Transcriptional rate and steady‐state changes of retinoblastoma mRNA in regenerating rat liver

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 Bg1II‐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‐β1, a known inhibitor of hepatocyte replication, just before partial hepatectomy caused no significant change in the modulation of the transcripts through 24 hr. The results indicate that the steady‐state expression of the 2.8‐ and 4.7‐kb retinoblastoma mRNA transcripts in the regenerating rat liver is regulated predominately as an intranuclear event. However, in the absence of functional analysis, the significance of these findings in relation to the growth characteristics of the regenerating liver after partial hepatectomy remains unanswered. (HEPATOLOGY 1994;19:1214–1222.)