Analysis of the IGF axis in preneoplastic hepatic foci and hepatocellular neoplasms developing after low-number pancreatic islet transplantation into the livers of streptozotocin diabetic rats

Abstract

Preneoplastic hepatic foci have been demonstrated in liver acini, which drain the blood from intraportally transplanted pancreatic islets in streptozotocin-induced diabetic rats with mild persisting diabetes. In long-term studies of this animal model, hepatocellular adenomas and carcinomas (HCC) developed after a sequence of characteristic preneoplastic hepatic foci. In this experimental model, the local hyperinsulinism is thought to have a causative role. Because insulin and the insulin-like growth factor (IGF) axis are closely linked, an altered gene expression of the IGF axis components is likely. Therefore, preneoplastic hepatic foci and HCC were studied for the expression of IGF axis components. Glycogen-storing "early" preneoplastic hepatic foci were detectable several days after pancreatic islet transplantation. Northern blot analysis, in-situ hybridization, and immunohistochemical studies of these "early" lesions demonstrated increased expressions of IGF-I and IGF binding protein-4 (IGFBP-4) in altered parenchymal cells, and a decreased expression of IGFBP-1. IGF-II was not detected in these preneoplastic foci. HCC arising in this model had decreased expressions of IGF-I and IGFBP-4 but IGFBP-1 expression was not significantly altered. Some HCC showed a more than 100-fold overexpression of IGF-II, whereas other tumors were completely negative for IGF-II expression. Low IGF-I receptor expression was detected in preneoplastic foci and adjacent nonaltered liver tissue. However, HCC tissue consistently showed an increased IGF-I receptor expression, rendering these tissues susceptible to the mitogenic effects of IGF. The altered gene expression in glycogen-storing preneoplastic hepatic foci, especially the up-regulation of IGF-I and IGFBP-4 with the down-regulation of IGFBP-1, resemble the insulin-dependent regulation of these components in normal rat hepatocytes. These data agree with previous studies demonstrating a correspondence of the focal character, morphology, and enzyme pattern of preneoplastic hepatic foci with insulin effects on hepatocytes. The development from preneoplastic foci to HCC may be driven by insulin itself and/or an altered IGF axis component or yet unidentified factors.