Prolonged exposure to homocysteine results in diminished but reversible pancreatic β-cell responsiveness to insulinotropic agents

Background: Plasma homocysteine levels may be elevated in poorly controlled diabetes with pre-existing vascular complications and/or nephropathy. Since homocysteine has detrimental effects on a wide diversity of cell types, the present study examined the effects of long-term homocysteine exposure on the secretory function of clonal BRIN-BD11 β-cells. Methods: Acute insulin secretory function, cellular insulin content and viability of BRIN-BD11 cells were assessed following long-term (18 h) exposure to homocysteine in culture. RT-PCR and Western blot analysis were used to determine the expression of key β-cell genes and proteins. Cells were cultured for a further 18 h without homocysteine to determine any long-lasting effects. Results: Homocysteine (250-1000 μmol/L) exposure reduced insulin secretion at both moderate (5.6 mmol/L) and stimulatory (16.7 mmol/L) glucose by 48-63%. Similarly, insulin secretory responsiveness to stimulatory concentrations of alanine, arginine, 2-ketoisocaproate, tolbutamide, KCl, elevated Ca²⁺, forskolin and PMA, GLP-1, GIP and CCK-8 were reduced by 11-62% following culture with 100-250 μmol/L homocysteine. These inhibitory effects could not simply be attributed to changes in cellular insulin content, cell viability, H₂O₂ generation or any obvious alterations of gene/ protein expression for insulin, glucokinase, GLUT2, VDCC, or Kir6.2 and SUR1. Additional culture for 18 h in standard culture media after homocysteine exposure restored secretory responsiveness to all agents tested. Conclusion: These findings suggest that long-term exposure to high homocysteine levels causes a reversible impairment of pancreatic β-cell insulinotropic pathways. The in vivo actions of hyperhomocysteinaemia on islet cell function merit investigation.