Protein expression of the retinoblastoma (Rb) tumor suppressor gene product was examined by immunoblot analysis of nuclei isolated from regenerating rat liver after 70% partial hepatectomy (PH). Levels were almost undetectable in quiescent 0-h livers but increased 15- to 60-fold 3 to 24 h post-PH, 105-fold at 30 h, and 20- to 50-fold at 60 to 72 h post-PH. Expression returned to near baseline levels at 18, 42, and 48 h post-PH. A similar pattern of Rb protein expression in the regenerating liver was observed by indirect immunofluorescence microscopy, with peak nuclear expression at 30 h post-PH. Rb-related proteins with apparent molecular masses of 300, 156, and 74 kDa were detected in regenerating liver using mAbs to the Rb protein. Their expression increased 6- to 8-fold during regeneration, and only p156 returned to baseline levels at 60 h post-PH. Rb and its related proteins were detected in cultured primary hepatocytes, and although total protein levels did not change appreciably, there was a dramatic shift from cytosol into nuclei through 96 h. The half-life of the Rb protein was determined to be 1.9 h in regenerating liver and 2.2 h in cultured primary hepatocytes. Rb protein abundance in synchronized HUH-7 human hepatoma cells was cell cycle dependent and exhibited peak nuclear expression during S phase. Rb protein was detected primarily in its hyperphosphorylated state during liver regeneration and through the cell cycle of the HUH-7 cells. In vivo administration of transforming growth factor β1, an inhibitor of DNA synthesis in regenerating liver, resulted in reduced expression of Rb as well as its protein partners, cell cycle- dependent kinase 4 and cyclin E. The results suggest that in the regenerating rat liver and in synchronized HUH-7 cells, expression of Rb protein is modulated in a cell cycle-dependent fashion, remains primarily in a hyperphosphorylated state, and exhibits a relatively short half-life. The inhibition of Rb protein expression by transforming growth factor β1 may be linked to its simultaneous suppression of cell cycle-dependent kinase 4 and cyclin E protein levels.
|Original language||English (US)|
|Number of pages||14|
|Journal||Cell Growth and Differentiation|
|State||Published - Jan 1 1995|