Pathogenesis of feline diabetes mellitus.

The common form of spontaneous diabetes mellitus that occurs in domestic cats bears close resemblance clinically and pathologically to human type 2 diabetes mellitus (T2DM). For example, the typical diabetic cat is obese and middle-aged, and has low but detectable circulating insulin levels. However, the most striking similarity is the occurrence of islet amyloidosis (IA) in nearly all diabetic cats and in over 90% of humans with T2DM. IA in both humans and cats is derived from islet amyloid polypeptide (IAPP, or amylin) which is a hormone produced and secreted along with insulin by the pancreatic beta cells. Since all cats and humans normally produce IAPP, additional factors must be invoked in order to explain the development of IA. Several lines of evidence support the concept that IA is caused by chronically increased stimulus for beta cells to secrete IAPP (and insulin). For example, peripheral insulin resistance such as in chronic obesity results in increased IAPP and insulin secretion. A recent study, in which diabetes mellitus was induced in cats, demonstrated that IAPP hypersecretion was induced by treatment with a sulfonylurea drug and resulted in 4/4 cats in this group developing IA. In contrast, cats treated with insulin had low IAPP secretion and minimal IA developed in 1/4 cats. Several human-IAPP transgenic mouse models, in which there is IAPP overexpression, also support the notion that prolonged high expression of IAPP leads to IA. In vitro models of IAPP overexpression also support this mechanism for IA formation and by demonstrating an association between IA formation and beta cell toxicity, suggest a linkage between IA formation and loss of beta cells in T2DM. A recent study has indicated that intermediate-sized IAPP-derived amyloid fibrils can disrupt cell membranes and therefore, may be involved in the destruction of beta cells. Striking parallels between the pathogenesis of IA and beta-amyloid plaque formation in Alzheimer's disease suggest possible parallel pathogenetic mechanisms of cell death and provide potential avenues for future studies into the pathogenesis of IA.