Lipid transfer inhibitor protein (apolipoprotein F) concentration in normolipidemic and hyperlipidemic subjects

Abstract

Lipid transfer inhibitor protein (LTIP) is an important regulator of cholesteryl ester transfer protein function. We report the development of an immunoassay for LTIP and its use to quantify LTIP in plasma of varying lipid contents. A rabbit antibody against bacterially produced recombinant LTIP detected two LTIP isoforms in plasma differing in carbohydrate content. This antibody was used in a competitive, enzyme-linked immunoassay that uses partially purified LTIP bound to microtiter plates. To optimize LTIP immunoreactivity, plasma samples required preincubation in 1% Tween-20 and 0.5% Nonidet P-40. In normolipidemic plasma, LTIP averaged 83.5 mg/ml. LTIP was 31% higher in males than in females. LTIP was positively associated with HDL cholesterol in normolipidemic males but not in females. In hypertriglyceridemic males, LTIP was only 56% of control values, whereas in hypertriglyceridemic females, LTIP tended to increase. Additionally, in males with normal cholesterol and triglyceride (TG) < 200 mg/dl, LTIP varied inversely with plasma TG. Overall, we have confirmed the negative association between plasma TG levels and LTIP previously suggested by a small data set, but now we demonstrate that this effect is seen only in males. The mechanisms underlying this gender-specific response to TG, and why LTIP and HDL levels correlate in males but not in females, remain to be determined. In plasma, cholesteryl ester transfer protein (CETP) mediates the net transfer of cholesteryl ester (CE) from LDL and HDL to VLDL in return for triglyceride (TG) (1, 2). Several lines of evidence suggest that lipid transfer inhibitor protein (LTIP), also known as apolipoprotein F (3), alters this process and influences the capacity of CETP to remodel lipoproteins. We have shown that among normolipidemic subjects, LTIP activity levels in whole plasma associate negatively with the rate of lipid transfer between VLDL and LDL (4). The addition of exogenous LTIP to native plasma reduces the participation of LDL in lipid transfer events but leads to a dose-dependent increase in the efflux of CE from HDL to VLDL, resulting in HDLs that are markedly better substrates for lecithin:cholesterol acyltransferase (5). We have proposed that this increased CETP activity on HDL happens because plasma TG, not CETP, is rate-limiting for net lipid transfer in normal plasma (6, 7). Thus, in the presence of LTIP, less VLDL TG is expended by transfer to LDL, allowing the transfer from VLDL to HDL to be increased.We substantiated the foregoing observations through studies of patients undergoing continuous ambulatory peritoneal dialysis, who have very low LTIP activity (8). In these patients, the 2-fold preference of CETP for HDL as a lipid donor (compared with LDL) seen in normal plasma is absent. In separate studies, we determined that CETP actually displays no preference for total HDL as a substrate in assays with isolated lipoproteins; however, the addition of plasma levels of LTIP to these assays completely reconstitutes the 2-fold preference of C...

Molecular cloning of hamster lipid transfer inhibitor protein (apolipoprotein F) and regulation of its expression by hyperlipidemia

“…in hypercholesterolemic subjects but reduced in some hypertriglyceridemic individuals (6,8). As an initial inquiry into a possible basis for these changes in LTIP levels, we investigated the response of hamster LTIP to diet-induced hyperlipidemia.…”

“…To determine mRNA turnover, 1 mg/ml actinomycin D was added to cells following agonist treatment, and the decline in LTIP mRNA over time was determined. LTIP mass in conditioned cell media was determined by Western blot as previously described (6). Media was concentrated ?20-fold by cold acetone precipitation.…”

“…Hamster plasma was diluted 10-fold, combined with 53 SDSglycerol buffer (without b mercaptoethanol), and heated as previously described (6), then electrophoresed on 4-20% gradient polyacrylamide gels (Lonza, Rockland, ME)). Separated proteins were electrotransferred to polyvinylidene fluoride, and the membrane was blocked with PBS containing 5% dry milk plus 1% calf serum for 1 h at room temperature.…”

“…In several human studies, LTIP levels were shown to correlate negatively with triglyceride levels (6,8), although this correlation appears to occur only in males (6). However, LTIP levels are elevated in hypercholesterolemia (8).…”

Beyond the Genetics of HDL: Why Is HDL Cholesterol Inversely Related to Cardiovascular Disease?

“…This protein is also known as lipid transfer inhibitor protein which is able to inhibit CETP. Plasma apoF levels were found to be positively associated with HDL cholesterol in males but not in females (Morton et al 2008). The Rader group subsequently showed that overexpression of apoF in mice reduced HDL cholesterol levels by accelerating clearance from the circulation (Lagor et al 2009).…”