Fetuin-A impairs maturation of pig neonatal islet cell clusters

Introduction and aim:

Fetuin-A is a serum glycoprotein secreted from hepatocytes and upregulated in fatty liver. Previous observations suggest that fetuin-A impairs glucose-induced insulin secretion (GIIS) and TGFβR signalling, a pathway sustaining beta-cell maturation and differentiation. Therefore, we hypothesize that fetuin-A impairs islet cells differentiation via inhibition of TGFβR signalling. The effects of fetuin-A on beta-cell maturation were examined in pig neonatal islet cell clusters (NICCs).

Methods:

Freshly isolated, glucose-unresponsive NICCs were maturated in serum-free medium supplemented with 0.6 mg/ml human serum albumin (control) or fetuin-A. Insulin secretion was assessed in static incubations. Gene expression was analysed by RT-PCR, protein expression by western blotting and their subcellular localisation by confocal microscopy.

Results:

NICCs maturation was accompanied by increased expression of insulin, glucagon, somatostatin and PDX1. TGFBI mRNA, a target of TGFbR, and p16/INK4a protein, a beta-cell maturation marker, were upregulated suggesting activation of TGFβR/SMAD2 – 3 signalling. PDX1 and p16/INK4a proteins accumulated in nuclei upon maturation. Maturated NICCs displayed a modest GIIS which was potentiated by palmitic acid (2-fold) and forskolin (3-fold). When maturation was carried out in the presence of fetuin-A, GIIS was abolished, forskolin's effect was significantly lower and insulin, glucagon, PDX1 and TGFBI mRNA levels remained low. The reduced SMAD2 – 3 phosphorylation indicated impaired TGFbR signalling. Furthermore, the transcript of RanBP3L, a protein mediating the nuclear export of SMADs, was increased. In addition, fetuin-A inhibited nuclear accumulation of PDX1 and p16/INK4a.

Conclusion:

These observations suggest that fetuin-A may impair beta-cell maturation and differentiation via inhibition of TGFβR signalling.