Post-translational Modifications: The Signals at the Intersection of Exercise, Glucose Uptake, and Insulin Sensitivity

Abstract

Diabetes is a global epidemic, of which type 2 diabetes makes up the majority of cases. Nonetheless, for some individuals, type 2 diabetes is eminently preventable and treatable via lifestyle interventions. Glucose uptake into skeletal muscle increases during and in recovery from exercise, with exercise effective at controlling glucose homeostasis in individuals with type 2 diabetes. Furthermore, acute and chronic exercise sensitizes skeletal muscle to insulin. A complex network of signals converge and interact to regulate glucose metabolism and insulin sensitivity in response to exercise. Numerous forms of post-translational modifications (eg, phosphorylation, ubiquitination, acetylation, ribosylation, and more) are regulated by exercise. Here we review the current state of the art of the role of post-translational modifications in transducing exercise-induced signals to modulate glucose uptake and insulin sensitivity within skeletal muscle. Furthermore, we consider emerging evidence for noncanonical signaling in the control of glucose homeostasis and the potential for regulation by exercise. While exercise is clearly an effective intervention to reduce glycemia and improve insulin sensitivity, the insulin- and exercise-sensitive signaling networks orchestrating this biology are not fully clarified. Elucidation of the complex proteome-wide interactions between post-translational modifications and the associated functional implications will identify mechanisms by which exercise regulates glucose homeostasis and insulin sensitivity. In doing so, this knowledge should illuminate novel therapeutic targets to enhance insulin sensitivity for the clinical management of type 2 diabetes.

Keywords: acetylation; exercise; glucose; insulin; phosphorylation; ubiquitination.

Graphical Abstract

Figure 1.

Exercise training, insulin sensitivity, and glycemic control in type 2 diabetes. (A) Repeated endurance exercise training improves insulin sensitivity and glycemic control, often leading to type 2 diabetes remission. (B) Endurance exercise training is an umbrella term for various forms of exercise that lead to improvements in aerobic capacity. For example, endurance exercise encompasses moderate-intensity continuous exercise, in which a constant load is maintained for an extended period of time (ie, 50-70% Wmax for >30 minutes), high-intensity interval training, in which periods of high (ie, >90% Wmax), and low (ie, <50% Wmax) intensities are alternated for numerous intervals, and high-intensity exhaustive exercise, in which high-intensity exercise (ie, >80% Wmax) is performed to exhaustion.

Figure 2.

Insulin- and exercise-sensitive phospho-signaling in skeletal muscle. (A) Insulin stimulation induces autophosphorylation on the insulin receptor, which results in the recruitment and phosphorylation of IRS1, leading to PI3K activation (92). PI3K indirectly activates AKT via mTORC2 and PDK1 (92-94). AKT phosphorylates TBC1D1 and TBC1D4, which prevents their inhibition of the GLUT4 translocating Rab GTPases (97-101). Furthermore, AKT phosphorylates GSK3 leading to the activation of glycogen synthase (GS) and synthesis of glycogen. PI3K also activates RAC1, which promotes GLUT4 translocation via ARP2/3- and cofilin-mediated actin remodeling (95, 96). (B) Exercise regulates a range of phospho-signaling pathways. AMPK is phosphorylated and activated by CAMKK and liver kinase B1 (LKB1) (170, 255-259). AMPK orchestrates a shift towards catabolic processes: GLUT4 translocation is promoted via phosphorylation of TBC1D1 and TBC1D4 (97-99, 119, 123, 129), net glycogen breakdown via inhibition of GS (260), fatty acid oxidation via inhibition of acetyl-CoA carboxylase (ACC) (261), and inhibition of protein synthesis through negative regulation of mTORC1 activity (262). AMPK also promotes the expression of metabolic genes, including GLUT4, through myocyte enhancer factor 2 (MEF2) and peroxisome proliferator activated receptor gamma coactivator 1 alpha (PGC1α) activation (202, 263, 264), which can also be activated via CAMKII and p38MAPK (265-267). Accumulation of cyclic AMP (cAMP) activates PKA, which phosphorylates cyclic AMP-responsive element binding protein (CREB) and promotes transcription of metabolic genes (268, 269).

Figure 3.

Exercise- and insulin-sensitive signals converge on TBC1D1 and TBC1D4 to facilitate GLUT4 translocation. Exercise- and insulin-signaling cascades converge on TBC1D1 and TBC1D4. Insulin promotes AKT-mediated phosphorylation of TBC1D1 and TBC1D4, while AMPK and CAMKII mediate the exercise-induced phosphorylation of TBC1D1 and TBC1D4 on independent and overlapping sites to those targeted by AKT. Yellow phospho-sites represent insulin-induced phosphorylation. Red phospho-sites represent exercise-induced phosphorylation.

Figure 4.

Exercise regulates the ubiquitination or ubiquitin-like modification of glycolytic enzymes in skeletal muscle. Glycolytic enzymes are enriched within proteins with K-GG remnants (ubiquitin and ubiquitin-like modifications) regulated by exercise (184). K-GG remnants on ALDOA, GAPDH, PGAM1, PGAM2, and ENO3 are regulated by exercise. Blue ubiquitin sites represent K-GG remnants downregulated by exercise. Red ubiquitin sites represent K-GG remnants upregulated by exercise.

Figure 5.

Acetylation and palmitoylation in insulin signaling, GLUT4 translocation, and mitochondria. Acetylation and palmitoylation are apparent on numerous proteins in the insulin signaling cascade. Palmitoylation of GLUT4 by the palmitoyltransferase DHHC7 is critical for insulin-stimulated membrane translocation in adipocytes. Mitochondrial proteins are highly acetylated in skeletal muscle. Endurance exercise increases mitochondrial protein acetylation, including PDH, which the mitochondrial deacetylase SIRT3 opposes. Acetylation of PDH may regulate the flexibility between glucose and fatty acid oxidation. Exercise also increases histone acetylation, concomitant with the nuclear export of HDAC5, which may play a role in MEF2 activation and GLUT4 transcription.

Figure 6.

ADP ribosylation via the TNKS family of PARPs may regulate metabolism via GLUT4 translocation and inhibition PGC1α. TNKS ADP-ribosylates the GLUT4 storage vehicle-related protein IRAP and inhibition of TNKS PARP activity impairs GLUT4 translocation in adipocytes. Conversely, TNKS impairs skeletal muscle oxidative metabolism via PGC1α PARylation and degradation.

Figure 7.

Complex signals interact within exercising skeletal muscle to regulate insulin sensitivity, glucose uptake, and metabolism. Numerous post-translational modifications interact to control glucose homeostasis. Phospho-signaling emanating from CAMKK, AMPK, and CAMKII regulate GLUT4 translocation via TBC1D1/TBC1D4 and PIKFYVE. Palmitoylation and ADP-ribosylation of GLUT4 and IRAP, respectively, may also contribute to GLUT4 translocation. Ubiquitination and/or ubiquitin-like modification of enzymes within the glycolytic pathway are decreased during exercise, while the 26S proteasome is activated via phosphorylation of proteasomal subunits by PKA. Endurance exercise training induces mitochondrial hyperacetylation, including acetylation of PDH E1A, which may facilitate elevated fatty acid oxidation. (Ac), acetylation; (Ad), ADP ribosylation; (P), phosphorylation; (Pa), palmitoylation; (Ub), ubiquitination and ubiquitin-like modifications; (GTP), GTP-bound protein.?; –, unconfirmed in exercising/contracting skeletal muscle.