NOX2 activated by α₁-adrenoceptors modulates hepatic metabolic routes stimulated by β-adrenoceptors

Abstract

The NADPH oxidase (NOX) family of enzymes oxidase catalyzes the transport of electrons from NADPH to molecular oxygen and generates O₂^•−, which is rapidly converted into H₂O₂. We aimed to identify in hepatocytes the protein NOX complex responsible for H₂O₂ synthesis after α₁-adrenoceptor (α₁-AR) stimulation, its activation mechanism, and to explore H₂O₂ as a potential modulator of hepatic metabolic routes, gluconeogenesis, and ureagenesis, stimulated by the ARs. The dormant NOX2 complex present in hepatocyte plasma membrane (HPM) contains gp91^phox, p22^phox, p40^phox, p47^phox, p67^phox and Rac 1 proteins. In HPM incubated with NADPH and guanosine triphosphate (GTP), α₁-AR-mediated H₂O₂ synthesis required all of these proteins except for p40^phox. A functional link between α₁-AR and NOX was identified as the Gα₁₃ protein. Alpha₁-AR stimulation in hepatocytes promotes Rac1-GTP generation, a necessary step for H₂O₂ synthesis. Negative cross talk between α₁-/β-ARs for H₂O₂ synthesis was observed in HPM. In addition, negative cross talk of α₁-AR via H₂O₂ to β-AR-mediated stimulation was recorded in hepatocyte gluconeogenesis and ureagenesis, probably involving aquaporine activity. Based on previous work we suggest that H₂O₂, generated after NOX2 activation by α₁-AR lightening in hepatocytes, reacts with cAMP-dependent protein kinase A (PKA) subunits to form an oxidized PKA, insensitive to cAMP activation that prevented any rise in the rate of gluconeogenesis and ureagenesis.

Keywords::

• H₂O₂
• adrenaline
• gluconeogenesis
• ureagenesis
• NADPH oxidase

This paper was first published online on Early Online on 25 October 2011.