Cellular and Molecular Analysis of Hormone Production and Gene Expression in a Feline Insulinoma
Feline insulinoma is a rarely reported functional endocrine pancreatic tumor that produces hypoglycemia via excessive insulin secretion. Clinical signs, laboratory findings, and tumor histopathology in reported feline insulinoma cases are similar to those recognized in dogs with insulinoma. However, little is known about pathways responsible for the functional activity of these tumors. While some information is available about the pattern of hormone production exhibited by feline insulinomas, the molecular mechanisms that underlie the exaggerated insulin secretory response by the tumor have not been investigated. This study undertook analysis of a naturally-occurring feline insulinoma with these objectives: 1) Determine the pattern of hormone expression exhibited by the tumor; 2) Characterize tumor expression of select genes with important roles in glucose recognition and insulin secretion.
A pancreatic tumor obtained at the time of surgical resection from a cat with clinical signs and laboratory evidence of hypoglycemia and hyperinsulinemia was used for these studies. The clinical diagnosis of insulinoma was confirmed by histologic examination of the tumor. Normal (non-neoplastic) pancreatic tissue from the same cat served as control tissue for gene expression studies. Expression of select pancreatic peptide hormones was determined using immuno-histochemistry (IHC). Expression of GLUT2 (glucose transporter isoform 2), INS (insulin), HK1 (hexokinase 1), and GCK (glucokinase) genes was determined in tumor and control pancreas by routine RT-PCR and quantitative RT-PCR (qRT-PCR).
The tumor stained positive for chromogranin A and insulin. About 5% of tumor cells stained positive for somatostatin. Tumor staining for glucagon and pancreatic polypeptide was negative. GLUT2, INS, HK1, and GCK expression was detected by RT-PCR in tumor and in control pancreas. Quantitative expression analysis using qRT-PCR showed that tumor GCK expression was ~20-fold higher than in normal pancreas, while HK1 expression was equivalent in both tissues. The relative ratio of GCK to HK1 (GCK:HK1 ratio) was >20-fold higher in tumor than in normal pancreas.
In conclusion, the study results add to current understanding of feline insulinoma and offer new insights into the functional activity of these tumors. The most important finding is that GCK, a key gene involved in glucose recognition, is overexpressed in feline insulinoma. GCK overexpression is predicted to enhance glucose sensitivity in tumor cells, which may partly explain persistent insulin secretion despite hypoglycemia in cats with insulinoma.