Hui-Kang Liu

Abnormal protein expression in the corpus cavernosum impairs erectile function in type 2 diabetes

Study Type – Aetiology (case control)
Level of Evidence 3b

N-acetyl-GLP-1: a DPP IV-resistant analogue of glucagon-like peptide-1 (GLP-1) with improved effects on pancreatic β-cell-associated gene expression

Glucagon‐like peptide‐1(7‐36)amide (GLP‐1) is a key insulinotropic hormone with the reported potential to differentiate non‐insulin secreting cells into insulin‐secreting cells. The short biological half‐life of GLP‐1 after cleavage by dipeptidylpeptidase IV (DPP IV) to GLP‐1(9‐36)amide is a major therapeutic drawback. Several GLP‐1 analogues have been developed with improved stability and insulinotropic action. In this study, the N‐terminally modified GLP‐1 analogue, N‐acetyl‐GLP‐1, was shown to be completely resistant to DPP IV, unlike native GLP‐1, which was rapidly degraded. Furthermore, culture of pancreatic ductal ARIP cells for 72 h with N‐acetyl‐GLP‐1 indicated a greater ability to induce pancreatic β‐cell‐associated gene expression, including insulin and glucokinase. Further investigation of the effects of stable GLP‐1 analogues on β‐cell differentiation is required to assess their potential in diabetic therapy.

Administration of a Decoction of Sucrose- and Polysaccharide-Rich Radix Astragali (Huang Qi) Ameliorated Insulin Resistance and Fatty Liver but Affected Beta-Cell Function in Type 2 Diabetic Rats

The current investigation attempted to confirm the beneficial actions of a chemically characterized Radix Astragali decoction (AM-W) against type 2 diabetic (T2D) Sprague-Dawley (SD) rats. Using a case/control design, after 2 months of treatment with AM-W (500 mg/kg, daily i.p.) in T2D rats therapeutic outcomes were compared. Sucrose and Astragalus polysaccharides (ASPs) were shown to exist in nearly equal proportions in AM-W. Body weight loss, an improvement in insulin sensitivity, and an attenuation of fatty liver after AM-W administration in T2D rats were evident. Surprisingly, blood sugar, beta-cell function, and glucose tolerance in T2D rats did not improve with AM-W treatment. Further investigation indicated the deleterious effects of the addition of sucrose (100 and 500 μg/mL) and APSs (500 μg/mL) on glucose-stimulated insulin secretion and viability, respectively. In conclusion, a proper administration dosage and a reduction in the sucrose content are keys to maximizing the merits of this herb.

Function of a long-term, GLP-1-treated, insulin-secreting cell line is improved by preventing DPP IV-mediated degradation of GLP-1

Glucagon‐like peptide‐1 (GLP‐1) is an important insulinotropic hormone with potential in the treatment of type 2 diabetes. However, the short biological half‐life of the peptide after cleavage by dipeptidylpeptidase IV (DPP IV) is a major limitation. Inhibition of DPP IV activity and the development of resistant GLP‐1 analogues is the subject of ongoing research. In this study, we determined cell growth, insulin content, insulin accumulation and insulin secretory function of a insulin‐secreting cell line cultured for 3 days with either GLP‐1, GLP‐1 plus the DPP IV inhibitor diprotin A (DPA) or stable N‐acetyl‐GLP‐1. Native GLP‐1 was rapidly degraded by DPP IV during culture with accumulation of the inactive metabolite GLP‐1(9–36)amide. Inclusion of DPA or use of the DPP IV‐resistant analogue, N‐acetyl‐GLP‐1, improved cellular function compared to exposure to GLP‐1 alone. Most notably, basal and accumulated insulin secretion was enhanced, and glucose responsiveness was improved. However, prolonged GLP‐1 treatment resulted in GLP‐1 receptor desensitization regardless of DPP IV status. The results indicate that prevention of DPP IV action is necessary for beneficial effects of GLP‐1 on pancreatic β cells and that prolonged exposure to GLP‐1(9–36)amide may be detrimental to insulin secretory function. These observations also support the ongoing development of DPP‐IV‐resistant forms of GLP‐1, such as N‐acetyl‐GLP‐1.

Long-term beneficial effects of vanadate, tungstate, and molybdate on insulin secretion and function of cultured beta cells

Abstract: The ultratrace elements vanadate, tungstate, and molybdate exhibit significant antihyperglycemic effects in both type 1 and 2 diabetic animals, but possible effects on the function of pancreatic beta cells are understudied. In the present study, clonal BRIN BD11 cells were cultured for 3 days with each ultratrace element to establish doses lacking detrimental effects on viable beta cell mass. Vanadate treatment (4 μmol/L) had no effect on cellular insulin content but improved glucose-induced insulin secretory responsiveness. However, insulin secretion mediated by PKA and PKC activation was desensitized in vanadate-treated cells. Culture with tungstate (300 μmol/L) and molybdate (1 mmol/L) increased cellular insulin content and enhanced basal insulin release and the responsiveness to glucose and a wide range of other secretagogues. These observations suggest significant effects of ultratrace elements on pancreatic beta cells that may contribute to their antihyperglycemic action.

Effects of long-term exposure to nicotinamide and sodium butyrate on growth, viability, and the function of clonal insulin secreting cells.

The B vitamin nicotinamide (NIC), commonly known as niacin, is currently in trial as a potential means of preventing Type 1 diabetes in first‐degree relatives of affected individuals. Sodium butyrate (BUT) a common dietary micronutrient has also been reported to have beneficial effects on the differentiation and function of pancreatic beta cells. Cultured rat insulin‐secreting BRIN‐BD11 cells were used to investigate the effects of 3 days exposure to NIC (10 mM) and BUT (1 mM) both alone and in combination on beta cell function. Culture with NIC and/or BUT resulted in reduction of growth, insulin content and basal insulin secretion. BUT additionally decreased cell viability whilst NIC had no significant effect. Treatment with either agent abolished beta cell glucose sensitivity but insulin secretory responsiveness to a wide range of beta cell stimulators, including a depolarizing concentration of K+, elevation of Ca2 + and activation of adenylate cyclase and protein kinase C, were enhanced. These data illustrate that long term exposure to NIC and BUT has both positive and negative effects on the function of insulin‐secreting cells.

Pharmacological evaluation of insulin mimetic novel suppressors of PEPCK gene transcription from Paeoniae Rubra Radix

ETHNOPHARMACOLOGICAL RELEVANCE Paeoniae Rubra Radix (root of Paeonia lactiflora) has been frequently employed in Traditional Chinese Medicine (TCM) as and anti-diabetic therapy to enhance blood circulation and dissipate stasis. AIM OF THE STUDY Previously, we identified a novel hypoglycemic action of a crude extract from Paeoniae Rubra Radix, which also suppressed phosphoenolpyruvate carboxykinase (PEPCK) gene transcription. Therefore, the current investigation intended to elucidate potential active bio-constituents of this herb and mechanisms of action. MATERIALS AND METHODS Glucocorticoid receptor (GR) nuclear localization, the PEPCK messenger (m)RNA level, pregnane X receptor (PXR) mRNA expression, cAMP-responsive element-binding protein (CREB) serine phosphorylation and DNA binding were evaluated in dexamethasone (Dex) and 8-bromo-cAMP (CA)-stimulated H4IIE cells, while efficacy of agents was assessed in a stable cell line containing a green fluorescent protein (GFP) reporter driven by the PEPCK promoter. HPLC profiling, colorimetric assays, and NMR analysis were employed for chemical characterization purpose. RESULTS An extract of Paeoniae Rubra Radix lacking the insulin mimetic compound, 1,2,3,4,6-penta-O-galloyl-beta-d-glucose (PGG), and termed the non-PGG fraction (NPF), consisting of tannin polymers, suppressed PEPCK expression in the presence of an insulin receptor antagonist (HNMPA-AM(3)), suggesting the action of this fraction is independent of the insulin receptor. Furthermore, Dex-stimulated GR nuclear localization and transactivation were prevented by the NPF. Similarly, CA-stimulated CREB serine phosphorylation and DNA binding were also inhibited by the NPF in H4IIE cells. Hence NPF antagonizes both signaling pathways that induce PEPCK gene transcription. CONCLUSION In conclusion, the current study proposes that the potent suppressive activity on PEPCK gene transcription observed with Paeoniae Rubra Radix extract, can be attributed to at least two distinct components, namely PGG and NPF.

Schizandra arisanensis extract attenuates cytokine-mediated cytotoxicity in insulin-secreting cells

AIM To explore the bioactivity of an ethanolic extract of Schizandra arisanensis (SA-Et) and isolated constituents against interleukin-1β and interferon-γ-mediated β cell death and abolition of insulin secretion. METHODS By employing BRIN-BD11 cells, the effects of SA-Et administration on cytokine-mediated cell death and abolition of insulin secretion were evaluated by a viability assay, cell cycle analysis, and insulin assay. The associated gene and protein expressions were also measured. In addition, the bioactivities of several peak compounds collected from the SA-Et were tested against cytokine-mediated β cell death. RESULTS Our results revealed that SA-Et dose-dependently ameliorated cytokine-mediated β cell death and apoptosis. Instead of suppressing inducible nitric oxide synthase/nitric oxide cascade or p38MAPK activity, suppression of stress-activated protein kinase/c-Jun NH2-terminal kinase activity appeared to be the target for SA-Et against the cytokine mix. In addition, SA-Et provided some insulinotropic effects which re-activated the abolished insulin exocytosis in cytokine-treated BRIN-BD11 cells. Finally, schiarisanrin A and B isolated from the SA-Et showed a dose-dependent protective effect against cytokine-mediated β cell death. CONCLUSION This is the first report on SA-Et ameliorating cytokine-mediated β cell death and dysfunction via anti-apoptotic and insulinotropic actions.

Iterative exposure of clonal BRIN-BD11 cells to ninhydrin enables selection of robust toxin-resistant cells but with decreased gene expression of insulin secretory function.

Objectives: Prevention of pancreatic &bgr;-cell destruction combined with preservation of insulin secretory function is an important goal for cell-based diabetes therapy. This study describes the generation and characteristics of toxin-resistant &bgr;-cells. Methods: By using iterative exposures to ninhydrin, a new class of robust ninhydrin-tolerant insulin-secreting BRIN-BD11 ninhydrin-tolerant (BRINnt) cells was generated. Low- and high-passage BRINnt cells were used to evaluate &bgr;-cell function and tolerance against toxins in comparison with native BRIN-BD11 cells. Differences in viability, gene expression, insulin secretory function, antioxidant enzyme activity, DNA damage, and DNA repair efficiency were compared. Results: BRIN-BD11 ninhydrin-tolerant cells exhibited resistance toward ninhydrin and hydrogen peroxide but not streptozotocin (STZ). Both total superoxide dismutase (SOD) and catalase enzyme activities of BRINnt cells were significantly enhanced, and ninhydrin-induced DNA damage was decreased. BRIN-BD11 ninhydrin-tolerant cells also exhibited enhanced DNA repair efficiency. However, this was accompanied by loss of secretagogue-induced insulin release, decreased cellular insulin content, and deficits in insulin and glucose transporter 2 gene expression. Prolonged culture of BRINnt cells in the absence of ninhydrin reversed the degenerated function of BRINnt cells but restored ninhydrin susceptibility. Conclusions: These data illustrate dissociation between &bgr;-cell toxin resistance and secretory function, indicating difficulties in generation of robust and well-functioning cells using this approach.