Amino acid availability regulates the effect of hyperinsulinemia on skin protein metabolism in pigs

Authors: TUVDENDORJ, DEMIDMAA - University Of Texas Medical Branch; BORSHEIM, ELISABET - Arkansas Children'S Nutrition Research Center (ACNC); SHARP, CARWYN - University Of Texas Medical Branch; ZHANG, XIAOJUN - University Of Texas Medical Branch; BARONE, CARRIE - University Of Texas Medical Branch; CHINKES, DAVID - University Of Texas Medical Branch; WOLFE, ROBERT - University Of Texas Medical Branch

Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/29/2015
Publication Date: 7/17/2015
Citation: Tuvdendorj, D., Borsheim, E., Sharp, C.P., Zhang, X., Barone, C.M., Chinkes, D.L., Wolfe, R.R. 2015. Amino acid availability regulates the effect of hyperinsulinemia on skin protein metabolism in pigs. Journal of Biological Chemistry. 290(29):17776-17783.

Interpretive Summary: Skin has several functions, including protection of underlying organs. Skin protein metabolism is important for the maintenance of its function, and measurement of skin protein turnover may serve as a surrogate to understand of whole-body or lean-body protein metabolism. Hormones such as insulin (INS) are important regulators that promote net protein synthesis, especially in conjunction with the presence of amino acids (AAs; building blocks of protein). In this study, we evaluated the effects of INS and AAs, either alone or in combination, on skin protein metabolism in pigs by using stable isotope techniques to measure skin fractional synthesis. The fractional breakdown rate was also estimated using mathematical assumptions. Normal female Yorkshire swine were studied. There were four study groups as follows: Control (CNT), INS, AA, and INS + AA. Background blood samples were collected before a marker (stable isotope of phenylalanine) was continuously infused. Thereafter, the CNT group was continuously infused with 0.9% saline; INS group was infused with insulin and AA group infused with a mixture of amino acids (10% Travasol). The INS + AA group received both insulin and AA. Skin samples were collected before and after 3 hours of infusions. The results show the importance of measuring both synthesis and breakdown when evaluating protein metabolism: both synthesis and breakdown were elevated in the insulin group, but when looking at the net balance between synthesis and breakdown, there were no statistically significant differences between the groups. The data support our previous findings that net skin protein mass is maintained under different conditions that mimic post-meal conditions. These data also demonstrate that the substrate (e.g. AAs) fluxes in skin cells differ depending on the substrate availability, and that INS may can control the mechanism of protein synthesis but not the breakdownmodify the responses to AAs. In light of existing literature, the studies demonstrate that regulation of skin protein metabolism may differ from that in other metabolically-relevant tissues such as muscle, indicating that the use of readily-accessible skin tissue as a surrogate for assessment of whole-body protein turnover may not be appropriate.

Technical Abstract: The effects of amino acid supply and insulin infusion on skin protein kinetics (fractional synthesis rate (FSR), fractional breakdown rate (FBR), and net balance (NB)) in pigs were investigated. Four-month-old pigs were divided into four groups as follows: control, insulin (INS), amino acid (AA), and INS + AA groups based on the nutritional and hormonal conditions. l-[ring-(13)C6]Phenylalanine was infused. FBR was estimated from the enrichment ratio of arterial phenylalanine to intracellular free phenylalanine. Plasma INS was increased (p < 0.05) in the INS and INS + AA groups. Plasma glucose was maintained by infusion of glucose in the groups receiving INS. The interventions did not change the NB of skin protein. However, the interventions affected the FSR and FBR differently. An infusion of INS significantly increased both FSR and FBR, although AA infusion did not. When an AA infusion was added to the infusion of insulin (INS + AA group), FSR and FBR were both lower when compared with the INS group. Our data demonstrate that in anesthetized pigs INS infusion did not exert an anabolic effect, but rather it increased AA cycling into and out of skin protein. Because co-infusion of AAs with INS ameliorated this effect, it is likely that the increased AA cycling during INS infusion was related to AA supply. Although protein kinetics were affected by both INS and AAs, none of the interventions affected the skin protein deposition. Thus, skin protein content is closely regulated under normal circumstances and is not subject to transient changes in AAs or hormonal concentrations.