Endocrine Abstracts

Objective: Cystatin C (CysC) is a marker for estimation of glomerular filtration rate (GFR) and a predictor of mortality, beyond that provided by GFR. CysC increases with abdominal obesity and insulin resistance independently of GFR, not necessarily due to changes in renal clearance but possibly due to changes in production. Non-GFR determinants of CysC remain poorly defined. GH or thyroid hormone excess (acromegaly and hyperthyroidism) lead to concomitant increases in GFR and CysC levels, which can be reversed following successful treatment. We therefore studied whether GH and T3 increase the production of CysC by 3T3-L1 adipocytes in vitro.

Methods: 3T3-L1 cells were plated and grown for 5d in media containing serum, insulin, and dexamethasone (dex), then kept for 6d in serum-free test media. CysC release was assessed by ELISA and western analysis, responsiveness of the cells to insulin by measuring 14C-2-deoxyglucose (2DG) uptake.

Results: GH (10 nM) stimulated 2DG uptake after 6 h (1.2±0.2-fold) but not after 1 or 4d whereas T3 (10 nM) was stimulatory in longer term (after 4d, 1.4±0.1-fold). Insulin-stimulated 2DG uptake was attenuated by GH (4d) and enhanced by T3 (4d) pre-treatment. 3T3-L1 cells (much more than fibroblasts) express CysC and release the protein into the media where it accumulates in a time-dependent fashion. 10 nM GH- or T3-treated cells released (2.24±0.25 and 2.65±0.32 respectively) more CysC into the media than control-treated cells (1.82±0.22 μg CysC/mg protein x 4d). Among the hormones known and used to enhance adipocyte differentiation, insulin (100 nM, 1.5±0.3-fold) but not dex (100 nM) increased CysC production.

Conclusions: Our data confirm that GH attenuates 2DG uptake stimulation by insulin, and that T3 enhances basal and insulin-induced 2DG uptake in adipocytes. Moreover, insulin, GH, and T3 but not dex enhance the production of CysC by adipocytes in vitro, in line with in vivo observations.