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 microg/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) < or = 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.
Highlights
Lipid transfer inhibitor protein (LTIP) is an important regulator of cholesteryl ester transfer protein function
We determined that cholesteryl ester transfer protein (CETP) displays no preference for total HDL as a substrate in assays with isolated lipoproteins; the addition of plasma levels of LTIP to these assays completely reconstitutes the 2-fold preference of CETP for HDL that exists in native plasma [9]
Upon ultracentrifugation at d 5 1.21 to remove plasma lipoproteins, the 133 kDa protein was quantitatively recovered in the lipoprotein-free fraction, whereas the lower immunoreactive bands were recovered in the lipoprotein-containing fraction (Fig. 1B)
Summary
Lipid transfer inhibitor protein (LTIP) is an important regulator of cholesteryl ester transfer protein function. 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. We determined that CETP displays no preference for total HDL as a substrate in assays with isolated lipoproteins; the addition of plasma levels of LTIP to these assays completely reconstitutes the 2-fold preference of CETP for HDL that exists in native plasma [9] These data strongly suggest that the enhanced lipid transfer from HDL in control plasma, which generally has been attributed to a preferential interaction between CETP and HDL, is the consequence of LTIP activity.
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