Angiotensinogen Gene Activation by Angiotensin II Is Mediated by the Rel a (Nuclear factor-κB p65) Transcription Factor: One Mechanism for the Renin Angiotensin System Positive Feedback Loop in Hepatocytes

The renin-angiotensin system controls blood pressure through the enzymatic production of the vasopressor angiotensin II (All) from the angiotensinogen (AGT) precursor. Intravascular All production stimulates de novo synthesis of its precursor in a positive feedback loop through increased gene expression. In this study, we investigate the effects of All on AGT gene expression. At nanomolar concentrations, All activates transcription of the native AGT gene; this region is mapped to the AGT gene multihormone-inducible enhancer (-615 to -470). Within the multihormone-inducible enhancer, site-directed mutations of the acute-phase response element (APRE) that interfere with nuclear factor-κB (NF-κB) transcription factor binding also abolish All responsiveness. The APRE functions as a rapidly inducible All-inducible enhancer with peak reporter activity detected after a 4-h stimulation; this effect occurs only when the type 1 All receptor is expressed. All induces sequence-specific NF-κB binding to the APRE in HepG2 nuclear extracts. Moreover, All infusions of primary rat hepatocyte cultures produces a rapid 4-fold increase in sequence-specific NF-KB binding to the APRE. Antibodies against the transcriptional activator sub-unit, Rel A, quantitatively supershift the nucleoprotein complex, whereas antibodies to other NF-κB members do not, demonstrating that Rel A APRE-binding activity is All-inducible. Transient overexpression of Rel A(1-551) activates the AGT multihormone-inducible enhancer. All-inducible domains of Rel A were mapped by cotransfecting a chimeric GAL4-Rel A fusion protein with a reporter gene containing GAL4-binding sites. GAL4-Rel A(1-551) was an All-inducible transactivator. Deletion of the NH₂-terminal 254 amino acids of Rel A produces a constitutive transactivator, indicating that Rel A is activated by All in a manner dependent on its NH₂ terminus. These studies define one mechanism for the renin-angiotensin system positive feedback loop in hepatocytes.