Dissociation of amylin from insulin secretion in β-cells is mediated by changes in expression

Amylin is a 37 amino acid hormone normally co-secreted with insulin. It is the major component of pancreatic amyloid which appears responsible for islet destruction in type H diabetics. Since amylin also inhibits insulin secretion in vivo and in vitro, it may contribute directly and indirectly to the development of diabetes. Overproduction of amylin is likely to result from a dissociation between amylin and insulin regulation. While several studies have suggested that amylin and insulin (hormone and mRNA) are regulated in parallel, we have evidence of divergence. INSI is a rat β-cell line derived from an insulinoma which retains physiological glucose responsiveness. INS-1 cells treated with isobutylmethylxanthine (IBMX, 100 μM), which increases intracellular cAMP levels, induced a 10-fold rise in amylin secretion, nearly double that of insulin. Quantitative multiplex RT-PCR was developed to simultaneously measure both amylin and insulin mRNA using known concentrations of competitive, synthetic cRNA. IBMX (100 μM) increased amylin mRNA by 118% while insulin mRNA increased only 34% at 24 hours (P<0.05, N=5). Gel shift experiments, using probes homologous to the amylin promoter, show that IBMX induces protein-DNA binding to a CAAT domain. Since amylin secretion and mRNA levels increase in conjunction with increased promoter binding and the insulin promoter lacks a functional CAAT box, distinct elements may regulate amylin and insulin transcription independently. Differential regulation between amylin and insulin may explain pancreatic amyloid formation and the pathophysiology of type If diabetes.