CYCLIN A(2) AND C-MYC MESSENGER-RNA EXPRESSION IN ETHINYL ESTRADIOL-INDUCED LIVER PROLIFERATION

The time-course of c-myc and cyclin A(2) mRNA expression was determine d in the liver of male Sprague-Dawley rats during transient liver cell proliferation induced by a single dose of ethinyl estradiol (EE), and was compared to that during liver regeneration following two-thirds h epatectomy (PH). Cell proliferation was assessed in terms of 5'-bromod eoxyuridine (BrdU) labeling. EE administration and PH both increased B rdU labeling between 18 and 48 h, with peak values at 18 and 24 h. An early (2 h) increase in BrdU labeling was observed after EE but not PH . Maximal increases in cyclin A(2) mRNA levels and BrdU labeling coinc ided after both EE and PH, and cyclin A(2) mRNA expression was proport ional to the intensity of the proliferative response. In contrast, the degree of c-myc mRNA expression was similar after EE administration a nd PH, but the time course was different: c-myc gene expression rose c oncomitantly with DNA replication after EE, while after PH it increase d during the prereplicative phase. This indicates that the pattern of c-myc gene expression in the liver is strongly related to the type of proliferative response. (C) 1998 Elsevier Science Ireland Ltd. All rig hts reserved.