An L-arginine-dependent mechanism mediates kupffer cell inhibition of hepatocyte protein synthesis in vitro

Abstract

The hepatic failure associated with severe sepsis is characterized by specific, progressive, and often irreversible defects in hepatocellular metabolism (1). Although the etiologic microbe can often be identified, the direct causes and mechanisms of the hepatocellular dysfunction are poorly understood . We have hypothesized that Kupffer cells (KC), which interact with ambient septic stimuli, respond by providing signals to adjacent hepatocytes (HC) in sepsis . Furthermore, we have provided evidence (2, 3) that KC activated by LPS from Gram-negative bacteria can induce profound changes in the function of neighboring HC in coculture. In our model, coculture of either KC (2) or peritoneal macrophages (MO) (3) with HC normally promotes HC protein synthesis ([ 3 H]leucine incorporation) . The addition of LPS or killed Escherichia colt' to such cocultures induces a profound decrease in HC protein synthesis, as well as qualitative changes ([358]methionine, SDS-gel electrophoresis) in protein synthesis without inducing HC death (2, 3) . In this report we show that the inhibition in protein synthesis is mediated via an L-arginine-dependent mechanism.The metabolism of L-arginine by activated Mo to substances with cytostatic and even lethal effects on target cells is a relatively recent discovery. After the description by Stuehr and Marletta (4, 5) that LPS-triggered MO produced nitrite/nitrate (N02-/NO3-), Hibbs et al. (6,7) and Iyengar et al. (8) demonstrated that L-arginine was the substrate for the formation of both these nitrogen end products and citrulline. A role for the arginine-dependent mechanism in MO tumor cytotoxicity (6, 7) and microbiostatic activity (9) has been suggested. However, the in vivo functions of this novel MO mechanism have not yet been defined, but it is possible that there are both physiologic as well as pathologic roles. Our in vitro results raise the possibility that some metabolic responses to microbial invasion may be partially mediated by the L-arginine-dependent mechanism. What other metabolic responses are affected and the possible pathologic consequences remain to be studied.

Increased sensitivity to endotoxemia in the bile duct-ligated cirrhotic rat

“…27 Several studies have shown that both Kupffer cells and hepatocytes themselves synthesize NO. 28,29 In LPS treated rats portal and systemic plasma nitrite and nitrate levels (considered markers of NO production) are increased over 40-fold compared with normal rats as a result of iNOS induction by LPS. 30 Chronic exposure to sublethal levels of endotoxin, as occurs FIG.…”

Gadolinium blocks rat kupffer cell calcium channels: Relevance to calcium-dependent prostaglandin E2 synthesis and septic mortality

“…24 This possibility has been supported by the recognition of GD' s potent ability to block calcium channels in muscle cells, 26 endothelial cells, 27 and neuroendocrine cells. 28 The ability of the chelating agents, ethylene glycol-bis(␤-aminoethyl-ether)-N,N-tetraacetic acid, 29 and the orthophosphate salts, NaH 2 PO 4 and KH 2 PO 4 , 30 to reverse the GD blockage suggest that GD ions act extracellularly, probably near the mouth of the Ca ϩϩ channel. The failure of lathanide to affect the 340-nm and 380-nm excitation of fura-2-loaded cells also strongly suggested an extracellular effect.…”

“…1,7 Secondly, KC release tumor necrosis factor ␣ (TNF-␣) and prostaglandin E 2 (PGE 2 ), among other factors, in response to in vitro stimulation with LPS. 3,4,8,9 In addition, elevated blood levels of TNF-␣ 10 and enhanced monocyte release of PGE 2 11-13 characterize the human septic response. The critical role of TNF-␣ in the experimental septic response has been supported by the prevention of septic mortality with anti-TNF-␣ antibodies.…”

“…1 Its diverse functions are similar to those of other macrophage populations, including phagocytosis 1,2 and antigen presentation. 2 In addition, its secretory repertoire in response to lipopolysaccharide (LPS), 3,4 a component of the cell wall of gram-negative bacteria, includes proteolytic enzymes, eicosanoids, monokines, oxygen free radicals, and nitric oxide. Unlike other macrophage populations, KC may be exposed to gastrointestinally derived bacteria and endotoxin under physiological conditions 5 and appear to have adapted to this challenge by internalizing LPS by nonsaturable pinocytosis.…”

Nitric oxide reduces nontransferrin-bound iron transport in HepG2 cells

“…1,2 NO has been implicated in the regulation of a number of cell processes, 3 including the transport of Na ϩ across the cell membranes in proximal renal tubules, 4 and of HCO 3Ϫ and glutathione in the liver. 5 NO also plays an important role in iron homeostasis, mainly by modulating the activity of iron regulatory proteins (IRPs).…”