Detection of mRNAs present at low concentrations in rat liver by in situ hybridization: application to the study of metabolic regulation and azo dye hepatocarcinogenesis.

In situ hybridization on tissue sections was used to detect mRNAs present at low concentrations during metabolic adaptation and azo dye carcinogenesis in rat liver. The method consisted of hybridizing the slices at relatively high stringency with [35S]-labeled single-stranded probes derived from cDNA insert clones into the M13 phage. L-pyruvate kinase mRNA was proved to be present at very low concentrations in hepatocytes of fasted rats and to be relatively abundant in all hepatocytes after 18 hr of refeeding on a carbohydrate-rich diet. Aldolase A mRNA concentrations have been previously shown to increase markedly in liver of 3'-methyl DAB-fed rats, with a maximum at the fourth week. We demonstrate here, using our in situ hybridization technique, that this phenomenon is not due to re-expression of this "fetal marker" in hepatocytes but to its abundancy in proliferating small cells (i.e., so-called oval and transitional cells). Small amounts were also detected in sinusoidal cells. In normal liver, aldolase A mRNAs were detected only in some sinusoidal cells.