No constant relationship between islet amyloid polypeptide (IAPP) and insulin expression in insulinomas

The relationship between insulin and islet amyloid polypeptide (IAPP) expression was studied at the level of RNA and peptide in 3 patients operated on for insulinoma. Two patients had a solitary tumour; one patient suffered from the MEN-I syndrome. In the 3 tumours, as in normal pancreas, the same 2 IAPP-specific mRNA species, approximately 1600 and 2100 nucleotides, respectively, were detected. In 2 patients, the IAPP mRNA concentration of the tumour was lower than the insulin mRNA concentration; in the third patient, however, the specific IAPP hybridization signal was at least equal to that for insulin. Amyloid deposits were found in one solitary tumour and in the tumour from the MEN-I patient, both staining strongly positive with anti-IAPP antibodies; cytoplasmatic IAPP was weak. In conclusion, at least in some insulinomas, no constant relationship exists between insulin and IAPP expression.