DIFFERENTIAL REGULATION OF CYCLIN B1 RNA AND PROTEIN EXPRESSION DURING HEPATOCYTE GROWTH IN-VIVO

Cyclin genes and their products are important regulatory participants in the eukaryotic cell cycle. It is well established that cyclin B1 pr otein forms a complex with cyclin-dependent kinase 1 (CDK1), which, wh en activated, initiates mitosis. We have previously established that c yclin BI gene expression is posttranscriptionally regulated in regener ating rat liver after 70% partial hepatectomy (PH). We now report furt her characterization of cyclin B1 gene expression, as well as that of CDK1 and cdc25B, in this unique in vivo model of cell proliferation. C yclin B1 transcripts were detected by RNase protection through 96 h of liver regeneration and exhibited dramatic changes in steady-state lev els. Peak expressions occurred at 24-30 h, more significantly at 42-48 , and at 72 h. By Northern blot analysis, single transcripts for CDK1 and cdc25B were detected, and the temporal expression of both transcri pts during liver regeneration mirrored that of cyclin B1. By Western b lot and immunohistochemical analyses, cyclin B1 protein levels did not change significantly in either nuclear or cytoplasmic fractions, wher eas CDK1 protein levels paralleled their associated RNA expression. Cd c25B protein levels steadily decreased from 0 to 96 h after PH, In add ition, cytoplasmic protein levels of cyclin B1 exhibited a constant di stribution in subfractions of microsome- and polysome-associated and f ree proteins, Cyclin B1 RNA also localized to these three cytoplasmic subfractions. Finally, the apparent translational activity of cyclin B 1 transcripts was very similar at both 24 and 48 h after PH, in contra st to their respective mRNA half-lives, In a peroxisome proliferation model of hepatocyte growth and apoptosis, cyclin B1 and CDK1 proteins were induced in the absence of transcript up-regulation. Our results d emonstrate that cyclin B1 mRNA steady-state levels are regulated postt ranscriptionally in regenerating rat liver. Furthermore, the pattern o f cyclin B1 transcript expression is paralleled by that of the CDK1 ge ne, whereas their respective protein steady-state levels provide a str iking contrast. Finally, cyclin B1 is differentially regulated by an u ncoupling of transcript abundance and translational processing in two in vivo models of hepatocyte growth. The abundance of cyclin B1 protei n in nonreplicating cells suggests that cyclin B1 may be available for other cellular pathways in the hepatocyte.