Biosynthesis of complement protein D by HepG2 cells: a comparison of D produced by HepG2 cells, U937 cells and blood monocytes

Critical review of acylation-stimulating protein physiology in humans and rodents

In the last few years, there has been increasing interest in the physiological role of acylation-stimulating protein (ASP). Recent studies in rats and mice, in particular in C3 (-/-) mice that are ASP deficient, have advanced our understanding of the role of ASP. Of note, the background strain of the mice influences the phenotype of delayed postprandial triglyceride clearance in ASP-deficient mice. Administration of ASP in all types of lean and obese mice studied to date, however, enhances postprandial triglyceride clearance. On the other hand, regardless of the background strain, ASP-deficient mice demonstrate reduced body weight, reduced leptin and reduced adipose tissue mass, suggesting that ASP deficiency results in protection against development of obesity. In humans, a number of studies have examined the relationship between ASP, obesity, diabetes and dyslipidemia as well as the influence of diet, exercise and pharmacological therapy. While many of these studies have small subject numbers, interesting observations may help us to better understand the parameters that may influence ASP production and ASP action. The aim of the present review is to provide a comprehensive overview of the recent literature on ASP, with particular emphasis on those studies carried out in rodents and humans.

Synthesis of factor D by gastric cancer-derived cell lines

Synthesis of complement components in vitro by four human gastric cancer-derived cell lines, MKN28, MKN74, MKN45 and KATO-III, was studied. When these cells were cultured for 3 days without addition of any stimulator, 0.94 +/- 0.49, 2.10 +/- 0.59, 7.29 +/- 5.94 and 2.47+/- 1.34 ng of factor D/10(6) cells were detected in supernatants of MKN28, MKN74, MKN45 and KATO-III, respectively. Factor D production by these cells was reversibly inhibited by the presence of cycloheximide. Factors B, C3 and C2 were also detected in protein-free culture medium of these cell lines. Addition of tumour necrosis factor (TNF) to culture enhanced C3 and factor B secretion but depressed C2 secretion, without any distinct effect on factor D secretion. Since all cell lines tested secreted significant amounts of factor D without addition of any stimulator in medium, it is possible that factor D may be synthesized by gastric epithelial cells physiologically and constitutively. From a quantitative analysis of factor D secretion by these cells, factor D secreted by gastric tissue is likely to contribute to the factor D level in circulating blood. The possible mechanism of participation of complement system in inflammation of gastric epithelium was proposed. Thus, the present study may be significant for clarification of the mode of extrahepatic complement synthesis participating in mucosal immunity.

Participation of C3 and its ligands in complement activation

C3, the most abundant complement protein in blood, plays a central role in the activation sequence of the complement system as well as in host defense. Expression of the multiple functions of C3 requires its cleavage by highly specific enzymes termed C3 convertases. C3 in a conformationally altered form, C3H2O, resulting from the slow spontaneous hydrolysis of the internal thioester bond of native C3, initiates the assembly of a C3 convertase which continuously cleaves C3 in the blood at slow rates generating a constant supply of small amounts of C3b. When an activator of the alternative complement pathway is present, C3b becomes covalently attached to its surface via an ester or amide bond. Activator surface-bound C3b initiates the assembly of an "amplification" C3 convertase, C3bBb(P), which can efficiently activate C3 and generate additional convertase complexes on the surface of the activator. C3b generated by an amplification or classical pathway C3 convertase can also bind covalently to the noncatalytic subunit, C3b or C4b, respectively, resulting in the generation of a C5 convertase, an enzyme catalyzing the cleavage/activation of C5. In terms of participation in host defense, several fragments of C3, including C3a, C3b, iC3b, and C3dg, mediate a number of important functions such as increased vascular permeability, enhancement of phagocytosis, elimination of immune complexes, and perhaps also proliferative responses and/or differentiation of B cells.

Metabolism of complement factor D in renal failure

Factor D is an essential enzyme of the alternative pathway of complement. Its plasma concentration increases approximately tenfold in end-stage renal failure (ESRF). To analyze its metabolism in humans, we injected purified radiolabelled factor D into 5 healthy individuals and 12 patients with various renal diseases or renal failure. Fractional metabolic rates (FMR) and extravascular/intravascular distributions (EV/IV) were calculated using a compartmental model. The FMR was very rapid in normal individuals (mean 59.6%/hr; range 74.1 to 50.5), significantly diminished in the five patients with ESRF (5.7%/hr; 7.0 to 2.8; P less than 0.004), and correlated well with the creatinine clearance (r = 0.89; P less than 0.001). The extrarenal catabolic rate was not modified in renal failure. Despite a significant inverse correlation between plasma levels of factor D and creatinine clearance [r = 0.68; P less than 0.002], factor D levels were not a sensitive indicator of renal function because the synthesis rate (SR) varied widely from one individual to another (mean SR: 62.9 micrograms/kg/hr; 14.9 to 136.5). Factor D synthesis was not significantly altered by renal function, and did not correlate with C-reactive protein, suggesting that factor D is not an acute phase protein. The proportion of intact factor D elimination in the urine was increased in patients with tubular dysfunction (up to 15% compared to less than 0.2% in normal individuals) confirming that under normal circumstances factor D is filtered through the glomerulus and catabolized by tubular cells.(ABSTRACT TRUNCATED AT 250 WORDS)