Background: Familial hypercholesterolemia (FH) is characterized by an autosomal dominantly inherited deficiency of LDL-receptor expression on the cell surface, leading to excess plasma LDL-cholesterol and severe premature atherosclerosis. In patients with heterozygous FH, a major therapeutic objective of conventional drug therapy is to stimulate maximally the residual cellular capacity to produce LDL-receptors via inhibition of endogenous cholesterol synthesis. In contrast, LDL-immunoapheresis aims at reducing the plasma LDL-cholesterol level by extracorporeal elimination of LDL particles. The present study investigates whether LDL-immunoapheresis applied in addition to conventional drug therapy is able to further stimulate residual LDL-receptor expression capacity in patients with heterozygous FH via the withdrawal of external cholesterol supply, thereby exerting a second accessory lipid lowering effect.
Methods: LDL-receptor expression--calculated by transforming mean fluorescence intensities into numbers of antibody binding sites per cell (S/C)--was determined flow-cytometrically on peripheral blood monocytes before and after LDL-apheresis. For a comparison with the maximum obtainable receptor expression capacity, in vitro stimulation experiments under completely LDL deficient conditions were performed.
Results: Prior to LDL-apheresis, LDL-receptor density was comparable in patients (N = 7; 2014 +/- 359 S/C) and controls (N = 10; 1782 +/- 252 S/C). Under in vitro conditions LDL-receptor expression of controls exceeded that of patients with FH by 1.6 times. Immediately after apheresis, LDL-receptor expression significantly increased to almost the same level as obtained by in vitro stimulation (3640 +/- 423 S/C and 3632 +/- 572 S/C). The LDL-receptor expression in FH subsequent to LDL-apheresis exhibited two patterns of kinetics [Type 1: maximal receptor stimulation (288 +/- 70%; P < 0.07) already during apheresis; Type 2: highest receptor density 24 hours after treatment (149 +/- 11%; P < 0.01)].
Conclusions: These results demonstrate that despite drug therapy, LDL-apheresis significantly stimulates the residual LDL-receptor expression in FH via the reduction of available extracellular cholesterol resulting in delayed reappearance of hypercholesterolemia in between treatments.