Transthyretin constitutes a functional component in pancreatic β-cell stimulus-secretion coupling

Abstract

Transthyretin (TTR) is a transport protein for thyroxine and, in association with retinol-binding protein, for retinol, mainly existing as a tetramer in vivo. We now demonstrate that TTR tetramer has a positive role in pancreatic ␤-cell stimulus-secretion coupling. TTR promoted glucose-induced increases in cytoplasmic free Ca 2؉ concentration ([Ca 2؉ ]i) and insulin release. This resulted from a direct effect on glucose-induced electrical activity and voltagegated Ca 2؉ channels. TTR also protected against ␤-cell apoptosis. The concentration of TTR tetramer was decreased, whereas that of a monomeric form was increased in sera from patients with type 1 diabetes. The monomer was without effect on glucose-induced insulin release and apoptosis. Thus, TTR tetramer constitutes a component in normal ␤-cell function. Conversion of TTR tetramer to monomer may be involved in the development of ␤-cell failure͞ destruction in type 1 diabetes.type 1 diabetes ͉ Ca 2ϩ channels ͉ insulin release ͉ ␤-cell signal transduction ͉ apoptosis T ransthyretin (TTR) is a protein that is synthesized in the liver, the choroid plexus of the brain, and the endocrine pancreas (1, 2). It is a transport protein for thyroxine and, in association with retinol-binding protein, for retinol. It has been reported that 1-2% of plasma TTR circulates bound to highdensity lipoprotein (HDL) and that the association to the HDL vesicle occurs through binding to apolipoprotein A1 (3). TTR has a complex equilibrium between different quaternary structures in serum (4),but exists mainly as a tetrameric protein of 14-kDa subunits (160-380 mg͞liter) with only a small amount of TTR monomer present in vivo in normal individuals (5, 6). Consequently, measurements of TTR in serum by conventional methods mainly reflect the tetrameric form. The TTR amyloidoses are human diseases in which misfolded TTR protein aggregates in different tissues. Several point mutations in TTR have been related to familial amyloidotic polyneuropathy (FAP) (7). The fact that TTR is also produced within the pancreatic islet made us interested in evaluating a possible role of this protein in ␤-cell stimulus-secretion coupling. MethodsIdentification of TTR in Human Sera. Sera from type 1 diabetes (T1D) patients and control subjects were collected, identically sterile-processed, heat-inactivated by incubation at 56°C for 30 min, and stored frozen at Ϫ20°C until used. Changes in cytoplasmic free Ca 2ϩ concentration ([Ca 2ϩ ] i ) were tested in ␤-cells when they were depolarized with 25 mM KCl. Those diabetic sera that induced a higher increase in [Ca 2ϩ ] i than sera from controls were centrifuged, and the supernatant was passed through a 0.45-mm sterile filter. Samples were loaded on SepPak C 18 preconditioned with 0.1% trifluoroacetic acid. After application, the sample proteins were eluted with 60% acetonitrile in 0.1% trifluoroacetic acid. This procedure was repeated twice. The two fractions were pooled, and the volume was reduced by lyophilization.The lyophilized material was submitted to...

Proteomic analysis of conditioned media from glucose responsive and glucose non‐responsive phenotypes reveals a panel of secreted proteins associated with beta cell dysfunction

“…These results published by Refai and co-workers are supported by results presented in this paper [22]. The levels of TTR monomer was found to be substantially increased in the CM from the high-density glucose-nonresponsive cells compared with the glucose-responsive cells.…”

“…Refai and co-workers recently demonstrated that the TTR tetramer has a positive role in pancreatic beta-cell stimulus-secretion coupling. TTR promoted glucoseinduced increases in cytoplasmic free calcium concentration and insulin release [22]. Also reported was that the concentration of the TTR tetramer was decreased, whereas that of a monomeric form was increased in sera from patients with type I diabetes.…”

Contrasting effects of type 2 and type 1 diabetes on plasma RBP4 levels: The significance of transthyretin

“…Furthermore, higher plasma TTR concentrations were also observed in humans with obesity and/or insulin resistance (16). Increased levels of TTR mRNA and protein have been demonstrated in both pancreatic b-and a-cells during T2D (29)(30)(31)(32). Therefore, it is possible that elevated plasma TTR levels (possibly derived from pancreas) prevent the renal clearance of RBP4, leading to insulin resistance.…”

“…It is also possible that the decrease in plasma retinol and/or RBP4 could be a result of increased renal filtration due to low plasma TTR concentrations during T1D (40,41). It has been reported that plasma TTR concentrations were depressed in newly diagnosed T1D human subjects as well as in T1D rat models (29,32,41). Interestingly, the concentrations of monomeric form of TTR are higher in T1D when compared with normal subjects, but not the tetrameric TTR (32).…”

“…Interestingly, expression of TTR was reported in both pancreatic a-and b-cells and was shown to be upregulated during T2D, whereas it was downregulated during T1D (Table 1), providing a strong case that argue for pancreatic TTR role in the modulation of plasma RBP4 levels. Moreover, TTR is found to be stored in the secretory vesicles in pancreatic islets, thus it is likely that part of the plasma TTR is derived from the pancreas (29)(30)(31)(32). Furthermore, TTR is known to be inducible by insulinlike growth factor-1, and an insulin-responsive element has also been identified in the TTR promoter region (59).…”