Crosstalk between adipocytes and immune cells in adipose tissue in an obese inflammatory state: Role of contact-mediated signaling

Obesity is defined as heightened fat accumulation leading to health impairments. It has been directly correlated to cardiovascular disease, type II diabetes mellitus, and cancer. Heightened cytokine levels are found in serum and adipose tissue of obese subjects, including TNFalpha (tumor necrosis factor alpha), IL-6 (interleukin-6), and MCP-1 (monocyte chemoattractant protein-1), being characterized as a chronic low-grade inflammatory disease. In this dissertation, I have generated a novel co-culture model between adipocytes and immune cells (derived from splenocytes) that mimics inflammation seen in obese adipose tissue. This co-culture model allows for distinct evaluation between secreted paracrine factors (indirect cultures) and these factors plus direct cell contact. Paracrine signaling from both cell types increased the release of IL-6 and MCP-1, with a concomitant decrease of TNFalpha, whereas direct physical contact exacerbated the effects. The anti-inflammatory cytokine IL-10 (interleukin-10) did not play a role in the decreased secretion of TNFalpha. A time course study showed that direct and indirect co-cultures exhibited differential secretion rates, demonstrating cytokine-specific regulatory mechanisms. To determine specific cellular cytokine contributions, directly cultured cells were separated and analyzed showing both adipocytes and immune cells contribute significantly to inflammation. Adipocytes express MCP-1 and IL-6, whereas immune cells, TNFalpha and IL-6. Additionally, TNFalpha is necessary for this augmentation of IL-6 and MCP-1 secretion in direct contact. By use of non-toxic levels of signaling pathway inhibitors, I verified that the changes in cytokine secretions are mediated by NF-kappaB (nuclear factor kappa B) and MAPKs (mitogen-activated protein kinases). Specifically, NF-kappaB is the major signaling cascade for TNFalpha production, IL-6 is regulated by NF-kappaB, JNK (c-Jun N-terminal kinase), and p38, whereas MCP-1 by NF-kappaB, JNK, and MEK1. Analysis of the cell adhesion molecules on adipocytes identified 42 molecules. Direct contact with immune cells produced an up-regulation of Cadherin-1, claudin 4, 8, and 11, and down-regulation of Integrin alpha 6 in adipocytes. In conclusion, these results illustrate that direct contact and "crosstalk" between adipocytes and immune cells is paramount for exacerbation of inflammation in obesity. These changes are mediated by specific signaling cascades and cell adhesion molecules, which are important novel targets for this disease.