This study characterizes the mRNA expression of the retinoblastoma tumor suppressor gene in regenerating rat liver during 96 hr after 70% partial hepatectomy. A 960-bp BglII-OxaNI fragment of murine retinoblastoma cDNA was used to probe Northern blots of poly(A)(+)-enriched RNA isolated from regenerating liver. Two species of retinoblastoma mRNA, 2.8 kb and 4.7 kb long, were identified in control liver and exhibited an intensity ratio of 5:1, respectively. Expression of the 2.8-kb mRNA was reduced by 50% 1 hr after partial hepatectomy and was less than 10% of control values by 3 hr. The transcript began to reappear at 12 hr and returned to near-baseline levels by 24 hr. In contrast to the rapid disappearance of the 2.8-kb transcript, expression of the 4.7-kb mRNA increased 15-fold by 6 hr and returned to control levels by 18 hr after partial hepatectomy. Pretreatment of the animals with cycloheximide before partial hepatectomy completely stabilized steady-state levels of both mRNA transcripts through 6 hr. Nuclear run-on assays revealed a sixfold increase in transcription by 30 min and a return to near-baseline levels by 6 hr. The in vivo half-lives of the 2.8- and 4.7-kb transcripts in control livers were 39.5 and 41.2 min, respectively. The half-life of the 4.7-kb transcript 6 hr after partial hepatectomy was 39.1 min. Intravenous administration of transforming growth factor-beta 1, a known inhibitor of hepatocyte replication, just before partial hepatectomy caused no significant change in the modulation of the transcripts through 24 hr.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals Gene Expression Regulation