Acylation-stimulatory activity in hyperapobetalipoproteinemic fibroblasts: enhanced cholesterol esterification with another serum basic protein, BP II

miniSAGE: gene expression profiling using serial analysis of gene expression from 1 microg total RNA

The use of serial analysis of gene expression (SAGE) to determine gene expression profiles is increasing because the technique can provide absolute transcript numbers in a digital format and identify new genes. We developed a miniSAGE technique, which uses only 1 microg total RNA and reduces the amount of the starting material by 250- to 500-fold. Unlike the other modified SAGE methods, the miniSAGE technique does not require the additional PCR amplifications. The additional PCR amplifications potentially introduce bias and compromise the quantitative aspects of the SAGE method. Three key modifications in the miniSAGE technique are: (i) using the phase lock gel (PLG, Eppendorf) to increase the recovery and the purity of DNA material after each phenol extraction step; (ii) reducing the amount of linkers in the ligation, thereby minimizing their interference with SAGE ditag amplification and increasing the SAGE ditag yield; and (iii) employing the mRNA capture kit (Boehringer Mannheim) to allow the first five steps: mRNA isolation, cDNA synthesis, enzyme cleavage of cDNA, binding of the cleaved biotin-cDNA to the streptavidin-magnetic beads, ligating linkers to the bound cDNA, and the release of cDNA tags to occur within one tube to significantly reduce the loss of material between successive steps. Two fibroblast SAGE libraries have been successfully prepared. The preliminary analysis of 3838 tags from one library demonstrated a typical fibroblast gene expression pattern. This miniSAGE technique will permit a broader application of SAGE.

Fasting and postprandial plasma concentrations of acylation-stimulation protein (ASP) in lean and obese Pima Indians compared to Caucasians

OBJECTIVE

ASP stimulates the clearance of free fatty acids (FFA) from the circulation and the synthesis of triglycerides (TG) in adipose tissue. We tested whether fasting and post-prandial plasma ASP concentrations are increased in Pima Indians, a population with a very high prevalence of obesity, but a remarkably low prevalence of dyslipidemia.

RESEARCH METHODS AND PROCEDURES

Plasma concentrations of ASP, TG, FFA, total cholesterol (CHOL), and insulin (INS) were measured in 15 Pima Indians (P) and 15 Caucasians (C) closely matched for age, sex, and body weight [7 lean and 8 obese subjects, body mass index (BMI) cut-off 30 kg/m2], before and for 4 hours after a standard mixed meal (20% of daily caloric requirements, 41% carbohydrate, 44% fat, 15% protein).

RESULTS

Fasting ASP was positively related to percent body fat (dual energy X-ray absorptiometry; r=0.49, p<0.01) and to TG and FFA, independently of percent body fat (partial r = 0.42 and 0.46, respectively, both p<0.05). There were no differences in fasting TG, FFA, CHOL, INS, or ASP between lean C and lean P. In contrast, obese P had lower TG, lower CHOL, higher INS and, on average, 27% lower ASP compared to obese C. The ethnic difference in ASP remained after adjustment for TG, FFA, and percent body fat. ASP decreased in response to the meal in all four groups with no differences between groups. There was a significant inverse correlation between preprandial ASP and the change in FFA 60 minutes after the meal (r=-0.56, p<0.001).

DISCUSSION

Pima Indians do not have higher plasma ASP concentrations than Caucasians. Whether other alterations in the ASP-pathway, such as increased sensitivity of adipocytes to ASP, contribute to the high prevalence of obesity and low prevalence of dyslipidemia in Pima Indians, remains to be elucidated.

A critical evaluation of the putative role of C3adesArg (ASP) in lipid metabolism and hyperapobetalipoproteinemia

The acylation stimulating protein, ASP is a small, basic serum protein capable of stimulating triglyceride synthesis in cultured fibroblasts and adipocytes. Sequence analysis of ASP has shown that ASP is identical to C3adesArg the inactive fragment of the complement anaphylatoxin peptide, C3a. It has been proposed that C3adesArg (ASP) can be generated by mature adipocytes secreting the three complement proteins: complement protein C3, factor B and factor D (adipsin). There have also been indications that adipocytes may express a specific C3adesArg (ASP)-receptor that is distinct from the recently cloned C3a-receptor. This suggests that C3adesArg (ASP) acts as an adipocyte autocrine and that it plays a central role in the metabolism of adipose tissue. Based on these observations a hypothesis for the etiology of hyperapobetalipoproteinemia (hyperapoB) has been proposed. Hyperapobetalipoproteinemia (hyperapoB), is a familial lipoprotein disorder characterized by increased hepatic secretion of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) particles. If C3adesArg (ASP) function in the adipose tissue is impaired, a reduced rate of triglyceride synthesis will follow, generating an increased flux of fatty acids to the liver. In response to an increased flow of fatty acids, the liver will increase its production of VLDL particles yielding the phenotype of hyperapoB. This review critically assesses this hypothesis and the potential role of C3adesArg (ASP) as a major determinant for triglyceride synthesis in the light of data collected in vitro and in vivo.

Haplotypes of the ApoA-I/C-III/A-IV gene cluster and familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is the most frequent familial lipoprotein disorder associated with premature coronary heart disease. However, no genetic defect(s) underlying FCHL has been identified. A linkage between FCHL and the apoA-I/C-III/A-IV gene cluster has been reported but not verified in other populations. A recent study identified FCHL susceptibility haplotypes at this gene cluster. To study whether such haplotypes are also associated with FCHL susceptibility in Finns, we studied 600 well-defined Finnish FCHL patients and their relatives belonging to 28 extended FCHL families by using haplotype, linkage, sib-pair, and linkage disequilibrium analyses. The genotypes of the MspI polymorphisms were associated with total serum cholesterol (P<0.01) and apoB (P<0.05) levels in spouses, which represent the general Finnish population. However, no evidence of direct involvement of any of these loci or their specific haplotypes in the expression of FCHL in the Finnish FCHL families was found.

Plasma acylation stimulating protein (ASP) as a predictor of impaired cellular biological response to ASP in patients with hyperapoB

BACKGROUND

The objective of this study was to examine specific membrane binding of [125I]-acylation stimulating protein (ASP) in cultured human skin fibroblasts obtained from normal subjects and patients with hyperapoB. ASP is a small basic protein isolated from human plasma that stimulates triglyceride synthesis (TGS) and glucose transport (GT) in human skin fibroblasts and adipocytes.

DESIGN

In the present study, three groups were studied: normal (NASP-NB) subjects, hyperapoB subjects with normal plasma ASP (NASP-HB) and hyperapoB subjects with high plasma ASP (HASP-HB).

RESULTS

ASP-induced TGS in fibroblasts from HASP-HB subjects was significantly less than in the two control groups with normal plasma ASP (NASP-NB and NASP-HB). Similarly, ASP stimulation of GT was less in HASP-HB fibroblasts than in the NASP-HB fibroblasts or the NASP-NB subjects. Insulin-induced TGS was similar in all three groups as was insulin-stimulated GT. As well, protein kinase C-mediated stimulation was equivalent among the three groups both for GT and for TGS. There was no significant difference in the binding affinity (Kd) of [125I]-ASP to intact cells in any group. By contrast, binding of [125I]-ASP revealed a significantly lower Bmax of the HASP-HB cell lines than the NASP-NB cells and the NASP-HB cells.

CONCLUSION

A decrease in the ASP cell-surface receptor concentration is responsible for decreased ASP stimulation of TGS, and GT and may contribute to the inefficient postprandial triglyceride (TG) clearance in HASP-HB subjects.

Differential effect of genistein on the stimulation of cholesterol production by basic protein II in normal and hyperapoB fibroblasts

We studied further the basis for the abnormal effect of human serum basic protein II (BP II) on cholesterol production in hyperapobetalipoproteinemia (hyperapoB) fibroblasts and whether this effect involves protein tyrosine kinase phosphorylation (TKP). Genistein, a specific inhibitor of TKP was used as a probe. Compared with normal cells, BP II stimulated significantly the cellular mass of total cholesterol (6.4-fold), unesterified cholesterol (3.6-fold), and esterified cholesterol (6.7-fold) in hyperapoB fibroblasts. The addition of genistein to BP II in hyperapoB cells markedly inhibited these abnormal stimulatory effects of BP II on cell sterol mass. In normal cells, the addition of genistein to BP II produced an opposite effect: a marked stimulation in the mass of total (5.5-fold) and esterified (18.3-fold) cholesterol and a decrease in unesterified cholesterol (3.4-fold). These effects of genistein on the formation of cellular cholesterol by BP II were both time and concentration dependent. The inhibition of the stimulatory effect of BP II on cholesterol production by genistein in hyperapoB cells may be mediated through 3-hydroxy 3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme of cholesterol biosynthesis, since the rate of incorporation of [14C]acetate, but not [3H]mevalonate, into unesterified cholesterol was decreased by genistein in the hyperapoB cells. When the mass of cell total cholesterol in the cells treated with BP II was subtracted from those treated with BP II plus genistein, a negative number was produced in each of the six hyperapoB cell lines, while each of the normal cell lines retained a positive number. The mean difference for the mass of total cholesterol between the hyperapoB and normal fibroblasts under these conditions was 128.2 nmol/mg cell protein, a difference that was separated by >3 SD. This study supports further the tenet that there is a defect in the response of hyperapoB cells to BP II and that this defect results in an abnormality in cholesterol metabolism that appears mediated through a protein TKP-mediated process.

Abnormal protein tyrosine phosphorylation in fibroblasts from hyperapobetalipoproteinemia subjects

The stimulatory effects of three normal human serum basic proteins (BP), BP I (Mr 14,000, pI 9.10), BP II (Mr 27, 500, pI 8.48), and BP III (Mr 55,000, pI 8.73) on cellular triglyceride and cholesterol formation require intact protein-tyrosine kinase phosphorylation (TKP). Here we examined whether there is an abnormality in TKP in cultured fibroblasts from 11 patients with hyperapobetalipoproteinemia (hyperapoB) that manifest two acylation-stimulatory defects, decreased stimulation of triglyceride synthesis by BP I but enhanced formation of cholesterol by BP II. Soluble and insoluble proteins in Triton X-100 extracts were isolated by immunoprecipitation with a monoclonal anti-phosphotyrosine antibody (MAPA) bound to agarose beads and by ultracentrifugation, respectively, from confluent fibroblasts after incubation for 24 h in supplemented serum-free and lipid-free medium (DMEM/F12). Western blots of insoluble proteins showed that group (Gp) II (Mr 36,000-55,000) and Gp III (Mr 14,000-35,000) from hyperapoB cells, grown in DMEM/F12 medium without BP, had significantly decreased reactivity to MAPA. No significant differences in reactivity to MAPA were detected between normal and hyperapoB cells for Gp I (Mr 97-120,000). BP II, but not BP I or BP III, reversed the decreased reactivity of Gp II and Gp III to MAPA in hyperapoB cells. Sodium vanadate, an inhibitor of phosphotyrosine phosphatases, did not reverse the deficiency in TKP or the 50% deficiency in the stimulation of mass triglyceride by BP I in hyperapoB cells. Tyrosine-phosphorylated Erk-2, a mitogen-activated protein kinase, identified as one of the proteins in Gp II, was significantly decreased in hyperapoB cells. These results provide further evidence for abnormal protein TKP in hyperapoB cells and suggest a possible link between atherosclerotic changes in hyperapoB patients and growth factors upstream from mitogen-activated protein kinase.

Phenotype expression in familial combined hyperlipidemia

Familial combined hyperlipidaemia (FCHL) is one of the most common hereditary disorders predisposing to early coronary death. The affected family members have elevations of serum total cholesterol, triglycerides or both. Despite intensive research efforts the genetic and metabolic defects underlying this complex disorder are still unknown. To dissect the metabolism and genetics of FCHL the phenotype of an individual must be precisely defined. We assessed the influence of different diagnostic criteria on the phenotype definition and studied factors affecting the phenotype expression in 16 large Finnish families (n = 255) with FCHL. The fractile cut-points used to define abnormal lipid values had a profound influence on the diagnosis of FCHL. If the 90th percentile cut-point was used, approximately 45% of the family members were affected, in concord with the presumed dominant mode of transmission for FCHL. If the 95th percentile was used only 22% of study subjects were affected. To characterize the metabolic differences or similarities between the different lipid phenotypes, we determined very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL), low density lipoprotein (LDL) and high density lipoprotein (HDL) particles separated by ultracentrifugation. In linkage analysis no single ultracentrifugation variable could discriminate reliably affected family members from non-affected family members. Our data emphasizes the need for re-evaluation of FCHL diagnostic criteria. Preferably, the diagnosis should be based on a single, reliable metabolic marker.

Serum complement and familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is one of the most common inherited lipid disorders. Resistance of adipocytes to the effects of acylation stimulating protein (ASP) may contribute to ineffective triglyceride synthesis and thereby prolonged postprandial lipemia and increased fatty acid flux to the liver seen in FCHL patients. Interestingly, ASP is identical to C3a-desArg, fragment of the third component of complement. We examined the relationships between serum levels of complement components C3 and C4 and markers of lipid and glucose metabolism in 11 large FCHL families (n = 53). Median serum C3 levels were 38% higher in affected compared to non-affected male FCHL family members (1.90 g/l vs. 1.38, P = 0.0027). The strongest correlations were observed between serum complement C3 and apolipoprotein B levels, reaching 0.77 in males. These relations were not confounded by obesity or impaired glucose tolerance. In conclusion, serum levels of the main complement components C3 and C4 correlated significantly with serum lipid levels. Further studies are needed to clarify the importance of disturbances in the complement system on the pathogenesis of FCHL and other lipid disorders.

Inhibition of protein tyrosine kinase alters the effect of serum basic protein I on triacylglycerols and cholesterol differently in normal and hyperapoB fibroblasts

We studied whether the stimulatory effect of human serum basic protein I (BP I) on the formation of cell triacylglycerols and cholesterol may be mediated through protein tyrosine kinase in normal fibroblasts, and whether there was a deficiency in such a process in cells from subjects with hyperapobetalipoproteinemia (hyperapoB). Genistein, a highly specific inhibitor of tyrosine kinase phosphorylation, was used as a probe. When BP I (428.0 nmol/L) alone was added to F-12 medium without genistein, the mean mass of cell triacylglycerols doubled in six normal cell lines from healthy subjects, an effect that was decreased by 50% in six cell lines from subjects with hyperapoB (P = .007). The addition of genistein with BP I to normal cells decreased the stimulation of triacylglycerol formation by BP I by about 50% (P = .008), whereas genistein had little effect in the BP I-treated hyperapoB cells. The effect of genistein on the stimulation of triglyceride and cholesterol production by BP I was shown to be both time and concentration (92.5 nmol/mL medium nadir) dependent. In normal fibroblasts. BP I stimulated the rate of incorporation of both [14C]acetate (P = .0001) and [3H]mevalonolactone (P = .002) into unesterified cholesterol, an effect that was markedly deficient in the hyperapoB cells (P = .0001 for [14C]acetate and P = .0002 for [3H]mevalonolactone). In normal but not hyper-apoB cells, genistein inhibited the significant stimulation by BP I of the rates of both [14C]acetate (P = .0001) and [3H]mevalonolactone (P = .04) incorporation into unesterified cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic predictors of FCHL in four large pedigrees. Influence of ApoB level major locus predicted genotype and LDL subclass phenotype

The genetic basis of familial combined hyperlipidemia (FCHL) has eluded investigators for 20 years, despite the apparent segregation of FCHL as an autosomal dominant disorder affecting 1% to 2% of individuals. Etiologic heterogeneity and additive effects of traits controlled by other genetic loci have been suggested. Two traits have been implicated in FCHL. The first is the predominance of a small, dense low-density lipoprotein (LDL), LDL subclass phenotype B, which segregates as a mendelian trait. The second is a mendelian locus with large effects on apolipoprotein (apo) B levels that is defined by complex segregation analysis (predicted apoB level genotype). This study shows that these factors appear to be separate genetic effects, both of which aid in the prediction of FCHL in four large pedigrees. The results suggest that FCHL may be best predicted by a threshold model in which apoB level genotype and LDL subclass phenotype each act to increase the risk of FCHL. Heterogeneity in the transmission of apoB levels among families is suggested, supporting the etiologic heterogeneity of FCHL. These results emphasize the advantages inherent in the study of large pedigrees when disease heterogeneity is suspected.

The effect of three serum basic proteins on the mass of lipids in normal and hyperapoB fibroblasts

We studied whether serum basic proteins (BPs) produce abnormal changes in the mass of cellular lipids in fibroblasts from patients with hyperapobetalipoproteinemia (hyperapoB) and if inhibition or stimulation of protein kinase C affects these processes. In normal cells, BP I increased the mean mass of triglycerides about twofold, whereas there was significantly less stimulation in hyperapoB cells (P < .005). The increase in the mass of cell cholesteryl esters seen in normal cells with BP I was also significantly reduced in hyperapoB cells (P < .005). In contrast, BP II abnormally stimulated the mass of cell cholesteryl esters sixfold in hyperapoB cells (P < .005). BP I also stimulated the mass of total phospholipids about twofold in normal cells, an effect that was reduced by about one third in hyperapoB cells (P = .08). No abnormality was found in hyperapoB cells with BP III. H-7, an inhibitor of protein kinase C, decreased the effects of BP I and BP II in normal and hyperapoB cells. C:8, an analogue of diacylglycerol, activated protein kinase C and stimulated triglyceride formation in normal (fourfold) and hyperapoB (fivefold) cells in the absence of BP I. When added with C:8, BP I further increased triglyceride production 1.5-fold in normal cells but not in hyperapoB cells. Two cellular abnormalities in lipid metabolism in hyperapoB fibroblasts were found, one with BP I, another with BP II. Protein kinase C activity was not deficient in hyperapoB cells, and the defect(s) may occur at another, perhaps earlier, step in the pathway.

The adipsin-acylation stimulating protein system and regulation of intracellular triglyceride synthesis

We have previously characterized an activity from human plasma that markedly stimulates triglyceride synthesis in cultured human skin fibroblasts and human adipocytes. Based on its in vitro activity we named the active component acylation stimulating protein (ASP). The molecular identity of the active serum component has now been determined. NH2-terminal sequence analysis, ion spray ionization mass spectroscopy, and amino acid composition analysis all indicate that the active purified protein is a fragment of the third component of plasma complement, C3a-desArg. As well, reconstitution experiments with complement factors B, D, and complement C3, the components necessary to generate C3a, have confirmed the identity of ASP as C3a. ASP appears to be the final effector molecule generated by a novel regulatory system that modulates the rate of triglyceride synthesis in adipocytes.

Substrate delivery as a determinant of hepatic apoB secretion

The evidence that apoB particles secreted by the liver can differ in number and composition has been reviewed. No evidence has yet emerged that changes in apoB100 itself affect the rate of its secretion from the liver. The metabolic role of apoB appears to be the prevention of lipid accumulation within the liver cell: when delivery of lipid to the liver increases, apoB secretion will increase pari passu. This reality in no way detracts from the critical role played by the LDL receptor in determining the number of LDL particles in plasma, nor does it diminish the potential importance of intracellular processes such as 7 alpha-hydroxylase activity to also mediate LDL receptor activity. However, it should be obvious that variation in catabolism by itself cannot explain all that has been observed in physiological and pathological studies. On the contrary, the whole process must be taken into account--the rate at which apoB particles are added to the circulation, the rate at which they are converted to LDL, and the rate at which they are irreversibly removed from plasma--if we are to understand and appreciate this most peculiar and most important of transport systems.

Incorporation of oleic acid and eicosapentaenoic acid into glycerolipids of cultured normal human fibroblasts

Confluent skin fibroblasts from normal humans were incubated in serum free medium with up to 100 nmole/mL eicosapentaenoic acid (EPA; bound to albumin in a 4.6:1 ratio) and compared with cells incubated with oleic acid (OA) at similar concentrations. The rate of [14C]OA incorporation into triacylglycerol (TG) (nmol/mg/h) was approximately 5-fold greater than that of [14C]EPA. The mass of TG formed after incubation of fibroblasts with EPA was also significantly lower than that formed with OA (43.2 +/- 9.3 vs. 59.5 +/- 6.6 micrograms/mg cell protein, respectively, P = 0.006). The addition of excess, unlabeled EPA reduced the rate of incorporation of [14C]OA into TG whereas unlabeled OA stimulated incorporation of [14C]EPA into TG. When the cells were preincubated with human serum basic proteins (BP I, II and III), the mass of TG formed (compared to baseline) was significantly higher with the basic proteins whether OA or EPA was studied. Only BP I significantly stimulated the mass of cell phospholipids, an effect that occurred with either OA or EPA in the medium. The results suggest that in cultured normal human fibroblasts, OA is a better substrate for TG synthesis than EPA and that this effect may be accentuated by the presence of the basis proteins.