A rat liver gap junction (GJ) cDNA probe that detects mRNA encoding the 32 Kd GJ-protein (connexin 32) was employed to study GJ-protein gene expression in rat liver tumors induced by a single exposure to diethylnitrosamine (DEN) followed by exposure to 2-acetylaminofluorene (AAF)/CCl.,/AAF or induced by systemic administration of N-ethyl-N-hydroxyethylnitrosamine (EHEN). All carcinomas generated by these carcinogens showed markedly reduced levels of GJ-protein mRNA. This may indicate that GJ-protein levels and gapjunctional intercellular communication (GJIC) capacity are also severely compromised. Moreover, all hyperplastic nodules also showed a reduced level of GJ-protein mRNA. Taken together with our earlier finding that the liver tumor promoter phenobarbital inhibits GJ-protein gene expression, these results suggest that deranged GJIC is a relatively early event in liver multistage carcinogenesis. A range of other cDNA probes was also used to characterize gene expression in the DEN-induced tumors. Induction of expression was seen for glutathione S-transferase (placental form) (GST-P), y-glutamyltranspep tidase (GGT), and c-raf but not for c-Ha-ras or c-myc.
Key words. diethylnitrosamine, GJ-protein mRNA, cDNA probes, carcinomas, carcinogens
From the time of discovery of the gap junction (GJ) as a channel providing direct cell-cell exchange of ions and low MW molecules, it has become clear that this intercellular communication (GJIC) system is intimately involved in the basic processes of tissue differentiation and homeostasis [1,2]. This has been due in part to the observations of decreased GJ number or function in various cancers [ 2 4 ] . More recently, reports that certain tumor promoters inhibit GJIC or reduce GJ number have led to the hypothesis that tumor development may occur if there is disturbance of GJIC Abbreviations used. AAF, 2-acetylaminofluorene; DEN, diethylnitrosamine; EHEN, N-ethyl-N-hydroxyethylnitrosamine; GGT, y-glutamyltranspeptidase; GJ, gap junction; GJIC, gapjunctional intercellular communication; GST-P, glutathione S-transferase (placental form).