Novel link between inflammation and impaired glucose transport during equine insulin resistance

Although insulin resistance (IR) has been increasingly recognized in horses, a clear understanding of its pathophysiology is lacking. The purpose of the present study was to determine the early pathologic changes in IR horses by characterizing alterations in proteins that play key roles in innate immunological responses and inflammatory pathways, and by identifying potential links with glucose transport and insulin signaling. Visceral (VIS) and subcutaneous (SC) adipose tissue and skeletal muscle (SM) biopsies were collected from horses, which were classified as insulin-sensitive (IS) or IR based on the results of an insulin-modified frequently sampled intravenous glucose tolerance test. Protein expression of Toll-like receptor 4 (TLR-4), suppressor of cytokine signaling 3 (SOCS-3) and tumor necrosis factor alpha (TNF-α) were quantified by Western blotting in VIS and SC adipose depots and SM, as well as insulin receptor substrate 1 (IRS-1). To better characterize the potential relationship between inflammation, IR and impaired glucose transport, we correlated active cell surface glucose transporter 4 (GLUT-4) content (measured by a cell surface biotinylated assay) with individual- and tissue-specific data related to inflammation. IR was associated with a significantly increased expression of TLR-4 and SOCS-3 in SM and VIS tissue, without a significant change in SC site. We also observed a significant increase in TNF-α in VIS, but not in SC, tissue of IR vs. IS horses. There was no difference in total content or serine phosphorylation of IRS-1 for any sampling site in IR compared to IS horses. We further observed a significant positive correlation between TLR-4 content and SOCS-3, as well as a significant negative correlation between SOCS-3 content and GLUT-4 trafficking. Taken together, the data suggested a pro-inflammatory state in SM and VIS, but not SC, adipose depot during compensated IR. In addition, SOCS-3 appears to be a novel link between inflammation and dysregulated glucose metabolism and insulin sensitivity during the early pathogenesis of insulin resistance.