Upstream eventDecreased, Mitochondrial Fatty Acid Beta Oxidation
Decreased, Ketogenesis (production of ketone bodies)
Key Event Relationship Overview
AOPs Referencing Relationship
Life Stage Applicability
Key Event Relationship Description
The KE, “mitochondrial fatty acid beta oxidation” catabolizes short, medium and long chain fatty acids (<C20) into acetyl-CoA and ATP. The production of acetyl-CoA monomers is important as they serve as fundamental units for metabolic energy production (ATP) via the citric acid cycle followed by electron-transport chain mediated oxidative phosphorylation (Nelson and Cox, 2000A). Acetyl-CoA is also a fundamental units of energy storage via gluconeogenesis (Nelson and Cox, 2000B) and lipogenesis (Nelson and Cox, 2000C). The liver plays a key role in processing the fundamental energy substrate, acetyl-CoA, into metabolic currencies that contribute to the systemic cellular energy needs of the whole organism. The liver represents a key organ involved in systemic energy distribution given its ability to synthesize glucose (an ability shared only with the kidney, Gerich et al 2001) as well as its exclusive role in the generation of ketone bodies (Cahill 2006, Sengupta et al 2010, Kersten 2014). This is especially important for the metabolic energy needs of the brain which can only use glucose and the ketone body, β-hydroxybutyrate for cellular energy production (Cahill 2006, Owen 2005, Kersten 2014). Therefore, the KE, “ketogenesis (production of ketone bodies)” is critical to supporting general systemic energy homeostasis in fasting events (Cahill 2006, Evans et al 2004, Sengupta et al 2010).
Evidence Supporting this KER
The KER scores for weight of evidence for the KE, “mitochondrial fatty acid beta oxidation” -> the KE, “ketogenesis (production of ketone bodies)” was considered “strong” given that the former serves as a primary source of substrate for the latter (Badman et al. 2007, Potthoff et al. 2009). Interference with ketogenesis, for example by PPARα inhibition, has been demonstrated to inhibit β-hydroxybutyrate production (measured in serum) during fasting events in mice (Badman et al 2007, Potthoff 2009, Sengupta et al 2010). The quantitative understanding score for this KER was considered “moderate” given that weight of evidence for the individual KEs was robust and the results in a study by Badman et al (2007) indicating that metabolism of fatty acid substrates (measured as liver triglycerides) that would otherwise contribute to β-hydroxybutyrate production was additionally inhibited under PPARα knockout.
Biological plausibility of this KER is strong given the supporting relationships cited in the literature described in the previous bullets above.
Include consideration of temporal concordance here
As described in the previous sections, there is a fundamental linkage between KEs given that the KE, “mitochondrial fatty acid beta oxidation” produces raw materials that are used in the KE, “ketogenesis (production of ketone bodies)”. It is less clear how essential the former is to a sustainable throughput of the latter especially given that the latter can utilize substrates that can be produced by various other cellular energy processing pathways in addition to mitochondrial fatty acid beta oxidation.
Uncertainties and Inconsistencies
Additional investigations tracing substrate processing, specifically from sources resulting from the KE, “mitochondrial fatty acid beta oxidation” under control as well as starvation conditions would supplement current understanding of the connections between the KE, “mitochondrial fatty acid beta oxidation” and the KE, “ketogenesis (production of ketone bodies)”.
Quantitative Understanding of the Linkage
Is it known how much change in the first event is needed to impact the second? Are there known modulators of the response-response relationships? Are there models or extrapolation approaches that help describe those relationships?
As discussed in the previous sections, the degree to which the, KE “mitochondrial fatty acid beta oxidation” affects the KE, “ketogenesis (production of ketone bodies)” is not well described, neither are modulators of the response-response relationships. Certainly, the pathways are interrelated and connected by PPARalpha as the master regulator of each process, so additional modulators related to resource availability and cellular signaling require exploration. We are not currently aware of any models available to extrapolate results among KEs.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
The relationships described herein have been primarily established in human and rodent models.
Badman MK, Pissios P, Kennedy AR, Koukos G, Flier JS, Maratos-Flier E: Hepatic fibroblast growth factor 21 is regulated by PPARalpha and is a key mediator of hepatic lipid metabolism in ketotic states. Cell metabolism 2007, 5(6):426-437.
Cahill GF, Jr. Fuel metabolism in starvation. Annu Rev Nutr 2006, 26:1-22.
Evans RM, Barish GD, Wang YX: PPARs and the complex journey to obesity. Nat Med 2004, 10(4):355-361.
Gerich JE, Meyer C, Woerle HJ, Stumvoll M: Renal gluconeogenesis: its importance in human glucose homeostasis. Diabetes Care 2001, 24(2):382-391.
Kersten S. 2014. Integrated physiology and systems biology of PPARalpha. Molecular Metabolism 2014, 3(4):354-371.
Nelson DL, Cox MM 2000A. The Citric Acid Cycle. Lehninger Principles of Biochemistry. 3rd Edition. Worth Publishers. New York, NY. p567-592.
Nelson DL, Cox MM 2000B. Carbohydrate Biosynthesis. Lehninger Principles of Biochemistry. 3rd Edition. Worth Publishers. New York, NY. p722-764.
Nelson DL, Cox MM 2000C. Lipid Biosynthesis. Lehninger Principles of Biochemistry. 3rd Edition. Worth Publishers. New York, NY. p770-814.
Owen OE: Ketone bodies as a fuel for the brain during starvation. Biochem Mol Biol Educ 2005, 33(4):246-251.
Potthoff MJ, Inagaki T, Satapati S, Ding X, He T, Goetz R, Mohammadi M, Finck BN, Mangelsdorf DJ, Kliewer SA et al: FGF21 induces PGC-1α and regulates carbohydrate and fatty acid metabolism during the adaptive starvation response. Proceedings of the National Academy of Sciences 2009, 106(26):10853-10858.
Sengupta S, Peterson TR, Laplante M, Oh S, Sabatini DM: mTORC1 controls fasting-induced ketogenesis and its modulation by ageing. Nature 2010, 468(7327):1100-1104.