Distribution of fibronectin and fibronectin-binding proteins, AGp110 and integrin α5β1, during chemically induced hepatocarcinogenesis in adult rats

Abstract

We have used single- and double-label immunocytochemistry to examine the distribution of AGp110, integrin α5β1 and fibronectin in adult rat liver during carcinogenesis induced by aflatoxin B1 or diethylnitrosamine. In normal liver fibronectin and the fibronectin integrin receptor α5β1 are localized on all three domains of the parenchymal cell surface: sinusoidal, lateral and canalicular. In contrast, AGp110, a non-integrin monomeric glycoprotein with fibronectin receptor properties, is confined to the bile canalicular (apical) plasma membrane of hepatocytes. Hepatocarcinogenesis induced by anatoxin B1 causes altered cell foci to form in the parenchyma, followed by enlargement of these foci to form pre-neoplastic nodules and finally hepatocellular carcinomas of either poorly differentiated, trabecular or adenocarcinoma morphology. Expression of AGp110 decreased to a minimal level, at first selectively in altered cell foci, from the 9th week of treatment, and then indiscriminately in poorly differentiated carcinomas. The same lesions that were deficient in AGp110 also displayed a reduced level of fibronectin and α5β1, although the observed change in AGp110 demarcated altered foci and poorly differentiated tumour lesions more sharply, since expression of α5β1 and fibronectin, though substantially reduced, was still faintly apparent on the cell surface. Small acinar structures, observed in late hyperplastic nodules and in trabecular carcinomas, exhibited even, pericellular staining of fibronectin and α5β1, including prominent staining of the lumen area, whereas staining of AGp110 appeared to be confined to the lumen. In larger ducts of overt adenocarcinomas, fibronectin and α5β1 were distributed along the basal surface of the epithelium and AGp110 on the apical domain. Tumours induced by diethylnitrosamine and promoted with ethinyl estradiol displayed similar histology and staining patterns for all three proteins as that described for aflatoxin B1. Finally, comparisons between AGp110 and cytokeratin 19, a selective tumour marker, indicated that whereas loss of AGp110 occurs in poorly differentiated lesions and tumours, expression of cytokeratin 19 is associated with acinar and glandular structures found in late hyperplasia and with trabecular and pseudoglandular tumours. The results indicate that loss of differentiation in either hyperplastic or neoplastic lesions correlates with reduced expression of fibronectin and of its receptors α5β1 and AGp110. On the basis of morphological similarities and staining patterns in the pre-neoplastic and neoplastic state, we deduce that hepatocellular carcinomas derive from differentiated hepatocytes.