MECHANISM BY WHICH ANGIOTENSIN-II STABILIZES MESSENGER-RNA FOR ANGIOTENSINOGEN

The most important specific regulatory mechanism for hepatic angiotensinogen synthesis and secretion is its stimulation by angiotensin II, the effector peptide of the renin-angiotensin system. In the circulating system, this octapeptide is thought to stimulate hepatic angiotensinogen synthesis through a positive feedback loop. In the present study, we have identified the intracellular mechanisms leading to an increase in angiotensinogen messenger RNA (mRNA) and secretion. In a [H-3]uridine-dependent pulse and chase system as well as in hepatocytes in which de novo synthesis of mRNA has been blocked by actinomycin D or 5,6-dichlorobenzimidazole riboside, angiotensin II significantly increased the half-life of angiotensinogen mRNA. In contrast, no effect of angiotensin II on the transcription of angiotensinogen mRNA could be observed in a nuclear run-on assay with nuclei from pretreated hepatocytes, whereas dexamethasone, as a positive control, increased the transcription fivefold to sevenfold. We have isolated a 12-kD protein from the polysomal fraction of isolated hepatocytes, which has an affinity to the nontranslated 3' tail of angiotensinogen mRNA. For in vitro transcription of this mRNA fragment, the DNA sequence coding for the nontranslated 3' tail was excised from the vector pRAG 16 and cloned into the transcription vector pGEM 5zf+. Molecular weight and isoelectric point of the mRNA-binding protein correspond to the parameters of a cytosolic protein that becomes phosphorylated by decreased cyclic AMP concentrations as analyzed in [P-32]orthophosphate-loaded hepatocytes. In a cytosolic incubation system in which the polysomal fraction was integrated, the mRNA-binding protein increased the half-life of angiotensinogen mRNA significantly. Thus, there is evidence that the angiotensin II-induced stimulation of hepatic angiotensinogen synthesis depends on mRNA stabilization.