Insulin Regulation of Proteostasis and Clinical Implications Haleigh A. James, Brian T. O'Neill, K. Sreekumaran Nair  Cell Metabolism  Volume 26, Issue 2, Pages 310-323 (August 2017) DOI: 10.1016/j.cmet.2017.06.010 Copyright © 2017 Terms and Conditions

Figure 1 Simplified Whole-Body Model of Proteostasis Amino acids, derived from diet, degradation of endogenous proteins, and in vivo synthesis (nonessential amino acids only), are distributed through cellular compartments and the bloodstream. They travel via blood vessels to different tissues, where they may be incorporated into proteins via protein synthesis (PS) and become part of the whole-body proteome. Within the tissues, protein degradation (PD) releases amino acids back into the amino acid pool, where they may be recycled via acylation to tRNA for further PS. The amino acids that are not directed for PS are oxidized, releasing carbon dioxide (CO2) and nitrogen in the process. The rate of protein turnover, which includes the processes of PS and PD, is tissue-dependent and variably influenced by insulin. Cell Metabolism 2017 26, 310-323DOI: (10.1016/j.cmet.2017.06.010) Copyright © 2017 Terms and Conditions

Figure 2 Protein Flux between Gut, Splanchnic Tissue, and Skeletal Muscle during Postabsorptive, Postprandial, and Insulin-Deficient States (A) In the nondiabetic postabsorptive (fasting) state, protein degradation (PD) exceeds protein synthesis (PS) in the muscle, leading to efflux of amino acids into the systemic pool and uptake by the splanchnic tissue where PS exceeds PD. This allows continued synthesis of necessary proteins, such as clotting factors, in the liver even when amino acids are not actively being ingested. (B) In the nondiabetic postprandial (mixed-meal fed) state, a high-dose amino acid load from the gut triggers increased PS and decreased PD in the splanchnic bed. Insulin is also secreted and inhibits PD in the muscle. PS increases in muscle owing to the additive effects of insulin and amino acids. (C) In the insulin-deficient state, muscle PD is greatly increased and PS is not affected. This leads to transfer of a large amount of amino acids from muscle to the splanchnic bed, where both PS and PD are increased, but PS exceeds PD, resulting in net positive protein balance, presumably as a mechanism to allow continued synthesis of necessary proteins and perhaps to deal with the stress incurred by the absence of insulin. Cell Metabolism 2017 26, 310-323DOI: (10.1016/j.cmet.2017.06.010) Copyright © 2017 Terms and Conditions

Figure 3 The Effects of Insulin and Amino Acids on Phenylalanine Balance and Protein Dynamics in Skeletal Muscle and Splanchnic Bed The top panels show net phenylalanine balance, and the bottom panels show changes in protein synthesis (PS) and protein degradation (PD), with use of labeled phenylalanine and tyrosine as tracers across the leg and splanchnic beds in fasting healthy individuals during infusion of normal saline (NS), insulin alone (Ins), insulin + baseline replacement of amino acids (LoAA/Ins), insulin + high-dose physiologic amino acids (HiAA/Ins), and somatostatin + baseline replacement of insulin, glucagon, and growth hormone + high-dose physiologic amino acids (SRIH/AA) or saline (SRIH/NS). The SRIH/AA treatment shows the effects of high-dose amino acids alone, since insulin, glucagon, and growth hormone are maintained at baseline levels. Phenylalanine is an essential amino acid that cannot be synthesized by humans, and it is disposed of in muscle exclusively by incorporation into proteins (protein synthesis). Therefore, phenylalanine balance represents the difference between PS and PD in muscle. In liver, it is converted to tyrosine, so an independent tyrosine tracer was used along with the phenylalanine tracer to measure splanchnic PS and PD. (A and B) NS infusion led to a statistically significant negative phenylalanine balance in the leg, while insulin infusion balanced phenylalanine in the leg. Adding amino acids to insulin increased phenylalanine balance in a dose-dependent manner in both the muscle and splanchnic bed. (C) PD exceeds PS in the leg during NS infusion, but PS increases and exceeds PD when both insulin and high-dose physiologic amino acids (HiAA/Ins) and when high-dose amino acids alone (SRIH/AA) are infused. (D) In the splanchnic bed, PD and PS are nearly equal during NS and insulin infusion, but PS increases in a dose-dependent manner and exceeds PD when amino acids are added to insulin or when high-dose amino acids alone are infused. Data are represented as mean ± SEM. ∗Indicates that phenylalanine balance is different between baseline and intervention (p < 0.05). #Indicates that the rate of PD is different than the rate of PS (p < 0.05). Adapted from Nygren and Nair (2003). Cell Metabolism 2017 26, 310-323DOI: (10.1016/j.cmet.2017.06.010) Copyright © 2017 Terms and Conditions

Figure 4 Mechanisms for the Regulation of Protein Turnover in Muscle by Insulin and Insulin-like Growth Factor 1 (A) In the fed state, muscle growth and maintenance of muscle mass are stimulated by insulin, amino acids, and, to a lesser extent, insulin-like growth factor 1 (IGF-1). Insulin engages the insulin receptor and signals via the IRS-PI3K-Akt pathway to suppress translocation of forkhead box O (FoxO) isoforms 1, 3, and 4, and inhibit their transcriptional activity. Inhibition of FoxOs suppresses proteasomal and autophagy-lysosomal protein degradation. In addition, mammalian target of rapamycin complex 1 (mTORC1) is activated by amino acids and Akt to enhance protein synthesis, which ultimately leads to net protein gain and muscle growth. (B) Decreased insulin signaling, as occurs with fasting or in diabetes, increases FoxO isoform translocation and transcription of critical mediators of ubiquitin-proteasome and autophagy-lysosome systems, leading to a marked increase in protein degradation that outweighs protein synthesis, leading to muscle atrophy and a high-protein-turnover state. IGF-1R, insulin-like growth factor-1 receptor; IRS, insulin receptor substrates; PI3K, phosphoinositidase-3 kinase. Cell Metabolism 2017 26, 310-323DOI: (10.1016/j.cmet.2017.06.010) Copyright © 2017 Terms and Conditions

Figure 5 Protein Biogenesis and Homeostasis Protein synthesis begins with transcription of genes into mRNA, which is followed by translation into polypeptides. Insulin facilitates both of these processes, and amino acids are necessary for translation. Polypeptides are folded into specific configurations in order to become functional proteins, but they may become dysfunctional if misfolded. Properly folded proteins may undergo reversible posttranslational modifications (PTMs), which aid in various protein functions, or they may undergo irreversible PTMs (oxidation, glycation, etc.) from oxidative or other environmental stress. The proteins damaged by irreversible PTMs, as well as misfolded ones, are usually targeted for proteasome-mediated degradation or autophagy, and amino acids may be recycled. If excess misfolding occurs or environmental stress overwhelms the system, damaged proteins may accumulate and/or aggregate, leading to disease. LMW, low molecular weight; ROS, reactive oxygen species. Cell Metabolism 2017 26, 310-323DOI: (10.1016/j.cmet.2017.06.010) Copyright © 2017 Terms and Conditions