Most of the interleukin 1 receptor antagonist, cathepsin S, macrophage migration inhibitory factor, nerve growth factor, and interleukin 18 release by explants of human adipose tissue is by the non–fat cells, not by the adipocytes

Abstract

The present studies were designed to compare the relative release of interleukin 1 receptor antagonist (IL-1Ra), cathepsin S, macrophage migration inhibitory factor (MIF), nerve growth factor (NGF), and interleukin 18 (IL-18) by adipocytes as compared with the non–fat cells present in subcutaneous and omental adipose tissue from morbidly obese gastric bypass patients as compared with obese abdominoplasty patients. The release of IL-1Ra, cathepsin S, and MIF by explants of human adipose tissue incubated for 48 hours averaged 6, 9, and 19 pmol/g, respectively, and was far greater than the release of NGF (0.05 pmol/g) or IL-18 (0.006 pmol/g). The release by human adipocytes of IL-1Ra, cathepsin S, and MIF was 0.13, 0.32, and 2.6 pmol/g, respectively, over 48 hours, whereas NGF release was 0.003 and IL-18 0.001 pmol/g. Only the total release of MIF by human adipose tissue explants was enhanced, whereas that of IL-18 was significantly reduced in explants from morbidly obese women. Most of (55%-73%) the release of IL-1Ra, cathepsin S, MIF, NGF, and IL-18 was by the adipose tissue matrix, whereas release by stromal-vascular (SV) cells was 3% to 28% of total release over 48 hours by the adipose tissue matrix, SV cells and adipocytes. The release of NGF by adipocytes was 42%, that of MIF was 27%, and for the other factors 15% or less of release over 48 hours by the adipose tissue matrix, SV cells, and adipocytes. Our results suggest that the non–fat cells in human adipose tissue contribute to most of the release of NGF, IL-18, IL-1Ra, cathepsin S, and MIF seen during primary culture of adipose tissue explants from obese women.

Introduction

Previously, we reported that resistin [1], monocyte chemoattractant protein 1 [2], transforming growth factor β1 [3], interleukin 8 (IL-8) [4], vascular endothelial growth factor [4], interleukin 6 (IL-6) [4], prostaglandin E₂ [4], tumor necrosis factor α [4], hepatocyte growth factor [4], interleukin 1β (IL-1β) [4], and interleukin 10 (IL-10) [4] were released by human adipocytes at levels of 11% or less of that by the adipocytes plus non–fat cells of human adipose tissue. The present studies were designed to extend the studies examining interleukin 1 receptor antagonist (IL-1Ra) [5], cathepsin S [6], macrophage migration inhibitory factor (MIF) [7], and interleukin 18 (IL-18) [8] release by human adipose tissue because of recent reports that they are inflammatory response proteins whose circulating levels are elevated in obesity. Nerve growth factor (NGF) was also examined because it is secreted by murine 3T3-L1 adipocytes [9]. The focus of these studies was the relative contribution of adipocytes as contrasted to the non–fat cells present in human adipose tissue to the in vitro release of IL-1Ra, cathepsin S, MIF, NGF, and IL-18.

Interleukin 1 receptor antagonist levels are elevated in human obesity [5], [10]. This protein is a physiologic antagonist of IL-1α and IL-1β because it competes with them for binding to their receptors. The IL-1Ra protein is induced by many of the same stimuli that enhance release of IL-1β in cells and by interferon β. The reason that IL-1Ra levels are elevated in obesity is still unclear, but could possibly involve feedback termination of the IL-1β–induced inflammatory response in adipose tissue.

Cathepsin S is an elastolytic cysteine protease secreted by mononuclear cells [11] that is active at neutral pH [12], [13]. Circulating levels of cathepsin S have a positive correlation with body mass index (BMI) [6], and the level of cathepsin messenger RNA (mRNA) expression is elevated in subcutaneous adipose tissue from obese subjects [14]. The elevated levels of cathepsin S in human obesity have recently been postulated to be a link between obesity and atherosclerosis [14]. The greater expression of cathepsin S message in the adipose tissue of obese individuals may reflect elevated levels of macrophages, mast cells, and other cells involved in the immune response [15], [16].

Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine whose serum concentration is elevated in obesity [17]. Uniquely, the circulating concentration of MIF is at least 1000-fold greater than that of IL-6, IL-8, IL-10, or IL-1β. Church et al [7] found that in obese individuals with elevated MIF values, participation in physical activity and a dietary-focused weight management resulted in reductions in both weight and MIF. The presence of MIF in the adipocytes of rodent adipose tissue was originally reported by Hirokawa et al [18]. More recently, Skurk et al [19] reported that both human preadipocytes and adipocytes release MIF.

Esposito et al [8] reported that the mean IL-18 serum value was 14 pmol/L in obese women (BMI of 34), which was higher than the value of 7 pmol/L seen in nonobese women (BMI of 24). The AtheroGene investigators suggested that serum IL-18 level is a predictor of death from cardiovascular causes in patients with coronary artery disease [20], [21]. These data suggest that IL-18 could be a link between obesity and increased risk of both diabetes and cardiovascular disease. Skurk et al [22] have even postulated that the increase in serum IL-18 seen in obese humans is due to elevated release by adipocytes.