Aberrant fatty acid metabolism in the skeletal muscle contributes to insulin resistance in zinc transporter 7 (Znt7) knockout mice

Authors: Huang, Liping; TEPAAMORNDECH, SURAPUN - National Center For Genetic Engineering And Biotechnology (BIOTEC); Kirschke, Catherine; Newman, John; Keyes, William; Pedersen, Theresa

Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2017
Publication Date: 5/19/2018
Citation: Huang, L., Tepaamorndech, S., Kirschke, C.P., Newman, J.W., Keyes, W.R., Pedersen, T.L. 2018. Aberrant fatty acid metabolism in the skeletal muscle contributes to insulin resistance in zinc transporter 7 (Znt7) knockout mice. Journal of Biological Chemistry. 293(20):7549-7563. doi:10.1074/jbc.M117.817692.
DOI: https://doi.org/10.1074/jbc.M117.817692

Interpretive Summary: ZnT7 (Slc30a7) is a zinc transporter which presents in many cell types in the body including mice and humans. It functions to transport zinc ions into the Golgi apparatus, a cellular component involved in sorting, secretion and intracellular transport of proteins via vesicles (vehicles). Znt7 knockout mice (KO, deleting the gene from the mouse genome) are mildly zinc deficient and lean. Despite the lean phenotype, adult male Znt7 KO mice are prone to insulin resistance. We hypothesized that fat partitioning from the fat tissue to the non-fat tissues was the cause for insulin resistance in Znt7 KO mice. We used biological and biochemical methods including fatty acid and oxylipin profiling, electron microscopy, immunohistochemistry, quantitative RT-PCR, and Western blot analysis to identify the underlying mechanism of insulin resistance in Znt7 KO mice. We found that insulin resistance in Znt7 KO mice was primarily associated with increased intracellular fatty acid levels in the skeletal muscle, which subsequently promoted intracellular lipid accumulation and bioactive lipid mediator productions. We demonstrated that level of Fabp3 protein, a vehicle for fatty acid transport inside a cell, was dramatically increased in Znt7 KO muscle cells accompanied with increased levels of Cd36 and Slc27a4, two fatty acid transporters in the skeletal muscle. In addition, we demonstrated that the fatty acid oxidative capacity of Znt7 KO muscle cells was increased evident by the enlarged mitochondria and increased mRNA expression of key enzymes involved in the fatty acid mitochondrial shuttle and ß-oxidation. We conclude that increased fatty acid uptake in the Znt7 KO skeletal muscle is the key factor that contributes to the excessive intracellular lipid deposit and elevated production of bioactive lipid mediators. These mediators may play key roles in oxidative stress and inflammation leading to insulin resistance.

Technical Abstract: ZnT7 (Slc30a7) is a widely expressed zinc transporter involved in sequestration of zinc into the Golgi apparatus and vesicular compartments. Znt7 knockout (KO) mice are mildly zinc deficient and lean. Despite the lean phenotype, adult male Znt7 KO mice are prone to insulin resistance. We hypothesized that fat partitioning from the adipose tissue to the non-adipose tissues was the cause for insulin resistance in Znt7 KO mice. We used biological and biochemical methods including fatty acid and oxylipin profiling, electron microscopy, immunohistochemistry, quantitative RT-PCR, and Western blot analysis to identify the underlying mechanism of insulin resistance in Znt7 KO mice. We found that insulin resistance in Znt7 KO mice was primarily associated with increased intracellular fatty acid levels in the skeletal muscle, which subsequently promoted intracellular lipid accumulation and bioactive lipid mediator productions. We demonstrated that the expression of Fabp3 was dramatically up-regulated in Znt7 KO muscle cells accompanied with increased expression of Cd36 and Slc27a4, two fatty acid transporters in the skeletal muscle. In addition, we demonstrated that the fatty acid oxidative capacity of Znt7 KO muscle cells was increased evident by the enlarged mitochondria and increased mRNA expression of key enzymes involved in the fatty acid mitochondrial shuttle and ß-oxidation. We conclude that increased fatty acid uptake in the Znt7 KO skeletal muscle is the key factor that contributes to the excessive intracellular lipid deposit and elevated production of bioactive lipid mediators. These mediators may play key roles in oxidative stress and inflammation leading to insulin resistance.