A Path to Insulin Resistance in Diet-Induced Obesity
Increasing evidence indicates that inflammation in adipose tissue (AT) contributes to insulin resistance and other metabolic abnormalities frequently observed in obesity. We know that macrophages in AT play an important role in obesity-associated inflammation and that obese AT can activate proinflammatory CD8+cytotoxic T cells. Now, Vanderbilt Basic Sciences investigators Arion Kennedyand Alyssa Hasty and their colleagues report a novel mechanism by which CD8+T cell activation can occur in AT. They began their work by feeding mice either a high fat diet (HFD) or a low fat diet (LFD) until the HFD-fed mice demonstrated substantial weight gain and insulin resistance. AT from HFD-fed mice exhibited a 3-fold increase in the density of CD8+T cells when compared to AT from LFD-fed mice. The researchers isolated these T-cells and conducted deep sequencing of their genomic DNA in order to characterize the repertoire of T cell receptors (TCRs) expressed by the cells. The T cell uses its TCR to interact with antigen presented by macrophages or dendritic cells, leading to activation of the T cell and clonal expansion. Genomic rearrangements result in a huge repertoire of potential TCRs, only one of which is expressed by a given T cell. The deep sequencing results revealed higher TCR clonality in the T cell population from HFD-fed as compared LFD-fed mice, indicating that certain T-cell populations had undergone activation and clonal expansion as a result of the HFD. In addition, CD8+T cells from the AT of HFD-fed mice were more likely than those from LFD-fed mice to exhibit public clonotypes – that is TCR sequences shared by more than one mouse. Further analysis of the data indicated that sequences of the AT CD8+T cell TCR expected to interact directly with antigen were more likely to contain positively charged or nonpolar amino acids in HFD-fed than LFD-fed mice, suggesting that these TCRs would most likely interact with an antigen bearing nonpolar regions and/or a negative charge. This led the investigators to hypothesize that the antigen was isolevuglandins – a product of oxidation of polyunsaturated fatty acids within cell phospholipids. Isolevuglandins meet the structural criteria of both a negative charge and nonpolar regions. Their levels increase under conditions of inflammation and oxidative stress, and the investigators found that the HFD led to higher isolevuglandin content in the lipids of AT macrophages in their mice. Treatment of bone marrow-derived macrophages with t-butyl hydroperoxide to induce isolevuglandin formation generated antigen presenting cells that were capable of activating CD8+T cells in co-culture. The findings suggest that oxidative stress in obese adipose tissue leads to formation of isolevuglandins that, in turn, activate CD8+T cells, thereby exacerbating the inflammation and promoting insulin resistance.The work is published in the journal Diabetes [W. J. McDonnell, et al. (2018) Diabetes, published online September 4, DOI:10.2337/db180040].