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

The most important specific regulatory mechanism for hepatic angiotens inogen synthesis and secretion is its stimulation by angiotensin II, t he effector peptide of the renin-angiotensin system. In the circulatin g system, this octapeptide is thought to stimulate hepatic angiotensin ogen synthesis through a positive feedback loop. In the present study, we have identified the intracellular mechanisms leading to an increas e in angiotensinogen messenger RNA (mRNA) and secretion. In a [H-3]uri dine-dependent pulse and chase system as well as in hepatocytes in whi ch 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 angi otensin II on the transcription of angiotensinogen mRNA could be obser ved in a nuclear run-on assay with nuclei from pretreated hepatocytes, whereas dexamethasone, as a positive control, increased the transcrip tion fivefold to sevenfold. We have isolated a 12-kD protein from the polysomal fraction of isolated hepatocytes, which has an affinity to t he nontranslated 3' tail of angiotensinogen mRNA. For in vitro transcr iption of this mRNA fragment, the DNA sequence coding for the nontrans lated 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 cytosol ic protein that becomes phosphorylated by decreased cyclic AMP concent rations as analyzed in [P-32]orthophosphate-loaded hepatocytes. In a c ytosolic incubation system in which the polysomal fraction was integra ted, the mRNA-binding protein increased the half-life of angiotensinog en mRNA significantly. Thus, there is evidence that the angiotensin II -induced stimulation of hepatic angiotensinogen synthesis depends on m RNA stabilization.