Beneficial Effect Of 1,25-Dihydroxyvitamin D₃ On Streptozotocin-Treated Rat Pancreatic Islets

OBJECTIVE: In this study we investigated the effect of 1,25-(OH)2D3 on streptozotocin-induced damage to rat pancreatic islet cells. METHODS: Islet cells, isolated from neonatal Wistar rats, were cultured and processed as follows. Normal control group: Islet cells were incubated in normal growth media. 1,25-(OH)2D3 group: Islet cells were treated with 1,25-(OH)2D3 at a final concentration of 10-6 M in the culture media. STZ group: Islet cells were incubated in the presence of 2.2 mM concentration of streptozotocin and then the medium was removed after 30 minutes. Normal growth media was added and the cells were allowed to recover for 36 hours. 10-8M 1,25-(OH)2D3+STZ group: Islet cells were incubated in the medium containing 10-8M 1,25-(OH)2D3 for 30 minutes and then streptozotocin was added. After exposure to 10-8M 1,25-(OH)2D3 and streptozotocin for 30 minutes, the cells were then incubated in new normal growth media for 36 hours. 10-6M 1,25-(OH)2D3+STZ group: Islet cells were processed as described above with a different 10-6M concentration of 1,25-(OH)2D3. After 36 hours recovery period, analysis were performed. The islet insulin contents were measured by radioimmunoassay. Apoptosis was confirmed by transmission electron microscopy and the TUNEL method was used to quantify the number of apoptotic cells. A20 mRNA was determined by RT-PCR in rat islet cells RESULTS: 1. 1,25-(OH)2D3 group: Both insulin content and the rate of islet cells apoptosis did not differ significantly from that of normal control group. The expression of A20 was observed in islet cells of 1,25-(OH)2D3 group but not of normal control group. 2. STZ group: Insulin content exhibited a significant decrease compared with normal control group, and the rate of islet cells apoptosis, a significant increase (25.36± 2.37% vs 8.13±1.24%), P<0.01. A20 mRNA in islet cells was not monitored by RT-PCR. 3. 10-8M 1,25-(OH)2D3+STZ group and 10-6M 1,25-(OH)2D3+STZ group: Insulin contents showed significantly higher values than in STZ group. Apoptosis rates lowered to(15.18± 1.20)% and(13.85±1.28)% respectively ,compared with STZ group, P<0.01. We also observed a stimulation of A20 in both groups. No significant different effects were observed between these 2 groups. CONCLUSION: 1. STZ can induce apoptosis in isolated rat islet beta cells in vitro. 2. 1,25-(OH)2D3 protects rat beta cells against STZ-induced apoptosis. 3. 1,25-(OH)2D3 can induce the expression of A20 in islet cells that may play a role in the different pathways implicated in the beneficial effect of 1,25-(OH)2D3 on STZ-treated islet cells.