Mechanisms of Preconditioning Against Surgical Stress by Short-Term Dietary Protein Restriction
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Harputlugil, Eylul. 2016. Mechanisms of Preconditioning Against Surgical Stress by Short-Term Dietary Protein Restriction. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.Abstract
Dietary restriction (DR) or reduced food intake without malnutrition encompasses a variety of dietary interventions including reduction of calories or specific macronutrients such as amino acids or total protein. Even though DR is known to result in various beneficial health effects, including extended longevity and increased stress resistance, the underlying nutritional and genetic requirements remain incompletely understood. Previous studies in lower organisms, as well as in mammals, point to the importance of the restriction of dietary protein intake in DR benefits. Previously our lab has shown that short-term restriction of intake of individual essential amino acids protects against hepatic ischemia reperfusion injury (IRI) via activation of the amino acid deprivation sensing protein, general control non-derepressible 2 (GCN2). Interestingly, GCN2 was no longer required for the protective effects of total dietary protein restriction (PR) against hepatic IRI. We thus investigated the potential role of a distinct amino acid sensing pathway involving the mechanistic target of rapamycin complex 1 (mTORC1) kinase, which is normally repressed upon PR. To test the hypothesis that reduced mTORC1 signaling is required for benefits of PR against hepatic IRI, we used a mouse model with liver specific deletion of the mTORC1 repressor gene tuberous sclerosis complex 1 (TSC1), leading to constitutive hepatic mTORC1 activation (LTsc1KO). Although one week of PR was able to reduce circulating growth factors and amino acid levels and activate hepatic GCN2 signaling in both LTsc1KO and WT mice, LTsc1KO mice failed to gain the preconditioning benefits of PR against hepatic IRI. To understand the molecular mechanism underlying the genetic requirement for the TSC complex in PR benefits, we focused on the observation that PR improved hepatic insulin sensitivity in WT but not LTsc1KO mice. Additional data from liver specific insulin receptor knockout (LIrKO) mice and in WT mice using pharmacological PI3K inhibition by wortmannin indicated a partial requirement for post-reperfusion insulin/Akt signaling in PR-mediated protection from hepatic IRI. In addition to defects in insulin signaling important for PR action, LTsc1KOs also failed to upregulate hepatic production of another potent protective molecule, hydrogen sulfide (H2S), which was increased in WT mice upon PR and required for PR-mediated protection from hepatic IRI. Finally, we investigated the mechanisms of regulation of hepatic H2S production. Using an in vitro model of increased H2S production upon serum deprivation, we identified growth hormone (GH) as a negative regulator of H2S production through JAK/STAT signaling, and increased autophagy as the likely source of free cysteine, the substrate for cystathionine gamma lyase (CGL)-mediated H2S production. The mechanistic details of how in vivo PR translates into increased H2S production, and the potential role of mTORC1, GH signaling and autophagy in this process remain to be fully elucidated.Terms of Use
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http://nrs.harvard.edu/urn-3:HUL.InstRepos:26718744
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