Thiazolidinediones (TZDs) have been suggested to act beneficially on pancreatic islet function and on beta-cell viability but data concerning direct effects on isolated islets are controversial. Therefore, we have examined parameters of pancreatic insulin and glucagon secretion and biosynthesis in TZD-exposed rat pancreatic islets under physiological glucose level conditions and under conditions of glucolipotoxicity. Primary rat islets were incubated for 2.5 h with or without troglitazone (10 microM) in 5.6 mM glucose (standard glucose levels) and 16.7 mM glucose (high glucose levels); a subgroup was additionally treated with oleate (200 microM) to simulate acute glucolipotoxicity. Insulin and glucagon secretion, intracellular content and their respective mRNAs were quantified. Newly synthesized insulin was determined by pulse-labeling experiments. Troglitazone reduced insulin secretion at standard and high glucose levels by about one-third (P<or=0.05). Insulin content was decreased at 5.6 mM glucose but increased at 16.7 mM glucose by the presence of troglitazone (P<or=0.05). Newly synthesized insulin mRNA and preproinsulin mRNA decreased by about 20% at standard glucose levels (P<or=0.05). Glucagon secretion was augmented by troglitazone in islets under high glucose conditions by an additional 50% (P<or=0.05). No clear beneficial troglitazone effects were observed under glucolipotoxic conditions. The reduced insulin secretion and biosynthesis at standard glucose levels can be interpreted as an insulin-sparing effect. Troglitazone effects were less pronounced at high glucose alone or in combination with oleate. From a clinical point of view, these results indicate a greater benefit of troglitazone for beta-cell function in hyperinsulinemic, but normoglycemic patients with insulin resistance or early type 2 diabetes without major insulin secretion deficits and/or pronounced hyperglycemia.
LC Bollheimer, S Troll, H Landauer, CE Wrede, J Scholmerich and R Buettner
S Gahr, M Merger, LC Bollheimer, CG Hammerschmied, J Scholmerich and SR Hugl
We investigated the role of hepatocyte growth factor (HGF) in beta-cell growth and its complex intracellular signal transduction pathways. Cell proliferation was measured in the beta-cell line INS-1 using [3H]thymidine incorporation. Activation of mitogenic signaling proteins was assessed using co-immunoprecipitation, immunoblot analysis and specific protein activity inhibitors in proliferation assays. HGF (1 x 375 nM) increased INS-1 cell proliferation in the presence of 3-24 mM glucose up to 45-fold vs unstimulated controls. HGF exceeded the effect of glucose alone (2 x 2-fold at 3 mM glucose and 1 x 7-fold in the presence of 15 mM glucose). The HGF-induced INS-1 cell proliferation was further increased by addition of IGF-I or GH. Stimulation with HGF activated the JAK-2/STAT-5 pathway with a subsequent activation of phosphatidylinositol-3'-kinase (PI3'K). PI3'K activation was necessary for HGF- and glucose-stimulated INS-1 cell proliferation. The effect of PI3'K was mediated via 70 kDa S6 kinase and protein kinase B, which showed maximum activation in the presence of 3-6 mM glucose. Protein kinase C was essential for HGF-induced INS-1 cell proliferation. The HGF effect was also mediated at low glucose concentrations via insulin receptor substrate 4 (IRS-4) whereas other IRS proteins did not show any activation. High glucose concentrations also showed an increased IRS-4/PI3'K binding and therefore activation. In conclusion, beta-cell proliferation is mediated via complex interacting signal transduction pathways. HGF, in contrast to other growth factors, seems to be of importance particularly in the presence of low glucose concentrations and therefore takes a special role in this complex concert.
B Zietz, W Drobnik, H Herfarth, C Buechler, J Scholmerich and A Schaffler
Plasminogen activator inhibitor-1 (PAI-1) levels were found to be associated with obesity indicating that adipocytes influence PAI-1 plasma levels. In addition, the 4 G/5 G promoter polymorphism of the PAI-1 gene may modulate PAI-1 transcription. We investigated the transcriptional regulation of the human PAI-1 gene in adipocytes and analyzed the genetic contribution of the 4 G/5 G polymorphism. The PAI-1 promoter was analyzed using electrophoretic mobility shift assays (EMSAs) and luciferase reporter gene assays. A putative binding site for the upstream stimulatory factor-1/2 (USF-1/2) at the polymorphic region of the PAI-1 promoter was identified. The binding of USF-1/2 was studied using nuclear extracts prepared from adipocytes and was similar in all the promoter variants as analyzed by EMSA. A 257 bp PAI-1 promoter fragment including the 4 G/5 G site was transcriptionally active in adipocytes and was not influenced by the polymorphism. The present data indicate for the first time that USF-1/2 is transcriptionally active in differentiated adipocytes. However, USF-1/2 binding activity and PAI-1 transcription are not influenced by the 4 G/5 G-allele. These data possibly explain the observation that PAI-1 secretion from adipose tissue is not influenced by the PAI-1 promoter polymorphism.
R Buettner, K G Parhofer, M Woenckhaus, C E Wrede, L A Kunz-Schughart, J Schölmerich and L C Bollheimer
High-fat (HF)-diet rodent models have contributed significantly to the analysis of the pathophysiology of the insulin resistance syndrome, but their phenotype varies distinctly between different studies. Here, we have systematically compared the metabolic and molecular effects of different HF with varying fatty acid compositions. Male Wistar rats were fed HF diets (42% energy; fat sources: HF-L – lard; HF-O – olive oil; HF-C – coconut fat; HF-F – fish oil). Weight, food intake, whole-body insulin tolerance and plasma parameters of glucose and lipid metabolism were measured during a 12-week diet course. Liver histologies and hepatic gene expression profiles, using Affymetrix GeneChips, were obtained. HF-L and HF-O fed rats showed the most pronounced obesity and insulin resistance; insulin sensitivity in HF-C and HF-F was close to normal. Plasma ω-3 polyunsaturated fatty acid (ω-3-PUFA) and saturated fatty acid (C12-C14, SFA) levels were elevated in HF-F and HF-C animals respectively. The liver histologies showed hepatic steatosis in HF-L, HF-O and HF-C without major inflammation. Hepatic SREBP1c-dependent genes were upregulated in these diets, whereas PPARα-dependent genes were predominantly upregulated in HF-F fed rats. We detected classical HF effects only in diets based on lard and olive oil (mainly long-chain, saturated (LC-SFA) and monounsaturated fatty acids (MUFA)). PUFA- or MC-SFA-rich diets did not induce insulin resistance. Diets based on LC-SFA and MUFA induced hepatic steatosis with SREBP1c activation. This points to an intact transcriptional hepatic insulin effect despite resistance to insulin’s metabolic actions.