Mast cells participate in chronic low-grade inflammation within adipose tissue

Increased mast cell abundance in adipose tissue of metabolic syndrome: relevance to the proinflammatory state and increased adipose tissue fibrosis

Metabolic Syndrome (MetS) affects 35% of American adults > 40 yr and portends an increased risk for both atherosclerotic cardiovascular disease (ASCVD) and diabetes. The role of mast cells in the proinflammatory state of MetS is not well elucidated. We propose that mast cells in subcutaneous adipose tissue (SAT) of MetS patients without diabetes or clinical ASCVD contribute to insulin resistance and inflammation. Matched controls ( n = 15) and MetS ( n = 19) subjects were recruited from Sacramento, CA, and selected based on Adult Treatment Panel III criteria. SAT biopsy was performed on all subjects and processed for immunohistochemistry. The SAT sections were stained using Astra Blue stain and tryptase stain for mast cells. Fasting blood was obtained for chemistries and biomarkers. Abundance of mast cells (Astra Blue stain) in SAT of MetS subjects compared with controls was increased 2.5-fold ( P < 0.0001). Mast cells correlated positively and significantly with waist circumference, glucose, triglycerides, homeostatic model of assessment-insulin resistance (HOMA-IR), AT insulin resistance, leptin, interleukin (IL)-1β, IL-6, chemerin, p38 MAPK activity, and nuclear factor κB activity in circulating monocytes. Mast cells also correlated significantly with markers of fibrosis and angiogenesis. Tryptase staining of mast cells in AT revealed a significant increase ( P = 0.008) with similar correlations. We make the novel observation that there are increased mast cells in SAT of MetS, and these mast cells correlate with insulin resistance (hepatic and adipose tissue), inflammation, and AT fibrosis. Hence, these immune cells appear to occupy a pivotal role in the pathogenesis of MetS.

Leptin stimulates tissue rat mast cell pro-inflammatory activity and migratory response

Objective

The aim of this study was to determine whether leptin, a member of the adipocytokines involved in immune and inflammatory response regulation, may influence some aspects of mast cell biology.

Materials and methods

Experiments were done in vitro on fully mature tissue rat mast cells isolated from the peritoneal cavity, and leptin was used at concentrations 0.001–100 ng/ml. The effect of leptin on mast cell degranulation (histamine release assay), intracellular Ca²⁺ level (fluorimetry), pro-inflammatory mediator release (ELISA technique), surface receptor expression (flow cytometry and confocal microscopy), and migration (Boyden microchamber assay) was estimated.

Results

Leptin was found to stimulate mast cells to degranulation and histamine release. It induced the intracellular Ca²⁺ increase, as well. In response to leptin stimulation, mast cells generated and released cysLTs and chemokine CCL3. Leptin-induced upregulation of CYSLTR1 and CYSLTR2 surface expression was observed. Moreover, this adipocytokine stimulated mast cells to migratory response, even in the absence of extracellular matrix (ECM) proteins.

Conclusions

Our observations clearly documented that leptin promotes the pro-inflammatory activity of mast cells, and it thereby engages these cells in the inflammatory processes.

An intracrine view of sex steroids, immunity, and metabolic regulation

Background

Over the past two decades, parallel recognition has grown of the importance of both sex steroids and immune activity in metabolic regulation. More recently, these discrete areas have been integrated in studies examining the metabolic effects of sex steroid immunomodulation. Implicit in these studies has been a traditional, endocrine model of sex steroid delivery from the gonads to target cells, including immune cells. Thus, research to date has focused on the metabolic effects of sex steroid receptor signaling in immune cells. This endocrine model, however, overlooks the extensive capacity of immune cells to generate and metabolize sex steroids, enabling the production of sex steroids for intracrine signaling – that is, sex steroid production for signaling within the cell of origin. Intracrine function allows highly cell-autonomous regulation of sex steroid exposure, and sex steroid secretion by immune cells could confer paracrine signaling effects in neighboring cells within metabolic tissues. In this review, immune cell intracrinology will denote sex steroid production within immune cells for either intracrine or paracrine signaling. This intracrine capacity of immune cells has been well established, and prior work has supported its importance in autoimmune disorders, trauma, and cancer. The potential relevance of immune cell intracrine function to the regulation of energy balance, body weight, body composition, and insulin sensitivity has yet to be explored.

Scope of review

The following review will detail findings to date regarding the steroidogenic and steroid metabolizing capacity of immune cells, the regulation of immune cell intracrine function, and the biological effects of immune-derived sex steroids, including the clinical relevance of immune cell intracrinology in fields other than metabolism. These findings will serve as the basis for a proposed model of immune cell intracrinology constituting a new frontier in metabolism research.

Major conclusions

The development of highly sensitive mass spectrometric methods for sex steroid measurement and quantitation of metabolic flux now allows unprecedented ability to interrogate sex steroid production, metabolism and secretion by immune cells. Immune cell intracrinology could reveal key mechanisms underlying immune cell-mediated metabolic regulation.

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Sex steroids exert immunomodulatory effects that may influence metabolic health.

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Immune cells can synthesize, modify, and metabolize sex steroids.

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Immune cell-derived sex steroids may play intracrine, autocrine, paracrine, and possibly even endocrine roles.

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Immune cell steroidogenesis is a largely unexplored area of metabolism research.