;30% energy deficit compared with weight maintenance. The lack of increased muscle proteolysis with acute HA exposure or after chronic HA exposure, in combination with severe energy deficit, suggests that muscle protein breakdown is not a major contributor to muscle loss during HA sojourns.…”

“…Moreover, during energy deficit at SL, high-quality protein intake alone or in recovery from exercise seems to suppress or maintain proteolytic activity compared with rates observed when in energy balance. Stimulating anabolism while suppressing or maintaining proteolysis provides the underlying conditions for a net improvement in protein balance (15,23,24) and the attenuation of muscle loss (5,26).…”