Improved adipose tissue metabolism after 5-year growth hormone replacement therapy in growth hormone deficient adults: The role of zinc-α2-glycoprotein

Abstract

Growth hormone (GH) supplementation therapy to adults with GH deficiency has beneficial effects on adipose tissue lipid metabolism, improving thus adipocyte functional morphology and insulin sensitivity. However, molecular nature of these effects remains unclear. We therefore tested the hypothesis that lipid-mobilizing adipokine zinc-α2-glycoprotein is causally linked to GH effects on adipose tissue lipid metabolism. Seventeen patients with severe GH deficiency examined before and after the 5-year GH replacement therapy were compared with age-, gender- and BMI-matched healthy controls. Euglycemic hyperinsulinemic clamp was used to assess whole-body and adipose tissue-specific insulin sensitivity. Glucose tolerance was determined by oGTT, visceral and subcutaneous abdominal adiposity by MRI, adipocyte size morphometrically after collagenase digestion, lipid accumulation and release was studied in differentiated human primary adipocytes in association with GH treatment and zinc-α2-glycoprotein gene silencing. Five-year GH replacement therapy improved glucose tolerance, adipose tissue insulin sensitivity and reduced adipocyte size without affecting adiposity and whole-body insulin sensitivity. Adipose tissue zinc-α2-glycoprotein expression was positively associated with whole-body and adipose tissue insulin sensitivity and negatively with adipocyte size. GH treatment to adipocytes in vitro increased zinc-α2-glycoprotein expression (>50%) and was paralleled by enhanced lipolysis and decreased triglyceride accumulation (>35%). Moreover, GH treatment improved antilipolytic action of insulin in cultured adipocytes. Most importantly, silencing zinc-α2-glycoprotein eliminated all of the GH effects on adipocyte lipid metabolism. Effects of 5-year GH supplementation therapy on adipose tissue lipid metabolism and insulin sensitivity are associated with zinc-α2-glycoprotein. Presence of this adipokine is required for the GH action on adipocyte lipid metabolism in vitro.

Keywords:

• adipocyte size
• adipose tissue
• glucose tolerance
• growth hormone deficiency
• growth hormone replacement therapy
• insulin sensitivity
• lipolysis

Abbreviations:

• ACC1, acetyl-CoA carboxylase 1
• BSA, bovine serum albumin
• DGAT, diacylglycerol acyltransferase
• DMEM, Dulbecco's Modified Eagle Medium
• EHC, euglycemic hyperinsulinemic clamp
• FAS, fatty acid synthase
• FABP4, fatty acid binding protein 4
• FBS, fetal bovine serum
• FFA, free fatty acids
• GAPDH, glyceraldehyde-3-phosphate dehydrogenase
• GH, growth hormone
• GLUT4, glucose transporter 4
• HSL, hormone sensitive lipase
• GHD, growth hormone deficiency
• IGF-1, insulin-like growth factor 1
• IRS1, insulin receptor substrate 1
• MRI, magnetic resonance imaging
• oGTT, oral glucose tolerance test
• PPARGC1A, peroxisome proliferator-activated receptor 1 gamma coactivator 1 α
• rhGH, recombinant human growth hormone
• RPL13A, ribosomal protein L13a
• TG, triglycerides
• ZAG, zinc-α2-glycoprotein.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

Authors would like to express their gratitude to all those who contributed to this work (in alphabetical order) A. Banarova, V. Belan, A. Dlesk, J. Fedeles, R. Imrich, Z. Kunakova, M. Kuzma, A. Mitkova, A. Penesova, M. Pura, L. Rossmeislova, M. Srbecky and L. Straub as well as to all study volunteers for their cooperative attitude and genuine interest in our work and in their own metabolic health.

Funding

This work was supported by Pfizer Global Investigator Initiated Research Grant, EFSD New Horizons, Slovak Research and Development Agency Grant 0122-06; Scientific Grant Agency of the Slovak Academy of Sciences grants # 2/0198/11 and 2/0174/12 and National Scholarship Program of the Slovak Republic.