The inhibitory effect of ghrelin on sepsis-induced inflammation is mediated by the MAPK phosphatase-1

Hepatocellular dysfunction occurs early in sepsis and this appears to be caused by Kupffer cell-derived TNF-α production from the liver as a result of the increased release of the sympathetic neurotransmitter, norepinephrine, from the gut. Ghrelin, a novel stomach-derived peptide, is down-regulated in sepsis and administration of ghrelin into rodents decrease pro-inflammatory cytokines, attenuates hepatic and other organ injuries and improves survival. Ghrelin's beneficial effect in sepsis is mediated by the inhibition of the sympathetic nervous system (SNS), as evidenced by the reduced gut-derived norepineprine (NE) release in sepsis after ghrelin treatment. Recent data suggest that MKP-1, the MAPK phosphatase-1, is involved in the innate immune responses. To determine that the beneficial effect of ghrelin in sepsis is mediated by MKP-1, rats were subjected to sepsis by cecal ligation and puncture (CLP) alone, or treated with ghrelin, beginning at 5-h post-CLP and liver tissues were harvested and examined for MKP-1 mRNA and protein expression. CLP alone produced a significant decrease in MKP-1 gene expression in liver tissues at 20 h after CLP (P<0.05). MKP-1 mRNA was decreased by 30-40% at 2 and 5 h after CLP, but not statistically significant. MKP-1 protein expression was significantly decreased as early as 2 h after CLP and remained low at 5-20 h after CLP. While septic rats treated with vehicle produced significant decreases from sham rats, ghrelin treatment improved both mRNA and protein from vehicle group (0.58±0.069 vs. 0.91±0.16, P<0.05; 0.14±0.027 vs. 0.22±0.017, P=0.013), respectively. Since ghrelin's inhibitory effect is mediated by the SNS, we hypothesized that NE treatment in Kupffer cells may downregulate MKP-1. Kupffer cells were treated with NE and examined for MKP-1. Treatment with NE for 60 min showed an average of 46.9% decrease in MKP-1 mRNA expression compared to untreated cells (P<0.001). Likewise, NE treatment in RAW264.7 cells produced significantly lower MKP-1 mRNA than that of control cells. To further confirm the effect of NE on MKP-1, normal rats were infused with NE for 2 h through the portal vein and MKP-1 mRNA from the liver was examined. Infusion with NE produced a significant 73.7% decrease in MKP-1 mRNA. Therefore, ghrelin's inhibitory effect on gut-derived NE release in sepsis leading to the downregulation of pro-inflammatory cytokines is mediated by MKP-1.