Induction of cytochrome P-4502B1-related mouse cytochrome P-450 and regulation of its expression by epidermal growth factor/transforming growth factor alpha in primary hepatocyte culture.

Phenobarbital-dependent induction of mouse cytochrome P-450 (Cyp) orthologous to rat CYP2B1 and its modulation by hepatotrophic growth factors were examined in primary hepatocyte cultures. Compared to rat hepatocytes, induction in mouse hepatocytes was more rapid and effective. Ligands of the EGF receptor, epidermal growth factor, and transforming growth factor alpha inhibited induction on the basis of protein expression and CYP2B-associated 7-pentoxyresorufin-O-depentylase activity. Furthermore, EGF led to repression of accumulation of corresponding mRNA under phenobarbital, an effect not blocked by inhibition of protein synthesis under cycloheximide. Ligands of the EGF receptor may contribute towards the decrease in hepatic CYP expression observed during (pre)neoplastic development and regeneration.

Ultrastructural localization of laminin subunits during the onset of mesoderm formation in the mouse embryo.

Using ultrastructural immunogold histochemistry on LR-Gold-embedded 6- and 7-day-old mouse embryos we investigated the appearance of the A- and B1-chains of the laminin molecule during mesoderm formation. With the help of antibodies against the A-chain and the E4 fragment of the B1-chain of the laminin molecule we were able to detect the subunits in vivo. Staining for the E4 fragment of the short arm of the laminin molecule from day 6 was negative. In contrast, strong staining for the A-chain of laminin was observed. Our results show, that the A-chain of laminin appears before the B1-chain in the 6-day-old mouse embryo before a basement membrane is seen between the ectodermal and entodermal cell layers. Furthermore, the staining pattern indicates, that the laminin molecule changes its orientation in the basement membrane of the ectoderm during mesoderm formation. On day 7 staining for the A-chain of laminin and for the E4 fragment was seen in a random distribution throughout the entire basement membrane, whereas in areas were the onset of mesoderm formation was taking place, the E4 fragment was restricted to the edge of the disintegrating basement membrane.

Regulation of cytochrome P-450 CYPIA1 gene expression and proto-oncogene expression by growth factors in primary hepatocytes.

The effect of growth factors on the cytochrome P-450 (CYPIA1) gene expression was studied in primary mouse hepatocytes. Of the three growth factors used, i.e. epidermal growth factor (EGF), transforming growth factor alpha (TGF alpha) and insulin, only EGF or TGF alpha completely blocked CYPIA1 expression in the presence of the CYPIA1 inducer 3-methylcholanthrene (3-MC). This repression was not linked to cell cycle progression of the hepatocyte because insulin was active to induce 'early immediate genes' and DNA replication as well as EGF/TGF alpha but failed to suppress CYPIA1 expression. A specific EGF/TGF alpha receptor-mediated function may repress CYPIA1 gene expression and contribute to the acquisition of a xenobiotic drug resistance phenotype.