Acylation-stimulating protein.
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Animals↗
Biomedical subjects
K M Cianflone
Publications and source records attributed to K M Cianflone.
The relation between triglyceride synthesis in peripheral tissues and postprandial plasma triglyceride levels: preliminary evidence of a role for acylation stimulating protein.
The present study examines the hypothesis that the rate at which peripheral tissues synthesize triglycerides is a key determinant of the rate at which they are removed from plasma. The cells of greatest interest in this regard would, of course, be the adipocytes. However, serial sampling of this tissue is not possible in man. We have approached the question indirectly, by studying triglyceride synthesis in human mononuclear cells before and after an oral fat load. In addition, plasma levels of Acylation Stimulating Protein (ASP) were measured after an overnight fast and 4 h after ingestion of the oral fat load. Similar measurements were made in the same subjects after fasting overnight and with the fast extended for 4 additional hours. With the extended fast, no parameter measured changed significantly. However, after the oral fat load, the following changes were found to be significant: plasma triglycerides increased (63.6 +/- 31.1 vs 101.5 +/- 43 mg/dl, p less than 0.005) as did d less than 1.006 g/ml triglycerides (42.3 +/- 22.8 vs 78.0 +/- 39.7 mg/dl, p less than 0.005) and plasma ASP (10.4 +/- 2.2 vs 14.7 +/- 2.3 mg/dl, p less than 0.005). In addition, the rate of triglyceride synthesis in the mononuclear cells increased significantly (1.22 +/- 0.3 vs 1.52 +/- 0.3 nmol oleate/mg cell protein per h, p less than 0.005). There was also a significant inverse relation between the increase in the d less than 1.006 triglycerides between 0 and 4 h and the increase in triglyceride synthesis in the mononuclear cells (r = 0.91, p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)
Acylation↗
Levels of acylation stimulating protein in obese women before and after moderate weight loss.
Acylation stimulating protein (ASP) is a small (MW 14,000) basic (pI 9.0) protein which has only recently been purified from human plasma. Since ASP is the most potent known stimulant of triglyceride synthesis in human adipose tissue, the present study was designed to determine if plasma ASP was elevated in patients with moderate obesity, and if so, whether this level changed with weight loss. Fasting plasma ASP levels were determined by competitive ELISA immunoassay in 10 obese women before weight loss, immediately after weight loss, and 3 months after maintaining weight reduction. Their plasma ASP results were compared to 17 age and sex-matched lean controls. With weight loss, plasma ASP decreased significantly: 19.6 +/- 10.7 mg/dl before weight loss vs 15.0 +/- 9.5 mg/dl after weight loss vs 13.8 +/- 7.7 mg/dl 3 months after being weight stable, P less than 0.05 initial vs final value. Nevertheless, plasma ASP was significantly higher than the control value at all three times. Thus, before weight loss, the average ASP in the obese group was four times that in the control group (19.6 +/- 10.7 vs 5.1 +/- 3.6 mg/dl, P less than 0.0005) while even 3 months after weight loss, it remained almost three times above the control group (13.8 +/- 7.7 vs 5.1 +/- 3.6 mg/dl, P less than 0.0005). The data suggest, therefore, that an elevated plasma level of ASP is common in obesity, that the level of plasma ASP may reflect the fat cell mass present in an individual, and raises the possibilities that ASP may play a role in initiation or maintenance of the obese state.
Adult↗
Impaired response of fibroblasts from patients with hyperapobetalipoproteinemia to acylation-stimulating protein.
Acylation-stimulating protein (ASP) is a small, basic, human plasma protein that markedly stimulates triglyceride synthesis in human adipocytes and cultured human skin fibroblasts. The present studies examine the response to ASP of cultured skin fibroblasts from normal subjects patients with hyperapobetalipoproteinemia, patients with familial hypercholesterolemia, and patients with hypertriglyceridemia without hyperapobetalipoproteinemia. Triglyceride synthesis induced by ASP did not differ significantly among the normals, the patients with familial hypercholesterolemia, and the patients with hypertriglyceridemia with normal low density lipoprotein (LDL) apolipoprotein B levels; however, on average, it was markedly reduced in the patients with hyperapobetalipoproteinemia. In all groups studied, evidence of specific saturable binding of radioiodinated ASP was present. Binding, however, was significantly reduced in the groups with hyperapobetalipoproteinemia whereas the other three groups were indistinguishable. By contrast, LDL-specific binding was reduced only in the patients with familial hypercholesterolemia. There was a significant direct relation between the degree of ASP binding and the triglyceride synthesis inducible by ASP. In addition, with the exception of the patients with familial hypercholesterolemia, there was an inverse relation between both ASP-specific binding and ASP-induced triglyceride synthesis in fibroblasts to LDL levels in plasma whereas no relation was evident to plasma high density lipoprotein and very low density lipoprotein.
Adolescent↗
Regulation of apoB secretion from HepG2 cells: evidence for a critical role for cholesteryl ester synthesis in the response to a fatty acid challenge.
Secretion of hepatic apoB lipoproteins removes excess triglyceride from the liver. However, the mechanism by which synthesis of apoB, which occurs on the rough endoplasmic reticulum, is coordinated with synthesis of triglyceride, which takes place in the smooth endoplasmic reticulum, is not known. To examine this question, we have manipulated intracellular synthesis of triglyceride and cholesteryl ester in HepG2 cells and determined the impact of these maneuvers on apoB secretion. Since cholesteryl ester is the only major lipid class synthesized in the rough endoplasmic reticulum, our hypothesis was that, in response to a fatty acid challenge, synthesis of cholesteryl ester rather than synthesis of triglyceride would be the immediate trigger to apoB secretion. Oleate complexed to bovine serum albumin caused intracellular triglyceride synthesis to increase 6-fold and cholesteryl ester synthesis to increase almost 3-fold, while apoB secretion into the medium increased by 2.5-fold (P less than 0.0125) at all time points between 4 and 24 h. Addition of acylation stimulating protein to the medium further stimulated both triglyceride and cholesteryl ester synthesis (58% and 108%, respectively) above oleate alone and this resulted in a 50% increase in apoB secretion (P less than 0.0025). By contrast, both progesterone and 2-bromooctanoate inhibited triglyceride and cholesteryl ester synthesis and these effects were associated with reduced apoB secretion. Lovastatin inhibited cholesteryl ester synthesis (45%, P less than 0.0025); however, at the doses used, triglyceride formation was unaffected. Under these circumstances, apoB secretion was reduced by 25% (P less than 0.05). Similarly, 58-035 (an inhibitor of acyl CoA:cholesterol acyltransferase) on the one hand reduced cholesteryl ester synthesis markedly (59%, P less than 0.005), but on the other increased triglyceride synthesis though not statistically significantly (65%, P NS), and again this resulted in decreased apoB secretion (25%, P less than 0.005). Control experiments established that changes in low density lipoprotein catabolism did not contribute importantly to the quantity of apoB in the medium. Taken together, the data indicate that, at least in HepG2 cells, there are parallel changes in cholesteryl ester synthesis and apoB secretion and suggest that it is cholesteryl ester synthesis, not triglyceride synthesis, that is the immediate regulator of apoB secretion when these cells are exposed to an increased influx of fatty acids. However, alternative or additional regulatory mechanisms, such as, for example, a role for acylation of apoB, are not excluded by these studies.
Apolipoproteins B↗
Purification and characterization of acylation stimulating protein.
We have purified to homogeneity and analyzed the amino acid composition of a small (Mr 14,000), basic (pI 9.0) protein from human plasma. This has been named acylation stimulating protein (ASP) because it markedly stimulates triacylglycerol synthesis in human adipocytes. As well, it stimulates triacylglycerol synthesis in human skin fibroblasts cultured from normal individuals. Characteristic saturation curves for the cell metabolic responses to ASP were observed in both cell types with higher stimulation of oleate incorporation into triacylglycerol being observed in adipocytes. The stimulation of triacylglycerol synthesis was much greater with ASP than with insulin. Neither fatty acid binding protein nor albumin was able to mimic the ASP effect.
Adipose Tissue↗