Glucocorticoid Metabolism in Human Orbital Adipose Tissue

Abstract

Purpose:: Glucocorticoids (GCs) have a profound effect on adipose biology increasing tissue mass causing central obesity. The pre-receptor regulation of GCs by 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) that activates cortisol from cortisone, has been postulated as a fundamental mechanism underlying the metabolic syndrome mediating adipocyte hyperplasia and hypertrophy in the omental (OM) depot. Orbital adipose tissue (OF) is the site of intense inflammation and tissue remodelling in several orbital inflammatory disease states. To date, no studies have evaluated 11ß-HSD1 and endogenous glucocorticoid bioavailability in OF. The aim of this study is to describe features of the GC metabolic pathways in normal human OF depot and to compare with subcutaneous (SC) and OM depots.

Methods:: An automated, histomorphometric analysis was established to compare three optimal parameters (cell area, perimeter, diameter) for each adipose depot. Anti-CD68 staining and subsequent macrophage count was also performed. 11ß-HSD1 was characterised in OF using immunohistochemistry, real-time RT-PCR analysis and specific enzyme assays. Real-time RT-PCR was carried out on whole adipose tissue samples for evaluating the expression of hexose-6-phosphate (H6PDH), glucocorticoid receptor-α (GRα), fatty acid binding protein 4 (FABP4) and glycerol-3-phosphate dehydrogenase (G3PDH) genes.

Results:: OF adipocytes were significantly smaller than OM and SC adipocytes (p<0.001). The whole tissue depot demonstrated a higher resident CD68⁺ population (mRNA=p<0.05; protein=p<0.001) and expression of GRα mRNA (p<0.05). Conversely, 11ß-HSD1 and H6PDH mRNA, together with markers of late adipocyte differentiation (FABP4, G3PDH) were significantly lower in OF. These findings were supported by greater 11ß-HSD1 activity generating cortisol in both SC and OM primary cultures of preadipocytes, compared to OF.

Conclusions:: The OF depot has distinct GC biological features that distinguish it from SC and OM depots. Orbital adipocytes are smaller; less differentiated, and express low levels of 11ß-HSD1 and H6PDH but abundant GRα. The depot harbours a large resident CD68⁺ population. These characteristics define an orbital microenvironment that has the potential to respond to sight-threatening orbital inflammatory disease.

Keywords: autoimmune disease • corticosteroids • immunomodulation/immunoregulation