Insulin inhibits hepatocyte iNOS expression induced by cytokines by an Akt-dependent mechanism

Abstract

Harbrecht BG, Nweze I, Smith JW, Zhang B. Insulin inhibits hepatocyte iNOS expression induced by cytokines by an Akt-dependent mechanism. Am J Physiol Gastrointest Liver Physiol 302: G116-G122, 2012. First published October 28, 2011 doi:10.1152/ajpgi.00114.2011.-Hepatocyte inducible nitric oxide synthese (iNOS) expression is a tightly controlled pathway that mediates hepatic inflammation and hepatocyte injury in a variety of disease states. We have shown that cyclic adenosine monophosphate (cAMP) regulates cytokine-induced hepatocyte iNOS expression through mechanisms that involve protein kinase B/Akt. We hypothesized that insulin, which activates Akt signaling in hepatocytes, as well as signaling through p38 and MAPK p42/p44, would regulate iNOS expression during inflammation. In primary rat hepatocytes, insulin inhibited cytokine-stimulated nitrite accumulation and iNOS expression in a dose-dependent manner. Inhibition of MAPK p42/p44 with PD98059 had no effect on iNOS activation, whereas SB203580 to block p38 reversed insulin's inhibitory effect. However, insulin did not increase p38 activation and inhibition of p38 signaling with a dominant negative p38 plasmid had no effect on cytokine-or insulin-mediated effects on iNOS. We found that SB203580 blocked insulin-induced Akt activation. Inhibition of Akt signaling with LY294002 or a dominant negative Akt plasmid increased cytokine-stimulated nitrite production and iNOS protein expression and blocked the inhibitory effects of insulin. NF-B induces iNOS expression and can be regulated by Akt, but insulin had no effect on cytokine-mediated IB␣ levels or NF-B p65 translocation. Our data demonstrate that insulin inhibits cytokine-stimulated hepatocyte iNOS expression and does so through effects on Aktmediated signaling. nitric oxide; nitric oxide synthase; sepsis; inflammation; liver HEPATIC DYSFUNCTION WAS IDENTIFIED decades ago as a component of the multiple organ failure syndrome (10). Liver dysfunction can be manifested in a variety of ways and is associated with increased morbidity and mortality in critically ill patients (13,18,25). Nitric oxide synthesis is an important pathway in the host's response to inflammation and mediates hepatic dysfunction and hepatic injury in models of shock and organ failure (19,28). Proinflammatory cytokines stimulate hepatic nitric oxide production, and we have shown that glucagon and cyclic AMP inhibit hepatocyte inducible nitric oxide synthase (iNOS) expression (14 -16). The role of other counterregulatory mediators produced in shock and sepsis on iNOS regulation remain incompletely studied (8). Glucagon and cAMP regulate hepatocyte iNOS expression through intracellular signaling pathways involving 20,44). Our laboratory and others have demonstrated that cAMP activates protein kinase B/Akt in hepatocytes (9,27,43,45). Akt-mediated signaling changes have profound effects on hepatocyte function and physiology (9, 27, 43), and we have shown that the cAMP-induced changes in iNOS expression are mediated in part through Akt-induce...

Zinc deficiency exacerbates while zinc supplement attenuates cardiac hypertrophy in high-fat diet-induced obese mice through modulating p38 MAPK-dependent signaling

“…In line with the above observations, the inhibition of p38 MAPK also suppressed the expression of NF-κB, GATA4, and MEF2. Previous studies have shown that Akt activation caused cardiac hypertrophy in HFD feeding (Unsold et al, 2015, Zeidan et al, 2011, and SB203580 at a higher concentration not only influenced the p38 MAPK function, but also affected Akt function to certain degree (Harbrecht et al, 2012, Lali et al, 2000. However, in the present study, we found that Akt phosphorylation levels were not affected by SB203580, confirming that Akt signaling was not involve in the induction of pathological cardiac hypertrophy in HFD/obesity mice.…”

Resveratrol decreases nitric oxide production by hepatocytes during inflammation

“…Hepatocyte viability was assessed using the 3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay (14). …”

“…RSV has been shown to activate several of these intracellular signaling pathways, including Akt (6, 8, 17). Since RSV activates Akt, and we know that Akt downregulates iNOS expression and activation (14, 15), we hypothesized that RSV would decrease inflammation-induced hepatocyte iNOS activation.…”

Calcium-mediated signaling and calmodulin-dependent kinase regulate hepatocyte-inducible nitric oxide synthase expression

“…Cell viability was measured using the 3-(4,5 dimethlythiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (MTT) (25). After the indicated incubation, media was replaced with 5 mg MTT/ml in 70% ethanol diluted 1:50 with culture media, incubated for 30 minutes, removed, and 0.5 ml DMSO added.…”