Extrapancreatic insulin-producing cells in multiple organs in diabetes

Insulin-producing cells normally occur only in the pancreas and thymus. Surprisingly, we found widespread insulin mRNA and protein expression in different diabetic mouse and rat models, including streptozotocin-treated mice and rats, ob/ob mice, and mice fed high-fat diets. We detected in diabetic mice proinsulin- and insulin-positive cells in the liver, adipose tissue, spleen, bone marrow, and thymus; many cells also produced glucagon, somatostatin, and pancreatic polypeptide. By in situ nucleic acid hybridization, diabetic, but not nondiabetic, mouse liver exhibited insulin transcript-positive cells, indicating that insulin was synthesized by these cells. In transgenic mice that express GFP driven by the mouse insulin promoter, streptozotocin-induced diabetes led to the appearance of GFP-positive cells in liver, adipose tissue, and bone marrow; the fluorescent signals showed complete concordance with the presence of immunoreactive proinsulin. Hyperglycemia produced by glucose injections in nondiabetic mice led to the appearance of proinsulin- and insulin-positive cells within 3 days. Bone marrow transplantation experiments showed that most of the extrapancreatic proinsulin-producing cells originated from the bone marrow. Immunoreactive proinsulin- and insulin-positive cells were also detected in the liver, adipose tissue, and bone marrow of diabetic rats, indicating that extrapancreatic, extrathymic insulin production occurs in more than one species. These observations have implications for the regulation of insulin gene expression, modulation of self-tolerance by insulin gene expression, and strategies for the generation of insulin-producing cells for the treatment of diabetes.