DIFFERENTIAL REGULATION OF MULTIPLE GAP JUNCTION TRANSCRIPTS AND PROTEINS DURING RAT-LIVER REGENERATION

The mRNA and protein expression of alpha1 (connexin 43), beta1 (connexin 32), and beta2 (connexin 26) gap junction genes were examined in the regenerating rat liver after 70% partial hepatectomy (PH). Expression of beta1 and beta2 steady-state mRNA levels changed minimally until 12 h after PH when both transcripts decreased to approximately 15% of baseline values. A similar decrease in assembled connexin levels was detected by immunoblot and indirect immunofluorescence at 18 h after PH. Both transcripts simultaneously increased between 24 and 42 h and again rapidly decreased by 48 h post-PH. Beta1 and beta2 assembled gap junction protein expression increased at 48 h post-PH and rapidly decreased by 56 h. By 72 to 84 h post-PH, beta1 and beta2 mRNA and assembled protein expression returned to near baseline levels and were maintained. Interestingly, inhibition of protein synthesis with cycloheximide completely inhibited disappearance of the beta2 transcript, in contrast to beta1 mRNA which was unaffected. Nuclear run-on assays showed no change in transcriptional rates for either gene during the regenerative period. However, both beta1 and beta2 transcripts exhibited significantly decreased mRNA half-lives at 12 h post-PH (3.8 and 3.7 h, respectively) relative to those at 0 h (10.9 and 6.1 h, respectively). Surprisingly, although the transcriptional rate for alpha1 was similar to that observed for beta2, no alpha1 transcripts were detectable by northern or RNase protection analysis. The results suggest that in the regenerating rat liver, beta1 and beta2 gap junction genes are not regulated at the transcriptional level. Rather, the cyclical modulation of their steady-state transcripts is regulated primarily by posttranscriptional events of which mRNA stability is at least one critical factor in the control process.