Graduate Thesis Or Dissertation
 

Regulation of Skeletal Muscle Insulin Action and Fatty Acid Trafficking with Obesity and Exercise

Public Deposited

Downloadable Content

Download PDF
https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/37720k49j

Descriptions

Attribute NameValues
Creator
Abstract
  • Altered skeletal muscle fat metabolism is linked to changes in skeletal muscle insulin sensitivity. Whereby the accumulation of bioactive signaling lipids (i.e., diacylglycerols and ceramides) within skeletal muscle are negatively associated with insulin sensitivity and the exercise-induced changes to fatty acid trafficking are related to improved skeletal muscle insulin action. However, the mechanisms that contribute to these observations remain to be completely elucidated. The overall aim of this dissertation was to investigate potential underlying mechanisms that contribute to altered skeletal muscle fat metabolism and/or insulin sensitivity in response to lipid/obesity-induced insulin resistance, exercise training, and a single session of exercise. The primary mechanisms of focus were the role of skeletal muscle Ras-related C3 botulinum toxin substrate 1 (Rac1) in insulin- and exercise-stimulated glucose regulation and the contribution of long-chain acyl-coenzyme a synthetases (ACSL) to skeletal muscle fat metabolism. Rac1 is required for normal insulin-stimulated glucose transporter 4 (GLUT4) translocation and evidence suggest it may be negatively regulated by lipids. Therefore, the primary aim of my second chapter was to investigate the potential role for diacylglycerols and ceramides as negative regulators on insulin-stimulated Rac1 activation. Cultured muscle cells were treated overnight with or without fatty acids, one of which is known to induced insulin resistance (i.e., palmitate). Overnight fatty acid treatments were followed by a time course approach to measure insulin-stimulated Rac1 activation (GTP-binding), other components of insulin signaling, and functional output measure of insulin action, GLUT4 translocation. Overnight palmitate treatment resulted accumulation of diacylglycerols and ceramides and almost complete ablation of insulin-stimulated GLUT4 translocation. However, impaired GLUT4 translocation occurred independent of changes to insulin-stimulated Rac1-GTP binding. Phosphorylation of a Rac1 downstream effector protein p21-activated kinase (PAK) was blunted by palmitate treatment as was Akt phosphorylation, which stimulates GLUT4 translocation independent of Rac1. Collectively, we interpret our findings to indicate that palmitate-induced down regulation of PAK1 activation and GLUT4 translocation occur independent to insulin-stimulated Rac1-GTP binding. Evidence suggest ACSLs may regulate fat oxidation and fat storage within skeletal muscle. Whereby specific skeletal muscle isoform ACSL1 is suggested to contribute to fat oxidation and ACSL6 may play a role in fat storage. However, it is unknown if ACSLs are regulated by diet-induced obesity and/or aerobic exercise training. The primary aims of my third chapter were to investigate skeletal muscle ACSL isoform protein expression following high fat diet (HFD)-induced obesity and aerobic exercise training, and determine potential roles for ACSL1 and 6 with measures of fat metabolism in mice. In mouse gastrocnemius muscle, protein abundance for 4 of the 5 known ACSL isoforms was detected. HFD-induced obesity resulted in a non-significant increase in ACSL1, significantly greater ASCL6, and no change in ACSL4 or ACSL5. Aerobic exercise training resulted in decreased ACSL4 protein abundance, greater ACSL6, and no changes in ACSL1 or ACSL5. Skeletal muscle ACSL1 protein abundance was not related to measures of whole-body fat oxidation at rest, whereas ACSL6 was positively associated with intramyocellular lipid content. Taken together, we interpret our findings to demonstrate ACSLs undergo isoform specific regulation by diet and exercise and ACSL6 may be a regulator of skeletal muscle fat storage. Model systems implicate ACSLs as key regulators of skeletal muscle fat oxidation and fat storage; however, such roles remain underexplored in humans. The primary aims of my fourth chapter were to determine the protein expression of ACSL isoforms in skeletal muscle at rest and in response to acute exercise, and identify relationships between skeletal muscle ACSLs and measures of fat metabolism. In vastus lateralis biopsy samples collected from relatively lean sedentary adults, protein abundance for 4 of 5 known ACSL isoforms was detected. ACSL isoforms were largely unaltered by acute exercise aside from a transient increase in ACSL5 15 minutes post-exercise which returned to resting levels by 120 minutes. Skeletal muscle ACSL1 was not related to measures of resting whole-body fat oxidation. ACSL1 did tend to be positively related to whole-body fat oxidation during exercise, when skeletal muscle a major determinant of whole-body substrate oxidation. Skeletal muscle ACSL6 was positively related to skeletal muscle triacylglycerol concentration suggesting a role in the regulation of fat storage. We interpret our findings to indicate the protein abundance for ACSLs undergo isoform specific regulation by acute exercise and provide further evidence for their role in skeletal muscle fat metabolism in humans. A single session of exercise improves insulin sensitivity in most adults. Rac1 facilitates GLUT4 translocation and is activated by both insulin and exercise (i.e., mechanical stress). However, it is unknown whether insulin-stimulated Rac1 activation is further enhanced by prior exercise. The primary aims of my fifth chapter were to determine the effects of previous exercise on insulin sensitivity, Rac1 signaling, and other components of insulin signaling in the hours after exercise in relatively lean sedentary adults. A single session of moderate-intensity exercise improved measures of whole-body insulin sensitivity. Observed improvements in insulin sensitivity occurred independent of enhanced insulin-stimulated Rac1 activation, phosphorylation of its downstream effector protein PAK, or insulin-stimulated Akt activation post-exercise. Exercise induces activation of AMPK and its downstream effector protein TBC1D1 and contribute to glucose uptake in the hours after exercise. AMPK-specific activation of TBC1D1 was increased 15 minutes post-exercise and remained elevated at 180 minutes compared with basal measures and the rest trial, which closely coincided with when measures of insulin sensitivity during the steady state of the hyperinsulinemic-euglycemic clamp (i.e., 270-300 minutes post-exercise). Collectively, we interpret our results to suggest the mechanisms independent of insulin-stimulated Rac1 signaling such as TBC1D1 may contribute to the insulin sensitizing effects of exercise. This collection of studies indicate that lipid-induced insulin resistance occurs independent to impairments in insulin-stimulated Rac1 activation. Skeletal muscle ACSLs are related to measures of fat metabolism in mice and humans and demonstrate isoform specific regulation by diet-induced obesity and aerobic exercise training in mice, but are largely unaltered by acute exercise in humans. These isoform specific changes in ACSL protein abundance may contribute to the altered skeletal muscle fat metabolism with diet-induced obesity and/or aerobic exercise training. Lastly, the insulin sensitizing effects occur independent to enhanced insulin-stimulated activation of Rac1 post-exercise.
License
Resource Type
Date Issued
Degree Level
Degree Name
Degree Field
Degree Grantor
Commencement Year
Advisor
Committee Member
Academic Affiliation
Rights Statement
Publisher
Peer Reviewed
Language
Embargo reason
  • Pending Publication
Embargo date range
  • 2020-05-26 to 2022-06-27

Relationships

Parents:

This work has no parents.

In Collection:

Items

Thumbnail Title Date Uploaded Visibility Actions
file details StierwaltHarrisonD2020.pdf 2020-05-26 Public Download