High glucose causes apoptosis in cultured human pancreatic Islets of Langerhans - A potential role for regulation of specific Bcl family genes towardan apoptotic cell death program

Type 2 diabetes is characterized by insulin resistance and inadequate insul in secretion. In the advanced stages of the disease, beta -cell dysfunction worsens and insulin therapy maybe necessary to achieve satisfactory metabo lic control. Studies in autopsies found decreased beta -cell mass in pancre as of people with type 2 diabetes. Apoptosis, a constitutive program of cel l death modulated by the Eel family genes, has been implicated in loss of b eta -cells in animal models of type 2 diabetes. in this study, we compared the effect of 5 days' culture in high glucose concentration (16.7 mmol/l) v ersus normal glucose levels (5.5 mmol/l) or hyperosmolar control (mannitol 11 mmol/l plus glucose 5 mmol/l) on the survival of human pancreatic islets . Apoptosis, analyzed by flow cytometry and electron and immunofluorescence microscopy, was increased in islets cultured in high glucose (HG5) as comp ared with normal glucose (NG5) or hyperosmolar control (NG5+MAN5). We also analyzed by reverse transcriptase-polymerase chain reaction and Western blo tting the expression of the Bcl family genes in human islets cultured in no rmal glucose or high glucose. The antiapoptotic gene Bcl-2 was unaffected b y glucose change, whereas Bcl-xl was reduced upon treatment with HG5. On th e other hand, proapoptotic genes Bad, Bid, and Bik were overexpressed in th e islets maintained in HG5. To define the pancreatic localization of Bcl pr oteins, me performed confocal immunofluorescence analysis on human pancreas . Bad and Bid mere specifically expressed in beta -cells, and Bid was also expressed, although at low levels, in the exocrine pancreas. Bik and Bcl-xl were expressed in other endocrine islet cells as well as in the exocrine p ancreas. These data suggest that in human islets, high glucose may modulate the balance of proapoptotic and antiapoptotic Eel proteins toward apoptosi s, thus favoring beta -cell death.