Insulin induces a shift in lipid and primary carbon metabolites in a model of fasting-induced insulin resistance

Abstract

Introduction Prolonged fasting in northern elephant seals (NES) is characterized by a reliance on lipid metabolism, conservation of protein, and reduced plasma insulin. During early fasting, glucose infusion previously reduced plasma free fatty acids (FFA); however, during late-fasting, it induced an atypical elevation in FFA despite comparable increases in insulin during both periods suggestive of a dynamic shift in tissue responsiveness to glucose-stimulated insulin secretion. Objective To better assess the contribution of insulin to this fasting-associated shift in substrate metabolism. Methods We compared the responses of plasma metabolites (amino acids (AA), FFA, endocannabinoids (EC), and primary carbon metabolites (PCM)) to an insulin infusion (65 mU/kg) in early- and late-fasted NES pups (n = 5/group). Plasma samples were collected prior to infusion (T0) and at 10, 30, 60, and 120 min post-infusion, and underwent untargeted and targeted metabolomics analyses utilizing a variety of GC-MS and LC-MS technologies. Results In early fasting, the majority (72%) of metabolite trajectories return to baseline levels within 2 h, but not in late fasting indicative of an increase in tissue sensitivity to insulin. In late-fasting, increases in FFA and ketone pools, coupled with decreases in AA and PCM, indicate a shift toward lipolysis, beta-oxidation, ketone metabolism, and decreased protein catabolism. Conversely, insulin increased PCM AUC in late fasting suggesting that gluconeogenic pathways are activated. Insulin also decreased FFA AUC between early and late fasting suggesting that insulin suppresses triglyceride hydrolysis. Conclusion Naturally adapted tolerance to prolonged fasting in these mammals is likely accomplished by suppressing insulin levels and activity, providing novel insight on the evolution of insulin during a condition of temporary, reversible insulin resistance.

Can offsetting the energetic cost of hibernation restore an active season phenotype in grizzly bears (Ursus arctos horribilis)?

“…Our results therefore reveal a previously unknown feature of the insulin resistance in hibernating bears, namely, that it progressively increases throughout hibernation. A similar progressive change has been observed in elephant seal pups over several months of fasting (Olmstead et al, 2017).…”

Exogenous GLP-1 stimulates TCA cycle and suppresses gluconeogenesis and ketogenesis in late-fasted northern elephant seals pups

“…Despite this contradictory evidence, the metabolomics data suggests that ketogenesis is suppressed suggesting that in fasting seal pups, insulin out-competes glucagon for the regulation of ketogenesis. This is further substantiated by previous studies that demonstrate that: 1) insulin infusion in fasting seal pups suppresses b-hydroxybutyric acid (17) and 2) the natural increase in ketones ( 8) is associated with reduced insulin (31,33).…”

“…However, reductions in plasma levels do not necessarily imply a state of nonfunctionality. For example, despite the fasting-induced reductions in insulin, we have demonstrated that exogenous insulin infusion in late-fasted seal pups (when levels are the lowest) induced time-dependent changes in the phosphorylation of insulin receptor and downstream signaling proteins in adipose and muscle (although the cellular signaling events were suppressed compared with early-fasted pups in the same study) (7) and in the metabolome (17) suggesting that the tissues remain insulin sensitive. Given that GLP-1 facilitates GSIS and induces GLP-1 receptor-mediated cellular effects independent of insulin, we took a similar approach as with insulin and have shown that exogenous GLP-1 in late-fasted seal pups induced profound changes in their hormone profile and time-dependent changes in insulin receptor phosphorylation, again suggesting that tissues remain responsive to GLP-1 (7).…”

“…All entries in Bin-Base were matched against the Fiehn mass spectral library using retention index and mass spectrum information or the National Institute of Standards and Technology (NIST) 11 commercial library. Quality control procedures and data quality efforts have been described previously (17). Metabolites were reported if present in at least 50% of the samples.…”

Learning to starve: impacts of food limitation beyond the stress period

“…Of particular interest will be the utilization of combined next-generation approaches [e.g. transcriptomics (Khudyakov et al, 2015a,b) proteomics, phosphoproteomics and metabolomics (Olmstead et al, 2017); reviewed in Ng'oma et al, 2017] in future studies, as these will provide a clear picture of the molecular mechanisms underlying starvation responses. Combined next-generation studies have been used in many systems to more precisely establish critical biological responses through identification of enriched pathways overlapping between methods (e.g.…”